GEMOC ARC National Key Centre

Ming Zhang and Suzanne Y. O'Reilly, GEMOC Macquarie

The global-scale mantle convection cells in the asthenosphere are not geochemically homogeneous.  The heterogeneity is most prominently reflected in the isotopic compositions (Pb-Sr-Nd) of the mid-ocean ridge basalts (MORB) that are direct partial melts from the underlying asthenosphere. Of particular relevance to Australia's geodynamic evolution from about 100 million years, are the distinctive geochemical signatures of the asthenosphere beneath the Pacific Ocean (Pacific MORB) and Indian Ocean (Indian MORB). Therefore, delineation of the boundary between the two distinct mantle reservoirs and any change in that boundary with time provide information about the patterns of global-scale asthenospheric mantle convection. This information has also allowed us to track large-scale mantle chemical reservoirs such as the distinctive Gondwana lithospheric mantle, and hence better understand the geodynamic evolution of the Australian continent from the time of Gondwana dispersal.

Pb-Sr-Nd isotope data for Cenozoic basalts in eastern Australia (Zhang et al, 1999) indicate that Pacific-MORB type isotopic signatures characterise the lava-field basalts (55-14 Ma) in southeastern Australia, whereas Indian-MORB type isotopic signatures characterise younger basalts (6-0 Ma) from northeastern Australia.  This discovery helps to constrain the changing locus of the major asthenospheric mantle convection cells represented by the Pacific and Indian MORB sources during and following the breakup of the eastern part of Gondwana, and locates, for the first time, the boundary of these convection cells beneath the Australian continent. This extends previous work in the SW Pacific back-arc basins (eg Hickey-Vargas et al., 1995) and the Southern Ocean (Lanyon et al., 1995) that indicates that the I- and P-MORB mantle convection cells have been moving in opposite directions since the early Tertiary. These new data also indicate that the Indian-MORB source is a long-term asthenospheric reservoir beneath most of the Gondwana lithosphere and that the westward migration of the Pacific MORB source may have been associated with the Tasman Sea opening (ca 85-60 Ma) along a broad front southeast of the Australian continent.

Independent dynamic modelling by Gurnis et al. (1998) produced a west-pointing V-shaped boundary between I-MORB and P-MORB in the Southern Ocean, consistent with the data from this study, and explains the present-day geochemical and geophysical characteristics of the Australia Antarctic Discordance (AAD) in the Southern Ocean.

References
Gurnis, M., Müller, R.D, and Moresi, L., 1998, Cretaceous vertical motion of Australia and the Australian-Antarctic discordance: Science, p. 1499-1504.
Hickey-Vargas, R., Hergt, J.M., and Spadea, P., 1995, The Indian Ocean-type isotopic signature in western Pacific marginal basins: origin and significance, in Bebout, G.E. et al., eds., Active Margins and Marginal Basins of the Western Pacific: Geophysical Monograph 88, p. 175-197.
Lanyon, R., Crawford, A.J., and Eggins, S.M., 1995.  Western migration of Pacific Ocean upper mantle into the Southern Ocean region between Australia and Antarctica: Geology, 23, 511-514.
Zhang M., O'Reilly, S.Y. and Chen, D.G., 1999.  Location of Pacific and Indian MORB mantle in two time-slices: evidence from Pb-Sr-Nd isotopes for Cenozoic Australian basalts. Geology 27, 39-43.