HR: 11:40h
AN: U12A-06 INVITED    [Abstracts]
TI: Geochemical Consequence of Extraction of Incipient CO₂-rich melts from Earth's Deep Upper Mantle
AU: * Dasgupta, R
EM: rajdeep@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AU: Withers, A C
AF: Dept of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AU: McDonough, W F
AF: Dept of Geology, University of Maryland, College Park, MD 20742, United States
AU: Hirschmann, M M
AF: Dept of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AB: The initiation of partial melting beneath mid-oceanic ridges and ocean islands likely produces incipient carbonatitic melts. These highly mobile melts generated at great depth may not only affect geophysical properties of deep upper mantle but can also influence Earth's geochemical evolution by releasing incompatible parent and/or daughter elements, heat producing elements, and volatiles. But constraints on the fractionation of the key trace elements between the peridotitic residue and carbonatitic melts are lacking at conditions of initiation of partial melting beneath ridges.
Experiments at 6.6-8.6 GPa and 1265-1470 °C on carbonated peridotite doped with a blend of trace elements produced cpx + garnet + magnesite ± opx ± olivine + carbonatitic melt (cbL) similar in composition to that expected at the solidus of carbonated peridotite (Ca# = 0.52 at 6.6 GPa and 0.45 at 8.6 GPa; Na₂O = ~4 wt.% at 6.6 GPa and ~2.5 wt.% at 8.6 GPa). Compared to previous studies at lower pressures (2.0-4.6 GPa), D^cpx/cbL from the present study are smaller for elements substituting into the cpx M2 site, especially trivalent cations ( D_Lu = 0.17, D_La = 0.006). D^cpx/cbL for U (0.001) and Th (0.002) are also noticeably lower than the previous estimates. In contrast, D^garnet/cbL values are higher for REEs ( D_Lu = 4.6, D_La = 0.039) and HFSEs, U (0.023) and Th (0.017).
Our estimate of D^{peridotite/cbL} indicates that incipient carbonatite extraction from the deep upper mantle will produce a residue with a more depleted Rb/Sr, U/Pb, Th/U, and enriched Sm/Nd, which may evolve to produce the most common of the mantle end member components, PREMA. Metasomatic implantation of deep carbonatitic melt into the lithosphere can generate a high-μ (²³⁸U/²⁰⁴Pb) signature, whereas mixing of carbonatite with elevated Th/U can contribute to the observed ²³⁰Th-excesses in MORBs. However, carbonatites derived by incipient melting of depleted (DMM) mantle have limited trace element enrichments, ~10-100×primitive mantle (PM), compared to natural magnesio-carbonatites (~100-1000×PM). Thus, natural carbonatites either derive from highly enriched sources or become enriched in trace elements by extensive melt/rock reaction during ascent. Owing to much deeper intersection of carbonated peridotite solidus with solid mantle adiabat, the volume of the mantle contributing to the highly incompatible elements and volatiles is likely much larger than the volume that supplies the major elements of basalts, thus the abundance of various incompatible trace elements might be overestimated for the upper mantle basalt source regions.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Union [U]
MN: 2007 Fall Meeting