A xenolith-derived geotherm for Southeastern Australia and its geophysical implications

Well-equilibrated garnet websterite xenoliths from two Quaternary maars in southeastern Australia have been used to construct a paleogeotherm. The P-T curve goes through 800°C, 7 kbar; 900°C, 10 kbar; 1050°C, 15 kbar; 1200°C, 25 kbar. All available P-T data from other eastern Australian xenolith localities lie close to this geotherm, suggesting that a similar thermal regime has characterized parts of this region at various times from the Permian to the present. The position and curvature of the geotherm implies convective heat transport, related to lithospheric thinning and diking of the crust by magmas. The geotherm is consistent with high measured heat flows and with the low magnetic signature of the region as shown by MAGSAT data. Published measurements of seismic velocity beneath the region show a gradient from V_p ~ 7.0 km/sec to V_p ~ 7.8 km/sec from 25-55 km depth, where V_p increases rapidly to {slanted equal to or greater-than} 8.0 km/sec. Calculation of V_p for the xenolith rock types suggests that the mantle from 25 to 55 km consists of spinel lherzolite interleaved with spinel- and garnet-pyroxenites and basaltic cumulates. The seismically defined "Moho" beneath southeastern Australia coincides with the depth of the spinel lherzolite-garnet lherzolite transition predicted by the xenolith-derived geotherm.