Characterizing the Importance of Mafic Sources for Mantle-Derived Magmatism

Carlson这项研究正在研究镁铁质（辉石/石榴石/云母为主的岩石类型）相对于橄榄岩（橄榄石为主的岩石类型）来源于幔源岩浆的重要性。 虽然与橄榄岩相比，镁铁质物质通常只存在少量的体积，但地幔中的镁铁质物质可能含有大部分地幔的不相容元素，包括钠，铁，钙和铝等主要元素。 因此，地幔中的镁铁质成分可以影响地幔中产生的熔体的组成，其影响程度与其体积不成比例。 鉴于熔体中不相容元素丰度可用于推断地幔性质，如温度、整体成分和地球动力学历史，在只考虑橄榄岩源的模型中，镁铁质物质参与熔融过程可能导致关于地幔性质的错误结论。检测镁铁质源组分的贡献是困难的，因为地幔熔体的许多性质将受到以橄榄岩围岩为主。 常用的放射性同位素系统，如铷-锶，钐-钕和铀-铅不是镁铁质来源的敏感指标，因为镁铁质和橄榄岩材料可以有类似的父女儿比，这些系统。 相比之下，锇的比例是显着不同的镁铁质和橄榄岩的材料，这使得这个放射性系统非常敏感，反映丰富的镁铁质物质的来源，地幔衍生熔体。 此外，当检测到镁铁质源贡献时，镥-铪同位素系统潜在地能够区分由浅源产生的镁铁质源和浅源产生的镁铁质源。（尖晶石稳定性）熔融，而不是深熔（石榴石稳定性）熔融。为了确定镁铁质源物质是否对幔源岩浆有显著贡献，这项研究是检验在美国西部、意大利、巴西和西伯利亚的各种构造环境中喷发的镁铁质-碱性岩浆的锇和铪同位素组成。这些组合物代表小体积的地幔熔体，可能会优先采样地幔镁铁质成分。 定义这些小体积熔体的化学特性，可以归因于镁铁质来源，将允许这些签名更明确地确定在较大的体积岩浆（洋脊，海洋岛屿，大陆溢流玄武岩），从镁铁质来源的熔体的百分比可能是小的相比，从周围的橄榄岩熔体。 这项研究还将产生来自南部非洲和美国西部地幔的镁铁质捕虏体的放射性同位素数据，以更好地确定地幔中镁铁质物质的同位素组成的预期范围。 了解幔源岩浆源物质的成分特征，对于它们作为地幔温度变化、地幔成分不均匀性以及地幔对流和混合的地球动力学示踪剂的应用至关重要。

EAR-0106475CarlsonThis 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.