A. W. R. Bevan

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 Mossbauer 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 Fe0 and Fe2+, thus the abundance of ferric iron is directly related to the level of terrestrial weathering. Mossbauer spectroscopy identifies two broad types of ferric alteration: paramagnetic phases (akaganeite, lepidocrocite, and goethite), and magnetically ordered (principally magnetite and maghemite). OC finds show a range in the percentage of total Fe existing as Fe3+ 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.

An anomalous basaltic meteorite from the innermost main belt

The Meteorite Who Fell to Earth Orbital data is available for only a handful of meteorites. Some are found long after they fell to Earth. Others are recovered after they have been observed falling through the atmosphere, but their trajectories are rarely recorded. Bland et al. (p. 1525) used a photographic camera network located in the Australian desert to track a fireball in the sky, find the meteorite, and establish its orbit. The meteorite is a basaltic achondrite; most such rocks have been traced to the major asteroid Vesta. In this case, the meteorites isotopic composition and orbital properties suggest a distinct parent asteroid—a different source of basaltic material residing in the innermost main belt. This meteorite’s composition and orbital properties are such that it cannot be traced to the parent asteroid. Triangulated observations of fireballs allow us to determine orbits and fall positions for meteorites. The great majority of basaltic meteorites are derived from the asteroid 4 Vesta. We report on a recent fall that has orbital properties and an oxygen isotope composition that suggest a distinct parent body. Although its orbit was almost entirely contained within Earth’s orbit, modeling indicates that it originated from the innermost main belt. Because the meteorite parent body would likely be classified as a V-type asteroid, V-type precursors for basaltic meteorites unrelated to Vesta may reside in the inner main belt. This starting location is in agreement with predictions of a planetesimal evolution model that postulates the formation of differentiated asteroids in the terrestrial planet region, with surviving fragments concentrated in the innermost main belt.

Chapter 2.2 Early Solar System Materials, Processes, and Chronology

Publisher Summary This chapter presents the early solar system materials, processes, and chronology. The products of early solar system processes are a unique source of information about the mechanisms of the construction of small planetesimals and the planets, and the kinds of materials from which they might have accreted. The available chemical, isotopic, and astronomical evidence suggests that the materials seen as meteorites originated within the solar system, and that the great majority appears to be fragments of asteroids in solar orbits between Mars and Jupiter. Essentially, only two major categories of meteorites are recognized, which include meteorites that contain chondrules, the chondrites, and the nonchondritic meteorites that do not. The nonchondritic meteorites include those meteorites that lack chondrules and have textures and chemistries that show that they formed by partial, or complete, igneous differentiation of their parent bodies, or are breccias of igneous debris. The abundance of iron in chondrites and the distribution of the element between reduced and oxidized phases distinguish a number of groups of chondrites. Ordinary chondrites are the most abundant meteorites observed to fall and quickly recovered, accounting for more than 80% of the modern meteorite flux.

Diamonds in Western Australia

Western Australia has risen to world prominence as a diamond producer, famous for its pink diamonds, only since the discovery of the diamondiferous Argyle (AK1) lamproite pipe in the Kimberley region, in October 1979. Today, Argyle Diamonds mine on average around 30 million carats of diamonds per year, representing about a quarter of the world’s current total diamond production. Yet diamonds were first discovered in Western Australia in 1895, at Nullagine in the Pilbara region, by gold prospectors who were working surficial Tertiary conglomerates (Groom 1896). These alluvial deposits were worked sporadically during the early part of the twentieth century (Simpson 1951), but it was not until 1998 that a probable primary source rock, the Brockman kimberlite dike, was discovered in the region, about 60 kilometers north of Nullagine (Wyatt et al. 2003). For the first half of the twentieth century, Western Australia was not seen as having much potential for the discovery of diamond fields (Prider 1989). Apart from the diamonds discovered in the east Pilbara region, fleeting glimpses of diamond potential came from the world of academia, where, during the 1930s and ’40s, Rex Prider, of the University of Western Australia, was working on an unusual suite of volcanic rocks from the northern edge of the Canning Basin, adjacent to the Kimberley region. These were the leucite lamproites of the Fitzroy Valley and adjacent Lennard Shelf—a rare group of K-Mg-rich silicasaturated igneous rocks. Standing out from the sediments of the Canning Basin as a series of low hills, the leucite lamproites are the remnants of small volcanic vents that were an onshore expresFigure 2.