Episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the Larderello-Travale Geothermal Field, Italy

Microscale oxygen isotope analysis (O-18/O-16) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the Larderello-Travale geothermal field (LTGF), Italy, record stages of magmatic crystallization, metamorphic-hydrothermal replacement and fluid-rock interaction. The large range in infra- and intergrain white mica delta O-18 values between 1 and 14 parts per thousand reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180-450 degrees C present-day temperatures; p.d.T.). This variability reflects the large-scale circulation of both (1) magmatic, syn-intrusive to early contact metamorphic hydrothermal fluids with high-delta O-18 values, and (2) meteoric fluids with delta O-18 values of - 7 parts per thousand during a post-intrusive, late hydrothermal stage. Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (mu m-scale), including detrital grains (delta O-18 = 12-14 parts per thousand; high Na, low Mg), partially altered white mica (delta O-18 = 8-9 parts per thousand) and late hydrothermal white mica (1-6 parts per thousand; low Na, mid Mg). The late hydrothermal white mica has similar delta O-18 values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a delta O-18 of -6 to 0.5 parts per thousand, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous delta O-18 values of ca. 9 parts per thousand likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 degrees C), or (2) a large spread in delta O-18 from 2 to 12 parts per thousand within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300 degrees C, present day temperature; hereafter p.d.T.). This contrasting delta O-18 signature of white mica is also recorded in granite cored at up to 4.6 km depth. The Carboli granite contains white mica with a homogeneous magmatic delta O-18 of 10 +/- 0.6 parts per thousand, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in delta O-18 from 4 to 10 parts per thousand. A pronounced intragrain delta O-18 variability of up to 6 parts per thousand occurs in white mica domains with higher Fe-Mg-Ti halos around inclusions of chloritized biotite, as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4.6 km. In the Porto Azzurro granite on Elba, Italy, altered white mica has delta O-18 values of 2.6 parts per thousand down from 10 parts per thousand in unaltered grains. The distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration (at depth, sample and grain scale). Secondly, the actual preservation of these mu m-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350 degrees C where most of the heterogeneous white mica is found. Selective, sample- and grain-scale fluid penetration occurs episodically and anisotropically, on micro- and megascale, along faults, fractures and cleavages, producing lower delta O-18 white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation.