Small-Volume U-Pb Zircon Geochronology by Laser Ablation-Multicollector-ICP-MS

• Main Authors: Johnston, Scott, Gehrels, George, Valencia, Victor, Ruiz, Joaquin
• Other Authors: ARIZONA UNIV TUCSON DEPT OF GEOSCIENCES
• Format: Text
• Language: English
• Published: 2008
• Subjects: Geology; Geochemistry and Mineralogy; *LASERS; *ABLATION; MONITORING; CRYSTALS; MASS SPECTROMETRY; VIABILITY; STANDARDS; STRUCTURAL GEOLOGY; GREENLAND; SHALE; NONDESTRUCTIVE TESTING; REPRINTS; HIGH RESOLUTION; *U-PB GEOCHRONOLOGY ZIRCON; LA-ICP-MSO; MESOPROTEROZOIC-CRETACEOUS; OLIGOCENE-AGED GRAIN; MUDSTONES; Greenland
• Online Access: http://www.dtic.mil/docs/citations/ADA515753; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA515753

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 um in diameter by 4-5 um in depth (b3 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 ~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. Published in the Journal of Chemical Geology 2008.