Volume 3, Issue 10 p. 1-14
Free Access

Correlated geophysical, geochemical, and volcanological manifestations of plume-ridge interaction along the Galápagos Spreading Center

R. S. Detrick

R. S. Detrick

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02453 USA

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J. M. Sinton

J. M. Sinton

School of Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Road,, Honolulu, Hawaii, 96822 USA

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G. Ito

G. Ito

School of Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Road,, Honolulu, Hawaii, 96822 USA

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J. P. Canales

J. P. Canales

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02453 USA

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M. Behn

M. Behn

MIT-WHOI Joint Program in Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543 USA

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T. Blacic

T. Blacic

Department of Geology, University of California, Davis, Davis, California, 95616 USA

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B. Cushman

B. Cushman

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02453 USA

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J. E. Dixon

J. E. Dixon

Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, 33149 USA

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D. W. Graham

D. W. Graham

College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, 97331 USA

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J. J. Mahoney

J. J. Mahoney

Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02453 USA

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First published: 10 October 2002
Citations: 111

Abstract

[1] As the Galápagos hot spot is approached from the west along the Galápagos Spreading Center there are systematic increases in crustal thickness and in the K/Ti, Nb/Zr, 3He/4He, H2O, and Na2O content of lavas recovered from the spreading axis. These increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth, residual mantle Bouguer gravity anomaly, magma chamber depth, average lava Mg #, Ca/Al ratio, and the frequency of point-fed versus fissure-fed volcanism. Magma chamber depth and axial morphology display a “threshold” effect in which small changes in magma supply result in large changes in these variables. These correlated variations in geophysical, geochemical, and volcanological manifestations of plume-ridge interaction along the western Galápagos Spreading Center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes, and the consequences of these variations on magma supply, axial thermal structure, basalt chemistry, and styles of volcanism.

1. Introduction

[2] A significant portion of the global mid-ocean ridge system is influenced by mantle plumes [Schilling, 1991; Ito and Lin, 1995a]. Characterizing the distinctive gradients in geophysical and geochemical anomalies along plume-influenced ridges can help elucidate mantle processes such as mantle flow, source mixing, decompression melting, and melt migration in ways that are not possible along more “normal” ridge systems. Hot spot-related variations in magma production rates at a more or less constant spreading rate can also be used to examine the influence of magma supply on a wide range of crustal accretion processes, including the formation of axial topography, magma chamber properties, and style of volcanism. In this report we describe results from a combined geophysical and petrological investigation of an ∼800-km-long section of the hot spot-influenced western Galápagos Spreading Center (GSC) that provides new insight into the effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on crustal accretion processes.

2. G-PRIME Experiment

[3] The east-west striking GSC separates the Cocos and Nazca plates in the eastern equatorial Pacific (Figure 1). The GSC is an intermediate spreading-rate ridge with full opening rates increasing from 45 mm/yr at 98°W to ∼56 mm/yr at 91°W [DeMets et al., 1990]. At 91°W the GSC lies ∼200 km north of the Galápagos Archipelago, the western end of which marks the probable center of the Galápagos mantle plume [White et al., 1993; Toomey et al., 2001]. The proximity of the Galápagos hot spot has had a strong influence on the GSC. This can be seen in along-axis variations in bathymetry and gravity [Ito and Lin, 1995b], axial morphology [Canales et al., 1997], in the chemical and isotopic composition of GSC basalts [Schilling et al., 1976, 1982; Fisk et al., 1982; Verma and Schilling, 1982; Verma et al., 1983], and in the long history of rift propagation along the GSC [Hey, 1977; Wilson and Hey, 1995].

Details are in the caption following the image
Map of the western Galápagos Spreading Center (GSC) and Galápagos swell based on integration of available multibeam bathymetry [Canales et al., 1997; Hey et al., 1992; this study] and satellite-derived seafloor topography data [Smith and Sandwell, 1997]. Water depths range from >3500 m (blue) to <1000 m (red). The axis of the east-west trending, intermediate-spreading rate GSC is shown by the thin white band. At 91°W, the GSC lies ∼200 km north of the Galápagos Archipelago, the western end of which marks the probable center of the Galápagos mantle plume [White et al., 1993; Toomey et al., 2001]. The Galápagos swell is apparent in a gradual shoaling of sea floor depths along the western GSC over a distance of ∼800 km. The Wolf-Darwin lineament (WDL) is a volcanic chain located on the southern flank of the GSC. Location of seismic refraction (black) and multichannel reflection (blue) lines, and lava sampling stations (red circles) obtained on the G-PRIME expedition are shown. White box indicates location of bathymetry map detail shown in Figure 3a.

[4] In April–May 2000 the Galápagos Plume-Ridge Interaction Multidisciplinary Experiment (G-PRIME) conducted an extensive geophysical and petrological investigation of the western GSC between 90.5°W and 98°W (Figure 1). The first modern multibeam bathymetry data were collected along the GSC between 90.5°W and 93.3°W, and west of 95°W, defining the location of the spreading axis and constraining the variation in axial morphology. In order to determine the thickness and internal structure of the crust, ∼1400 km of multichannel seismic reflection data were obtained along the GSC between 91.25°W and 95°W, and three wide-angle seismic refraction experiments were carried out along parts of the GSC with distinctly different axial morphologies (rift valley, transitional morphology, and axial high).

[5] With the tectonic and structural context provided by these geophysical surveys, rock samples were collected at 91 stations along the GSC between 90.5°W and 98°W, supplementing previous collections along the GSC between 83°W and 101°W [Schilling et al., 1976, 1982] and near 95°W [Christie and Sinton, 1981, 1986; Hey et al., 1992], to provide comprehensive petrological, geochemical, and He, Sr, Nd, and Pb isotopic data on lava compositions. Major and minor element abundances on these samples presented in this paper are University of Hawaii electron microprobe analyses of natural glasses; Nb/Zr was determined by X-ray fluorescence on whole rock samples at the University of Hawaii. Forty-five individual glasses were analyzed for dissolved H2O and CO2 by Fourier transform spectrometer (FTIR) at the University of Miami. Thirty-six basalt glass samples were analyzed for 3He/4He at Oregon State University. All geochemical data presented in this paper are summarized in Table 1.

Table 1. Galápagos Spreading Center: Glass and Whole Rock Dataa
Glass Data Whole Rock Data
Glass label MORB-type Longitude, °W Mg# K/Ti Na8.0 Ca8.0/Al8.0 H2O8.0 3He/4He R/Ra Sample MORB-type Longitude, °W Nb/Zr
EW2D N 90.821 62.9 0.049 0.124 7.75 EW2-1 N 90.821 0.019
EW3C E 90.983 47.2 0.157 2.45 0.869 EW7-4 E 91.275 0.086
EW4D T 91.060 58.3 0.115 2.51 0.788 0.248 7.80 EW10-1 E 91.364 0.111
EW5C E 91.076 51.1 0.159 2.52 0.864 EW11-1 E 91.404 0.108
EW6D T 91.183 54.7 0.112 2.25 0.853 7.97 EW15-1 T 91.607 0.091
EW7D-a E 91.275 50.9 0.173 2.57 0.838 0.307 7.98 EW16-2 E 91.746 0.087
EW7D-b T 91.275 54.5 0.136 2.61 0.831 EW17-1 E 91.803 0.098
EW8C E 91.357 39.1 0.240 2.61 0.824 EW17-4 E 91.803 0.099
EW9D-a T 91.322 51.8 0.128 2.53 0.834 0.279 8.07 EW19-1 E 91.957 0.098
EW9D-b T 91.322 57.0 0.123 2.57 0.775 EW20-1 E 92.012 0.099
EW10D E 91.364 38.4 0.239 2.53 0.833 0.354 EW25-1 E 92.320 0.105
EW11D-a E 91.404 30.9 0.440 2.43 0.149 7.30 EW28-1 E 92.519 0.096
EW11D-b E 91.404 33.5 0.333 2.59 EW29-1 T 92.618 0.073
EW12D E 91.486 36.2 0.237 2.56 0.835 0.359 7.31 EW32-2 T 92.880 0.068
EW13D E 91.560 39.0 0.271 2.48 0.833 EW33-1 T 92.971 0.075
EW14C E 91.608 33.0 0.223 2.25 0.848 EW38-2 T 93.268 0.068
EW15D T 91.607 58.0 0.161 2.20 0.803 0.305 7.37 EW41-1 T 93.093 0.074
EW16D E 91.746 53.1 0.200 2.95 0.741 0.480 7.71 EW41-7 T 93.093 0.071
EW17D-a E 91.803 57.6 0.264 3.23 0.688 0.555 7.77 EW45-2 T 93.352 0.063
EW17D-b E 91.803 59.4 0.255 3.21 0.664 EW48-4 T 93.657 0.075
EW18D E 91.881 57.2 0.198 2.51 0.819 EW49-1 T 93.871 0.073
EW19D E 91.957 38.6 0.240 2.77 0.817 0.432 7.55 EW50-1 T 93.777 0.082
EW20D-a E 92.012 60.2 0.225 2.61 0.709 EW56-2 T 94.240 0.057
EW20D-b E 92.012 58.4 0.231 2.72 0.737 0.423 8.05 EW58-2 T 94.350 0.061
EW21D E 92.051 50.8 0.212 2.83 0.821 EW62-1 T 94.662 0.056
EW22D E 92.157 51.5 0.254 2.90 0.853 EW63-1 N 94.747 0.038
EW23D E 92.222 50.4 0.194 2.56 0.854 7.54 EW63-2 T 94.747 0.052
EW24D E 92.244 53.6 0.193 2.43 0.826 EW67-1 T 95.033 0.060
EW25D-a E 92.320 55.7 0.233 2.45 0.821 0.324 7.71 EW69-1 T 95.213 0.061
EW25D-b E 92.320 56.9 0.226 2.47 0.804 EW70-1 T 95.317 0.051
EW26C E 92.373 54.5 0.402 2.45 0.816 EW71-1 N 95.602 0.030
EW27D E 92.427 46.4 0.151 2.27 0.857 EW73-1 T 95.702 0.047
EW28D E 92.519 53.7 0.189 2.31 0.865 0.261 7.45 EW77-2 N 96.191 0.029
EW29D T 92.618 50.8 0.135 2.20 0.840 0.233 7.38 EW79-1 N 96.725 0.035
EW30D E 92.695 43.5 0.149 2.30 0.867 EW80-2 N 96.630 0.018
EW31D T 92.822 47.8 0.135 2.32 0.832 7.39 EW83-5 N 96.818 0.026
EW32D T 92.880 49.4 0.118 1.98 0.841 EW87-1 N 97.100 0.026
EW33D T 92.971 48.8 0.123 2.07 0.829 0.192 6.98 EW88-1 N 97.192 0.031
EW34D-b T 93.010 51.9 0.120 2.17 0.851 EW92-1 N 97.782 0.028
EW34D-a T 93.010 52.9 0.122 2.10 0.844 AL-1538-3 T 95.42 0.058
EW35D-a T 93.049 48.1 0.112 2.24 0.845 AL-1538-4 T 95.42 0.056
EW35D-b T 93.049 47.1 0.113 2.27 0.865 AL-1539-2 T 95.44 0.056
EW36D T 93.431 51.7 0.130 2.17 0.825 AL-1540-4 T 95.47 0.059
EW37D T 93.353 53.9 0.116 2.13 0.859 AL-1541-2 T 95.48 0.062
EW38D T 93.268 49.0 0.116 2.07 0.848 0.217 6.83 AL-1544-1 T 95.46 0.050
EW39D-b T 93.218 54.4 0.153 2.18 0.859 AL-1544-2 E 95.46 0.065
EW39D-a T 93.218 48.4 0.122 2.13 0.848 AL-1544-3 N 95.46 0.049
EW40D T 93.159 62.0 0.096 AL-1545-1 T 95.37 0.059
EW41D-a T 93.093 54.6 0.113 2.19 0.838 AL-1545-3 T 95.37 0.056
EW41D-b T 93.093 55.5 0.119 2.17 0.842 0.182 AL-1545-4 N 95.37 0.037
EW41D-c T 93.093 53.0 0.124 2.13 0.851 0.180 7.09 AL-1545-5 T 95.38 0.061
EW42D-a N 93.203 63.1 0.083 AL-1545-6 N 95.38 0.034
EW42D-b T 93.203 62.1 0.098 AL-1546-2 T 95.46 0.055
EW43D N 93.257 62.6 0.068 0.132 6.90 AL-1549-1 T 95.38 0.061
EW44D T 93.296 60.7 0.098 AL-1549-3 N 95.38 0.038
EW45D T 93.352 58.0 0.099 2.27 0.800 0.202 AL-1549-4 N 95.38 0.037
EW46D T 93.494 57.4 0.119 2.15 0.860 6.89 AL-1549-5 N 95.38 0.038
EW47D T 93.564 49.7 0.131 2.26 0.841 AL-1549-6 T 95.38 0.061
EW48D-a N 93.657 58.2 0.081 2.24 0.845 AL-1550-2 N 95.54 0.042
EW48D-b T 93.657 49.7 0.137 2.29 0.840 0.222 AL-1550-4 T 95.54 0.045
EW49D T 93.871 48.0 0.120 2.29 0.834 0.220 AL-1551-3 N 95.48 0.026
EW50D T 93.777 57.0 0.142 2.21 0.843 0.206 6.89 AL-1552-8 N 95.53 0.031
EW51D T 93.962 56.3 0.093 2.29 0.874 AL-1554-1 N 95.48 0.034
EW52D T 94.062 50.9 0.121
EW53D T 94.129 44.9 0.105
EW54C T 94.170 60.7 0.127
EW55D-a T 94.220 52.5 0.133
EW55D-b T 94.220 49.6 0.143
EW55D-c T 94.220 48.7 0.144
EW56D T 94.240 54.0 0.105
EW57D T 94.275 48.6 0.149
EW58D T 94.350 51.0 0.106
EW59D T 94.434 56.2 0.105
EW60D T 94.600 55.9 0.095
EW61D T 94.538 56.4 0.111
EW62D T 94.662 56.8 0.093
EW63D-a N 94.747 56.6 0.068
EW63D-b T 94.747 56.7 0.099
EW64D T 94.824 53.6 0.118
EW65D T 94.908 55.1 0.102
EW66D N 94.978 59.1 0.086
EW67D T 95.033 57.3 0.111
EW68D-a T 95.143 53.7 0.114
EW68D-b T 95.143 54.6 0.114
EW69D T 95.213 53.9 0.107
EW70D T 95.317 55.1 0.093
EW71D-a N 95.602 65.3 0.058
EW71D-b N 95.602 64.2 0.050
EW72C N 95.620 61.9 0.082
EW73D T 95.702 64.0 0.108
EW74D N 95.793 62.4 0.068
EW75D N 95.876 53.8 0.079
EW76D N 96.126 61.1 0.077
EW77D-a N 96.191 59.9 0.062
EW77D-b N 96.191 58.6 0.059
EW78D-scor E 96.332 48.3 0.249
EW79D N 96.725 61.9 0.059
EW80D N 96.630 62.9 0.040
EW81D N 96.697 67.4 0.045
EW82D N 96.775 65.1 0.031
EW83D N 96.818 61.7 0.051
EW84D N 96.877 59.1 0.061
EW85D N 96.962 59.0 0.054
EW86D-a N 96.996 60.2 0.064
EW86D-b N 96.996 59.9 0.064
EW87D N 97.100 61.6 0.055
EW88D N 97.192 62.4 0.060
EW89D N 97.358 60.0 0.059
EW90C-a N 97.258 50.7 0.067
EW90C-b N 97.258 55.3 0.059
EW91D N 97.604 63.7 0.057
EW92D N 97.782 60.7 0.071
AL1538_PR T 95.420 51.1 0.102
AL1539-a_PR T 95.440 51.9 0.102
AL1539-b_PR T 95.440 48.3 0.104
AL1540-a_PR T 95.460 46.1 0.109
AL1540-b_PR T 95.470 52.2 0.104
AL1541_PR T 95.480 52.1 0.107
AL1544_PR N 95.460 51.6 0.090
AL1545-a_PR T 95.370 51.8 0.103
AL1545-b_PR N 95.370 62.0 0.070
AL1549-a_PR T 95.380 52.7 0.108
AL1549-b_PR N 95.380 62.1 0.072
AL1551-a_NG N 95.480 62.1 0.066
AL1551-b_NG T 95.480 62.3 0.096
AL1554-a_NG N 95.510 62.3 0.062
AL1554-b_NG N 95.510 59.9 0.061
AL1554-c_NG N 95.510 65.4 0.071
AL1554-d_NG N 95.510 65.3 0.053
AL1555_DR N 95.660 61.2 0.073
AL1557-a_PR T 95.460 45.9 0.111
AL1557-b_PR T 95.460 49.6 0.106
AL1557-c_PR T 95.480 52.1 0.105
A6_PR T 95.370 52.6 0.104
A13_DR N 95.620 58.5 0.084
  • a For glass samples, station number, and type, (D)redge or Wax (C)ore shown. Samples from same station with significantly similar major element data were grouped together. H2O and 3He/4He data were determined from one sample representative of the group. All samples from R/V Maurice Ewing cruise EW00-04 except samples 1538–1557 which are a reanalysis of Alvin samples collected from 95W propagating rift area by Hey et al. [1992] and dredges A6 and A13.
  •   PR, propagating rift; DR, dying rift; NG, North Graben. MORB type based on K/Ti ratio: N, N-MORB; T, T-MORB; E, E-MORB. Mg#, atomic (Mg/Mg+Fe). Na8.0, Ca8.0/Al8.0, and H208.0 are derived from oxide values of Na20, CaO, Al2O3 and H2O corrected to 8.0 wt. % MgO. He R/Ra, (3He/4He)/atmospheric ratio. Gas trapped in vesicles was released by in vacuo crushing, and the separated helium was analyzed by mass spectrometry. Nb/Zr determined on whole rock samples; MORB type based on K/Ti ratios of the corresponding glass samples.

3. Results

3.1. Swell Topography and Crustal Thickness

[6] The Galápagos hot spot swell is apparent in the gradual shoaling of sea floor depths along the western GSC over a distance of ∼800 km (Figure 1). Ridge axis depths along the western GSC decrease from ∼3500 m at 97.7°W to <1700 m at 91.4°W (Figure 2a). Part of this ∼1800 m change in axial depth is a result of a systematic change in axial morphology. Far from the Galápagos hot spot (west of 95°W), the GSC is associated with a 20–40 km wide, 400–1000 m deep rift valley resembling that of the slow-spreading Mid-Atlantic Ridge. Within ∼300 km of the Galápagos hot spot (east of ∼92.7°W) the GSC is associated with a 400–700 m axial high typical of the fast-spreading East Pacific Rise. These differences in axial topography can account for a significant amount of the depth variation observed along the western Galápagos swell. Canales et al. [2002] found that a low-pass filter with a cutoff wavelength of 85 km effectively removes the short-wavelength contribution of variations in axial topography while preserving the longer wavelength anomaly associated with the hot spot swell. They found that ∼60% of the observed variation in axial depth along the western Galápagos swell is related to changes in axial morphology. After correcting for these effects, the maximum swell amplitude near 91°W is estimated to be ∼700 m (Figure 2a). Coinciding with this topographic swell is a regional mantle Bouguer anomaly (MBA) that becomes increasingly negative along the GSC toward the hot spot, with a minimum of −70 mGal at ∼91.25°W (Figure 2b).

Details are in the caption following the image
Geophysical, volcanological, and geochemical variations along the western GSC showing the following: (a) depth of ridge axis (black line) and Galápagos swell (blue line); swell depth is corrected for variations in axial topography along western GSC. (b) Mantle Bouguer anomaly (black line) and residual mantle Bouguer anomaly (RMBA, red line; red dots where unconstrained by crustal thickness measurements). (c) Crustal thickness along GSC from Canales et al. [2002] including estimates from ocean bottom seismic refraction experiments (red horizontal bars) and two-way Moho reflection travel times converted to crustal thickness (dots); black line is best fitting polynomial function to crustal thickness data. (d) Abundance of small seamounts in axial zone of GSC determined from high-resolution multibeam bathymetry data; seamount abundance calculated in ten minute longitude bins. (e) Mg # (100[atomic MgO/(MgO+FeO)]) for lava samples from GSC. (f) K/Ti. (g) Nb/Zr. In Figures 2e–2g, blue squares denote normal mid-ocean ridge basalts (N-MORB), inverted red triangles enriched MORB (E-MORB), and green circles transitional MORB (T-MORB). Solid symbols are samples from the GSC; open symbols are from the 91°W fracture zone. The domains of axial morphology (rift valley, transitional, axial high) along the GSC are shown for reference.

[7] Crustal thickness variations along the western Galápagos swell are constrained by three ridge-parallel, wide-angle seismic refraction profiles at 97°W, 94.25°W, and 92°W, and by two-way Moho reflection times observed on multichannel seismic reflection profiles collected on ridge-parallel, off-axis profiles between 91.5°W and 95.5°W (Figure 1). Crustal thickness along the western GSC increases from 5.6±0.2 km at 97°W to ∼7.9 km at 91.5°W, an increase of ∼2.3 km (Figure 2c). The significant thickening of the crust east of 94°W indicates that the primary effect of the hot spot on melt productivity beneath the GSC is confined to a distance less than ∼400 km from the center of the hot spot.

[8] The effects of crustal thickening on the along-axis swell and gravity anomalies can be calculated assuming that the smoothed crustal thickness profile (best fitting polynomial function shown as black curve in Figure 2c) is compensated by local Airy isostasy. Assuming average water, crust, and mantle densities of 1000 kg/m3, 2770 kg/m3, and 3300 kg/m3, respectively, Canales et al. [2002] showed that, on average, crustal thickness variations support ∼50% of the swell depth anomaly east of 94°W. After computing the gravitational effects of the crust, Canales et al. [2002] also found that the crust accounts for an average of ∼60% of the observed MBA east of 94°W. That portion of the swell topography not explained by crustal thickness variation is called the residual mantle swell anomaly, or RMSA, and is shown in Figure 3b for the most plume influenced part of the ridge east of 94°W. The remaining gravity anomaly is the residual MBA, or RMBA. The RMBA becomes increasingly negative toward the east reaching a minimum of ∼−25 mGal at 92.25°W, where it coincides with the largest (∼300 m) RMSA (Figure 3b). Both of these anomalies occur near the intersection of the GSC and the Wolf-Darwin seamount chain, rather than farther to the east where the ridge crest is closer to the hot spot. A secondary minimum in RMBA and peak in RMSA occurs near 91.3°W where another small seamount chain intersects the GSC (Figure 3b); however, these anomalies are less well constrained because of a lack of seismic control on crustal thickness east of 91.5°W (Figure 2c).

Details are in the caption following the image
Correlated variations in morphology, axial structure, and lava chemistry along the most plume-influenced portion of the western GSC east of 94°W, showing the following: (a) bathymetry (purple-blue, deep; yellow-red, shallow) from Canales et al. [1997] and Smith and Sandwell [1997], and this study; location of fracture zone (FZ) at 91°W and propagating rift (PR) at 93.3°W shown for reference. (b) Red line (dashed where unconstrained by crustal thickness) shows residual mantle swell anomaly (RMSA), that portion of the swell topography not predicted by crustal thickness variation; RMBA (black line and dots), from Figure 2b, shown for comparison. (c) Two-way travel time to base of seismic layer 2A (blue) and top of axial magma chamber (red) from multichannel seismic reflection data (black dots show picks from cross-axis profiles, all other picks are from along-axis profiles). (d) 3He/4He, (e) H2O8, (f) Na8, and (g) Ca8/Al8 (note reversed scale), for GSC lavas. Symbols in Figures 3d–3g are same as in Figure 2. Note that west of 92.7°W the axial high disappears and a distinct transitional morphology lacking either an axial high or axial valley is present. This change in axial morphology correlates with significant changes in axial structure (Figure 3c) and lava chemistry (Figures 3d–3g). The largest residual swell anomaly (Figure 3b), shallowest axial magma chamber (Figure 3c), and peaks in 3He/4He, H2O, and Na8 (Figures 3d–3f) all occur between 91.8°W and 92.25°W near where the Wolf-Darwin lineament and a second similar seamount chain intersect the GSC. A secondary peak in both residual swell anomaly and 3He/4He occurs near 91.3°W where a third seamount chain intersects the GSC.

[9] The negative RMBA indicates that anomalously low mantle densities are present beneath the GSC with the lowest densities near the hot spot. The ratio of the variation in RMBA and RMSA is well explained by calculations that assume the excess swell topography is isostatically compensated by mantle density variations confined to depths of 50–100 km [Canales et al. 2002]. We see no need to invoke additional dynamic topography, unrelated to mantle density variations, such as those that may be caused by regional asthenospheric pressure gradients [e.g., Phipps Morgan et al., 1995; Yale and Phipps Morgan, 1998]. The simplest case of passive mantle upwelling and adiabatic decompression melting requires only a small increase in mantle potential temperature of ∼30°C to thicken the crust by ∼2.3 km, as observed along the western GSC between 98°W and 91°W. The density reduction associated with this increase in mantle temperature, as well as the increased mantle depletion [Oxburgh and Parmentier, 1977] due to melting, can account for part, but not all, of the mantle density variation required to explain the RMBA and excess swell topography. This excess buoyancy may be caused by melt in the mantle and/or depletion due to melting of the upwelling plume far from the ridge axis and the subsequent transport of this depleted residuum northward toward the GSC.

3.2. Depth of Axial Magma Lens, Thickness of Seismic Layer 2A, and Axial Morphology

[10] At an intermediate spreading-rate ridge like the GSC, lithospheric thermal structure and its effect on axial morphology are expected to be particularly sensitive to small changes in magma supply. The along-axis variation in crustal thickness (Figure 2c) reveals a systematic change in magma supply, while the depth at which magma resides in the crust at the ridge axis (Figure 3c) constrains thermal gradients in the axial lithosphere. West of ∼95°W, where a rift valley characterizes the axial morphology, we find the thinnest crust along the western GSC (5.6–6 km). Between 95°W and 92.7°W, where axial morphology is transitional, displaying neither an axial valley nor a topographic high, crustal thickness is 6–7 km and the top of an axial magma chamber (AMC) is at depths of 2.5–4.25 km (1.0–1.7 two-way travel time (TWTT)). East of 92.7°W, where the GSC is associated with an axial high, the AMC is significantly shallower (0.55–0.9 s TWTT or ∼1.38–2.25 km depth), and the crust is 7–8 km thick. Although the variation in magma supply along the GSC is very gradual, the transition from one type of topographic regime to another is fairly abrupt. For example, the appearance of the axial high near 92.7°W occurs over an along-axis distance of only ∼20 km (Figure 3a). The development of the axial high correlates remarkably well with a rapid shoaling of the AMC by >1 km and an approximate halving of the thickness of seismic layer 2A from 0.3–0.5 s TWTT (∼300–500 m) to 0.15–0.35 s TWTT (∼150–350 m), over the same along-axis distance. The correlation between AMC depth and axial topography supports the notion that axial topography is directly linked to the thermal structure and thus the strength of the axial lithosphere [Chen and Morgan, 1990]. The abrupt changes in both AMC depth and axial morphology, despite only modest changes in crustal thickness, support the hypothesis of Phipps Morgan and Chen [1993] of a threshold effect in which small changes in magma supply lead to significant changes in axial thermal structure, magma chamber depth, and axial morphology.

3.3. Seamount Abundance and Magma Supply-Related Variations in Eruptive Style

[11] The number of small seamounts or volcanic edifices present in the axial zone of the GSC has been determined using high-resolution multibeam bathymetric data and a numerical algorithm that identifies closed, concentric contours that meet certain shape and height criteria (Figure 2d). Seamount densities have been calculated using a maximum likelihood model [Smith and Cann, 1992]. The number of seamounts in the axial zone decreases significantly as the Galápagos hot spot is approached, suggesting a change from dominantly point-source to fissure-fed volcanism as magma supply increases. West of 95.5°W, where magma supply is lower, the number of seamounts per unit area (∼279±16 per 103 km2) is similar to values observed at the slow-spreading Mid-Atlantic Ridge [Smith and Cann, 1992; Magde and Smith, 1995]. In contrast, east of 92.7°W, where magma supply is higher, seamount density (50±9 per 103 km2) is similar to observations at the fast-spreading East Pacific Rise [Abers et al., 1988]. Thus the western GSC displays the same range in seamount density observed along the global mid-ocean ridge system suggesting that both spreading rate and magma supply are important factors controlling the style of constructional volcanism (point source versus fissure fed eruptions) at oceanic spreading centers.

3.4. Geochemical Constraints on Source Composition and Mantle Melting

[12] As the Galápagos hot spot is approached from the west along the GSC there are systematic variations in basalt chemistry, first described by Schilling et al. [1982], that correlate with the geophysical variations described above (Figures 2 and 3). Although sample spacing in this study (<10 km) is much closer than the 40–50 km by Schilling et al. [1982], and our samples are more precisely located on the ridge axis using multibeam bathymetry, we find a similar long-wavelength pattern of geochemical variation as reported by Schilling and colleagues. For example, average lava Mg # (atomic MgO/(MgO+FeO)) decreases progressively from west to east with the lowest Mg # (most fractionated) lavas erupting along the most plume influenced portion of the ridge (Figure 2e). This indicates an eastward increase in the degree of crystallization that correlates with the presence and shoaling of the AMC. Incompatible elements such as K2O, TiO2, and H2O all increase along the western GSC toward the 91°W fracture zone, indicating the presence of a plume source enriched in volatiles and other incompatible elements as described by Schilling et al. [1982].

[13] Variations in ratios of incompatible elements including K/Ti and Nb/Zr, and isotopes such as 3He/4He, largely reflect differences in the mantle source undergoing melting. These ratios therefore provide information on the relative proportion of plume-derived versus ambient mantle in GSC magmas. In this paper, basalts with a K/Ti ratio <0.09 have been designated as N-MORB; T-MORB have 0.09< K/Ti <0.15, and E-MORB have K/Ti >0.15. The propagating rift tip at 95.5°W appears to define the western limit of plume-affected mantle on the basis of variations in K/Ti (Figure 2f) and Nb/Zr (Figure 2g). West of 95.5°W, within the rift valley domain, chemically normal mid-ocean ridge basalt, or N-MORB, with K/Ti <0.09 is the dominant rock type recovered. In this distal region, which is unaffected by the Galápagos plume, Nb/Zr is <0.04 and 3He/4He is close to 7 times atmospheric values (RA). Between 95.5°W and 92.7°W, K/Ti values of 0.09–0.15 define compositions that are transitional between N-MORB and incompatible-element enriched basalts or E-MORB. These transitional basalts (or T-MORB) have Nb/Zr and 3He/4He (Figure 3d) ratios that generally increase eastward reaching values of ∼0.08 and ∼7.4 RA, respectively, near 92.7°W. East of 92.7°W, E-MORB is dominant, with K/Ti ratios >0.15, Nb/Zr > 0.09, and 3He/4He >7.5 RA. Although the Galápagos Islands represent a “high 3He/4He” hot spot, having 3He/4He ratios up to 30 times the atmospheric values in the western and southern parts of the archipelago [Graham et al., 1993; Kurz and Geist, 1999], the highest 3He/4He ratios observed along the western GSC are only 8.0–8.1 RA, similar to basalts from many ridges with no plume influence [Graham, 2002].

[14] Basalt concentrations of incompatible elements such as H2O, Na, Ca, and Al may in part reflect source composition, but these elements are also strongly influenced by the partial melting process [Jacques and Green, 1980; Langmuir et al., 1992]. East of ∼92.7°W, incompatible elements H2O8 (Figure 3e) and Na8 (Figure 3f) (H2O and Na2O corrected for magmatic differentiation to 8 wt% MgO, respectively) show maximum values, and Ca8/Al8 ratios (Figure 3g) are minimum. This is consistent with the results of Schilling et al. [1982] and Fisk et al. [1982] who also report high Na2O, low CaO/Al2O3, and high H2O along the GSC near 91°–92°W. Although some of these chemical variations can be accounted for by increasing incompatible element enrichment in the mantle source east of 92.7°W, variations in Na8, and especially Ca8/Al8, are more likely to be controlled by variations in the extent of melting than by source compensation. Thus, paradoxically, the region east of 92.7°W with the thickest crust and the strongest plume source contribution has major element compositions (e.g., high Na2O and low CaO/Al2O3) that suggest mean extents of partial melting that are relatively low.

4. Discussion

[15] The regional geophysical, geochemical, and volcanological correlations observed along the western GSC clearly reflect the combined effects of changes in mantle source composition and melt generation processes on the thickness, composition, and structure of oceanic crust. The maximum values in K/Ti (>0.4), Nb/Zr (>0.10), H2O8 (> 0.5 wt%), 3He/4He (8.1 RA), Na8 (∼3.2), crustal thickness (7.9 km), and residual swell anomaly (∼300 m), and minima in axial depth (<1700 m), Ca8/Al8 (<0.7), Mg # (<40), magma chamber depth (<1.5 km) and residual gravity anomaly (−25 mGal) all occur between 91.3°W and 92.25°W indicating a maximum plume influence in this region. The increased melt production beneath the most plume-influenced part of the GSC results in a hotter, weaker axial lithosphere leading to formation of an axial-high morphology, the stabilization of axial magma chambers at increasingly shallow crustal depths, and the dominance of fissure-fed over point-source volcanism.

[16] The increased melt production along the most plume-influenced part of the GSC can be related to melting from a larger region of a hydrous, and otherwise incompatible element-enriched mantle, with slightly elevated temperature (Figure 4). The recognition of the importance of water in expanding the zone of partial melting, leading to enhanced melt production was first noted in the Galápagos region by Schilling et al. [1982] and Fisk et al. [1982] and also in the region around the Azores hot spot [Schilling et al., 1980; Bonatti, 1990]. By depressing the solidus [Kushiro, 1968], the presence of excess water increases the depth at which melting begins and expands the volume of mantle undergoing melting [Schilling et al., 1980; Plank and Langmuir, 1992]. We reconcile the association of increased melt production with lower mean extents of melting along the most plume-influenced part of the GSC by noting that the extent of melting within the expanded zone is likely to be low [Braun et al., 2000]. Thus the total melt volume close to the hot spot has, in addition to the contribution from normal anhydrous melting, a contribution from a large volume undergoing low extents of partial melting. Although the total melt production is increased, the mean extent of melting for the total melt volume is reduced. The decrease in the proportion of plume-affected mantle westward along the GSC results in a progressive decrease in the amount of low-degree melts derived from the hydrous melting region and thus a decrease in crustal thickness and incompatible element enrichment (Figure 4b).

Details are in the caption following the image
The geophysical, geochemical, and volcanological correlations observed along the western GSC can be explained by the combined effects of changes in mantle source composition and melt generation processes on the thickness, composition, and structure of oceanic crust as Galápagos plume mantle feeds the GSC, and spreads laterally along the ridge axis. With available geophysical and geochemical constraints, we do not know if the GSC is sampling a broad, radially spreading plume head or whether there is channelized flow of plume material to and along the GSC. (a) Isostatic support for the Galápagos swell (ΔH = 0.7km) comes from variations in crustal thickness (ΔC = 2.3 km) along the GSC, thermal buoyancy associated with along-axis mantle potential temperature variations (ΔT = 30°C), and a compositionally buoyant mantle distributed to mantle depths of 50–100 km (Za). The increased melt production beneath the plume-influenced part of the GSC results in a hotter, weaker axial lithosphere leading to formation of an axial high morphology, the stabilization of axial magma chambers at increasingly shallow crustal depths, and the dominance of fissure-fed over point-source volcanism. Cross-sections for “normal” (distal) and plume-influenced (proximal) portions of the GSC are shown in Figures 3b and 3c, respectively. Melting from a larger region of a hydrous, and otherwise incompatible-element-enriched mantle, with slightly elevated temperatures can explain both the thicker crust, and the increase in K/Ti, Na2O, Nb/Zr, 3He/4He, and H2O content of lavas along the most plume-influenced part of the GSC. See text for discussion.

[17] The low 3He/4He ratio of GSC lavas suggests that even the most plume-affected mantle beneath the GSC has been degassed of its most volatile components. One explanation of these low 3He/4He ratios that has been suggested for other plume-ridge systems [Poreda et al., 1993; Graham et al., 1999] is a small amount of melting in the upwelling plume. If this slightly depleted plume mantle is subsequently transported to the ridge axis and undergoes additional decompression melting associated with plate spreading, it will acquire an anomalous chemical buoyancy that could explain the excess swell topography observed along the western GSC (Figure 3b). However, this explanation requires a mechanism that decouples He from K, H2O and other incompatible elements enriched in GSC basalts during plume upwelling, melting, and lateral transport to the ridge. Alternatively, the plume material that is present beneath the GSC may have an inherently low 3He/4He ratio. White et al. [1993] concluded that the Galápagos plume is heterogeneous and chemically zoned with distinctive northern, central, and southern components. Graham et al. [1993] and Kurz and Geist [1999] reported relatively low 3He/4He ratios for the Wolf-Darwin and Pinta lavas, respectively, in contrast to Fernandina which has a high 3He/4He ratio. In this case the low 3He/4He ratios observed at the GSC reflect sampling of the low 3He/4He component of the plume source and the excess mantle buoyancy observed beneath the western GSC must arise from another mechanism, such as melt retention in the mantle. Additional geochemical and geophysical data from the Galápagos region are required to distinguish between these two alternatives.

[18] The largest residual swell anomaly (Figure 3b), the shallowest axial magma chamber (Figure 3c), the highest values of 3He/4He, H2O8, and Na8 (Figures 3d3f), and the lowest values of Ca8/Al8 (Figure 3g) all occur between 91.8°W and 92.25°W near where the Wolf-Darwin lineament and a second similar seamount chain intersect the GSC (Figure 3a). A secondary peak in both residual swell anomaly and 3He/4He occurs near 91.3°W where a third seamount chain intersects the GSC. The anomalies near 92°W coincide with the approximate location of a maximum in 87Sr/86Sr and a minimum in 143Nd/144Nd previously reported by Verma and Schilling [1982] and Verma et al. [1983]. These geophysical and geochemical anomalies indicate a strong thermal and chemical plume influence in this area, supporting the Morgan [1978] hypothesis of channeling of plume material to the GSC along the Wolf-Darwin lineament. However, the association of bathymetric, geochemical, and geophysical anomalies with the intersection of two other small seamount chains with the GSC (e.g., the peaks in H2O8 and Na8 near 91.8°W, and in 3He/4He, K/Ti, Nb/Zr and residual swell depth at 91.3°W) may indicate a more complicated pattern of plume-ridge interaction with channelized flow of plume material to the ridge in at least three different locations between 91.3°W and 92.25°W. Alternatively, these seamount chains may be lithospheric cracks that are sampling a broad, radially spreading plume head that is being entrained by upwelling beneath the GSC. Additional isotope analyses of basalts from the GSC and these seamount chains and improved geophysical constraints on mantle properties beneath the GSC and its southern ridge flank will be required to more fully understand how the Galápagos plume and ridge are interacting.

5. Conclusions

[19] The correlated variations in geophysical, geochemical, and volcanological manifestations of plume-ridge interaction observed along the western Galápagos Spreading Center can be explained by the combined effects of changes in mantle temperature and source composition on melt generation processes, and the consequences of these variations on magma supply, axial thermal structure, basalt chemistry, and styles of volcanism. Key elements of this interpretation include (1) isostatic support for the Galápagos swell by a combination of crustal thickening (2.3 km between 98°W and 90.5°W), thermal buoyancy associated with a comparatively small along-axis variation in mantle potential temperature (∼30°C), and a compositionally buoyant, melt-depleted mantle; (2) melting from a larger region of a hydrous, and otherwise incompatible-element-enriched mantle, with slightly elevated temperatures, to explain both the thicker crust, and the increase in K/Ti, Na2O, Nb/Zr, 3He/4He, and H2O content of lavas along the most plume-influenced part of the GSC; and (3) the association of increased melt production with a hotter, weaker axial lithosphere leading to formation of an axial high morphology, the stabilization of axial magma chambers at increasingly shallow crustal depths, and the dominance of fissure-fed over point-source volcanism. Our results document a clear link between magma supply and variations in axial morphology, AMC depth and volcanic style that are largely independent of spreading rate. Along the western GSC even small changes in magma supply result in changes in axial thermal structure that have pronounced effects on axial morphology and crustal accretion processes.

Acknowledgments

[20] We thank the officers, crew and scientific complement on R/V Maurice Ewing leg EW00-04 for their professionalism, expert help, and hard work collecting the data presented in this report. We also thank John Lupton for access to his helium isotope lab, which is supported by the NOAA Vents Program. We are very grateful to the Ecuadorian government and the Parque Nacional Galápagos for permission to work in their waters. We thank Bill White, Jean-Guy Schilling, and Doug Toomey for reviews of a previous version of this manuscript, and Charlie Langmuir for discussions concerning the relationships among hydrous melting, total melt production and mean extents of melting. This research was supported by NSF OCE-9819117 to Woods Hole Oceanographic Institution, NSF OCE-9818632 to University of Hawaii, and NSF OCE-9818886 to Oregon State University. This is Woods Hole Oceanographic Institution Contribution number 10715 and SOEST contribution 5994.