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A Russian record of a Middle Ordovician shower : extraterrestrial chromite in Volkhovian-Kundan (lower Darriwilian) strata at Lynna River, St. Petersburg Region

Lindskog, Anders (2011) In Dissertations in Geology at Lund University
Department of Geology
Abstract
Numerous fossil meteorites and high concentrations of sediment-dispersed extraterrestrial chromite (EC) grains with typical ordinary chondritic composition have previously been documented from 467±1.6 Ma Ma lower Kundan (Middle Ordovician) strata. These finds most likely reflect a temporarily enhanced influx of ordinary chon- dritic matter, following the disruption of the L chondrite parent body in the asteroid belt 470±6 Ma. In this study, an uppermost Volkhovian-lower Kundan limestone/marl succession at Lynna River, St. Petersburg region, northwest- ern Russia, has been sampled and its extraterrestrial component determined through chromite grain extraction and analysis. Seven samples, forming two separate sample sets, were collected at... (More)
Numerous fossil meteorites and high concentrations of sediment-dispersed extraterrestrial chromite (EC) grains with typical ordinary chondritic composition have previously been documented from 467±1.6 Ma Ma lower Kundan (Middle Ordovician) strata. These finds most likely reflect a temporarily enhanced influx of ordinary chon- dritic matter, following the disruption of the L chondrite parent body in the asteroid belt 470±6 Ma. In this study, an uppermost Volkhovian-lower Kundan limestone/marl succession at Lynna River, St. Petersburg region, northwest- ern Russia, has been sampled and its extraterrestrial component determined through chromite grain extraction and analysis. Seven samples, forming two separate sample sets, were collected at the locality. Four samples from strata around the Lenodus variabilis-Yangtzeplacognathus crassus conodont zone boundary yielded a total of 397 EC grains in 51.5 kg of rock (average ~7.71 EC grains kg-1). A successive increase in the concentration of EC and other heavy mineral grains upwards in this sample set likely traces sea-level rise. Three stratigraphically lower-lying samples, from strata around the Baltoniodus norrlandicus-L. variabilis conodont zone boundary, which corresponds to the Volkhov-Kunda boundary, yielded only two EC grains in 38.2 kg of rock (average ~0.05 EC grains kg-1). This lack of EC enrichment contrasts with preceding studies at other localities, which have consistently identified a notable EC component in the lowermost L. variabilis conodont zone. Discrepancies in the timing of the initial EC enrichment between localities, as well as variations in concentration within individual stratigraphies, may be ex- plained by differential deposition, however, the predictable increase in EC abundance in lower Kundan strata is best explained by an enhanced influx of ordinary chondritic matter. A strong similarity in average EC chemical compo- sition between this and other comparable studies indicates that all or most EC grains in these Middle Ordovician strata share a common source – most likely the L chondrite parent body. (Less)
Abstract (Swedish)
Tidigare studier har dokumenterat ett flertal fossila meteoriter samt höga koncentrationer av utomjordisk kromit (ett järn-kromoxidmineral) i cirka 467 miljoner år gamla, mellanordoviciska sedimentlager.

Fynden antas vara resultaten av ett tillfälligt förhöjt inflöde av meteoritiskt material, efter uppbrytningen av en kropp i asteroidbältet – de så kallade L-kondriternas föräldrakropp – för omkring 470 miljoner år sedan. I den här studien undersöktes innehållet av utomjordisk kromit (bild nedan) i sedimentprover från en lagerföljd innefattande de två ordoviciska etagerna volkhov och kunda, vid floden Lynna, i St. Petersburgregionen i nordvästra Ryssland. Sju prover, i två separata provserier, valdes ut vid lokalen. Fyra prover med... (More)
Tidigare studier har dokumenterat ett flertal fossila meteoriter samt höga koncentrationer av utomjordisk kromit (ett järn-kromoxidmineral) i cirka 467 miljoner år gamla, mellanordoviciska sedimentlager.

Fynden antas vara resultaten av ett tillfälligt förhöjt inflöde av meteoritiskt material, efter uppbrytningen av en kropp i asteroidbältet – de så kallade L-kondriternas föräldrakropp – för omkring 470 miljoner år sedan. I den här studien undersöktes innehållet av utomjordisk kromit (bild nedan) i sedimentprover från en lagerföljd innefattande de två ordoviciska etagerna volkhov och kunda, vid floden Lynna, i St. Petersburgregionen i nordvästra Ryssland. Sju prover, i två separata provserier, valdes ut vid lokalen. Fyra prover med totalt 51,5 kg sten från lager i undre kunda innehöll sammanlagt 397 utomjordiska kromitkorn, vilket ger ett medelvärde på cirka 7,71 korn/kg. Detta är oerhört höga halter av utomjordisk kromit, i jämförelse med typiska bakgrundshalter. En successiv ökning i koncentrationerna av utomjordisk kromit och andra tungmineral uppåt i denna provserie spårar sannolikt en havsnivåhöjning.

Tre lägre liggande prover med totalt 38,2 kg sten, från lager kring volkhov-kundagränsen innehöll endast två utomjordiska kromitkorn, vilket ger ett medelvärde på endast 0,05 korn/kg. Detta står i tydlig kontrast mot tidigare studier vid andra lokaler, vilka alla har påvisat en förhöjd kromitkoncentration i understa kunda. Skillnader i vid vilken nivå den första kromitanrikningen sker i lagerföljderna vid olika lokaler, liksom koncentrationsvariationer inom lagerföljder, kan förklaras med olikformig sedimentation, men den förutsägbara ökningen av utomjordisk kromit i lager från undre kunda förklaras bäst med att en tid med förhöjt inflöde av meteoritiskt material inföll.

En god överensstämmelse mellan den kemiska sammansättningen hos kromitkorn från den här studien och kromit från andra liknande studier indikerar att alla (eller de flesta) korn i dessa mellanordoviciska lager har samma ursprungskälla – sannolikt L-kondriternas föräldrakropp. (Less)
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author
Lindskog, Anders
supervisor
organization
alternative title
Ett ryskt sedimentarkiv över ett mellanordoviciskt meteoritregn : utomjordisk kromit i volkhov-kundalager (undre Darriwilian) vid floden Lynna, St. Petersburgregionen
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Lynna River, extraterrestrial chromite, fossil meteorites, Volkhov, Kunda, Darriwilian, heavy minerals, Lenodus variabilis, Lynna, utomjordisk kromit, fossila meteoriter, tungmineral
publication/series
Dissertations in Geology at Lund University
report number
287
language
English
id
2277908
date added to LUP
2012-01-09 10:55:27
date last changed
2012-01-09 10:55:27
@misc{2277908,
  abstract     = {Numerous fossil meteorites and high concentrations of sediment-dispersed extraterrestrial chromite (EC) grains with typical ordinary chondritic composition have previously been documented from 467±1.6 Ma Ma lower Kundan (Middle Ordovician) strata. These finds most likely reflect a temporarily enhanced influx of ordinary chon- dritic matter, following the disruption of the L chondrite parent body in the asteroid belt 470±6 Ma. In this study, an uppermost Volkhovian-lower Kundan limestone/marl succession at Lynna River, St. Petersburg region, northwest- ern Russia, has been sampled and its extraterrestrial component determined through chromite grain extraction and analysis. Seven samples, forming two separate sample sets, were collected at the locality. Four samples from strata around the Lenodus variabilis-Yangtzeplacognathus crassus conodont zone boundary yielded a total of 397 EC grains in 51.5 kg of rock (average ~7.71 EC grains kg-1). A successive increase in the concentration of EC and other heavy mineral grains upwards in this sample set likely traces sea-level rise. Three stratigraphically lower-lying samples, from strata around the Baltoniodus norrlandicus-L. variabilis conodont zone boundary, which corresponds to the Volkhov-Kunda boundary, yielded only two EC grains in 38.2 kg of rock (average ~0.05 EC grains kg-1). This lack of EC enrichment contrasts with preceding studies at other localities, which have consistently identified a notable EC component in the lowermost L. variabilis conodont zone. Discrepancies in the timing of the initial EC enrichment between localities, as well as variations in concentration within individual stratigraphies, may be ex- plained by differential deposition, however, the predictable increase in EC abundance in lower Kundan strata is best explained by an enhanced influx of ordinary chondritic matter. A strong similarity in average EC chemical compo- sition between this and other comparable studies indicates that all or most EC grains in these Middle Ordovician strata share a common source – most likely the L chondrite parent body.},
  author       = {Lindskog, Anders},
  keyword      = {Lynna River,extraterrestrial chromite,fossil meteorites,Volkhov,Kunda,Darriwilian,heavy minerals,Lenodus variabilis,Lynna,utomjordisk kromit,fossila meteoriter,tungmineral},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {A Russian record of a Middle Ordovician shower : extraterrestrial chromite in Volkhovian-Kundan (lower Darriwilian) strata at Lynna River, St. Petersburg Region},
  year         = {2011},
}