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

Bulle, Florian; Rubatto, Daniela; Ruggieri, Giovanni; Luisier, Cindy; Villa, Igor M.; Baumgartner, Lukas (2020). Episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the Larderello-Travale Geothermal Field, Italy. Chemical geology, 532, p. 119288. Elsevier 10.1016/j.chemgeo.2019.119288

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Microscale oxygen isotope analysis (18O/16O) 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 intra- and inter-grain white mica δ18O values between 1–14 ‰ reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180–450 °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-δ18O values, and (2) meteoric fluids with δ18O values of –7 ‰ during a post-intrusive, late hydrothermal stage.
Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (μm-scale), including detrital grains (δ18O = 12–14 ‰; high Na, low Mg), partially altered white mica (δ18O = 8–9 ‰) and late hydrothermal white mica (1–6 ‰; low Na, mid Mg). The late hydrothermal white mica has similar δ18O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ18O of –6 to 0.5 ‰, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous δ18O values of ca. 9 ‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in δ18O from 2 to 12 ‰ within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300 °C, present day temperature; hereafter p.d.T.). This contrasting δ18O 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 δ18O of 10 ± 0.6 ‰, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in δ18O from 4–10 ‰. A pronounced intra-grain δ18O variability of up to 6 ‰ 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 δ18O values of 2.6 ‰ down from 10 ‰ 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 μm-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350 °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 δ18O white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation.

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