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Understanding igneous processes through zircon trace element systematics : prospects and pitfalls

Hafnadóttir, Marín Ósk LU (2014) In Dissertations in Geology at Lund University GEOR02 20131
Department of Geology
Abstract (Swedish)
Zirkon från magmatiska bergarter extraherades från fem granitoider i Birimianterrängen i Ghana., och detritala zirkoner provtogs från sex platser i centrala Västgrönland. Av dessa härrör en population från palaeoproterozoikum i Prøvens magmatiska komplex. De återstående fem detritala populationerna härrör från Arke-iska källor i Västgrönland. Zirkon-populationerna analyserades med LA-ICP-MS-teknik för in situ-bestämning av spårämnen för att utvärdera hur magmatiska processer kan spåras i zirkonsammansättningen. Zirkonpopulationerna är berikade i LREE i varierande grad, vilket härrör från sekundära processer, varpå en sållningsprocess genomför-des för att identifiera förändrade zirkoner. Urvalet grundas på förhöjd lantan-, uran- och... (More)
Zirkon från magmatiska bergarter extraherades från fem granitoider i Birimianterrängen i Ghana., och detritala zirkoner provtogs från sex platser i centrala Västgrönland. Av dessa härrör en population från palaeoproterozoikum i Prøvens magmatiska komplex. De återstående fem detritala populationerna härrör från Arke-iska källor i Västgrönland. Zirkon-populationerna analyserades med LA-ICP-MS-teknik för in situ-bestämning av spårämnen för att utvärdera hur magmatiska processer kan spåras i zirkonsammansättningen. Zirkonpopulationerna är berikade i LREE i varierande grad, vilket härrör från sekundära processer, varpå en sållningsprocess genomför-des för att identifiera förändrade zirkoner. Urvalet grundas på förhöjd lantan-, uran- och toriumkoncentrationer i zirkon, samt låga Ce-anomalier. Kvarvarande spårelementdata visade att zirkon-populationerna genererades i konti-nentala magmor, eftersom dessa populationer har högre U/Yb och generellt lägre Y-koncentrationer än oceaniska zirkoner. Vidare är U/Yb relativt konstant från Hadean till Palaeoproterozoisk tid, vilket kan tyda på att plattektonik började tidigare än vad som allmänt antas. Negativa Eu-anomalier och Sr-koncentrationer i zirkon indikerar pla-gioklasfraktionering i källan och därmed djup av uppsmältning . De arkeiska zirkonerna har i allmänhet ingen mindre negativ Eu-anomalier och högre Sr-koncentrationer än zirkon från den Prøvens magmatiska komplex och Birimianterrängen, liksom Hadeiska zirkoner från Jack Hills och Mount Narryer i Australien. Denna skillnad kan tillskrivas plagioklasfraktionering i källan av hadeiska och proterozoiska bergarter. Generellt visar arkeiska bergar-ter inte tecken på plagioklasfraktionering då de bildats på djup där plagioklas inte är stabilt. En stor del av den arke-iska populationen överlappar med andra populationer, vilket indikerar plagioklasfraktionering. Detta kan tyda på att en del magmor bildades på grundare djup än vad som allmänt antas för arkeiska bergarter. Granat är ett mineral som är stabilt vid större djup, och ingår i restitmaterialet vid uppsmältning. Ett försök att hitta granatsignaturer i zirkon visade sig dock fruktlöst eftersom zirkon har högre fördelningskoefficienter för de elementer som är kompa-tibla i både granat och zirkon. (Less)
Abstract
Zircon were extracted from five granitoids in the Birimian terrane in Ghana, and detrital zircon were sampled from six locations in Central West Greenland. Of these, one population is derived from the Palaeoprotero-zoic Prøven Igneous Complex. The remaining five populations are derived from the Archean basement in Central West Greenland. Zircon populations were analyzed with the LA-ICP-MS technique for in situ trace element abun-dances in order do assess igneous processes from the zircon compositions. Zircon populations are variably LREE enriched, which stems from secondary processes, and a screening process was implemented to identify altered zir-con. Screening is based on increased Lanthanum, Uranium and Thorium concentrations within the... (More)
Zircon were extracted from five granitoids in the Birimian terrane in Ghana, and detrital zircon were sampled from six locations in Central West Greenland. Of these, one population is derived from the Palaeoprotero-zoic Prøven Igneous Complex. The remaining five populations are derived from the Archean basement in Central West Greenland. Zircon populations were analyzed with the LA-ICP-MS technique for in situ trace element abun-dances in order do assess igneous processes from the zircon compositions. Zircon populations are variably LREE enriched, which stems from secondary processes, and a screening process was implemented to identify altered zir-con. Screening is based on increased Lanthanum, Uranium and Thorium concentrations within the zircon as well as low Ce-anomalies. Remaining trace element data demonstrated that the zircon populations were generated in conti-nental crust magmas, as the populations have higher U/Yb and generally lower Y concentrations than oceanic zir-con. Additionally, the U/Yb is relatively constant from the Hadean to the Palaeoproterozoic, which may indicate an earlier onset of plate tectonics than commonly depicted. Negative Eu-anomalies and Sr concentrations within zircon grains provide constraints on plagioclase fractionation in the source and consequently depth of melting. The Ar-chean zircon generally have none to less negative Eu-anomalies and higher Sr concentrations than zircon from the Prøven Igneous Complex and Birimian terrane, as well as Hadean zircon from the Jack Hills and Mount Narryer in Australia. This difference might be ascribed to plagioclase fractionation in the source of the Hadean and Palaeopro-terozoic rocks. Generally, Archean rocks do not display plagioclase fractionation signatures due to genesis at depths where plagioclase is not stable. However, a large portion of the Archean population overlaps with the other populat-ions indicating plagioclase fractionation. This may be due to them being derived from magmas that were generated at shallower depths than generally portrayed for Archean rocks. To constrain the source depth, garnet was a feasible phase, as it is stable at greater depths than plagioclase. An attempt to find garnet signatures in zircon, however, pro-ved futile, as zircon has higher partition coefficients than garnet for the elements garnet and zircon co-incorporate. (Less)
Please use this url to cite or link to this publication:
author
Hafnadóttir, Marín Ósk LU
supervisor
organization
course
GEOR02 20131
year
type
H2 - Master's Degree (Two Years)
subject
keywords
plagioclase fractionation. zirkon, hydrothermal alteration, precambrian, zircon, Ghana, hydrotermal-omvandling, plagioklasfraktionering
publication/series
Dissertations in Geology at Lund University
report number
376
language
English
id
4252112
date added to LUP
2014-01-23 14:39:04
date last changed
2014-01-27 11:10:30
@misc{4252112,
  abstract     = {{Zircon were extracted from five granitoids in the Birimian terrane in Ghana, and detrital zircon were sampled from six locations in Central West Greenland. Of these, one population is derived from the Palaeoprotero-zoic Prøven Igneous Complex. The remaining five populations are derived from the Archean basement in Central West Greenland. Zircon populations were analyzed with the LA-ICP-MS technique for in situ trace element abun-dances in order do assess igneous processes from the zircon compositions. Zircon populations are variably LREE enriched, which stems from secondary processes, and a screening process was implemented to identify altered zir-con. Screening is based on increased Lanthanum, Uranium and Thorium concentrations within the zircon as well as low Ce-anomalies. Remaining trace element data demonstrated that the zircon populations were generated in conti-nental crust magmas, as the populations have higher U/Yb and generally lower Y concentrations than oceanic zir-con. Additionally, the U/Yb is relatively constant from the Hadean to the Palaeoproterozoic, which may indicate an earlier onset of plate tectonics than commonly depicted. Negative Eu-anomalies and Sr concentrations within zircon grains provide constraints on plagioclase fractionation in the source and consequently depth of melting. The Ar-chean zircon generally have none to less negative Eu-anomalies and higher Sr concentrations than zircon from the Prøven Igneous Complex and Birimian terrane, as well as Hadean zircon from the Jack Hills and Mount Narryer in Australia. This difference might be ascribed to plagioclase fractionation in the source of the Hadean and Palaeopro-terozoic rocks. Generally, Archean rocks do not display plagioclase fractionation signatures due to genesis at depths where plagioclase is not stable. However, a large portion of the Archean population overlaps with the other populat-ions indicating plagioclase fractionation. This may be due to them being derived from magmas that were generated at shallower depths than generally portrayed for Archean rocks. To constrain the source depth, garnet was a feasible phase, as it is stable at greater depths than plagioclase. An attempt to find garnet signatures in zircon, however, pro-ved futile, as zircon has higher partition coefficients than garnet for the elements garnet and zircon co-incorporate.}},
  author       = {{Hafnadóttir, Marín Ósk}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Dissertations in Geology at Lund University}},
  title        = {{Understanding igneous processes through zircon trace element systematics : prospects and pitfalls}},
  year         = {{2014}},
}