The H2O content of granite embryos

Water quantification in natural granitic systems has been an on-going challenge owing to its extremely fugitive character from melts and magmas during cooling and ascent. Here we approach this problem studying granites in their source region and, making use of considerable advances in secondary ion mass spectrometry, we present the first NanoSIMS analyses on (remelted) nanogranites. Nanogranite inclusions are totally crystallized melt inclusions (MI) hosted in peritectic phases of anatectic rocks, and represent the embryos of the upper-crustal S-type granites. We measure water contents of 6.5±1.4 wt% for low-temperature, low-pressure (~700 °C, ~5 kbar) granitic melts produced in the anatectic metasedimentary crust at Ronda (S Spain). We demonstrate that MI reflect, when the entire population from the same host is considered, the water content of melt in the specific volume of rock surrounding the growing peritectic host. Mean water values for the selected peritectic garnets range from 5.4 to 9.1 wt%. This range is in rather good agreement with experimental models for granitic melts at the inferred P-T conditions. Our study, however, provides an unprecedented natural snapshot of the partially-melted continental crust, and documents for the first time the occurrence of water content heterogeneities of the anatectic melts at the source region. These heterogeneities are interpreted to reflect the birth of granitic melts under conditions of mosaic equilibrium, where the distinct fractions of melt experience different buffering assemblages –i.e. different compositional microdomains in the source rock induce changes to melting reaction stoichiometry at the microscale. Peritectic phase, in turn, seems to play a fundamental role in recording and preserving the pristine geochemical signatures of crustal melts at the source region. These results confirm the need for small-scale geochemical studies on natural samples to improve our quantitative understanding of crustal melting