EarthRef.org Reference Database (ERR) -- Brenan & Watson 1991

In order to further assess the potential of molten carbonate as a transport agent in the mantle, gamma-ray spectroscopy was used to determine partition coefficients (Ds) for several trace elements (Ba, Cs, Sr, Be, Ce, Gd, and Yb) between lherzolite phases (Cr-diopsidic clinopyroxene, Fo90 olivine) and synthetic carbonate melt at 1150-degrees-C and 1.5 GPa. Trace elements were added as radiotracers, and D values were determined either by mass balance involving whole-sample and acid-washed mineral activities or by combining the latter with the activity of carbonate melt leachate. Results from 2-4 day forward experiments as well as from reversal experiments involving pre-doped minerals and radiotracer-free melt are in general accord and indicate mineral/melt equilibrium was closely approached. Average clinopyroxene/melt D values from mass balance are 0.00047 (Ba), 0.00023 (Cs), 0.043 (Sr), 0.17 (Be), 0.16 (Ce), 0.50 (Gd), and 0.30 (Yb). Ytterbium and strontium were the only elements for which olivine melt D values were determined, and averages of 0.10 (Yb) and 0.00020 (Sr) are reported. Partition coefficients determined in these experiments show an overall similarity to D values from studies involving low-Si melt compositions. This is an expected result in light of the near unity of two-liquid (carbonate/silicate) partition coefficients determined for some of these elements in the high-pressure experiments of HAMILTON et al. (1989). The apparent lack of melt compositional preference among the trace elements examined in this study indicates that the levels of those elements in the spectrum of melts that can be produced from volatile-bearing lherzolite (i.e., carbonatites through to more Si-rich compositions) may be adequately modelled with a single set of D values. We illustrate that carbonate melt interaction with depleted lherzolite may markedly raise levels of large ion lithophile (LIL) elements in clinopyroxene from such rocks with only subtle mineralogical change. This result further substantiates the suggestion of GREEN and WALLACE (1988) that ephemeral carbonatites may account for the LIL element-enriched character of otherwise refractory mantle xenoliths.