Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island, South Atlantic

Details Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island, South Atlantic Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island, South Atlantic

Aug 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..60...79h&link_type=abstract

Earth and Planetary Science Letters, Volume 60, Issue 1, p. 79-85.

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New Sr and Pb isotope data are presented for a selection of lavas and associated coarse-grained blocks from Ascension Island. K-Ar dates for the lavas range up to 1.5+/-0.2 Ma. Initial 87Sr/86Sr ratios are consistent with earlier measurements and for most rocks are ca. 0.7029, but range up to 0.7135 in the case of the most evolved lavas and blocks. Pb isotope data are also consistent with earlier measurements, but the Pb in two gabbroic blocks is less radiogenic than Pb in the other rocks. It is suggested that these gabbroic blocks crystallized from a magma of tholeiitic composition whose source was similar to that of mid-oceanic ridge basalt whereas the lavas and other blocks crystallized from mildly alkaline magmas derived from a source further from the crest of the Mid-Atlantic Ridge. The high 87Sr/86Sr ratios result from contamination of the most silicic magma by radiogenic Sr from pelagic sediments. These data and their interpretation are consistent with the petrological and geochemical observations that the granite blocks are the coarse-grained equivalents of the volcanic suite [11] and not fragments of relict continental material [2,3].

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