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Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India

Whitehouse, Martin J.; Kumar, G. R. Ravindra and Rimsa, Andrius LU (2014) In Contributions to Mineralogy and Petrology 168(2). p.1042-1042
Abstract
Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U-Th-Pb and rare earth elements (RE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520-510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle-to heavy-RE depletion in ca. 570 Ma zircon rims suggests... (More)
Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U-Th-Pb and rare earth elements (RE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520-510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle-to heavy-RE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent Pb-207/Pb-206 age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from <5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Secondary ion mass spectrometry, Zircon, Ion imaging, Ion tomography, Ultrahigh-temperature metamorphism, Kerala Khondalite Belt
in
Contributions to Mineralogy and Petrology
volume
168
issue
2
pages
1042 - 1042
publisher
Springer
external identifiers
  • wos:000341181000008
  • scopus:84905290035
ISSN
0010-7999
DOI
10.1007/s00410-014-1042-2
language
English
LU publication?
yes
id
ec521d30-13a7-4717-91e9-9f132d501c7f (old id 4716962)
date added to LUP
2014-11-06 13:01:21
date last changed
2017-08-27 03:27:17
@article{ec521d30-13a7-4717-91e9-9f132d501c7f,
  abstract     = {Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U-Th-Pb and rare earth elements (RE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520-510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle-to heavy-RE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent Pb-207/Pb-206 age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from &lt;5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism.},
  author       = {Whitehouse, Martin J. and Kumar, G. R. Ravindra and Rimsa, Andrius},
  issn         = {0010-7999},
  keyword      = {Secondary ion mass spectrometry,Zircon,Ion imaging,Ion tomography,Ultrahigh-temperature metamorphism,Kerala Khondalite Belt},
  language     = {eng},
  number       = {2},
  pages        = {1042--1042},
  publisher    = {Springer},
  series       = {Contributions to Mineralogy and Petrology},
  title        = {Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India},
  url          = {http://dx.doi.org/10.1007/s00410-014-1042-2},
  volume       = {168},
  year         = {2014},
}