We report exceptionally negative δ238U values for spring water (−2.5‰ to −0.8‰) and travertine calcite (−3.2‰ to −1.1‰) from an area where the Jemez lineament intersects the western margins of the Rio Grande rift, west-central New Mexico (southwestern United States). The highest anomalies come from the southern margins of the Valles Caldera and are related to upwelling CO2-charged spring water forming travertine mounds along joints and faults. The anomaly likely occurs due to CO2 lixiviation of uranium in a deep-seated reduced environment where 235U is preferentially leached along a long flow path through Precambrian granitic basement, resulting in spring water with exceptionally low δ238U values inherited by the calcite that precipitated near or at the surface at relatively low temperatures, i.e., ∼40 °C (modern temperatures). The lowest δ238U values are preserved in settings where upwelling waters are least diluted by oxidized aquifer groundwaters. Given these low δ238U values in travertine are associated with and possibly indicators of upwelling CO2 related to tectonic and magmatic activity, studies such as ours may be used to identify this association far back in time.

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