HR: 15:25h
AN: V52D-08    [PDF]
TI: High Precision Osmium Isotope Evidence From Chondrites for Late Accretion and Evolution of the Earth's Mantle
AU: * Brandon, A
EM: alan.d.brandon1@jsc.nasa.gov
AF: NASA Johnson Space Center, Mail Code SR, Houston, TX 77058
AU: Puchtel, I
AF: Univeristy of Chicago, Department of Geophysical Sciences, Chicago, IL 60637
AB: Prevailing models for early differentiation in terrestrial planets call for magma oceans generally concurrent with core formation. Consequently, extraction of a metal-rich core must have resulted in strong depletion of the silicate portion of Earth in highly siderophile elements (HSE), including Re, Os, and Pt. Efficient degassing of a magma ocean could leave the interior of the Earth depleted in volatiles. A result of such processes is that subsequent late accretion may be necessary to replenish both volatiles and HSE to their estimated and measured bulk silicate Earth concentrations. If one type of material was the supplier, then late accretion material should be volatile-rich, and have Os isotope compositions and HSE concentration ratios similar to the Earth's mantle. At present, such type of material is not known. Alternatively, if different types of materials led to volatile and HSE replenishment, then volatile-poor material could supply HSE during late accretion. The Os isotopic compositions of the Earth's mantle can provide a fingerprint for the type of material accreted. Thus, tracing mantle evolution over Earth's history is critical to constraining the Os isotopic composition of the earliest terrestrial mantles, and hence the types of materials present during late accretion. To address the issues surrounding late accretion and terrestrial mantle evolution, volatile-rich carbonaceous, and volatile-poor enstatite and ordinary chondrites, were measured for high-precision Os isotopic compositions. These data combined with those from previously published data for abyssal peridotites, new data for 2.7 Ga Pyke Hill komatiites, show the following. The Earth's primitive upper mantle has evolved with Pt/Os and Re/Os ratios similar to those in enstatite and ordinary chondrites, suggesting that late accretion materials that replenished HSE in terrestrial mantles were volatile-poor. Different materials are therefore required to re-seed the interiors of terrestrial planets with volatiles. These volatile-rich materials must essentially be depleted in HSE.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting