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Extraterrestrial chromite distribution across the mid-Ordovician Puxi River section, central China: Evidence for a global major spike in flux of L-chondritic matter

Cronholm, Anders LU and Schmitz, Birger LU (2010) In Icarus 208(1). p.36-48
Abstract
Previous studies of mid-Ordovician limestone in Sweden have shown that over a stratigraphic interval representing a few million years there is a two orders-of-magnitude enrichment in fossil L-chondritic meteorites (empty set = 1-21 cm) and sediment-dispersed extraterrestrial chromite (EC) grains (>63 mu m). This has been interpreted as a dramatic increase in the flux of L-chondritic matter to Earth following the breakup of the L-chondrite parent body, which based on Ar-Ar gas retention ages (470 +/- 6 Ma) of recently fallen meteorites occurred at about this time. Here we show that the general trend in the distribution of sediment-dispersed EC grains can be reproduced in the Puxi River section in central China. A total of 288 kg of... (More)
Previous studies of mid-Ordovician limestone in Sweden have shown that over a stratigraphic interval representing a few million years there is a two orders-of-magnitude enrichment in fossil L-chondritic meteorites (empty set = 1-21 cm) and sediment-dispersed extraterrestrial chromite (EC) grains (>63 mu m). This has been interpreted as a dramatic increase in the flux of L-chondritic matter to Earth following the breakup of the L-chondrite parent body, which based on Ar-Ar gas retention ages (470 +/- 6 Ma) of recently fallen meteorites occurred at about this time. Here we show that the general trend in the distribution of sediment-dispersed EC grains can be reproduced in the Puxi River section in central China. A total of 288 kg of limestone was searched for chrome spinels. In samples spanning the lower 8 m of the section, representing the Paroistodus originalis and Lenodus antivariabilis conodont zones, a total of 110 kg of limestone yielded only one EC grain. Similarly to the Swedish sections, EC grains begin to be common in the overlying L. variabilis Zone and remain common throughout the upper 9 m of the section, representing the L. variabilis, Yangtzeplacognathus crassus and L. pseudoplanus zones. In this part of the section 178 kg of limestone yielded 290 EC grains, with an average chemical composition very similar to chromite from recent L chondrites. In most of the beds over this interval one finds 1-4 EC grains per kilogram rock, a clear two orders-of-magnitude enrichment relative to the lower part of the section. Small bed-by-bed variations in the EC content over the upper interval most likely reflect small variations in sedimentation rates. The Puxi River section contains only very rare terrestrial chrome spinel grains, which can be distinguished already by their rounded, abraded appearance compared to the angular, pristine extraterrestrial spinels. In the mid-Ordovician, based on paleoplate reconstructions, the Puxi River site was positioned at mid-latitudes on the southern hemisphere a couple of thousand kilometers east of the Swedish sites. The prominent enrichment of EC grains over the same stratigraphic interval in China and Sweden is supporting evidence for a dramatic increase in the flux of L-chondritic matter to Earth shortly after the disruption of the L-chondrite parent body in the asteroid belt. (C) 2010 Elsevier Inc. All rights reserved. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Interplanetary dust, Meteorites, Asteroids
in
Icarus
volume
208
issue
1
pages
36 - 48
publisher
Academic Press
external identifiers
  • wos:000278838200005
  • scopus:77953535425
ISSN
0019-1035
DOI
10.1016/j.icarus.2010.02.004
language
English
LU publication?
yes
id
9b979d01-aed1-4b60-b0d2-abb2827484d2 (old id 1630477)
date added to LUP
2010-07-21 15:41:25
date last changed
2018-05-29 10:34:14
@article{9b979d01-aed1-4b60-b0d2-abb2827484d2,
  abstract     = {Previous studies of mid-Ordovician limestone in Sweden have shown that over a stratigraphic interval representing a few million years there is a two orders-of-magnitude enrichment in fossil L-chondritic meteorites (empty set = 1-21 cm) and sediment-dispersed extraterrestrial chromite (EC) grains (>63 mu m). This has been interpreted as a dramatic increase in the flux of L-chondritic matter to Earth following the breakup of the L-chondrite parent body, which based on Ar-Ar gas retention ages (470 +/- 6 Ma) of recently fallen meteorites occurred at about this time. Here we show that the general trend in the distribution of sediment-dispersed EC grains can be reproduced in the Puxi River section in central China. A total of 288 kg of limestone was searched for chrome spinels. In samples spanning the lower 8 m of the section, representing the Paroistodus originalis and Lenodus antivariabilis conodont zones, a total of 110 kg of limestone yielded only one EC grain. Similarly to the Swedish sections, EC grains begin to be common in the overlying L. variabilis Zone and remain common throughout the upper 9 m of the section, representing the L. variabilis, Yangtzeplacognathus crassus and L. pseudoplanus zones. In this part of the section 178 kg of limestone yielded 290 EC grains, with an average chemical composition very similar to chromite from recent L chondrites. In most of the beds over this interval one finds 1-4 EC grains per kilogram rock, a clear two orders-of-magnitude enrichment relative to the lower part of the section. Small bed-by-bed variations in the EC content over the upper interval most likely reflect small variations in sedimentation rates. The Puxi River section contains only very rare terrestrial chrome spinel grains, which can be distinguished already by their rounded, abraded appearance compared to the angular, pristine extraterrestrial spinels. In the mid-Ordovician, based on paleoplate reconstructions, the Puxi River site was positioned at mid-latitudes on the southern hemisphere a couple of thousand kilometers east of the Swedish sites. The prominent enrichment of EC grains over the same stratigraphic interval in China and Sweden is supporting evidence for a dramatic increase in the flux of L-chondritic matter to Earth shortly after the disruption of the L-chondrite parent body in the asteroid belt. (C) 2010 Elsevier Inc. All rights reserved.},
  author       = {Cronholm, Anders and Schmitz, Birger},
  issn         = {0019-1035},
  keyword      = {Interplanetary dust,Meteorites,Asteroids},
  language     = {eng},
  number       = {1},
  pages        = {36--48},
  publisher    = {Academic Press},
  series       = {Icarus},
  title        = {Extraterrestrial chromite distribution across the mid-Ordovician Puxi River section, central China: Evidence for a global major spike in flux of L-chondritic matter},
  url          = {http://dx.doi.org/10.1016/j.icarus.2010.02.004},
  volume       = {208},
  year         = {2010},
}