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The Wyoming Province of Laurentia, which hosts some of the oldest known crustal material on Earth including zircon 207Pb/206Pb ages up to 3.96 Ga in the Beartooth Mountains, Montana, has been subjected to multiple periods of orogenesis and burial from Proterozoic time to present. We present new zircon U-Pb geochronology and zircon (U-Th)/He thermochronology from Archean-Proterozoic metamorphic rocks exposed in the Bridger Range, Montana, to resolve details of their origins and reconstruct their deep-time tectonothermal history. Zircon U-Pb geochronology and cathodoluminescence imaging, paired with whole rock geochemistry and petrography, was obtained from four metamorphic samples including quartzofeldspathic and garnet-biotite gneisses proximal to the “Great Unconformity” (GU), where Archean-Proterozoic metamorphic rocks are unconformably overlain by ~7.5-9 km of compacted Phanerozoic strata. Single grain 207Pb/206Pb ages range from 4099 ± 44 Ma to 1776 ± 24 Ma, extending the age of known crustal material in the northern Wyoming Province into the Hadean and recording high-grade conditions during the Paleoproterozoic Great Falls/Big Sky orogeny. Zircon (U-Th)/He thermochronology from five metamorphic samples proximal to the GU record cooling ages ranging from 705 Ma to 10.3 Ma, reflecting the variable He diffusivity of individual zircon grains with a large range of radiation damage as proxied by effective uranium (eU) concentrations, which range from ~5 to ~3000 ppm. A negative correlation between cooling age and eU is observed across the five samples suggesting the zircon (U-Th)/He system is sensitive to Proterozoic through Miocene thermal perturbations. Ongoing thermal history modeling seeks to reconstruct the temperature-time histories of these metamorphic rocks, including testing whether this dataset is sensitive to thermal effects imparted by the rifting of Rodina and erosion related to Cryogenian glaciation (i.e., hypotheses related to formation of the GU), and the onset of modern, active extension. These datasets and models provide crucial new constraints on the obscured Proterozoic tectonic history of the northern Wyoming Province and have important implications for our understanding of the formation of early crustal material on Earth.
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Uncertainty and sensitivity analysis for spatial and spectral processing of Pb isotopes in zircon by atom probe tomography
Measuring 207Pb/206Pb ratios by atom probe tomography (APT) has provided new insight into the nanoscale behavior of trace components in zircon, and their relationship to time, temperature and structure. Here, we analyze three APT data sets for a 3.77 Ga zircon from the Beartooth Mountains, USA, and apply systematic ranging approaches to understand the spatial and spectral uncertainties inherent in 207Pb/206Pb analysis by APT. This zircon possesses two, 100% concordant U-Pb analyses by secondary ion mass spectrometry (SIMS), indicative of closed U-Pb systematics on the micron scale since crystallization. APT data sets contain sub-spherical Pb-rich (>0.25% atomic) domains with diameter <15 nm. Broadly consistent Pb-rich regions are defined in applying six different permutations of the two most common cluster identification algorithms. Measured 207Pb/206Pb ratios within Pb-rich domains vary between 0.794±0.15 (±2σ) and 0.715±0.052 depending on the ranging approach, cluster definition protocol and number of clusters interrogated. For the bulk APT data sets, 207Pb/206Pb = 0.353±0.18; this is indistinguishable from the bulk 207Pb/206Pb ratio by SIMS (0.367±0.0037), and statistically distinct from the 207Pb/206Pb ratio within clusters. Bulk and clustered 207Pb/206Pb ratios are consistent with Pb clustering at ~2.8 Ga, during protracted metamorphism and magmatism in the Beartooth Mountains. 10.1002/9781119227250.ch16
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- Award ID(s):
- 1658823
- PAR ID:
- 10352132
- Editor(s):
- Moser, DE; Corfu, F; Darling, J; Reddy, SM; Tait, K
- Date Published:
- Journal Name:
- Geography monograph series
- ISSN:
- 1037-7158
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Wyoming Province of Laurentia, which hosts some of the oldest known crustal material on Earth including zircon 207Pb/206Pb ages up to 3.96 Ga in the Beartooth Mountains, Montana, has been subjected to multiple periods of orogenesis and burial from Proterozoic time to present. We present new zircon U-Pb geochronology and zircon (U-Th)/He thermochronology from Archean-Proterozoic metamorphic rocks exposed in the Bridger Range, Montana, to resolve details of their origins and reconstruct their deep-time tectonothermal history. Zircon U-Pb geochronology and cathodoluminescence imaging, paired with whole rock geochemistry and petrography, was obtained from four metamorphic samples including quartzofeldspathic and garnet-biotite gneisses proximal to the “Great Unconformity” (GU), where Archean-Proterozoic metamorphic rocks are unconformably overlain by ~7.5-9 km of compacted Phanerozoic strata. Single grain 207Pb/206Pb ages range from 4099 ± 44 Ma to 1776 ± 24 Ma, extending the age of known crustal material in the northern Wyoming Province into the Hadean and recording high-grade conditions during the Paleoproterozoic Great Falls/Big Sky orogeny. Zircon (U-Th)/He thermochronology from five metamorphic samples proximal to the GU record cooling ages ranging from 705 Ma to 10.3 Ma, reflecting the variable He diffusivity of individual zircon grains with a large range of radiation damage as proxied by effective uranium (eU) concentrations, which range from ~5 to ~3000 ppm. A negative correlation between cooling age and eU is observed across the five samples suggesting the zircon (U-Th)/He system is sensitive to Proterozoic through Miocene thermal perturbations. Ongoing thermal history modeling seeks to reconstruct the temperature-time histories of these metamorphic rocks, including testing whether this dataset is sensitive to thermal effects imparted by the rifting of Rodina and erosion related to Cryogenian glaciation (i.e., hypotheses related to formation of the GU), and the onset of modern, active extension. These datasets and models provide crucial new constraints on the obscured Proterozoic tectonic history of the northern Wyoming Province and have important implications for our understanding of the formation of early crustal material on Earth.more » « less
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Abstract To evaluate the mechanisms driving nanoscale trace element mobility in radiation-damaged zircon, we analyzed two well-characterized Archean zircons from the Kaapvaal Craton (southern Africa): one zircon remained untreated and the other was experimentally heated in the laboratory at 1450 °C for 24 h. Atom probe tomography (APT) of the untreated zircon reveals homogeneously distributed trace elements. In contrast, APT of the experimentally heated zircon shows that Y, Mg, Al, and Pb+Yb segregate to a set of two morphologically and crystallographically distinct cluster populations that range from 5 nm tori to 25 nm toroidal polyhedra, which are confirmed to be dislocation loops by transmission electron microscopy (TEM). The dislocation loops lie in {100} and {001} planes; the edges are aligned with <100>, <101>, and <001>. The largest loops (up to 25 nm diameter) are located in {100} and characterized by high concentrations of Mg and Al, which are aligned with <001>. The 207Pb/206Pb measured from Pb atoms located within all of the loops (0.264 ± 0.025; 1σ) is consistent with present-day segregation and confirms that the dislocation loops formed during our experimental treatment. These experimentally induced loops are similar to clusters observed in zircon affected by natural geologic processes. We interpret that differences in cluster distribution, density, and composition between experimentally heated and geologically affected zircon are a function of the radiation dose, the pressure-temperature-time history, and the original composition of the zircon. These findings provide a framework for interpreting the significance of clustered trace elements and their isotopic characteristics in zircon. Our findings also suggest that the processes driving cluster formation in zircon can be replicated under laboratory conditions over human timescales, which may have practical implications for the mineralogical entrapment of significant nuclear elements.more » « less
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- Journal Article:
- https://doi.org/10.1002/9781119227250.ch16
