Computer Science
Scientific paper
Aug 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.1997n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 8, pp.1997-2007
Computer Science
6
Scientific paper
Sr, Nd and Pb isotopic compositions are presented for a series of 290 to 250 Ma old basaltic to granitic rocks from the Oslo rift, southeast Norway. A large group of basalts, larvikites and rhomb porphyry lavas cluster within a restricted range of initial isotope ratios: t Sr : -3 to -16, t Nd : + 3.3 to + 4.2, ( 206 Pb / 204 Pb ) I : 18.9 to 19.3, ( 207 Pb / 204 Pb ) I : 15.59 to 15.66, ( 208 Pb / 204 Pb ) I : 38.6 to 39.1. The remainder of the rocks trend towards higher initial Sr and lower initial Nd and Pb isotopic ratios. Data are tested against mantle-crust mixing models. The Oslo rift magmatic rocks originated from a somewhat heterogeneous, mildly depleted source located in the subcrustal lithosphere. The isotopic character of this source has been inherited by the rocks showing uniform isotopic ratios, which have thus not suffered significant crustal contamination. The rest of the mafic to intermediate rocks and the syenites were contaminated in the intermediate to lower crust, which increased slightly their Sr-, but decreased their Nd- and Pb-isotopic ratios. As much as 40-50% contamination is suggested for some of the samples. Strong increases in radiogenic Sr, seen primarily in the granites, are attributed to contamination in the upper crust. The mildly depleted mantle proposed as the source for the Oslo rift mafic rocks has an isotopic character which can only be explained in terms of a multistage history which included periods of relatively high U/Pb and Th/Pb ratios, resulting from a relatively short depletion history in the subcontinental lithosphere and metasomatic enrichment of the mantle in selected elements.
Neumann Else-Ragnhild
Tilton George R.
Tuen Elisabeth
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