GEMOC ARC National Key Centre

Suzanne Y. O'Reilly1, W.L. Griffin1.2, O. Gaul1 and N. Pearson1
1. GEMOC Macquarie, 2. CSIRO Exploration and Mining

4-D Lithosphere Mapping (O'Reilly and Griffin, 1996) combines geophysical datasets and geological, geochemical and petrophysical information from mantle-derived rocks and minerals with tectonic syntheses to track the architecture, geochemical composition, thermal state and nature of major mantle events through time in different lithosphere domains.  As mantle-derived materials are not available for large regions of Australia, well-constrained lithosphere sections from other localities globally can provide analogues. Lithosphere section interpretations in Australia can be extended laterally using geophysical modelling (including seismic, gravity, MAGSAT).

Geochemical tomography sections have been constructed using 4-D Lithosphere Mapping techniques based on garnet geothermobarometry (garnet is the most common mantle mineral and contains the most pressure/temperature information) for different tectonic domains in Australia including two transects (one southern and one northern) across the Tasman Line, and sections in the Kimberley Block and surrounding mobile belts.  On the regional/global-scale, these reveal thicker lithosphere and lower geothermal gradients with increasing tectonothermal age of the lithosphere domains.  For example, in the southern lithosphere transect from Jugiong in Phanerozoic eastern Australia to the Eyre Peninsula in Proterozoic South Australia, the paleogeotherm decreases westward from greater than 50 mW/m2 at Jugiong to around 40 mW/m2 in South Australia and the depth to the lithosphere/asthenosphere (LAB) boundary changes from about 100km to 180 km.  Furthermore, geochemical tomography sections for timeslices in the Jurassic and Permian near to and east of the Port Augusta section of this traverse indicate lithosphere thinning from about 180 to 150 km , interpreted as due to Pangean rifting in this region.

These physical changes are paralleled by higher MgO and lower CaO and Al2O3 contents for progressively older lithospheric mantle, consistent with the global secular variation in mantle composition documented by Griffin et al. (1999). Olivine is the most abundant mantle mineral in all sections and the Fe/Mg ratio of olivine is important in controlling the physical properties of lithospheric regions (density, Vp, Vs).  Because olivine is rarely preserved (and contains no pressure or temperature information), we have developed a technique for inverting a garnet-olivine Fe-Mg exchange geothermometer to calculate the Fe/Mg of olivine coexisting with each garnet grain.  Application of this inversion to the southern Australian transect show high Mg# olivine in the shallow parts of the Proterozoic westerly sections with more Fe-rich olivine in the Phanerozoic east.  There also is an overall trend to lower Mg with increasing depth in each section.  Olivine density varies inversely with Mg# and the combination of density and geotherm differences makes thick old (Proterozoic and Archean) lithosphere buoyant relative to asthenosphere in contrast to young (Phanerozoic) Fe-rich mantle sections, which become more dense than asthenosphere at relatively low thickness (about 100 km).  Olivine Mg# also affects Vp and Vs (the higher the Mg#, the lower the density, and the higher the Vp and Vs) and thus is important in seismic interpretation, especially tomography. The increase in Fe with depth in all sections may reflect exchange with asthenospheric melts near the LAB and reinforces gravitational stability of the lithospheric mantle column.

At the micron-scale, GEMOC has recently developed in-situ Re-Os dating techniques for mantle sulfides: sulfide inclusions in primary mantle silicates can provide the age of the last major melting episode, commonly the lithospheric mantle formation age.  Preliminary results suggest there may be relict Proterozoic or Archean lithospheric mantle domains in northern Tasmania and the New England region, the latter coinciding with the deep cold signature for SKIPPY models in this area.

4-D Lithosphere Mapping using integrated data at global, regional, outcrop and micron scales has applications to defining important present-day Australian lithosphere domains relevant to mineral exploration and to tracking the change in lithosphere architecture and composition through the geological evolution of the Australian lithosphere.
 

References

O'Reilly, S.Y. and Griffin, W.L. 1996.  4-D lithospheric mapping: a review of the methodology with examples.  Tectonophysics 262, 3-18.

Griffin, W.L., O'Reilly, S.Y. and Ryan, C.G. 1999.  The composition and origin of subcontinental lithospheric mantle.  In: Y.Fei, C.M. Bertka and B.O. Mysen (eds.) Mantle Petrology:  Field observations and high-pressure experimentation : A tribute to Francis R. (Joe) Boyd, Geochemical Society Special Publication #6, The Geochemical Society (Houston).  pp. 13-45