Isotopic analysis of mineral phases to unravel the origin of altered volcanic rocks: an example from the Leucite Hills lamproites

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.