Helium-Carbon Dioxide Relationships in Mid-Ocean Ridge Basalts and Upper Mantle Geodynamics

P.I.：Michael Proposal Number：98-11161机构：塔尔萨大学标题：大洋中脊玄武岩和上地幔地球动力学中的氦-二氧化碳关系随着地质时期的推移，碳和水通过洋壳和沉积物的俯冲循环，可能是大量地幔深处的循环。相比之下，大洋岩石的氦同位素系统学支持地幔热柱来自地球深处的假设，这些地区相对没有脱除原始挥发性物质的气态。因此，火山活动期间释放的主要挥发物，如二氧化碳和水，在多大程度上是年轻的，而不是被板块构造循环利用的，这是地球化学中的一个基本问题。氦的地球动力学循环与碳的地球动力学循环密切相关。大洋中脊系统的3He/4He比值存在系统的地理变化，但其确切原因并不总是被很好地理解。在某些情况下，它们可能与来自深部地幔的高3He/4He物质的局部上涌有关。在其他情况下，它们可能是由于U和Th的放射性衰变导致的~4He加成的差异。原则上，可以使用对二氧化碳/氦比变化的准确了解来区分这些可能性，但这种方法的充分发展取决于对地幔二氧化碳/氦比细微尺度变化的更好定义。本研究的目的是通过对大洋中脊玄武岩的测量，准确地确定大西洋、太平洋和印度洋下上地幔区域的3He/4He和CO2/He比值。

P.I.: MichaelProposal Number: 98-11161 Institution: University of TulsaTitle: Helium-carbon dioxide relationships in mid-ocean ridge basalts and upper mantle geodynamicsCarbon and water have been recycled over geologic time through subduction of oceanic crust plus sediments, possibly in significant quantities to the deep mantle. In contrast, helium isotope systematics of oceanic rocks support the hypothesis of mantle plumes rising from regions deep in the Earth which are relatively undegassed of their primordial volatile inventory. The extent to which major volatiles released during volcanism, such as carbon dioxide and water, are juvenile rather than recycled by plate tectonics is therefore a fundamental question in geochemistry. The geodynamic cycle of helium is closely linked to that of carbon. There are systematic geographic variations in 3He/4He ratio along the mid-ocean ridge system, but their exact cause is not always well understood. In some cases they may be tied to localized upwelling of high 3He/4He material from the deep mantle. In other cases they may be due to differences in the addition of 4He from the radioactive decay of U and Th. In principle, an accurate knowledge of variations in the carbon dioxide/helium ration cna be used to distinguish between these possibilities, but the full development of such an approach depends on a better definition of fine-scale variations in the mantle CO2/He ratio. The purpose of this study is to accurately determine the 3He/4He and CO2/He rations for upper mantle regions beneath the Atlantic, Pacific and Indian Ocean from measurements on mid-ocean ridge basalts.