OXYGEN CONCENTRATIONS IN FLUORITE: POTENTIAL OXYGEN FUGACITY METER?

Substitution solid solution of oxygen for fluorine in fluorite (CaF2) is proposed. Measurement and calibration of this process means that oxygen concentrations in fluorite could serve as an oxygen fugacity meter.

Mineral chemistry was obtained by electron probe microanalysis (EPMA) on a Cameca SX50 instrument. Fluorite was analyzed under the conditions of 15 keV accelerating voltage, 15 nA beam current, 2 µm beam size, and 20 seconds peak counting time. Oxygen determinations have been made by using a ODPb crystal on oxygen Ka. Difficulties in the analyses are that oxygen Ka has a relatively wide peak, therefore the selection of a wider background position and longer counting time (50 seconds) has been used in the counting procedure.

Significant variations in oxygen concentration are observed within a single grain of fluorite, with values ranging from zero to 3.94 wt. %. Data from samples from a magmatic environment give distinctly lower oxygen contents, generally less than 1.70 wt. %. Analytical traverses made within grains show regions of relatively constant concentrations interspersed by significant discontinuities of very low oxygen. Efforts are underway to calibrate oxygen concentrations in terms of oxygen fugacity. Oxidation states of incorporated trace elements may aid in this endeavor, thus Fe and Mn concentrations have been determined simultaneously. Mn is present in variable amounts, but at oxygen values greater than 1.75 wt.%, no detectable Mn is incorporated into fluorite. Mn may be oxidized to a higher state, one which is not favorable for incorporation into fluorite. Thus calibration of the oxygen meter may be possible.

For samples from the Rio Grande rift, dewatering of rift basins is likely the source of the low oxygen component of the mineralizing solutions, and meteoric waters is the source of the more highly oxygenated conditions. These short periods of low oxygen may be associated with tectonic events.

Conclusions of this study are that 1) variations of oxygen concentrations in fluorite can be detected by EPMA, 2) results of oxygen variations in natural samples show areas of moderately uniform conditions interspersed by dramatic discontinuities characterized by low oxygen, 3) the geochemistry of fluorite may document tectonic history.