Isotope Geochemistry of the Oceanic Mantle Near the Bouvet Triple Junction
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We have measured the isotopes of helium and lead in three suites of dredged basalts from the Southwest Indian Ridge, the America-Antarctic Ridge, and the Mid-Atlantic Ridge near the Bouvet triple junction. The magmatic He-3/He-4 ratios range from 6.5 to 14.2 times atmospheric (R-a), which encompasses the entire range of existing MORE data, and differs significantly from the commonly accepted value for depleted MORE of roughly eight times atmospheric (Ka). Several samples from near the Bouvet triple junction have extremely low helium concentrations, low He-3/He-4 ratios, and display internal isotopic disequilibrium between glass and vesicles. These basalt glasses are, therefore, unique among the sample suite in having a significant contribution from post-eruptive accumulation of radiogenic He-4. Although these data do not reflect the mantle source, they can be used to calculate eruption ages for the basalts, which fall between 7 and 100 kyr, and demonstrates the potential utility of He-U ages for MORB. Lead isotopic compositions from the dredged basalt glasses range from depleted MORE to those of Bouvet Island itself (Pb-206/Pb-204 of 18.2-19.6). The results are consistent with a mantle plume origin of Bouvet Island in that high He-3/He-4 and Pb-206/Pb-204 ratios are observed both on Bouvet island and the ridge segment immediately adjacent to it. The American-Antarctic Ridge has lower He, Sr and Pb isotope ratios than those of the Southwest Indian Ridge, which suggests a lesser influence of the plume. The dredge samples from near the triple junction have relatively low He-3/He-4 ratios (7.4 +/- 0.2 R-a), suggesting that the plume is presently centered beneath Bouvet Island rather than beneath the triple junction. The data from the ridge segment adjacent to Bouvet Island, on the Southwest Indian Ridge, define a trend toward high helium, strontium, and lead isotope ratios, which could be explained by mixing processes. A single dredge from one ridge segment at 7 degrees E, between the Islas Orcadas and Shaka Fracture Zones, defines most of the helium isotopic variability found throughout the region and defines an isotopic composition unique in the region. The highest He-3/He-4 ratios. were obtained from this ridge segment (similar to 14 R-a) and are associated with relatively unradiogenic strontium and lead isotopic compositions. The remarkable isotopic variability over short distances along the Southwest Indian Ridge and the America-Antarctic Ridge may be related to their slow spreading rates and the lack of steady state magma chambers which could homogenize geochemically distinct magma batches. Alternatively, there is a different scale of heterogeneity in the mantle beneath this region. Copyright (C) 1998 Elsevier Science Ltd.