THE PRESSURE AND TEMPERATURE STABILITY LIMITS OF LAWSONITE - IMPLICATIONS FOR H2O RECYCLING IN SUBDUCTION ZONES

The stability relations of lawsonite, CaAl2Si2O7(OH)2.H2O, have been i nvestigated at pressures of 6 to 14 GPa and temperatures of 740 to app roximately 1150-degrees-C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite + H2O to determine the maximum stabili ty of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of approximately 13.5 GPa at 800-degrees-C, and between appro ximately 6.5 and 12 GPa at 1000-degrees-C. Its composition does not ch ange with pressure or temperature. All lawsonite reactions have grossu lar, vapour and two other phases in the system Al2O3-SiO2-H2O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH pha ses was used to relate the reactions to each other. At the lowest pres sures studied lawsonite breaks down to grossular + kyanite + coesite vapour in a reaction passing through approximately 980-degrees-C at 6 GPa and approximately 1070-degrees-C at 9 GPa. Above 9 GPa the reacti ons coesite = stishovite and kyanite + vapour = topaz-OH are crossed. The maximum thermal stability of lawsonite is at approximately 1080-de grees-C, at approximately 9.4 GPa. At higher pressures the lawsonite b reakdown reactions have negative slopes. The reaction lawsonite = gros sular + topaz-OH + stishovite + vapour passes through approximately 10 70-degrees-C at 10 GPa and approximately 1010-degrees-C at 12 GPa. At 14 GPa, approximately 740-840-degrees-C, lawsonite is unstable relativ e to the assemblage grossular + diaspore + vapour + a hydrous phase wi th an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO3(OH). Two experim ents on the bulk composition lawsonite + pyrope [Mg3Al2Si3O12] show th at at 10 GPa the reaction lawsonite = Gr-Py(ss) + topaz-OH + stishovit e + vapour is displaced down temperature from the end-member reaction by approximately 200-degrees-C for a garnet composition of Gr20Py80. C alculations suggest similar temperature displacements for reaction bet ween lawsonite and Gr-Py-Alm garnets of compositions likely to occur i n high-pressure eclogites. Temperatures in subduction zones remain rel atively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its cr ystallisation in blueschists and eclogites, up to pressures of at leas t 10 GPa. Lawsonite contains 11.5 wt% H2O, which when released may tri gger partial melting of the slab or mantle, or be incorporated in hydr ous phases such as the aluminosilicates synthesised here. These phases may then transport H2O to an even greater depth in the mantle.