2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 2
Presentation Time: 1:45 PM


BARTELS, Alexander, Institute for Mineralogy, University of Hannover, Callinstrasse 3, Hannover, D-30167, Germany, LINNEN, Robert L., Earth and Environmental Sciences, University of Waterloo, Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON ON, Canada and HOLTZ, Francois, Institute for Mineralogy, University of Hannover, Hannover, D-30167, Germany, rlinnen@uwaterloo.ca

A common style of mineralization in LCT pegmatites is Ta oxides associated with mica replacement (micas typically range from muscovite to lepidolite in composition). Mica rather than K-feldspar can be stable in highly fluxed silicate melts at low temperature thus the metasomatism may be caused by melts rather than aqueous fluids. In order to test this possibility a series of experiments were conducted at 200 MPa, 700 to 600°C and at fluid oversaturated and undersaturated conditions using a synthetic pegmatite glass, pure H2O and K-feldspar. The glass is peraluminous with 5.5, 2.8, 1.5 and 2.8 wt% F, P2O5, Li2O and B2O3, respectively, and 200 and 1400 ppm MnO and Ta2O5, respectively. No replacement textures were observed in any of the fluid oversaturated runs. For H2O undersaturated melts no mica replacement was present at 700°C, but at 675°, 650°, 625° and 600°C the mica content increased. The mica occurs as crystals in the quenched glass (a liquidus phase), but it is also observed to have replaced the K-feldspar crystal, particularly in the 600°C H2O undersaturated experiment. Preliminary analyses indicate that the micas contain roughly 1500 ppm Ta2O5, for an apparent mineral-melt partition coefficient of about 1. The experiments demonstrate that magmatic metasomatism is a viable process. In addition, natural micas rarely contain more than a few hundred ppm Ta2O5, thus these values may also reflect the maximum Ta contents of evolved melts.