HR: 0800h
AN: T41E-1354    [Abstracts]
TI: Trace Element Distribution Between Orthopyroxene and Clinopyroxene in Peridotites From the Gakkel Ridge: a SIMS and NanoSIMS Study
AU: * Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: MPI Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Snow, J
EM: jesnow@uh.edu
AF: Univ. Houston, Dept. of Geosciences 4700 Calhoun Dr., Houston, TX 77024 United States
AU: Mostefaoui, S
EM: smail@mpch-mainz.mpg.de
AF: MPI Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Hoppe, P
EM: hoppe@mpch-mainz.mpg.de
AF: MPI Chemie, PO Box 3060, Mainz, 55020 Germany
AB: Clinopyroxenes in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, clinopyroxenes always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene cannot significantly host most incompatible trace elements, exsolution will lead to enrichment in the clinopyroxene lamellae. In order to address a possibly exsolution-controlled partitioning between clinopyroxene and orthopyroxene, we have obtained major (EPMA) and trace (SIMS) element mineral compositions on fourteen plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of orthopyroxene lamellae in clinopyroxene porphyroclasts lies around fifteen percent of the original clinopyroxene. For the light to middle rare earth elements, the enrichment in the measured clinopyroxene exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into orthopyroxene. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C (1). The bulk clinopyroxene can be reconstructed using the lamellar proportions and their relative partitioning. The implications of such a reconstruction are that the clinopyroxene rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle-melting models proposed thus far for mid-ocean ridges. (1). Hellebrand et al., Contrib. Mineral. Petrol. (in press)
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1065 Major and trace element geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3651 Thermobarometry
SC: Tectonophysics [T]
MN: Fall Meeting 2005