Fluid CO2 inclusions in olivine and pyroxene and their behaviour under high pressure and temperature conditions

Abstract

The ultrabasic xenoliths from Hirzstein (Northern Hessia, Germany) are rich in primary and secondary fluid CO2 -inclusions. The densities of the primary inclusions range from 1.00-1.08 g/cm3 in olivine to 1.11-1.17 g/cm3 in pyroxene. Massive cylinders 4.5 mm high and 4 mm in diameter from the xenolith were equilibrated during up to 48 h under PT conditions between 30-100 kbar and 1 000-1 400 °C. Besides a slight shrinking no reaction has been observed below 50 kbar ; at higher pressures the cavities collapse under formation of carbonates, first in olivine, then in pyroxene. At pressures above 70 kbar partly a greyish substance which could not yet be identified, partly fused rims resp. melts have been detected in the collapsed cavities. The molten parts are extremely enriched in potassium. They have presumably been formed by a High-Pressure Fluid of intermediate silicate-carbonate composition which incorporates all incompatible elements. Small (< 5 µm) roundish inclusions with a CO2-free fluid of still unknown composition may persist until pressures of 100 kbar. The different reactions and their petrological implications are discussed.