SIMS ANALYSES OF MINOR AND TRACE-ELEMENT DISTRIBUTIONS IN FRACTURE CALCITE FROM YUCCCA MOUNTAIN, NEVADA, USA

Fracture-lining calcite samples from Yucca Mountain, Nevada, obtained as part of the extensive vertical sampling in studies of this site as a potential high-level waste repository, ha ie been characterized acco rding to microbeam-scale (25-30 mu m) trace and minor element chemistr y, and cathodoluminescent zonation patterns. As bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases, a technique for analysis by secondary ion mass spe ctrometry (SIMS) of minor (Mn, Fe, Sr) and trace (REE) elements in cal cite was developed and applied to eighteen calcite samples from four b oreholes and one trench. SIMS analyses of REE in calcite and dolomite have been shown to be quantitative to abundances <1 x chondrite. Altho ugh the low secondary ion yields associated with carbonates forced hig her counting times than is necessary in most silicates, Mn, Fe, Sr, an d REE analyses were obtained with sub-ppm detection limits and 2-15% a nalytical precision. Bulk chemical signatures noted by Vaniman (1994) allowed correlation of minor and trace element signatures in Yucca Mou ntain calcite with location of calcite precipitation (saturated vs. un saturated zone). For example, upper unsaturated zone calcite exhibits pronounced negative Ce and Eu anomalies not observed in calcite collec ted below in the deep unsaturated zone. These chemical distinctions se rved as fingerprints which were applied to growth zones in order ro ex amine temporal changes in calcite crystallization histories; analyses of such fine-scale zonal variations are unattainable using bulk analyt ical techniques. In addition, LREE (particularly Ce) scavenging of cal cite-precipitating solutions by manganese oxide phases is discussed as the mechanism for Ce-depletion in unsaturated zone calcite. (C) 1997 Elsevier Science Ltd.