Abstract
The distribution pattern of the trace elements Rb, Sr, Nd and Sm for Yamato 984028 (Y984028) is consistent with its classification as a lherzolitic shergottite. The Sm–Nd mineral isochron of this lherzolitic shergottite defines its age to be 170 ± 10 Ma for an initial ɛ Nd = +11.6 ± 0.2. The corresponding Rb–Sr mineral isochron yields an identical age of 170 ± 9 Ma and an initial 87Sr/ 86Sr = 0.710389 ± 0.000029. The concordant Sm–Nd and Rb–Sr isochron ages suggest that Y984028 crystallized 170 ± 7 Ma ago contemporaneously with five other lherzolitic shergottites and ten enriched basaltic and olivine-phyric shergottites. The age, Sr- and Nd- isotopic signatures further suggest that Y984028 and Y-793605, and also probably Y000097 could come from a single magmatic body. Using a two-stage evolution model, the time-averaged 87Rb/ 86Sr-ratio for the mantle source of the parent magma of Y984028 is ∼0.182, within the range of 0.178–0.182 that has been reported for other lherzolitic shergottites. The corresponding time-averaged 147Sm/ 144Nd-ratio for the source mantle of its parent magma is super-chondritic at ∼0.217, implying its source was a depleted mafic part of the Martian mantle similar to that of diabasic shergottite Northwest Africa (NWA) 1460. Rb, Sr, Sm and Nd distributions in Y984028 are likely produced by pyroxene and olivine accumulation, probably from a NWA 1460-like parental melt, in an intrusive magma body.
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