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Small-Volume U-Pb Zircon Geochronology by Laser Ablation-Multicollector-ICP-MS
Chemical Geology
  • Scott Johnston, University of Arizona
  • George Gehrels, University of Arizona
  • Victor Valencia, University of Arizona
  • Joaquin Ruiz, University of Arizona
Publication Date
2-1-2009
Abstract

U–Pb zircon geochronology is hampered by problems acquiring meaningful geologic ages on zoned grains that retain isotope signatures from multiple growth or thermal events. We present a new method using laser ablation-multicollector-inductively coupled plasma-mass spectrometry to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12–14 µm in diameter by 4–5 µm in depth (< 3 ng of zircon). Using Channeltron multipliers to monitor Pb intensities in conjunction with a total ion counting method and errors calculated as function of the number of counts, the small-volume technique reproduced published ages on eight Mesoproterozoic–Cretaceous secondary zircon standards precise and accurate within 2%, and an age not, vert, similar 1 Ma too young on a Oligocene-aged grain. Two initial applications of the small-volume technique — the detrital zircon provenance of fine-grained mudstones and shales and the creation of zircon U–Pb age maps to investigate the detrital and metamorphic history of a granulite-facies paragneiss — demonstrate the utility of this technique to a variety of geologic problems and confirm the viability of laser ablation-multicollector-inductively coupled plasma-mass spectrometry as a tool for high spatial resolution U–Pb geochronology.

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Citation Information
Scott Johnston, George Gehrels, Victor Valencia and Joaquin Ruiz. "Small-Volume U-Pb Zircon Geochronology by Laser Ablation-Multicollector-ICP-MS" Chemical Geology Vol. 259 Iss. 3-4 (2009) p. 218 - 229
Available at: http://works.bepress.com/scjohnst/1/