FIRST NANOSIMS ANALYSIS OF NANOGRANITES: THE WATER CONTENT OF GRANITIC MAGMA EMBRYOS

Formation, extraction and ascent of hydrous granitic magmas to upper crustal levels represent the most important mechanisms for the reworking of the Earth’s continental crust. In this scenario, water is of prime importance in the formation and evolution of granitic magmas. However, water quantification in natural granitic systems has been an on-going challenge in granite petrology. The primary difficulty stems from water exsolution and diffusion from cooling magmas.

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. Because these MI correspond to the first batches of melt, whose subsequent segregation, redistribution and accumulation out from the source regions result in the formation of granitic bodies at shallow crustal levels, they represent the embryos of the upper-crustal granitic magmas. The novel and intriguing approach resulting from the combination of nanogranites and NanoSIMS permits in situ measurements of the water content for any specific granitic melt produced in the partially melted Earth’s continental crust.

We measured water contents in the range 4.7-9.8 wt% for low-temperature (700 °C) granitic melts produced in the anatectic metasedimentary crust at Ronda (S Spain). We demonstrated that MI reflect, when the entire population from the same host is considered, the water contents of the specific fraction of melt trapped by growing peritectic phase. Our study has documented for the first time the occurrence of water content heterogeneities of the anatectic melts at the source region. The most likely explanation is that the birth of granitic magmas may occur under conditions of “mosaic equilibrium”. Despite the volume factor between granites and their embryos may exceed twenty-five orders of magnitude, in some particular circumstances the water content of upper-crustal granitic magmas can be inferred directly by their embryos.

These results confirm the need for small-scale geochemical studies on natural samples to improve our quantitative understanding of crustal melting and formation of silicic magmas.