An occurrence of metastable cristobalite in spodumene-hosted crystal-rich inclusions from Jiajika pegmatite deposit, China

Abstract

Laser Raman spectroscopic analyses showed the occurrence of cristobalite, together with carbonic aqueous fluid phases, in the crystal-rich inclusions (CIs) hosted in spodumene in Jiajika pegmatite deposit, China. In these CIs, cristobalite always coexists with zabuyelite, and occasionally with other minerals, including calcite, spodumene, and possibly cookeite. The rare occurrences of cristobalite in plutonic rocks are commonly considered to be resulted from the lack of H2O, which was thought to be the catalyzer for the conversion of cristobalite to quartz. However, this assumption is not supported by our observed coexistence of cristobalite with aqueous fluid in the CIs of Jiajika pegmatite deposit. The results of microthermometric analyses showed that the carbonic aqueous fluids in CIs had salinities of less than 7wt.% NaCl equivalent, in which the CO2 phase(s) occupied 60–80vol.% with the bulk CO2 densities of near 0.70g/cm3. Therefore, the estimated internal pressure of CIs are between 200 and 290MPa at the initial dissolving temperature of daughter minerals (cristobalite at ~400°C), much lower than the entrapment pressures of at least 440MPa. The occurrences of cristobalite in CIs could be ascribed to (1) the pressure decrease within CIs from the entrapment pressures of greater than 440MPa to 200–290MPa at ~400°C, mainly resulted from the crystallization of zabuyelite, and (2) the extension of the cristobalite stability to low temperatures due to the presence of lithium and H2O in the inclusions. More study is needed for the formation mechanisms of cristobalite in plutonic rocks and the effect of H2O on the conversion of cristobalite to quartz.