Studies of magmatism by trace element partitioning between clinopyroxene and silicate melt, uranium-series disequilibria in lavas from subduction zones, and non-traditional stable isotopes.

摘要

Magmatism is important for generating Earth's crust, controlling the chemical evolution of the crust and mantle, causing significant natural hazards, and forming concentrated ore deposits. However, there remain many important questions about magmatic processes that require better constraint. Such issues include how trace elements are fractionated during magmatism, the time-scale of magmatism, and the role of a temperature gradient in magma differentiation and the plausible evidence to fingerprint this differentiation process.; These are the three foci of this dissertation. In chapter 2, I report results of experimentally investigating the effects of melt structure on the partitioning of rare earth elements (REE) between clinopyroxene and SiO2-Al2O3-alkali rich melts. I observed that heavy and middle REEs are compatible in clinopyroxene and a good relationship between clinopryoxene/meltDREE and the proportion of Ca2+ in network modifier cations in melt. This indicates that proportion of Ca2+ is an important parameter in controlling partitioning of REEs between mineral and SiO2-rich melt.; In chapter 3, I determine the U-series disequilibria of Kick'em Jenny submarine volcano in the Southern Lesser Antilles. My results reinforce the previously observed global negative correlation between 231Pa excess and subduction rate. I quantitatively model this trend by combining in-growth melting models with a physical model whereby melting rate could be a linear function of subduction rate. In chapter 4, I apply a numerical model of simulating the effect of AFC and ageing on U-series disequilibria in young lavas. This model can reproduce the observed correlations between U-series disequilibria and fluid addition index in subduction zone lavas (e.g., 226Ra excess vs. Sr/Th), which have been used as evidence in the literature to argue that fluid addition generates some of the U-series disequilibria in subduction zones. Therefore, the results challenge previous implications of time-scales of magmatic process in convergent margins.; Finally, in chapter 5, I report results of Fe-Mg isotopic analyses of a thermal migration experiment. Significant mass-dependent Fe-Mg isotopic fractionations occur in the melt-present portion of the experiment. I conclude that the positive co-variation of Fe-Mg isotopes could be an important fingerprint of magma differentiation in a temperature gradient.