Chew, D.

Corfu, F.

Ganerød, M.

Meade, F.

Troll, V.

English

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       Chew, D., Ganerød, M., Troll, V., Corfu, F. and Meade, F. (2008) U-Pb TIMS zircon age constraints on the Tardree Rhyolite zircon fission track standard. On Track Forum, 16 (1). http://eprints.gla.ac.uk/53835/  Deposited on: 11 August 2011   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk U-Pb TIMS zircon age constraints on the Tardree Rhyolite zircon fission track standard Written by Dave Chew    Friday, 07 November 2008  David Chew1, Morgan Ganerød2, Valentin Troll3, Fernando Corfu4 and Fiona Meade1  The Tardree Rhyolite in NE Ireland forms part of the British and Irish Palaeogene Igneous Province, associated with the opening of the North Atlantic and the onset of the present Iceland Plume.  In NE Ireland, igneous activity comprised mainly voluminous eruptions of flood basalts (Antrim Plateau lavas).  A major pause in volcanism, represented by the Interbasaltic Formation, was accompanied by weathering of the basalt pile, localised eruptions of tholeiitic basalt (including the famous Giant's Causeway) and minor rhyolite domes and flows including the Tardree Rhyolite (Emeleus & Preston, 1969; Old, 1975).  Figure 1 shows the sampling site, an abandoned quarry on the south-east flank of Tardree Hill that is thought to be part of a rhyolite dome complex [Irish National Grid reference IJ19219413].  Columnar jointing is clearly visible.     The Tardree Rhyolite Formation has yielded a zircon fission track age of 65.2 ± 0.8 Ma (Fitch & Hurford, 1977).  This age determination employed the external detector method and a value of 6.85 × 10-17 yr-1 for the decay constant (λf) as the ζ-calibration method had not been adopted at that time.  Tardree Rhyolite zircons exhibit major U zonation, typically with a U-rich core and a U-poor rim.  Figure 2 is taken from Tagami et al. (2003).  It illustrates the typical spontaneous fission track distribution in a Tardree Rhyolite zircon, with a high track density core and low track density rim. Dobson (2006) reported LA-ICPMS data which demonstrates that the high U cores typically have U concentrations 40 times those of the rims.  Uncorrected (U-Th)/He ages of the Tardree zircon are 56.2 ± 5.4 Ma (1σ) (Tagami et al., 2003) and 57.8 ± 1.2 Ma (2σ) (Dobson, 2006).  Mean α-ejection corrected ages are irreproducible and significantly older due to the high U-cores and strong zoning (Tagami et al., 2003; Hourigan et al.2005, Dobson, 2006).       Figure 1. Field photograph of the sampled quarry within the Tardree Rhyolite [Irish National Grid reference IJ19219413].   Page 1 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...   Both zircon fission track and (U-Th)/He zircon studies of the Tardree Rhyolite use the U-Pb zircon age of 58.4 ± 0.7 Ma of Gamble et al. (1999) for the eruption age of the complex.  This age determination employed an ion micro-probe (SHRIMP), using the SL13 zircon standard.   However, several other methods have yielded older ages for the complex.  Meighan et al. (1988) report a Rb-Sr whole rock isochron of 60.3 ± 2.3 Ma.  Unpublished 40Ar-39Ar sanidine ages of 60.7 ± 0.6 Ma (data by Lux & Gibson quoted in Meighan et al., 1988) and 59.1 and 60.2 Ma (data by Hurford & Hammerschmidt quoted in Tagami et al., 2003) are all demonstrably older than the U-Pb SHRIMP zircon age of Gamble et al. (1999).  100 kg of Tardree Rhyolite was sampled from the abandoned quarry site in Fig. 1 to provide a substantial fraction for a zircon reference material.  The sample was crushed and sieved to below 300 microns and a heavy fraction was obtained using a Gemeni mineral separation table.  An aliquot of this heavy fraction was separated using heavy liquids and a magnetic separator.  No apatite was found in the non-magnetic fraction between 2.9 and 3.3 g/cm3, but the >3.3 g/cm3 non-magnetic fraction yielded abundant zircon.  The zircon fraction is comprised of prismatic grains up to 300 microns long with aspect ratios of between 3 and 10:1.  A small aliquot of zircon was analysed by isotope dilution thermal ionization mass spectrometry (TIMS) at the Geological Museum in Oslo, Norway.  The zircon grains were pre-treated using the chemical abrasion method ("CA-TIMS") of Mattinson (2005).  This method employs high temperature treatment of the zircon grains (in the range of 850°C for 36 h) to anneal zircon lattice radiation damage from natural alpha, alpha recoil, and spontaneous fission processes.  The annealed zircon fraction is then subjected to a partial dissolution step in concentrated HF and HNO3 at 220°C for 16 h which removes zircon sub-domains with high U and Th concentrations. These sub-domains are invariably slightly to strongly discordant due to Pb loss.  The remaining portion of the zircon crystal (in samples that lack inheritance) are completely free from Pb loss.   Six single grain zircon analyses were undertaken (Fig. 3), four of which employed the CA-TIMS approach.  These four single grains yield a Concordia age of 61.23 ± 0.11 Ma (2σ, MSWD = 1.01).  Two grains were treated by conventional mechanical abrasion to remove outer zones which may have undergone Pb loss.  These two grains yield a Concordia age of 60.97 ± 0.39 Ma, slightly younger than the CA-TIMS age.  The CA-TIMS age is taken as the crystallisation age of the Tardree Rhyolite and it is recommended that this value is adopted as the fission track age of the zircon standard.  Aliquots of the Tardree Rhyolite are available and can be obtained by emailing chewd@tcd.ie .       Figure 2. A photograph of etched spontaneous fission tracks in a Tardree Rhyolite zircon from Tagami et al. (2003)  Page 2 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...   1Department of Geology, Trinity College Dublin, Ireland  2Geological Survey of Norway (NGU), Norway 3Department of Earth Sciences, Uppsala University, Sweden 4Department of Geosciences, University of Oslo, Norway   References      Dobson K. J. (2006) The zircon (U-Th)/He thermochronometer: development and application of thermochronometers in igneous provinces, University of Glasgow. Emeleus C. H. and Preston J. (1969) Field excursion guide to the Tertiary Volcanic Rocks of Ireland. Symposium on volcanoes and their roots, 70. Fitch F. J. and Hurford A. J. (1977) Fission track dating of the Tardree Rhyolite, Co. Antrim. Proceedings of the Geologists' Association 88, 267-274. Gamble J. A., Wysoczanski R. J., and Meighan I. G. (1999) Constraints on the age of the British Tertiary Volcanic Province from ion microprobe U-Pb (SHRIMP) ages for acid igneous rocks from NE Ireland. Journal of the Geological Society, London 156, 291-299.     Figure 3.  U-Pb Concordia diagram of ID-TIMS data for the dated Tardree Rhyolite sample. Zircon grains treated by chemical abrasion are represented by the red uncertainty ellipses.  Hourigan, J. K., Reiners, P. W. and Brandon, M. T. (2005)  U-Th zonation-dependent alpha-ejection in (U-Th)/He chronometry. Geochimica et Cosmochimica Acta 69, 3349-3365. Mattinson J. M. (2005) Zircon U-Pb chemical abrasion ("CA-TIMS") method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology 220(1-2), 47-66. Meighan I. G., McCormick A., Gibson D., Gamble J. A., and Graham I. J. (1988) Rb-Sr isotopic determinations and the timing of Tertiary Central Complex magmatism in NE Ireland. In Early Tertiary Volcanism and the Opening of the NE Atlantic. Geological Society, London, Special Publications, Page 3 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...  Close Window Vol. 39 (ed. A. C. Morton and L. M. Parson), pp. 349-360. Geological Society of London Speical Publication. Old R. A. (1975) The age and field relationships of the Tardree Tertiary rhyolite complex, Co. Antrim, Northern Ireland. Bulletin of the Geological Survey of Great Britain 51, 21-40. Tagami T., Farley K. A., and Stockli D. F. (2003) (U-Th)/He geochronology of single zircon grains of known Tertiary eruption age. Earth and Planetary Science Letters 207(1-4), 57-67.   Last Updated ( Monday, 01 December 2008 ) Page 4 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...

ages for acid igneous rocks from NE Ireland.

The zircon (U-Th)/He thermochronometer: development and application of thermochronometers in igneous provinces,

U-Pb TIMS zircon age constraints on the Tardree Rhyolite zircon fission track standard

Enlighten: Publications

 
 
 
 
 
 
 
Chew, D., Ganerød, M., Troll, V., Corfu, F. and Meade, F. (2008) U-Pb 
TIMS zircon age constraints on the Tardree Rhyolite zircon fission track 
standard. On Track Forum, 16 (1).
 
http://eprints.gla.ac.uk/53835/ 
 
Deposited on: 11 August 2011 
 
 
Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
U-Pb TIMS zircon age constraints on the Tardree Rhyolite zircon fission track 
standard 
Written by Dave Chew    
Friday, 07 November 2008 
 
David Chew1, Morgan Ganerød2, Valentin Troll3, Fernando Corfu4 and Fiona Meade1  
The Tardree Rhyolite in NE Ireland forms part of the British and Irish Palaeogene Igneous Province, associated with the opening of the North Atlantic 
and the onset of the present Iceland Plume.  In NE Ireland, igneous activity comprised mainly voluminous eruptions of flood basalts (Antrim Plateau 
lavas).  A major pause in volcanism, represented by the Interbasaltic Formation, was accompanied by weathering of the basalt pile, localised eruptions 
of tholeiitic basalt (including the famous Giant's Causeway) and minor rhyolite domes and flows including the Tardree Rhyolite (Emeleus & Preston, 
1969; Old, 1975).  Figure 1 shows the sampling site, an abandoned quarry on the south-east flank of Tardree Hill that is thought to be part of a rhyolite 
dome complex [Irish National Grid reference IJ19219413].  Columnar jointing is clearly visible.  
   
The Tardree Rhyolite Formation has yielded a zircon fission track age of 65.2 ± 0.8 Ma (Fitch & Hurford, 1977).  This age determination employed the 
external detector method and a value of 6.85 × 10-17 yr-1 for the decay constant (λf) as the ζ-calibration method had not been adopted at that time.  
Tardree Rhyolite zircons exhibit major U zonation, typically with a U-rich core and a U-poor rim.  Figure 2 is taken from Tagami et al. (2003).  It 
illustrates the typical spontaneous fission track distribution in a Tardree Rhyolite zircon, with a high track density core and low track density rim. Dobson 
(2006) reported LA-ICPMS data which demonstrates that the high U cores typically have U concentrations 40 times those of the rims.  Uncorrected (U-
Th)/He ages of the Tardree zircon are 56.2 ± 5.4 Ma (1σ) (Tagami et al., 2003) and 57.8 ± 1.2 Ma (2σ) (Dobson, 2006).  Mean α-ejection corrected ages 
are irreproducible and significantly older due to the high U-cores and strong zoning (Tagami et al., 2003; Hourigan et al.2005, Dobson, 2006). 
   
  
 
Figure 1. Field photograph of the sampled quarry within the Tardree Rhyolite 
[Irish National Grid reference IJ19219413].   
Page 1 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...
04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...
   
Both zircon fission track and (U-Th)/He zircon studies of the Tardree Rhyolite use the U-Pb zircon age of 58.4 ± 0.7 Ma of Gamble et al. (1999) for the 
eruption age of the complex.  This age determination employed an ion micro-probe (SHRIMP), using the SL13 zircon standard.   However, several 
other methods have yielded older ages for the complex.  Meighan et al. (1988) report a Rb-Sr whole rock isochron of 60.3 ± 2.3 Ma.  Unpublished 40Ar-
39Ar sanidine ages of 60.7 ± 0.6 Ma (data by Lux & Gibson quoted in Meighan et al., 1988) and 59.1 and 60.2 Ma (data by Hurford & Hammerschmidt 
quoted in Tagami et al., 2003) are all demonstrably older than the U-Pb SHRIMP zircon age of Gamble et al. (1999).  
100 kg of Tardree Rhyolite was sampled from the abandoned quarry site in Fig. 1 to provide a substantial fraction for a zircon reference material.  The 
sample was crushed and sieved to below 300 microns and a heavy fraction was obtained using a Gemeni mineral separation table.  An aliquot of this 
heavy fraction was separated using heavy liquids and a magnetic separator.  No apatite was found in the non-magnetic fraction between 2.9 and 3.3 
g/cm3, but the >3.3 g/cm3 non-magnetic fraction yielded abundant zircon.  The zircon fraction is comprised of prismatic grains up to 300 microns long 
with aspect ratios of between 3 and 10:1.  A small aliquot of zircon was analysed by isotope dilution thermal ionization mass spectrometry (TIMS) at the 
Geological Museum in Oslo, Norway.  The zircon grains were pre-treated using the chemical abrasion method ("CA-TIMS") of Mattinson (2005).  This 
method employs high temperature treatment of the zircon grains (in the range of 850°C for 36 h) to anneal zircon lattice radiation damage from natural 
alpha, alpha recoil, and spontaneous fission processes.  The annealed zircon fraction is then subjected to a partial dissolution step in concentrated HF 
and HNO3 at 220°C for 16 h which removes zircon sub-domains with high U and Th concentrations. These sub-domains are invariably slightly to 
strongly discordant due to Pb loss.  The remaining portion of the zircon crystal (in samples that lack inheritance) are completely free from Pb loss.   
Six single grain zircon analyses were undertaken (Fig. 3), four of which employed the CA-TIMS approach.  These four single grains yield a Concordia 
age of 61.23 ± 0.11 Ma (2σ, MSWD = 1.01).  Two grains were treated by conventional mechanical abrasion to remove outer zones which may have 
undergone Pb loss.  These two grains yield a Concordia age of 60.97 ± 0.39 Ma, slightly younger than the CA-TIMS age.  The CA-TIMS age is taken as 
the crystallisation age of the Tardree Rhyolite and it is recommended that this value is adopted as the fission track age of the zircon standard.  Aliquots 
of the Tardree Rhyolite are available and can be obtained by emailing chewd@tcd.ie .  
   
  
Figure 2. A photograph of etched spontaneous fission tracks in a Tardree Rhyolite 
zircon from Tagami et al. (2003)  
Page 2 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...
04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...
   
1Department of Geology, Trinity College Dublin, Ireland 
 
2Geological Survey of Norway (NGU), Norway 
3Department of Earth Sciences, Uppsala University, Sweden 
4Department of Geosciences, University of Oslo, Norway  
 
References      
Dobson K. J. (2006) The zircon (U-Th)/He thermochronometer: development and application of thermochronometers in igneous provinces, University of 
Glasgow. 
Emeleus C. H. and Preston J. (1969) Field excursion guide to the Tertiary Volcanic Rocks of Ireland. Symposium on volcanoes and their roots, 70. 
Fitch F. J. and Hurford A. J. (1977) Fission track dating of the Tardree Rhyolite, Co. Antrim. Proceedings of the Geologists' Association 88, 267-274. 
Gamble J. A., Wysoczanski R. J., and Meighan I. G. (1999) Constraints on the age of the British Tertiary Volcanic Province from ion microprobe U-Pb 
(SHRIMP) ages for acid igneous rocks from NE Ireland. Journal of the Geological Society, London 156, 291-299. 
    
Figure 3.  U-Pb Concordia diagram of ID-TIMS data for the dated Tardree 
Rhyolite sample. Zircon grains treated by chemical abrasion are represented by 
the red uncertainty ellipses.  
Hourigan, J. K., Reiners, P. W. and Brandon, M. T. (2005)  U-Th zonation-dependent alpha-ejection in (U-Th)/He chronometry. Geochimica et 
Cosmochimica Acta 69, 3349-3365. 
Mattinson J. M. (2005) Zircon U-Pb chemical abrasion ("CA-TIMS") method: Combined annealing and multi-step partial dissolution analysis for 
improved precision and accuracy of zircon ages. Chemical Geology 220(1-2), 47-66. 
Meighan I. G., McCormick A., Gibson D., Gamble J. A., and Graham I. J. (1988) Rb-Sr isotopic determinations and the timing of Tertiary Central 
Complex magmatism in NE Ireland. In Early Tertiary Volcanism and the Opening of the NE Atlantic. Geological Society, London, Special Publications, 
Page 3 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...
04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...
  
Close Window 
Vol. 39 (ed. A. C. Morton and L. M. Parson), pp. 349-360. Geological Society of London Speical Publication. 
Old R. A. (1975) The age and field relationships of the Tardree Tertiary rhyolite complex, Co. Antrim, Northern Ireland. Bulletin of the Geological Survey 
of Great Britain 51, 21-40. 
Tagami T., Farley K. A., and Stockli D. F. (2003) (U-Th)/He geochronology of single zircon grains of known Tertiary eruption age. Earth and Planetary 
Science Letters 207(1-4), 57-67.  
 
Last Updated ( Monday, 01 December 2008 ) 
Page 4 of 4Ontrackforum - the platform of the international thermochronology community - U-Pb TIMS zir...
04/01/2009http://www.ontrackforum.org/index2.php?option=com_content&task=view&id=53&pop=1&pag...


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U-Pb TIMS zircon age constraints on the Tardree Rhyolite zircon fission track standard

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