A SR AND ND ISOTOPIC STUDY OF THE SOURCES AND AGE OF SALTS FROM THE DRY VALLEYS OF ANTARCTICA

A large number of surface salt deposits are found in the Antarctic Dry Valleys.  Due to the high stability of land surfaces in this environment (some may be as old as 15-20 Ma), it is possible that these salts have remained unaltered since the time of their original deposition from liquid water.  The two main goals of this research are to investigate the possibility of dating these salts using the Rb/Sr isochron method and to determine their source using Nd and Sr isotopes as provenance indicators.  The Nd isotope data on Antarctic salts reported here are the first such measurements known to the authors.  Salts were collected from various locations within the Dry Valleys by Prof. David Marchant of Boston University and by Prof. Huiming Bao of Louisiana State University.  The identity of the salt minerals was determined using XRD analysis.  The most common minerals were gypsum and halite.  Polyhalite, a potassium calcium sulfate salt detected in some samples, is of particular interest in the dating aspect of this study due to the possibility of a high Rb content.  Different salt fractions were cleaned and separated based upon physical characteristics, uv fluorescence, solubility, grain size, and magnetism.  These fractions were analyzed with an ICP-MS at Boston University to determine concentrations of Nd, Rb, and Sr. So far, one sample, which contains gypsum, halite, and antarcticite, has yielded salt fractions with ⁸⁷Rb/⁸⁶Sr ratios high enough (< 0.25) that they may be conducive for geochronology.  Age uncertainties attainable with such a sample for the age range of interest would be about +/- 4 Ma.  Efforts to date these salts by Rb/Sr are ongoing.  TIMS Sr and Nd isotope measurements were performed at the University of California-Berkeley and the Massachusetts Institute of Technology.  Sr isotopes (which range from .7101 to .7196) of many of the salts suggest the influence of seawater and weathering of local soils, particularly for buried soil salts, though surface cobble salts can't be explained by these sources alone.  εNd values of the salts fall within a range of -1.28 to -5.28.  These Nd isotope data, which are less prone to fractionation during weathering and transport than Sr, may be consistent with a source of local dolerite rock, or atmospheric dust with South American loess provenance as has been suggested for the Nd in Antarctic ice cores.