Temperature and Time Marking of Crystal Growth during Crystal-Liquid Partitioning Experiments: A New Technique and Applications to Anorthite-Liquid Partitioning

Abstract

All techniques for the experimental measurement of crystal-liquid partition coefficients (D) have inherent limitations, especially for incompatible elements with D <0.1. Experiments in which a starting material is inserted either directly at subliquidus temperatures or heated briefly above the liquidus and then dropped to the temperature of interest, tend to yield very rapid crystal growth rates. To avoid high values (relative to equilibrium) for measured incompatible trace element partition coefficients, complete diffusive re-equilibration between crystals and glass must occur over the duration of the experiment. In addition, trace element analysis of the small crystals or the equilibrated rims of larger crystals produced in such runs is very difficult. Fractional crystallization experiments have the advantage of minimizing crystal growth rates (e.g. Benjamin et al., 1980; Simon et al., 1994). As long as interface equilibrium is maintained between the liquid and growing crystal, equilibrium partition coefficients can be reliably calculated from fractional crystallization experiments. The disadvantage of this approach is that the measured partition coefficients can only be associated with a range in temperatures, and it is difficult to make inferences about the temperature dependence of the partition coefficients. In this abstract we report improved 'temperature and time marking' fractional crystallization experiments which counter some of the limitations of the standard fractional crystallization approach and have wide general applicability. mental apparatus consists of a 3mm OD alumina tube inside a 7mm OD alumina tube with Pt hanging wires between them. The starting powder (~ I00 mg) was put in a Pt boat suspended directly under the inner tube, and the assembly was hung in the hot-spot of a Deltech 1 atm. furnace. Small (~ 1 mg) pellets of SrCO3 and BaCO3 were made with polyvinyl alcohol and dropped down the inner tube at different times during the cooling history to act as temperature and time markers during crystal growth. The thermal history used here consisted of an initial 2 hour hold 30~ above the liquidus then step cooling to 10~ below the liquidus where the charge was held for 24 hours. This was followed by cooling at 2~ an additional 60~ below the liquidus in 20~ increments. The SrCO3 and BaCO3 pellets were dropped after the first and second cooling increments, respectively, and a 24 hour hold was placed on the sample immediately after the spikes were introduced to allow the sample to homogenize before the next stage of cooling and crystallization was begun. At the end of the cooling series, the sample was held for an additional 12 hours. Sr, Ba, Mg, and Ti abundances in anorthite and coexisting glass were determined by electron microprobe analysis using operating conditions optimized for acceptable counting statistics. A series of line scans were done on several anorthite crystals (e.g. Fig. la) in order to image the Sr and Ba variations in conjunction with Mg and Ti abundances.