EXPERIMENTAL-DETERMINATION OF TRACE-ELEMENT PARTITIONING BETWEEN PARGASITE AND A SYNTHETIC HYDROUS ANDESITIC MELT

In order to more fully establish a basis for quantifying the role of a mphibole in trace-element fractionation processes, we have measured pa rgasite/silicate melt partitioning of a variety of trace elements (Rb, Ba, Nb, Ta, i-If, Zr, Ce, Nd, Sm, Yb), including the first published values for U, Th and Pb. Experiments conducted at 1000 degrees C and 1 .5 GPa yielded large crystals free of compositional zoning. Partition coefficients were found to be constant at total concentrations ranging from similar to 1 to >100 ppm, indicating Henry's Law is operative ov er this interval. Comparison of partition coefficients measured in thi s study with previous determinations yields good agreement for similar compositions at comparable pressure and temperature. The compatibilit y of U, Th and Pb in amphibole decreases in the order Pb > Th > U. Par tial melting or fractional crystallization of amphibole-bearing assemb lages will therefore result in the generation of excesses in U-238 act ivity relative to Th-230, Similar in magnitude to that produced by cli nopyroxene. The compatibility of Pb in amphibole relative to U or Th i ndicates that melt generation in the presence of residual amphibole wi ll result in the long-term enrichment in Pb relative to U or Th in the residue. This process is therefore incapable of producing the depleti on in Pb relative to U or Th inferred from the Pb isotopic composition of MORE and OIB. Comparison of partition coefficients measured in thi s study with previous values for clinopyroxene allows some distinction to be made between expected trace-element fractionations produced dur ing dry (cpx present) and wet (cpx + amphibole present) melting. Rb, B a, Nb and Ta are dramatically less compatible in clinopyroxene than in amphibole, whereas Th, U, Hf and Zr have similar compatibilities in b oth phases. Interelement fractionations, such as D-Nb/D-Ba are also di fferent for clinopyroxene and amphibole. Changes in certain ratios, su ch as Ba/Nb, Ba/Th, and Nb/Th within comagmatic suites may therefore o ffer a means to discern the loss of amphibole from the melting assembl age. Elastic strain theory is applied to the partitioning data after t he approaches of Beattie and Blundy and Wood and is used to predict am phibole/melt partition coefficients at conditions of P, T and composit ion other than those employed in this study. Given values of D-Ca, D-T i and D-K from previous partitioning studies, this approach yields amp hibole/melt trace-element partition coefficients that reproduce measur ed values from the literature to within 40-45%. This degree of reprodu cibility is considered reasonable given that model parameters are deri ved from partitioning relations involving iron and potassium-free amph ibole.