HR: 1330h
AN: V32C-1027    [PDF]
TI: Double-dating single-grain zircons by (U-Th)/He and U/Pb
AU: * Reiners, P W
EM: peter.reiners@yale.edu
AF: Yale University, Geology and Geophysics PO Box 208109, New Haven, CT 06520 United States
AU: Campbell, I S
EM: Ian.Campbell@anu.edu.au
AF: Australian National University, RSES Mills Road, Canberra, ACT 0200 Australia
AU: Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Yale University, Geology and Geophysics PO Box 208109, New Haven, CT 06520 United States
AU: Allen, C
EM: charlotte.allen@anu.edu.au
AF: Australian National University, RSES Mills Road, Canberra, ACT 0200 Australia
AB: Dating of single zircon crystals by both the (U-Th)/He and U/Pb methods (He-Pb* double-dating) provides high-temperature (crystallization or high-grade metamorphism) and low-temperature ($\sim$180 $\deg$C) ages. Advantages of He-Pb* double-dating of detrital zircons includes greatly improved resolution of source terrain provenance, as well as constraints on depositional ages of active-margin sedimentary rocks by identification of first-cycle (e.g., volcanic) zircons of the youngest-age population. Although several analytical methods can be used for He-Pb* double-dating, our work shows that measurement of U/Pb ages by laser ablation ICP-MS, followed by bulk He extraction/measurement and U-Th measurement provides the best results, with several additional advantages. Besides U/Pb age determination, LA-ICP-MS provides spatial and depth resolution of U/Pb ages and U-Th concentration variations. Variations in single-grain U/Pb ages can date post-formation metamorphic or overgrowth events that further constrain the high-temperature history of the zircons. Depth-resolved U-Th zonation from LA-ICP-MS can also be used to customize alpha-ejection corrections, which commonly improves accuracy of (U-Th)/He ages by 5-10% (or more, for extremely zoned grains). We have applied the He-Pb* double-dating technique to a variety of detrital and volcanic zircons (the latter as standards), including samples from the Mississippi River delta, Navajo sandstone, Kamchatkan flysch, and alluvium from the Ganges and Indus rivers. Ganges and Indus zircons have U/Pb ages ranging from 25 to 2700 Ma, but 60% of grains have (U-Th)/He ages between 0.30 and 4.3 Ma, with most of the rest between 15-30 Ma. Some zircons with relatively old U/Pb ages also preserve rims with U/Pb ages of 15-30 Ma. These data suggest that most sediment draining the Himalayas originates from regions exhuming at rates greater than $\sim$1-2 km/myr, containing crust of a wide variety of pre-Himalayan formation ages that was metamorphosed in the mid-Tertiary.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 1815 Erosion and sedimentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting