Eric S. Anderson Abstract

Flourine Partitioning Between Granitic Melt and Biotite

ANDERSON, Eric S., and WOLF, Michael B., glwolf@augustana.edu, Geology Dept., Augustana College, Rock Island, IL 61201

Fluorine contents of natural biotites might be useful as a monitor of early fluorine contents of degassed magmas if biotite-melt partition coefficients can experimentally determined. Experiments were conducted at 650-750°C, 200 MPa(H2O), for 965-hour durations to measure the partitioning of fluorine between granitic melt and coexisting biotite. Biotite was added to four different gel compositions with initial F contents of 0.0, 0.7, 1.4, and 3.1 wt.% F. The runs were initially held at 850°C for 212 hrs to break down the starting biotite before temperatures were lowered to allow growth of new, compositionally distinct biotites.

At high contents of F in melt (≥0.85 wt.%) D_F(Bt/melt) is not temperature-dependent, as also found by Icenhower and London (1997). However, F-poor melt (0.2 wt %) shows a decrease in DF(Bt/melt) as temperature increases. The low-F trend may intersect the high-F trends at ~800°C; therefore, DF(Bt/melt) may be independent of F content at temperatures ≥800°C. The DF(Bt/melt) increases with increasing Mg# [Mg/(Mg+Mn+Fe)] of biotite for all melt compositions, probably due to an Mg-F affinity in the octahedral site. There appears to be a greater effect of Mg# on DF(Bt/melt) in F-poor melts; the slopes are similar the I+L(‘97) trend, however the DF(Bt/melt) values are lower. The Mg# of biotites at 650°C and 700°C is ~ 0.65, but drops to ~0.45 at 750°C, possibly due to differences in the quantity and composition of crystallizing magnetite as temperature increases.