Samuel Niedermann

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.

ATMOSPHERIC NOBLE GASES IN VOLCANIC GLASSES FROM THE SOUTHERN LAU BASIN : ORIGIN FROM THE SUBDUCTING SLAB?

Noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island arc. 3He/4He ratios are slightly below average MORB (6.8–7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe have isotopic compositions very similar to air. Together with previously published data from the Valu Fa Ridge and other spreading segments in the Lau Basin, our data show a systematic latitudinal variation of increasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere-like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H2O concentrations. Based on these observations and mass balance arguments, we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust. Our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar He and Ne. In addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its higher diffusivity, and thus He transport from the subducting slab into the mantle wedge is probably insignificant. We propose that the 3He/4He ratios lower than MORB observed in the VFR lavas result from radiogenic ingrowth of He in a highly depleted, and hence degassed, mantle wedge after the enrichment of U and Th released from the downgoing slab.

Correction to “Evidence for ascending upper mantle‐derived melt beneath the Cheb basin, central Europe”

[1] In the paper ‘‘Evidence for ascending upper mantlederived melt beneath the Cheb basin, central Europe’’ by K. Brauer et al. (Geophysical Research Letters, 32, L08303, doi:10.1029/2004GL022205, 2005), a false sampling date was accidentally given in the table. The first sampling date of location 14 Bad Brambach, Eisenquelle was 1992/04/27 instead of 1994/07/07. In consequence of this mistake one point of Figure 2 is not correct. GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L18304, doi:10.1029/2005GL024344, 2005