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Seventh Annual V. M. Goldschmidt Conference
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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 ([email protected]), 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 traceelement compositions of >8000 mantle-derived Crpyrope 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.
References: 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