Abstract
Uranium-helium “ages” have been determined for a suite of 25 whole-rock basalts from Madeira and Porto Santo islands in the Madeira Archipelago. We include petrographic descriptions of these samples. Uranium measurements were by delayed-neutron activation analysis. Helium measurements were by isotopic dilution in an all-metal system characterized by very low argon and helium blanks. For 19 of the samples “preferred” K Ar ages were also obtained. None of the rocks are concordant in these two ages; all have lost helium. The uranium-helium “ages” are best described, on the average, as equal to 74% of the K Ar ages minus 0.2 m.y. Nevertheless in our suite of young, chemically similar lavas the correlation between the K Ar age and the helium content is good enough to make the helium measurement worthwhile as a check on a possibly anomalous argon age, especially in a system where the additional labor required to make the helium is minimal. The Columbia River basalt from which standard samples BCR-1 and BCR-2 were prepared was also dated. Its uranium-thorium-helium “age”, 4.45 ± 0.24 m.y. shows that this rock has retained even less of its radiogenic helium (30%) than the island basalts.
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