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

The Rb-Sr 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 Rb-Sr 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 Pb-Pb 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, Lu-Hf 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.