Geochronology and isotope transport systematics in a subsurface granite from the Larderello-Travale geothermal system (Italy)

A monzogranite, here referred to as the Montieri pluton, was encountered along two depth profiles from ca. 2 to 4 km depth in wells of the Larderello-Travale geothermal system, Italy. We obtained high-resolution petrographic and microchemical data on seven fresh samples of the Montieri pluton. These data are required to correctly interpret the combined isotope data for the Rb-Sr and K-Ar systems. Biotite and K-feldspar from all samples were analyzed by Rb-Sr and 39Ar-40Ar. Rb-Sr analyses gave extremely variable two-point Rb-Sr apparent ages and Sr initials, indicating that biotite did not equilibrate with feldspar. Bulk dissolution of K-feldspars does not define a Rb-Sr alignment that could have proved a common origin of all K-feldspar samples from a single magma batch. The leachable fractions of K-feldspar separates have much more homogeneous 87Sr /86Sr ratios around 0.712, which may reflect the isotopic composition of a late-stage circulating fluid. Because of the anatectic origin of the granites from the Larderello-Travale geothermal system, it is likely that every K-feldspar separate contains at least three isotopically distinct feldspar generations: relics of the Hercynian gneiss basement, Pliocene magmatic minerals, and hydrothermal retrogression products. Such heterogeneity can be confirmed and quantified by petrographical observations and electron microprobe analyses. The 39Ar-40Ar age spectra of the biotite samples show some internal discordance. Because of deviations from stoichiometry, biotite discordance can be attributed both to chlorite intergrowths, as predictable from the active fluid circulation, and to the presence of multiple biotite generations, such as have already been documented from other micas from the Larderello-Travale geothermal system and as indeed confirmed by high-resolution petrography. Total K-Ar biotite ages on cluster around 3 Ma, and broadly agree with a Rb-Sr age obtained by regressing only the biotite analyses. This suggests that the Montieri pluton, whose known volume is at least 25 km3, was emplaced at that time. The K-feldspar spectra are strongly discordant; no step age approaches the zero-age which would be predicted by Ar diffusivity modelling. The extraneous Ar in the feldspars is not excess Ar, but inherited Ar instead. Ar inheritance in the feldspar separates correlates with inheritance of Sr. Arrhenius trajectories of the apparent Ar diffusivities for all seven feldspars are astonishingly identical, despite the heterochemism amongst samples. However, the apparent diffusivity calculated from the stepheating experiments lead to irreproducible models for the thermal history along the depth profile, as well as time-scales shorter than the historical record. This indicates that Ar diffusivities calculated from laboratory experiments must not be extrapolated to geological conditions.