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

摘要

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid-ocean ridge basalt glasses from the Mid-Atlantic Ridge Discovery 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 44 degrees-53 degrees S part of the Mid-Atlantic Ridge includes the Discovery and Shona bathymetric and geochemical ridge anomalies [Douglass et al., 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal-related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along-strike He, Ne, and Ar isotopic patterns, also consistent 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, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere 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, the sublithospheric mantle should have a K-40/Ar-36 ratio 2-5 times lower than the convective mantle and a U-238/He-3 ratio 2-3 times higher. [References: 94]