Ultramafic xenoliths: Clues to Earth's late accretionary history

Abstract

Fourteen spinel lherzolites, for which extensive trace element data are available, may be divided into three groups depending upon the percentage loss of basaltic partial melt; averages are slightly depleted, −7% melt (Ca/Sa > 0.09); moderately depleted, −13% (0.09 > Ca/Si > 0.06); and strongly depleted, −20% (0.06 > Ca/Si). Rhenium abundances of the groups are correlated with percent depletion, and the intercept of 0.0071 × Cl chondrite corresponding to undepleted mantle is identical to that independently derived from 187Os/186Os in osmiridium of known age. The distribution of Re in mafic and ultramafic rocks is apparently closely related to S and Se abundances. Lherzolite data suggest that the abundances of highly siderophile elements (Os, Re, Ir, Pd, Au) and chalcogenic elements (S, Se, Te) in a primary basaltic melt are significantly higher than those of average oceanic ridge basalts but may be similar to those of trace element‐rich Indian Ocean ridge basalts. The estimated S content (∼1000 ppm) of a primary basaltic melt is compatible with experimentally measured S solubilities at high temperature and pressure. On the basis of lherzolite depletion and using published trace element data for spinel lherzolites and ocean ridge basalts, estimated abundances in ppb (or × 103/Cl chondrite abundances) in pristine upper mantle are Os, 3.1 (5.9); Re, 0.26 (7.1); Ir, 3.4 (7.1); Pd, 4.5 (8.1); Au, 1.01 (8.4); S, 200,000 (3.2); Se, 57 (3.1). The trace element pattern closely resembles that of the CM2 chondrites, but not of the CO3, CV3 or H chondrites. The pattern is a reasonable match, except for Au, with Apollo 17 lunar breccias that contain a “group 2” ancient meteoritic component, suggesting that rather similar objects bombarded earth and moon during the first 600 m.y. of their history.