Geophysical evidence for chemical variations in the Australian continental mantle

The relative density-to-shear velocity scaling (¿) provides a diagnostic for the presence of compositional variations in the mantle. We invert shear-wave velocity from a recent 3-D model and gravity anomalies for radial profiles of ¿ of the uppermost mantle beneath Australia. We performed calculations for the three major tectonic provinces that constitute the continent, and found significant differences between them. The ¿ profile for the Phanerozoic region can be explained by thermal variations alone. In contrast, negative values of ¿ suggest that variations in composition are important between ~75 and ~150 km depth in the Proterozoic continental lithosphere (central Australia). It is likely that chemical variations are also required to explain the inferences for the Archean craton (west Australia), but poor tomographic resolution precludes a definitive conclusion. The scaling factors found are consistent with chemical depletion of deep Precambrian lithosphere, which supports a tectosphere model for the Australian continental root