Castelli, D.

Connolly, J. A. D.

Ferrando, S.

Frezzotti, M. L.

Groppo, C.

Maffeis, A.

English

Institutional Research Information System University of Turin

40  HP-UHP fluid inclusion evolu�on predicted by molecular and electroly�c fluid models: implica�ons for HP-UHP metamorphic fluid composi�on  Maffeis A.1*, Ferrando S.1, Connolly J.2, Groppo C.1,3, Frezzo� M. L.4, Castelli D.1 1University of Torino, Department of Earth Sciences; Via Valperga Caluso, 35, 1-10125 Torino, Italy (andrea.maffeis@unito.it) 2ETH Zurich, Department of Earth Sciences, Ins�tute for Geochemistry and Petrology; Clausiusstrasse 25, CH 8092, Zurich, Switzerland 3C.N.R. – I.G.G., Sec�on of Torino, Via Valperga Caluso 35, 1-10125, Torino, Italy 4University of Milano-Bicocca, Department of Environment and Earth Sciences; Piazza della Scienza 4, 20126 Milano, Italy. ___________________________________________________________________________ Subduc�on fluids are fundamental in regula�ng long-term chemical cycles and their reliable characterisa�on is of primary importance for our understanding of processes related to mantle wedge metasoma�sm, oxida�on and mel�ng. UHP fluids are composed by solvent C-O-H-N-S molecular vola�les and by solute non-vola�le elements (e.g., Si, Al, Ca, Mg, Fe, K, Na, Cl) bounded to inorganic and organic species. Both direct (fluid inclusion, FI) and indirect (thermodynamic modelling, TM) approaches to study these fluids have reliability issues due to the complexity of the inves�gated processes. The chemical fingerprint of UHP FI can be easily modified by post-trapping processes (e.g., solvent loss, chemical interac�on with host mineral), while TM of solute-bearing fluids at UHP condi�ons is s�ll in its infancy. In this work, we apply and compare the data obtained by FI study and TM on UHP FI (Fig. 1) trapped within peak diopside from a chemically simple marble from the UHP Brossasco-Isasca Unit of the Dora-Maira Massif (Western Alps). Classical molecular-fluid TM allowed to model post-trapping reac�ons between FI and host diopside, and then to discriminate among daughter, step-daughter and incidentally trapped minerals present within the inclusions. Electroly�c-fluid TM allowed to model the chemical composi�on of the peak solute-bearing aqueous fluid (H2O: 96.3 mol%/88.5 wt%; solutes: 3.7 mol%/11.34 wt%; CO2+H2S+CH4: 0.09 mol%/0.17 wt%) generated by progressive rock dissolu�on. The comparison between the modelled composi�on with that reconstructed by FI study allows to recognise the type and the extent of post-trapping chemical re-equilibra�ons (up to 35 mol% of host diopside contamina�on and 18-99 mol% of the original H2O lost by diffusion and decrepita�on; Fig. 1) occurred within UHP FI. Applying this mul�disciplinary approach, we demonstrate that the most impac�ng FI post-trapping process is the H2O loss, with consequent preserva�on of the geochemical informa�on in those FI lacking relevant post-trapping host-diopside chemical contamina�on (Fig.1). Moreover, we also show how FI complemented with electroly�c fluid TM can be employed to retrieve geochemical informa�on on deep subduc�on fluids and the fluid and rock geochemistry of C-bearing systems (especially regarding all the mul�ple C-oxida�on states contemporaneously in equilibrium at the same �me). Finally, this work demonstrates the poten�ality of this type of mul�disciplinary work in developing a more complete and meaningful geologic model of the petrologic processes occurring during deep subduc�on, thanks to the ability of FI studies and TM to access different and complementary informa�on, that would not be accessible otherwise.   

HP-UHP fluid inclusion evoluon predicted by molecular and electrolyc fluid models: implicaons for HP-UHP
metamorphic fluid composion

https://iris.unito.it/bitstream/2318/1806037/1/Maffeis_et_al_2021_MSG40.pdf

HP-UHP fluid inclusion evoluon predicted by molecular and electrolyc fluid models: implicaons for HP-UHP

metamorphic fluid composion

HP-UHP fluid inclusion evoluon predicted by molecular and electrolyc fluid models: implicaons for HP-UHP metamorphic fluid composion

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