Guy F. Atkinson Distinguished Lecture Series: "Lead and Oxygen Isotopic Zoning in Granulite-grade zircons, Kapuskasing Uplift, Canada: a Micro-Record of the Creation of Archean Crust."
| What | Event |
|---|---|
| When |
2009-02-12 from 15:30 to 17:00 |
| Where | INSCC Auditorium |
| Contact Email | kristin.christensen@utah.edu |
| Contact Phone | 581-7162 |
Guest Speaker:
John Bowman
University of Utah
Dept. Geology and Geophysics
Abstract:
Zircon is an indispensable tool for precise dating of events in Earth history, and recent advances in zircon oxygen isotope analysis have laid the foundation for its further use as a precise archive of solid and surface Earth evolution (Valley, 2003). Full application of this technique in the field of continental evolution is hampered, however, by uncertainty as to whether primary (igneous) zircon oxygen isotopic compositions (ZOIC) survive the ‘wet’ (PH20 >70 bars) high temperature regional metamorphic cycles common to modern and ancient plate margins. Experimentally determined ‘wet’ diffusion rates are relatively rapid (D=10-22 at 600°C; i.e., faster than Pb) and predict re-setting of ZOIC at upper amphibolite facies with little effect on U-Pb ratios (Cherniak and Watson, 1997). We have conducted a definitive test for rapid, ‘wet’ oxygen diffusion in zircon by making in situ U-Pb, oxygen, and trace element measurements of initially low δ18O igneous (detrital) zircons in a high δ18O paragneiss unit as it strikes 15km across a regional amphibolite (‘wet’) to granulite (‘dry’) metamorphic gradient in the Kapuskasing Uplift (Neoarchean Superior Province, Canada) – one of the rare such exposures where this is possible worldwide.
We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from this lower crustal paragneiss. Detrital igneous zircon cores with δ18O values of 5.1‰ to 7.1‰ record creation of primitive to increasingly evolved crust from 2.85+/0.02 Ga to 2.67+/0.02 Ga. Sharp, chemical unconformity between cores and higher δ18O (8.4‰ to 10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga saw ~80 million years of high-temperature (~700o to 650oC) nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512+/−8 Ma (T = 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted, high-grade regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with deep crustal zircon.
John Bowman
University of Utah
Dept. Geology and Geophysics
Abstract:
Zircon is an indispensable tool for precise dating of events in Earth history, and recent advances in zircon oxygen isotope analysis have laid the foundation for its further use as a precise archive of solid and surface Earth evolution (Valley, 2003). Full application of this technique in the field of continental evolution is hampered, however, by uncertainty as to whether primary (igneous) zircon oxygen isotopic compositions (ZOIC) survive the ‘wet’ (PH20 >70 bars) high temperature regional metamorphic cycles common to modern and ancient plate margins. Experimentally determined ‘wet’ diffusion rates are relatively rapid (D=10-22 at 600°C; i.e., faster than Pb) and predict re-setting of ZOIC at upper amphibolite facies with little effect on U-Pb ratios (Cherniak and Watson, 1997). We have conducted a definitive test for rapid, ‘wet’ oxygen diffusion in zircon by making in situ U-Pb, oxygen, and trace element measurements of initially low δ18O igneous (detrital) zircons in a high δ18O paragneiss unit as it strikes 15km across a regional amphibolite (‘wet’) to granulite (‘dry’) metamorphic gradient in the Kapuskasing Uplift (Neoarchean Superior Province, Canada) – one of the rare such exposures where this is possible worldwide.
We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from this lower crustal paragneiss. Detrital igneous zircon cores with δ18O values of 5.1‰ to 7.1‰ record creation of primitive to increasingly evolved crust from 2.85+/0.02 Ga to 2.67+/0.02 Ga. Sharp, chemical unconformity between cores and higher δ18O (8.4‰ to 10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga saw ~80 million years of high-temperature (~700o to 650oC) nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512+/−8 Ma (T = 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted, high-grade regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with deep crustal zircon.
