HR: 14:25h
AN: V23B-04    [Abstracts]
TI: Oxidation State of Iron in Feldspars from Felsic to Intermediate Volcanic Rocks as an Indicator of Magma Oxygen Fugacity
AU: * Low, P C
EM: low@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003 United States
AU: Dyar, M D
EM: mddyar@amherst.edu
AF: Department of Earth and Environment, Mount Holyoke College, 50 College Street, South Hadley, MA 01075 United States
AU: Seaman, S J
EM: sjs@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, 233 Morrill Science Center, Amherst, MA 01003 United States
AU: Delaney, J S
EM: jsd@rci.rutgers.edu
AF: Department of Geological Sciences, Wright Geological Laboratory, Busch Campus, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 United States
AB: The technique of synchrotron micro-XANES (X-ray-absorption near-edge spectroscopy) was used to obtain pre-edge and main-edge XANES spectra of feldspar crystals from the Bishop Tuff (California, USA), Quizapu volcano (Chile), Novarupta (Alaska, USA), and Timber Mountain (Nevada, USA). Micro-XANES was used to analyze Fe³⁺/σFe in feldspars on polished thin sections with a beam size of 10x20 μm. A feldspar-specific calibration line based on feldspars with known Fe³⁺/σFe was developed to relate the centroid energy of the Fe pre-edge envelope to Fe³⁺/σFe in feldspars. It can be used to measure Fe³⁺ and Fe²⁺ contents of unknown feldspars with an uncertainty of roughly ±6% in individual grains and <±2% in traverses within single grains. The goal of this study is to identify and empirically calibrate any correlation between Fe³⁺/σFe in feldspar, and the oxygen fugacity of the melt from which the feldspar crystallized. Feldspar was chosen as the subject of this study because of its ubiquity in crustal igneous rocks across a large compositional range. Traverses across phenocrysts indicate that most commonly in dacitic to rhyodacitic rocks, plagioclase feldspar records decreasing Fe³⁺/σFe as crystallization proceeds, and that later populations of feldspar crystals record lower Fe³⁺/σFe than earlier populations. This result is likely to reflect the effect of co-precipitating Fe-Ti oxides, typically magnetite, in melts of these compositions, which would drive the melt to less oxidizing conditions as crystallization proceeds. The feldspar crystals analyzed in this study consistently show a direct positive correlation between oxygen fugacity, typically independently determined on the basis of Fe-Ti oxide equilibrium, and Fe³⁺/σFe determined by micro-XANES. However, data from many more samples will be needed to establish a quantitative correlation between Fe³⁺/σFe and oxygen fugacity.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005