Climate and rock weathering: A study of terrestrial age dated ordinary chondritic meteorites from hot desert regions

Ordinary chondrites (OC) recovered from the desert areas of Roosevelt Count y, New Mexico, the Nullarbor Region of Western Australia, and the Algerian and Libyan Sahara, for which C-14 terrestrial ages have been determined, we re examined by Fe-57 Mossbauer spectroscopy. OC were chosen as a standard s ample to investigate weathering processes as their well constrained trace a nd bulk element chemistry, normative mineralogy and isotopic composition de fine a known, pre-weathering, starting composition. Given that terrestrial ages are known, it is possible to compare (initially very similar) samples that have been subsequently weathered in a range of climatic regimes from t he present day to > 44 ka BP. In addition, recently fallen equilibrated OC contain iron only as Fe-0 and Fe2+, thus the abundance of ferric iron is di rectly related to the level of terrestrial weathering. Mossbauer spectroscopy identifies two broad types of ferric alteration: par amagnetic phases (akaganeite, lepidocrocite, and goethite), and magneticall y ordered (principally magnetite and maghemite). OC finds show a range in t he percentage of total Fe existing as Fe3+ from zero to over 80%. However, oxidation is comparable between fragments of the same OC separated since th eir time of fall (i.e., paired meteorites). Our results indicate several features of meteorite weathering that may resu lt from climatic or geomorphologic conditions at the accumulation site: (1) Saharan samples rue, overall, less weathered than non-Saharan samples, whi ch may be related to the relatively recent age (ca. 20 ka) of the Saharan a ccumulation surface; (2) broad differences between sites in the rate of wea thering, arising from regional differences in climate; (3) consistent diffe rences in the weathering products between samples that fell during humid pe riods and those that fell during more arid periods (those falling during hu mid periods contain a higher proportion of magnetically ordered ferric oxid es); (4) one region (the Nullarbor) that shows a variation in the total amo unt of ferric species that closely matches the climatic record for this are a of Australia for the last 30 ka. Points (3) and (4) may be related to the identification of a rapid initial weathering phase: the majority of weathe ring occurs in the first few hundred years after fall, followed by passivat ion of weathering by porosity reduction. Porosity reduction, and the associ ated restriction in the ability of water to penetrate the sample, appears t o be the mechanism whereby a weathering assemblage formed during the brief initial period of oxidation is preserved through subsequent climatic cycles over the terrestrial lifetime of the sample. Copyright (C) 1998 Elsevier S cience Ltd.