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

英质岩浆是大陆地壳的主要组成部分，也是地球上经常发生的最具爆炸性的火山喷发的来源。长英质岩浆的水浓度和氧化程度是影响其演化和喷发行为的主要因素。在这项研究中，同步光谱学将用于探索斜长石晶体保存有关氧化状态和岩浆水浓度信息的可能性。将测量名义上无水斜长石中的Fe 3+/G Fe和H2浓度，并将它们分别用作产生晶体的岩浆的氧逸度和水浓度的代用物。斜长石晶体中铁氧化态的同步加速器微xanes （x射线吸收近边结构）光谱是基于测量与氧结合的铁离子和铁离子之间阳离子/氧键的小能量差异。同步加速器微傅里叶变换红外光谱，像传统的傅里叶变换红外光谱一样，提供了吸收光的波长与矿物中化学键的性质之间的相关性。斜长石中H2的同步加速器微红外分析将利用非常明亮的光源，消除了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Andean Altiplano Puna火山复合体的Lower Purico火成岩（~1 Ma）上进行测试，其中最大岩浆水浓度的独立测量已经由SIMS和FTIR确定，氧逸度的测定已经由deSilva（1989, 1991）和Schmitt等人（1998）根据结晶温度和矿物平衡进行。除了测试这些新技术外，本研究的目标还包括深入了解氧逸度、水浓度、矿物组合以及在大范围地质时间内硅质岩浆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.