Martian meteorites are relatively young objects widely considered to reflect melting of the mantle of Mars over the past 1.4 Gyr. As such, their trace element compositions partly reflect the complex history of melting in the martian mantle. Hafnium (Hf) and tungsten (W) having different bulk distribution coefficients during mantle melting and thus may be fractionated by such processes. This explains why Hf/W, as measured in martian meteorites, is decoupled from W isotopic compositions, which would have been produced by 182Hf decay during the first 50 Myrs of the solar system. In contrast, little fractionation is expected among Ba, Th, and W, all of which have a similar incompatibility during melting in Earth's mantle. Assuming this is also true for the martian mantle, Ba/W and Th/W may be used as a proxy for the degree of metal segregation. The W isotopic compositions indeed show some relationship with the degree of mantle W depletion predicted to have been caused by core formation, as deduced from measured Ba/W or Th/W. These indices, as well as the W isotopic variations themselves, provide evidence of heterogeneity probably caused by early core formation, which can be compared with the effects generated by early partial melting as recorded by the 146Sm-142Nd system. Shergottites such as Zagami with no Nd isotopic effect and hence no indication of Hf/W fractionation from partial melting appear to define rapid timescales for accretion and core formation of about one million years, implying a runaway growth mechanism of accretion for Mars. This is consistent with certain dynamical models and argues against early migration of Jupiter-sized objects through the inner solar system. Melting and core formation on Mars appears to have continued for at least 10 Myrs as recorded in the W and Nd isotopic compositions of some other martian meteorites. This accretion and core formation history is strikingly different from that of the Earth. The last major stage of Earth accretion is thought to be the Moon-forming Giant Impact, the most recent Hf-W age estimates for which are in the range 35 to 50 Myrs after the start of the solar system. The significantly more protracted rates of Earth accretion and core formation deduced from U-Pb are either incorrect or require some additional late process that removed Pb from the silicate Earth. Changes in the mechanisms and partitioning associated with core formation are indeed predicted from the stability in the mantle of sulfur-rich metal before, and sulfide after the Moon-forming Giant Impact. The most recent estimates of the Pb isotopic composition of the bulk silicate Earth then imply protracted cooling of the uppermost mantle by several thousand degrees K over about 30 Myrs postdating the Giant Impact. In contrast the bulk silicate Mars did not achieve the same level of Pb depletion, which is consistent with the absence of such a mechanism in smaller planets lacking perovskite and late Giant Impact events.
|Publication status||Published - 2005|
|Event||2005 AGU Fall Meeting - San Francisco, USA, United States|
Duration: 5 Dec 2005 → 9 Dec 2005
|Conference||2005 AGU Fall Meeting|
|City||San Francisco, USA|
|Period||5/12/05 → 9/12/05|