Cratonic Mantle Roots, Relicts of a more Chondritic Archean Mantle.

Details Cratonic Mantle Roots, Relicts of a more Chondritic Archean Mantle. Cratonic Mantle Roots, Relicts of a more Chondritic Archean Mantle.

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...v51a05f&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #V51A-05

Physics

1025 Composition Of The Mantle, 3640 Igneous Petrology, 3655 Major Element Composition

Scientific paper

A fundamental mass balance problem has not been resolved by those who would derive cratonic mantle roots from a Pyrolitic primitive mantle source by extensive partial melting. Archean high-Mg magmas are too low in Al and rich in Si to leave behind a refractory residue with a composition of the garnet harzburgite xenoliths thought to be derived from the mantle roots beneath Archean cratons. This problem is particularly acute for the Si-rich harzburgites of the Kaapvaal and Siberian cratons, but remains even for cratons whose mantle roots approach the composition of modern MORB harzburgite. Accepting a Pyrolitic primitive mantle for the Archean leaves us in the embarrassing position of having komatiitic magmas with no complimentary depleted-mantle residue, and residual cratonic mantle roots, representing large degrees of melting, and extending to hundreds of kilometers beneath continental cratons, but with no complementary lavas. These problems would be resolved if the fertile mantle in the Archean were richer in Si and Fe than fertile mantle today, more similar in composition to chondritic meteorites. The lever rule is consistent with modern Pyrolite mantle being derived by approximately 20 percent fractionation of Mg-Fe perovskite from the composition of carbonaceous chondrites to the lower mantle since the Hadean. Forsaking conventional geodynamic wisdom that the Earth has been in a steady state since the Hadean, and accepting the possibility that the Earth's convecting upper mantle has become poorer in Fe and Si since the Archean, not only provides a simple way of relating Archean high Mg lavas to cratonic mantle roots, but could lead to new models for the nature of the lower mantle sources of modern hot-spot volcanism.

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