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First field-scale occurrence of Si-Al-Na–rich low-degree partial melts from the upper mantle

¹ Département de Géologie, U.M.R. 6524 CNRS, Université Blaise Pascal 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France
² Laboratoire de Minéralogie, Université Paul Sabatier 39, allées Jules Guesdes, 31000 Toulouse, France
³ Département de Géologie, U.M.R. 6524 CNRS, Université Blaise Pascal 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France

Felsic dikes intruding an orogenic lherzolite body from the North Pyrenean zone (France) show great compositional similarities to some of the Si-Al-Na(K)–rich glasses found as microscopic inclusions in peridotite xenoliths. As shown by Sr-Nd isotopes, these silicic magmas originated from a mantle reservoir that was moderately depleted on a time- integrated basis. Their Si-Al-Na–rich bulk compositions match closely those predicted by experimental studies relevant to near-solidus melting of mantle peridotite. Specifically, the dikes are closely similar to low-degree melts from sodic harzburgite. On this basis, an olivine-orthopyroxene source that had been metasomatized by addition of sodic-carbonatite melts is inferred, as also suggested by peculiar trace element features (extreme fractionation of the light rare earth elements, high Nb contents). This new occurrence lends support to the existence of high-silica liquids in the mantle and provides evidence for their efficient extraction to form relatively large batches.

Key Words: upper mantle • felsic dike • low-degree partial melting • alpine peridotite • silicic composition • glass inclusions

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