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Tokyo Institute of Technology, Department of Earth and Planetary Sciences, Tokyo, Japan
Lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth approximately 70 km. To determine the maximum pressure stability of lawsonite in the MORB+H 2 O system, experiments were carried out using a 1000 ton uniaxial multi-anvil apparatus (SPI-1000). Mixtures of synthetic gel+2wt% H 2 O were used for the starting materials with the average MORB composition. Experimental P-T conditions were T = 700-900 degrees C and P = 5.5-13.5 GPa. Run durations were 12 and 24 h. Lawsonite was synthesized stably up to 10 GPa and T < 700 degrees C in the stishovite stability field, and <900 degrees C at 8 GPa and 750 degrees C at 5.5 GPa in the coesite stability field, with a steep positive slope for the lawsonite-out reaction. The lawsonite-out reaction in the coesite stability field changes to have a gentle negative slope in the stishovite stability field. The reaction leading to the disappearance of lawsonite is a continuous reaction due to the compositional enlargement of garnet toward the grossular end-member with increasing T and P. Lawsonite disappears when the tie line connecting grossular-rich garnet with omphacitic clinopyroxene reaches the bulk composition on the conventional AC(FM) ternary diagram.
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