UC San Diego

Detrital Cr-spinels from the Jack Hills region of Western Australia co-exist in conglomerates with the oldest terrestrial zircons (≤4.4 Ga). In this study, these Cr-spinel are examined to determine their age and tectonic setting using a new geochemical approach of combining highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and Re-Os isotope systematics with major, minor, and trace element mineral chemistry.The Cr-spinels exhibit high ZnO (≤13 wt.%), low average Mg# (15), and moderate average Cr# (61), consistent with chemical modification during metamorphism. The measured 187Os/188Os ratios (0.10517-0.10667) of Cr-spinel separates are strongly sub-chondritic (γOschon = -6.8 to -12). When corrected for Re ingrowth, the initial 187Os/188Os ratios (0.09218-0.09924) lie at or close to the Solar System initial or are modestly supra-chondritic. The time of rhenium depletion age (TRD), which is a minimum model age, indicates formation of the Cr-spinels ~3.23 ±0.16 billion years ago (2σ uncertainty). The Cr-spinels have between 34 to 53 ng/g Os and the HSE abundance patterns show significant Pt and Pd depletion relative to Os, Ir, and Ru.