Chromium Isotope tracing of subduction-zone fluids: Analysis of eclogite-facies veins from the Monviso Massif, Italy
Work at the cutting edge of applying novel isotope geochemistry to understand subduction zone processes.
At subduction zones, the downgoing oceanic crust and hydrated mantle lithosphere undergo complex metamorphic devolatilisation processes to release aqueous fluids and melts that are responsible for generating intermediate to deep earthquakes and for promoting mantle melting to produced arc magmatism.
High- and ultrahigh-pressure metamorphic rocks found in collisional orogenic zones are seen as excellent analogues of deeply subducted lithosphere, and therefore can provide crucial information on how crust and mantle rocks devolatilise during subduction.
The Monviso Massif in the western Italian Alps (see image below) is one of the best exposed and studied sections of oceanic lithosphere that was subducted to ~80 km depth. Monviso preserves eclogite-facies metabasalts and metagabbros that are cut by Cr-rich high-pressure veins formed by fluid infiltration during subduction. These rocks provide a unique record of devolatilisation of subducted oceanic crust and fluid migration under high-pressure conditions, although interpreting fluids sources for these veins has been an ongoing challenge.
This project will use a new geochemical tracer, stable Cr isotopes (53Cr/52Cr ratios), as a proxy to better understand (i) the sources, (ii) redox conditions, and (iii) hydrothermal mobility of Cr in the fluids that produced the high-pressure veins at Monviso. Specifically, the project aims to test if Cr, and by inference also other metals, that are enriched in the above veins have been sourced from fluids originating from the dehydration of subducted mafic/ultramafic rocks and marine sediments.
To address this, Cr isotopic analysis will be conducted on vein minerals and their host metagabbros, as well as putative fluid source rocks, such as serpentinites and metasedimentary rocks. Results of this project will represent one of the most omprehensive and unique metal isotope records of deep subduction processes, which in turn will contribute to improving our understanding of element recycling through convergent margin.