Low 3He/ 4He ratios in basalt glasses from the western Southwest Indian Ridge (10°-24°E)

Abstract

3He/ 4He ratios and total helium concentrations have been measured in a suite of basalt glasses from the ultra-slow spreading (0.8 cm/yr half-rate) western Southwest Indian Ridge (SWIR). The study area is divided into two subregions, a supersegment between 9° and 16°E, where poorly defined ridge segments lie at a highly oblique angle to the regional spreading direction, and a supersegment between 16° and 24°E, where ridge segments are nearly orthogonal to the spreading direction. Although earlier geochemical studies suggested that the western oblique supersegment may be affected by the Bouvet plume more than 700 km to the west, there is no evidence for elevated 3He/ 4He in the region. In fact, 3He/ 4He throughout the entire study area ranges from 6.3 to 7.3 R a, significantly below normal mid-ocean ridge basalt helium isotopic ratios of 8±1 R a. The preferred explanation for low 3He/ 4He ratios is recycled crustal or lithospheric material in the SWIR mantle source, although thorough evaluation of this explanation awaits additional geochemical data. 4He concentrations for the oblique supersegment are generally lower than those for the orthogonal supersegment, which may reflect variable extents of magmatic degassing along the ridge. The low 3He/ 4He ratios are not attributable to radiogenic ingrowth following post-eruptive degassing, because 4He concentrations are too high (>0.4 μccSTP/g). In addition to low ratios, 3He/ 4He measurements for the western SWIR are characterized by low variability, which contradicts predictions of an inverse relationship between spreading rate and helium isotope standard deviation. 3He/ 4He ratios for the orthogonal supersegment increase systematically from west to east, and are positively correlated with mantle Bouguer anomaly, a correlation which is opposite to that observed in other ridge environments. Overall, the range in 3He/ 4He isotopic values for the SWIR is significantly different from that measured along other ridges, suggesting that there may be systematic variability of 3He/ 4He ratios between ridge systems.