Characterizing the Importance of Mafic Sources for Mantle-Derived Magmatism

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

EAR-0106475 Carlson This study is examining the importance of mafic (pyroxene/garnet/mica dominated rock types) as opposed to peridotitic (olivine-dominated rock types) sources for mantle-derived magmas. Though generally present only in volumetrically small amounts compared to peridotite, mafic material in the mantle potentially can contain a large fraction of the mantle's incompatible elements, including major elements such as sodium, iron, calcium and aluminum. Consequently, mafic components in the mantle can influence the composition of melts produced in the mantle to a degree disproportionate to their volume. Given that the incompatible element abundances in melts are used to infer mantle properties such as temperature, bulk-composition, and geodynamic history, the participation of mafic material in the melting process may lead to erroneous conclusions regarding mantle properties in models where only peridotitic sources are considered. Detecting the contribution of mafic source components is difficult because many of the properties of mantle melts will be influenced by the predominance of peridotitic wall rock. Commonly used radiogenic isotope systems such as rubidium-strontium, samarium-neodymium and uranium-lead are not sensitive indicators of a mafic source since both mafic and peridotitic materials can have similar parent daughter ratios for these systems. In contrast, the ratio of rhenium to osmium is dramatically different between mafic and peridotitic materials, which makes this radiometric system very sensitive to reflecting the abundance of mafic material in the source of a mantle derived melt. In addition, when a mafic source contribution is detected, the lutetium-hafnium isotopic system potentially is capable of discriminating between a mafic source produced by shallow (spinel stability) melting as opposed to deep (garnet stability) melting. In order to determine whether or not mafic source materials contribute significantly to mantle derived magmas, this study is examining the osmium and hafnium isotopic composition of mafic-alkalic magmas erupted in a variety of tectonic settings in the western US, Italy, Brazil, and Siberia. These compositions represent small volume melts of the mantle that may preferentially sample a mantle mafic component. Defining the chemical characteristics in these small volume melts that can be attributed to mafic sources will allow these signatures to be more unambiguously identified in larger volume magmas (ocean ridge, ocean island, and continental flood basalts) where the percentage of melt from a mafic source may be small in comparison to melt from surrounding peridotite. The study will also produce radiogenic isotope data for mafic xenoliths from the mantle beneath southern Africa and the western US to better define the expected range of isotopic compositions of mafic material in the mantle. Understanding the compositional characteristics of the source materials of mantle-derived magmas is critical to their use as tracers of mantle temperature variations, compositional heterogeneity in the mantle, and the geodynamics of mantle convection and mixing.