(a) SEM-Backscattered electron microphotograph of sample MBK3 (1.3 GPa-1200 °C) showing a microdike filled with quenched melt (“glass”) into the graphite container (black). (b) Texture and paragenesis of partially molten peridotite MBK3; solid phases are as follows: Ol is grey, Opx is slightly darker than Ol, Cpx is light grey, and Spl is white. The interstitial glass is barely visible in b due to its low fraction (1.7%); the dark voids correspond to grains plucked off during sample preparation. Scale bars: 60 μm in a and 20 μm in b. (c) Composition of partial melts of fertile peridotites between 1 and 1.5 GPa plotted in the total alkali-silica diagram. Our partial melting experiments at 1.3 GPa are shown by the red circles (open circles: MBK+; solid circles: MBK); the open green squares are for the experiments with MBK at 1 GPa. The open red circle with a central dot is reversal experiment MBK+13: partial melting experiment MBK+6 and its reversal MBK+13 are surrounded by the red dashed line. The small black crosses correspond to fertile peridotite MM3 at 1 GPa; the diamonds correspond to a K2O-free analogue of MM3 at 1 and 1.5 GPa (open and solid diamonds, respectively). The solid triangle is the near-solidus melt of MORB-pyrolite MPY at 1.5 GPa. The blue symbols correspond to Heldburg phonolite and to the glass analyzed in the crystallization experiment CrysPho2 (crystallization of a composition equivalent to partial melt MBK+6 at 1 GPa-1150 °C). The star labeled “DGA-40” corresponds to a trachyte melt in equilibrium with ol + opx + cpx at 1.2 GPa and 1150 °C. Melt compositions computed by pMELTS for MBK+ at 1.3 GPa are shown by the blue curve (the theoretical trend is shifted to the left of the experimental data because of a slight underestimation of SiO2 by pMELTS). The grid corresponds to the chemical classification of volcanic rocks, with the main fields of interest labeled as follows: (0) Phonotephrite; (1) Tephriphonolite; (2) Phonolite; (3) Trachyte; (4) Trachyandesite; (5) Basaltic trachyandesite; (6) Trachybasalt; (7) Basalt; (8) Basaltic andesite.

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