V022-01
Fluids during (ultra-)high temperature metamorphism: new perspectives and changes in paradigm

Thursday, 10 December 2020: 07:00
Virtual
Bruna B B Carvalho1, Omar Bartoli2, Bernardo Cesare1, Tommaso Tacchetto3, Omar Gianola4, Fabio Ferri5, Laszlo Elod Aradi6,7 and Csaba Szabo8, (1)Università degli studi di Padova, Dipartimento di Geoscienze, Padova, Italy, (2)Università degli Studi di Padova, Dipartimento di Geoscienze, Padova, Italy, (3)Curtin University, School of Earth and Planetary Sciences, Perth, Australia, (4)University degli studi di Padova, Dipartimento di Geoscienze, Padova, Italy, (5)EiT Raw materials, Rome, Italy, (6)Eotvos Univ Budapest ELTE, Budapest, Hungary, (7)Lithosphere Fluid Research Lab, Eotvos University, Budapest, Budapest, Hungary, (8)Lithosphere Fluid Research Laboratory, Institute of Geography and Earth Sciences, Eötvös Loránd University, Budapest, Hungary
Recorded Presentation
Abstract:
Fluid regime during high-grade metamorphism and anatexis remains a debated topic in the literature. Most works argue for a dry lower crust and fluid-absent melting, yet others suggest that fluids may be essential agents in anatexis and granulite metamorphism. Fluid inclusions (FI) may help to elucidate such controversies.

Here we interrogate primary FI in peritectic garnet from three high- to ultra-high temperature metamorphic terranes [Ivrea Zone (NW Italy), Gruf Complex (Central Alps) and Athabasca Granulite Terrane (Canada)] to assess the fate of COH fluid inclusions in these metamorphic terranes, and the implication of such findings for a reconsideration of evidence for fluids in the deep crust.

Petrography and micro-Raman spectroscopy have shown that FI occur together with nanogranitoid inclusions in the same clusters regularly distributed in the core of peritectic garnets. Thus, they were trapped at the same anatectic event (Carvalho et al 2020 EPSL). All FI are multiphase, composed of a fluid and a solid assemblage (Figure 1). The latter usually contains siderite (Sid), magnesite (Mgs), pyrophyllite (Prl), calcite (Cal), kaolinite (Kaol), corundum (Cor), quartz (Qz), dolomite, and sometimes graphite (Gr),biotite (Bt) and muscovite (Ms). In the fluid phase, CO2 is the most common component and no free H2O has been detected. Methane and N2 are also present in samples from Ivrea Zone and Athabasca. The volume of the different phases and the composition of the fluid (CO2–H2O system) were estimate based on the 3D reconstruction of inclusions. In Ivrea, the XCO2 varies from 0.55 - 0.7, whereas in Athabasca the proportions were notably higher >0.95; Gruf resulted in XCO2 = 0.45.

Phase equilibria modelling in the Ca-free FMAS-CO2-H2O system was done to evaluate post-entrapment behavior of a fluid inclusion trapped in granulitic garnet. Results shows that the fluid reacts with the host to form metastable step-daughter phases, and whatever retrograde path followed by the host rock, primary CO2 or COH FI must change their nature to a multiphase assemblage.

Our findings undermine the main pillar of the theory of carbonic fluid-assisted metamorphism, but we offer a new perspective to identify such a process: only multiphase FI in peritectic minerals represent true evidence for the presence of fluids in the deep crust.

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