Study of lamproites from Leucite Hills, Wyoming, indicates that
the isotopic compositions of some specimens have been modified due to the
alteration and/or the presence of secondary carbonate impurities within the
whole rocks. Leachate test shows that while phlogopite lamproites are not
affected by secondary processes, the transitional madupitic lamproites from
Middle Table Mountain and one madupitic lamproite from Pilot Butte
localities are most affected by the mentioned processes. Comparing the Sr
and Nd isotopic compositions of sedimentary enclaves with those of
lamproites also pinpoints affected samples. Based on such observation,
mineral grains from a number of samples were separated and analysed for Sr
and Nd isotopic compositions. The analysis of clinopyroxene, apatite and
perovskite from the Leucite Hills lamproites provides the most reliable mean
for determining the isotopic composition of the affected samples. It also
shows that the the whole rock samples from Middle Table Mountain are not
representative of their parental melts and that the transitional madupitic
lamproites should be classified as madupitic lamproites. Variation of
isotopic ratios on Nd vs Sr isotopic diagram suggests that the madupitic
lamproites have originated from a mantle source that was geochemically
different from that of the phlogopite lamproites.