New Mexico Geological Society Annual Spring Meeting — Abstracts

We report on the geochemistry of playas from the Estancia Valley, New Mexico, as part of a generic study of playas from the western United States. To date some eighty solid playa samples have been analyzed by neutron activation analysis, primarily for trace elements, and some forty waters analyzed by various methods for TDS and constftuent chemistry. In addition, a few samples have been analyzed by stable isotopic analysis (C.J. Yapp, analyst) for D/H ratios. Solid playa samples are divided into those that are from surface, gypsiferous crusts; from clay mineral-rich zones 0.5-2 meters below the surface; from 0 to 0.5 meters (us ually mixtures of clays with gypsiferous samples); and from dune deposits. Careful in.spection of rare earth element (REE)/shale composite plots indicates mechanical mixing of surface with subsurface materials with no redoxcontrolled reactions active for the REE. This interpretation is corroborated by some 30 elemental variation diagrams.

The playas are representative of both closed hydrologic basins and partially-to-largely open hydrologic basins. In the former del-D values
are -3 to -4 0/00. whereas for the latter a range in del-D from -30 to -77 is noted. Elemental concentrations in all samples show the following:
(1) Most samples yield concentrations of As, Br, Set Sb, I, S, CI, U, and Cs above crustal average values for these elements. (2) Br; I, CI, Sr, U, are enriched above average shale values, and As, Sb, and Se marginally enriched above shale values. (3) Elements depleted in the playa samples include the REE, Fe, Mn, Co, Cr, Ta, Th, and Ba. (4) No trace elements are either enriched or depleted in the dune deposits relative to playa samples. Gradual accumulation of highly mobile elements (I, Cl, Br, U, Cs, Se, As, Sb) in the playas is evident, and the depletion of the elements Mn, Ree, Fe, Ta, Th, Co, Cr is due to their removal from transporting solutions far removed from the playas. These last elements do correlate with authigenic sulfides in the subsurface as expected. A complete material balance between sol.id and water samples is being attempted at present.