Noble gas evidence for a lower mantle component in MORBs from the southern East Pacific Rise: Decoupling of helium and neon isotope systematics

Abstract

Abundances and isotopic compositions of all noble gases have been determined in basalt glasses from different latitudes on the 13–23°S section of the East Pacific Rise. In this region earlier work has identified isotopic signatures of Sr, Nd, and Pb as well as He which indicate a plume-like component admixed to the depleted MORB mantle between about 15.8° and 20.7°S (Bach et al., 1994; Mahoney et al., 1994; Kurz et al., 1997), although incompatible trace element ratios yield no such evidence. Our noble gas data confirm plume-like isotope ratios for He, Ne, and probably Ar, whereas Kr and Xe compositions are atmospheric. Interestingly, He and Ne data do not correlate very well, as the plume-like Ne component can be traced farther south than plume-like He. A similar decoupling of isotopic patterns has been observed for He vs. the radiogenic isotopes (Sr, Nd, Pb) by Mahoney et al. (1994) and is confirmed here. Mixing models suggest that between 13.0° and 15.8°S, a small amount of undegassed material from the lower mantle is admixed to the depleted MORB mantle, whereas from 15.8° to 20.7°S a considerable, but constant proportion of plume material, which prior to mixing has been degassed to progressively larger extents proceeding to the south, was present in the magma source. South of 20.7°S the lower mantle component is virtually absent. Several processes are discussed to explain these observations, such as passive mantle heterogeneities (Mahoney et al., 1994), plume-ridge interaction (e.g., Poreda et al., 1993), or a leaking lower mantle. Based on our neon isotope data and references from the literature, we discuss the evidence for a lower 20 Ne 22 Ne ratio in the MORB reservoir than in the plume source, as might be expected if the nucleogenic 21 Ne 22 Ne production ratio in the mantle is smaller than predicted.