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Journal of Petrology Advance Access originally published online on November 3, 2005
Journal of Petrology 2006 47(3):481-504; doi:10.1093/petrology/egi083
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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

ETIENNE MÉDARD1,*, MAX W. SCHMIDT2, PIERRE SCHIANO1 and LUISA OTTOLINI3

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

RECEIVED JANUARY 14, 2005; ACCEPTED OCTOBER 10, 2005

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 > 1190°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.

KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite

* Corresponding author. Present address: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-1212, Cambridge, MA 02139, USA. Telephone: (1) 617 253 2876. Fax: (1) 617 253 7102. E-mail: emedard{at}mit.edu


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