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

Abstract

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°30′S, thus extending the database for the 50°-53°S Shona section [Moreira et al., 1995]. The 44°-53°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 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal-type, low 206Pb/204Pb component named LOMU (low μ, where μ=238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, 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 40K/36Ar ratio 2-5 times lower than the convective mantle and a 238U/3He ratio 2-3 times higher. Copyright 2000 by the American Geophysical Union.