[1] Improved insights into the processes of hot spot-ridge interaction along the Galápagos Spreading Center (GSC) are revealed by geochemical data between $91°W and 98°W. Principal components analysis reveals that >87% of the total isotopic variability can be explained with only two mantle source components. The ''depleted'' component has lower ratios of highly to moderately incompatible elements, higher Nd isotopic ratios, and lower Sr and Pb isotopic ratios. The second component is relatively enriched in incompatible elements, has more radiogenic Pb and Sr and less radiogenic Nd, and is comparable to the C or ''common'' mantle component observed at many locations in the Pacific. The enriched component's signature is strongest nearest the hot spot at $92°W and diminishes with distance from the hot spot to 95.5°W. Near 95.5°W, lava compositions change sharply, becoming dominated by the depleted component and remaining so farther west, to 98°W. Thus, the Galápagos hot spot most clearly influences the composition of the GSC between 91°W and 95.5°W. The depleted component between 91°W and 98°W differs from that evident at the Galápagos Archipelago, along the GSC east of 91°W, and along the East Pacific Rise. This suggests some form of compositional zoning in the regional mantle. If the depleted materials are intrinsic to the Galápagos mantle plume, then the plume is bilaterally zoned and feeds a depleted component to the GSC at 91°W-98°W that is distinct from the depleted material elsewhere in the region. This possibility is supported by melting models in which the Galápagos plume is a uniform mixture of a depleted matrix and fine-scale enriched veins. The expression of the more fusible veins is predicted to be enhanced nearest the hot spot ($92°W), where plume-like upwelling drives rapid flow and melting deeper in the melting zone (where the veins are melting). With increasing westward distance from the hot spot, the deep, plume-driven flow is predicted to decrease, as does the expression of the enriched veins in lava compositions. The model therefore adequately explains the compositional and crustal variations from 92°W to 95.5°W. The average model composition of the plume in this region does not differ significantly from that of the ambient mantle beneath other ridges not influenced by hot spots.

A total of 16 Ontong Java Plateau (OJP) basalt samples from Ocean Drilling Program Legs 192 and 130 were analysed for major, trace and platinum-group elements (PGEs; Ir, Ru, Rh, Pt and Pd). Major- and trace-element compositions determined by our study confirm Leg 192 shipboard analyses that indicated a new group of more primitive or ‘Kroenke-type’ basalts, with higher MgO, Ni and Cr, and lower incompatible-element, abundances than the more common Kwaimbaita-type basalts. The PGE abundances quantified here extend the range of the continuum of compositions found in previously analysed OJP basalts and are similar to those present in some komatiites. The PGEs, therefore, cannot be used to differentiate definitively between OJP basalts groups. The two samples analysed from Leg 130 (one from Site 803 and one from Site 807) are akin to the Kwaimbaita-type basalts.Low-temperature alteration has not affected Pd abundances in the Leg 192 basalts as it has in the Solomon Island and the Leg 130 samples. Elemental abundances and ratios along with petrography reveal that the OJP basalts have not experienced sulphide saturation. Positive correlations of Ir and Ru with Cr and Ni attest to the lithophile behaviour of the PGEs and lend more credence to studies suggesting compatibility of these elements in oxide and silicate phases, such as Cr-spinel and olivine. Estimates of sulphur abundance in the mantle, degree of partial melting and pressure of melt initiation were used in conjunction with the model of Mavrogenes & O’Neill to calculate a minimum initial excess temperature of +185–+235°C (1465–1515°C at 3.5–4.0 GPa) above ambient mantle for the OJP source. This is in broad agreement with a fossil geotherm preserved in megacrysts and peridotite xenoliths found in pipe-like intrusives of alnöite that outcrop on the island of Malaita, Solomon Islands. Using the PGEs as a guide, the OJP basalts were modelled using a three-source component melt mix: a 10% garnet peridotite melt of primitive mantle composition, which then passed through the garnet-spinel transition and melted a further 20%, a 30% partial melt of fertile upper mantle and 0–1% of outer core material. The core component was included only in the plume source, and the ratio of plume source to upper mantle source was 19: 1. It is evident from this study that the PGE contents of at least some of the OJP basalts are too high to be generated by primitive mantle sources alone. A PGE-enriched component is required and we suggest that this is outer core material. While a sulphide-rich mantle component could also increase the PGE abundances (assuming that the sulphide is exhausted during partial melting), the sulphur-undersaturated nature of these basalts argues against this. Variations in the amount of outer core in the source (from 0 to 1 wt%) and degree of fractional crystallization can account for the entire range in PGE abundances of OJP basalts.

[1] Major and trace element concentrations; Sr, Nd, and Pb isotopic ratios; and plagioclase and olivine compositions of samples from 92°35 0 W to 94°10 0 W on the Galápagos Spreading Center (GSC) constrain magmatic processes associated with the 93.25°W overlapping spreading center (OSC). Significant variations in parental magma compositions and extent of low-pressure fractionation over short along-axis distances suggest small, discontinuous, or poorly mixed magma chambers. Mineralogical, petrological, and geochemical data indicate this OSC is characterized by low overall melt supply. Regional along-axis geochemical gradients along the western GSC (e.g., in 206 Pb/ 204 Pb, Nb/Zr, and La (8) /Sm (8) ) are interrupted by increased variability within the OSC, consistent with less efficient mixing of melts in this region. Despite a long-term history of westward propagation, bathymetric and geochemical data are inconsistent with recent propagation of this offset; the eastern limb appears to have recently retreated and left behind a series of abandoned ridges within the overlap zone. The eastern limb shows two progressive westward trends of decreasing ratios of highly to moderately incompatible elements, one outside and one within the overlap zone. The overlapping portion of the western limb also has low ratios of these elements. These patterns are consistent with previously depleted mantle being preferentially melted beneath this offset.

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The geochemical reference material BHVO‐1 was analysed by a variety of techniques over a six year period. These techniques included inductively coupled plasma‐mass spectrometry and atomic emission spectroscopy (ICP‐MS and ICP‐AES, respectively), laser ablation ICP‐MS and spark source mass spectroscopy. Inconsistencies between the published consensus values reported by Gladney and Roelandts (1988, Geostandards Newsletter) and the results of our study are noted for Rb, Y, Zr, Pb and Th. The values reported here for Rb, Y, Zr and Pb are generally lower, while Th is higher than the consensus value. This is not an analytical artefact unique to the University of Notre Dame ICP‐MS facility, as most of the BHVO‐1 analyses reported over the last ten to twenty years are in agreement with our results. We propose new consensus values for each of these elements as follows: Rb = 9.3 ± 0.2 μg g‐1 (compared to 11 ± 2 μg g‐1), Y = 24.4 ± 1.3 μg g‐1 (compared to 27.6 ± 1.7 μg g‐1), Zr = 172 ± 10 μg g‐1 (compared to 179 ± 21 μg g‐1), Pb = 2.2 ± 0.2 μg g‐1 (compared to 2.6 ± 0.9 μg g‐1) and Th = 1.22 ± 0.02 μg g‐1 (compared to 1.08 ± 0.15 μg g‐1).