The Secular Evolution of the Subcontinental Lithospheric Mantle and its implications

W.L. Griffin1,2, S.Y. O'Reilly1 and M. Gregoire1

1. GEMOC, Macquarie, 2. CSIRO EM

Major changes in the composition of subcontinental lithospheric mantle (SCLM) with time can be inferred by comparing the compositions of peridotite xenoliths in Cenozoic basaltic rocks and in kimberlites that penetrate Archean cratons. The Archean SCLM is more strongly depleted, but differs from Phanerozoic SCLM in having lower Ca/Al and higher Si/Mg at any Mg# (Boyd, 1989). Other comparisons (Boyd, 1996) suggest that Phanerozoic SCLM is similar in many respects, but not all, to abyssal peridotites formed at mid-ocean ridges.

Lithosphere mapping using garnet concentrates from kimberlites and other volcanic rocks has produced images of the thermal state and lithostratigraphy of the lithospheric mantle to depths of 150 to 250 km in >35 localities worldwide. These sections also show consistent differences between Archean and younger lithosphere.

We have used major- and trace-element analyses of >13,000 peridotitic garnet xenocrysts in volcanic rocks worldwide, to examine secular trends in SCLM composition. The average composition of such xenocrysts is strongly correlated with the tectonothermal age of the continental crust penetrated by the host volcanic rocks. Garnet composition can in turn be correlated with lithology by comparison with data from xenoliths, and used to estimate the relative abundances of different rock types in individual mantle sections. Strongly subcalcic harzburgites, representing extremely depleted compositions, are found only in SCLM beneath Archean terrains. Mildly subcalcic harzburgites are common beneath Archean terrains, less abundant beneath Proterozoic terrains, and essentially absent beneath terrains with tectonothermal ages <1 Ga. Lherzolites (defined as all clinopyroxene-bearing peridotites) are the most common rock type even in Archean mantle, and make up essentially all of the lithospheric mantle beneath younger terrains. Garnets from lherzolites show a decrease of mean Cr content and Zr/Y, and a rise in mean Y and Y/Ga, with decreasing crustal age. Observed correlations between garnet composition and xenolith bulk-rock chemistry, and modelling using empirical element distribution coefficients, show that these changes in garnet composition reflect a rise in the average (cpx+gnt) and cpx/gnt of the peridotitic subcontinental lithosphere, from Early Proterozoic time to the present. This trend, coupled with the decrease in the relative abundance of harzburgites through time, requires that the average composition of subcontinental lithospheric mantle has become progressively less depleted in basaltic components throughout Earth's history, corresponding to a progressive decrease in the average degree of melt extraction from the material that became lithosphere. This change suggests an evolution in fundamental large-scale Earth processes, probably related to secular cooling.

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).

While the garnet data are consistent with xenolith data from Archean and Phanerozoic terrains, there are few xenolith suites from Proterozoic terrains, and xenoliths in Cenozoic basalts may sample mantle perturbed by the lithospheric thinning that accompanies the volcanism. Studies of orogenic peridotite massifs, and especially garnet peridotite massifs, may provide critical data on the Proterozoic to early Phanerozoic evolution of SCLM composition. Further studies of these bodies, such as the Moldanubian garnet peridotites (Becker, 1996), may shed further light on the secular evolution of subcontinental mantle, and on the processes that have produced it.

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