THERMOBAROMETRY OF >4 GA ZIRCONS: TOWARDS A HADEAN GEOTHERM

Hadean detrital zircons from Western Australia's Jack Hills play a key role in understanding the first 500 Ma years of Earth history. One underexploited aspect of these ancient zircons is their diverse assemblage of mineral inclusions. Previous studies have reported inclusions in >4 Ga zircons including quartz, K-feldspar, biotite, chlorite, amphibole, muscovite, albite, feldspar, rutile, apatite, FeOOH, Ni-rich pyrite, thorite, and monazite. Thus the inclusion suite appears to represent both hydrated metaluminous and peraluminous granitic assemblages. Crystallization thermometry of Hadean Jack Hills zircons has documented a dominant peak in the distribution at 680±25°C. If the inclusion suite permits barometric estimates, results could be coupled with crystallization temperatures to estimate geotherms for igneous processes during the Hadean. An examination of 175 >4 Ga zircons indicates that, on average, ~20% have inclusions that intersect the surface (approx. 45% qtz, 40% musc, 10% bio, and 5% hbl). We have analyzed chemical compositions of 5 to 15 µm-sized inclusions of muscovite, biotite and hornblende from zircons yielding crystallization temperatures in the range of 650-750°C. Provided that appropriate buffering assemblages are present, several geobarometers, such as Al-in-hornblende, Ti-in-muscovite, and Si-Al occupancy in white mica, can be used to estimate pressure. For example, a 4.03 Ga zircon with a crystallization temperature of 695°C yields an Al-in-hornblende pressure of ~7 kbars. While not all the components of the buffering assemblage are as yet documented in this grain, this result is consistent with a geotherm of ~30°C/km. This value is surprising low given the much higher radioactive heat generation during the Hadean, but would be a minimum estimate if the parent granitoid had buoyantly ascended from its anatectic source prior to crystallization. Future work includes documenting the 3D distribution of mineral inclusions within these ancient zircons and a continued search for inclusion suites permitting the utilization of other geobarometers to confirm our preliminary results.