Paleotransport of lanthanides and strontium recorded in calcite compositions from tuffs at Yucca Mountain, Nevada, USA

Details Paleotransport of lanthanides and strontium recorded in calcite compositions from tuffs at Yucca Mountain, Nevada, USA Paleotransport of lanthanides and strontium recorded in calcite compositions from tuffs at Yucca Mountain, Nevada, USA

Nov 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.4417v&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 60, Issue 22, pp.4417-4433

Mathematics

Logic

5

Scientific paper

Secondary calcite occurs in both saturated and unsaturated hydrologic zones (SZ and UZ, respectively) in the tuffs at Yucca Mountain, Nevada, USA. In the upper UZ, the major constituents of the calcite crystal structure (C, O) have surface origins. At greater depth there is a "barren zone," straddling the water table, where calcite is rare and mixing of surface and subsurface sources may occur. Deep in the SZ, distinctive Mn calcites reflect deep sources, including Ca released as analcime or albite formed and carbonates derived from underlying Paleozoic rocks. In the UZ and in the barren zone, above the deep Mn calcites, variations in calcite lanthanide chemistry can be used to distinguish rhyolitic from quartz-latitic sources. Lanthanide ratios and Sr contents of calcites record the chemical evolution of waters flowing through the UZ and upper SZ. Variations in calcite chemistry in the UZ and in the barren zone show that (1) Sr, which is readily exchanged with clays or zeolites, is essentially removed from some flowpaths that are in contact with these minerals and (2) traces of Mn oxides found in the tuffs have a significant effect on groundwater chemistry in the UZ and in the barren zone by removing almost all Ce from solution (evidenced by characteristic Ce depletions in calcite throughout this zone). Extreme Ce removal may be a result of Ce oxidation (Ce 3+ Ce 4+ ) at the surfaces of some Mn oxides, particularly rancieite. Higher Sr contents and lack of Ce depletions in the deeper Mn calcites reflect different ages, origins, and transport systems. The calcite record of lanthanide and Sr transport in the UZ shows that minor minerals (clays and zeolites) and even trace minerals (Mn oxides) will affect the compositions of groundwaters that flow over distances greater than a few tens of meters.

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