FY 1997 Correlated He and Sr isotope ratios in South Atlantic near-ridge seamounts and implications for mantle dynamics Graham, D.W., P. Castillo, J.E. Lupton, and R. Batiza Earth Planet. Sci. Lett., 144(3–4), 491–504, doi: 10.1016/S0012-821X(96)00172-0 (1996) 4He/3He and 87Sr/86Sr ratios are highly anti-correlated for a suite of seamount glasses from both sides of the Mid-Atlantic Ridge at 26°S; the linear correlation coefficient (r2) is 0.99 for 5 localities at 3 different seamounts. The seamounts are located on crust up to 2.5 myr old, and have 4He/3He as low as 65,400 (3He/4He = 11 RA) and 87Sr/86Sr as high as 0.70350. These isotopic values are significantly lower and higher, respectively, than those for basaltic glasses recovered from 13 localities along the adjacent ridge axis, where the lowest 4He/3He ratio is 92,000 (3He/4He = 7.8 RA) and the highest 87Sr/86Sr is 0.70258. Geophysical studies and the small (1–2%) degree of helium isotope disequilibrium between vesicles and glass for three seamount lavas suggest that the seamounts formed on or near the ridge axis. Because no off-ridge hotspots are present in this area, formation of the seamounts probably involved capture by the ridge of a passive mantle heterogeneity or ‘blob’ during rift propagation and tectonic evolution of the Moore fracture zone. The He—Sr—Nd—Pb isotopic results for the seamounts show a general trend toward compositions observed for the Réunion hotspot in the Indian Ocean. Collectively, the seamount and ridge axis results are somewhat enigmatic. In addition to the highly correlated He and Sr isotopes at the seamounts, a fair correlation exists between He and Nd isotopes (r2 = 0.70). In contrast, a correlation between He and Pb isotopes is absent for the seamount glasses, while an independent, positive correlation exists between 4He/3He and 206Pb/204Pb for axial lavas. Apparently, different processes are responsible for the seamount He—Sr—Nd isotope relationships and for the nearby ridge He—Pb isotope relationship. If these relations are only of local significance and result from complications inherent in multi-stage mixing of more than two mantle components, then they imply that the upper mantle may contain domains with variable 4He/3He ratios, in some cases significantly lower than 80,000 (3He/4He> 9 RA). On the other hand, binary mixing adequately explains the linear He—Sr isotope trend in the seamount lavas. This linear trend suggests similar 3He/86Sr ratios in the local MORB mantle source and in the source region of the low 4He/3He blob, which is most likely the lower mantle or the transition zone region. This similarity in 3He/86Sr is inconsistent with a lower mantle 3He/86Sr ratio that exceeds the upper mantle ratio by at least a factor of 50, deduced from geochemical models of mantle evolution. Consequently, rare gas models invoking a steady-state upper mantle and quasi-closed lower mantle may be inappropriate if applied at length scales on the order of ∼ 100 km, characteristic of mid-ocean ridge segments. Feature Publications | Outstanding Scientific Publications Contact Sandra Bigley | Help
