Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts

The distribution of F between amblygonite (Amb, LiAlPO4F)-montebrasite (Mbr , LiAlPO4OH) solid solutions and metaluminous haplogranitic melt has been c alibrated at 585 degreesC and 200 MPa H2O. The partition coefficient for F between the crystalline phase and melt, D-F(Mbr/melt), is linear between 0 to similar to 10 wt% F in amblygonite, which contains 13 wt% F at the end-m ember: C-F(Mbr) = 3.65 C-F(melt) + 0.07, r(2) = 0.995, n = 6. Values of D-F(Amb/melt) decrease sharply above 10 wt% F in amblygonite as t he amblygonite reaches saturation in F at 200 MPa H2O. In natural occurrenc es, however, the vast majority of primary amblygonite-montebrasite solid so lutions contain similar to4-7 wt% F, well within the linear range of the ca librated exchange reaction, and the montebrasite-bearing assemblages are am ong the last to crystallize. If the F contents of the montebrasite are magm atic, then these most-fractionated residual melts of the LCT (Li-Cs-Ta, and mostly peraluminous S-types) rare-element class generally contained up to similar to1.0-1.8 wt% F near the end of their crystallization. The modest F contents of pegmatites are consistent with the common association of Li al uminosilicates and with the general paucity of topaz in these occurrences. In topaz-bearing granites of Western Europe, however, high-F amblygonite (s imilar to 10-11 wt% F) reflects >3 wt% F in melt during crystallization of these magmatic phases.