Abstract
Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.
Highlights
Aqueous fluids play a critical role during metamorphic processes in the Earth’s crust and upper mantle acting as catalysts for chemical reactions, affecting stable mineral assemblages and controlling mineral textures and element distribution at the micro scale (e.g. Erambert and Austrheim 1993; Ague 2003; Parsons and Lee 2009; Putnis 2009; Beinlich et al 2010; Jamtveit and Austrheim 2010; Putnis and Austrheim 2010)
Garnet in eclogite AV16-47 from Malone Valley shows a major element zonation with XSps decreasing from core to rim (0.12–0.01), typical of prograde metamorphism, and a patchy zoning in Ca, with XGrs decreasing toward the rim (0.40–0.30) (Fig. 2)
Major and trace element zoning in garnet combined with oxygen isotope investigation in garnet, quartz and phengite at the microscale were used to explore the nature of the multistage fluid–rock interaction of the Sesia Zone
Summary
Aqueous fluids play a critical role during metamorphic processes in the Earth’s crust and upper mantle acting as catalysts for chemical reactions, affecting stable mineral assemblages and controlling mineral textures and element distribution at the micro scale (e.g. Erambert and Austrheim 1993; Ague 2003; Parsons and Lee 2009; Putnis 2009; Beinlich et al 2010; Jamtveit and Austrheim 2010; Putnis and Austrheim 2010). Metamorphic fluids generally escape the rock system with which they have reacted. They may leave behind distinctive geochemical and isotopic signatures in minerals that can assist in reconstructing their paths, sources and the exchange between different systems. As fluid–rock interaction can occur during several events separated in time and under different physicochemical conditions, an approach that combines field observations, petrological and geochemical characterization of key metamorphic minerals and geochronology constitutes the basis for determining P–T-time-fluid paths (Martin et al 2011, 2014a; Baxter et al 2017; Rubatto and Angiboust 2015; Gerrits et al 2019)
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