Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages   总被引：1，自引：0，他引：1  

Timmons?M.?EricksonEmail authorView author&#;s OrcID profile  Nicholas?E.?Timms  Christopher?L.?Kirkland  Eric?Tohver  Aaron?J.?Cavosie  Mark?A.?Pearce  Steven?M.?Reddy 《Contributions to Mineralogy and Petrology》2017,172(2-3):11

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), \(\left( 10\bar{1} \right)\), \(~\{110\}\), \(\left\{ 212 \right\},\) and type two (irrational) twin planes with rational shear directions in \([0\bar{1}\bar{1}]\) and \([\bar{1}\bar{1}0]\). SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a ²⁰⁷Pb/²⁰⁶Pb age of 2010?±?15 Ma (2σ, n?=?9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259?±?5 Ma (2σ, n?=?7), which is consistent with previous impact age estimates of 255?±?3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts.  相似文献   

Systemic geopolitical modeling. Part 2: subjectivity in prediction of geopolitical events     

Nicholas J. Daras  John Th. Mazis 《GeoJournal》2017,82(1):81-108

This paper studies subjective priorities for the data amounts in the processing of geopolitical data accoding to Mazis I. Th., theoretical paradigm of Systemic Geopolitical Analysis. After defining geopolitical plans and geopolitical focus sets, they are introduced geopolitical preferences and geopolitical management capacities. The geopolitical rational choice is studied, as well as the geopolitical preference-capacity distributions. Then, they are investigated geopolitical contrasts of subjective priorities by several geopolitical operators, and it is shown that there are cores and equilibriums of geopolitical contrasts, the study of which may provide useful information.  相似文献   

The fluvial and geomorphic context of Indian Knoll,an Archaic shell midden in West‐Central Kentucky     

Darcy F. Morey  George M. Crothers  Julie K. Stein  James P. Fenton  Nicholas P. Herrmann 《Geoarchaeology》2002,17(6):521-553

Indian Knoll is the largest Archaic shell midden excavated by WPA archaeologists in Kentucky. Situated in a large alluvial valley, the site is not associated with a known river shoal as might be expected, making its fluvial and geomorphic setting of interest. Based on sediment cores and auger samples, undisturbed portions of the site remain despite extensive excavations. In undisturbed portions, a shell‐bearing layer is overlain by a shell‐free midden layer. Profiles of organic matter and calcium carbonate content for both layers are similar to those of other Green River shell middens. New radiocarbon determinations date the shell deposit at 5590–4530 cal yr B.P. Analysis of mussel species collected from the Indian Knoll indicates that shell fishing took place in a swiftly flowing, shallow to moderately deep setting of the main river channel. Overall, the prehistoric river setting adjacent to Indian Knoll was characterized by deeper water on average with variable but finer‐grained substrate compared to other Green River shell midden sites. © 2002 Wiley Periodicals, Inc.  相似文献