The Application of Fluid Inclusion Geothermometry Based on Secondary Inclusions in Primary Igneous Quartz Crystals

Abstract

Primary igneous quartz crystals in the cores and cuttings of volcanic rocks from the Wairakei-Tauhara geothermal field usually have been fractured.Secondary fluid inclusions have formed along many of the fracture planes as the fractures have healed. These inclusions exhibit little variation in homogenization temperature within single fracture planes, suggesting that annealing was rapid. Due to the large size (up to 5mm diameter) and common occurrence of primary igneous quartz crystals at Wairakei-Tauhara, more detailed and complete fluid inclusion geothermometry of the field is now possible compared to the previous use of hydrothermal minerals only. This, in turn, allows a better evaluation of past variations in the thermal regime of the field . Results for samples from well TH1, for example, show that temperatures of over 280?C have occurred at 975m depth in the past, but that it has since cooled to the present 250?C.