First-principles calculation of iron and silicon isotope fractionation between Fe-bearing minerals at magmatic temperatures: The importance of second atomic neighbors

Ségolène Rabin, Marc Blanchard, Carlos Pinilla, Franck Poitrasson, Michel Grégoire

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15 Citations (Scopus)

Abstract

In order to elucidate the processes involved in iron and silicon isotopes partitioning during magmatic differentiation, it is essential to know the precise value of equilibrium fractionation factors between the main minerals present in the evolving silicic melts. In this study, we performed first-principles calculations based on the density functional theory to determine the equilibrium iron and silicon isotopes fractionation factors between eleven relevant silicate or oxide minerals in the context of magmatic differentiation, namely: aegirine, hedenbergite, augite, diopside, enstatite, fayalite, hortonolite, Fe-rich and Fe-free forsterites, magnetite and ulvospinel. Results show that Fe2+-bearing silicate minerals display significant differences in iron isotope fractionation factors that cannot be neglected, even at high temperature (1000 °C). Various physical and chemical parameters control the iron isotopic fractionation of silicate minerals. However, the main parameter, after temperature and the iron oxidation state, is the nature and number of iron second neighbors (i.e. the local chemical composition around Fe atoms). This conclusion is also valid for silicon isotopes. In the investigated nesosilicates and inosilicates, silicon isotope reduced partition function ratios (also called β-factors) show no correlation with the average Si-O bond length, which remains almost constant, but Si β-factors are correlated with the local chemical composition of the minerals. Fractional crystallization is one of the mechanisms, which could explain the evolution of iron isotopic compositions during magmatic differentiation. Using the present theoretical set of equilibrium fractionation factors allows us to assess the impact of inter-mineral isotopic fractionations, and shows that pyroxene appears to be the main mineral phase driving the isotopic evolution to a heavier signature in the most evolved lavas.

Original languageEnglish
Pages (from-to)101-118
Number of pages18
JournalGeochimica et Cosmochimica Acta
Volume304
DOIs
Publication statusPublished - 1 Jul 2021

Bibliographical note

Funding Information:
We thank the associate editor Dr. Fang Huang and the two anonymous reviewers for their efforts to improve our manuscript. This work was supported by the ECOS-NORD/COLCIENCIAS French-Colombian cooperation program (Project number: C17U01). CP also acknowledges funding from MINCIENCIAS through research grants No. 2015-710-51568. This work was also supported through a grant to FP from the “Programme National de Planétologie-PNP” of CNRS/INSU, co-funded by CNES. Calculations were performed using the HPC resources from CALMIP (Grant 2019 – P1037). SR deeply thanks CALMIP staff for their support.

Funding Information:
We thank the associate editor Dr. Fang Huang and the two anonymous reviewers for their efforts to improve our manuscript. This work was supported by the ECOS-NORD/COLCIENCIAS French-Colombian cooperation program (Project number: C17U01). CP also acknowledges funding from MINCIENCIAS through research grants No. 2015-710-51568. This work was also supported through a grant to FP from the “Programme National de Planétologie-PNP” of CNRS/INSU, co-funded by CNES. Calculations were performed using the HPC resources from CALMIP (Grant 2019 – P1037). SR deeply thanks CALMIP staff for their support.

Publisher Copyright:
© 2021 Elsevier Ltd

Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.

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