EarthRef.org Reference Database (ERR) -- Mcculloch 1994

A primitive measured Sr-87/Sr-86 ratio of 0.700510 +/- 9 is reported for an early Archean (3450 Ma) barite from the Pilbara Block of Western Australia. The low Rb-87/Sr-86 ratio in the barite (0.00015) enables a precise initial Sr-87/Sr-86 ratio of 0.700502 +/- 10 to be calculated at 3450 Ma. This initial ratio is the same as that previously inferred for mineral separates from Onverwacht Group basalts of similar age and, if reliable, implies a relatively high Sr-87/Sr-86 ratio of 0.70050 +/- 2 for the early Archean mantle. Assuming that the upper-mantle has become progressively depleted in Rb/Sr due to the growth of continental crust, then this allows an estimate to be placed on the Earth's initial Sr ratio of 0.69940 +/- 10 (relative to NBS 987 = 0.71021). This ratio, designated 'BEBI' (Bulk Earth Best Initial), is significantly higher than that measured for the Moon (LUNI = 0.69900 +/- 2) or the well-characterised achondrite Angra dos Reis (ADOR = 0.69893 +/- 2). Assuming a Rb-87/Sr-86 ratio of approximately 0.4 (approximately 50% of chondrites) for the terrestrial precursors gives an accretion interval for the Earth of approximately 60-100 Ma after ADOR and approximately 40-80 Ma after the formation of the Moon. This chronology is compatible with either an early (approximately 4540 Ma) impact origin for the Moon or, alternatively, a late-stage (approximately 4480 Ma) giant impact, with the Moon being derived predominantly from a low Sr-87/Sr-86 (0.6990) impactor. In both scenarios the final accretion of the Earth was completed relatively late at approximately 4480 Ma. An approximately 60-100 Ma timescale for the accretion of the Earth is consistent with Pb isotopic constraints for core formation and also allows an extended interval for cooling of the terrestrial magma ocean, and hence avoids the dilemma of a highly differentiated early Earth.