THERMOBAROMETRY OF DEPLETED PERIDOTITES

An assessment was made of the possibility of using olivine thermobarometry for xenoliths of depleted peridotites from the Udachnaya pipe (Yakutia), represented by 76 megacrystalline dunites and 5 megacrystalline harzburgites. Specially developed and tested method of microprobe analysis was used to determine Al and Ca microimpurities in the olivine samples studied. PT-parameters of megacrystalline dunite xenoliths were calculated using the equations of olivine, garnet, and olivine-garnet thermobarometers. Enstatite geobarometers were additionally used for some megacrystalline harzburgites. Comparison of the obtained data showed that the geothermobarometric pair of FinnertyRigden barometer (Ca in olivine) for wide PT-range peridotites and thermometer [Bussweiler et al., 2017] (Al in olivine) for garnet peridotites is most suitable for assessing the PT-parameters of the megacrystalline dunite collection studied, though some of the high PT-parameters (>60 kbar and 1200 °C) are questionable. Garnet barometer [Grütter et al., 2006] (CaO – Cr2O3 ratio in pyrope) does not always agree with olivine geothermometers. This may be caused by the necessity to adjust the barometer equations for a wider range of compositions, as well as by the possible imbalance between garnet and olivine. We admit that more accurate results can be obtained by using this barometer in combination with garnet thermometers based on the determination of Ni in garnet. There is not enough data on harzburgites to draw certain conclusions. However, it should be recognized that the traditional pair of McGregor enstatite barometer and O’Neill-Wood garnet-olivine thermometer still remains the most acceptable for estimating the PT-parameters of megacrystalline harzburgites, though with some temperature underestimation.