Microthermometric behavior of crystal-rich inclusions in spodumene under confining pressure

Abstract

Abstract A hydrothermal diamond anvil cell (HDAC) was used to observe the microthermometric behavior of solid + liquid + vapor inclusions in spodumene from the Tanco pegmatite, Manitoba, under confining pressure. At 25 °C, these inclusions commonly contain a carbonate mineral (zabuyelite, rarely calcite or nahcolite), quartz, a phyllosilicate (cookeite), and an aqueous carbonic fluid phase. Heating spodumene-hosted inclusions to temperatures between 600 and 680 °C in a HDAC resulted in total or partial dissolution of the contained solid phases, followed by homoepitaxial growth of new spodumene on the inclusion walls, which reduced the inclusion volume by up to 31%. At room temperature, the homogenized inclusions contain only an aqueous fluid phase, CO2 liquid, and CO2 vapor. Inclusions that failed to homogenize at 680 °C, or leaked during heating, contain partially dissolved minerals with or without an aqueous carbonic fluid. The volume of spodumene formed within an inclusion during experimental re-heating, as determined by the difference in inclusion size before and after total dissolution of the contained solid phases, was used to estimate the volume of zabuyelite, quartz/cristobalite, and cookeite produced by the reaction The relative volumes of the calculated reaction products approximate the proportions of zabuyelite, quartz/cristobalite, and cookeite in inclusions prior to heating. The absence of silicate glass in the quenched homogenized inclusions indicates that they do not represent the crystallized products of an entrapped hydrous silicate melt that wetted the surface of spodumene during its growth. Large changes in inclusion volume and composition during experimental re-heating shows that the inclusions are neither isochoric nor isoplethic systems and as such are unsuitable for estimating the P-T conditions of trapping. Readers should therefore exercise caution when using thermobaric estimates of pegmatite crystallization inferred from microthermometric measurements of presumably primary melt inclusions in spodumene.