Rapid weathering in Holbrook: An iron‐57 Mössbauer spectroscopy study

Abstract

Abstract— Samples of Holbrook (an L6 chondrite that fell in Arizona in 1912) were analysed by 57Fe Mössbauer spectroscopy to determine the proportion of the total Fe that had been converted to Fe3+ by weathering processes. Although samples recovered in 1912 showed negligible (<1%) alteration, a spectrum recorded from a specimen recovered in 1968 showed that 9.7% of the total Fe was present as Fe3+. Given the existence of samples in similar semiarid environments with terrestrial ages >40 ka, a linear rate of weathering is unrealistic, rapidly producing very high levels of oxidation in samples with low terrestrial ages. However if weathering follows an approximate power law, then we can model a rapid initial phase and the levels of oxidation that are more typical in samples with much longer terrestrial residence times recovered from Roosevelt County. Our analysis, together with work on terrestrial age dated samples, indicates that hot desert weathering of ordinary chondrites is at least a two‐stage process, with an initial rapid phase lasting <500 years before oxidation is passivated.