Rhenium‐osmium isotope systematics and platinum group element concentrations in oceanic crust from DSDP/ODP Sites 504 and 417/418

Abstract

Rhenium, Os, 187Os/188Os, Ir, Ru, Pt, and Pd analyses of 43 samples from DSDP/ODP Hole 504B in the eastern equatorial Pacific crust with an age of ∼6.7 Myr are used to calculate the composition of a composite section through the upper oceanic crust. Weighted average Re (1526 pg/g), Os (21 pg/g), 187Os/188Os (0.213), 187Re/188Os (353), initial 187Os/188Os (0.173), Ir (9 pg/g), Pt (304 pg/g), and Pd (265 pg/g) are similar to a composite sample from DSDP Sites 417/418 in the western Atlantic with an age of ∼120 Myr (2153 pg Re/g, 33 pg Os/g, 187Os/188Os = 1.042 ± 0.016, 187Re/188Os = 349; 187Os/188Osi = 0.344, 25 pg Ir/g, 698 pg Ru/g, 804 pg Pt/g, and 2001 pg Pd/g). These sections complement existing data for ODP Site 735B (11–12 Myr) and, combined, provide the best available information on Re‐PGE‐Os isotope systematics of the upper and middle oceanic crust. Weighted average Os concentrations for samples from the volcanic zone, the transition zone with stockwork mineralization, and the sheeted dike complex decrease from 30 pg/g to 18 pg/g. Rhenium concentrations are highest in the transition zone (3867 pg/g), leading to 187Re/188Os of 885, compared to values of 178 for the volcanic zone and 393 for the sheeted dike complex. The present‐day and initial 187Os/188Os are highest in the transition zone (0.329 and 0.230, respectively). This zone also has the highest average Pt (899 pg/g) and Pd (523 pg/g) concentrations. Of all lithologic units, breccias are most enriched in Re (4874 pg/g) and depleted in Os (8 pg/g), Ir (3 pg/g), Pt (131 pg/g), and Pd (173 pg/g) and have the most radiogenic present‐day and initial 187Os/188Os (0.710 and 0.324, respectively). We contend that this is indicative of intense alteration, loss of PGE, and early addition of hydrogenous Os. Pillow lavas and massive flows and dikes of the volcanic zone and the sheeted dike complex are also affected by additions of hydrogenous Os, causing superchonditic initial 187Os/188Os. The weighted average Os/Ir of 2.4 for the entire core is significantly higher than values reported for mantle peridotites and estimates of the upper mantle and most likely indicates fractionation during melting and melt extraction or intracrustal differentiation rather than differential mobility of Os and Ir during alteration. Addition of hydrogenous Re to altered oceanic crust, although locally important, does not increase the Re concentration of oceanic crust by more than a few percent. The magnitude of mantle‐derived Os loss to seawater cannot be reliably estimated from our data, but a steady state mass balance of hydrogenous Os indicates that the average loss cannot exceed 10% of the crustal inventory. Similarly, average addition of hydrogenous Os does not exceed 10% of the primary crustal inventory. This, however, leads to a significant increase in 187Os/188Os from depleted upper mantle values to ∼0.17. The characteristic fractionated PGE pattern and 187Re/188Os of fresh MORB are not fundamentally changed during hydrothermal alteration, and the magnitude of radiogenic ingrowth during storage of subducted oceanic crust for prolonged periods in the mantle is primarily determined by magmatic processes at the ocean ridges and, possibly, additional processing in subduction zones.