Geochemistry, mineralogy, and chronology of K-metasomatism of volcanic rocks near Socorro, New Mexico, USA


1New Mexico Bureau of Geology, New Mexico Tech, Socorro, NM, 87801, USA (
2Souder, Miller and Associates, 401 17th St, Ste 4, Las Cruces, NM, USA (

Potassium metasomatism, thought to be caused by deep circulation of alkaline-saline brines, affects an area of roughly 1000 km2 near Socorro, New Mexico, increasing the K2O content of local volcanic rocks by up to 10 wt.%. Petrographic and microbeam analyses indicate that during the chemical changes associated with metasomatism, Na-rich phases, primarily plagioclase, are dissolved and replaced by secondary mineral phases, mainly adularia and clay minerals. Bulk geochemical analyses of altered rocks compared to fresh rocks demonstrate consistent chemical patterns, including enrichments of K2O, Fe2O3, As, Rb, Pb, Sb, Ba, and Cs and depletions of MgO, CaO, Na2O, Eu and Sr. Analyses of alteration phases hand-picked from relict plagioclase grains provide the clearest picture of the chemical processes that occur during alteration, and allow quantitative correlations between the presence of alteration phases and the abundance of elements such as K2O, Rb, CaO, Na2O, Eu and Sr. This analysis also suggests that variations of elements such as As, Pb, Sb, Ba, and Cs are related to hydrothermal alteration, either overprinting, or overprinted by, potassic metasomatism. A final chemical signature observed in hand-picked alteration mineral suites is that the REE content of samples from a low-REE rock unit increase during metasomatism, whereas there is a significant decreases in samples from a unit with higher initial REE contents, suggesting that the variation in REE contents may be related to equilibration between the rocks and the metasomatizing fluid. Results suggest that the enrichment of REE may be roughly related to the abundance of metasomatism-derived clay minerals in the sample.
The chronology of metasomatism determined by 40Ar/39Ar analysis of hand-picked secondary potassium feldspar, indicates that metasomatic alteration began at around 15 Ma, and continued to around 7 Ma. The areas that show the most intense effects of alteration yield the youngest ages, suggesting that this may represent the deepest, most central and longest-lived part of the playa system. The age range determined here is consistent with inferred timing of playa deposition from independent geological evidence.