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Journal of Petrology Advance Access published online on November 3, 2005
Journal of Petrology, doi:10.1093/petrology/egi083
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received January 14, 2005
Accepted October 10, 2005

Article

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

ETIENNE MÉDARD 1*, MAX W. SCHMIDT 2, PIERRE SCHIANO 1, and LUISA OTTOLINI 3

1 LABORATOIRE MAGMAS ET VOLCANS, UNIVERSITÉ BLAISE-PASCAL-CNRS, OPGC, 5 RUE KESSLER, F-63038 CLERMONT-FERRAND, FRANCE
2 INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, ETH, SONNEGGSTRASSE 5, CH-8092 ZÜRICH, SWITZERLAND
3 CNR-ISTITUTO DI GEOSCIENZE E GEORISORSE, SEZIONE DI PAVIA, VIA FERRATA 1, I-27100 PAVIA, ITALY

* To whom correspondence should be addressed.
ETIENNE MÉDARD, E-mail: emedard{at}mit.edu


  Abstract

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (~1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5-1·0 GPa, 1200-1350°C, Pt-graphite capsules) investigated the melting behaviour of a model amphibole-olivine-clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1180°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma-rock interaction (assimilative recycling) in the arc crust.

Keywords: experimental melting; subduction zone; ultra-calcic melts; wehrlite.
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