HR: 1340h
AN: V13B-1477    [Abstracts]
TI: Melting Of Amphibole-Bearing Wehrlites: An Experimental Study On The Origin Of Ultracalcic Nepheline-Normative Melts In Arc Settings.
AU: * Medard, E
EM: E.Medard@opgc.univ-bpclermont.fr
AF: Lab. Magmas et Volcans, UBP-CNRS-OPGC, 5, rue Kessler, Clermont-Ferrand, 63038 France
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Inst. Min. Pet., ETH, Sonnegstrasse 5, Zuerich, 8092 Switzerland
AU: Schiano, P
EM: p.schiano@opgc.univ-bpclermont.fr
AF: Lab. Magmas et Volcans, UBP-CNRS-OPGC, 5, rue Kessler, Clermont-Ferrand, 63038 France
AB: Ultramafic olivine+clinopyroxene$\pm$amphibole cumulates have been widely documented in island arc settings (Alaskan-type ultramafic-mafic complexes, xenoliths in basaltic arc lavas) and probably constitute the lower part of the arc crust. Due to the low melting temperature of amphibole ($\sim$1100 $\deg$C), such cumulates may melt during intrusion of primary mantle magmas. Accordingly, we have investigated melting conditions and melt compositions for a model amphibole-wehrlite, to assess the possible role of such cumulates in island arc magma genesis. First melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and Al$_{2}$O$_{3}$-rich. With increasing melt fraction, melts become ultracalcic while remaining strongly nepheline-normative. The ultracalcic melts coexist with olivine and clinopyroxene and are generated at temperatures of $>=$ 1200 $\deg$C (1.0 GPa), at least 100 $\deg$C lower than temperatures previously required to generate ultracalcic liquids from other lithologies. The melt fraction required to obtain ultracalcic liquids depends on the amount of amphibole in the source, and may amount to only a few percent in cumulates containing interstitial amphibole only. The experimental melts have strong compositional similarities to natural nepheline-normative ultracalcic melt inclusions and whole rocks from arc settings (Schiano et al., G3, 2000). The high CaO contents (up to 18 wt%), low SiO$_{2}$ contents (down to 43 wt%) and high CaO/Al$_{2}$O$_{3}$ ratios (up to 1.2) of these natural ultracalcic melts span the range of experimental partial melts from amphibole-wehrlites. Phase relations suggest that they originate by melting according to 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. It is proposed that nepheline-normative ultracalcic melt inclusions and lavas in island arc settings are tracers of interaction between high-MgO basalts and the arc crust as the temperature difference between hot (1200-1300 $\deg$C) incoming picro-basaltic magma and the necessary melting temperature is sufficient for generation of nepheline-normative ultracalcic melts from amphibole-bearing cumulates.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting