NORRIS Tim B., and WOLF, Michael B., glwolf@augustana.edu, Geology Dept., Augustana College, Rock Island, IL 61201

Experiments were conducted to study the effects of cooling rates and fluorine content of melt on the nucleation and growth of alkali feldspar in water-undersaturated granitic melts. Three sets of haplogranitic gels were made in which only the F content was varied: F-absent, 0.7 wt.% F, and 2.0 wt.% F. A small amount (~5 wt.%) of very fine-grained biotite was added to each charge to study biotite nucleation and growth as well. Water (~3 wt.%) was added to each charge. Two groups of cooling experiments were done at 200 MPa(H2O).

One group, with three sets of runs, started at 800°C, and then each set was cooled at 10, 20, or 40°C/day (each set contained three runs with different F contents). Due to time constraints, sufficiently long durations and low temperatures were not achieved in the more slowly cooled runs. No feldspar grew in the three runs cooled at 10°C/day down to 670°C or the three runs cooled at 20°C/day down to 610°C, regardless of F content. However, F affected the nucleation and growth of feldspar in the three runs cooled at 40°C/day down to 480°C. Feldspar grew in neither the F-absent nor F-rich run (2 wt.% F), but abundant skeletal feldspar (>80 mm long, some in radial sprays) grew in the run with 0.7 wt.% F.

The three sets of runs in the second group were held at 850°C for four days, then held at 700°C for three days before they were each cooled at a different rate: 5, 15, or 30°C/day. The 5 and 15°C/day sets are still being cooled, but the data will be presented at the meeting. The 30°C/day set was cooled to 100°C. Feldspar grew in all three runs of this set; however, the amount of F in the melt affected both the quantity and morphology of feldspar growth. In the F-absent run, a single, relatively large (150 x 20 mm) feldspar crystal grew. Feldspar growth was most abundant in the run with 0.7 wt.% F. Skeletal feldspar >100 mm long, commonly in radial sprays, occupies ~half of the charge. In the run with 2 wt.% F, the quantity of feldspar lies between that of the lower F runs. The morphology of the feldspar has elements of both of the lower F runs; smaller skeletal feldspar crystals (>100 mm long) grew off of one large feldspar (100 x 200 mm), with the whole cluster ~350 mm in diameter.

A small amount of F (0.7 wt.%) enhances the nucleation and growth of feldspar in rapidly cooled, water-undersaturated granitic melts. The addition of more F does not further enhance feldspar growth but appears to inhibit it, relative to the low F melts.