Abstract

A carbonatite sample from Phalaborwa, South Africa, consists of apatite, magnetite and a calcite-dolomite ‘perthite’ which is interpreted as being due to exsolution of dolomite from a high-Mg calcite precursor. Carbon and oxygen isotope data indicate that the carbonates are equilibrated. In situ ion-microprobe analyses for Fe, Mn, Na, Si, Y, the REEs, Pb, Th and U give the following average concentrations (in ppm) in the sequence apatite, calcite, dolomite: Fe 98, 1680, 8190; Mn 61, 510, 615; Na 1171, 627, 125; Si 368; 1.6, 0.2; Sr 4447, 5418, 2393; Ba 37, 2189, 75; La 1245, 300, 67; Y 121, 50, 5.8; Pb 16, 5.4, 1.4; Th 20, 0.02, 0; U 2.4, 0, 0.01. The concentrations are reasonably uniform in both apatite and dolomite, but in calcite are more variable. Na, Si, Y, the REEs, Pb, Th and U partition into apatite relative to both carbonates (and, hence, the precursor carbonate); KD ap/cc for REE decreases from ~4 for La to ~2 for Tm. There is almost equal partitioning of Sr between apatite and calcite. During separation of dolomite from calcite, Sr and Ba partition strongly into calcite and all the other analysed elements, except Fe and Mn, also preferentially enter calcite. The REEs prefer calcite relative to dolomite, and the KD dol/cc is reasonably constant, only varying from 0.23 to 0.17. Sr, Ba and Pb in the carbonates, and their partitioning between the calcite and dolomite, differ from other carbonatite carbonates reported in the literature.

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