Abundances of K, Rb, Sr and Ba in some ultramafic rocks and minerals

The abundances of K, Rb, Sr and Ba have been determined in several eclogites and a garnet peridotite, and in their constituent minerals, by isotopic dilution analyses. Leaching experiments on two volcanic eclogites, and comparison of other whole rock and mineral data, indicate that substantial fractions of these four elements reside in secondary minerals or alteration phases and are highly labile. Abundance data obtained from such whole rock samples should not be used directly to estimate upper mantle abundances of K, Rb, Sr and Ba. In the separated minerals the concentrations of these elements appear to be related to their environment of crystallization. The abundance and distribution of K, Rb, Sr and Ba in these minerals are such that it is difficult to generate the trace-element characteristics of oceanic tholeiites by simple partial melting of a garnet-two pyroxene-olivine assemblage. Involvement of amphibole in the partial melting process appears to be a satisfactory alternative.