Characterizing the Importance of Mafic Sources for Mantle-Derived Magmatism

这项研究正在检验基性岩（辉石/石榴石/云母为主的岩石类型）与橄榄岩（橄榄石为主的岩石类型）对幔源岩浆的重要性。虽然与橄榄岩相比，地幔中的基性物质通常只以很小的体积存在，但它们可能含有很大一部分地幔不相容元素，包括钠、铁、钙和铝等主要元素。因此，地幔中的基性成分可以影响地幔中产生的熔体的组成，其程度与其体积不成比例。鉴于熔体中不相容元素丰度被用来推断地幔性质，如温度、总体组成和地球动力学历史，在只考虑橄榄岩源的模型中，基性物质参与熔融过程可能会导致关于地幔性质的错误结论。由于地幔熔体的许多性质将受到橄榄岩围岩优势的影响，因此检测基性源组分的贡献是困难的。常用的放射性成因同位素系统，如铷-锶、钐-钕和铀-铅不是基性来源的敏感指标，因为基性和橄榄岩材料在这些系统中可能具有相似的母-子比率。相比之下，基性物质和橄榄岩物质的铼锇比有显著差异，这使得该辐射测量系统对反映幔源熔体中基性物质的丰度非常敏感。此外，当检测到基性源贡献时，镥铪同位素系统可能能够区分由浅（尖晶石稳定性）熔融产生的基性源和由深（石榴石稳定性）熔融产生的基性源。为了确定基性源物质是否对幔源岩浆有重要贡献，本研究对美国西部、意大利、巴西和西伯利亚不同构造背景下爆发的基性碱性岩浆的锇和铪同位素组成进行了研究。这些成分代表了地幔的小体积熔体，可能优先采样地幔基性成分。确定这些可归因于基性源的小体积岩浆的化学特征，将使这些特征在更大体积的岩浆（洋脊、洋岛和大陆洪水玄武岩）中得到更明确的识别，在这些岩浆中，来自基性源的熔体的百分比可能比来自周围橄榄岩的熔体小。这项研究还将产生来自非洲南部和美国西部地幔的基性捕虏体的放射性成因同位素数据，以更好地确定地幔中基性物质同位素组成的预期范围。了解幔源岩浆源物质的组成特征对于它们作为地幔温度变化、地幔成分非均质性以及地幔对流和混合地球动力学的示踪剂至关重要。

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.