Petrographic and trace element analysis of uranium‐rich tufa calcite, middle Miocene Barstow Formation, California, USA

Abstract

An integrated petrographic and spectroscopic (X‐ray diffraction, phosphor imaging and synchrotron X‐ray absorption spectroscopy) study of tufas from the Miocene Barstow Formation, California, relates sample morphology, mineralogy and geochemical composition. The tufas, composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters, have textures similar to those of microbially mediated terrestrial stromatolites and travertines. The tufas have elevated concentrations of a number of trace elements including Mn, Fe, Sr and U. Synchrotron X‐ray fluorescence analyses show that U concentrations can exceed 500 p.p.m. X‐ray absorption spectroscopy indicates that the U in these samples is incorporated as U(IV). It is suggested here that alkaline lake waters had a high U/Ca ratio and tufa calcite formed where groundwater or (possibly epithermal) springs brought in Ca and trace elements such as Sr. The rapid, and possibly microbially mediated, precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine‐scale inclusions.

Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California

“…1). Petrography of this sample is similar to tufas from the Owl Canyon Campground locality (Cole et al, 2004). Some tufa is present as allochthonous, cobble-size pieces dispersed in the alluvium, but some of the larger tufa mounds appear to be in situ.…”

“…This fabric consists of patches of micritic and bladed or radial fi brous calcite. Both of these textures often appear in the same hand specimen (Cole et al, 2004). Based on phosphor imaging (Cole et al, 2003), the densely laminated calcite often contains higher concentrations of U (Fig.…”

“…Calcite spar and chalcedony fi ll in much of the primary porosity. Phosphor imaging of the hand specimen shows moderately elevated but variable U concentra-tions in the framework calcite and lower concentrations in the sparry calcite and chalcedony (Cole et al, 2004).…”

“…3C) to map the distribution of U (Cole et al, 2003), which aided in obtaining ranges in U/Pb ratios for samples. Thin sections of these samples were evaluated with transmitted and refl ected light petrography and cathodoluminescence for evidence of alteration or secondary phases (Cole et al, 2004). Portions of the tufa samples that appeared altered or contained visible detritus were avoided during sampling for U-Pb analyses.…”

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U–Th series and REEs

“…At T = 5°C, the relationship between D Sr and R is slightly different and a 2-fold variation of D Sr would correspond to almost a 10-fold variation in the calcite growth rate (Tang et al, 2008). The influence of calcite growth rate on incorporation of Sr was proposed by Chiquet et al (1999) for pedogenic carbonates in Spain and also invoked by Cole et al (2004) to account for high U and Sr concentrations in tufas, with in that case also include a possible effect of microbial mediation. If one considers that the growth rate of calcite is the preponderant mechanism for explaining the U and Sr contents in the Maddur and Mule Hole pedogenic carbonates, the experiments of Tang et al (2008) would indicate that the relative growth rates of the carbonate-cemented lumps could have been up to several orders of magnitude lower than for the nodules of the black soils.…”