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

Abstract

Ordinary chondrites (OC) recovered from the desert areas of Roosevelt County, New Mexico, the Nullarbor Region of Western Australia, and the Algerian and Libyan Sahara, for which 14C terrestrial ages have been determined, were examined by 57Fe Mössbauer spectroscopy. OC were chosen as a standard sample to investigate weathering processes as their well constrained trace and bulk element chemistry, normative mineralogy and isotopic composition define 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 the present day to > 44 ka BP. In addition, recently fallen equilibrated OC contain iron only as Fe 0 and Fe 2+, thus the abundance of ferric iron is directly related to the level of terrestrial weathering. Mössbauer spectroscopy identifies two broad types of ferric alteration: paramagnetic phases (akaganéite, lepidocrocite, and goethite), and magnetically ordered (principally magnetite and maghemite). OC finds show a range in the percentage of total Fe existing as Fe 3+ from zero to over 80%. However, oxidation is comparable between fragments of the same OC separated since their time of fall (i.e., paired meteorites). Our results indicate several features of meteorite weathering that may result from climatic or geomorphologic conditions at the accumulation site: (1) Saharan samples are, overall, less weathered than non-Saharan samples, which may be related to the relatively recent age (ca. 20 ka) of the Saharan accumulation surface; (2) broad differences between sites in the rate of weathering, arising from regional differences in climate; (3) consistent differences in the weathering products between samples that fell during humid periods and those that fell during more arid periods (those falling during humid periods contain a higher proportion of magnetically ordered ferric oxides); (4) one region (the Nullarbor) that shows a variation in the total amount of ferric species that closely matches the climatic record for this area 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 weathering occurs in the first few hundred years after fall, followed by passivation of weathering by porosity reduction. Porosity reduction, and the associated restriction in the ability of water to penetrate the sample, appears to 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.