Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11a1584b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11A-1584
Physics
[1027] Geochemistry / Composition Of The Planets, [1028] Geochemistry / Composition Of Meteorites
Scientific paper
Mechanisms for the emplacement of highly siderophile elements (HSE) in Earth's mantle have been debated for several decades. The chief conundrum is accounting for the high absolute and chondritic relative abundances of these elements in the terrestrial mantle, despite their strong tendency to partition into metal during core formation. Two end member models are most frequently discussed with respect to this issue. In the first model, abundances of HSE in planetary mantles are controlled by partitioning between segregating metal and silicate at high pressures, where some or all of the HSE may be considerably less siderophile, as may be appropriate for the base of a terrestrial magma ocean. A major weakness of this model is the generally chondritic HSE ratios in the mantle, which would require conditions under which the metal-silicate partitioning of all HSE would converge to approximately the same values. In the second model, termed late accretion, core extraction removes >99% of HSE from the Earth's mantle. The mantle is subsequently reseeded with HSE via continued accretion of 0.5 to 1% by mass of additional material. This model has been questioned because the timing of late accretion is poorly defined, and the mechanisms that can rapidly mix the late accreted materials to homogeneity within the mantle are difficult to envision. To examine this issue, 23 mafic to ultramafic shergottite meteorites from Mars, were measured for 187Re-187Os isotopes and HSE abundances. The objective is to gain insights on the early chemical evolution of the martian mantle to address the issue of HSE controls on the mantles of terrestrial bodies, with Mars serving as an important point of comparison to Earth. The shergottites display calculated initial 187Os/188Os ratios that correlate with the initial 143Nd/144Nd. Shergottites from mantle sources with long-term melt-depleted characteristics (initial ɛ143Nd of +36 to +40) have chondritic initial γ187Os ranging from -0.5 to +2.5. Shergottites from long-term enriched sources, with initial ɛ143Nd of ~ -7, are characterized by suprachondritic γ187Os values of +5 to +15. The initial γ187Os and ɛ143Nd variations do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot arid-desert finds, Antarctic finds, or observed falls. Modeling indicates that the correlation between Nd and Os isotopes is unlikely to be the result of the participation of martian crust. More likely, this correlation relates to mixing between depleted and enriched reservoirs that formed from a martian magma ocean at ca. 4.5 Ga, formed at variable stages during solidification of a magma ocean. The expanded database for the shergottite HSE abundances suggests that their martian mantle sources have similar HSE abundances to the Earth's mantle, consistent with prior studies. The relatively high HSE abundances in both planetary mantles likely cannot be accounted for by high pressure-temperature metal-silicate partitioning at the bases of magma oceans, as has been suggested for Earth. If the HSE were instead supplied by late accretion, this event must have occurred prior to the crystallization of the last martian magma ocean.
Brandon Alan D.
Puchtel Igor S.
Walker Ray J.
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