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Zircon U-Pb, Hf and O isotope constraints on the growth versus recycling of continental crust in the Grenville orogen, Ohio, USA

Petersson, Andreas (2010) In Dissertations in Geology at Lund University
Department of Geology
Abstract (Swedish)
Detta arbetet har hanterat sex olika borrkärnor från den Prekambriska berggrunden i Ohio, USA. Denna berggrund är täckt med 800 – 5000 meter av Paleozoiska avlagringar och på grund av detta vet vi väldigt lite om den. Arbetet har gått ut på att karakterisera och datera dessa bergarter samt att via isotop analyser av syre och hafnium ta reda på bergarternas sammansättning och evolution.

Borrkärnorna har krossats och mineralet zirkon har separerats ut. Zirkon är ett unikt mineral med många egenskaper som gör det passande för geologiska och geokemiska undersökningar, då det oftast bibehåller sin kemiska signatur från kristallisationstillfället.

Via sönderfallet av uran till bly har åldern på dessa bergarter fastställts med hjälp av en... (More)
Detta arbetet har hanterat sex olika borrkärnor från den Prekambriska berggrunden i Ohio, USA. Denna berggrund är täckt med 800 – 5000 meter av Paleozoiska avlagringar och på grund av detta vet vi väldigt lite om den. Arbetet har gått ut på att karakterisera och datera dessa bergarter samt att via isotop analyser av syre och hafnium ta reda på bergarternas sammansättning och evolution.

Borrkärnorna har krossats och mineralet zirkon har separerats ut. Zirkon är ett unikt mineral med många egenskaper som gör det passande för geologiska och geokemiska undersökningar, då det oftast bibehåller sin kemiska signatur från kristallisationstillfället.

Via sönderfallet av uran till bly har åldern på dessa bergarter fastställts med hjälp av en masspektrometer. Zirkonernas kristallisationsåldrar ligger i ett spann mellan 1643±54 Ma och 1025±8 Ma med ytterligare kristallisationstillfällen runt 1230 Ma och 1450 Ma. Dessa åldrar reflekterar magmatisk kristallisation respektive metamorf omkristallisation av zirkon.

Genom att analysera zirkonernas syreisotoper har en indikation på hur stor del, omarbetad jordskorpa, eller metamorfa fluider som bidragit till magmorna fåtts. Analyserna av hafniumisotoper indikerar när moderbergarterna lämnade manteln, det vill säga när ny kontinentalskorpa bildades. Det ger även en indikation på ifall dessa moderbergarter blandats med juvenila mantelkomponenter eller omarbetad krusta vid senare kristallisationstillfällen.

Via dessa analyser samt gemen noggranna mikroskoperingsundersökningar av både bergarter och zirkoner har vi nu en klarare bild på denna berggrund och hur den bildats. (Less)
Abstract
The Grenville orogen forms a major, continental scale, Mesoproterozoic orogenic belt along the present day eastern North American continent, from Labrador in the north to Mexico in the south. It is, however, mainly exposed in the northeastern parts of North America. Most of the orogen forms a basement to thick piles of Phanerozoic sedimentary rocks, and the geological evolution of the extensive covered parts of the orogen is poorly known. In this thesis, samples of the Precambrian basement from six drill-cores in Ohio (USA) were examined. Zircon was separated for U-Pb, Hf- and O-isotope analyses and the first robust and reproducible isotope-data for this part of the unexposed Grenville orogen is presented here. Ages were obtained using... (More)
The Grenville orogen forms a major, continental scale, Mesoproterozoic orogenic belt along the present day eastern North American continent, from Labrador in the north to Mexico in the south. It is, however, mainly exposed in the northeastern parts of North America. Most of the orogen forms a basement to thick piles of Phanerozoic sedimentary rocks, and the geological evolution of the extensive covered parts of the orogen is poorly known. In this thesis, samples of the Precambrian basement from six drill-cores in Ohio (USA) were examined. Zircon was separated for U-Pb, Hf- and O-isotope analyses and the first robust and reproducible isotope-data for this part of the unexposed Grenville orogen is presented here. Ages were obtained using SIMS (Secondary Ionization Mass Spectrometry) analysis. Zircon U–Pb dates are interpreted as magmatic and metamorphic zircon crystallization ages that span from 1643 ±54 Ma to 1025 ±8 Ma, with additional crystallization ages around 1450 Ma and 1230 Ma. In situ zircon LA-MC-ICP-MS (Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry) Lu-Hf analyses were obtained from all six samples. Zircon εHf (t: 1650-1025 Ma) values that range from 1.9 ±1.2 to 9.0 ±0.6 are explained by reworking of a single crustal reservoir that was derived from the mantle at 1640 ±40 Ma and that evolved with a 176Lu/177Hf value of 0.014. One sample, Lake, deviates from this trend and its spread in εHf t=1230 Ma indicates a 0-33 wt% contribution of a depleted mantle component to the tDM = 1650 Ma reservoir. Zircon (SIMS) 18O weighted mean values range from 5.36 ±0.65‰ to 10.88 ±0.21‰, which corresponds to juvenile mantle and recycled crustal oxygen signatures respectively. There is a broad negative correlation between zircon Th/U and 18O and age respectively. The general increase in 18O with time and progressive metamorphic recrystallization is best explained by the increasing effect of heavy 18O metamorphic fluids during continent-continent collision. Together, the new data presented here suggests that the crustal evolution of this part of the North American continent is significantly older, and that the influence of metamorphic fluids on the O-isotope system in zircon is stronger than previously reported. (Less)
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author
Petersson, Andreas
supervisor
organization
alternative title
Uran-bly, hafnium och syre -isotopanalyser av prekambrisk zirkon från Ohio, USA : modellering av nytillväxt kontra omarbetning av kontinental jordskorpa i den Grenvilliska orogenen
year
type
H2 - Master's Degree (Two Years)
subject
keywords
zircon, U-Pb, geochronology, Hf-isotopes, O-isotopes, Grenville, crustal growth, North-American craton, zirkon, Hf-isotoper, O-isotopeer, kontinenttillväxt, nordamerikanska kratonen
publication/series
Dissertations in Geology at Lund University
report number
265
funder
Vetenskapsrådet
language
English
additional info
John. M. Hanchar, Memorial University Newfoundland, Kanada.
Mark Baranoski, Geological Survey of Ohio, USA.
This project was enabled through the financial support
from the Swedish Science Council through grant VR#2008-3447.
id
2295246
date added to LUP
2012-01-16 14:11:27
date last changed
2014-10-24 03:44:46
@misc{2295246,
  abstract     = {The Grenville orogen forms a major, continental scale, Mesoproterozoic orogenic belt along the present day eastern North American continent, from Labrador in the north to Mexico in the south. It is, however, mainly exposed in the northeastern parts of North America. Most of the orogen forms a basement to thick piles of Phanerozoic sedimentary rocks, and the geological evolution of the extensive covered parts of the orogen is poorly known. In this thesis, samples of the Precambrian basement from six drill-cores in Ohio (USA) were examined. Zircon was separated for U-Pb, Hf- and O-isotope analyses and the first robust and reproducible isotope-data for this part of the unexposed Grenville orogen is presented here. Ages were obtained using SIMS (Secondary Ionization Mass Spectrometry) analysis. Zircon U–Pb dates are interpreted as magmatic and metamorphic zircon crystallization ages that span from 1643 ±54 Ma to 1025 ±8 Ma, with additional crystallization ages around 1450 Ma and 1230 Ma. In situ zircon LA-MC-ICP-MS (Laser Ablation Multi Collector Inductively Coupled Plasma Mass Spectrometry) Lu-Hf analyses were obtained from all six samples. Zircon εHf (t: 1650-1025 Ma) values that range from 1.9 ±1.2 to 9.0 ±0.6 are explained by reworking of a single crustal reservoir that was derived from the mantle at 1640 ±40 Ma and that evolved with a 176Lu/177Hf value of 0.014. One sample, Lake, deviates from this trend and its spread in εHf t=1230 Ma indicates a 0-33 wt% contribution of a depleted mantle component to the tDM = 1650 Ma reservoir. Zircon (SIMS) 18O weighted mean values range from 5.36 ±0.65‰ to 10.88 ±0.21‰, which corresponds to juvenile mantle and recycled crustal oxygen signatures respectively. There is a broad negative correlation between zircon Th/U and 18O and age respectively. The general increase in 18O with time and progressive metamorphic recrystallization is best explained by the increasing effect of heavy 18O metamorphic fluids during continent-continent collision. Together, the new data presented here suggests that the crustal evolution of this part of the North American continent is significantly older, and that the influence of metamorphic fluids on the O-isotope system in zircon is stronger than previously reported.},
  author       = {Petersson, Andreas},
  keyword      = {zircon,U-Pb,geochronology,Hf-isotopes,O-isotopes,Grenville,crustal growth,North-American craton,zirkon,Hf-isotoper,O-isotopeer,kontinenttillväxt,nordamerikanska kratonen},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Zircon U-Pb, Hf and O isotope constraints on the growth versus recycling of continental crust in the Grenville orogen, Ohio, USA},
  year         = {2010},
}