Iron in Plagioclase: Synthesis Experiments with Applications to Lunar Reflectance Spectroscopy
Abstract
Plagioclase is the most abundant mineral on the Moon’s surface, comprising >90% of the anorthositic rocks that dominate the crust as primary crystallization products of the lunar magma ocean. Understanding the compositional variability of plagioclase across the lunar surface may inform various aspects of the Moon’s evolution, such as the extent of homogeneity of magma ocean crystallization, or the effect of low-grade metamorphism on mineral chemistry during prolonged cooling. Information about plagioclase compositions can be obtained remotely from near-infrared reflectance (NIR) data, which is sensitive to the particular coordination environment of Fe2+ cations in the mineral structure. As high spatial and spectral resolution NIR data have only recently begun to come available for the Moon, robust laboratory characterizations of the compositional controls on the optical properties of plagioclase are now necessary. Here we present preliminary results of experiments to synthesize various plagioclase compositions under lunar conditions. Powdered synthetic anorthite and albite glasses were used as endmember starting materials, and mixed in varying proportions with controlled amounts of Fe2O3. The mixtures were homogenized by melting at 1550C in a platinum capsule, and quenched in air after 2 hours. Samples were then sintered at 1400C for at least 24 hours in a gas-mixing furnace (CO/CO2) at a pO2~2^-10. Electron microprobe analyses confirm that the anorthite and albite starting endmembers can be combined and homogenized with sufficient accuracy to generate well-controlled plagioclase compositions, and that the sintering process allows the Fe2+ to be incorporated into the plagioclase homogenously. No contaminating phases have been observed. While previous laboratory and remotely-sensed NIR reflectance data typically show only one prominent Fe2+ absorption band (near 1250 nm) [e.g. Adams and Goullaud, 1978], the spectra of samples presented here display an additional major absorption centered at ~2000 nm. Both absorptions are present in spectra of samples with varying Na content (anorthite and bytownite compositions), diffusion time (one day and one week), and cooling rate (1 and 8 degrees C per minute). These results are consistent with Fe2+ cations being distributed between two coordination environments in plagioclase [e.g., Appleman et al., 1971; Hofmeister and Rossman, 1984]. Future spectral analyses of these samples will provide more quantitative constraints on the relative distribution of Fe2+ within the mineral structure. Additionally, we have begun a set of experiments to more closely examine the role of plagioclase bulk FeO content in directly controlling the relative strengths of the two prominent absorption bands. To do this, we are synthesizing a series of plagioclases with constant An# that have varying FeO contents. The results of this systematic investigation are intended to provide a basis for extracting compositional information from remote measurements of plagioclase on the lunar surface.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V21E2370C
- Keywords:
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- 1042 GEOCHEMISTRY / Mineral and crystal chemistry;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology;
- 3934 MINERAL PHYSICS / Optical;
- infrared;
- and Raman spectroscopy;
- 5464 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Remote sensing