INTRACRYSTALLINE FE-2+-MG ORDERING IN AUGITE - EXPERIMENTAL-STUDY AND GEOTHERMOMETRIC APPLICATIONS

An augite crystal from a basaltic dyke on Vulcano Island (Aeolian Archipelago) was equilibrated at temperatures in the range 670-1180-degrees-C and then quenched to room temperature. Following each experiment, the partitioning of Fe2+-Mg between the M1 and M2 sites of the clinopyroxene was determined by crystal structure refinement. These Fe2+-Mg partitioning data were then used to calibrate a new clinopyroxene geothermometer which estimates closure temperatures for the exchange reaction. The new geothermometer was then applied to six clinopyroxenes from a basaltic andesite lava flow on Salina Island (Aeolian Archipelago). Calculated closure temperature are : 600-degrees-C for samples near the base of the flow, 793-degrees-C for the sample from the top of the flow, and 513-degrees-C (the lowest temperature found) for the sample nearest the middle of the flow. A simplified mathematical cooling model for lava flows (Jaeger, 1968) was then used in conjunction with these clinopyroxene equilibration temperatures to calculate cooling rates at different heights in the Salina Island lava.