Collaborative Research: Using Osmium-Lead isotope variations in mid-ocean ridge and abyssal peridotite sulfides to understand fundamental properties of Earth's mantle

University Of Texas At Austin, Austin TX

Investigators

Abstract

Fundamental knowledge of how the Earth works and how and why continents move across its surface over time is critical for our understanding of seafloor volcanism. This movement is driven by forces deep in the Earth and are transmitted to the crust by movement and convection in the mantle. Chemical and isotopic variations in volcanic rocks, generated along Earth's mid-ocean ridge system provide insights into the composition and evolution of Earth's mantle. Abyssal peridotites, direct samples of the mantle, are occasionally exposed though tectonic processes along the mid-ocean ridge. These rocks are a complement to the composition of Earth's mantle provided by mid-ocean ridge lavas. Recent studies have revealed discrepancies in the picture of mantle composition and evolution painted by these two different types of rocks. The discrepancy likely results from the failure of current models to account for complexities in the processes by which melts are generated and extracted from the mantle. This research tests the hypothesis that the differences between these two independent sources of information can be explained by the preferential melting of chemically distinct veins in the mantle and the chemical interaction of these enriched melts with the surrounding mantle material during the ascent of the magma to the surface. Isotopic analysis of small sulfide inclusions in both abyssal peridotites and in the lavas erupted at mid-ocean ridges can help test this "marble cake mantle" hypothesis. Results of the research will enhance our understanding of how oceanic crust, which covers nearly ¾ of the Earth's surface, forms. The project will also support the education and training of two graduate students from two Texas universities, one of which is a minority-serving institution and the other is the first Tier-1 university to be designated as Hispanic-serving. Collaboration between students and investigators at the two institutions will complement ongoing efforts to expand outreach in science to underserved communities. This research examines Osmium (Os) and Lead (Pb) isotope variations in sulfides from abyssal peridotites and mid-ocean-ridge basalts. Os-isotopes in magmatic sulfides, while slightly more radiogenic than average abyssal peridotites, overlap with values in peridotite-derived sulfides. Grain-scale Os- and Pb-isotope heterogeneity documented in many peridotites are postulated to reflect either the long-term isolation and evolution of phases with variable parent/daughter ratios or the recent metasomatism by isotopically-enriched melts. This research tests the latter hypothesis and implicates eclogite/pyroxenite melt generation as the ultimate source of radiogenic Os- and Pb-isotope signatures in both interstitial and magmatic sulfides. This work examines Os- and Pb-isotopes in interstitial and included sulfides from exceptionally fresh abyssal peridotites from the Gakkel Ridge, an ultra-slow mid-ocean ridge spreading center that exposes significant amounts of virtually unaltered mantle rock. X-ray CT imaging will be used to examine the size, spacing, and textural relationships of sulfides and other phases prior to sulfide extraction and analysis. These data will be integrated with sulfide Os-Pb analyses from Gakkel and other North Atlantic mid-ocean ridge basalts spanning a wide range in composition. Specific questions being addressed include: (1) do grain-scale Os- and Pb-isotope variations in peridotite interstitial and included sulfides reflect "internal isochrones" and long-term preservation of heterogeneities in isolated, Os- and Pb-rich phases or do they represent recent metasomatic overprinting from eclogite- or pyroxenite-derived melts percolating through the mantle; (2) do Os- and Pb-isotopes in sulfides from mid-ocean ridge basalts correlate with other petrologic or geochemical signals potentially related to pyroxenite melting (e.g., Nickel-in-olivine, Na/Ti, or other long-lived radiogenic tracers); (3) do areas of low melt productivity preferentially sample mafic components during melt generation; and (4) are there systematic differences in the Os-isotope signatures of sulfides from Gakkel Ridge basalts versus basalts from faster spreading ridge segments with higher melt productivity. A primary goal of this work is to use Os- and Pb-isotope variations in magmatic and mantle sulfides to constrain the role of lithologic heterogeneity and reactive melt transport in the generation of both mid-ocean ridge basalts and abyssal peridotites. The integration of Os- and Pb-isotope data from both sulfides from the basalts and those from abyssal peridotites with other geochemical and petrologic data from the same samples will allow the role that lithologic heterogeneity plays in mid-ocean ridge basalt generation to be determined in addition to better gauging the role of recent and ancient mantle melting and melt/rock reaction in generating chemical and isotopic variability in abyssal peridotites. Results of the work will dramatically improve our ability to use mid-ocean ridge basalt chemistry to infer the complex depletion and refertilization history of Earth's convecting upper mantle.