Research ArticlePHYSICAL SCIENCE

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

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Science Advances  02 Sep 2016:
Vol. 2, no. 9, e1601318
DOI: 10.1126/sciadv.1601318
  • Fig. 1 Cathodoluminescence (CL) image, analysis spots, and EBSD orientation map.

    (A) CL image of analyzed zircon showing oscillatory zoning in the core. Arrows labeled with an “M” point to atom probe analysis locations; large spots mark laser ablation split stream (LASS) analysis locations; 206Pb/238U dates are shown in white; the concordance of each spot analysis is reported below each date. Uncertainty is conservatively estimated at 2%. (B) Pole figure showing the orientation data for the analyzed zircon grain. Red dots show the orientation of major crystallographic planes constrained by EBSD. The blue dot shows the orientation of atom probe specimen tips.

  • Fig. 2 Tera-Wasserburg plot of zircon dates from LASS and APM data.

    Ellipses show results from LASS core and rim analyses with 2σ uncertainty. The discordia line (dashed) constructed from the LASS data indicates an upper intercept of 2144 ± 33 Ma with a lower intercept of 148.6 ± 3.0 Ma. The lower intercept agrees with the timing of clustering of Pb in the zircon core (Fig. 3). The light blue field marks the range of published U-Pb dates measured from zircon rims formed at this time (23, 30). The 207Pb/206Pb dates were calculated from the atom probe data (boxes with 1σ uncertainty); dashed lines project onto concordia. The green box is the 207Pb/206Pb date from all clusters; the dark blue box is the 207Pb/206Pb date corrected for encapsulation at 160 Ma. Yellow diamonds show the modeled effect of Pb loss (%) on U-Pb dates for a zircon that crystallized at 2.15 Ga and was metamorphosed at 150 Ma; italicized numbers correspond to % Pb loss.

  • Fig. 3 APM mass spectrum.

    Representative atom probe microscopy (APM) mass spectrum acquired from tip M1. Peaks are color-coded by molecular species and/or element. Inset: Mass spectrum for atoms local to cluster IV (Fig. 4) showing distinct mass peaks for 206Pb and 207Pb. 208Pb (if present) coincides with a Si2O3 molecular species and cannot be quantified.

  • Fig. 4 APM reconstructions from the discordant 2.1-Ga zircon.

    (A and B) Reconstructed tips showing clustered distributions of Pb (green) atoms without co-clustering of U (yellow) atoms (A). Each tip measures between 200 and 500 nm in length. Roman numerals correspond to clusters shown in (B) and table S2. All tips are shown at the same scale; arrangement follows the sequence across the zircon shown in Fig. 1A. Rotating 3D projections are included in videos S1 to S4. (B) Individual Pb atoms (large green spheres) in clusters are shown with U (yellow) and Y (orange) atoms. All clusters are shown at the same scale. Cluster VI is also shown with an isoconcentration surface defined at 0.11 atomic % Pb to illustrate the toroidal morphology.

  • Fig. 5 Schematic history of the zircon.

    0: Grain crystallized (c. 2.1 Ga). Inferred Pb accumulation in the crystal (green curve) and temperature decrease (purple curve) as the source material was exhumed. 1: Before metamorphism (Pre-metm), Pb is randomly distributed throughout the specimen. 207Pb/206Pb evolves according to U decay constants (blue curve). Radiation damage accumulates below the critical amorphization temperature [magenta bar; 360°C for zircon with 1000 ppm U (1, 6)]. 2: During the prograde portion of the high-temperature (T) metamorphic event at c. 150 Ma, >75% of Pb is lost from the crystal (sharp drop on the solid green curve). The remaining Pb diffuses into dislocation loops, resulting in heterogeneously distributed Pb with high concentrations (2.0 to 5.5 atomic %) within the dislocation loops (dashed green line). 3: Pb continues to accumulate in the crystal after Pb loss (gray dashed line), thus evolving the bulk ratio within the tip (solid green line). Because the zircon lost Pb, newly accumulated Pb has a larger effect on the 207Pb/206Pb ratio of the whole tip (blue curve). The Pb ratio within the clusters does not change significantly (blue dashed line) because there is no detected U within the clusters.

  • Table 1 Atom probe data from Pb clusters.

    Tip refers to the specimen number (see Figs. 1 and 3). Roman numerals correspond to clusters shown in Fig. 3. Pb counts are the number of background-corrected 206Pb and 207Pb atoms counted per cluster. Background-corrected 207Pb/206Pb ratios are reported with 1σ uncertainty. 207Pb/206Pb dates for each cluster are reported with 1σ uncertainty. The sum of the clusters is labeled “Total.”

    TipClusterPb counts
    (atoms)
    Corrected 207Pb/206Pb
    ratio ±1σ
    Date (Ma)
    ±1σ
    206Pb207Pb
    M5I564840.149 ± 0.0192334 ± 210
    II176240.134 ± 0.0332151 ± 443
    M4VIII93150.160 ± 0.0532456 ± 590
    VI9001420.157 ± 0.0152424 ± 163
    III7861070.136 ± 0.0162177 ± 221
    M2IV8241080.131 ± 0.0152111 ± 202
    VII126170.134 ± 0.0382151 ± 515
    M1V87110.123 ± 0.0462000 ± 713
    Total35565070.142 ± 0.00742258 ± 90

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1601318/DC1

    Supplementary Methods

    table S1. Extended data table for LASS-ICP-MS analysis of zircon.

    table S2. Acquisition parameters for atom probe analysis on the LEAP 4000X HR at Curtin University.

    table S3. Reconstructions were performed with the following parameters using the “voltage evolution” algorithm in IVAS 3.6.12.

    table S4. Extended data table from the LEAP analysis (matrix and clusters).

    fig. S1. Chondrite-normalized REE plot for the zircon core (green) and rim (black) analyses.

    fig. S2. U portion of mass spectrum from tip M1 (analytical run 0654) showing two of the four mass peaks used to quantify the U content.

    video S1. This video shows a 360° rotation about the z axis of reconstructed atom probe data from M1 (see Fig. 1 for location).

    video S2. This video shows a 360° rotation about the z axis of reconstructed atom probe data from M2 (see Fig. 1 for location).

    video S3. This video shows a 360° rotation about the z axis of reconstructed atom probe data from M4 (see Fig. 1 for location).

    video S4. This video shows a 360° rotation about the z axis of reconstructed atom probe data from M5 (see Fig. 1 for location).

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Methods
    • Legend for table S1
    • table S2. Acquisition parameters for atom probe analysis on the LEAP 4000X HR at Curtin University.
    • table S3. Reconstructions were performed with the following parameters using the “voltage evolution” algorithm in IVAS 3.6.12.
    • Legend for table S4
    • fig. S1. Chondrite-normalized REE plot for the zircon core (green) and rim (black) analyses.
    • fig. S2. U portion of mass spectrum from tip M1 (analytical run 0654) showing two of the four mass peaks used to quantify the U content.
    • Legends for videos S1 to S4

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    Other Supplementary Material for this manuscript includes the following:

    • table S1 (Microsoft Excel format). Extended data table for LASS-ICP-MS analysis of zircon.
    • table S4 (Microsoft Excel format). Extended data table from the LEAP analysis (matrix and clusters).
    • video S1 (.mp4 format). This video shows a 360? rotation about the z axis of reconstructed atom probe data from M1 (see Fig. 1 for location).
    • video S2 (.mp4 format). This video shows a 360? rotation about the z axis of reconstructed atom probe data from M2 (see Fig. 1 for location).
    • video S3 (.mp4 format). This video shows a 360? rotation about the z axis of reconstructed atom probe data from M4 (see Fig. 1 for location).
    • video S4 (.mp4 format). This video shows a 360? rotation about the z axis of reconstructed atom probe data from M5 (see Fig. 1 for location).

    Files in this Data Supplement:

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