GEMOC ARC National Key Centre

S. Y. O'Reilly¹, W. L. Griffin^1,2, Y. H. Poudjom Djomani¹ and P. Morgan^1,3.

1 Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia (sue.oreilly@mq.edu.au),
2 CSIRO Exploration and Min-ing, PO Box 136, North Ryde, NSW 1670, Australia,
3 Dept. of Geology, N. Arizona Univ., Flagstaff, USA
 

 Introduction: The interpretation of geophysical datasets reflects remote responses to the physical prop-erties of Earth's interior at the present day. Translating this information into actualistic Earth models requires input from petrological and geochemical observations and measurements using tangible samples of the deep-earth materials. In recent years exciting new tools, collectively termed 4-D Lithosphere Mapping [1], have been sequentially developed to track the evolution of lithospheric mantle, based on the integration of geochemical and petrophysical properties of mantle-derived materials with tectonic syntheses and geophysical datasets. This methodology can provide some important constraints on fundamental questions such as the compositional structure of sub-continental lithospheric mantle (SCLM) formed at different times, the lateral variability of SCLM composition and its effects on tectonics, and the extent to which lithospheric mantle can be recycled into the convecting mantle, or is irreversibly differentiated from it. One of the most important results has been the demonstration (quantified using a large database of xenolith and garnet and chromite xenocryst compositions) that the composition of the SCLM varies in a systematic way correlated with the age of the last major tectonothermal event in the overlying crust (lower MgO and higher CaO and Al₂O₃ with decreasing age) [2].

The compositional variations in different SCLM volumes result in differences in the density and elastic properties of lithospheric mantle of different age [3]. Archean mantle roots are highly buoyant; they cannot be delaminated but require mechanical disaggregation (lithospheric thinning and/or rifting) and infiltration of upwelling fertile material to be destroyed or transformed. This buoyancy, combined with the refractory nature of Archean SCLM may explain the thickness and longevity of Archean lithospheric keels.  However, 4-D Lithosphere Mapping (using mantle fragments sampled by magmas of different ages in some regions such as eastern China) documents the dispersion of old, buoyant SCLM and the upwelling of young fertile asthenospheric mantle accompanied by lithospheric thinning and heating, consistent with detailed tomographic imaging.  Application of gravity modelling techniques to determine regional elastic thickness in the part of the Archean craton in Siberia most densely penetrated by Mesozoic kimberlites, shows a N-S lithosphere-scale structurally weak zone within the lithospheric mantle that indicates lithosphere modification by deformation. Typical Phanerozoic SCLM sections (about 100km thick) are buoyant under conditions of high geothermal gradient (e.g. during their formation). However, they are at best neutrally buoyant after cooling to typical stable conductive geotherms and vulnerable to Rayleigh-Taylor instability and will tend to delaminate and sink. Asthenospheric material welling up into the resulting "space" will cool to form a new, little-depleted SCLM; this will raise geotherms and may cause melting in the overlying crust. As this new SCLM cools down, it in turn will become unstable, and start the cycle again. This cyclic delamination may explain the ubiquitous presence of fertile xenolith suites in young basalts erupted through Paleozoic-Mesozoic orogenic belts [2].

Olivine is the most abundant mineral in all mantle sections and the Fe/Mg ratio of olivine is important in controlling the physical properties of the lithosphere (density, Vp, Vs).  Increasing Mg % of olivine results in decreasing density but increasing seismic velocity. Because olivine is rarely preserved (and contains no P or T 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 xenocrystic garnet grain [4]. Application of this inversion to determine the compositional variation of olivine in a spatial context for well-characterised SCLM regions such as the Siberian traverse, the Kaapvaal Craton, the Slave Craton and southern Australia independently confirms higher-Mg olivine in Archean sections with more Fe-rich olivine in the Phanerozoic. There is also a decrease in the mg# of olivine, paralleling an increase in Zr and Ti of garnet with depth in many SCLM sections worldwide, and we suggest that this pattern reflects the transformation of SCLM over time by addition of asthenosphere-derived material.

The contrasting properties of different mantle do-mains require lateral contrasts in composition, density, thickness and seismic response in the present-day SCLM. They also suggest a secular evolution in Earth's geodynamics from Archean to Proterozoic time, and an increased importance for lithosphere-delamination processes in Phanerozoic orogens. The intrinsic buoyancy of Archean SCLM places major constraints on the tectonic behaviour of old continents; some dynamic models indicate that without this buoyancy in their cratonic cores, continents would not have survived on the convecting Earth.

[1] O'Reilly et al., GSA Today, in press, April 2001.
[2] Griffin et al., (1999) Geoch. Soc. Pub.6, p13-43
[3] Poudjom Djomani et al. (2001) EPSL 184 605-621
[4] Gaul et al. (2000) EPSL, 182, 223-235