Oxygen and Hydrogen Fugacity of the Purico Ignimbrite Magmas, Northern Chile

EAR-0001081SeamanFelsic岩浆是大陆地壳关键成分的来源，也是地球上通常发生的最具爆炸性的火山喷发的来源。影响长英质岩浆演化和喷发行为的主要因素是水的含量和氧化程度。在这项研究中，同步加速器光谱将被用来探索斜长石晶体保存有关晶体生长的岩浆的氧化状态和水浓度的信息的可能性。将测量名义上无水斜长石中Fe3/GFe和H2的浓度，并探索它们作为产生晶体的岩浆的氧逸度和水浓度的代用。斜长石晶体中铁的氧化态的同步微XANES(X射线吸收近边结构)光谱是基于测量与氧结合的亚铁离子和铁离子之间阳离子/氧键的微小能量差。与常规FTIR光谱一样，同步加速器微型FTIR(傅里叶变换红外)光谱提供了吸收的光的波长与矿物中化学键的性质之间的关联。同步加速器显微FTIR分析斜长石中的氢将利用非常明亮的光源，消除去除和抛光定向单晶的需要，并允许在其空间背景下分析晶体。这些技术将在安第斯阿尔蒂普诺-普纳火山杂岩中的下普里科褐沸岩(~1 Ma)上进行试验，SIMS和FTIR已经独立地测定了那里的最大岩浆水浓度，DeSilva(1989,1991)和Schmitt等人根据结晶温度和矿物平衡测定了氧逸度。(1998年)。除了测试这些新技术，这项研究的目标还包括深入了解氧逸度、水浓度、矿物组合和硅质岩浆在大范围地质时代的喷发方式之间的相互作用。

EAR-0001081SeamanFelsic magmas are the sources of the keystone components of continental crust, and of the most explosive volcanic eruptions that commonly occur on Earth. The major factors in the evolution and eruptive behavior of felsic magmas are their water concentrations and their degree of oxidation. In this study, synchrotron spectroscopy will be used to explore the possibility that plagioclase crystals preserve information about the oxidation state and the water concentration of the magmas from which the crystals grew. Both Fe 3+/G Fe and H2 concentrations in the nominally anhydrous plagioclase will be measured and their use as proxies, for, respectively, oxygen fugacity and water concentration of the magmas that produced the crystals, will be explored. Synchrotron microXANES (X-ray absorption near-edge structure) spectroscopy of oxidation state of Fe in plagioclase crystals is based on measuring small energy differences in cation/oxygen bonds between ferrous and ferric ions that bond to oxygen. Synchrotron microFTIR (Fourier transform infrared) spectroscopy, like conventional FTIR spectroscopy, provides a correlation between the wavelength of light absorbed and the nature of chemical bonds in the mineral. Synchrotron microFTIR analysis of H2 in plagioclase will utilize a very bright source, eliminating the need to remove and polish oriented single crystals, and allowing for the analysis of crystals in their spatial context. These techniques will be tested on the Lower Purico ignimbrite (~1 Ma) in the Andean Altiplano Puna Volcanic Complex, for which independent measurements of maximum magmatic water concentrations have already been determined by SIMS and FTIR, and oxygen fugacity determinations have been made on the basis of crystallization temperatures and mineral equilibria by deSilva (1989, 1991) and Schmitt et al. (1998). Along with testing these new techniques, goals of this study include acquiring insight into the interplay of oxygen fugacity, water concentration, mineral assemblage, and the eruption style of siliceous magmas across a large range of geologic time.