PARTITIONING OF TRACE-ELEMENTS BETWEEN OLIVINE AND AQUEOUS FLUIDS AT HIGH P-T CONDITIONS - IMPLICATIONS FOR THE EFFECT OF FLUID COMPOSITION ON TRACE-ELEMENT TRANSPORT

In order to investigate the effect of fluid composition on trace-element transport at high pressure and temperature, olivine/fluid partition coefficients (D) were measured in experiments involving aqueous carbonate-, chlorine- and NaOH-bearing fluids at 1.0 GPa and 1000-degrees-C. Radiotracers of Ce, Gd, Yb, Ba, Sr, Cs, and Na were used to monitor trace-element levels. D values were calculated by mass balance using the trace-element contents of acid-washed olivines from run-products combined with whole-sample abundances. A single reversal experiment in which pre-doped olivine was annealed in tracer-free water yielded results consistent with forward experiments involving olivine + H2O. This result suggests crystal/fluid equilibrium was approached over the 5-day durations used for all experiments. Partition coefficients were found to vary with fluid chemistry and, with the exception of the D's for Na, olivine/fluid values are markedly lower than those determined for olivine/H2O. Interelement ratios in the fluid also vary with fluid chemistry as exemplified by enhanced partitioning of Ce relative to Gd into Na, K or Cs carbonate-bearing compositions. Consideration of the trace-element exchange equilibria between olivine and fluid suggests that variation in the activity coefficient for aqueous trace-element and charge-balancing cations is likely to account for the interfluid variation in D values. Na partitioning is interpreted to be coupled to the formation of Na-Al complexes as has been suggested in previous studies. These data indicate that, although trace elements do not partition strongly into H2O coexisting with olivine, the addition of dissolved salts dramatically changes this result. Such variation in D values for different fluid chemistries suggests that fluid composition, in addition to residual mineralogy, may affect both absolute trace-element abundances and interelement fractionation in high P-T aqueous fluids.