Experimental constraints on high pressure melting in subducted crust

Synthesis piston-cylinder experiments were carried out from 2.0 to 4.5 GPa and 850 to 1150 degreesC in order to determine phase and melting relations in a model composition for subducted crust (K2O-CaO-MgO-Al2O3-SiO2-H2O). As subduction zone magmas are enriched in H2O and large ion lithophile elemen ts (LILE). this study concentrates on the stability of phases that host the se elements. Biotite and phengite were found to be the stable phases able t o transport H2O and LILE to mantle depths. At pressures below 3.0 GPa bioti te is stable to higher temperatures than phengite and melting related to bi otite breakdown occurs at about 950 degreesC. At higher pressure, only phen gite melting occurs along the reaction phengite+clinopyroxene+coesite --> g arnet+kyanite+melt+K-feldspar, which has a positive slope to 1050 degreesC, 4.5 GPa. Biotite reacts to phengite under subsolidus conditions with the c onservation of LILE and H2O stored in the rock. The stability of phengite t o high pressures and temperatures prevents liberation of LILE and H2O by 'f luid absent. melting in subduction zones with a normal thermal gradient. It is suggested that these elements are probably released by melting in the p resence of fluids, which derive from dehydration of the mafic or ultra-mafi c layer of the slab. The experiments demonstrate that melts produced by mic a melting or by addition of small amounts of H2O at lower temperatures are granitic in composition and display an increase of K2O with increasing temp erature. Determination of trace element partitioning between melt and resid ue indicates that the heavy rare earth elements will be incorporated in gar net and strongly enriched in the residue whereas the LILE preferentially en ter the melt even if there is phengite in the residue. The light rare earth elements (LREE) are not significantly enriched in the granitic melts becau se of small amounts of LREE-rich allanite in the residue. Such hydrous gran itic melts could participate in the metasomatism of the mantle wedge and mi ght be partly responsible for the characteristic tract element patterns of subduction zone magmas. (C) 2001 Elsevier Science B.V. All rights reserved.