Silicate-perovskite and majorite signature komatiites from the Archean Abitibi Greenstone Belt: Implications for early mantle differentiation and stratification

Details Silicate-perovskite and majorite signature komatiites from the Archean Abitibi Greenstone Belt: Implications for early mantle differentiation and stratification Silicate-perovskite and majorite signature komatiites from the Archean Abitibi Greenstone Belt: Implications for early mantle differentiation and stratification

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9915799x&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. B8, p. 15,799-15,812

Physics

12

Earth Mantle, Earth Planetary Structure, Geochemistry, Perovskites, Silicates, Stratification, Aluminum Oxides, Chemical Composition, Fractionation, Olivine, Planetary Evolution, Tables (Data), Titanium Oxides, X Ray Spectroscopy

Scientific paper

Three komatiite-tholeiite sequences from the Archean Abitibi Southern Volcanic Zone (SVZ), which are separated by major terrane boundaries, show systematic differences in composition, rare earth element (REE) patterns and high field strength element (HFSE)/REE fractionation. Komatiites from Tisdale Township are Al-undepleted, with MgO = 12-24%, Al2O3/TiO2 = 13-17, CaO/Ai2O3 = 1-1.3, and high Ni (722-1275 ppm) and Cr (1875-2820 ppm) contents. Mg-tholeiites and Fe-tholeiites are spatially associated with komatiites. Both komatiites and Mg-tholeiites have flat REE patterns, (La/Yb)n = 0.6-1.2, and Nb/Nb(*), Zr/Zr(*), and Hf/Hf(*) approx. equals 1 (calculated using primitive mantle normalized values and adjacent REEs in the incompatability sequence of elements in mid-ocean ridge basalt; MORB), suggesting that they may have formed by high-degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths (less than 300 km). Munro Township komatiites are Al-undepleted, with MgO = 22-30%, Al2O3/TiO2 = 17-20, CaO/Al2O3 = 1.0-1.2, and high Ni (831-1590 ppm) and Cr (1250-2760 ppm) contents. They have light rare earth element (LREE) depleted patterns ((La/Yb)n = 0.2-0.4), and Nb/Nb(*), Zr/Zr(*), and Hf/Hf(*) greater than 1, consistent with having been derived from Mg-perovskite enriched sources at depths greater than 700 km below the transition zone, whereas spatially associated tholeiites have (La/Yb)n = 1-2, and Nb/Nb(*), Zr/Zr(*), and Hf/Hf(*) approx. equals 1, suggesting a shallow undepleted mantle source, distinct from the komatiites. Komatiites and tholeiites from Boston Township are characterized by MgO-11-29%, Al2O3/TiO2 = 4.5-5.4, CaO/Al2O3 = 1.4-2.5, high Ni (203-3420 ppm) and Cr (194-1965 ppm) contevts; Al, Sc, Y, and heavy rare earth element (HREE) depletion, and pronounced negative normalized Zr and Hf anomalies (Zr/Zr(*) and Hf/Hf(*) less than 1), in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300-600 km.

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