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GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, VOL. 4, NO. 1, 1005, doi:10.1029/2002GC000333, 2003

Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time

Kent C. Condie

Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico, 87801, USA


Abstract

Ratios of elements with similar incompatibilities in the mantle can be used to characterize magma sources through time. Nb/Y and Zr/Y distributions in oceanic basalts support the existence of a long-lived, deep depleted source in mantle. Zr/Y, Nb/Y, Zr/Nb, and Nb/Th ratios in oceanic basalts and komatiites suggest that depleted and recycled components, together probably with an enriched component, were present in the deep mantle by 3.5 Ga. Low Zr/Nb and Hf/Sm ratios and high La/Yb and Nb/Y ratios in some plume basalts and Al-depleted komatiites may reflect majorite fractionation. High Zr/Nb ratios and low Nb/Y ratios in Archean Al-undepleted komatiites may record partial melting of a Mg-perovskite source in deep mantle plumes in which Mg-perovskite crystallizes and accumulates in komatiite melts during ascent. Oceanic greenstone basalts show a gradual increase in the Nb/Th ratio with time with a relatively sudden increase at about 2 Ga. This trend is consistent with gradual continental growth and with a major episode of continental growth at 2.7 Ga. Nb/Th ratios in some Early Archean basalts may record extraction of up to 25% of the present volume of continental crust from the early upper mantle. An alternative explanation for the rapid increase in Nb/Th in oceanic basalts at 2 Ga is that a catastrophic 2.7-Ga event in the mantle changed the composition or/and location of the primary volume of mantle from which continental crust was extracted.

Received 18 February 2002; accepted 12 August 2002; published 14 January 2003.

Keywords: Mantle plumes; mantle plume events; continental growth rate; mantle sources; komatiites.

Index Terms: 1010 Geochemistry: Chemical evolution; 1065 Geochemistry: Trace elements (3670); 1645 Global Change: Solid Earth; 8125 Tectonophysics: Evolution of the Earth.

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Citation: Condie, K. C. (2003), Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time, Geochem. Geophys. Geosyst., 4(1), 1005, doi:10.1029/2002GC000333.