Thermal and Compositional Anomalies of the Australian Upper Mantle From Seismic and Gravity Data

Abstract

To discern temperature and compositional variations of the Australian upper mantle, we apply an integrative technique, which jointly interprets seismic tomography and gravity data. The final thermal model, obtained by changing the upper mantle composition according to the density variations, shows temperatures higher by 100-150°C in the Archean and Proterozoic upper mantle, with respect to the initial model based on a uniform "fertile" composition. In the North and West Australian cratons, the upper mantle is cold, with composition depleted in heavy constituents. This suggests the presence of an Archean lithosphere, which remained relatively undisturbed through the Proterozoic. Central Australia is predominantly characterized by a thick, low-temperature lithosphere having a more fertile composition. Its shallow part is characterized by a thin layer of low-velocity mantle, which is interpreted by our results as a thermal anomaly. However, this high-temperature anomaly is hard to reconcile with the tectonic history of the region. A low-density mineral phase, such as amphibole, may reduce the density relative to our assumed composition. Furthermore, we observe larger iron depletion in the Western Australian Craton than in the Proterozoic terranes. At the depths larger than 150 km, the depletion becomes negligible beneath the Proterozoic regions, while it also persists in the Western Australian Craton at the depths larger than 200 km.

Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

“…Marked in blue and in red are the P–T regions geologically incoherent and coherent with (spinel-bearing) mantle lithologies, respectively. The modelled P–T locus crosses at ~ 2.3 GPa/1190 K the geotherm calculated for this study (blue thick line) from the analysis of surface heat flow data and combining seismic tomography gravity data 88 , 89 . This P–T path is slightly “cooler” with respect to the Australian geotherm’s field (grey field) from literature 86 , 87 .…”

“…The Australian mantle domain that stabilized the Mount Leura xenolith mineral assemblages under study is placed in this region. Using the recent thermal model of the crust and upper mantle of the Australian continent, obtained from the analysis of surface heat flow data and combining seismic tomography and gravity data 88 , 89 , it was possible to calculate the geotherm of this mantle column down to 200 km of depth in the proximity of the xenolith location (38°14′41.0"S; 143°09′27.5"E). The new proposed geotherm (Fig.…”

“…The new proposed geotherm (Fig. 5 ) takes into account the mantle xenolith’s petrological evidence (part of this study) of the extensive re-fertilization that occurred during the Proterozoic tectonic events 88 , 89 . The resulting geotherm, specifically related to the mantle column of the Mount Leura xenoliths, is slightly “cooler” than the Australian mantle geotherms’ field (2.3 GPa /1290 K) and intersects the modelled P – T locus of the amphibole-phlogopite equilibrium at ~ 2.3 GPa/1190 K. This P – T point represents the plausible thermal-baric conditions of the mantle region (~ 70 km depth) related to the Mount Leura xenolith provenance.…”

The Lithospheric Architecture of Australia From Seismic Receiver Functions

“…Some of the MLDs observed in central Australia (MULG has an MLD at 75 km, WRAB has an MLD at 71 km, and WRKA has an MLD at 74 km) may be explained by this thermal anomaly, but other stations in central Australia (OOD, INKA) have MLDs either above or below this anomaly. Most of western and central Australia is relatively cold at MLD depths (300°C–500°C), indicating that a thermal anomaly is not able to explain the origin of MLDs in these regions (B. Kennett et al., 2018; Tesauro et al., 2020).…”

“…The SAC has some Archean gneissic terranes in the Gawler Craton, rimmed by Proterozoic orogens and basins (Cawood & Korsch, 2008; Conor & Preiss, 2008; Daly, 1998). Unlike the other Australian cratons, the SAC has a higher temperature, thinner lithosphere with slower wave speeds, and a more enriched mantle (Mg# ∼89.5); enrichment has been interpreted as due to the possible refertilization of the mantle during the Proterozoic, while the thinned lithosphere is more likely due to the detachment of the SAC from Antarctica (Rawlinson et al., 2016; Tesauro et al., 2020; Yoshizawa & Kennett, 2015). Despite these differences, there is still a marked change between the SAC and Phanerozoic lithosphere to its east, with a thickening of the Moho and seismic lithosphere to the west accompanied by changes in reflectivity (Liang & Kennett, 2020).…”