Experimental crystallization of the Macusani Obsidian, with applications to lithium-rich granitic pegmatites

Abstract

Experiments at 200 MPa with the peraluminous (S-type) rhyolite obsidian from Macusani, Peru, asses the dynamics of crystallization starting with non-vesicular solid glass cores as compared to earlier experiments starting with powders of the same composition. Textures, spatial zonation of feldspars and quartz, and their compositional relations are substantively different and more clearly revealed in the solid-core experiments. The new experiments with solid cores possess more sharply bounded segregation of feldspathic and quartz-rich domains of crystallization, a shift from a predominance of feldspars to increasing mica with initial H2O content > 6wt %, and the simultaneous crystallization of solvus pairs of plagioclase and alkali feldspar at opposite ends of the melt volume. As in other similar studies, the maximum in the rate of crystallization, a function principally of crystal growth rate, occurs at a liquidus undercooling of ~200 ± 50°C. Both experimental studies with the Macusani obsidian apply to the chemical, textural, and spatial zonation of minerals within granitic pegmatites, particularly the Li-rich peraluminous pegmatites of S-type granite affinity. The new results have now reproduced and can account for the following features of pegmatites: (1) feldspathic outer zones and quartz-rich to pure quartz cores; (2) massive fine-grained border zones, followed by coarsening wall zones with unidirectional solidification texture, culminating in central domains of more isotropic fabric and coarse grain size; (3) locally, alternating laminations of mineral assemblages as in layered pegmatites and layered aplites; (4) a steady decrease in the An content of plagioclase from margin to core via subsolidus isothermal fractional crystallization; (5) spatial segregation of plagioclase and alkali feldspar along opposite margins of the melt body via far-field chemical diffusion; (6) an inward increase in the size of individual crystals by ~102; (7) albite+lepidolite bodies as the latest primary assemblage, and following the crystallization of pure quartz bodies. All of these experimental results followed from the appreciable undercooling of the melt prior to the onset of crystallization. All of the features cited, except for the formation of miarolitic cavities, are entirely igneous in origin, owing nothing to the simultaneous occurrence of an aqueous solution.