Computer Science
Scientific paper
Nov 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979e%26psl..45..263b&link_type=abstract
Earth and Planetary Science Letters, Volume 45, Issue 2, p. 263-274.
Computer Science
5
Scientific paper
Rb-Sr and Pb isotopic data are reported for some of the Karroo volcanics from Swaziland. Linear arrays of Karroo data are found in the Rb-Sr and Pb/Pb isochron diagrams. Certain of these linear arrays, if interpreted as isochrons, give ages considerably in excess of the age normally accepted for the Karroo and must be interpreted as ``erupted isochrons''. It is argued that these ``erupted isochrons'' represent mixing lines resulting from contamination of mantle-derived magma by continental crust. It is unlikely that the Rb-Sr ``erupted isochron'' has any age significance because the Rb/Sr ratios were probably fractionated at the time or eruption, both during the contamination event and during subsequent fractionation. ``Erupted isochrons'' in the Pb/Pb isochron diagram are much more likely to have time significance because relatively recent U/Pb fractionation events do not significantly affect the Pb isotopic compositions. The Pb/Pb ``erupted isochron'' has an apparent age of 3260 +/- 60 m.y. This age is very similar to that of the Kaapvaal craton on which the volcanics rest and it is argued that the ``erupted isochron'' approximately dates the formation of the Kaapvaal craton. Six Pb isotope analyses of separated feldspar crystals are also reported. Three of these were demonstrably out of isotopic equilibrium with the magma at the time of eruption. This shows that isotopic heterogeneities on the scale of a few millimetres may be preserved between crystals and their host magma. The nature of the crust which contaminated the magma is considered and it seems most likely that the magma was contaminated by heterogeneous crust of uniform age. Present address: Department of Earth Sciences, Leeds University, Leeds, U.K.
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