Silica solid solution and zoning in natural plagioclase

Abstract

Microprobe analyses of plagioclase from some lunar basalts have anomalous stoichiometry as compared to terrestrial feldspars. The anomaly indicates the presence of the "excess silica" substitution, [ ]Si_4O_8. This coupled substitution leaves a vacancy in the feldspar lattice and is effectively SiO_2, dissolved in the feldspar. Detailed study indicates that core-rim [ ]Si_4O_8, zoning is a common phenomenon in plagioclase from lunar basalts. Anorthite-albite zoning is also typically present, so plots of [ ]Si_4O_8 as a function of anorthite content provide a convenient measure of the [ ]Si_4O_8 variations present. Most lunar basaltic plagioclase shows a monotonic increase in [ ]Si_4O_8 with albite content, typically ranging from 0 mole percent [ ]Si_4O_8 in the cores to 5-7 percent in the rims. This zoning is produced by two cooperating processes: a falling temperature of formation, and growth from a progressively siliceous liquid. In addition, rapid growth and delayed nucleation of plagioclase and silica phase appear to be important to the disequilibrium incorporation of [ ]Si_4O_8. By contrast, plagioclase from a wide variety of terrestrial occurrences lacks [ ]Si_4O_8. The Absence of [ ]Si_4O_8 in many terrestrial igneous plagioclases is primarily related to the presence of H_2O, which decreases the liquidus temperatures relative to those in lunar rocks. Terrestrial basalts which have comparable crystallization temperatures to their lunar counterparts lack [ ]Si_4O_8 in part because of an absence of strong in situ differentiation. In addition, plagioclase typically crystallizes prior to pyroxene (the reverse of the mare basalts), so kinetic effects have a reduced role. If a melt is in equilibrium with cristobalite (or tridymite or quartz), the silica activity is buffered, and the [ ]Si_4O_8, content of the plagioclase is directly related to the temperature. [ ]Si_4O_8, incorporation, however, is typically not an equilibrium process, so there is little hope of using it directly as a geothermometer.