Secular Variation in the Composition of Subcontinental Lithospheric Mantle

W. L. Griffin1,2, S. Y. O'Reilly1, C. G. Ryan2, O. Gaul1 and D. A. Ionov: 1National Key Centre for Geochemical Evolution and Metallogeny of Continents, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia (bill.griffin@mq.edu.au), 2CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia.

A synthesis of modal and compositional data for mantle-derived peridotites, and the major- and trace-element compositions of >8000 mantle-derived Cr-pyrope garnets, indicates that lithospheric mantle has formed episodically from the Archean to the present. The data document a secular and apparently irreversible change in the chemical composition of newly-created lithospheric mantle throughout the Earth's history. This change suggests an evolution in fundamental large-scale Earth processes. It also has important implications for the interpretation of seismic tomography, and means that lithosphere erosion or replacement will have major tectonic consequences.

The average composition of peridotitic garnet xenocrysts from volcanic rocks is strongly correlated with the tectonothermal age of the continental crust penetrated by the eruptions. Garnet composition can be correlated with lithology by comparison with data from mantle-derived 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 lithospheric mantle 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 (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 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, suggest 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 indicates 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 lithospheric mantle.

The Archean-Proterozoic boundary represents a major change in the processes that form continental lithospheric mantle; since 2.5 Ga there has been a pronounced, but more gradual, secular change in the nature of these processes. Actualistic models of lithosphere formation based on modern processes may be inadequate, even for Proterozoic time. The correlation between mantle composition and crustal age indicates that the continental crust and the underlying lithospheric mantle are formed together, and generally stay coupled together for periods of eons. The stability and thickness of Archean lithospheric mantle is directly related to its low density, which in turn reflects both its high degree of geochemical depletion and its low Mg/Si. The latter characteristics produce high seismic velocities, and compositional factors may account for at least half of the velocity contrast between Archean and younger areas, seen in seismic tomography. The higher density and mantle heat flow of younger, less depleted mantle lithosphere imposes severe limits on its thickness and ultimate stability, because the cooler upper parts of Proterozoic or Phanerozoic lithospheric sections will be negatively buoyant relative to the underlying asthenosphere. The replacement of Archean lithosphere by Phanerozoic lithosphere, as in eastern China, (Griffin et al., 1997) may initially lead to large-scale uplift due to thermal effects, but ultimately will lead to subsidence and basin formation, as a result of the changes in thickness and density of the lithospheric column.

Reference

Griffin, W.L., Andi, Z., O'Reilly, S.Y. and Ryan, C.G. 1997. Phanerozoic evolution of the lithosphere beneath the Sino-Korean Craton. In: Mantle Dynamics and Plate Interactions in East Asia (Flower, M., Chung, S.L., Lo, C.H. and Lee, T. Y. eds) American Geophysical Union Spec. Publ., in press

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