Pyroxenites isotopic ratios

Decoupling between He and Ne is also required, since only MORE He isotope ratios are observed in the apatites. Also, Matsumoto et al. (2000) subsequently reported similar Ne characteristics in a garnet pyroxenite with MORE He/ He ratios. Since all the Ne data do overlap with MORE Ne when two sigma errors are considered (rather than the one sigma errors routinely reported for noble gas analyses), the possibility that there is hotspot Ne in the xenoliths idea requires confirmation. 

Figure 5. Histogram Th/U for clinopyroxenes in peridotites and pyroxenites from the Ronda peridotite massif Concentrations were measured by isotope dilution mass spectrometry in acid-leached clinopyroxenes. This histogram shows that pyroxenites do not have larger Th/U ratios than peridotites. Thus, the correlation found between ( °Th/ U) and Th/U cannot be explained by mixing of peridotite and pyroxenite melts as advocated in Sigmarsson et al. (1998). Data from Hauri et al. (1994) and Bourdon and Zindler (unpublished). It can be shown with a simple Student t-test that the two populations are indistinguishable.

Figure 24 Chondrite-normalized abundances of REEs in a wall-rock harzburgite from Lherz (dotted lines— whole-rock analyses), compared with numerical experiments of ID porous melt flow, after Bodinier et al. (1990). The harzburgite samples were collected at 25-65 cm from an amphibole-pyroxenite dike. In contrast with the 0-25 cm wall-rock adjacent to the dike, they are devoid of amphibole but contain minute amounts of apatite (Woodland et al., 1996). The strong REE fractionation observed in these samples is explained by chromatographic fractionation due to diffusional exchange of the elements between peridotite minerals and advective interstitial melt (Navon and Stolper, 1987 Vasseur et al, 1991). The results are shown in (a) for variable t t ratio, where t is the duration of the infiltration process and t the time it takes for the melt to percolate throughout the percolation column (Navon and Stolper, 1987). This parameter is proportional to the average melt/rock ratio in the percolation column. In (b), the results are shown for constant f/fc but variable proportion of clinopyroxene at the scale of the studied peridotite slices (<5 cm). All model parameters may be found in Bodinier et al. (1990). As discussed in the text, this model was criticized by Nielson and Wilshire (1993). An improved version taking into account the gradual solidiflcation of melt down the wall-rock thermal gradient and the isotopic variations was recently proposed by Bodinier et al. (2003).

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