Day 3 Road Log: New Mexico travertine quarries and Salado Arroyo Carbonic Spring
— L.C. Crossey, K. Karlstrom, A. Priewisch, J Ricketts, B.A. Frey, S.G. Lucas, D. McCraw, A. Williams, P.L. Miller, J. Lardner, and V. Blomgren

This half-day trip visits one of the most spectacular travertine accumulations this side of Rome. Travertine (fresh-water carbonate deposits) forms according to the following reactions (Pentecost, 2005; Crossey et al., 2006, 2009):

extCO_2(g) + H₂O + CaCO_3(limestone) → Ca₂⁺_(aq) + 2HCO₃ ^-_(aq)

Ca₂⁺_(aq) + 2HCO₃ ^-_(aq) → CO_2(g)↑ + H₂O + CaCO_{3(travertine)}

Where extCO₂ is an “external” CO₂ source (e.g. soil gas and/or magmatically derived volatiles). For waters to become supersaturated with respect to CO₂, this an “external” CO₂ source (e.g. soil gas and/or magmatically derived volatiles) is needed to make waters aggressive enough to dissolve calcite in regional aquifers. The precipitation phase of travertine involves degassing of CO₂ due to pressure drop as artesian waters move upward and/or turbulence, e.g. due tosuch as small waterfalls, step-pools, and cataracts in a stream. Biological influences can also facilitate travertine precipitation by changing the saturation state of the water or through the trapping of fine-grained carbonate by algae. The Belen travertine is interpreted here as having formed primarily due to degassing of CO₂, but biologic processes likely also contributed to travertine formation.

The age of the travertines is summarized by Priewisch et al. (2014; 2016, this volume) based on U-series dating. This method (Asmerom et al., 2010) can be used to reliably measure a geologic age back to ca. 500–600 ka (Edwards et al., 1987). The system ²³⁴U - ²³⁰Th returns to secular equilibrium within analytical resolution after ~6–8 half-lives of the daughter isotope ²³⁰Th, which has a half-life of 75,700 yrs (Cheng et al., 2000). For samples outside of the U-series range, model ages were calculated by using a range of assumed δ²³⁴U values corresponding to values from dated samples that had robust U-series dates within the same location. The validity of the model ages thus relies on the assumption that spring chemistry was similar through time in each area. Travertine samples were dated at Sheherazade, Temple Cream, and Vista Grande quarries (Priewisch et al., 2014). Two samples at Sheherazade, T11 and T12, gave U-series ages of 663 ± 169 ka and 709 ± 140 ka (Fig. 1 SH_A,B) that overlap within error and are considered to be usable, though imprecise, ages. T10 from Temple Cream (Fig. 2A TC_VG A) gave a robust U-series age of 254 ± 2.7 ka while sample T9 is in secular equilibrium and older than 2–1.5 Ma (Fig. 2A TC_VG A). A robust U-series age of 435 ± 13.5 ka was obtained from sample T7 at Vista Grande while a model age was calculated for sample T8 at the same location that ranges from 690 to 560 ka (Fig. 2B TC_VG B). For Temple Cream and Vista Grande, the younger travertine samples are inferred to be infillings that formed when groundwater degassed in factures within the existing travertine mound due to high hydraulic head.