Lambart, S. and Laporte, D. and Provost, A. and Schiano, P. (2012) Fate of Pyroxenite-derived Melts in the Peridotitic Mantle: Thermodynamic and Experimental Constraints. Journal of Petrology, 53 (3). pp. 451-476. ISSN 0022-3530 http://resolver.caltech.edu/CaltechAUTHORS:20120327-143602255
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We performed a thermodynamic and experimental study to investigate the fate of pyroxenite-derived melts during their migration through the peridotitic mantle. We used a simplified model of interaction in which peridotite is impregnated by and then equilibrated with a finite amount of pyroxenite-derived liquid. We considered two pyroxenite compositions and three contexts of pyroxenitic melt impregnation: (1) in a subsolidus lithospheric mantle; (2) beneath a mid-ocean ridge (MOR) in a subsolidus asthenospheric mantle at high pressure; (3) beneath a MOR in a partially molten asthenospheric mantle. Calculations were performed with pMELTS at constant pressure and temperature with a melt–rock ratio varying in the range 0–1. Concurrently, a series of impregnation experiments was performed at 1 and 1·5 GPa to reproduce the final stages of the calculations where the melt–rock ratio is 1. Incoming melt and host-rocks react differently according to the melt composition and the physical state of the surrounding mantle. Whereas clinopyroxene (Cpx) is systematically a reaction product, the role of olivine (Ol) and orthopyroxene (Opx) depends on the incoming melt silica activity a^0_(SiO2): if it is lower than the silica activity Formula of a melt saturated in Ol and Opx at the same pressure P and temperature T, Opx is dissolved and Ol precipitates, and conversely if a_(SiO2) > a^0_(SiO2). Such contrasted reactions between pyroxenitic melts and peridotitic mantle may generate a large range of new lithological heterogeneities (wehrlite, websterite, clinopyroxenite) in the upper mantle. Also, our study shows that the ability of pyroxenite-derived melts to migrate through the mantle depends on the melting degree of the surrounding peridotite. The reaction of these melts with a subsolidus mantle results in strong melt consumption (40–100%) and substantial Cpx production (with some spinel or garnet, depending on P). This is expected to drastically decrease the system permeability and the capacity of pyroxenite-derived melts to infiltrate neighbouring rocks. In contrast, melt migration to the surface should be possible if the surrounding mantle is partially melted; although liquid reactivity varies with composition, melt consumption is restricted to less than 20%. Hence, magma–rock interactions can have a significant impact on the dynamics of melting and magma migration and should not be neglected when modelling the partial melting of heterogeneous mantle.
|Additional Information:||© 2012 The Author. Published by Oxford University Press. Received February 7, 2011. Accepted November 16, 2011. Advance Access Publication January 23, 2012. The authors are grateful to Jacques Kornprobst, Emeritus Professor at Université Blaise Pascal, Clermont-Ferrand, for providing the two pyroxenites from Beni Bousera used in this study. This study has benefited from discussions with Michel Pichavant, Othmar Müntener, Etienne Me¤dard and Muriel Laubier. Special thanks are due to the following persons: Jean-Louis Fruquière and Frank Pointud for manufacturing the piston-cylinder assemblages and for maintenance operations; Jean-Luc Devidal for technical assistance with the electron microprobe and for the preparation of synthetic gels; Jean-Marc Hénot for technical assistance with the scanning electron microscope; Mhammed Benbakkar for the ICP-AES analyses of pyroxenites and synthetic gels. We also thank Tetsu Kogiso, Bruno Scaillet and an anonymous reviewer for their constructive reviews. This work was supported by the program DyETI of the Institut National des Sciences de l’Univers (INSU-CNRS) and by the Agence Nationale de la Recherche ‘MIME’ grant (ANR-07-BLAN-0130-01 to A.P.).|
|Subject Keywords:||experimental petrology; melt-peridotite interactions; MORB genesis; pMELTS; pyroxenite|
|Official Citation:||S. Lambart, D. Laporte, A. Provost, and P. Schiano Fate of Pyroxenite-derived Melts in the Peridotitic Mantle: Thermodynamic and Experimental Constraints J. Petrology (2012) 53(3): 451-476 doi:10.1093/petrology/egr068|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||27 Mar 2012 22:08|
|Last Modified:||27 Mar 2012 22:08|
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