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Genèse de magmas riches en calcium dans les zones de subduction et sous les rides médio-océaniques : approche expérimentale.

Abstract : CaO-rich, Al2O3-poor ultracalcic primitive melts occur in all geodynamic environments. Some of them do not present a "garnet signature" and should originate at P < 3 GPa. They are subdivided into a "nepheline-normative” alkaline-rich, silica-poor group uniquely found in arcs and in "hypersthene-normative” melts associated with tholeiitic basalts (MORB, OIT, IAT). The high CaO contents (to 19.0 wt%) and CaO/Al2O3 ratios (to 1.8) exclude an origin from fertile lherzolites. Experimental investigation of the liquidus of a hypersthene-normative and a nepheline-normative ultracalcic melt results in olivine+clinopyroxene saturation at distinct pressure-temperature conditions. Our results in combination with melting experiments from the literature suggest that hypersthene-normative melts result from melting of a refractory olivine+clinopyroxene±orthopyroxene source (either refractory lherzolite or cumulate wehrlite) in the upper mantle and their presence indicate elevated (~1400 °C) mantle temperatures. Contrasting, nepheline-normative ultracalcic melts form from crustal wehrlitic sources at lower temperatures (~1200 °C). To account for the high alkaline and low silica contents, and the relatively low temperatures, source wehrlites must have contained amphibole. Melting experiments on amphibole-wehrlites that frequently occurs at the base of the arc crust show that melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and become ultracalcic with increasing melt fraction, with strong compositional similarities to natural nepheline-normative ultracalcic melts. Such melts should thus originate by melting according to amphibole+clinopyroxene = melt+olivine in the arc crust at temperatures near 1200 °C. They reflect interactions between high-Mg basalts and crustal amphibole-bearing cumulates that are likely to melt during intrusion of hot high-Mg basalts, due to the characteristically low melting temperature of amphibole (~1100 °C).
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https://tel.archives-ouvertes.fr/tel-00077726
Contributor : Etienne Médard <>
Submitted on : Wednesday, May 31, 2006 - 9:00:24 PM
Last modification on : Thursday, February 25, 2021 - 11:43:38 AM
Long-term archiving on: : Monday, April 5, 2010 - 10:06:00 PM

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  • HAL Id : tel-00077726, version 1

Citation

Etienne Médard. Genèse de magmas riches en calcium dans les zones de subduction et sous les rides médio-océaniques : approche expérimentale.. Géologie appliquée. Université Blaise Pascal - Clermont-Ferrand II, 2004. Français. ⟨tel-00077726⟩

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