B. Buhn et al., Rare-earth element systematics of carbonatitic fluorapatites, and their significance for carbonatite magma evolution, CONTR MIN P, 141(5), 2001, pp. 572-591
Magmatic fluorapatites of five African carbonatite complexes were analyzed
for rare-earth (REE) and trace elements by electron microprobe and high-res
olution synchrotron micro-XRF to explore the fluorapatite composition durin
g different stages of carbonatite magma evolution. Early crystallized fluor
apatites have La concentrations mostly below 1,500 ppm and low Sigma REE. T
hey display convex-upward shaped REE patterns with (La/Nd)(cn)less than or
equal to1 and low (La/Yb)(cn) ratios < 100. In contrast, fluorapatites from
fractionated carbonatites have straight REE patterns with (La/Nd), and (La
/Yb)(cn) generally above 100, and have La up to 1 wt% at a high <Sigma>REE.
Model calculations with the fractionating mineral assemblage fluorapatite
+ calcite clinopyroxene suggest REE distribution coefficients for fluorapat
ite/carbonatite melt with a positive slope throughout from La to Lu, in ord
er to meet the relationships observed in the natural fluorapatites. The cal
culations oppose closed system conditions of magma fractionation along the
liquid lines of descent, but suggest periods of instantaneous fluorapatite
crystallization. Fluorapatite trace element characteristics are therefore t
hought to be indicative for carbonatite evolution, and can reflect the rela
tive degree of magma fractionation. We suggest that the (Eu/Eu*)(cn) and Y
evolution in the fluorapatites is a manifestation of an aqueous fluid immis
cibly coexisting with the carbonatite magma from early evolution on, which
is able to continuously extract divalent Eu and Y from the carbonatite magm
a.