Application of Sr isotopes in secondary silicate minerals to paleogroundwater hydrology: An example from K-metasomatized rocks in the western U.S.

Strontium isotope analyses of secondary silicate minerals represent a largely unexploited method of investigating paleogroundwater hydrology. Guided by cathodoluminescent imaging, we obtained Sr isotopic data from physical microsamples of primary and secondary silicate minerals in a K-metasomatized, mid-Tertiary ash flow tuff at Socorro, New Mexico. The intent was to test the utility of silicate Sr isotopic data in paleohydrologic studies and to assess models for the basin-scale paleogroundwater flow responsible for the K-metasomatism in the Socorro area. Secondary adularia in the upper Lemitar tuff from a location at the southwestern extent of K-metasomatized rocks contain Sr with significantly higher 87Sr/86Sr ratios (∼ 0.720) than igneous feldspars (∼ 0.709) at the approximate time of the Late Cenozoic metasomatism ( 6 Ma). Estimates of saline brine 87Sr/86Sr ratios obtained from Late Cenozoic evaporites in the region are much lower (0.712) than the adularia values and suggest that such brines were not responsible for the K-metasomatism in this area. Groundwaters that had passed through high 87Sr/86Sr Precambrian rocks prior to entering the tuffs are more likely K-metasomatizing fluids, but isotopic data from altered upper Lemitar Tuff from throughout the Socorro area are required to fully test this possibility.