Rb-Sr isotope systematics of the oldest crustal rock in the solar system

Abstract

The RbSr isotopic system is an important cosmochemical tool for constraining the evolution of moderately volatile (Rb) and refractory (Sr) elements in the earliest Solar System. Presented here are RbSr isotopic systematics of twenty-two fractions of the evolved achondrite Erg Chech 002, the oldest crustal rock yet identified in the Solar System. Although the data show a large amount of scatter, likely due to terrestrial alteration, an errorchron yields an age within uncertainties of published PbPb ages. Initial 87Sr/86Sr ratios of both the errorchron and of individual mineral fractions show that Erg Chech 002 formed from materials evolving with a typical CI-chondrite nebular Rb/Sr ratio and that any Rb depletion on its parent body must have occurred almost simultaneously with the crystallization of the meteorite. Additionally, LuHf systematics of bulk Erg Chech 002 overlap with the chondritic range. Other early-formed parent bodies similarly show initial 87Sr/86Sr ratios close to a CI-chondrite evolution line. Despite nucleosynthetic variations of 84Sr/86Sr being evident between inner and outer Solar System materials, no variations are detectable in 87Sr/86Sr for Erg Chech 002 or other early-formed parent bodies. Although the initial 87Sr/86Sr of Erg Chech 002 overlaps with that of all other early-accreted bodies, the initial 87Sr/86Sr of asteroid 4 Vesta (BABI) remains the most precise and relevant initial value for the bulk silicate Earth.