The Bholghati (howardite) consortium: An overview

Abstract

The Bholghati (howardite) has had a complex history. The consortium studies indicate that eucrite clasts show evidence of rapid crystallization followed by prolonged subsolidus annealing. Dark clasts are carbonaceous CM2 type. Bholghati bulk composition can be modeled by 55% eucritic, 45% diogenitic, and 3% dark clast components. The eucritic clasts show a LREE depleted pattern relative to HREEs, which is not typical of a normal eucrite. The LREE depletion requires two-stage melting from a chondritic source. The volatile/mobile trace element patterns in dark and eucritic clasts are highly variable, probably due to volatile redistribution. The eucritic clast shows excess fission Xe, which is attributed to in situ decay of Pu-244. The cosmic-ray exposure age is 10–17 Ma. The Rb-Sr, Sm-Nd, and K-Ar ages of Bholghati bulk and eucritic clasts indicate 4.53 Ga as crystallization age, 2–3 Ga as metamorphic event, and probably a minor thermal event < 1 Ga. The Bholghati evolution scenario is 1) early multiple magmatic events (4.53 Ga ago), producing eucrites and diogenites; 2) a metamorphic event (2–3 Ga ago) and prolonged subsolidus annealing; 3) fragmentation and low-temperature mixing of eucrites and diogenites; 4) low-velocity impact (< 1 Ga. ago) and admixing of carbonaceous material; 5) disruption of regolith and ejection of Bholghati 10–17 Ma ago; and 6) Bholghati fell on the Earth in 1905.