Beryllium-10 in Australasian tektites: Constraints on the location of the source crater     

P. Ma  K. Aggrey  C. Tonzola  C. Schnabel  P. de Nicola  J.T. Wasson  L. Brown  R. Middleton 《Geochimica et cosmochimica acta》2004,68(19):3883-3896

By using accelerator mass spectrometry we have measured the ¹⁰Be concentrations of 86 Australasian tektites. Corrected to the time of tektite production ∼0.8 My ago, the ¹⁰Be concentrations (10⁶ atom/g) range from 59 for a layered tektite from Huai Sai, Thailand, to 280 for an australite from New South Wales, Australia. The average value is 143 ± 50. When tektites are sorted by country, their average measured ¹⁰Be concentrations increase slowly with increasing distance from Southeast Asia, the probable location of the tektite producing event, from 59 ± 9 for 6 layered tektites from Laos to 136 ± 20 for 20 splash-form tektites from Australia. The lowest ¹⁰Be concentrations for tektites fall on or within a contour centered off the shore of Vietnam, south of the Gulf of Tonkin (107°E; 17°N), but also encompassing two other locations in the area of northeastern Thailand previously proposed for the site of a single tektite-producing impact. The ¹⁰Be concentrations of layered tektites show only a weak anticorrelation (R ∼ −0.3) with the numbers of relict crystalline inclusions.Loosely consolidated, fine-grained terrestrial sediments or recently consolidated sedimentary rocks are the most likely precursor materials. Dilution of sediments with other kinds of rock raises problems in mixing and is not supported by petrographic data. Sedimentary columns that have the right range of ¹⁰Be concentrations occur off the coasts of places where sedimentation rates are high relative to those in the deep sea. A single impact into such a region, 15 to 300 m thick, could have propelled near-surface, high-¹⁰Be material farthest—to Australia—while keeping the deeper-lying, low-¹⁰Be layers closer to home. We do not rule out, however, other proposed mechanisms for tektite formation.  相似文献   

Volatile compounds in shergottite and nakhlite meteorites     

James L. Gooding  Kwesi E. Aggrey  David W. Muenow 《Meteoritics & planetary science》1990,25(4):281-289

Abstract— Vacuum pyrolysis and quadrupole mass spectrometry were used to measure evolved-gas profiles and total concentrations of H₂O, CO₂, CO, SO₂, S₂, H₂S, HCl, Cl, and hydrocarbons in both exterior and interior samples of shergottites (ALHA77005, EETA79001, and Shergotty), a nakhlite (Nakhla), and eucrites (ALHA81001, EETA79004, and Pasamonte). Eucrites were analyzed as control samples to monitor effects of terrestrial weathering and contamination, relative to properties sought for the shergottite-nakhlite parent body. In contrast with eucrites, shergottites and Nakhla contain large proportions of their sulfur as oxidized sulfur compounds. Sulfate occurs in all shergottite and Nakhla samples and carbonate was confirmed in EETA79001 and Nakhla. Carbonate and sulfate abundances are inversely correlated but total chlorine abundance varies directly with fractional sulfate abundance. Most of the volatile compounds seem to be anhydrous, based on low bulk water contents in the meteorites (<0.1% H₂O), although Nakhla might contain significant water that is chemically associated with chlorine. Traces of saturated and unsaturated hydrocarbons in some samples are most likely terrestrial contaminants. The indigenous volatile compounds indicate that the shergottite-nakhlite parent body was highly oxidizing and supported aqueous geochemistry during at least part of its history.  相似文献   

Responding to the consequences of uncertainty in geographical data     

Aggrey Agumya  Gary J. Hunter 《International journal of geographical information science》2013,27(5):405-417

The presence of uncertainty in geographical data has the potential to expose users to undesirable consequences in their decision making. Accordingly, our efforts to understand uncertainty seek to (a) avoid the use of data that are not suitable for their intended purpose (that is, data whose consequences are unacceptable), (b) to reduce any undesirable consequences to an acceptable level, and (c) to devise ways of living with undesirable data when the adverse consequences caused by poor data do not alter our ultimate decision choice. To assist this task, we propose an approach where the adverse consequences of uncertainty caused by the use of unsuitable geographical data are expressed in terms of risk. In this paper we first show that risk management offers geographical data users a range of options for responding to the adverse consequences of data uncertainty, and secondly we present and discuss the various risk response options.  相似文献   

Exposure history of the St‐Robert (H5) fall     

I. Leya  R. Wieler  K. Aggrey  G. F. Herzog  C. Schnabel  K. Metzler  A. R. Hildebrand  M. Bouchard  A. J. T. Jull  H. R. Andrews  M‐S. Wang  T. E. Ferko  M. E. Lipschutz  J. F. Wacker  S. Neumann  R. Michel 《Meteoritics & planetary science》2001,36(11):1479-1494

Abstract— The compositionally typical H5 chondrite St‐Robert has an exposure age, 7.8 Ma, indistinguishable from that of the main cluster of H chondrites. Small values of the cosmogenic ²²Ne/²¹Ne ratio in interior samples imply a pre‐atmospheric radius on the order of 40 cm. Sample depths based on tracks and the production rates of Bhattacharya et al. (1973) range from 6 to ～40 cm and are generally larger than depths estimated from published ⁶⁰Co activities, perhaps because the track production rates adopted are too high. Depth profiles of the production rates of ¹⁴C, ³⁶Cl, ²⁶Al, ¹⁰Be, and ²¹Ne in stony material show increases with depth and reach levels 5% to 15% higher than expected from modeling calculations. The maximum concentrations in St‐Robert are, however, generally comparable to those measured for the L5 chondrite, Knyahinya, whose pre‐atmospheric radius of ～45 cm is thought to lead to the maximum possible production rates in chondrites. We infer that the pre‐atmospheric radius of St‐Robert was within 5 cm of the value that supports maximum production rates (i.e., 45 ± 5 cm). This radius corresponds to a pre‐atmospheric mass of (1.3 ± 0.4) × 10³ kg. The agreement of exposure ages for St‐Robert obtained in several different ways and the similarity of the depth profiles for ¹⁴C, ²⁶Al, ¹⁰Be, and ²¹Ne argue against a lengthy pre‐exposure of St‐Robert on the parent body and against a two‐stage exposure after launch from the parent body. Following Morbidelli and Gladman (1998), we suggest that St‐Robert was chipped from deep in its parent body, spent the next 7–8 Ma without undergoing a major collision, was nudged gradually into an orbital resonance with Jupiter, and then traveled quickly to Earth.  相似文献