IODP Site U1437 is located in the Izu rear-arc region, approximately 330 km west of the Izu-Bonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6–15 Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]-type, Rift-type, and volcanic front [VF]-type). RASC-type has the most enriched Nd and Hf isotope and fluid-immobile trace element ratios and dominates from 9 to 6 Ma. Rift-type, dominant from 15 to 9 Ma, is similar to VF-type in Nd-Hf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VF-type. Less common and randomly distributed VF-type sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15 Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a melt-rich slab component generated at ~125 km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial post-Shikoku basin magmatism in the Izu rear-arc generated Rift-type magmas for about 6 million years before the distinctive RASC-type magmatism began, which then became increasingly enriched.

Key Points

The geochemistry of the volcaniclastics and hyaloclastites of IODP Site U1437 is reported
Two magma types and one magma type of distal origin existed in the Izu rear-arc after spreading in the Shikoku Basin ended
The initial post backarc basin magmatism generated magmas similar to Quaternary Rift-type and evolved to rear-arc seamount chain type

Plain Language Summary

We explore the history of magma genesis in the Izu rear-arc, Japan, by studying the geochemistry of volcaniclastic sediments that were obtained by ocean drilling. This paper presents their Sr-Pb-Nd-Hf isotope ratios and numerical models of magma genesis. We identify the presence of three geochemical types that change through time. Rift-type formed soon after cessation of seafloor spreading in the Shikoku Basin at ~15 Ma and came from a more fertile mantle source with a less hydrous slab component than at the contemporaneous volcanic front. This type is most like basalts from rifts behind the current volcanic front and was unknown before drilling. RASC-type dominates from 9 to 6 Ma and has more enriched Nd and Hf isotope and fluid-immobile trace element ratios that are like those of rocks dredged from the coeval rear-arc seamount chains. VF-type is uncommon, randomly dispersed, and interpreted as derived from the distal volcanic front. The results confirm that the arc front has migrated trenchward since the Miocene.

1 Introduction

Studying oceanic arcs is important for understanding the genesis of continents in subduction zones (e.g., Tamura et al., 2016, 2019; Taylor & McLennan, 1985). The Izu arc is a very suitable place in which to study oceanic arcs for many reasons: (1) the weak influence of crustal contamination relative to mature island arcs (Savov et al., 2006; Stern et al., 2003; Tollstrup et al., 2010); (2) many samples are available from across the arc including the forearc, active magmatic arc, rear-arc rifts and cross chains, and backarc basin (Shikoku Basin); (3) the composition of the subduction inputs (the mantle wedge, subducting sediment, and altered oceanic crust [AOC]) is well known (Chauvel et al., 2009; Hauff et al., 2003; Kelley et al., 2003; Plank et al., 2007); and (4) no slab material has been removed into an accretionary complex, resulting in complete subduction. Many geochemical studies have been conducted in this arc (e.g., Heywood et al., 2020, and references therein), and numerous sites have been drilled in the arc front and forearc regions by ODP Legs 125 and 126 and IODP Leg 352, but there had been no drilling in the rear-arc region prior to IODP Exp. 350 (Tamura et al., 2015).

By “rear-arc” we mean the region between a volcanic front and a backarc basin formed by sea floor spreading. In the Izu case, this region is about 150 km wide, most of which is part of the ~25 km thick arc edifice. The easternmost segment of the rear arc has been the locus of rifting since 2.8 Ma (Ishizuka, Uto, et al., 2002) and may eventually evolve into another backarc basin like the Mariana Trough to the south.

A clear geochemical difference between the arc front and rear-arc regions in Izu is well known (e.g., Hochstaedter et al., 2001; Ishizuka, et al., 2003; Kimura et al., 2010; Tollstrup et al., 2010). The most striking feature is that samples from the rear-arc region have higher concentrations of K₂O and greater light rare earth element (LREE) enrichment than arc front samples that have low K₂O and LREE-depleted patterns (DeBari et al., 2020; Tamura et al., 2015). This geochemical difference is thought to reflect differences in the magma genesis conditions related to the depth of the subducting slab. However, the geochemical characteristics are known only from widely spaced dredged samples dating back to the Neogene. Therefore, one of the scientific objectives of IODP Expedition 350 was to identify temporal changes in the geochemical characteristics of the Izu rear-arc (Tamura et al., 2015).

The rear-arc region in Izu has been subdivided into segments that display distinct geochemical signatures (e.g., Heywood et al., 2020; Hochstaedter et al., 2001). The Active Rifts (AR) directly behind the volcanic front and some of the <2.8 Ma Back Arc Knolls (BAK) are referred to as “Rift-type” by Heywood et al. (2020); they are found 13–50 km behind the volcanic front and have La/Yb = 1.2–2.2 (extensional zone of Figure 1). These Rift-type magmas are more geochemically enriched than volcanic front magmas (VF, La/Yb < 1.2) but not as enriched as the Rear Arc Seamount Chains (RASC-type, La/Yb > 2.2).

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

The structure of the Izu-Ogasawara-Mariana arc system (a) and map of the Izu arc with bore hole position of IODP Site U1437 (b) after Tamura et al. (2015).

IODP Site U1437 is located in the Izu rear-arc region adjacent to Manji Seamount (Figure 1) that is part of a RASC. It was drilled in 2014 to 1806.5 mbsf (Busby et al., 2017; Tamura et al., 2015). RASC-type tephras are found in some intervals in Unit I and in all of Unit II (~4.3 Ma; Heywood et al., 2020). Inferred RASC-type tephras are also found in Unit III (Wurth, 2019; Wurth & DeBari, 2019) and Units IV and V (Sato et al., 2020; Wurth, 2019).

Although drilling was not deep enough to reach rocks of Oligocene or Eocene age, the deepest stratigraphic units (VI and VII) are older than 9 Ma and document the earliest volcanism in the region shortly after spreading in the Shikoku Basin ended. Sato et al. (2020) discovered that rocks of these units are not RASC-type but instead share geochemical characteristics with volcanic front or Rift-type magmas.

Based on study of tuffaceous muds that overlie Unit VI, Gill et al. (2018) suggested that Rift-type magmatism coexisted with RASC-type magmatism throughout the Neogene. Sato et al. (2020) suggested that the geochemical change from Unit VII to Unit V records variations in the subduction component with time.

In order to test these hypotheses, we selected coarse volcaniclastic and in situ hyaloclastite samples from Units IV to Unit VII, including some of those studied by Sato et al. (2020), and analyzed them for major and trace elements and Sr, Nd, Hf, and Pb isotopes. We will compare them to similar results for the tuffaceous mudstones (Gill et al., 2018) and will evaluate the temporal evolution of mantle sources and magmatism in the rear-arc region using these data.

2 Site U1437 and Samples

IODP Site U1437 is located at 31.790 N, 139.026°E, ~330 km west of the Izu-Bonin trench axis, ~90 km west of the arc front volcanoes, and on the southeast slope of the 7 Ma Manji seamount at 2,117 mbsl (Ishizuka, Uto, et al., 2002; Figure 1). Its stratigraphy consists of seven lithologic units (Units I–VII) with one intrusive rhyolite sheet (Igneous Unit 1) within Unit VI (Figure 2; Busby et al., 2017; Tamura et al., 2015). This study focuses on the four units deeper than 1,000 mbsf (Units IV to VII) that include volcaniclastics and hyaloclastites and consist mostly of coarse lava clasts, lapilli-tuff, tuff, and tuffaceous mudstones. Biostratigraphic and magnetostratigraphic control down to 1,320 mbsf (Unit V) indicates that the age of Units IV to V is 6–9 Ma (Busby et al., 2017; Tamura et al., 2015). Zircons from Igneous Unit 1 have U-Pb ages of 13.9 Ma (Schmitt et al., 2018). One zircon age from a felsic lapilli-tuff in Unit VII at 1,656 mbsl has a weighted average U-Pb age (triplicate analyses) of 15.4 ± 0.8 Ma (Schmitt et al., 2018). Although further evaluation of zircons from the volcaniclastic units is necessary to check for contamination from drilling mud (Schmitt et al., 2018), the geochemical characteristics of Unit VII volcaniclastics are expected to reflect the mantle source of early Izu-rear arc magmatism soon after cessation of Shikoku Basin spreading at ~15 Ma (Okino et al., 1994).

Geochemical and lithological data for the samples analyzed in this study are given in Table 1. Data from Sato et al. (2020) are plotted in our figures and therefore included in Table 1 for completeness. Most samples are from tuffs to lapillistones, and we analyzed individual clasts whenever possible (see Table 1 for details about each sample). Most clasts are crystalline rock fragments, not glass. They are nonvesicular to sparsely vesicular and contain phenocrysts of plagioclase > clino-pyroxene phenocrysts. Samples that lack some data were too small for complete analysis. Most samples from Units VI and VII are variably altered to gray-green colors due to low-grade metamorphism. However, there are no amphiboles or phosphate minerals in the analyzed samples. There is no correlation between P₂O₅ and CaO, MgO, Sr, or REE (rare earth element), showing no effect of phosphate. The stratigraphic position of the samples analyzed in this study is shown in Figure 2.

Table 1. Results of Major and Trace Elements and Sr-Nd-Hf-Pb Isotopic Analyses for U1437 Samples

Unit	IV	IV	IV	IV	IV	IV	V	V	V	V	V	V	VI	VI	VI	VI	VI	VI	VI	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	VII	1 (in VI)	1 (in VI)	1 (in VI)	1 (in VI)	1 (in VI)
Hole	U1437D	U1437D	U1437D	U1437D	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E	U1437E
Core	64R	67R	69R	70R	6R	6R	17R	17R	19R	21R	25R	25R	28R	35R	35R	41R	41R	41R	41R	42R	42R	43R	44R	45R	47R	51R	51R	53R	54R	56R	56R	56R	57R	57R	57R	58R	58R	59R	59R	60R	61R	66R	66R	66R	69R	70R	70R	70R	71R	71R	72R	75R	78R	79R	79R	79R	35R	35R	35R	35R	35R
Section	1	CC	2	CC	3	3	2	2	2	6	3	3	1	3	3	2	2	4	4	4	4	2	1	1	1	1	1	1	2	3	5	5	1	1	1	1	2	1	2	2	1	4	5	5	3	2	4	6	2	2	2	3	1	1	2	2	1	1	2	2	2
Position (cm)	118–121	1–4	64–67	19–20	100–102	106–109	122–124	125–127	0–2	43–44	54–56	59–60	9–11	60–62	60–63	19–21	19–21	119–122	129–133	95–97	106–112	98–101	15–17	80–83	39–41	40–42	78–81	64–66	87–89	0–3	123–125	128–130	58–60	58–60	71–75	34–36	36–38	62–64	60–64	60–66	10–14	101–103	97–99	103–105	9–11	101–105	129–131	35–37	13–15	69–71	109–112	70–72	129–131	46–48	37–41	38–40	110–113	123–125	13–16	44–46	48–51
Depth (mbsf)	1018.98	1048.43	1058.24	1075.06	1119.9	1119.95	1216.01	1215.99	1234.03	1259.88	1293.87	1293.91	1320.09	1391.63	1391.62	1448.2	1448.2	1452.01	1452.11	1461.55	1461.68	1468.27	1475.86	1486.22	1505.3	1544.21	1544.58	1563.85	1575.38		1598.48	1598.54	1602.78	1602.79	1603.1	1612.25	1613.7		1623.68	1633.42	1641.22	1695.05	1696.52	1696.57	1712.27	1721.415	1724.49	1726.5	1729.91	1730.47	1740.62	1770.72	1793.6	1797.27	1798.58	1798.58	1389.22	1389.34	1389.67	1389.97	1390.02
Lithology of sample	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapillistone	Lapillistone	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Lapilli-tuff	Moderately quartz-feldspar phyric rhyodacite clast (lapilli 2-64 mm)	Moderately quartz-feldspar phyric rhyodacite clast	Highly pyroxene feldspar phyric andesite clast	Highly pyroxene feldspar phyric andesite clast (lapilli 2-64 mm)	Basalt glass clast	Lapilli-tuff	Vitric lapilli-tuff	Lapilli-tuff	Highly pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Vitric lapilli-tuff	Vitric lapilli-tuff	Vitric lapilli-tuff	Highly pyroxene-feldspar phyric andesite clast (volcanic block >64 mm)	Andesite clast	Vitric lapilli-tuff	Sparsely pyroxene feldspar phyric andesite clast (lapilli 2-64 mm)	Dacite clast	Highly feldspar phyric andesite clast	Highly feldspar phyric andesite clast (volcanic block >64 mm)	Feldspar phyric andesite clast	Feldspar phyric andesite clast (volcanic block >64 mm)	Moderately feldspar phyric andesite clast (lapilli 2-64 mm)	Moderately feldspar phyric andesite clast (volcanic block >64 mm)	Moderately feldspar phyric andesite clast (volcanic block >65 mm)	Andesite clast	Moderately feldspar phyric andesite clast (lapilli 2-64 mm)	Aphyric andesite clast (lapilli 2-64 mm)	Lapilli-tuff	Highly pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Moderately pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Moderately pyroxene feldspar phyric andesite clast	Highly amphibole pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Highly pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Moderately feldspar pyroxene phyric andesite clast (lapilli 2-64 mm)	Highly pyroxene feldspar phyric andesite clast (lapilli 2-64 mm)	Moderately pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Highly feldspar pyroxene phyric andesite clast (volcanic block >64 mm)	Moderately pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Sparsely pyroxene phyric andesite clast (volcanic block >64 mm)	Moderately pyroxene feldspar phyric andesite clast (lappili 2-64 mm)	Moderately pyroxene feldspar phyric andesite clast (lapilli 2-64 mm)	Moderately pyroxene feldspar phyric andesite clast	Moderately pyroxene feldspar phyric andesite clast (volcanic block >64 mm)	Intrusive sheets	Intrusive sheets	Intrusive sheets	Intrusive sheets	Intrusive sheets
Nature of sample					Whole sample			Whole sample	Whole sample	Whole sample	Whole sample			Single clast	Single clast		Single clast	Single clast	Whole sample	Whole sample	Whole sample	Single clast	Whole sample	Whole sample	Whole sample	Single clast		Whole sample	Single clast	Single clast		Single clast		Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Whole sample	Single clast	Single clast		Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Single clast	Single clast		Single clast	Whole rock	Whole rock	Whole rock	Whole rock	Whole rock
Host rock					-			-	-	-	-			lappilistone			tuff breccia		-		-	lappilistone				lappili tuff			lapilli tuff			lapilli tuff		lappilistone	lappilistone	tuff breccia	tuff breccia		lapillituff	tuff breccia	-	tuff breccia & lapilli tuff	tuff breccia		tuff breccia	volcanic breccia	lappilistone	lapillituff	not recovered	not recovered	tuff breccia	lapillituff	not recovered	lappilistone		lappilistone		lapilli tuff, lapillistone		lapilli tuff, lapillistone
Sampling ID	HAME	HAME	HAME	HAME	SATO	HAME	HAME	SATO	SATO	SATO	SATO	HAME	HAME	SATO	DEBA	HAME	SATO	DEBA	SATO	DEBA	SATO	SATO	DEBA	DEBA	DEBA	SATO	HAME	DEBA	SATO	DEBA	HAME	SATO	HAME	SATO	SATO	SATO	SATO	DEBA	SATO	SATO	SATO	SATO	SATO	HAME	SATO	SATO	SATO	SATO	SATO	SATO	SATO	SATO	SATO	SATO	HAME	SATO	DEBA	SATO	DEBA	SATO	DEBA
Acid leached (M:Major, T:Trace, I:Isotope)	I	I	I	I	I	I	I	I	I	I	I	I	I	I		I	I		I		I					I	I				I	I	I	I	I	I	I			I	I	I	I	I	I	I	I	I	I	I	I			I	I	I		I		I
Type^a	RASC-type	RASC-type	RASC-type	RASC-type		VF-type			RASC-type	RASC-type		VF-type	VF-type	Rhyolite	Rhyolite			Rift-type		Rift-type	Rift-type		Rift-type	Rift-type	Rift-type		Rift-type	Rift-type		Rift-type	Rift-type						Rift-type	Rift-type		Rift-type				Rift-type	Rift-type		Rift-type	Rift-type		Rift-type				Rift-type			Rhyolite	Rhyolite	Rhyolite	Rhyolite	Rhyolite
Major element wt %
SiO₂					54.41			52.38	52.04	52.21	57.93			71.62	72.94		51.13	51.77	53.22	50.43	50.50	55.55	53.16	52.15	51.22	49.94		59.26	56.37	68.26		49.06		52.95	52.04	50.61	49.76	58.94	52.15	52.24	51.37	52.86	56.87		53.32	55.91	52.94	51.34	56.49	52.44	53.18	56.61	56.18	53.87		53.92	77.89	77.32	77.92	74.00	76.13
TiO₂					0.99			1.21	0.86	1.07	0.71			0.25	0.24		1.03	0.94	0.97	0.99	0.91	0.75	1.01	0.85	0.75	0.93		0.80	1.29	0.73		0.96		0.88	1.13	1.05	0.88	0.94	1.06	1.14	0.98	1.10	0.95		1.09	0.96	1.05	1.30	1.07	1.14	0.81	0.78	0.80	1.14		1.12	0.28	0.29	0.27	0.29	0.26
Al₂O₃					17.00			18.81	17.61	17.72	18.96			15.52	14.78		19.67	19.17	18.22	20.67	22.16	19.01	18.46	20.41	19.34	19.98		16.01	19.24	15.04		21.79		19.31	19.75	20.30	20.33	19.19	19.02	17.24	19.97	19.37	16.79		18.99	18.99	21.14	19.62	18.20	19.67	18.68	16.73	17.73	21.18		21.16	11.01	11.99	11.93	13.81	13.14
Fe₂O₃					11.66			10.27	10.42	11.23	8.95			1.94	1.86		8.90	11.31	10.42	10.93	9.53	5.60	11.18	9.64	10.48	9.12		9.54	7.08	2.84		9.27		7.87	8.72	9.08	9.96	6.40	10.50	11.70	10.43	8.72	9.35		9.32	8.39	7.35	10.04	8.26	9.31	8.27	6.19	6.65	6.28		6.30	2.96	2.14	2.42	1.99	1.85
MnO					0.18			0.13	0.13	0.16	0.26			0.04	0.04		0.23	0.26	0.23	0.16	0.14	0.19	0.15	0.14	0.15	0.22		0.12	0.18	0.08		0.18		0.17	0.15	0.14	0.20	0.17	0.31	0.32	0.11	0.25	0.22		0.20	0.20	0.16	0.21	0.22	0.17	0.21	0.19	0.14	0.10		0.08	0.04	0.03	0.04	0.05	0.03
MgO					5.03			4.95	5.00	4.25	2.09			0.22	0.32		4.22	6.84	7.24	6.36	5.96	2.83	7.36	6.54	7.98	5.96		6.15	3.39	0.08		3.92		3.94	3.95	3.90	4.59	1.70	3.70	4.83	6.78	4.69	3.93		4.62	3.42	3.39	4.77	2.85	4.00	5.88	4.68	5.11	3.67		3.80	0.60	0.22	0.47	0.22	0.24
CaO					7.46			8.50	9.30	8.78	2.97			2.06	1.99		9.85	8.14	7.65	9.17	9.80	12.40	6.19	9.11	7.57	11.23		6.46	6.86	3.34		11.83		9.64	9.50	10.76	11.87	4.81	8.17	7.83	9.27	8.06	5.69		6.81	7.25	9.09	8.16	7.47	9.03	10.59	10.53	8.61	7.78		7.92	0.88	0.60	0.57	1.24	0.81
Na₂O					1.49			2.85	2.80	2.92	7.94			5.46	5.06		3.58	1.62	1.24	1.63	1.27	2.03	1.50	1.41	2.07	2.64		1.45	3.70	7.50		2.96		4.53	3.93	3.46	2.38	7.82	4.38	3.10	1.36	4.50	5.47		5.15	4.23	4.34	4.25	4.42	3.68	2.76	2.73	3.07	5.01		5.25	6.07	6.64	6.72	7.55	7.29
K₂O					1.63			0.48	1.24	1.37	0.44			2.64	2.92		0.88	0.49	0.72	0.15	0.10	0.23	1.83	0.31	1.31	0.11		0.34	1.53	0.62		0.20		0.39	0.69	0.47	0.23	0.35	0.60	1.36	0.12	0.62	0.64		0.53	0.83	0.63	0.59	0.87	0.48	0.17	0.22	0.45	0.89		0.35	0.36	0.32	0.31	0.55	0.60
P₂O₅					0.11			0.16	0.21	0.23	0.14			0.06	0.04		0.20	0.11	0.16	0.28	0.10	1.24	0.12	0.15	0.11	0.16		0.07	0.31	1.32		0.15		0.14	0.25	0.18	0.12	0.37	0.27	0.46	0.08	0.25	0.19		0.23	0.21	0.21	0.20	0.19	0.23	0.27	0.98	0.54	0.26		0.22	0.09	0.10	0.07	0.08	0.06
Total					99.96			99.74	99.61	99.94	100.39			99.81	100.18		99.69	100.64	100.07	100.77	100.47	99.83	100.95	100.71	100.99	100.29		100.21	99.95	99.82		100.32		99.82	100.11	99.95	100.32	100.68	100.16	100.22	100.47	100.42	100.10		100.26	100.39	100.30	100.48	100.04	100.15	100.82	99.64	99.28	100.18		100.12	100.18	99.65	100.73	99.78	100.39
LOI					8.10			5.95	11.29	5.12	7.33			2.32	1.37		3.42	8.83	6.08	10.19	5.20	3.69	9.63	10.22	11.12	2.73		9.60	11.53	1.33		2.50		1.41	3.92	3.05	1.30	1.83	1.95	2.14	4.30	1.88	2.49		2.64	2.30	1.51	2.55	1.68	2.33	2.33	1.82	3.07	2.05		2.11	2.80	2.27	1.65	2.53	0.96
Trace element by XRF (ppm)
Sc															4.53			37.92		28.16			31.50	30.44	25.00			29.90		21.97								25.91																			7.43		5.94		6.24
V					337.80			295.20		316.30	88.00			26.60	31.22		311.10	261.26	283.00	278.90	342.00	261.20	282.90	258.37	228.80	292.70		232.80		78.11		349.90		275.40	333.70	362.60	337.90	218.47	361.40	389.10	360.40	277.80	234.50		278.00	251.80	296.30	395.90	316.40	325.50	322.40	301.20	359.00	247.80		254.50	45.04	35.40	31.19	40.30	39.50
Cr					13.80			21.60		16.60	8.60			5.50	3.05		64.40	64.94	53.40	12.14	16.70	75.50	29.20	28.57	39.90	112.30		16.70		3.55		85.40		159.00	73.80	107.40	76.10	17.43	17.20	5.10	41.10	24.10	45.30		25.60	12.30	29.80	16.90	17.50	15.30	195.90	336.20	193.80	25.30		23.30	4.40	4.90	1.98	3.30	3.27
Ni					13.20			27.90		17.40	6.40				0.89		22.40	25.64	40.90	10.65	12.20	18.90	16.70	15.27	24.20	46.40		12.40		4.53		32.10		50.70	21.90	49.60	31.50	9.16	17.30	10.70	18.30	12.60	21.70		13.70	9.40	18.80	14.50	11.00	10.80	51.10	54.60	50.70	24.30		20.90	1.76		1.78		1.88
Cu					126.30			47.60		41.70	41.20			2.90	0.79		48.90	154.44	118.50	62.98	62.40	69.40	64.90	56.44	47.70	64.50		48.70		9.06		89.80		40.50	85.30	35.60	54.80	41.27	78.40	57.60	57.30	60.00	33.30		40.00	32.30	53.50	41.90	25.80	38.40	57.90	63.10	26.20	49.30		20.70	2.64	2.80	1.78	2.00	2.18
Zn					111.80			102.50		109.50	137.80			21.60	21.97		95.30	87.62	108.20	72.14	85.00	151.00	80.00	60.77	68.30	90.60		66.30		72.20		90.90		129.10	110.30	82.60	76.50	90.52	108.20	175.90	87.70	100.30	92.80		96.30	95.20	88.30	118.80	98.00	89.80	83.10	102.40	98.30	79.90		62.50	39.76	29.70	32.97	27.60	25.44
Rb					14.00			6.60		8.20	5.40			24.20	25.41		6.70	7.52	8.50	3.18	2.30	2.40	12.90	3.84	9.20	1.90		3.90		5.42		2.90		5.10	7.20	4.90	2.00	3.35	6.60	10.10	3.20	7.90	8.20		5.80	8.50	6.80	6.60	8.80	5.00	3.70	4.60	8.50	6.90		3.60	2.25	2.80	1.78	4.10	3.37
Sr					507.40			373.00		324.80	67.20			121.80	111.31		267.00	125.53	161.20	173.93	208.00	249.10	129.20	156.42	280.10	254.30		134.30		98.89		274.00		223.30	275.70	297.00	268.80	194.24	278.20	219.30	219.70	283.10	220.60		262.50	276.10	296.90	280.10	257.10	338.40	230.70	267.90	264.60	308.30		286.20	7.62		6.63		7.72
Y					24.40			26.60		30.30	37.80			11.10	12.81		36.50	30.29	25.20	42.19	19.00	34.00	23.20	25.31	24.00	27.70		20.70		59.40		21.50		26.70	28.90	22.90	19.10	49.64	32.00	70.80	18.10	34.50	27.90		35.90	34.00	33.80	35.50	29.90	32.10	25.30	22.30	34.90	28.20		23.80	21.69	19.60	18.81	16.70	18.51
Zr					57.10			113.70		88.30	76.20			107.10	103.62		94.30	48.40	63.20	56.00	52.50	35.90	59.20	44.60	41.20	79.40		55.00		112.29		54.00		77.40	70.80	64.80	49.70	149.13	68.60	74.00	60.20	92.90	88.90		102.90	89.20	92.00	94.20	84.10	92.40	44.80	42.20	48.50	98.00		96.80	91.45	104.90	103.46	106.70	102.56
Nb					0.70			3.20		2.50				0.90	0.59		1.50	0.00	0.70	0.30	1.00	0.60	0.60	0.20	0.60	0.90		0.00		2.07		0.50		1.00	1.20	1.20	0.80	1.28	1.00	1.00	0.90	1.30	1.40		1.50	0.90	1.10	1.20	1.10	1.00	0.60	0.80	0.80	1.70		1.50	0.98	0.90	1.29	0.90	1.49
Ba					237.30			114.10		133.70	50.00			267.20	280.43		64.60	45.54		14.93		37.80	55.00	17.93	113.60			27.60		93.97		26.20		74.90	96.00	77.50	38.50	105.89	223.70	81.40		113.80	86.00		108.60	79.10	61.00	98.50	122.60	77.00	29.30		52.60	288.40		90.00	10.75		12.67		23.76
Pb					2.90			2.80		3.80	6.50			2.10	2.46		2.20	3.07	6.40	1.69	2.10	2.60	3.40	0.89	5.20			2.40		6.01		1.90		3.10	2.00	2.40	1.90	3.15	1.60	2.40	2.10	3.30	2.90		3.20	1.90	3.30	1.70	1.60	3.00	1.50	2.20	1.40	2.50		3.20	2.05	3.30	4.16	1.30	1.78
Th								2.90							1.28			0.00		0.20			0.00	0.00	1.20			0.20		1.08					1.70			0.30																			0.78		1.09		0.89
Trace element by ICP-MS (ppm)
Sc	23.0	26.4	17.0	24.1					24.7	29.6					4.25			40.1		30.0	22.3		32.2	31.5	24.6			31.7		22.7							30.4	26.0		32.1					26.8		23.8	32.5		28.3				26.0			7.31	3.85	5.75		5.79
V	156	191	141	216		332						57.9	242														276				367													205
Cr	29.11	123	6.70	7.23		18.2						5.21	13.9														108			23.4	86.7													29.8
Co	17.9	18.3	13.0	19.2		35.9			33.4	29.0		21.1	18.7								19.6						25.7			10.4	30.4						27.6			20.0				16.2	18.8		17.4	23.8		21.7				16.1				1.78
Ni	8.44	27.39	3.24	5.50		10.8			27.8	16.2		20.7	6.88								9.64						38.4				31.5						31.4			8.35				14.0	11.2		15.6	13.5		9.90				21.4				1.11
Cu	26.2	39.6	11.0	36.2		113			42.9	38.6		49.3	69.2								57.4						60.5			0.00	91.4						50.7			56.1				32.7	37.5		49.1	40.8		35.1				44.1				3.15
Zn	94.1	91.3	81.2	88.8		115						95.9	97.4														91.6				102													81.2
Rb	7.48	10.2	15.2	7.38		11.6			10.9	7.12		6.05	10.4		23.9			7.09		2.09	1.34		12.0	2.92	8.06		1.44	2.39		4.96	2.27						1.15	2.45		8.92				11.2	4.66		5.53	5.30		4.06				5.94			1.78	1.49	1.46		2.94
Sr	382	388	371	439		455			359	316		83.12	308		110			129		179	190		133	161	279		264	138		99.5	283						260	197		211				196	242		279	267		320				293			7.99	6.47	6.76		8.00
Y	36.3	23.1	30.9	27.9		29.1			23.3	29.8		25.7	30.7		11.6			30.5		43.0	17.2		23.8	25.3	22.2		32.84	20.4		57.8	26.3						18.1	49.4		69.2				29.0	34.2		32.0	34.9		30.7				26.6			20.9	16.2	17.7		16.9
Zr	119	107	139	114		57.8			65.4	82.7		47.0	54.9		104			50.4		59.2	49.6		61.2	46.8	44.8		80.9	57.3		113	61.3						49.7	154		75.2				84.0	102		89.8	96.0		92.7				96.8			92.1	93.6	102		103
Nb	6.68	6.60	6.25	6.31		0.59			2.47	2.44		0.46	0.40		0.88			0.72		0.86	0.63		0.89	0.77	1.32		1.03	0.74		1.41	0.85						0.63	1.86		1.02				1.15	1.20		1.15	1.29		1.24				1.47			0.82	0.80	0.87		0.85
Cs	0.13	0.21	0.16	0.13		0.23			0.21	0.18		9.72	0.39		0.09			0.30		0.08	0.06		0.13	0.08	0.34		0.03	0.09		17.0	0.04						0.02	0.14		0.08				0.22	0.14		0.15	0.10		0.30				0.11			0.03	0.02	0.03		0.03
Ba	105	390	163	106		219			156	133		107	210		289			45.0		16.3	13.3		59.6	16.3	116.70		31.6	27.5		22.0	47.3						48.5	103		95.4				139	116		76.2	108		73.8				297			13.6	11.7	12.1		22.6
La	14.7	9.25	16.1	12.9		3.35			7.33	7.87		3.38	2.73		4.37			4.76		7.88	2.94		4.20	3.82	4.04		5.17	3.35		11.7	4.04						3.13	8.68		11.4				5.33	5.92		4.71	4.19		6.17				5.46			5.71	6.03	5.43		3.66
Ce	32.6	20.7	34.6	27.9		8.38			17.4	20.4		8.13	7.77		11.9			10.6		17.4	8.07		10.9	10.1	10.7		14.9	9.29		8.96	11.6						8.50	23.9		26.8				14.6	17.8		13.5	13.3		16.3				14.8			15.7	14.2	14.4		10.5
Pr	4.50	2.93	4.64	3.78		1.42			2.39	3.01		1.32	1.41		1.69			1.67		2.65	1.30		1.76	1.61	1.68		2.35	1.51		3.60	1.85						1.38	3.84		4.00				2.41	2.80		2.27	2.26		2.52				2.29			2.19	1.88	2.02		1.55
Nd	22.3	14.2	20.9	17.7		8.16			10.8	14.3		7.03	8.08		7.16			8.42		13.2	6.74		9.06	8.13	8.66		11.7	7.62		17.4	9.81						7.04	18.9		19.8				12.3	14.1		11.6	11.5		12.6				11.6			9.40	8.05	8.51		6.73
Sm	5.66	3.68	5.13	4.67		2.78			2.85	3.96		2.38	2.82		1.91			2.87		4.06	2.13		2.94	2.82	2.85		3.47	2.55		5.19	3.02						2.19	5.88		5.58				3.60	4.20		3.65	3.71		3.79				3.30			2.36	1.90	2.15		1.88
Eu	1.78	1.25	1.46	1.51		1.00			0.97	1.22		0.86	1.07		0.61			1.06		1.51	0.79		1.04	1.06	1.01		1.20	0.89		1.57	1.07						0.81	1.71		1.87				1.21	1.34		1.26	1.26		1.25				1.19			0.73	0.57	0.68		0.60
Gd	6.51	4.14	5.44	5.17		3.87			3.33	4.66		3.17	4.01		1.90			4.06		5.71	2.77		3.67	3.75	3.59		4.44	3.15		6.62	3.73						2.83	7.30		7.83				4.42	5.18		4.60	4.70		4.64				3.90			2.63	2.06	2.35		2.21
Tb	1.04	0.68	0.87	0.81		0.70			0.57	0.79		0.60	0.75		0.33			0.74		1.02	0.49		0.67	0.66	0.62		0.81	0.56		1.21	0.67						0.50	1.29		1.34				0.78	0.90		0.81	0.85		0.80				0.65			0.46	0.36	0.43		0.41
Dy	6.41	4.18	5.56	5.04		4.67			3.83	5.14		4.13	5.07		2.07			4.94		6.92	3.22		4.55	4.34	3.86		5.57	3.85		8.29	4.59						3.26	8.20		9.33				5.16	5.86		5.46	5.77		5.28				4.30			3.19	2.18	2.82		2.80
Ho	1.34	0.86	1.12	1.02		1.02			0.86	1.12		0.90	1.12		0.43			1.10		1.55	0.69		0.97	0.92	0.82		1.21	0.84		1.90	0.99						0.70	1.77		2.18				1.12	1.27		1.18	1.26		1.13				0.97			0.70	0.55	0.62		0.61
Er	4.10	2.66	3.53	3.10		3.06			2.65	3.40		2.74	3.42		1.29			3.19		4.28	2.08		2.69	2.63	2.13		3.59	2.35		5.70	2.95						2.10	4.91		6.85				3.32	3.83		3.56	3.82		3.39				3.16			2.14	1.82	1.89		1.83
Tm	0.59	0.39	0.54	0.47		0.45			0.39	0.50		0.40	0.50		0.20			0.47		0.63	0.30		0.39	0.38	0.31		0.50	0.35		0.88	0.42						0.30	0.73		0.99				0.48	0.57		0.52	0.56		0.50				0.49			0.35	0.30	0.29		0.29
Yb	3.91	2.66	3.77	3.15		2.98			2.51	3.38		2.74	3.28		1.43			2.94		3.83	2.00		2.55	2.34	1.91		3.29	2.17		5.81	2.81						2.03	4.67		6.49				3.16	3.80		3.48	3.68		3.28				3.39			2.43	2.11	2.07		2.05
Lu	0.59	0.40	0.58	0.47		0.46			0.39	0.52		0.40	0.50		0.24			0.47		0.61	0.30		0.40	0.38	0.32		0.50	0.35		0.95	0.43						0.31	0.73		1.02				0.49	0.58		0.52	0.56		0.50				0.53			0.41	0.35	0.36		0.35
Hf	3.32	2.94	3.73	3.14		1.90			1.79	2.38		1.66	1.93		2.91			1.62		1.80	1.49		1.86	1.40	1.28		2.55	1.67		3.15	2.00						1.50	4.39		2.23				2.59	2.96		2.62	2.76		2.67				2.72			2.55	2.61	2.81		2.78
Ta						0.06			0.19	0.17		0.04	0.05		0.11			0.06		0.08	0.04		0.08	0.07	0.15		0.10	0.07		0.13	0.08						0.04	0.18		0.08				0.10	0.09		0.08	0.09		0.09				0.11			0.11	0.08	0.11		0.11
Tl									0.19	0.07											0.02																0.01			0.08					0.04		0.06	0.06		0.04				0.06				0.01
Pb	3.20	2.94	3.38	3.17		2.83			9.24	5.03		15.1	3.79		2.31			2.57		1.83	1.88		3.61	1.48	5.30		1.39	2.47		4.52	1.76						1.51	3.67		1.69				2.25	2.89		2.24	1.71		2.13				2.31			2.42	2.79	3.56		1.89
Th	1.89	1.74	2.32	1.97		0.29			1.41	1.30		0.20	0.20		1.21			0.30		0.47	0.33		0.46	0.37	0.48		0.59	0.39		0.91	0.45						0.34	1.27		0.57				0.66	0.75		0.65	0.69		0.67				0.82			1.01	0.99	1.11		1.12
U	0.62	0.56	1.07	0.66		0.21			0.53	0.52		0.25	0.18		0.44			0.18		0.38	0.22		0.20	0.18	0.29		0.27	0.15		0.49	0.19						0.12	0.54		0.23				0.24	0.34		0.31	0.28		0.35				0.38			0.46	0.42	0.48		0.47
Measured isotope ratios (leached sample)^b
⁸⁷Sr/⁸⁶Sr	0.703267	0.703032	0.703033	0.703015		0.706169	0.703342		0.703057	0.703142		0.703759	0.703518			0.703106					0.703121						0.703094				0.703129		0.703306				0.703059			0.703126				0.703651	0.703420		0.703230	0.703410		0.703274				0.703475	0.703308			0.704133
2SE	0.000007	0.000009	0.000009	0.000009		0.000009	0.000010		0.000008	0.000008		0.000008	0.000008			0.000009					0.000007						0.000012				0.000008		0.000009				0.000006			0.000007				0.000011	0.000007		0.000007	0.000008		0.000007				0.000006	0.000006			0.000007
¹⁴³Nd/¹⁴⁴Nd	0.513026	0.512930	0.512997	0.513003		0.513068	0.513026		0.513006	0.513047		0.513110	0.512896			0.513086					0.513114						0.513087										0.513100			0.513097					0.513080		0.513100	0.513090		0.513097				0.513091				0.513113
2SE	0.000007	0.000007	0.000009	0.000007		0.000008	0.000012		0.000008	0.000008		0.000009	0.000009			0.000011					0.000009						0.000014										0.000009			0.000008					0.000008		0.000009	0.000009		0.000009				0.000008				0.000006
²⁰⁶Pb/²⁰⁴Pb	18.2767	18.2953	18.2745	18.2778		18.2979	18.3213		18.3979	18.3186		18.4489	18.3128			18.2741					18.2462						18.2182				18.2399		18.2158				18.2306			18.2086				18.2398	18.2433		18.2609	18.2382		18.2392				18.2519	18.2335			18.5303
2SE	0.0003	0.0003	0.0004	0.0003		0.0003	0.0003		0.0013	0.0009		0.0003	0.0003			0.0003					0.0025						0.0003				0.0004		0.0003				0.0024			0.0010				0.0002	0.0010		0.0010	0.0008		0.0011				0.0011	0.0002			0.0009
²⁰⁷Pb/²⁰⁴Pb	15.5125	15.5259	15.5063	15.5165		15.5448	15.5310		15.5620	15.5238		15.6001	15.5302			15.5203					15.4917						15.4846				15.4974		15.4813				15.4858			15.4914				15.4879	15.4951		15.5020	15.4971		15.4969				15.5001	15.4912			15.5230
2SE	0.0002	0.0003	0.0004	0.0002		0.0002	0.0003		0.0011	0.0009		0.0002	0.0002			0.0002					0.0026						0.0003				0.0002		0.0003				0.0025			0.0009				0.0002	0.0009		0.0008	0.0007		0.0010				0.0009	0.0002			0.0008
²⁰⁸Pb/²⁰⁴Pb	38.1660	38.2220	38.1382	38.1634		38.2729	38.2492		38.4047	38.2123		38.5712	38.2155			38.1681					38.0545						38.0403				38.0987		38.0312				38.0335			38.0429				38.0689	38.0691		38.1030	38.0719		38.0690				38.0868	38.0740			38.2847
2SE	0.0007	0.0009	0.0014	0.0007		0.0007	0.0007		0.0035	0.0023		0.0007	0.0007			0.0007					0.0042						0.0008				0.0008		0.0009				0.0034			0.0024				0.0007	0.0024		0.0023	0.0020		0.0027				0.0023	0.0007			0.0022
¹⁷⁶Hf/¹⁷⁷Hf	0.283180	0.283182	0.283190	0.283217		0.283243	0.283203		0.283195	0.283208		0.283247	0.283270			0.283256					0.283239						0.283246				0.283238		0.283236				0.283239			0.283239				0.283253	0.283242		0.283241	0.283234		0.283242				0.283241	0.283241			0.283237
2SE	0.000004	0.000004	0.000003	0.000004		0.000003	0.000003		0.000005	0.000004		0.000004	0.000004			0.000004					0.000004						0.000004				0.000004		0.000003				0.000004			0.000004				0.000004	0.000005		0.000005	0.000005		0.000005				0.000005	0.000003			0.000005

Note. Major and trace elements with italic numbers are from Sato et al. (2020).
^a The samples with Nb/Yb ratio of <0.2, 0.2–0.7, and >0.7 are classified as VF-type, Rift-type, and RASC-type. The blanks are without trace element data.
^b 2SE: two standard error.

3 Analytical Methods

Samples were prepared and analyzed in three laboratories. Briefly, concentrations were measured using unleached samples after obviously altered material was removed, whereas isotope ratios were measured on strongly leached material. Most major and trace element concentrations were measured using XRF and solution ICPMS, Sr isotope using TIMS, Nd isotope using TIMS and MC-ICPMS, and Pb and Hf isotopes using MC-ICPMS. We normalized ⁸⁷Sr/⁸⁶Sr to 0.710251 for SRM 987 (Miyazaki & Shuto, 1998), ¹⁴³Nd/¹⁴⁴Nd to 0.512115 for JNdi-1 (Tanaka et al., 2000), ¹⁷⁶Hf/¹⁷⁷Hf to 0.282160 for JMC-475 (Patchett et al., 2004), and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios to 16.9416, 15.5000, and 36.7262 for SRM 981 (Baker et al., 2004). The trace element data determined by ICPMS were used in figures. Complete details are given in supporting information.

4 Results

4.1 Major and Trace Elements

The major and trace element concentrations of samples from the upper part of Unit IV to the lower part of Unit VII are presented in Table 1 and shown in Figures 3-5. Because the samples analyzed for major and trace element analyses in this study were not leached in acid, they are variably affected by alteration. In the following discussion and in Figure 3, the major elements are normalized to 100% on a volatile-free basis, with total iron calculated as FeO. Although Sato et al. (2020) only eliminated the samples with high MnO (>0.3 wt%) and/or P₂O₅ (>0.4 wt%) from their discussion using major elements, we eliminate these samples for all discussion and plotting on figures. However, samples without major element data are included in the trace element and isotope figures.

Most samples from Units IV and V are medium-K and tholeiitic in composition (Figure 3). Their Nb/Yb ratios are 0.7–1.0 in Unit V and 1.5–2.5 in Unit IV, like D-MORB to N-MORB and E-MORB, respectively (Figures 4c and 4d). They have typical arc-like trace element enrichments and depletions, but they have more LREE enrichment and less negative Nb and Ta anomalies than for Units VI and VII. Even the tuffaceous mudstones from Units IV and V share many of these traits, especially D68R2/44-45 from Unit IV (Gill et al., 2018). Because these traits are similar to those of the RASCs in general and the nearby Manji Seamount in particular (Hochstaedter et al., 2001; Ishizuka, Yuasa, et al., 2002), we refer to these traits with Nb/Yb > 0.7 as the RASC magma type (or “RASC-type”) following Heywood et al. (2020). These are the same traits found in shallower units within the core (Heywood et al., 2020; Wurth, 2019; Wurth & DeBari, 2019).

Unit VI and VII samples deeper in the core differ in important ways from RASC-type traits. They are low-K to medium-K tholeiitic and calc-alkaline basalts and basaltic andesites (Figure 3). Three Unit VII samples (diamond symbols; E44R1/15-17, E47R1/39-41, and E54R2/87-89) with medium-K or high-K have ~10 wt% LOI are from the same 100 m thick hyaloclastite interval (1,475–1,575 mbsf) and have similar ratios of fluid-immobile elements as other Unit VII samples (E54R2/87-89 has no trace data). We attribute their higher K₂O concentrations to alteration that affects little else. Trace element patterns have arc-like signatures with enrichments of Ba, Th, U, K, Pb, and Sr relative to REE and depletions of HFSE (e.g., Gill, 1981; Pearce et al., 1995; Figure 4). REE patterns of samples from Unit VII are relatively flat, and some are concave with a maximum at Nd. This differs from both the LREE-enriched rear-arc seamount chains and the LREE-depleted Quaternary volcanic front basalts (Figure 5) but is similar to the Oligocene (first arc) volcanic rocks (Tamura et al., 2010). Most Nb/Yb ratios are 0.3–0.5, at the low end of D-MORB (Yang et al., 2019). We refer to these geochemical characteristics as “Rift-type” because of their similarity to the much younger rocks dredged from the extensional AR and BAK region 13–50 km behind the Quaternary volcanic front (Heywood et al., 2020). Although Units VI and VII are clearly older than the modern Rift-type magmas, we use the same terminology because of their similar geochemical characteristics. This will become useful in evaluating models for their origin.

The rhyolitic peperite of Igneous Unit 1 that intrudes stratigraphic Unit VI has low-K and Nb/Yb ~ 0.4 (Figures 3 and 4e). Its REE pattern and some of its trace element ratios reflect strong differentiation like negative anomalies in Sr and Ti (Figures 4e and 5e). Two different rhyolite clasts in lapillistones just below the peperite have much higher K₂O that we attribute to alteration but display similar S-shaped REE patterns and positive Zr and Hf anomalies to those of the peperite (Figures 4e and 5e).

Three lapilli-tuffs, one each from Units VI, V, and IV, differ from both our Rift-type and RASC-type categories: E6R3/106-109, E25R3/59-60, and E28R2/9-11, respectively (Figures 4f and 5f). All have lower La/Yb and Nb/Yb ratios than even Rift-type samples and are similar to those of the Quaternary volcanic front (<1.3 and <0.2, respectively). Their high Ba/(Th,REE) ratios also are more similar to basalts from the Quaternary volcanic front than the extensional zone. Consequently, we interpret them as derived from a geological setting similar to the current volcanic front, either as airfall or density current deposits. We refer to these traits with Nb/Yb < 0.2 as the “volcanic front-type” (VF-type) because of their geochemical similarity to the younger magmas from the Quaternary volcanic front. REE patterns in isolated clinopyroxene crystals from two Unit V intervals, including E25R3/54-60, indicate the same thing (Wurth, 2019; Wurth & DeBari, 2019), and a meter thick interval above E21R3 also was interpreted as a density current deposit from the volcanic front (Andrews et al., 2017). All of these samples are from within a 56 m thick interval near the base of Unit V that contains little tuffaceous mudstone (<0.5 m) and includes many reversely graded, coarse-tailed deposits. To evaluate whether a thick sequence of sediments from the volcanic front might be present at this depth, we inspected the K and Th contents at 10 cm intervals throughout that depth range based on NGR data for core (see Gill et al., 2018). The concentration profiles are the same as for the RASC-rich intervals above them. Consequently, we cannot determine the distribution or frequency of sediment from the volcanic front versus from more proximal Rift-type sources without more complete geochemical data.

Because some of the samples from Units V and IV are tuff and lapillistone, they might include foreign components such as loess from a continental source, as in the tuffaceous mudstones (Gill et al., 2018). To evaluate this possibility, we checked their Th concentrations because Th is a good indicator of a foreign source. Figure 6 shows that all of our samples, and even one of the mudstones, have arc-like Th-Nb systematics, and three other mudstones are only slightly displaced toward loess. However, to be conservative, most of our discussion relies solely on our data for discrete volcaniclastic intervals or clasts. Igneous Unit 1 Rhyolites have higher Th at the same Nb contents and differ from the trend of other samples.

4.2 Sr-Nd-Pb-Hf Isotopes

The Sr-Nd-Pb-Hf isotope ratios of the Unit IV to VII samples and Igneous Unit 1 samples are given in Table 1 and shown in Figures 7-9. The samples used in these analyses were strongly leached as described in supporting information. Our Sr-Nd-Hf-Pb isotope ratios for tuffaceous mudstones from the same units were leached by the same method and are shown for comparison (Gill et al., 2018).

The Unit VII Rift-type samples have ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.703059 to 0.703651, 0.513080 to 0.513114, and 0.283234 to 0.283253, respectively (Figures 7 and 8). Although the ⁸⁷Sr/⁸⁶Sr ratio of seawater after 20 Ma is known to be >0.7084 (Richter et al., 1992), it appears that the strong leaching treatment effectively removed most products of seawater alteration. However, the rhyolite of Igneous Unit 1 has a strong apparent seawater signal (this igneous unit has peperite texture on its top and bottom, suggesting that it was intrusive into wet unconsolidated sediments; syneruptive seawater alteration or assimilation of unconsolidated mud may increase its ⁸⁷Sr/⁸⁶Sr ratios), and most of the Unit VII samples have slightly higher ⁸⁷Sr/⁸⁶Sr relative to ¹⁴³Nd/¹⁴⁴Nd than Indian Ocean MORB that may indicate slight residual seawater alteration (Figure 8c).

¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf values of the Rift-type and VF-type samples are higher than those of RASC-type samples (Figure 7) except for the relatively low ¹⁴³Nd/¹⁴⁴Nd in E28R1/9-11 (not shown), the most depleted sample that we attribute to the volcanic front. We cannot evaluate the potential for alteration to explain this exceptional result because the sample was too small for a major element analysis. Consequently, we will not discuss it further. ⁸⁷Sr/⁸⁶Sr values are similar between Rift-type and RASC-type samples (Figure 8). The ¹⁴³Nd/¹⁴⁴Nd ratios of RASC-type samples are lower than Rift-type samples just as the RASC-type samples are more enriched in K₂O and incompatible trace elements. Their ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios lie within the field of Indian Ocean MORB with ¹⁴³Nd/¹⁴⁴Nd = 0.51293 to 0.51303 (Figures 8a–8c) that correlate negatively with Nb/Yb (Figure 8d). However, ¹⁴³Nd/¹⁴⁴Nd ratios in general, and especially D67RCC/1-4, are slightly lower relative to ¹⁷⁶Hf/¹⁷⁷Hf than in MORB (Figure 7b).

²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of the Unit VII samples (Rift-type) are quite unradiogenic within the narrow ranges of 18.209–18.261, 15.481–15.502, and 38.031–38.103, respectively (Figure 9). RASC-type samples have more radiogenic Pb isotopes than those of Rift-type samples, with Unit V RASC-type being the most radiogenic of all (²⁰⁶Pb/²⁰⁴Pb = 18.32–18.40; Figure 9).

Igneous Unit 1 rhyolite is the only rock type that deviates from the trend of others in Pb isotopes just as in Sr isotopes (Figure 9). We attribute its anomalously high ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr ratios to syneruptive seawater alteration or assimilation of unconsolidated mud that increased its U/Pb and ⁸⁷Sr/⁸⁶Sr ratios.

The VF-type sediment samples have ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios that are indistinguishable from the Rift-type samples (except for the relatively low ¹⁴³Nd/¹⁴⁴Nd in E28R1/9-11(not shown)), just as there is extensive overlap between lavas dredged from the modern volcanic front and rift environments (Tollstrup et al., 2010). However, their Sr and Pb isotopes are more radiogenic than in the Rift-type samples, again mimicking differences between lavas from the two environments.

Some tuffaceous mudstones from Units IV and V (Gill et al., 2018) have ¹⁷⁶Hf/¹⁷⁷Hf ratios similar to those of the coarser volcaniclastics, but their ¹⁴³Nd/¹⁴⁴Nd ratios are lower and their ⁸⁷Sr/⁸⁶Sr ratios are higher, as also is true for mudstones with Rift-like geochemistry. Their Pb isotopes are only slightly more radiogenic than in separated igneous clasts from the same stratigraphic unit.

Although some geochemical characteristics of Oligocene volcaniclastics and lavas are similar to the Unit VII samples, their Pb, Nd, and Hf isotope ratios differ and the Oligocene samples have lower Cs, Rb, and Ba contents (Straub et al., 2010; Taylor & Nesbitt, 1998; Yogodzinski et al., 2018).

4.3 Summary

Major and trace element and Sr-Nd-Hf-Pb isotopes separate three geochemical types. Rift-type samples have Nd and Hf isotope ratios as high as at the Quaternary volcanic front (Figure 7b) but the least radiogenic Sr (Figure 8c) and Pb (Figures 9c and 9d) and incompatible trace element ratios intermediate between those of the Quaternary volcanic front and RASC (see section 5.1 and Figure 10). VF-type sediments have the most radiogenic Sr and Pb and the most depleted trace element ratios. RASC-type sediments have the most enriched Nd and Hf isotopes and highest K₂O, Nb/Yb, and LREE. Unit VII consists exclusively of Rift-type rocks, whereas Units IV and V have mostly but not exclusively RASC-type rocks. The Igneous Unit 1 rhyolitic peperite within Unit VI is a strongly differentiated Rift-type intrusion. Tuffaceous mudstones maintain this contrast with three mudstones in Units IV and V being RASC-type and two Rift-type or VF-type. Therefore, the initial rear-arc volcanism at Site U1437 was Rift-type and RASC-type magmatism became more enriched in time, from Unit V to Unit IV. We found one VF-type coarse sediment in each of Units VI, V, and IV (E28R1/9-11 of Unit VI is not shown in Figures 7-11).

5 Discussion

5.1 Discriminating Mantle and Slab Components

Island arc magma is generated through the partial melting of a variably depleted underlying mantle wedge that contains components from the subducting slab (e.g., Gill, 1981; Tatsumi & Eggins, 1995). The concentration of trace elements in arc magmas reflects the mass fraction and nature of slab components added to the mantle source and the degree of melting of the source mantle. Ratios of isotopes and trace elements, especially those that are highly incompatible, can characterize magma source compositions because they are less affected by melting and crystal fractionation than absolute concentrations. Although the influence of secondary alteration may exist in our samples, the trace/trace element plots primarily show mixing between the wedge mantle and the slab components.

We will explore this topic using element ratio versus Nb/Yb ratio diagrams relative to data for global MORB (Arevalo & McDonough, 2010; Yang et al., 2019) as a proxy for the local Ambient Mantle Wedge (AMW; Figure 10). Woodhead et al. (2012) defined the isotopic characteristics of the AMW for the Mariana arc system. Because some Shikoku Basin basalts share the same traits (Hickey-Vargas, 1991; Ishizuka et al., 2009; Straub et al., 2010), we infer that similar AMW is present beneath the Izu arc.

Figure 10a shows Hf/Yb versus Nb/Yb ratios. Note that almost all of our samples have Nb/Yb ratios in the range of D-MORB, with only Unit IV RASC-type samples reaching the range of N-MORB. However, most of the Nb/Yb ratios of Rift-type Unit VI and VII samples are >0.25 and not as depleted as those of the Izu Quaternary volcanic front that are mostly <0.25 when Nb concentrations are well calibrated. Note also that Hf/Yb-Nb/Yb ratios lie mostly within the MORB field. Because the subducting sediment is zircon-free pelagic clay, its Hf/Yb ratio lies below the MORB array, whereas, if anything, our samples lie on the high side of the array. Therefore, the AMW shortly after the Shikoku Basin opened was very depleted but less so than at the current volcanic front. Hf in our samples is unaffected by slab components. The younger RASC-type AMW was more enriched than the initial Rift-type AMW, and any zircon in the subducting sediment broke down before the slab component was released, as suggested by Hirai et al. (2018) for the recent rift basalts that do not have negative Hf anomalies.

Figure 10b shows Ba/Yb vs Nb/Yb ratios. The deviations from the MORB array of Ba/Yb at a given Nb/Yb show the addition of fluid-rich slab components (Pearce et al., 2005). Four separate clusters are present with increasing Nb/Yb. VF-type samples that we attribute to derivation from the volcanic front have the lowest Nb/Yb. Rift-type samples have higher Nb/Yb and a wider range of Ba/Yb. Units V and IV have still higher Nb/Yb, respectively, and Unit IV samples deviate the least from the MORB array. Sr/Yb, Pb/Yb, and U/Yb show similar systematics (supporting information Figures S1a–S1c).

Figures 10c and 10d show Th/Yb and La/Yb versus Nb/Yb ratios. Both are elevated above the MORB array, Th more so than La. A striking feature of both is that they are roughly parallel to the MORB array. Although the differences in trace element ratios between VF-type and RASC-type could be explained by a reduction in degree of mantle melting from 20% to 2%, this would not explain the differences between Rift-type and RASC-type because both reflect similar degrees of melting nor would it explain the isotopic differences. Nd/Yb ratios show similar systematics but are less displaced from the MORB array (Figure 10e).

Identifying the trace element and isotopic characteristics of the wedge mantle component is indispensable for discriminating and investigating the slab components. The plots of Nd/Yb and Hf/Yb against Nb/Yb suggest that the Nd and Hf isotope ratios should be suitable indicators of wedge mantle composition (Figures 10e and 10a). Figures 8c and 8d confirm that ¹⁴³Nd/¹⁴⁴Nd values are as expected for Indian MORB mantle (Class & Lehnert, 2012) at a given level of enrichment, as measured by Nb/Yb ratios. However, Figure 7b shows that some Rift-type and RASC-type volcaniclastics have slightly lower ¹⁴³Nd/¹⁴⁴Nd than expected relative to ¹⁷⁶Hf/¹⁷⁷Hf ratios for the AMW defined by Woodhead et al. (2012; red dashed line) or for Shikoku Basin basalts (tilted crosses). Although the deviation is ≤0.0001 and the elevation of LREE/Yb ratios above the MORB array is slight, we infer that a small amount of Nd but not Hf was added to the mantle wedge by the slab component. However, a key isotopic effect is the more enriched character of the RASC-type AMW. It is similar isotopically to the source of the Kinan Seamounts even though the very high Nb/Yb ratios of those basalts reflect much smaller degrees of melting.

In detail, the trace element ratios of the Rift-type samples overlap those of the much younger Rift-type dredged lavas (previously called BAK and AR), and the RASC-type clastic samples overlap the RASC-type lavas that were dredged from seamounts including Manji adjacent to the drill site. The Rift-type samples also overlap the Nd-Hf isotope ratios of the Quaternary volcanic front, especially Hachijo-jima at the same latitude (Freymuth et al., 2016), but they have less radiogenic Sr and Pb isotopes. That is, the AMW for the Rift-type samples has similar Nd and Hf isotopes to that of the volcanic front, but it is less depleted in incompatible trace elements and lacks evidence of a fluid-rich slab component. All isotope ratios of most RASC-type clastic samples overlap those of the dredged RASC-type lavas. The exception is D67RCC/1-4 whose Nd and Sr isotopes are more enriched than any of the dredged lavas, suggesting more sediment in the slab component. With one exception (D64R1/118-121), the youngest samples in Unit IV have the lowest ¹⁴³Nd/¹⁴⁴Nd ratios as well the highest Nb/Yb ratios. Therefore, AMW enrichment ± the amount of sediment added to the mantle increased with time. Although Pb isotopes in Rift-type samples are less radiogenic than at most of the Quaternary volcanic front (they overlap those of Miyake-jima), they are similar to those of the dredged Rift-type lavas. However, they are considerably more radiogenic than the Pb in basalts from the Mariana Trough or Shikoku Basin, showing that Pb is very sensitive to the addition of even small amounts of slab component. This is confirmed by the negative correlation between ²⁰⁶Pb/²⁰⁴Pb and Ce/Pb ratios stretching between subducting sediment and Shikoku Basin basalts that have Ce/Pb ~ 17 (Figure 11). VF-type and RASC-type of Unit V samples have values closest to the sediment. The influence of sediment on the Pb isotope ratio is larger than that of AOC, because the ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of AOC are low and deviate from the trend of our samples and sediment (Figure 9).

The AMW is isotopically “Indian” (Pearce et al., 1999) or Philippine Sea Plate type, as it has been since the inception of subduction (Yogodzinski et al., 2018).

5.2 Evolution of the Source Composition

The Sr, Nd, Hf, and, Pb isotope ratios of the Unit VII and VI samples have a narrow range, indicating a common magma source from ~15 to 9 Ma (Figures 7-9). Hf and Nd isotopes lie close to depleted AMW values, similar to those of Hachijo-jima at about the same latitude. Nb/Yb ratios are more enriched than at the Quaternary volcanic front, and most Ba/Yb, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁶Pb/²⁰⁴Pb ratios are lower. These attributes make the samples most like Rift-type basalts 13–50 km behind the volcanic front. They have some enrichments of Ba/Yb, Th/Yb, and La/Yb relative to the MORB array, significantly more radiogenic Pb than in Shikoku Basin or Mariana Trough basalts and slightly more radiogenic Sr than in Indian Ocean MORB with the same ¹⁴³Nd/¹⁴⁴Nd. The low Sr and Pb concentrations of the wedge mantle (depleted MORB mantle [DMM] with Sr = 7.664 ppm, and Pb = 0.018 ppm: Workman & Hart, 2005) means that Sr and Pb isotope ratios are easily affected by the slab component. As shown in Figure 11, the Pb isotope ratio is more sensitive to small amounts of slab component than the Sr isotope ratio.

Consequently, the mantle source for the earliest magmatism in the Izu rear-arc contained more slab component than the mantle source during backarc spreading but less than at the modern volcanic front, and the slab component was more melt like. The rocks are intermediate both in distance from the trench and in geochemistry. Because of their similarity to younger Pleistocene Rift-type basalts <13–50 km behind the Izu volcanic front (Heywood et al., 2020), we refer to the in situ hyaloclastite as the “Rift-type.” This extensive initial Rift-type volcanism was unknown before IODP Expedition 350 drilling.

In contrast, 6–9 Ma Units V and IV clastic samples are more enriched in Nb/Yb and all isotopes and are similar to dredged RASC lavas of the same age and younger. We therefore refer to those clasts as the “RASC-type.” A first-order discovery from Site U1437 is the transition from Rift-type to RASC-type with time (Sato et al., 2020; Wurth, 2019; Wurth & DeBari, 2019). Our isotope data for these samples indicate that the mantle source changed with time in two respects. First, Hf ± Nd and Sr isotopes stayed within the AMW array but became more enriched. We attribute this to preferential initial melting of old enrichments in the mantle wedge, as have previous authors (e.g., Hochstaedter et al., 2001; Ishizuka et al., 2003; Tollstrup et al., 2010). Either the mantle wedge became more enriched with time after spreading ceased in the Shikoku Basin or lower degrees of melting with time preferentially melted more enriched mantle components. Second, the mass fraction of a melt-like slab component, or the percent of sediment in the slab component, or both, increased with time. Evidence for this is seen in the lower ¹⁴³Nd/¹⁴⁴Nd ratios relative to ¹⁷⁶Hf/¹⁷⁷Hf and Nb/Yb ratios, the higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd that lie within the Indian Ocean MORB array, and the more sediment-like Pb isotopes and Ce/Pb ratios, in RASC-type clasts. At this point we cannot determine the relative importance of the two processes.

Sato et al. (2020) concluded that Unit VII might have erupted at either a volcanic front (like the Oligocene volcanic front) or backarc rift zone. The distinction has little effect on their conclusion, with which we agree, that Site U1437 was within tens of kilometers of the Mid-Miocene volcanic front so that the front migrated tens of kilometers toward the trench with time. However, our new isotopic data and ABS modeling are inconsistent with Unit VII having the geochemical characteristics of Izu volcanic front magmas throughout the Neogene, after opening of the Shikoku Basin. In addition to lacking as much relative Cs, Rb, and Ba enrichment and Nb + LREE depletion as the front, Unit VII basalts lack the enrichment in radiogenic Sr and Pb and require smaller degree melts of mantle that was enriched by more melt-like slab components than in a typical frontal setting. This is an especially robust conclusion considering the possible effects of alteration. Therefore, we believe that they erupted in an extensional zone behind the volcanic front.

Figure 12 presents a tectonic model that is consistent with our conclusions about the evolution of source compositions. Units VI and VII were erupted on attenuated arc crust in an extensional zone that extended from near the boundary between backarc (Shikoku Basin) crust in the west to the volcanic front in the east. There was some RASC volcanism further west on old Shikoku Basin crust before 14 Ma (Ishizuka et al., 2009), but we found no evidence of it at Site U1437. Instead, early extensional volcanism was replaced by RASC volcanism starting around 9 Ma. Subsequent loci of both RASC and VF volcanism migrated trenchward as inferred by Ishizuka, Uto, et al. (2002), Wurth and DeBari (2019), and Sato et al. (2020), presumably reflecting steepening of the slab dip or trench rollback or both.

5.3 Modeling Magma Genesis

It is common to use Sr, Nd, Hf, and Pb isotope ratios ± trace element ratios to estimate the source composition of arc magmas. Conventional mixing calculations using these constraints often suggest that some combination of the AMW, plus fluid or melt from subducted sediment and/or AOC, can explain many aspects of arc magmas. However, there are many variables so that the calculated result is not unique. We approach this topic by using first the Primary Magma Calculator v. 2 (PRIMACALC2) coded by Kimura and Ariskin (2014) and then the Arc Basalt Simulator v. 4 (ABS4) coded by Kimura et al. (2014). Details about both are in supporting information S2. We assumed the unmodified mantle (DMM), sediment, and AOC compositions given in supporting information Table S1, all of which are commonly used in models for the Izu arc (e.g., Tollstrup et al., 2010). We assumed water-saturated conditions for the slab, which resulted in melting its top 1.0 km (i.e., both sediment and AOC). The depth of final slab dehydration or melting and the P-T conditions of mantle melting are free variables that are calculated by fitting the model results to the calculated primary melt.

For this exercise we chose two representative Rift-type samples, 23 m apart in Unit VII and with quite similar compositions: E69R3/9-11 and E70R6/35-37. They are slightly altered, basaltic clasts with trace element and isotope ratios that are typical of Units VI and VII. In detail, they have somewhat high Ba/Yb and ⁸⁷Sr/⁸⁶Sr with respect to Nb/Yb and ¹⁴³Nd/¹⁴⁴Nd respectively, indicating that they reflect the high end of added slab component for Unit VII samples. E69R3/9-11 also has slightly low ¹⁴³Nd/¹⁴⁴Nd with respect to ¹⁷⁶Hf/¹⁷⁷Hf. These two samples used for ABS4 calculations are shown as blue crosses in our figures. The measured and corrected compositions, the average and one standard deviation of ABS4 model melt compositions, and the representative slab liquid and mantle residue component for each sample are given in supporting information Table S2 and shown in Figures S2 and S3. The trace element pattern and isotope ratios of both samples are reproduced well by the ABS4 models (Figures S2 and S3), the details of which are discussed in supportng information S2. We were unable to create suitable models for the RASC-type clasts.

A great feature of ABS4 models is that many physical parameters of magma genesis are constrained (Kimura et al., 2014). Table S2 lists the most important of them: (1) the depth at which the slab component is released from the slab [Slab P]; (2) the slab surface temperature at that depth [Slab T]; (3) the mass fraction of the slab component that is derived from AOC [Fliq (AOC)], from sediment [Fliq (SED)], and from the base of the mantle wedge that is dragged down with the slab and retains slab components released updip of the final slab component release [Fliq (DMM)]; (4) the mantle melting pressure [P]; (5) the mantle melting temperature [T]; (6) the extent of the prior depletion of the DMM mantle [%Prior depletion]; (7) the mass fraction of slab component added to the mantle [Fslb liq.%]; (8) the H₂O content of the slab component [H₂O% in SLBliq, that is, how fluid like or melt like it is]; (9) the H₂O content of the modified mantle [H₂O% in PERID]; and (10) the degree of mantle melting [F]. Results of simple binary mixing between the ambient mantle, and the calculated slab component that is the product of many of these variables and is given in Table S2, are shown in Figures 7b, 8b, 9c, and 9d.

In order to evaluate the geodynamic implications of the ABS4 results for Unit VII, we will compare them to ABS4 results for basalts from known locations in the Izu arc: the volcanic front, Rift-type basalts in the rear-arc 13–50 km behind the volcanic front, and RASC-type basalts even further from the plate boundary. Solutions for representative examples of each are given in supporting information Table S3 and compared to our results for Unit VII in Figure 13. Kimura et al. (2010) provide similar results for the Izu volcanic front and rear-arc using an older version of ABS that assumed slab dehydration instead of melting.

Despite the differences between E69R3/9-11 and E70R6/35-37, all physical parameters calculated for them overlap between their maximum and minimum values, indicating that the results are robust for the conditions under which Unit VII Rift-type basalts were generated. About half of the slab component is derived from AOC, 16–37% from sediment, and 17–29% from the base of the modified mantle wedge. The slab component is generated between 3.6 and 4.1 GPa (120–140 km) and 773–792°C, well above the wet basalt solidus. The degree of flux melting of the mantle wedge is low, between 1.5% and 4.2%, at 1.4 to 1.6 GPa and 1160 to 1185°C. The mass fraction of slab component (i.e., wet melt) added to the mantle wedge is correspondingly low: 0.4% and 0.6%.

These conditions differ from those obtained from ABS4 models for both the volcanic front and RASC (Figure 13). Magma genesis beneath the Izu volcanic front involves a shallower, more fluid-like slab component and a much higher degree of mantle flux melting that we attribute to more slab component. However, the calculated depth of mantle melting and the largely Nd-Hf-free nature of the slab component are similar.

Likewise, conditions for Unit VII differ from those calculated for a somewhat RASC-like basalt from Toshima, a rear-arc island behind Oshima. The slab component in Toshima is more from AOC (73%), and it is generated deeper (~160 km) at ~810°C. Mantle melting fraction is as low as for Unit VII, but three times the mass fraction of slab component is required because it is drier melt.

ABS results for Rift-type rear-arc basalts (previously called BAK or AR) lie between these two extremes and are most similar to the physical conditions for Unit VII samples. Specifically, the intermediate P-T conditions for generating the slab component, the melt-like character of the slab component (low %H₂O in slab liquid), and the small mass fraction of slab component required, all indicate that Unit VII magmas were generated above an already dehydrated slab ~125 km deep.

ABS results can explain many of the general geochemical features of our Units VI and VII Rift-type samples. Adding less than 0.6% of the calculated slab component to a DMM-type mantle from which ~1.5 wt.% melt had been extracted can explain many of the features (Figures 7b, 8c, 9c, and 9d). These include the elevation of trace element ratios above the MORB array, more radiogenic Pb than in Shikoku Basin basalts, and Hf ± Nd isotope ratios remaining close to AMW values. The slightly high ⁸⁷Sr/⁸⁶Sr relative to ¹⁴³Nd/¹⁴⁴Nd has a flatter slope than predicted and may reflect sea water alteration rather than the slab component. The RASC-type samples from Units IV and V require both an ambient mantle more enriched than DMM and a more melt-like slab component capable of transporting Nd, as discussed in section 5.1.

We infer that a volcanic front existed several tens of kilometers closer to the trench than the U1437 Site from 15 to 6 Ma, during the time when our samples formed (Figure 12). The three samples that we attribute to the volcanic front are examples of its volcaniclastic products. Although we do not have ABS results for Unit IV or V RASC-type samples, we infer that conditions similar to those for Toshima basalts, or an even deeper slab, are required for them.

In addition, because ABS is a mass balance program, it retains useful information about the concentration of trace elements in the residue of subduction zone slab and mantle sources. Kimura et al. (2016) used this attribute of ABS to compare the isotopic composition of those sources through time to various kinds of ocean island basalt (OIB). The residues from all of the mantle wedge compositions in Table S2, that is, from all mantles modified by diverse kinds of flux melting throughout the Neogene history of the arc, share many traits. All except the residues of high degree melts beneath the volcanic front have Rb/Sr and U/Pb ratios even lower than in DMM but higher Lu/Hf ratios. Sm/Nd ratios in residues from Unit VII, BAK, and AR (i.e., Rift-type magmatism) are higher than for DMM; that is, the residual mantle is even more LREE depleted than DMM. In contrast, the residues of melting beneath both the volcanic front and rear-arc seamount chains are more LREE enriched because of the added slab flux. Consequently, trenchward advection of rear-arc mantle results in mantle beneath the volcanic front that is more depleted than DMM in Rb with respect to Sr, U with respect to Pb, and Hf with respect to Lu, as proposed by Hochstaedter et al. (2001). Rift-type volcanism has the same result for Nd with respect to Sm. This differs from what was proposed for subarc mantle by Donnelly et al. (2004) to explain the source of E-MORB. They attributed the higher Rb/Sr, U/Pb, Hf/Lu, and Nd/Sm of E-MORB relative to N-MORB sources to processes in subarc mantle wedges. However, our calculations show that these ratios in the residues of melting throughout most of the mantle wedge are even lower than in DMM. Overall, Izu rear-arc volcanism leaves mantle residues that become more Indian like, and more depleted in fluid-immobile elements, through time.

6 Conclusions

Three types of volcaniclastic sediments and in situ hyaloclastites characterized the Izu rear-arc at Site 1437, based on our new geochemical data. These were generated from 15 to 6 Ma, after spreading in the Shikoku Basin ended. In order of decreasing distance of their source from the plate boundary, they are RASC-type, Rift-type, and VF-type. Rift-type in situ hyaloclastites dominate from 15 to 9 Ma and RASC-type sediments from 9 to 6 Ma. Both are from proximal sources. Distal volcanic front-derived sediments (VF-type) are less common and randomly distributed. RASC-type sediments have the most enriched Nd and Hf isotopes and fluid-immobile trace element ratios. VF-type sediments have the most radiogenic Sr and Pb and the highest Ba/(Th,LREE) ratios. Rift-type hyaloclastites are similar to the volcanic front in Nd-Hf isotopes but have the least radiogenic Sr and Pb, and intermediate Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than at the volcanic front. ABS models for mafic Unit VII samples yield parameters of magma genesis that are similar to those for basalts in the modern rift environment but different from both volcanic front and RASC. The models indicate addition of ~1% of a melt-rich slab component generated at ~125 km to a Philippine Sea Plate ambient mantle that was more depleted than DMM. The initial post backarc basin magmatism in the Izu rear-arc was Rift-like in composition for about 6 Ma before the distinctive RASC-type magmatism began, which then became increasingly enriched with time.

Acknowledgments

We thank all R/V JOIDES Resolution crew, technical staff, and science party during IODP Expedition 350. We thank M. Bizimis, J. Pearce, and two anonymous reviewers for helpful comments. We also thank K. N. Wurth for insights provided by her MS thesis at Western Washington University. Thanks also go to W. Zhang, H. Higuchi, and M. Kanazawa, of JAMSTEC, for their support in the laboratory work and Y. Hirai of JAMSTEC for his constructive discussion. Funding for S. DeBari was provided by the United States Science Support Program (PEA Grant T350A77). This work was supported by JSPS Kakenhi Grant JP17H02987.

Open Research

Data Availability Statement

All data for this paper are in Table 1 plus the supporting information. These data were submitted to the EarthChem Library repository (URLs: https://doi.org/10.26022/IEDA/111607, https://doi.org/10.26022/IEDA/111608, https://doi.org/10.26022/IEDA/111609).

Supporting Information

References

Andrews, G., Schindlbeck, J., Kaess, A., Kars, M., Brown, S, & Exp. 350 Scientists (2017). Fine-grained, Late Miocene volcaniclastic density current deposits in the Manji-Enpo volcano-bound basin (Unit V, IODP Expedition 350 Site U1437-Izu-Bonin Rear-Arc), AGU Chapman Conference - Submarine Volcanism, Hobart, Tasmania, 29 January – 3 February 2017, Abs. #202445.
Google Scholar
Arevalo, R. Jr., & McDonough, W. F. (2010). Chemical variations and regional diversity observed in MORB. Chemical Geology, 271, 70–85. https://doi.org/10.1016/j.chemgeo.2009.12.013
10.1016/j.chemgeo.2009.12.013
CASADSWeb of Science®Google Scholar
Baker, J., Peate, D., Waight, T., & Meyzen, C. (2004). Pb isotopic analysis of standards and samples using a ²⁰⁷Pb–²⁰⁴Pb double spike and thallium to correct for mass bias with a double-focusing MC-ICP-MS. Chemical Geology, 211, 275–303. https://doi.org/10.1016/j.chemgeo.2004.06.030
10.1016/j.chemgeo.2004.06.030
CASADSWeb of Science®Google Scholar
Busby, C. J., Tamura, Y., Blum, P., Guèrin, G., Andrews, G. D. M., Barker, A. K., Berger, J. L. R., Bongiolo, E. M., Bordiga, M., DeBari, S. M., Gill, J. B., Hamelin, C., Jia, J., John, E. H., Jonas, A. -S., Jutzeler, M., Kars, M. A. C., Kita, Z. A., Konrad, K., Mahoney, S. H., Martini, M., Miyazaki, T., Musgrave, R. J., Nascimento, D. B., Nichols, A. R. L., Ribeiro, J. M., Sato, T., Schindlbeck, J. C., Schmitt, A. K., Straub, S. M., Vautravers, M. J., Yamashita, M., & Yang, Y. (2017). The missing half of the subduction factory: Shipboard results from the Izu rear arc, IODP Expedition 350. International Geology Review, 59, 1677–1708. https://doi.org/10.1080/00206814.2017.1292469
10.1080/00206814.2017.1292469
ADSWeb of Science®Google Scholar
Chauvel, C., Marini, J. C., Plank, T., & Ludden, J. N. (2009). Hf-Nd input flux in the Izu-Mariana subduction zone and recycling of subducted material in the mantle. Geochemistry, Geophysics, Geosystems, 10, Q01001. https://doi.org/10.1029/2008GC002101
10.1029/2008GC002101
ADSWeb of Science®Google Scholar
Class, C., & Lehnert, K. (2012). PetDB expert MORB (Mid-Ocean Ridge Basalt) composition, version 1.0, Interdisciplinary Earth Data Alliance (IEDA). https://doi.org/10.1594/IEDA/100060 Accessed 2019-10-29.
Google Scholar
DeBari, S., Heywood, L., & Gill, J. (2020). Izu bonin across arc geochemical dataset between latitudes 32°40′ to 30°20′, version 1.0, Interdisciplinary Earth Data Alliance (IEDA). https://doi.org/10.1594/IEDA/111493
Google Scholar
Donnelly, K. E., Goldstein, S. L., Langmuir, C. H., & Spiegelman, M. (2004). Origin of enriched ocean ridge basalts and implications for mantle dynamics. Earth and Planetary Science Letters, 226(3–4), 347–366. https://doi.org/10.1016/j.epsl.2004.07.019
10.1016/j.epsl.2004.07.019
CASADSWeb of Science®Google Scholar
Freymuth, H., Ivko, B., Gill, J. B., Tamura, Y., & Elliott, T. (2016). Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc. Geochimica et Cosmochimica Acta, 186, 49–70. https://doi.org/10.1016/j.gca.2016.04.034
10.1016/j.gca.2016.04.034
CASADSWeb of Science®Google Scholar
Gill, J. B. (1981). Orogenic andesite and plate tectonics. Berlin, Germany: Springer Verlag. https://doi.org/10.1007/978-3-642-68012-0
10.1007/978-3-642-68012-0
Google Scholar
Gill, J. B., Bongiolo, E. M., Miyazaki, T., Hamelin, C., Jutzeler, M., DeBari, S., Jonas, A. -S., Vaglarov, B. S., Nascimento, L. S., & Yakavonis, M. (2018). Tuffaceous mud is a volumetrically important volcaniclastic facies of submarine arc volcanism and record of climate change. Geochemistry, Geophysics, Geosystems, 19, 1217–1243. https://doi.org/10.1002/2017GC007300
10.1002/2017GC007300
ADSWeb of Science®Google Scholar
Hart, S. R. (1984). A large-scale isotope anomaly in the Southern Hemisphere mantle. Nature, 309, 753–757. https://doi.org/10.1038/309753a0
10.1038/309753a0
CASADSWeb of Science®Google Scholar
Hauff, F., Hoernle, K., & Schmidt, A. (2003). Sr-Nd-Pb composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185): Implications for alteration of ocean crust and the input into the Izu-Bonin-Mariana subduction system. Geochemistry, Geophysics, Geosystems, 4(8), 8913. https://doi.org/10.1029/2002GC000421
10.1029/2002GC000421
ADSWeb of Science®Google Scholar
Heywood, L. J., DeBari, S. M., Gill, J. B., Straub, S. M., Schindlbeck-Belo, J. C., Escobar-Burciaga, R. D., & Woodhead, J. (2020). Across-arc diversity in rhyolites from an intra-oceanic arc: Evidence from IODP Site U1437, Izu-Bonin rear arc and surrounding area. Geochemistry, Geophysics, Geosystems, 21, e2019GC008353. https://doi.org/10.1029/2019GC008353
10.1029/2019GC008353
CASADSWeb of Science®Google Scholar
Hickey-Vargas, R. (1991). Isotope characteristics of submarine lavas from the Philippine Sea: Implications for the origin of arc and basin magmas of the Philippine tectonic plate. Earth and Planetary Science Letters, 107(2), 290–304. https://doi.org/10.1016/0012-821X(91)90077-U
10.1016/0012-821X(91)90077-U
CASADSWeb of Science®Google Scholar
Hickey-Vargas, R. (1998). Origin of the Indian Ocean–type isotopic signature in basalts from Philippine Sea plate spreading centers: An assessment of local versus large-scale processes. Journal of Geophysical Research, 103(B9), 20,963–20,979. https://doi.org/10.1029/98JB02052
10.1029/98JB02052
CASADSWeb of Science®Google Scholar
Hirai, Y., Yoshida, T., Okamura, S., Tamura, Y., Sakamoto, I., & Shinjo, R. (2018). Breakdown of residual zircon in the Izu arc subducting slab during backarc rifting. Geology, 46, 371–374. https://doi.org/10.11230/G39856.1
10.1130/G39856.1
CASADSWeb of Science®Google Scholar
Hochstaedter, A. G., Gill, J. B., Musakabe, M., Newman, S., Pringle, M., Taylor, B., & Fryer, P. (1990). Volcanism in the Sumisu Rift, I: Major element,volatile, and stable isotope geochemistry. Earth and Planetary Science Letters, 100(1–3), 179–194. https://doi.org/10.1016/0012-821X(90)90184-Y
10.1016/0012-821X(90)90184-Y
CASADSWeb of Science®Google Scholar
Hochstaedter, A. G., Gill, J. B., Peters, R., Broughton, P., Holden, P., & Taylor, B. (2001). Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab. Geochemistry, Geophysics, Geosystems, 2(7), 1019. https://doi.org/10.1029/2000GC000105
10.1029/2000GC000105
ADSWeb of Science®Google Scholar
Hochstaedter, A. G., Gill, J. B., Taylor, B., Ishizuka, O., Yuasa, M., & Morita, S. (2000). Across-arc geochemical trends in the Izu-Bonin arc: Constraintson source composition and mantle melting. Journal of Geophysical Research, 105(B1), 495–512. https://doi.org/10.1029/1999JB900125
10.1029/1999JB900125
CASADSWeb of Science®Google Scholar
Ishizuka, O., Geshi, N., Itoh, J. -I., Kawanabe, Y., & TuZino, T. (2008). The magmatic plumbing of the submarine Hachijo NW volcanic chain, Hachijojima, Japan: Long-distance magma transport? Journal of Geophysical Research, 113, B08S08. https://doi.org/10.1029/2007JB005325
10.1029/2007JB005325
Web of Science®Google Scholar
Ishizuka, O., Taylor, R. N., Milton, J. A., & Nesbitt, R. W. (2003). Fluid–mantle interaction in an intra-oceanic arc: Constraints from high-precision Pb isotopes. Earth and Planetary Science Letters, 211(3–4), 221–236. https://doi.org/10.1016/S0012-821X(03)00201-2
10.1016/S0012-821X(03)00201-2
CASADSWeb of Science®Google Scholar
Ishizuka, O., Taylor, R. N., Milton, J. A., Nesbitt, R. W., Yuasa, M., & Sakamoto, I. (2006). Variation in the source mantle of the northern Izu arc with time and space constraints from high-precision Pb isotopes. Journal of Volcanology and Geothermal Research, 156(3–4), 266–290. https://doi.org/10.1016/j.jvolgeores.2006.03.005
10.1016/j.jvolgeores.2006.03.005
CASADSWeb of Science®Google Scholar
Ishizuka, O., Uto, K., Yuasa, M., & Hochstaedter, A. G. (2002). Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealedby laser-heating ⁴⁰Ar/³⁹Ar dating. Journal of Volcanology and Geothermal Research, 120(1–2), 71–85. https://doi.org/10.1016/S0377-0273(02)00365-7
10.1016/S0377-0273(02)00365-7
ADSWeb of Science®Google Scholar
Ishizuka, O., Yuasa, M., Taylor, R. N., & Sakamoto, I. (2009). Two contrasting magmatic types coexist after the cessation of back-arc spreading. Chemical Geology, 266(3–4), 274–296. https://doi.org/10.1016/j.chemgeo.2009.06.014
10.1016/j.chemgeo.2009.06.014
CASADSWeb of Science®Google Scholar
Ishizuka, O., Yuasa, M., & Uto, K. (2002). Evidence of porphyry copper-type hydrothermal activity from a submerged remnant back-arc volcano of the Izu-Bonin arc: Implications for the volcanotectonic history of back-arc seamounts. Earth and Planetary Science Letters, (3–4), 381–399. https://doi.org/10.1016/S0012-821X(02)00515-0
10.1016/S0012-821X(02)00515-0
ADSWeb of Science®Google Scholar
Kelley, K. A., Plank, T., Ludden, J., & Staudigel, H. (2003). Composition of altered oceanic crust at ODP Sites 801 and 1149. Geochemistry, Geophysics, Geosystems, 4(6), 8910. https://doi.org/10.1029/2002GC000435
10.1029/2002GC000435
ADSWeb of Science®Google Scholar
Kimura, J. -I., & Ariskin, A. A. (2014). Calculation of water-bearing primary basalt and estimation of source mantle conditions beneath arcs: PRIMACALC2 model for WINDOWS. Geochemistry, Geophysics, Geosystems, 15, 1494–1514. https://doi.org/10.1002/2014GC005329
10.1002/2014GC005329
CASADSWeb of Science®Google Scholar
Kimura, J. -I., Gill, J. B., Kunikiyo, T., Osaka, I., Shimoshioiri, Y., Katakuse, M., Kakubuchi, S., Nagao, T., Furuyama, K., Kamei, A., Kawabata, H., Nakajima, J., van Keken, P. E., & Stern, R. J. (2014). Diverse magmatic effects of subducting a hot slab in SW Japan: Results from forward modeling. Geochemistry, Geophysics, Geosystems, 15, 691–739. https://doi.org/10.1002/2013GC005132
10.1002/2013GC005132
ADSWeb of Science®Google Scholar
Kimura, J. I., Gill, J. B., Skora, S., van Keken, P. E., & Kawabata, H. (2016). Origin of geochemical mantle components: Role of subduction filter. Geochemistry, Geophysics, Geosystems, 17, 3289–3325. https://doi.org/10.1002/2016GC006343
10.1002/2016GC006343
CASADSWeb of Science®Google Scholar
Kimura, J. -I., Kent, A. J. R., Rowe, M. C., Katakuse, M., Nakano, F., Hacker, B. R., van Keken, P. E., Kawabata, H., & Stern, R. J. (2010). Origin of cross-chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes. Geochemistry, Geophysics, Geosystems, 11, Q10011. https://doi.org/10.1029/2010GC003050
10.1029/2010GC003050
CASADSWeb of Science®Google Scholar
Kuritani, T., Yokoyama, T., Kobayashi, K., & Nakamura, E. (2003). Shift and rotation of composition trends by magma mixing: 1983 Eruption at Miyakejima Volcano, Japan. Journal of Petrology, 44(10), 1895–1916. https://doi.org/10.1093/petrology/egg063
10.1093/petrology/egg063
CASADSWeb of Science®Google Scholar
McDonough, W. F., & Sun, S. S. (1995). The composition of the Earth. Chemical Geology, 120(3–4), 223–253. https://doi.org/10.1016/0009-2541(94)00140-4
10.1016/0009-2541(94)00140-4
CASADSWeb of Science®Google Scholar
Miyashiro, A. (1975). Volcanic rock series in island arcs and active continental margins. American Journal of Science, 274(4), 321–355. https://doi.org/10.2475/ajs.274.4.321
10.2475/ajs.274.4.321
ADSWeb of Science®Google Scholar
Miyazaki, T., Kimura, J. -I., Senda, R., Vaglarov, B. S., Chang, Q., Takahashi, T., Hirahara, Y., Hauff, F., Hayasaka, Y., Sano, S., Shimoda, G., Ishizuka, O., Kawabata, H., Hirano, N., Machida, S., Ishii, T., Tani, K., & Yoshida, T. (2015). Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles. Geochemistry, Geophysics, Geosystems, 16, 3309–3332. https://doi.org/10.1002/2015GC005911
10.1002/2015GC005911
CASADSWeb of Science®Google Scholar
Miyazaki, T., & Shuto, K. (1998). Sr and Nd isotope ratios of twelve GSJ rock reference samples. Geochemical Journal, 32(5), 345–350. https://doi.org/10.2343/geochemj.32.345
10.2343/geochemj.32.345
CASADSWeb of Science®Google Scholar
Okino, K., Shimakawa, Y., & Nagaoka, S. (1994). Evolution of the Shikoku Basin. Journal of Geomagnetism and Geoelectricity, 46(6), 463–479. https://doi.org/10.5636/jgg.46.463
10.5636/jgg.46.463
ADSWeb of Science®Google Scholar
Patchett, P. J., Vervoort, J. D., Söderlund, U., & Salters, V. J. M. (2004). Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth. Earth and Planetary Science Letters, 222(1), 29–41. https://doi.org/10.1016/j.epsl.2004.02.030
10.1016/j.epsl.2004.02.030
CASADSWeb of Science®Google Scholar
Pearce, J. A., Baker, P. E., Harvey, P. K., & Luff, I. W. (1995). Geochemical evidence for subduction fluxes, mantle melting and fractional crystallization beneath the south sandwich island arc. Journal of Petrology, 36(4), 1073–1109. https://doi.org/10.1093/petrology/36.4.1073
10.1093/petrology/36.4.1073
CASADSWeb of Science®Google Scholar
Pearce, J. A., Kempton, P. D., Nowell, G. M., & Noble, S. R. (1999). Hf-Nd element and isotope perspective on the nature and provenance of mantle and subduction components in Western Pacific Arc-basin systems. Journal of Petrology, 40(11), 1579–1611. https://doi.org/10.1093/petroj/40.11.1579
10.1093/petroj/40.11.1579
CASADSWeb of Science®Google Scholar
Pearce, J. A., Stern, R. J., Bloomer, S. H., & Fryer, P. (2005). Geochemical mapping of the Mariana arc-basin system: Implication for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems, 6, Q07006. https://doi.org/10.1029/2004GC00895
10.1029/2004GC000895
CASADSWeb of Science®Google Scholar
Plank, T., Kelley, K. A., Murray, R. W., & Stern, L. Q. (2007). Chemical composition of sediments subducting at the Izu-Bonin trench. Geochemistry, Geophysics, Geosystems, 8, Q04I16. https://doi.org/10.1029/2006GC001444
10.1029/2006GC001444
Web of Science®Google Scholar
Richter, F. M., Rowley, D. B., & DePaolo, D. J. (1992). Sr isotope evolution of seawater: The role of tectonics. Earth and Planetary Science Letters, 109(1–2), 11–23. https://doi.org/10.1016/0012-821X(92)90070-C
10.1016/0012-821X(92)90070-C
CASADSWeb of Science®Google Scholar
Sato, T., Miyazaki, T., Tamura, Y., Gill, J. B., Jutzeler, M., Senda, R., & Kimura, J. -I. (2020). The earliest stage of Izu rear arc volcanism revealed by drilling at Site U1437, IODP Expedition 350. Island Arc, 29, e12340. https://doi.org/10.1111/iar.12340
10.1111/iar.12340
Web of Science®Google Scholar
Savov, I. P., Hickey-Vargas, R., D'Antonio, M., Ryan, J. G., & Spadea, P. (2006). Petrology and geochemistry of West Philippine Basin basalts and early Palau-Kyushu Arc volcanic clasts from ODP Leg 195, Site 1201D: Implications for the early history of the Izu-Bonin-Mariana arc. Journal of Petrology, 47(2), 277–299. https://doi.org/10.1093/petrology/egi075
10.1093/petrology/egi075
CASADSWeb of Science®Google Scholar
Schmitt, A. K., Konrad, K., Andrews, G. D. M., Horie, K., Brown, S. R., Koppers, A. A. P., Pecha, M., Busby, C. J., & Tamura, Y. (2018). ⁴⁰Ar/³⁹Ar ages and zircon petrochronology for the rear arc of the Izu-Bonin-Marianas intra-oceanic subduction zone. International Geology Review, 60, 956–976. https://doi.org/10.1080/00206814.2017.1363675
10.1080/00206814.2017.1363675
ADSWeb of Science®Google Scholar
Stern, R. J., Fouch, M. J., & Klemperer, S. (2003). An overview of the Izu-Bonin-Mariana subduction factory. In J. Eiler (Ed.), Inside the Subduction Factory, Geophysica, Monograph (Vol. 138, pp. 175–222). Washington, DC: American Geophysical Union.
10.1029/138GM10
Web of Science®Google Scholar
Straub, S. M., Goldstein, S. L., Class, C., Schmidt, A., & Gomez-Tuena, A. (2010). Slab and mantle controls of the Sr-Nd-Pb-Hf isotope evolution of the post-42 Ma Izu Bonin volcanic arc. Journal of Petrology, 51(5), 993–1026. https://doi.org/10.1093/petrology/egq009
10.1093/petrology/egq009
CASADSWeb of Science®Google Scholar
Sun, S. S., & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In A. D. Saunders, & M. J. Norry (Eds.), Magmatism in the Ocean Nasins, Special Publication (Vol. 42, pp. 313–345). Piccadilly, London: The Geological Society.
Google Scholar
Tamura, Y., Busby, C. J., Blum, P., & Expedition 350 Scientists (2015). Site U1437, proceedings of the international ocean discovery program volume 350, 142 pp. https://doi.org/10.14379/iodp.proc.350.104.2015
Google Scholar
Tamura, Y., Ishizuka, O., Aoike, K., Kawate, S., Kawabata, H., Chang, Q., Saito, S., Tatsumi, Y., Arima, M., Takahashi, M., Kanamaru, T., Kodaira, S., & Fiske, R. S. (2010). Missing Oligocene crust of the Izu-Bonin arc: Consumed or rejuvenated during collision? Journal of Petrology, 51(4), 823–846. https://doi.org/10.1093/petrology/egq002
10.1093/petrology/egq002
CASADSWeb of Science®Google Scholar
Tamura, Y., Ishizuka, O., Sato, T., & Nichols, A. R. L. (2019). Nishinoshima volcano in the Ogasawara Arc: New continent from the ocean? Island Arc, 28, e12285. https://doi.org/10.1111/iar.12285
10.1111/iar.12285
Web of Science®Google Scholar
Tamura, Y., Sato, T., Fujiwara, T., Kodaira, S., & Nichols, A. (2016). Advent of continents: A new hypothesis. Scientific Reports, 6, 33517. https://doi.org/10.1038/srep33517
10.1038/srep33517
CASADSPubMedWeb of Science®Google Scholar
Tamura, Y., Tani, K., Ishizuka, O., Chang, Q., Shukuno, H., & Fiske, R. S. (2005). Are arc basalts dry, wet, or both? Evidence from the Sumisu Caldera Volcano, Izu-Bonin arc, Japan. Journal of Petrology, 46(9), 1769–1803. https://doi.org/10.1093/petrology
10.1093/petrology/egi033
CASADSWeb of Science®Google Scholar
Tanaka, T., Togashi, S., Kamioka, H., Amakawa, H., Kagami, H., Hamamoto, T., Yuhara, M., Orihashi, Y., Yoneda, S., Shimizu, H., Kunimaru, T., Takahashi, K., Yanagi, T., Nakano, T., Fujimaki, H., Shinjo, R., Asahara, Y., Tanimizu, M., & Dragusanu, C. (2000). JNdi-1: A neodymium isotopic reference in consistency with LaJolla neodymium. Chemical Geology, 168(3–4), 279–281. https://doi.org/10.1016/S0009-2541(00)00198-4
10.1016/S0009-2541(00)00198-4
ADSWeb of Science®Google Scholar
Tatsumi, Y., & Eggins, S. (1995). Subduction zone magmatism (p. 211). Cambridge, UK: Blackwell Science.
Google Scholar
Taylor, R. N., & Nesbitt, R. W. (1998). Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin Arc, Japan. Earth and Planetary Science Letters, 164(1–2), 79–98. https://doi.org/10.1016/S0012-821X(98)00182-4
10.1016/S0012-821X(98)00182-4
CASADSWeb of Science®Google Scholar
Taylor, S. R., & McLennan, S. M. (1985). The continental crust: Its composition and evolution: An examination of the geochemical record preserved in sedimentary rocks (p. 312). Oxford: Blackwell Scientific Publications.
Google Scholar
Tollstrup, D., Gill, J., Kent, A., Prinkey, D., Williams, R., Tamura, Y., & Ishizuka, O. (2010). Across-arc geochemical trends in the Izu-Bonin arc: Contributions from the subducting slab, revisited. Geochemistry, Geophysics, Geosystems, 11, Q01X10. https://doi.org/10.1029/2009GC002847
10.1029/2009GC002847
Web of Science®Google Scholar
Woodhead, J., Stern, R. J., Pearce, J., Hergt, J., & Vervoort, J. (2012). Hf-Nd isotope variation in Mariana Trough basalts: The importance of ‘ambient mantle’ in the interpretation of subduction zone magmas. Geology, 40(6), 539–542. https://doi.org/10.1130/G32963.1
10.1130/G32963.1
CASADSWeb of Science®Google Scholar
Workman, R. K., & Hart, S. R. (2005). Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters, 231(1–2), 53–72. https://doi.org/10.1016/j.epsl.2004.12.005
10.1016/j.epsl.2004.12.005
CASADSWeb of Science®Google Scholar
Wurth, K., & DeBari, M. S. (2019). Clinopyroxene trace element chemistry as a proxy for magma compositional variations in the Izu Bonin rear arc over the last 15 million years, AGU Fall Meeting 2019 abstract, V51G-0203.
Google Scholar
Wurth, K.N. (2019). Clinopyroxene trace element chemistry as a proxy for magma compositional variations in the Izu Bonin rear arc over the last 15 million years, MS thesis, Western Washington University Graduate School Collection. 898, https://cedar.wwu.edu/wwuet/898
Google Scholar
Yang, A., Wang, C., Liang, Y., & Lissenberg, J. (2019). Mineral and melt compositions from experimental studies of reaction between mid-ocean ridge basalt and lower oceanic crust, Version 1.0, Interdisciplinary Earth Data Alliance (IEDA). https://doi.org/10.1594/IEDA/111370 Accessed 2019-11–27.
Google Scholar
Yogodzinski, G. M., Bizimis, M., Hickey-Vargas, R., McCarthy, A., Hocking, B. D., Savov, I. P., Ishizuka, O., & Arculus, R. (2018). Implications of Eocene-age Philippine Sea and forearc basalts for initiation and early history of the Izu-Bonin-Mariana arc. Geochimica et Cosmochimica Acta, 228, 136–156. https://doi.org/10.1016/j.gca.2018.02.047
10.1016/j.gca.2018.02.047
CASADSWeb of Science®Google Scholar
Yokoyama, T., Kobayashi, K., Kuritani, T., & Nakamura, E. (2003). Mantle metasomatism and rapid ascent of slab components beneath island arcs; evidence for ²³⁸U-²³⁰Th-²²⁶Ra disequilibria of Miyakejima volcano, Izu arc, Japan. Journal of Geophysical Research, 108, 2329. https://doi.org/10.1029/2002JB002103
10.1029/2002JB002103
CASADSWeb of Science®Google Scholar
Yokoyama, T., Kuritani, T., Kobayashi, K., & Nakamura, E. (2006). Geochemical evolution of a shallow magma plumb-ing system during the last 500 years, FMiyakejima volcano, Japan: Constraints from ²³⁸U−²³⁰Th−²²⁶Ra systematics. Geochimica et Cosmochimica Acta, 70(11), 2885–2901. https://doi.org/10.1016/j.gca.2006.02.027
10.1016/j.gca.2006.02.027
CASADSWeb of Science®Google Scholar

References From the Supporting Information

Deniel, C., & Pin, C. (2001). Single-stage method for the simultaneous isolation of lead and strontium from silicate samples for isotopic measurements. Analytica Chimica Acta, 426(1), 95–103. https://doi.org/10.1016/S0003-2670(00)01185-5
10.1016/S0003-2670(00)01185-5
CASWeb of Science®Google Scholar
Hamelin, C., Bezos, A., Doss, L., Escartin, J., Cannat, M., & Mevel, C. (2013). Atypically depleted upper mantle component revealed by Hf isotopes at Lucky Strike segment. Chemical Geology, 341, 128–139. https://doi.org/10.1016/j.chemgeo.2013.01.013
10.1016/j.chemgeo.2013.01.013
CASADSWeb of Science®Google Scholar
Johnson, D. M., Hooper, P. R., & Conrey, R. M. (1999). XRF analysis of rocks and minerals for major and trace elements on a single low dilution Li-tetraborate fused bead. Advances in X-Ray Analysis, 41, 843–867.
CASGoogle Scholar
Kimura, J. -I. (2017). Modeling chemical geodynamics of subduction zones using the Arc Basalt Simulator version 5. Geosphere, 13, 992–1025. https://doi.org/10.1130/GES01468.1
Web of Science®Google Scholar
Knaack, C., Cornelius, S. B., & Hooper, P. R. (1994). Trace element analyses of rocks and minerals by ICP-MS. Pullman, WA: Washington State University GeoAnalytical Lab.
Google Scholar
Miyazaki, T., Hanyu, T., Kimura, J. I., Senda, R., Vaglarov, B. S., Chang, Q., Hirahara, Y., Takahashi, T., & Kawabata, H. (2018). Clinopyroxene and bulk rock Sr–Nd–Hf–Pb isotope compositions of Raivavae ocean island basalts: Does clinopyroxene record early stage magma chamber processes? Chemical Geology, 482, 18–31. https://doi.org/10.1016/j.chemgeo.2017.12.015
10.1016/j.chemgeo.2017.12.015
CASADSWeb of Science®Google Scholar
Miyazaki, T., Vaglarov, B. S., Takei, M., Suzuki, M., Suzuki, H., Ohsawa, K., Chang, Q., Takahashi, T., Hirahara, Y., Hanyu, T., Kimura, J. -I., & Tatsumi, Y. (2012). Development of a fully automated open-column chemical-separation system—COLUMNSPIDER—and its application to Sr-Nd-Pb isotope analyses of igneous rock samples. Journal of Mineralogical and Petrological Sciences, 107(2), 74–86. https://doi.org/10.2465/jmps.110520
10.2465/jmps.110520
CASADSWeb of Science®Google Scholar
Pin, C., Briot, D., Bassin, C., & Poitrasson, F. (1994). Concomitant separation of strontium and samarium-neodymium for isotopic analysis in silicate samples, based on specific extraction chromatography. Analytica Chimica Acta, 298(2), 209–217. https://doi.org/10.1016/0003-2670(94)00274-6
10.1016/0003-2670(94)00274-6
CASWeb of Science®Google Scholar
Steenberg, L., Boroughs, S., & Knaack, C. (2017). Estimation of accuracy and precision for trace elements analyzed by in- ductively coupled plasma mass spectrometry (ICP-MS) at the Peter Hooper Geoanalytical Laboratory, Washington State University, Geological Society of America, Abstracts with Programs 49. Paper No. 69–3.
Google Scholar
Weis, D., Kieffer, B., Maerschalk, C., Barling, J., de Jong, J., Williams, G. A., Hanano, D., Pretorius, W., Mattielli, N., Scoates, J. S., Goolaerts, A., Friedman, R. M., & Mahoney, J. B. (2006). High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS. Geochemistry, Geophysics, Geosystems, 7, Q08006. https://doi.org/10.1029/2006GC001283
10.1029/2006GC001283
CASADSWeb of Science®Google Scholar

Citing Literature

Volume21, Issue10

October 2020

e2020GC009114

Filename	Description
ggge22324-sup-0001-Text_SI-S01.docxWord 2007 document , 672.1 KB	Supporting Information S1
ggge22324-sup-0002-Table_SI-S01.xlsxPDF document, 31.7 KB	Table S2
ggge22324-sup-0003-Table_SI-S02.xlsxPDF document, 21.6 KB	Table S3

The First 10 Million Years of Rear-Arc Magmas Following Backarc Basin Formation Behind the Izu Arc

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Site U1437 and Samples

3 Analytical Methods