NOBLE-GAS EVIDENCE FOR A LOWER MANTLE COMPONENT IN MORBS FROM THE SOUTHERN EAST PACIFIC RISE - DECOUPLING OF HELIUM AND NEON ISOTOPE SYSTEMATICS

Abundances and isotopic compositions of all noble gases have been dete rmined in basalt glasses from different latitudes on the 13-23 degrees S section of the East Pacific Rise. In this region earlier work has i dentified isotopic signatures of Sr, Nd, and Pb as well as He which in dicate a plume-like component admired to the depleted MORB mantle betw een about 15.8 degrees and 20.7 degrees S (Each 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 i sotope ratios for He, Ne, and probably Ar, whereas Kr and Xe compositi ons are atmospheric. Interestingly, He and Ne data do not correlate ve ry well, as the plume-like Ne component can be traced farther south th an plume-like He. A similar decoupling of isotopic patterns has been o bserved for He vs. the radiogenic isotopes (Sr, Nd, Pb) by Mahoney et al. (1994) and is confirmed here. Mixing models suggest that between 1 3.0 degrees and 15.8 degrees S, a small amount of undegassed material from the lower mantle is admired to the depleted MORB mantle, whereas from 15.8 degrees to 20.7 degrees S a considerable, but constant propo rtion of plume material, which prior to mixing has been degassed to pr ogressively larger extents proceeding to the south, was present in the magma source. South of 20.7 degrees S the lower mantle component is v irtually absent. Several processes are discussed to explain these obse rvations, such as passive mantle heterogeneities (Mahoney et al., 1994 ), plume-ridge interaction (e.g., Poreda et al., 1993), or a leaking l ower mantle. Based on our neon isotope data and references from the li terature, we discuss the evidence for a lower Ne-20/Ne-22 ratio in the MORB reservoir than in the plume source, as might be expected if the nucleogenic Ne-21/Ne-22 production ratio in the mantle is smaller than predicted. Copyright (C) 1997 Elsevier Science Ltd.