Nanogranite inclusions in migmatitic garnet: behavior during piston-cylinder remelting experiments

Nanogranites represent totally crystallized inclusions of anatectic melt trapped within peritectic minerals of migmatites and granulites. They have recently been discovered in several locations. This discovery opens new possibilities for investigating crustal melting processes, provided that an appropriate method for retrieving the information contained within nanogranite inclusions is available. Here, we describe a series of remelting experiments that have been performed at different temperatures and under dry, and H2O-added, conditions on nanogranite inclusions hosted in migmatitic garnet, using a piston-cylinder apparatus. The glasses obtained by quenching the sample from temperature that approaches the trapping temperature have compositions very similar to those of preserved glassy inclusions coexisting with nanogranites in the same cluster. No significant differences in H2O contents were observed for nanogranites rehomogenized under dry and wet conditions. Higher (50-100 degrees C) experimental temperatures resulted in dissolution of the host into the melt and inclusion decrepitation with the loss of volatiles. Therefore, piston-cylinder remelting experiments may eliminate inclusion decrepitation, maintaining the primary fluid contents in the originally trapped melt. These volatiles would otherwise be lost during remelting experiments at ambient pressure. By preventing volatile loss, the inclusion does not have to be overheated to achieve homogenization, and the compositions of quenched glasses so obtained can be assumed to be those of melts produced (and trapped as inclusions) during crustal anatexis. The experimental approach described here represents a promising technique for the successful rehomogenization of crystallized melt inclusions from high-pressure environments, such as the mafic continental crust.