Tracers mantle sources

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. 

Both enrichment (addition of metasomatic fluids) and depletion (extraction of magmas) therefore seem to generally produce lower K values in the mantle (the exceptions to this statement, significantly, seem to be confined to some continental alkaline volcanics and destructive plate margins, and may reflect a different nature of the fluids that enrich their sources). Variations of K ratios in the mantle, therefore, can indicate both enrichment and depletion and, as such, are not one to one geochemical tracers. 

Lipman, P. W., Hedge, C. E. Radiogenic tracers and the source of continental andesites a beginning at the San Juan volcanic field, Colorado. Proc. Andesites Conf, Internat. Upper Mantle Sci. Rept. 16, State Oregon Dept. Geol. Mineral. Ind. Bull. 65, 143—149 (1969a). 

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