HR: 17:25h
AN: U14A-06    [Abstracts]
TI: Global Variations in Abyssal Peridotite Compositions I: Isotopic and Trace Element Characteristics of the MORB-Source Mantle
AU: * Warren, J M
EM: jmwarren@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Shimizu, N
EM: nshimizu@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AB: Abyssal peridotites are typically assumed to be the residues of near-fractional melting of an initially uniform source composition. We use a global compilation of abyssal peridotites to assess the extent to which these assumptions are true. After filtering the dataset for samples with characteristics of melt addition (e.g., veins, high spinel TiO₂ concentrations), we observe considerable variation in nominally residual peridotites. As a function of spreading rate, peridotites at slow and ultra-slow spreading rates extend to more depleted and more enriched compositions than can be explained by a simple near-fractional melting model. Globally, trace elements in residual peridotite Cpx vary by 2-4 orders of magnitude over length-scales ranging from the grain-scale to the ridge segment-scale. Similarly, variations in spinel Cr# and peridotite modes occur at all length-scales. Variations at the sub-dredge scale cannot be the result of variations in the pressure-temperature regime during melting. We suggest instead that these variations reflect various melt-rock reaction processes during melt extraction. In addition, we suggest that some of the variation in peridotite composition reflects initial source heterogeneity. To constrain the role of source heterogeneity, we assess the isotopic range of abyssal peridotite Cpx. Using a global compilation (70 samples), we find that the average Nd isotopic composition of peridotite Cpx (0.513167) is similar to average MORB (0.51313; Su and Langmuir, 2003). Peridotites are observed to extend to similarly enriched isotopic compositions as those of normal-MORB, but to more depleted compositions than MORB. These depleted isotopic compositions should be accompanied by depleted elemental concentrations and thus be relatively undetectable in MORB. We interpret the range in peridotite isotopic compositions to reflect initial source chemical heterogeneity. Hence, the upper mantle is both initially heterogeneous before the onset of melting and undergoes processes that create further heterogeneities during melting and melt extraction.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Fall Meeting