GEMOC ARC National Key CentreSuzanne Y. O'REILLY, William L. Griffin, Yvette Poudjom Djomani and
Paul Morgan
GEMOC Macquarie
The lithosphere-asthenosphere boundary (LAB) is an interface that reflects the fundamental evolution of the Earth into major geochemical reservoirs driven by global thermal processes. The depth to the LAB varies significantly with the tectonothermal age of lithosphere domains and is a function of the geochemical nature of the lithospheric mantle column and its thermal state which determine its density and rheological response. The Earth's lithospheric mantle is non-convecting and does not mix or homogenise easily: therefore it carries a geochemical imprint of major melting events and subsequent fluid fluxes (metasomatism). As a generalisation, lithospheric mantle has undergone significant (probably multiple) melting and is depleted in "basaltic" components such as Fe, Al, Ca and Ti. Asthenosphere by contrast, is "fertile" (rich in basaltic components) and geochemically more homogeneous than lithosphere because it is convecting.
Mantle xenoliths and their disaggregated minerals (especially garnet,
chromite and diamond) brought to the surface in basalts and kimberlites,
can be used to construct sections of the rock types and their spatial distribution
with depth in the lithospheric mantle, to estimate empirical palaeogeotherms,
and to locate the depth to the LAB. This geochemically determined depth
coincides with estimations of depth using geophysical datasets, especially
seismic refraction and magnetotelluric. We have established that the mean
geochemical composition of the subcontinental lithospheric mantle shows
an episodic secular and apparently irreversible change from Archean to
Proterozoic to Phanerozoic (Griffin et al., 1999). Density and seismic
measurements and calculations for these mantle types reveal an increase
in density and decrease in Vp and Vs with decreasing age (Poudjom Djomani
et al., 2001). Geothermal gradients increase concomitantly and average
depths to the LAB decrease from about 220 km to 180 km to 100 km respectively.
Density and thermal differences in these distinctive mantle volumes determine
the relative buoyancy of the specific lithospheric mantle column relative
to asthenosphere. Thus the geochemical composition and thermal state of
lithospheric mantle may control the location of the LAB through time and
result in the different geophysical signatures.
REFRENCES :
Griffin, W.L., O'Reilly, S.Y., and Ryan, C.G., 1999. The composition
and origin of sub-continental 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, The Geochemical Society,
Special Publication No. 6, pp 13-43
REFERENCES:
Poudjom Djomani, Y. H., O'Reilly , S. Y. and Griffin, W. L, 2001. The density structure of subcontinental lithosphere through time. EPSL, in press.