HR: 09:00h
AN: V41D-05    [Abstracts]
TI: Fe$^{3+}$-in-Clinopyroxene Oxybarometry: Effects of Na on Iron Partitioning Behavior
AU: * McCanta, M C
EM: McCanta@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058
AU: Dyar, M D
AF: Mount Holyoke College, 50 College St, S. Hadley, MA 01075
AU: Delaney, J S
AF: Rutgers University, Busch Campus, Piscataway, NJ 08855
AB: Traditional methods for quantifying magmatic oxygen fugacity (fO2), such as coexisting Fe-Ti oxides, are not applicable to many natural systems. Therefore, to understand the fO2 variations that exist among magmatic suites, other oxybarometers must be developed to measure the fO2 of these magmas. It is well known that iron is the only major element to occur in multiple valence states, primarily as a function of fO2. We have used synchrotron microXANES methods to investigate the concentration of Fe$^{3+}$ in clinopyroxene (augite) as a potential oxybarometer for Martian magmas, for which determination of fO2 using established methods is difficult. A series of 1-atm, gas mixing experiments were run to examine the relationship between fO2 and the Fe$^{3+}$ content of clinopyroxene. The original starting composition was an SNC-type basaltic melt consisting of CMAS+Fe with liquidus augite. Experiments were conducted over a range of fO2 values from IW+4 to IW-2. Initial results suggest that at fO2 values $<$ QFM, insignificant amounts of Fe$^{3+}$ ($<$ 10%) are present in basaltic melts to be incorporated into augite at measurable levels. At higher fO2 values, the concentration of Fe$^{3+}$, both in the melt and coexisting augite, increases systematically with fO2, indicating the usefulness of this oxybarometer in high-fO2 magmas. New experiments have been run on a starting composition CMAS+Fe+Na to determine the effect of Na on the partitioning of Fe$^{3+}$ into clinopyroxene. Coupled substitution of Na and Fe$^{3+}$ for Ca and Mg or Fe$^{2+}$ is a dominant reaction in Na-bearing pyroxenes. Therefore, the availability of Na should have a significant effect on clinopyroxene Fe$^{3+}$ uptake. Our experiments have shown that indeed the addition of Na to the starting composition increases the amount of Fe$^{3+}$ incorporated into clinopyroxene. This effect, while relatively small, appears greatest under low fO2 conditions. It is possible that under high fO2 conditions, with more Fe$^{3+}$ present in the melt, the coupled substitution described above is not as important as the direct substitution of Fe$^{3+}$-Al$^{3+}$. The experiments demonstrate the potential use of the Fe$^{3+}$ content of clinopyroxene as an oxybarometer and indicate that the presence of Na has a measurable effect on Fe$^{3+}$ partitioning. Due to the chemical complexity of natural systems, the effect of Na on clinopyroxene Fe$^{3+}$ partitioning should not be dismissed when applying this oxybarometer.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting