PNNL

Abstract

IVB iron meteorites and ungrouped irons Tishomingo, Willow Grove, and Chinga are Ni-rich and volatile-depleted. Chemical similarities include enrichments of 10-100?CI for some highly (HSE) and moderately (MSE) siderophile elements. Yet certain siderophile elements ( P, W and Mo) are depleted relative to elements of similar volatility. Tishomingo contains a single stishovite grain, indicating exposure to a minimum shock pressure of 8-9 GPa. The presence of stishovite and of rare chromite as inclusions in troilite-daubreelite in IVB irons allows the determination of the oxygen isotopic composition of these meteorites. Mass independent oxygen isotopic compositions of IVB irons and Tishomingo suggest genetically different parent bodies. IVB irons and Tishomingo and are characterized by indistinguishable mass independent, genetic isotopic compositions of Mo consistent with the carbonaceous chondrite (CC) type genetic origin. Chinga reveals Mo isotopic composition consistent with a CC origin, but its Mo is depleted in s-process isotopes relative to IVB and Tishomingo, indicating a unique parent body. These isotopic constraints place these irons in the same, general nebular formational environment as CC, most likely the outer Solar System. Mo isotopic composition of Willow Grove, when corrected for exposure to cosmic rays (CRE), is consistent with a mixed CC-non-carbonaceous (NC) genetic origin, potentially indicating mixing between inner/outer Solar System materials early in nebular evolution. Collectively, genetic isotope data show that Tishomingo, Chinga, Willow Grove and the IVB irons sample four distinct parent bodies. IVB irons share a common oxygen, as well as Mo and 183W isotopic compositions with the South Byron trio (SBT) meteorites and the Milton pallasite, which are also Ni-rich, but relatively less depleted in volatile siderophile elements. Despite evidence for genetic differences, the HSE abundances of Tishomingo and Willow Grove are similar to some IVB and SBT, consistent with formation in planetesimal cores by moderate degrees of fractional crystallization from initial melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to group IVB, yet substantially higher Ni, implies formation on parent bodies with lower bulk HSE abundances. By contrast, the HSE abundances in Chinga are considerably lower than in IVB, and highly fractionated, and the processes responsible remain elusive. All three Ni-rich meteorites are characterized by CRE corrected 182W/184W, consistent with metal-silicate segregation on their respective parent bodies within 2-4 Ma of CAI formation. High temperature condensation contributed to the enrichment of HSE and MSE, and depletion of volatile siderophile elements, although parent body degassing may have played a role in the depletion of volatile siderophile elements. Oxidation of Fe from metal, likely during parent body accretion or core segregation, resulted in high Ni concentrations in these meteorites, also suggested for SBT and Milton. Oxidation of the parent bodies of Tishomingo and Willow Grove may have occurred at ~IW+1, as indicated by depletions in Mo and W, likely due to reaction of metal with water during differentiation. Late-stage reduction, reflected in the presence of Cr-bearing sulfides in Tishomingo and IVBs, may have resulted from exhaustion of the oxidant.