Tracer with basalt samples

Figure 9. Percentage of initial concentrations of tracers left in solution as a function of time for the sorption experiments with the basalt samples. ( ), U Np f AX Pu fax Am (O), Cm.

Boron concentrations and isotopes are also useful geochemical tracers of contamination in MORB. Boron concentrations are low (<2 ppm) in unaltered ocean floor basalt but high in altered basalts (>8 ppm B) (Spivack and Edmond 1987 Ryan and Langmuir 1993). Goldstein et al. (1989) measured B concentrations in their samples and found them to be less than 1.6 ppm, inconsistent with contamination. More recently, B isotopes have been used to assess contamination since large differences in 5 B are known to exist between seawater, sediments, and unaltered MORB. Sims et al. (2002) reported that 6 B for their 9°N EPR samples were inconsistent with incorporation of any seawater or seawater-derived material. 

While the effects of magma contamination and radiogenic production cannot be discounted completely from some of these samples, the general pattern seems consistent with a mantle region of lower He/" He ratios. As discussed above, it appears that these mantle domains directly provide the He seen in the continental lithosphere. In western Europe, the Cameroon Line, Meidob Hills and Kivu, data from other isotopic tracers in the host basalts are consistent with such a source, which also then provides a reasonable explanation for the xenolith He. However, such domains are sampled at only a small proportion of oceanic vents (see Graham 2002, this volume), and so it is unlikely that such a source has contributed to many of the other continental locations. 

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