Istituto di Geoscienze e Georisorse     
Medard E., Schmidt M., Schiano P., Ottolini L. Melting of amphibole-bearing wehrlites an experimental study on the origin of ultra-calcic nepheline-normative melts. In: Journal of Petrology, vol. 47 (3) pp. 481 - 504. Oxford University press, 2006.
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 (1100C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 05-10 GPa, 1200-1350C, 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 > 1190C at 10 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.
URL: http://https://academic.oup.com/petrology
Subject experimental melting
subduction zone
ultra-calcic melts

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