Testing Models for the Origin of 186Os/188Os and 187Os/188Os Isotope Variations in the Mantle: Core Signal, Recycled Components, or Intra-mantle Differentiation

University Of Texas At Austin, Austin TX

Investigators

Abstract

This project seeks to determine the physical and chemical process(es) responsible for generating variations in the osmium isotopic compositions of mantle-derived rocks. Previous studies havesuggested that some osmium isotope variations in lavas from Hawaii and Gorgona may reflect chemical interaction between the Earth?s mantle and core. If confirmed, this would have wide ranging implications for the chemical and thermal evolution of Earth?s interior. Testing this hypothesis through detailed study is therefore of high importance for the geologic community. Osmium isotopes are affected by the radioactive decay of rhenium and platinum, and so osmium isotope variations in geologic materials reflect long-term fractionation of the platinum/osmium and rhenium/osmium ratios in different portions of the Earth's interior. Several different processes in addition to core/mantle interaction can affect the osmium isotope compositions of different mantle reservoirs. These processes include recycling of oceanic crust and sediments into the mantle through plate tectonics, ancient mantle melting and melt extraction, and melting of distinct rock types in the mantle during melt generation today. Therefore, osmium isotope variations can provide important constraints on several geologic processes that have helped shape the chemical and physical evolution of Earth?s interior, but only if the different mechanisms for generating these isotopic variations can be distinguished. This study will examine correlations between Osmium isotopes and other geochemical tracers in lavas from Hawaii and Gorgona Island, thought to be related to deep-seated mantle plumes, and in several suites of mantle-derived peridotites that span a range in composition. These analyses will allow the supported researchers to test and confirm or refute several hypotheses that have been proposed to explain unusual enrichments in 186Os (an isotope generated by the decay of 190Pt) in some plume-derived lavas. By combining high-precision 186Os/188Os data with complementary isotopic, major, and trace element data, the supported research will test several proposed mechanisms for generation of elevated 186Os/188Os ratios in mantle materials. The selected lava suites are chosen because they either have been reported to possess elevated 186Os/188Os in earlier studies (Hawaiian Islands, Gorgona), or because they possess geochemical characteristics that suggest a significant role for pyroxenite or eclogite melting in their generation (Australs HIMU lavas). The peridotite samples will help constrain the long-term coupling (or lack thereof) between Pt/Os and Re/Os ratios in the upper mantle, and illuminate the role that melt depletion plays in the 190Pt-186Os evolution of the upper mantle. Combined, this study will place stronger constraints on the different models proposed for the origin of Pt/Os fractionation and 186Os/188Os variation in the mantle than is currently possible, thereby addressing an important and longstanding controversy in the Geological Sciences and increasing the utility of the 190Pt-186Os decay system in future geochemical studies.