Rare gas systematics on the southernmost Mid-Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have bee n measured for mid-ocean ridge basalt glasses from the Mid-Atlantic Ridge D iscovery section, centered at 47 degrees 30'S, thus extending the database for the 50 degrees-53 degrees S Shona section [Moreira et al., 1995]. The 4 4 degrees-53 degrees S part of the Mid-Atlantic Ridge includes the Discover y and Shona bathymetric and geochemical ridge anomalies [Douglass et al., 1 999], which also appear clearly in the rare gas isotopic record. In additio n to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal-relat ed component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degass ed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal ve ry consistent along-strike He, Ne, and Ar isotopic patterns, also consisten t with Xe data. These systematics provide evidence that plume argon has low Ar-40/Ar-36 and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal-type, low Pb-206/Pb- 204 component named LOMU (low mu, where mu=U-238/Pb-204) by Douglass et al. [1999], and has radiogenic He-4/He-3 and Ne-21/Ne-22, relatively elevated Ne-20/Ne-22, mildly radiogenic Ar-40/Ar-36, and low Xe isotopic ratios, pos sibly representing the Dupal rare gas signature. Interpretations of this co mponent as either recycled oceanic crust, or delaminated subcontinental lit hosphere are consistent with the rare gas systematics, In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, t he sublithospheric mantle should have a K-40/Ar-36 ratio 2-5 times lower th an the convective mantle and a U-238/He-3 ratio 2-3 times higher.