Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

Abstract The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensiv...

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Autores principales:	Costanza Bonadiman, Valentina Brombin, Giovanni B. Andreozzi, Piera Benna, Massimo Coltorti, Nadia Curetti, Barbara Faccini, Marcello Merli, Beatrice Pelorosso, Vincenzo Stagno, Magdala Tesauro, Alessandro Pavese
Formato:	article
Lenguaje:	EN
Publicado:	Nature Portfolio 2021
Materias:	Medicine R Science Q
Acceso en línea:	https://doaj.org/article/840c784f6ced414882045257231e3c6a
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spelling	oai:doaj.org-article:840c784f6ced414882045257231e3c6a2021-12-02T15:57:21ZPhlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient10.1038/s41598-021-90844-w2045-2322https://doaj.org/article/840c784f6ced414882045257231e3c6a2021-06-01T00:00:00Zhttps://doi.org/10.1038/s41598-021-90844-whttps://doaj.org/toc/2045-2322Abstract The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.Costanza BonadimanValentina BrombinGiovanni B. AndreozziPiera BennaMassimo ColtortiNadia CurettiBarbara FacciniMarcello MerliBeatrice PelorossoVincenzo StagnoMagdala TesauroAlessandro PaveseNature PortfolioarticleMedicineRScienceQENScientific Reports, Vol 11, Iss 1, Pp 1-17 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Medicine R Science Q
spellingShingle	Medicine R Science Q Costanza Bonadiman Valentina Brombin Giovanni B. Andreozzi Piera Benna Massimo Coltorti Nadia Curetti Barbara Faccini Marcello Merli Beatrice Pelorosso Vincenzo Stagno Magdala Tesauro Alessandro Pavese Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
description	Abstract The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.
format	article
author	Costanza Bonadiman Valentina Brombin Giovanni B. Andreozzi Piera Benna Massimo Coltorti Nadia Curetti Barbara Faccini Marcello Merli Beatrice Pelorosso Vincenzo Stagno Magdala Tesauro Alessandro Pavese
author_facet	Costanza Bonadiman Valentina Brombin Giovanni B. Andreozzi Piera Benna Massimo Coltorti Nadia Curetti Barbara Faccini Marcello Merli Beatrice Pelorosso Vincenzo Stagno Magdala Tesauro Alessandro Pavese
author_sort	Costanza Bonadiman
title	Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
title_short	Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
title_full	Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
title_fullStr	Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
title_full_unstemmed	Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
title_sort	phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
publisher	Nature Portfolio
publishDate	2021
url	https://doaj.org/article/840c784f6ced414882045257231e3c6a
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Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

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