Trapped phosphate melt inclusions in silicate-carbonate-hydroxyapatite from comb-layer alvikites from the Kaiserstuhl carhonatite complex (SW-Germany)

Abstract

Structural and crystal-chemical observations on acicular and skeletal silicate-carbonate-hydroxyapatite crystals (SCHap) from comb-layer alvikite dykes of the Kaiserstuhl carbonatite complex yield strong evidence for their being quenched products. As well as being intimately intergrown with calcite, they also contain distinctive primary phosphate inclusions. In contrast to the host apatite, these inclusions are free of Si, low in LREE but enriched in Sr and the volatiles F and S, and contain a high concentration of Na. This pronounced fractionation combined with the absence of x-ray diffraction and electron diffraction patterns suggests that the phosphate inclusions represent portions of a trapped residual phosphate melt. Therefore the SCHap are considered to be the direct liquidus phase of a carbonate-phosphate melt. That two phosphate phases coexist, however, presumes strong enrichment of phosphorous in the parental carbonatitic melt, since the eutectic would otherwise produce calcite and apatite only. The formation of sharply interfaced apatite- and calcite-comb-layers and the occurrence of trapped phosphate melt inclusions in the quench apatites argue for the existence of two immiscible liquids such as a phosphate-rich and a carbonate-rich melt.