Isotope Geochemistry of the Oceanic Mantle Near the Bouvet Triple Junction

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 3He/ 4He ratios range from 6.5 to 14.2 times atmospheric (R a), which encompasses the entire range of existing MORB data, and differs significantly from the commonly accepted value for depleted MORB of roughly eight times atmospheric (Ra). Several samples from near the Bouvet triple junction have extremely low helium concentrations, low 3He/ 4He 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 4He. 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 MORB to those of Bouvet Island itself ( 206Pb/ 204Pb of 18.2–19.6). The results are consistent with a mantle plume origin of Bouvet Island in that high 3He/ 4He and 206Pb/ 204Pb 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 3He/ 4He 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°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 3He/ 4He ratios were obtained from this ridge segment (∼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