Other
Scientific paper
Nov 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.107..272j&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 107, no. 2, Nov. 1991, p. 272-289.
Other
42
Basalt, Earth Mantle, Lanthanum, Niobium, Petrology, Thorium, Mineralogy, Rare Earth Elements, Secular Variations, Trace Elements
Scientific paper
The model of Hofmann et al. (1986) of secular variation of the composition of the upper mantle is tested through new spark-source mass spectrometric analyses of Nb, Th, REE, and other trace elements in Archaean to Tertiary komatiites and basalts. Most 3.4-Ga komatiites and basalts have Nb/Th between 7 and 9, values that stradddle the primitive mantle ratio of about 8. Several 2.7-Ga komatiites also have Nb/Th of 7-9, but most samples of this age have higher Nb/Th, in the range 10-15. It is argued that komatiites come from a source that evolved independently from the normal upper mantle, and that the depletion of Nb and Th resulted from fractionation of an unknown phase during the deep melting. Certain tholeiitic basalts do not show unusual Nb-Th-La fractionation but exhibit a secular increase in Nb/Th. It is proposed that this variation indicates a change in upper mantle compositions resulting from progressive withdrawal of continental crust during the past 3 Ga.
Arndt Nicholas T.
Hofmann Albrecht W.
Jochum Klaus Peter
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