GEOCHEMISTRY OF NEOPROTEROZOIC CARBONATES: RELATIONS BETWEEN D13C EXCURSIONS AND RARE EARTH ELEMENTS

Neoproterozoic carbonates from the Great Basin show statistically robust negative correlations between per mil d13C and rare earth element concentrations, especially among middle and heavy REE. The data set includes units associated with both Sturtian (~750 Ma) and Marinoan (~600 Ma) glaciations, namely limestones from the Kingston Peak and Johnnie Formations. Johnnie samples gave negative d13C values ranging from -9.6 to –11.1 per mil PDB and were strongly enriched in MREE and HREE. Kingston Peak samples gave mostly positive d13C values (+5.9 to +9.3 per mil PDB) and were not as enriched in MREE and HREE. A multivariate regression model was used to test and quantify relationships between trace element concentrations and per mil d13C. The model demonstrates that variance in trace element concentration is a function of the following variables: Al and Mn concentrations, Ca/Mg ratio, and per mil d13C. In the model, per mil d13C is negatively correlated to REE concentrations giving significant coefficients ranging from -0.64 to -0.96 for Fe, Sm, Eu, Tb, Gd, Dy, Ho, Er, Tm, Yb and Lu. In most regressions (except for those for Lu or Yb concentration), the d13C correlation coefficients were larger than the combined effects of all other variables. Correlation between independent variables (multicolinearity) was checked with stepwise and partial regressions, values for the coefficient of determination (R squared) ranged from 0.92 to 0.98, and all models yielded F and t statistics above critical values for 95% confidence level. Regression analysis therefore suggests that d13C excursions in the Great Basin section are accompanied by MREE and HREE enrichment. A possible explanation for this result is that carbonate lattice defect sites, occupied by REE cations, are more prevalent in carbonates with negative d13C values. These defect sites appear to favor REE with smaller carbonate partitioning coefficients and shorter ionic radii, and may be the result of rapid precipitation rates associated with d13C excursions.