Rhenium-osmium isotope systematics and platinum group element concentrations in oceanic crust

Abstract

Knowledge of the 187 Os/ 188 Os ratio as well as the inventories of rhenium and platinum group elements (PGE) in oceanic crust allows quantification of the proportion of recycled oceanic crust in oceanic basalt sources. Our knowledge is limited by the availability of well-characterized sections of oceanic crust, specifically of the plutonic, lower portion that has not been drilled in situ to the Moho. Here we report new data for plutonic rocks that compose the bottom 4680 m of an ocean crust section from the Oman ophiolite. Major and trace element data as well as mineral analyses indicate that Oman gabbros are primitive cumulates from melts similar to typical mid-oceanic ridge basalt. The mean weighted composition of this section (Re: 427 pg/g; Os: 55 pg/g; Ir: 182 pg/g; Pd: 2846 pg/g; Pt: 4151 pg/g; initial 187 Os/ 188 Os: 0.142) indicates significantly higher Os and lower Re concentrations than previously analyzed partial sections of ocean crust that lack cumulate lower crust [Deep Sea Drilling Project–Ocean Drilling Program (DSDP-ODP) Hole 504B, ODP Hole 735B], emphasizing that the lower, cumulate oceanic crust dominates the Os budget of oceanic crust. Analyses of mineral grain size fractions indicate that rhenium, PGE, and lead are enriched in the sulfur-rich, fine fraction. This corroborates the notion that small accessory phases, and the melt migration processes affecting them, control these elements’ budgets, distributions, and susceptibilities to alteration. The Re-Os-PGE inventories of a hypothetical 6.5-km-thick composite section that consists of 1825 m of DSDP Hole 504B−like upper oceanic crust and 4680 m of Oman-like lower ocean crust (Re: 736 pg/g; Os: 45 pg/g; Ir: 133 pg/g; Pd: 2122 pg/g; Pt: 2072 pg/g; initial 187 Os/ 188 Os: 0.146) provide a new comprehensive assessment of oceanic crust composition. Upon recycling and mixing with reasonable proportions of mantle peridotite, this composite requires at least 2 G.y. to develop sufficiently radiogenic 187 Os/ 188 Os to generate high μ (HIMU: μ = 238 U/ 204 Pb) basalts.