SR AND ND ISOTOPIC GEOCHEMISTRY OF PEGMATOID LAMPROITES FROM WALGIDEE HILLS, WEST KIMBERLEY, AUSTRALIA

For the first time, Sr and Nd isotopic compositions are reported for mineral separates and whole rocks from pegmatoid lamproites from the Walgidee Hills. The 20 Ma pegmatoid lamproites consist of large crystals (up to few mm) including clinopyroxene, apatite, perovskite, and potassic richterite. The lamproites, in particular the matrix, are strongly altered and most minerals have been replaced by secondary carbonate and clay. Because of such pervasive alteration, little geochemical data are available that can be used to decipher the origin of the melts parental to these lamproites and the nature of their sources. To obtain such information, unaltered mineral separates (i.e., clinopyroxene, apatite, perovskite and potassic richterite) from four whole rock samples were analyzed for Sr and Nd isotopic compositions. The initial 87Sr/86Sr ratios are: 0.71220-0.71240 (clinopyroxene), 0.71221-0.71234 (apatite), 0.71220-0.71240 (perovskite), and 0.71220-0.71244 (potassic richterite). The initial 143Nd/144Nd ratios are: 0.51206-0.51207 (clinopyroxene), 0.51207-0.51208 (apatite), 0.51208-0.51210 (perovskite), and 0.51209-0.51212 (potassic richterite). The isotopic data for individual mineral phases from the same sample do not differ from one another and hence are in isotopic equilibrium. In addition, the isotopic data from the whole rocks are in agreement with the data from the separated mineral phases. It is interesting, to note that initial Sr and Nd isotopic compositions vary significantly from one sample to another. The new isotopic data indicate: (1) the mineral phases were in isotopic equilibrium with their host magma at the time of their crystallization, (2) the alteration of some minerals to secondary carbonate and clay has not affected the Sr and Nd isotopic composition of the whole rock, possibly due to the high Sr and Nd concentrations of the whole rocks, (3) the isotopic data for the Walgidee Hills lamproites plot in the enriched quadrant on a Nd-Sr isotope diagram and reside close to the data from other lamproites for West Kimberly. It thus appears that these lamproites have originated from a common, time-integrated, enriched and heterogeneous mantle source.