Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops.
Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The (207)Pb/(206)Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.
Earth and Oceanographic Science, Bowdoin College, Brunswick, ME 04011, USA.
Emily M
EM
Peterman
Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.; Department of Applied Geology, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
Steven M
0000-0002-4726-5714
SM
Reddy
Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.; Department of Physics and Astronomy, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
David W
0000-0001-7433-946X
DW
Saxey
Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA.
David R
DR
Snoeyenbos
Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.; Department of Physics and Astronomy, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
William D A
WD
Rickard
Geoscience Atom Probe Facility, Advanced Resource Characterisation Facility, John de Laeter Centre, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.; Department of Applied Geology, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
Denis
D
Fougerouse
Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, USA.
Andrew R C
AR
Kylander-Clark
D019015:Geologic Sediments / Q000737:chemistry*; D005844:Geology; D007834:Lasers; D007854:Lead / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry*; D018707:Volcanic Eruptions / Q000032:analysis; D015040:Zirconium / Q000737:chemistry*
D017640:Silicates; D007854:Lead; C003784:zircon; D015040:Zirconium
APM; APT; Atom Probe Microscopy; LASS; U-Pb; cathodoluminescence; discordance; dislocation loop; geochronology; zircon