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

摘要

Microscale oxygen isotope analysis (~(18)O/~(16)O) 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 intergrain white mica δ~(18)O values between 1 and 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-δ~(18)O values, and (2) meteoric fluids with δ~(18)O 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 (δ~(18)O= 12-14‰; high Na, low Mg), partially altered white mica (δ~(18)O =8-9‰) and late hydrothermal white mica (1-6‰; low Na, mid Mg). The late hydrothermal white mica has similar δ~(18)O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ~(18)O 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 δ~(18)O values of ca. 9‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in δ~(18)O from 2 to 12‰ within and across grains of variable texture and chemistry in