Noble gases in mantle plumes.

Trieloff et al. (1) provided neon isotope data to argue that mantle neon originates as a trapped component in meteorites rather than having a pristine solar composition. That finding, if true, has important consequences for understanding the mechanism of volatile input into the mantle, because it rules out models involving direct incorporation of solar gases. The main argument that Trieloff et al. put forward was that a dunite from Hawaii and a basalt glass from Iceland show the same upper limit Ne/Ne value, 12.55 6 0.11, in the last stages of crushing. This value, although distinct from the solar wind value of Ne/Ne 5 13.8, is indistinguishable from the mixture of implanted neon from solar wind and solar energetic particles (Ne/Ne 5 11.2) that is trapped in lunar and meteorite grains (2). Trieloff et al. argued that the constant Ne/Ne value lower than solar wind cannot be due to the addition of an air contaminant (Ne/ Ne 5 9.8), because that would require a mechanism capable of contaminating the sample in proportion to the amount of Ne released in the final stages of crushing, which varied from 1.7 3 10 to 12.4 3 10 cm at standard temperature and pressure. Air-like noble gas contamination of samples is not removed by exposure to vacuum together with moderate heating, and requires a high-energy environment for release. There is no evidence that the first steps of the ball mill crushing technique employed by Trieloff et al. quantitatively remove this air contamination. Air-like noble gases account for lower Ne and Ar isotope values in almost all other reported samples. In basalt glass, air contamination is proportional to vesicularity, and air is almost certainly trapped within the matrix surrounding the vesicles (3). The final stages of crushing that release the last gas trapped in inclusions may also release the last vestiges of vesicularity-related contamination in proportion to the varying amounts of Ne released. This would provide a mechanism to produce the observed Ne/Ne “plateau” effect in the Iceland glass. Irrespective of whether the same mechanism applied to the dunite sample examined by Trieloff et al., any measured Ne/Ne values can only be considered lower limits on the mantle composition. Harrison et al. (4), using a pressure fracturing technique rather than a high-energy ball mill, reported Ne/Ne 5 13.75 6 0.32—indistinguishable from the solar value—in the same Iceland sample analyzed by Trieloff et al. The lower Ne/Ne values reported by Trieloff et al. thus may only represent the upper limit of the ball mill crushing technique. If the mantle were indeed dominated by the proposed trapped Ne component, no major portion of the mantle would contain Ne/Ne values greater than ;12.5. The mantle that supplies mid-ocean ridge basalts (MORB) provides a useful test case. Employing the same release technique as Trieloff et al., Moreira et al. (5) also found a maximum value of Ne/Ne ' 12.5 in the gas-rich MORB sample 2pD43, and showed a clear correlation between Ar/ Ar and Ne/Ne for different release steps. This correlation suggests that if the MORB-source mantle has Ne/Ne 5 12.5, then it would have Ar/Ar ' 25,000 (Fig. 1). Fisher reports Ar/Ar . 35,000 in MORB from the East Pacific Rise (6), and Burnard et al. report Ar/ Ar at up to 40,000 in another portion of the 2pD43 sample, with the gas released from individual vesicles using a laser (7). Combined with the Ne-Ar correlation (5), this value is consistent with solar Ne in the MORB-source (Fig. 1). The constancy of the Ne/Ne plateau and the similarity of that value with the average value found trapped in meteorites are important observations (1). Nevertheless, release techniques other than ball mill crushing show the mantle to have Ne/ Ne as high as 13.75 (4). The general conclusion that the mantle contains solar Ne (8) rather than the Ne trapped in meteorites cannot be overturned.