Secular Evolution of Sub-Continental Mantle: Geophysical Effects

W L Griffin (GEMOC, Macquarie Univ., Sydney 2109, and CSIRO-EM, Box 136, NSW 2113, Australia)

S Y O'Reilly, O Gaul, D Ionov (all at GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, Australia) and C G Ryan (CSIRO-EM, P.O. Box 136, NSW 2113, Australia)

Lithosphere mapping using garnet concentrates from kimberlites and other volcanic rocks shows consistent differences between Archean and younger lithosphere. Mantle sections beneath Archean terranes contain 10-60% garnet harzburgites, typically concentrated between 140 to 180 km depth and interspersed with depleted lherzolites. In Proterozoic and Phanerozoic sections, harzburgitic rocks are rare, and the dominant lherzolitic garnets are less depleted in LIL and HFSE elements. Lherzolitic garnets from Archean sections have high mean Zr/Y (³5) and low mean Y/Ga (<3), those from Phanerozoic areas have low mean Zr/Y (²1) and high mean Y/Ga (³4), and garnets from Proterozoic sections have intermediate values. Xenolith data and numerical modelling based on partition coefficients indicate that these differences reflect an increase in the average Cpx/Gnt ratio and the average modal (Cpx+Gnt) content of lithospheric mantle from Archean to Phanerozoic time.

The compositional evolution outlined here will result in differences in seismic signature between areas with mantle of different tectonothermal age. Typical Archean and Phanerozoic lherzolites have Vp of 8.1 and 7.8 km/sec at 600°C. At 1000°C the difference in Vp decreases to 7.8 and 7.6 respectively. If the harzburgitic component is considered, the average velocities beneath Archean cratons will be lowered further. The keels with high Vs and Vp, extending to depths of 250-450 km beneath the older (>1.7 Ga) parts of many cratons, commonly are interpreted as cooler than the mantle beneath younger cratons and Phanerozoic mobile belts, which have no significant Vs anomalies. However, at least part of the difference in seismic signature may be related to compositional differences. The harzburgites and highly depleted lherzolites of Archean mantle will contribute to the Vs and Vp anomaly beneath the >2.5 Ga cratons, but the roots of Early Proterozoic cratons do not contain such rocks; this suggests that moderately depleted lherzolites also can provide a seismic anomaly. In areas where Archean lithospheric mantle has been replaced by Phanerozoic material, dramatic changes in the seismic and gravity signatures and topography can occur, reflecting changes in both the density and the thermal state of the lithospheric mantle. An example is the eastern part of the Sino-Korean Craton (Griffin et al. 1996),