The high-pressure synthesis of lawsonite in the MORB+H ₂ O system

Lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth ~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 8C 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 °C in the stishovite stability field, and <900 °C at 8 GPa and 750 °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.