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AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

Article Volume 9, Number 4 2 April 2008 Q04004, doi:10.1029/2007GC001775 ISSN: 1525-2027

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Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply A. S. Davis and D. A. Clague Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-6039, USA ([email protected])

B. L. Cousens Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6

R. Keaten and J. B. Paduan Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-6039, USA

[1] High-density, precisely located, dive and rock-corer basalt samples from the 65-km-long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04– 0.36%) for lavas erupted in close proximity. Although they form a near-continuum in the higher MgO range, the samples can be separated into two groups; one is typical N-type mid-ocean ridge basalt (MORB) (K2O/TiO2 < 0.09), and the other is a more enriched T-MORB (K2O/TiO2 > 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N < 1 and Zr/Nb > 20 for N-MORB and (Ce/Yb)N > 1 and Zr/Nb < 20 for T-MORB. Samples collected from off-axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)-depleted N-MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N < 1), although isotopic ratios are comparable to those onaxis. Lavas erupted in the past, before the present-day deep axial valley formed on this part of the ridge, were more uniform N-MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE-depleted N-MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U-shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N-, T-, and E-MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E-MORB (Zr/Nb < 10, (Ce/Yb)N >1.0–3.0). Arctic E-MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

Copyright 2008 by the American Geophysical Union

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davis et al.: geochemistry of basalt from the gorda ridge

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Components: 15,608 words, 13 figures, 6 tables. Keywords: mid-ocean ridge; geochemistry; trace elements; isotopes; Gorda Ridge. Index Terms: 1032 Geochemistry: Mid-oceanic ridge processes (3614, 8416); 1065 Geochemistry: Major and trace element geochemistry; 3614 Mineralogy and Petrology: Mid-oceanic ridge processes (1032, 8416); 8416 Volcanology: Mid-oceanic ridge processes (1032, 3614). Received 1 August 2007; Revised 25 January 2008; Accepted 5 February 2008; Published 2 April 2008. Davis, A. S., D. A. Clague, B. L. Cousens, R. Keaten, and J. B. Paduan (2008), Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply, Geochem. Geophys. Geosyst., 9, Q04004, doi:10.1029/2007GC001775.

1. Introduction

processes [e.g., Reynolds et al., 1992; Castillo et al., 2000; Perfit et al., 1983, 1994].

[2] Chemical variability of mid-ocean ridge basalt (MORB) has been reported from many ridge segments, ranging from ultrafast to ultraslow spreading centers. Relationships between tectonic segmentation and geochemical compositions have been explored by numerous studies [e.g., Reynolds et al., 1992; Perfit et al., 1994; Sinton et al., 1991; Castillo et al., 2000] but few ridge segments are represented by high-density, precisely located samples. The majority of samples collected by dredging are imprecisely located and frequently widely spaced [e.g., Castillo et al., 2000; Davis and Clague, 1987, 1990; Dosso et al., 1993; Hellevang and Pedersen, 2005; Reynolds and Langmuir, 1997; Robinson et al., 2001]. Samples collected with rock coring devices [Reynolds et al., 1992] provide a magnitude better spatial resolution than dredging but lack in situ observations. Submersible and remotely operated vehicle (ROV) dive samples do not have either of these shortcomings, but areas studied with them still represent only a small fraction of the global ocean ridge system.

[4] Using the Monterey Bay Aquarium Research Institute’s (MBARI’s) ROV Tiburon and a rock corer, we collected 92 basalt samples along the axial valley of the 65-km-long, northern Gorda Ridge segment. To explore the temporal variability at the central part of the segment, twenty-two additional samples were collected over a distance of >4 km up the adjacent eastern valley wall. The off-axis samples from the farthest end of the traverse correspond to a maximum ocean crust age of about 150 ka [Goldstein et al., 1993]. We use the chemical and isotopic compositions to examine variability relative to ridge morphology along axis and through time on the central ridge segment. We compare our data to those of samples dredged from the entire ridge and from other ridge segments with different spreading rates and evaluate them with respect to magma supply, melting processes, and mantle heterogeneities.

[3] Differences in lava chemistry have been related to ridge morphologies, depth to ridge axis, crustal thickness, and mantle temperatures [e.g., Batiza et al., 1977; Klein and Langmuir, 1987; Langmuir et al., 1986; Niu and Batiza, 1993; Niu et al., 1999; Robinson et al., 2001]. Chemical and isotopic characteristics have been used to infer mantle sources and conditions of melting processes and melt transport [e.g., Castillo et al., 2000; Reynolds et al., 1992; Niu and Batiza, 1997; Perfit et al., 1983; le Roex et al., 1987; Zindler et al., 1984]. The finer resolution studies have shown greater chemical variability on a smaller scale than previously thought and shown greater complexity in identifying local versus global trends in ridge

[5] The Gorda Ridge is a 300-km-long segment of oceanic spreading center located off the coast of southern Oregon and northern California (Figure 1). To the north, the ridge is separated from the Juan de Fuca Ridge by the Blanco Fracture Zone at about 43°N, and to the south it terminates at the Mendocino Ridge at about 40°250N. The ridge has been divided into five morphological segments based on offsets in the ridge axis, but all are unusually deep, ranging from 3200 to 3900 m. Chadwick et al. [1998] named the segments North Gorda, Jackson (after the adjacent seamount chain), Central, Phoenix, and Escanaba, from north to south (Figure 1). Spreading rate varies along the ridge from 5.5 cm/a (full-rate) at North Gorda to 2.3 cm/a at Escanaba

2. Geologic Setting

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Figure 1. Map shows location of Gorda and southern Juan de Fuca ridges relative to the continental margin. The five segments [Chadwick et al., 1998] are labeled. Area of North Gorda segment shown in Figure 2 is enclosed in box.

[Riddihough, 1980]. Morphology along the ridge varies with spreading rate from a narrow V-shaped valley at North Gorda to a broad U-shaped valley at Escanaba. The latter is covered with thick terrigenous sediment (to 390 m [Vallier et al., 1973; Zuffa et al., 2000]), whereas the northern segments are sediment starved. [6] The North Gorda is 65 km long with an hourglass shape in plan view (Figure 2) and convex depth profile along axis (Figure 3a), with the shallowest depth near the center of the segment and the deepest part near the southern non-transform offset (Figure 3b). The morphology is similar to that of the slow spreading Mid-Atlantic Ridge [e.g., Fox et al., 1991]. [7] We have further subdivided the axis of North Gorda into three parts identified as north, central and south, and called N Axis, C Axis, and S Axis throughout this paper. The shallower C Axis of the

segment is proposed to be magmatically more robust than the deeper end segments [Chadwick et al., 1998]. The most recent eruption on Gorda Ridge occurred in 1996 on the C Axis of this segment [Chadwick et al., 1998; Rubin et al., 1998]. [8] Despite extensive surveys of North Gorda, no active hydrothermal vents were observed along the ridge axis. Even the site of the most recent eruption in 1996 supports no active vent system. However, there is an active, high-temperature hydrothermal vent field located on one of the fault blocks forming the step-like terraces of the eastern valley wall, about 3 km from the ridge axis [Rona et al., 1990].

3. Sampling and Analytical Methods [9] Samples in this study were collected on three cruises to the Gorda Ridge of the MBARI’s R/V 3 of 24


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Figure 2. Bathymetric map of North Gorda shows dive tracks and rock corer locations. Profiles along axis (A to A0) shown in Figure 3a and sections across axial valley shown in Figure 3b are indicated, and the three subsections of the segment axis referred to in the text are labeled. Contour interval is 500 m.

Western Flyer in 2000, 2002, and 2005. Samples were collected on dives with the remotely operated vehicle ROV Tiburon (Txxx-Rx) by breaking off outcrops with the manipulator arm or vacuuming glass using a suction device, affectionately known as the glass sucker (Txxx-GSx), from pillow and sheet flows too fragile to collect with the arm. Additional glass samples were collected with a wax-tipped rock-coring device [Reynolds et al., 1992]. [10] Two Tiburon dives (T885 and T886) were dedicated to exploring the 1996 eruption site [Clague et al., 2005]. Three other dives (T185, T187, T455) were done along the ridge axis, supplemented by rock coring at 29 sites spaced

3 km or less apart (Figure 2). Additional lava samples were collected on five dives on the eastern valley wall to the off-axis hydrothermal vent field, informally named Sea Cliff site [Rona et al., 1990]. Dives T186 and T454 started in the axial valley and collected samples on traverses up the fault scarp to the hydrothermal vent site. Dives T456 and T884 collected additional samples across the hydrothermal field. For completeness, we also include two samples collected on MBARI dives T695 and T696 to the hydrothermal vents to measure Raman spectra of vent fluids and hydrothermal precipitates and two samples from the axial valley near the western wall collected on Sea Cliff dive 771 in 1988 (771-x [Rona et al., 1990]). 4 of 24


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Figure 3. (a) Profile of depth versus latitude along the ridge axis and (b) depth versus distance across the axial valley at five locations at North Gorda. Location of the Sea Cliff hydrothermal vent site is indicated.

[11] Basaltic glass from the rims of pillow or sheet flow fragments and from the wax-tipped rock corer was handpicked under a binocular microscope and mounted in polished thin section grain mounts for analysis with a JEOL 8900 electron microprobe at the U.S. Geological Survey in Menlo Park. Analyses were performed with 15 kv accelerating voltage, 25 nAmps specimen current, using a 10 mm beam size and natural and synthetic glass and mineral standards [e .g., Davis et al . , 1994] (Table 1a). Each analysis is the average of at least five points. Where glass rims were thin or absent, whole rock pieces were cut with a rock saw and analyzed by x-ray fluorescence (XRF) in the

GeoAnalytical Laboratory of Washington State University [Johnson et al., 1999]. [12] For samples with thin or no glass rind, a portion of the rock powder was analyzed for trace elements by ICP-MS. Where enough glass was available, fragments were handpicked under the microscope for direct dissolution and also analyzed by ICP-MS [Knaack et al., 1994]. The ICP-MS analyses of glasses from the 2002 and 2005 cruises were done in the GeoAnalytical Laboratory; those from the 2000 cruise were done in the laboratory of the Ontario Geological Survey in Sudbury, Ontario. An internal standard using a Juan de Fuca glass 5 of 24


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Table 1a. Averaged Microprobe Analyses of Glass Standard VG-2 Compared to Reported Valuesa Oxide

VG-2 Standard

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 SO3 Total

50.81 1.85 14.06 11.84 0.20 6.71 11.12 2.62 0.19 n.a. n.a. 99.4

a

VG-2 (± std dev), n = 48 50.65 1.80 14.06 11.70 0.21 6.67 10.97 2.67 0.19 0.19 0.34 99.44

± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.08 0.08 0.13 0.03 0.18 0.08 0.10 0.01 0.05 0.04 0.49

Note: n.a., not analyzed.

(L13-82-22) was analyzed at both places and the inter laboratory difference for most samples is smaller than variations between different runs at the same laboratory (Table 1b). However, for highly depleted N-MORB (e.g., D4-15) with elements near detection limits the discrepancy between the two laboratories is significant. We included the low concentrations only if they plotted on a calibration curve relative to our internal standard. [13] Radiogenic isotope compositions of Sr, Nd, and Pb of selected glass samples were determined at Carleton University, Ottawa, utilizing techniques described by Cousens [1996a]. Prior to dissolution, glass splits for Pb and Nd were acid-washed in warm 1.5N HCl for 12 h, then rinsed three times with ultrapure H2O. Glass splits for Sr were acidwashed in hot 6N HCl for 4 d, then rinsed three times with H2O, followed by dissolution in HF/ HNO3. All Pb mass spectrometer runs are corrected for fractionation using NIST SRM981. The average ratios measured for SRM981 are 206Pb/204Pb = 16.890 ± 0.010, 207Pb/204Pb = 15.429 ± 0.013, and 208 Pb/204Pb = 36.502 ± 0.042 (2 sigma). The fractionation correction, based on the values of Todt et al. [1984], is +0.13%/amu. Sr isotope ratios are normalized to 86Sr/88Sr = 0.11940 to correct for fractionation. Two Sr standards are run at Carleton, NIST SRM987 (87Sr/86Sr = 0.710251 ± 18) and the Eimer and Amend (E&A) SrCO3 (87Sr/86Sr = 0.708032 ± 24). Nd isotope ratios are normalized to 146Nd/144Nd = 0.72190. 54 runs of the La Jolla standard average 143Nd/144Nd = 0.511876 ± 18.

4. Lava Compositions 4.1. Major Elements [ 14 ] The major element compositions of 110 glasses are given in Table 2 and a subset of whole

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rock compositions of samples with thin or no glass rims are given in Table 3. We have grouped the samples from North Gorda based on location into four groups, three from the three on-axis parts (N Axis, C Axis, S Axis) of the North Gorda segment and one off-axis. On the basis of morphology along the axis (Figures 2 and 3a), the samples collected from near the Blanco Fracture Zone to the shallower, domed section define the N Axis of the North Gorda segment, those from the domed part define the C Axis, and those from the dive in the deep basin at the southern end define the S Axis. The samples labeled off-axis include all collected from the base of the eastern valley wall up to and beyond the hydrothermal Sea Cliff site (Figure 3b). [15] Collectively, the samples are predominantly N-MORB spanning a larger compositional range (Figure 4) than those previously dredged from the Gorda Ridge [Davis and Clague, 1987, 1990]. The bulk of the compositions cluster between 8.5 and about 7% MgO, as previously observed on the basis of dredged samples, but a few have less. One sample (GR00RC11) with MgO of 4.2% is the Table 1b. Comparison of ICP-MS Analyses of Glass Standard L2-83-22a

Element

L2-83-22 WSU (± std.dev), n=6

L2-83-22 GSO (± std.dev), n=2

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ba Th Nb Y Hf Ta U Pb Rb Cs Sr Sc Zr

4.80 ± 0.08 13.19 ± 0.16 2.19 ± 0.03 11.60 ± 0.13 4.20 ± 0.09 1.42 ± 0.01 5.54 ± 0.19 1.00 ± 0.02 6.6 ± 0.06 1.42 ± 0.03 4.06 ± 0.08 0.61 ± 0.01 4.02 ± 0.05 0.64 ± 0.01 16.5 ± 0.3 0.22 ± 0.01 4.40 ± 0.11 42.20 ± 0.75 3.06 ± 0.06 0.36 ± 0.08 0.09 ± 0.01 0.40 ± 0.06 1.50 ± 0.10 0.01 ± 0.00 118 ± 2 48.5 ± 0.7 110 ± 2

4.69 ± 0.04 13.48 ± 0.02 2.26 ± 0.01 11.87 ± 0.16 4.20 ± 0.02 1.59 ± 0.01 5.66 ± 0.02 1.04 ± 0.01 6.82 ± 0.01 1.51 ± 0.01 4.29 ± 0.01 0.65 ± 0.01 4.08 ± 0.04 0.63 ± 0.01 16.05 ± 0.49 0.27 ± 0.00 4.26 ± 0.01 37.35 ± 0.07 3.18 ± 0.04 n.d. 0.11 ± 0.00 0.62 ± 0.09 1.49 ± 0.00 0.01 ± 0.00 120.63 ± 0.54 42.50 ± 0.64 113.28 ± 1.58

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Note: n.d., not detected.

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Table 2. Locations and Compositions of Basalt Glasses From North Gordaa Sample N Axis GR02RC1 GR02RC2 GR02RC3 GR02RC4 GR02RC5 GR02RC6 GR02RC7 GR02RC8 GR02RC9 GR02RC10 GR02RC11 GR02RC12 GR02RC13 C Axis GR00RC1 GR00RC2 GR00RC3 GR00RC4 GR00RC5 GR00RC6 GR00RC7a GR00RC7b GR00RC7c GR00RC8 GR00RC9 GR00RC10 GR00RC11 GR00RC12 GR00RC13 GR00RC14 GR00RC15 GR00RC16 GR00RC17 GR00RC18 83-D4-15 SC771R1 SC771R3 T185GS1 T185-R1a T185-R1b T185-R2 T885-R1 T885-R2 T885-R3 T885-R4 T885-R5 T885-R6 T885-R7 T885-R8 T885-R9 T885-R10 T885-R11 T885-R12 T886-R1 T886-R2 T886-R3 T886-R4 T886-R5 T886-R6

Lat., °N

Long., °W

42.798 42.817 42.823 42.843 42.862 42.873 42.892 42.906 42.913 42.933 42.948 42.969 42.979

126.720 126.702 126.690 126.691 126.681 126.668 126.658 126.650 126.642 126.632 126.620 126.609 126.602

3042 3084 3204 3298 3251 3190 3157 3163 3197 3190 3289 3192 3244

50.68 51.19 51.23 50.54 50.91 51.61 50.37 50.84 50.80 50.60 50.58 51.13 50.25

14.85 15.26 15.18 15.65 14.43 14.17 15.87 15.50 15.59 15.74 13.05 15.58 15.69

10.66 9.82 9.82 9.37 11.61 11.07 9.52 9.27 9.80 9.13 14.26 9.35 9.60

6.87 7.30 7.33 7.82 6.90 6.44 7.73 7.36 7.61 7.74 4.38 7.58 8.03

0.19 0.15 0.19 0.19 0.20 0.21 0.20 0.18 0.17 0.18 0.25 0.17 0.18

1.90 10.98 1.67 11.41 1.77 11.25 1.43 11.84 2.40 10.18 1.93 11.08 1.50 11.32 1.33 11.55 1.62 11.39 1.47 11.58 3.00 8.87 1.44 11.44 1.47 11.35

2.83 2.68 2.74 2.50 2.77 2.82 2.73 2.57 2.54 2.70 3.24 2.69 2.61

0.15 0.16 0.15 0.13 0.22 0.19 0.13 0.14 0.13 0.14 0.36 0.13 0.11

0.18 0.23 0.18 0.14 0.28 0.22 0.15 0.15 0.15 0.16 0.50 0.18 0.09

0.118 0.113 0.116 0.105 0.122 0.129 0.104 0.103 0.103 0.099 0.165 0.103 0.099

99.41 99.98 99.96 99.72 100.02 99.87 99.62 98.99 99.90 99.54 98.66 99.79 99.48

42.750 42.750 42.740 42.733 42.709 42.640 42.650 42.650 42.650 42.666 42.685 42.762 42.758 42.701 42.782 42.719 42.772 42.776 42.753 42.743 42.757 42.757 42.757 42.752 42.754 42.754 42.756 42.683 42.684 42.686 42.687 42.688 42.689 42.687 42.687 42.685 42.686 42.685 42.684 42.673 42.674 42.675 42.676 42.677 42.677

126.745 126.742 126.746 126.754 126.772 126.794 126.783 126.783 126.783 126.777 126.771 126.731 126.742 126.768 126.715 126.762 126.734 126.726 126.743 126.747 126.762 126.742 126.742 126.749 126.747 126.747 126.745 126.780 126.782 126.781 126.779 126.779 126.781 126.780 126.780 126.779 126.780 126.780 126.779 126.780 126.781 126.781 126.782 126.784 126.785

3036 3042 2998 2985 3081 3206 3046 3046 3046 3185 3002 3110 3090 3103 3060 3136 3044 3151 3088 3090 3223 3090 3090 3032 3057 3057 3065 3115 3105 3099 2987 3071 3119 3096 3079 3101 3099 3099 3134 3146 3151 3131 3079 3087 3100

50.82 50.40 50.40 50.36 50.74 51.52 50.90 50.68 50.11 50.79 51.35 50.68 50.39 51.29 50.69 50.44 51.27 50.49 50.08 49.97 49.40 50.17 50.28 50.66 51.07 50.91 51.02 51.23 50.65 50.96 51.17 50.91 51.07 50.93 51.01 51.03 50.97 50.97 50.66 50.53 51.30 51.37 50.95 51.10 50.85

15.16 15.13 16.04 16.30 15.39 15.19 15.69 15.48 16.32 15.93 14.57 16.25 16.49 14.94 15.58 16.39 14.79 16.18 16.52 16.50 16.90 16.52 16.52 14.91 15.14 15.09 15.50 15.88 15.57 16.24 15.61 15.85 15.10 15.85 15.99 16.08 16.12 16.03 15.66 16.30 15.49 14.84 15.73 15.61 15.74

10.23 10.12 9.05 8.73 9.84 8.97 8.76 9.68 9.07 8.74 10.85 8.46 8.58 10.34 9.59 8.54 10.35 8.56 8.80 8.83 8.02 8.66 8.59 10.74 10.05 10.19 9.31 8.63 9.74 8.37 8.77 8.76 9.29 8.78 8.52 8.60 8.55 8.52 9.76 8.44 8.76 9.60 8.54 8.75 8.86

7.16 7.60 7.96 8.12 7.26 7.32 7.91 7.48 8.28 7.82 6.55 7.97 7.98 6.85 7.70 8.04 6.68 8.04 7.98 7.94 9.17 7.95 7.97 7.07 7.02 7.14 7.53 7.70 7.46 7.93 7.58 7.81 7.40 7.86 7.82 7.83 7.98 7.89 7.54 8.26 7.54 7.02 7.71 7.74 7.71

0.20 0.20 0.17 0.17 0.18 0.18 0.16 0.18 0.19 0.17 0.18 0.16 0.15 0.19 0.18 0.16 0.17 0.16 0.17 0.16 0.15 0.16 0.16 0.19 0.17 0.19 0.18 0.16 0.16 0.16 0.14 0.16 0.18 0.16 0.17 0.16 0.15 0.15 0.15 0.16 0.16 0.17 0.14 0.15 0.18

1.78 1.47 1.38 1.27 1.63 1.33 1.20 1.55 1.20 1.19 2.00 1.14 1.26 1.87 1.63 1.19 1.97 1.16 1.27 1.29 0.94 1.25 1.26 1.91 1.78 1.76 1.50 1.21 1.57 1.15 1.28 1.19 1.29 1.23 1.23 1.21 1.21 1.24 1.59 1.13 1.30 1.54 1.22 1.29 1.22

2.67 2.33 2.47 2.43 2.69 2.81 2.54 2.69 2.42 2.59 2.79 2.65 2.59 2.77 2.54 2.58 2.74 2.61 2.57 2.59 2.61 2.58 2.59 2.67 2.70 2.70 2.59 2.85 2.73 2.79 2.86 2.81 2.86 2.78 2.81 2.78 2.75 2.80 2.69 2.40 2.85 2.70 2.81 2.78 2.81

0.15 0.13 0.20 0.15 0.13 0.11 0.12 0.13 0.14 0.13 0.17 0.08 0.15 0.15 0.14 0.13 0.18 0.07 0.16 0.17 0.04 0.16 0.15 0.18 0.15 0.15 0.13 0.10 0.18 0.09 0.10 0.09 0.09 0.09 0.10 0.10 0.10 0.09 0.17 0.16 0.10 0.14 0.08 0.09 0.08

0.17 0.13 0.15 0.12 0.15 0.12 0.12 0.16 0.11 0.12 0.21 0.11 0.13 0.20 0.17 0.12 0.22 0.09 0.13 0.14 0.06 0.14 0.14 0.21 0.21 0.19 0.15 0.13 0.18 0.12 0.12 0.11 0.12 0.11 0.11 0.12 0.10 0.11 0.17 0.12 0.13 0.17 0.11 0.12 0.11

0.115 0.115 0.103 0.098 0.110 0.102 0.096 0.112 0.089 0.101 0.120 0.099 0.097 0.116 0.105 0.101 0.125 0.105 0.101 0.097 0.090 0.100 0.100 0.109 0.115 0.114 0.113 0.084 0.095 0.083 0.089 0.093 0.094 0.091 0.084 0.089 0.083 0.085 0.097 0.082 0.089 0.099 0.088 0.096 0.090

99.83 99.83 99.85 99.85 99.83 99.85 99.84 99.81 99.86 99.85 99.82 99.85 99.85 99.82 99.84 99.84 99.81 99.85 99.85 99.86 99.28 99.85 99.85 99.83 99.84 99.81 99.83 100.03 99.76 99.87 99.82 100.00 99.85 100.07 99.88 100.10 100.08 99.98 99.92 100.04 99.93 99.39 99.61 99.92 99.89

Depth SiO2

Al2O3 FeO* MgO MnO TiO2

CaO Na2O K2O P2O5

11.37 12.20 11.93 12.10 11.71 12.20 12.34 11.67 11.93 12.27 11.03 12.25 12.03 11.10 11.51 12.15 11.31 12.38 12.07 12.17 11.90 12.16 12.09 11.18 11.43 11.38 11.81 12.07 11.43 11.98 12.09 12.22 12.36 12.19 12.04 12.11 12.07 12.09 11.45 12.45 12.20 11.75 12.24 12.19 12.24

S

Totals

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Table 2. (continued) Sample T886-R7 T886-R8 T886-R9 T886-R10 T886-R11 T886-R12 T886-R13 T886-R14 T886-R15 T886-R16 T187GS6 T187-R1a T187-R1b T187-R2 T187-R3 T187-R4 T187-R5 T187-R6 T187-R7 T187-R8 T187-R9 T187-R10 T187-R11 T187-R12 T187-R13 T187-R14 T187-R16 T187-R17 S Axis T455-GS1 T455-GS2 T455-GS3 T455-GS4 T455-GS5 T455-R1 T455-R2 T455-R3 T455-R5 T455-R6 T455-R8 T455-R9 T455-R10 T455-R11 Off-Axis T186-R1 T186-R5 T454-R2 T454-R5 T454-R6 T454-R7 T454-R8 T454-R9 T454-R10 T454-R12 T454-R13 T454-R14 T456-R4 T456-R6 T456-R7 T456-R9

Lat., °N

Long., °W

42.674 42.673 42.672 42.670 42.668 42.667 42.666 42.665 42.663 42.663 42.607 42.605 42.605 42.602 42.601 42.601 42.603 42.610 42.611 42.615 42.615 42.618 42.618 42.620 42.619 42.619 42.616 42.617

126.786 126.785 126.784 126.783 126.786 126.788 126.788 126.788 126.789 126.790 126.818 126.815 126.815 126.818 126.818 126.819 126.820 126.818 126.816 126.813 126.813 126.812 126.809 126.807 126.806 126.802 126.797 126.795

3130 3134 3158 3154 3148 3162 3166 3154 3045 3031 3292 3353 3353 3200 3164 3166 3168 3352 3343 3337 3324 3331 3365 3364 3223 3207 3160 3152

51.23 51.19 51.29 50.35 50.47 51.14 50.54 50.74 50.60 50.56 51.64 50.52 50.82 50.79 51.08 50.77 50.61 52.00 51.78 51.88 51.75 50.92 50.27 49.76 49.84 50.58 49.95 50.00

15.31 15.43 15.09 16.21 15.24 15.38 15.36 14.77 15.27 15.31 14.75 15.93 15.76 15.74 15.80 15.81 16.01 14.98 15.11 14.94 15.09 15.44 16.02 16.55 16.45 15.87 16.33 16.31

8.75 8.66 9.33 8.36 9.69 8.93 9.68 10.14 9.73 9.67 9.58 8.78 8.76 8.86 8.82 8.85 8.50 9.33 9.12 9.28 9.15 9.17 9.00 9.26 9.33 8.76 9.41 9.32

7.49 7.54 7.31 8.23 7.38 7.55 7.50 7.16 7.44 7.44 7.55 8.18 8.08 8.16 8.05 8.11 8.28 7.47 7.53 7.55 7.61 7.75 7.82 8.28 8.28 8.35 8.32 8.26

0.16 0.15 0.16 0.15 0.18 0.15 0.17 0.17 0.18 0.18 0.17 0.17 0.13 0.15 0.15 0.16 0.17 0.21 0.17 0.17 0.18 0.15 0.18 0.14 0.17 0.16 0.16 0.17

1.29 1.27 1.44 1.14 1.51 1.25 1.58 1.65 1.51 1.52 1.20 1.17 1.19 1.18 1.22 1.18 1.11 1.17 1.13 1.16 1.12 1.30 1.29 1.12 1.14 1.13 1.21 1.22

12.32 12.28 12.06 12.36 11.91 12.20 11.88 12.04 11.88 11.92 12.37 12.48 12.39 12.39 12.17 12.34 12.55 12.18 12.37 12.29 12.36 12.28 12.25 12.16 12.04 12.48 11.77 11.85

2.81 2.82 2.68 2.41 2.62 2.85 2.65 2.63 2.63 2.60 2.24 2.29 2.36 2.26 2.29 2.30 2.30 2.20 2.27 2.25 2.23 2.51 2.60 2.26 2.27 2.22 2.36 2.36

0.10 0.10 0.12 0.16 0.14 0.10 0.14 0.14 0.15 0.15 0.13 0.10 0.11 0.10 0.10 0.10 0.11 0.12 0.13 0.12 0.12 0.08 0.20 0.13 0.13 0.12 0.13 0.13

0.13 0.12 0.14 0.12 0.17 0.12 0.16 0.17 0.16 0.16 0.13 0.11 0.12 0.11 0.11 0.12 0.10 0.13 0.12 0.11 0.11 0.11 0.14 0.10 0.10 0.10 0.12 0.12

0.092 0.087 0.095 0.081 0.099 0.096 0.097 0.105 0.101 0.100 0.100 0.105 0.108 0.102 0.083 0.104 0.100 0.920 0.108 0.104 0.100 0.112 0.092 0.096 0.094 0.093 0.093 0.100

99.68 99.65 99.71 99.59 99.41 99.78 99.75 99.71 99.64 99.61 99.86 99.84 99.83 99.84 99.87 99.84 99.84 100.71 99.84 99.85 99.65 99.82 99.86 99.86 99.84 99.86 99.85 99.84

42.517 42.617 42.516 42.509 42.497 42.516 42.515 42.512 42.511 42.506 42.492 42.491 42.491 42.491

126.866 126.870 126.876 126.880 126.886 126.866 126.877 126.878 126.880 126.880 126.889 126.890 126.891 126.891

3615 3721 3850 3828 3804 3645 3841 3810 3783 3815 3796 3770 3752 3751

50.62 50.66 50.61 51.18 50.74 50.71 50.86 51.00 50.93 50.68 50.91 51.04 50.85 50.95

16.08 15.92 16.10 14.53 15.33 16.06 15.33 14.18 15.23 15.31 15.09 15.20 15.19 15.23

9.00 9.06 8.96 11.30 10.05 8.86 9.54 11.60 10.27 10.21 10.05 10.07 9.90 9.87

7.99 7.98 8.17 6.49 7.44 8.15 7.28 6.14 7.21 7.33 7.56 7.63 7.45 7.67

0.19 0.17 0.16 0.19 0.19 0.16 0.16 0.20 0.18 0.18 0.20 0.19 0.17 0.17

1.33 1.30 1.29 2.05 1.84 1.28 1.42 2.16 1.88 1.80 1.49 1.52 1.44 1.43

12.07 11.95 12.17 11.13 10.99 11.99 12.06 11.23 11.26 11.04 12.05 11.90 12.17 12.19

2.42 2.44 2.40 2.77 2.68 2.29 2.60 2.59 2.46 2.56 2.35 2.35 2.37 2.27

0.13 0.13 0.14 0.17 0.21 0.13 0.09 0.17 0.17 0.21 0.09 0.08 0.07 0.08

0.12 0.10 0.11 0.19 0.18 0.11 0.10 0.19 0.18 0.18 0.11 0.11 0.11 0.12

0.105 0.096 0.090 0.128 0.105 0.083 0.088 0.125 0.113 0.094 0.108 0.109 0.090 0.112

100.06 99.81 100.20 100.13 99.76 99.82 99.53 99.59 99.88 99.59 100.01 100.20 99.81 100.09

42.755 42.754 42.754 42.754 42.754 42.753 42.753 42.753 42.752 42.748 42.749 42.746 42.754 42.754 42.754 42.756

126.710 126.709 126.710 126.709 126.709 126.708 126.707 126.706 126.701 126.69 126.688 126.686 126.711 126.710 126.710 126.711

2788 2684 2749 2683 2659 2601 2618 2638 2716 2675 2643 2542 2745 2705 2687 2768

51.14 51.03 50.86 50.81 50.72 50.84 50.95 50.89 50.91 50.73 51.07 51.05 51.04 50.96 50.94 51.47

15.32 15.99 15.78 15.76 15.62 15.79 15.63 15.70 15.72 15.75 15.66 15.31 15.76 15.72 15.71 15.28

9.09 9.17 9.08 9.08 9.16 9.14 9.16 9.16 9.22 9.59 8.74 9.44 9.11 9.06 9.15 9.35

7.37 7.69 7.68 7.63 7.53 7.64 7.64 7.55 7.53 7.54 7.52 7.21 7.75 7.75 7.59 7.40

0.17 0.18 0.17 0.15 0.16 0.15 0.17 0.16 0.16 0.17 0.18 0.18 0.17 0.17 0.15 0.17

1.44 1.41 1.45 1.46 1.46 1.38 1.44 1.43 1.46 1.49 1.26 1.41 1.41 1.37 1.42 1.40

12.13 11.89 12.03 11.99 12.16 12.04 12.13 12.06 12.06 11.93 12.14 12.10 11.53 12.06 12.08 12.18

2.83 2.19 2.82 2.87 2.83 2.80 2.84 2.84 2.85 2.74 2.65 2.49 2.62 2.58 2.77 2.64

0.08 0.09 0.08 0.09 0.08 0.09 0.09 0.08 0.09 0.12 0.08 0.11 0.08 0.08 0.08 0.07

0.14 0.11 0.12 0.12 0.11 0.12 0.11 0.12 0.11 0.12 0.10 0.12 0.09 0.09 0.11 0.09

0.110 0.090 0.097 0.083 0.098 0.094 0.092 0.098 0.089 0.097 0.078 0.092 0.091 0.091 0.089 0.087

99.82 99.84 100.17 100.04 99.93 100.08 100.25 100.09 100.20 100.28 99.48 99.51 99.65 99.93 100.09 100.14

Depth SiO2

Al2O3 FeO* MgO MnO TiO2

CaO Na2O K2O P2O5

S

Totals

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Table 2. (continued) Sample T695-R1 T696-R3 T884-R4 T884-R7 a

Lat., °N

Long., °W

42.755 42.755 42.756 42.755

126.709 126.709 126.711 126.710

Depth SiO2 2701 2729 2772 2751

50.97 50.97 50.88 50.67

Al2O3 FeO* MgO MnO TiO2 15.67 15.62 15.26 15.66

8.90 8.87 8.98 9.04

7.76 7.69 7.50 7.77

0.16 0.15 0.17 0.15

1.38 1.33 1.37 1.35

CaO Na2O K2O P2O5 11.85 11.93 11.99 11.89

2.84 2.87 2.76 2.78

S

0.09 0.14 0.093 0.10 0.12 0.094 0.07 0.11 0.103 0.08 0.12 0.101

Totals 100.10 99.99 99.19 99.61

Compositions are in wt.%.

most evolved composition found on the Gorda Ridge to date (Figure 4). No high-MgO (>9%) glass like that dredged previously from the C Axis of North Gorda (D4-15 [Davis and Clague, 1987]) was found among the dive or rock-core samples. For samples with both glass and whole rock analyses the MgO contents are 0.5 to 1.2 weight percent higher for the corresponding whole rock samples, depending on amount of olivine present. Maximum compositional diversity along axis occurs at the shallowest, C Axis of North Gorda, proposed to be magmatically most active [Chadwick et al., 1998]. Compared to the samples from the C Axis of North Gorda, the adjacent off-axis samples are less diverse compositionally (Figure 5) and are concentrated at the high-MgO and high-Na2O end of the spectrum (Figure 4). [16] Typical for tholeiitic differentiation, FeO* (Figure 4a) and TiO2 (Figure 4b) show fairly well-defined trends increasing with decreasing MgO. A2O3 (Figure 4c) and CaO (Figure 4d) show the opposite trend, but with more scatter. Maximum scatter is observed for Na2O (Figure 4e) and K2O (Figure 4f) versus MgO. Observed trends are broadly consistent with variable fractional crystallization trends of predominantly plagioclase and olivine (±clinopyroxene at MgO < 7%). A plot of K2O versus TiO2 (Figure 5), suggests two trends, one with higher and one with somewhat lower K2O at comparable TiO2 contents. Although forming a continuum at the higher MgO range, samples with K2O/TiO2 < 0.09 are typical N-MORB, whereas those with K2O/TiO2 > 0.09 are T-MORB [e.g., Reynolds et al., 1992]. All of the glass compositions from off-axis are N-MORB with K2O/TiO2 < 0.09 (Figure 5), but some of the corresponding whole rock compositions have elevated K2O, presumably due to secondary alteration although it is not obvious in thin section.

4.2. Trace Elements [17] Trace element abundances of whole rock samples determined by XRF are included in Table 3.

Trace element abundances determined by ICP-MS for the same whole rock powders and selected glasses are given in Table 4, including the most primitive sample (D4-15) dredged from North Gorda [Davis and Clague, 1987] that had not previously been analyzed for a complete set of trace elements. The two trends suggested by K2O versus TiO2 (Figure 5) are better defined on Zr versus Nb (Figure 6a) and La versus Ba plots (Figure 6b). The majority of the samples have high Zr/Nb (20 – 50), typical of N-MORB [Basaltic Volcanism Study Project, 1981]; the most primitive composition of D4-15 has Zr/Nb as high as 52 (Table 4). A smaller number, mostly from the C Axis, have lower Zr/Nb ( 0.09). All off-axis glass samples have K2O/TiO2 < 0.09, but some whole rock samples have higher K2O contents. Dredge sample compositions from northern Gorda and Escanaba are again shown for comparison (N, NESCA; S, SESCA; sf, sediment free). E-MORB samples in sediment are off-scale. Symbols and data sources as in Figure 4.

K2O/TiO2 and a negative one with Zr/Nb (figures not shown) but the latter two have more scatter. A general, but not identical, positive correlation also exists between Pb isotopic ratios and these highly incompatible elements (Figure 10b) suggesting that more of an enriched component is included at smaller degrees of melting. Variable isotopic ratios and Zr/Nb ranging from 52 to 13.4 for lavas erupted in close proximity along the C Axis of North Gorda suggest small-scale heterogeneities are most pronounced at this part of the segment. The off-axis lavas have a narrower range of isotopic compositions, especially for 87Sr/86Sr (Figures 9a and 10a) that overlap with those from the ridge axis.

5. Discussion [19] Despite the many petrologic studies of the global mid-ocean ridge system, few ridge segments have been as intensively sampled as North Gorda and only rare studies include off-axis samples to explore temporal variations [e.g., Karsten et al., 1990; Perfit et al., 1994; Reynolds et al., 1992]. To evaluate the relative influences of source heterogeneities and melting processes, major and trace elements and isotopic compositions are needed on the same samples. This is more commonly done in recent

[21] Although off-axis samples were collected only along one part of the eastern axial graben wall, they represent systematic sampling over a distance of 4 km, corresponding to a temporal sequence from present to 150 ka. Located adjacent to the central bulge of this ridge segment, these lava flows present a window into the past of this chemically most diverse ridge axis segment. We saw no evidence for off-axis eruption in the form of lava flows draping the fault scarps and increasing palagonite and absence of glass rinds for some samples from the most distal end of the traverse indicate progressively older flows away from the axis, in agreement with increasing ages with distance from the axis determined for dredged lavas by Goldstein et al. [1993]. [22] The narrow range of higher MgO samples collected off-axis (Figures 4 and 11a) indicates less compositional variability and higher temperatures for most lava flows in the past on this ridge segment. All off-axis samples are depleted N-MORB with high Zr/Nb (50.5–26.5) and with concave downward LREE patterns (Figure 7). Some of the off-axis whole rock samples have higher K2O content (e.g., K2O/TiO2 > 0.09, Figures 4f and 5) than their glass rims but the low REE abundances of whole rock and corresponding glass rim are the same, within analytical precision. A narrow range of lower (La/Sm)N ( 12% [Byerly et al., 1976]). Na2O corrected for fractionation (Na8.0, Figure 12c), is mostly higher and more variable at North Gorda, suggesting smaller and more variable degrees of melting. However, Na2O is sensitive to degree as well as depths of melting [Klein and Langmuir, 1987]. Trace element abundances and ratios are less variable at Juan de Fuca Ridge where all samples are N-MORB, based on K2O/TiO2 (Figure 5) and Zr/Nb (Figure 6a). Chondrite-normalized La/Sm and Ce/Yb increase with decreasing degree of

partial melting but they are also affected by source composition. The flat trend shown by (Ce/Yb)N versus Ce abundance (Figure 8) for Juan de Fuca lavas is compatible with crystal fractionation, whereas a twofold change in this ratio at a given Ce abundance for North Gorda suggests different degrees of melting. Although there is a positive correlation between these and other highly incompatible element ratios (e.g., Ba/La) with isotopic ratios, the data suggest small, variable degree of melting but of sources that are heterogeneous on a small scale. Source heterogeneities are also present at southern Juan de Fuca Ridge but isotopic variation is smaller and compositions are more radiogenic, and are similar to those from the President Jackson seamount chain (Figure 9) [Davis and Clague, 2000] the only near ridge seamount chain off the Gorda Ridge. [28] Using >11,000 published MORB analyses, Rubin and Sinton [2007] presented a model for explaining melt variations largely in terms of spreading rate and magma supply. High melt 16 of 24

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supply at fast spreading ridges produces more differentiated lavas from shallower magma chambers but homogenizes elements related to melt processes and mantle sources to some extent in steady state magma reservoirs. With lower melt supply at slower spreading ridges, melt accumulates more episodically, resides in isolated reservoirs deeper in the crust, resulting in higher MgO lavas that retain more of the variability of the mantle source. At comparable spreading rate for North Gorda and southern Juan de Fuca Ridge, the chemical differences can be largely explained by lower melt supply at North Gorda. Figure 8. Chondrite-normalized Ce/Yb versus Ce abundance shows trends compatible with varying degrees of partial melting and/or source heterogeneities, especially for the C Axis part of the North Gorda segment. Off-axis samples for whole rock and glass compositions form a tight cluster with low (Ce/Yb)N and low Ce abundances compatible with larger degrees of melting. In contrast, compositions of Escanaba dredge samples (blue cross) span a large range in Ce abundances, and Ce/Yb ratios are mostly >1.0. Field for southern Juan de Fuca Ridge [Smith et al., 1994] shows that they are mostly related by crystal fractionation. Symbols and data sources as in Figure 4.

5.3. Escanaba Trough: Slower Spreading Rate and Low Magma Supply [29] If lava chemistry relates to spreading rate, the effects should be apparent for Escanaba, which has the slowest spreading rate for the Gorda Ridge. In fact, one E-MORB was recovered from an outcrop in southern Escanaba [Davis et al., 1994, 1998] but many more pyroclastic glass shards of E-MORB were found in the surface sediment in Escanaba [Clague et al., 2003; Davis and Clague, 2003]. To a first order, the major element compositions of most Escanaba lavas are similar to, but include

Table 5. Isotopic Compositions of Selected Glass Samplesa Sample N Axis GR02RC6 GR02RC11 C Axis GR00RC9 GR00RC16 GR00RC17 T885-R4 T886-R3 T886-R10 T187-R5 T187-R9 T187-R11 771-1 D4-15b S Axis T455-R2 T455-R6 Off-axis T186-R1 T454-R13 T884-R4

87

Sr/86Sr

143

Nd/144Nd

208

Pb/204Pb

207

Pb/204Pb

206

Pb/204Pb

0.702488 0.702428

0.513189 0.513190

37.867 37.663

15.492 15.450

18.380 18.260

0.702455 0.702410 0.702504 0.702347 0.702403 0.702509 0.702478 0.702439 0.702453 0.702468 0.702382 0.70246b

0.513227 0.513274 0.513192 0.513235 0.513212 0.513180 0.513229 0.513286 0.513177 0.513183 0.513303

37.509 37.508 37.719 37.758 37.657 37.450 37.538 37.549 37.704 37.805 37.239

15.415 15.444 15.442 15.466 15.464 15.427 15.396 15.404 15.471 15.480 15.349

18.173 18.090 18.374 18.351 18.195 18.029 18.240 18.242 18.295 18.407 17.941

0.702384 0.702486

0.513217 0.513181

37.639 37.783

15.454 15.479

18.206 18.336

0.702408 0.702340 0.702353

0.513255 0.513226 0.513229

37.485 37.754 37.758

15.433 15.477 15.466

18.066 18.230 18.351

a Uncertainties in Sr and Nd ratios are ±0.00002. Uncertainties in 206Pb/204Pb, 207Pb/204Pb, and respectively. b Dredge sample of Davis and Clague [1987]; higher 87Sr/86Sr on unleached sample.

208

Pb/204Pb are 0.010, 0.013, and 0.042,

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led Davis et al. [1998] to propose that this enrichment was likely due to assimilation of minute amounts of sediment. It may be difficult to distinguish between assimilation of small amounts of sediment during magma residence in sills beneath and within sediment versus melting variably enriched mantle sources. 87Sr/86Sr and 207Pb/204Pb isotopic compositions show similar, but not identical, positive correlations with incompatible trace elements as for the North Gorda samples, suggesting they are incorporated to a greater degree with smaller degree of melting. However, sources appear to be more radiogenic than at North Gorda and are similar to those tapped off-axis by the near ridge seamounts and at southern Juan de Fuca Ridge.

Figure 9. Sr, Nd, and Pb isotopic compositions for selected glasses from North Gorda plot at the less radiogenic end of the field for Pacific MORB. Lavas from Escanaba and the President Jackson seamounts as well as from the southern Juan de Fuca Ridge indicate more radiogenic mantle sources. Published Gorda (W) and Juan de Fuca (W) data are from White et al. [1987], and Gorda Pb isotopic data (C&T) are from Church and Tatsumoto [1975]. Pacific MORB field from White et al. [1987], Ito et al. [1987], and Castillo et al. [2000] and references therein.

fewer evolved compositions than, lavas from North Gorda. The most distinctive feature is the higher K2O of samples from the sediment-covered parts of the trough (Figures 4f, 5, and 12b), especially for the more evolved lavas at the NESCA site [Davis et al., 1994, 1998]. [30] The situation in southern Escanaba is complicated because the lava flows erupted from sills beneath a thick cover (hundreds of meters) of terrigenous sediment. The higher K2O and the highly variable 87Sr/86Sr and 207Pb/204Pb (Figures 9, 10) for a single large flow at NESCA

Figure 10. Chondrite-normalized La/Sm versus (a) 87Sr/86Sr and (b) 207Pb/204Pb show broad positive correlations but with more scatter for 207Pb/204Pb. N- to E-MORB from Escanaba plot along the same trend but with much higher abundances. Data for two ultraslow spreading ridges show more diverse isotopic compositions (data from Devey et al. [1994], Haase et al. [2003], Metz et al. [1991], and Muehe et al. [1993]). 18 of 24


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Figure 11. Distribution of North Gorda compositions along axis and with distance from axis up the eastern valley wall for (a) MgO and (b) (La/sm)N. Note that the MgO of whole rock (filled symbols) and glass analyses (open symbols) for the same samples can vary by 0.5 to 1.2%, depending on amount of olivine present. However, (La/Sm)N for whole rock and corresponding glass are within analytical precision. Symbols as in Figure 4.

Figure 12. Variations in lava compositions with latitude from the southern Juan de Fuca along Gorda Ridge to southern Escanaba for (a) MgO, (b) K2O/TiO2, (c) Na8.0, and (d) 87Sr/86Sr isotopes. The North Gorda segment shows the greatest variability in MgO and Na8.0 and lowest 87Sr/86Sr in a small area. Collectively, Escanaba samples also span a large range and include one E-MORB from outcrop; E-MORB compositions of glass grains in sediment are off-scale. Symbols and data sources as in Figure 8, except 87Sr/86Sr data of Eaby et al. [1984] are shown as orange stars. 19 of 24


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volcanic centers as at Escanaba, or some Arctic ridges. Spacing between volcanic centers, up to 100 km, is extreme at Gakkel Ridge [e.g., Michael et al., 2003]. [32] Incompatible trace element abundances are sensitive to different degrees of melting, whereas compatible trace elements reflect the major elements, which reflect largely crystallization processes. Typically, as degree of melting decreases the enriched mantle heterogeneities, apparently ubiquitous in the oceanic mantle [e.g., Cousens, 1996b; Ito et al., 1987; White et al., 1987; Zindler et al., 1984], are incorporated to a greater degree. Hence lavas erupted at ultraslow spreading ridges are predominantly E-MORB [Haase et al., 1996; Hellevang and Pedersen, 2005; Michael et al., 2003; Muehe et al., 1993, 1997; Robinson et al., 2001]. The linear array along the curved path shown by the (La/Sm)N and Ba/La versus Zr/Nb (Figure 13) of North Gorda basalts forms a continuum from N-MORB to E-MORB, overlapping with those from the ultraslow spreading Arctic ridges. Figure 13. (a) Ba/La and (b) (La/Sm)N versus Zr/Nb show well-defined trends along a curved path toward progressively more enriched compositions from N- to E-MORB, overlapping with those from ultraslow spreading ridges, suggesting a similar component is increased with decreasing degree of partial melting. Symbols and data sources as in previous figures.

5.4. Arctic Ridges: Ultraslow Spreading Rate and Low Magma Supply [31] Some studies have concluded that spreading rate is not directly related to degree of melting [Michael et al., 2003], while others proposed a close relationship between spreading rate and extent of melting [Niu and Hekinian, 1997]. Ultimately, spreading rate and ridge morphology must be dependent on regional magma supply. More frequent eruptions and/or more voluminous flows at fast spreading ridges like the EPR build larger volcanic structures that rise to shallower water depth. With decreasing spreading rate the depth to ridge axis increases, and discontinuous volcanic centers may alternate with amagmatic, faultdominated topography. At some ultraslow spreading ridges, magma supply is so low that along some segments no eruptions occur but peridotite mantle is exposed instead [Hellevang and Pedersen, 2005; Hellebrand et al., 2001; Dick et al., 2003; Michael et al., 2003]. Slow spreading may produce numerous small flows in close proximity, as at North Gorda, or somewhat larger flows with more widely spaced

[33] Zr/Nb, Ba/La, and La/Sm, should be unaffected by crystal fractionation and only slightly affected by partial melting, especially for elements with similar incompatibility like Ba, and La. Hence these element ratios should be more representative of source variations, whereas Ce/Yb is more sensitive to degree of melting for spinel peridotite. The small range of similar HREE and the lack of crossing HREE patterns (Figure 7) suggests that melts at North Gorda are generated in the spinel peridotite field and do not require residual garnet as proposed for some ultraslow spreading ridge lavas [Hellevang and Pedersen, 2005; Robinson et al., 2001; Dick et al., 2003]. [34] The correlations between (La/Sm)N, Zr/Nb, Ba/La, and K2O/TiO2 and Sr isotopic compositions are similar at the ultraslow spreading Arctic ridges, except that incompatible element abundances and ratios are much higher because the degree of melting at Arctic ridges is smaller. The 87Sr/86Sr isotopic ratios are also much more radiogenic and more variable along Arctic ridges but the Pb isotopic ratios, similar to those at North Gorda, plot at the very depleted end of the Pacific MORB Pb field (Figures 9 and 10). The variations at North Gorda can be explained by binary mixing of spinel peridotite with small, but variable amounts of variably enriched components. The process of including more of the enriched components with decreasing melting at North Gorda appears similar to that at ultraslow spreading Arctic ridges, con20 of 24


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tributing mostly an increase in K, LREE, Ba, 87 Sr/86Sr, and a decrease in Nb. [35] The ubiquitous presence of variably enriched components in the sub-oceanic mantle has long been recognized and much has been published about them. The enriched components have been proposed to be recycled, altered ocean crust [e.g., Niu and Batiza, 1997; Niu et al., 1999], peridotite metasomatized by deeper mantle plumes near hot spots [e.g., Dosso et al., 1993; le Roex et al., 1987; le Roux et al., 2002; Schilling, 1973] and/or recycled, subducted sediment [e.g., Hofmann, 1997; Rehkaemper and Hofmann, 1997], or recycled continental crust [e.g., Janney et al., 2005; Mahoney et al., 1992]. Near-ridge seamounts and off-axis eruptions on fast spreading ridges have shown that the mantle near fast spreading ridges is just as riddled with small-scale heterogeneities as near slow spreading ones [e.g., Castillo et al., 2000; Fornari et al., 1988; Niu and Batiza, 1997; Perfit et al., 1994; Zindler et al., 1984]. [36] As pointed out by many studies, at fast ridges less diversity is observed on axis because larger, more steady state, magma chambers tend to homogenize melts to some extent [e.g., Sinton and Detrick, 1992; Niu and Batiza, 1997; Rubin and Sinton, 2007] but lavas from off-axis and near transform faults that bypass these crustal reservoirs are more diverse and may include T- and E-MORB as well. Complexity at local sites can be created by variety, amount, and distribution of the enriched component present, the degree of partial melting, and depth and pressure at which melt is generated. [37] In some regions ‘‘hot spots’’ are impinging on spreading centers and result in significant trace element enrichment correlated with more radiogenic isotopes [e.g., Graham et al., 1999; Haase and Devey, 1996; Haase et al., 1997; Harpp and Geist, 2002; le Roux et al., 2002; Schilling, 1973; White et al., 1986]. However, at spreading center segments not affected by hot spots, smaller degrees of melting appear to incorporate more of the enriched components, also present in variable, but smaller amounts away from hot spots. The relationship between the increase in the highly incompatible elements with decreasing spreading rate suggests that changes in lava chemistry reflects changes in regional melt supply.

6. Conclusions [38] High-density sampling at the 65-km-long North Gorda segment reveals lava chemistry as

10.1029/2007GC001775

diverse as for the entire Gorda Ridge. Maximum diversity occurs at the C Axis part of the segment where the most recent eruption in 1996 occurred. Compared to off-axis samples that are all LREEdepleted N-MORB, samples from the shallower, C Axis part include T-MORB with a trend toward LREE-enrichment, K2O/TiO2 (>0.09) and lower Zr/Nb (