Critical evaluation of Re–Os and Pt–Os isotopic evidence on the origin of intraplate volcanism

Abstract

Evaluation of Re–Os and Pt–Os isotope systematics indicates that the mantle plume model does not provide a unique explanation of the 187Os/ 188Os– 186Os/ 188Os isotopic variation in intraplate volcanic rocks. The low Os contents of ocean floor basalt and sediment compositions results in the amount of recycled crust (up to 55%) required in a plume to explain the range in 187Os/ 188Os, being unrealistically high unless non-equilibrium melting conditions are invoked. The low Pt content of MORB also limits the Pt/Os ratio of recycled crust, requiring plume models to resort to interaction with the outer core to explain suprachondritic 186Os/ 188Os ratios. Generation of the sources of intraplate volcanism by mixing of sediment into the mantle similarly requires high amounts (30–50%) of crustal materials. Non-equilibrium melting regimes can be avoided when the sources for intraplate volcanism are formed by metasomatic processes at shallow level in the mantle. Wetspot compositions formed by fluid metasomatism at convergent margins could produce the range of 186Os/ 188Os– 187Os/ 188Os signatures in intraplate volcanism over 2 Gyrs if the fluid was derived principally from the slab sediment component and the Pt/Os and Re/Os ratios of the hydrous peridotite remained unmodified. However, the most feasible method of generating suprachondritic 186Os/ 188Os– 187Os/ 188Os is from partitioning of platinum group elements into pyroxenites precipitated from Mg-rich melts. Isotopic compositions of 186Os/ 188Os=0.119850, γOs=+10 could be generated from depleted mantle compositions in 150 Myrs in an oceanic perisphere domain containing 60–70% pyroxenite veins, or in a continental mantle section containing as little as 15% pyroxenite over 2 Gyrs. Osmium isotope systematics therefore do not prove the mantle plume over alternative models, as illustrated for the Columbia River Basalt Group, where 187Os/ 188Os ratios of up to 0.144 in the least crustally contaminated basalts can be explained with a source containing 15–35% Mesozoic pyroxenite.