Research ArticleGEOCHRONOLOGY

Seeing is believing: Visualization of He distribution in zircon and implications for thermal history reconstruction on single crystals

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Science Advances  10 Feb 2017:
Vol. 3, no. 2, e1601121
DOI: 10.1126/sciadv.1601121
  • Fig. 1 CL images and isotopic and U-Pb age maps.

    (A to O) CL images, He content, eU concentration, and U-Pb age maps (dots mark the center of ablation pits) generated for four representative crystals. ppm, parts per million; Ga, billion years; Ma, million years; ncc, nano–cubic centimeter; Discord, discordance. (P and Q) Transmitted and reflected light images, respectively, of crystal R-3 showing mineral and fluid inclusions. Mineral inclusions identified by energy-dispersive x-ray analysis include apatite (Ap), K-feldspar (Kfs), monazite (Mnz), quartz (Qtz), and titanite (Ti). Note that the location of the hot spot in the He map (N) corresponds to the location of voids after fluid inclusions (v) in (P) and (Q). (R) Close-up secondary electron image of a zircon ablated with square ablation pits. The ablation opened a fluid inclusion, releasing the fluid and leaving an empty cavity (black). Also note that He distribution in all crystals correlates well with CL intensities and eU distributions. Data used for He, eU, and U-Pb maps are shown in tables S1 and S2.

  • Fig. 2 CL image and Raman spectra.

    (A) Enlarged section of the CL image of crystal I2-9 scanned by Raman spectroscopy (for exact location within the grain, see Fig. 1A). (B and C) Color-coded Raman map (B) and corresponding Raman spectra (C) showing shift (parameter X in the inset) and increased width of peaks (FWHM) in the range of 950 to 1020 cm−1, indicating variable degrees of disorder in the zircon. a.u., arbitrary units. Note that the identified Raman domains correlate with the CL intensities (A) and eU map shown in Fig. 1K. However, the amorphous zone with the most severe radiation damage (blue in the Raman map) has negligible He retentivity and thus shows a negative correlation with the He map (Fig. 1J).

  • Fig. 3 Thermal history reconstruction.

    (A) Measured U-Th zoning profiles of sample M14-4 used for thermal history reconstruction. Fractional radial position: 0 corresponds to crystal core and 1 corresponds to crystal rim. (B) Thermal histories resulting in a ZHe age of 67.1 Ma, illustrating different styles of cooling: fast cooling through the ZHePRZ (green), slow cooling through the ZHePRZ (yellow), and a reheating to the ZHePRZ temperatures (purple). (C) He production-diffusion profiles calculated by the HeFTy software (59) using the diffusion algorithm of Guenthner et al. (10) corresponding to the thermal trajectories from (B) (color-coded). Red curve represents the He production-diffusion profile calculated from the measured He map. Note that the green curve [fast cooling through the ZHePRZ in (B)] is most similar to the red curve, suggesting that the fast cooling thermal trajectory is the most viable solution.

  • Table 1 Sample details.

    Origin, ages, internal structures, and α-ejection correction factors of analyzed crystals. Fth and Ftz, α-ejection correction factors for full crystals with assumed homogeneous and measured zoned distribution of U and Th, respectively, calculated using the equation of Farley et al. (29); age difference, the difference between the ZHe age corrected for α-ejection based on measured U-Th distribution (termed “true” age here) and the ZHe age corrected for α-ejection assuming homogeneity of U and Th (termed “conventional” age here). “+” and “−” mean that the true ZHe age is older or younger, respectively, than the conventional ZHe age. Ma, million years; N/A, not applicable.

    CrystalOriginMagmatic/
    metamorphic age
    Conventional
    ZHe ages (Ma)
    Zircon internal featuresFthFtzAge
    difference
    I2-9Leucogranite (India)Emplacement at
    2029 ± 65 Ma;
    overprint at
    536 ± 48 Ma (53)
    194.7 ± 20.6 to
    314.7 ± 18.9 (Table 2)
    Oscillatory zoning with two amorphous,
    inclusion-rich domains, overgrown by
    thin high–CL response rim; traversed
    by several radial fractures
    0.870.82+5%
    I2-1Leucogranite (India)Emplacement at
    2029 ± 65 Ma;
    overprint at
    536 ± 48 Ma (53)
    194.7 ± 20.6 to
    314.7 ± 18.9 (Table 2)
    Oscillatory zoned; low–CL response core
    with rounded terminations, overgrown
    by high–CL response rims containing faint
    indications of patchy and sector zoning
    0.880.90−2%
    M14-4Variscan batholith
    from Sardinia (Italy)
    Emplacement at
    320–290 Ma (52)
    67.1 ± 7.1; 73.7 ± 4.2
    (Table 2)
    Idiomorphically zoned; rim overgrowing
    high–CL response core with invaginated
    boundaries
    0.780.72+6%
    R-3Variscan granite
    from Bohemian
    Massif (Poland)
    Emplacement at
    312.5 ± 0.3 Ma (54)
    99.7 ± 6.7 to
    271.2 ± 24.8 (26)
    Oscillatory zoning; low–CL response
    mineral inclusions; convoluted
    boundaries between some
    growth zones
    N/AN/AN/A
  • Table 2 Zircon (U-Th)/He data.

    TAU, total analytical uncertainty; ESR, equivalent sphere radius in micrometers; Ft, α-ejection correction factor calculated using the equation of Farley et al. (29), assuming homogeneous distribution of U and Th and corrected for the mineral portion removed by polishing. Crystals marked with asterisk were used for isotopic mapping.

    Sample
    code
    232Th
    (ng)
    ±
    (%)
    238U
    (ng)
    ±
    (%)
    147Sm
    (ng)
    ±
    (%)
    4He
    (ncc)
    ±
    (%)
    TAU
    (%)
    Th/
    U
    Raw age
    (Ma)
    ±1 σ
    (Ma)
    ESR
    (μm)
    FtCorrected
    age (Ma)
    ±1 σ
    (Ma)
    MS14-4*0.4361.42.9041.90.00514.021.1881.72.50.5357.71.5540.8667.17.1
    MS14-21.4451.42.9331.90.00217.927.1002.02.70.4967.71.8930.9273.74.2
    I2-1*0.6692.03.5832.40.00711.594.7171.62.80.33204.25.81500.95215.022.8
    I2-30.4201.40.8191.90.00224.825.4752.42.90.51223.86.6850.91245.414.2
    I2-60.7732.03.6842.40.00317.6138.3022.53.30.21286.29.6830.91314.718.9
    I2-9*1.4071.42.2271.90.00514.455.9751.62.30.53177.24.2830.91194.720.6
    R-3*0.9122.01.6712.30.0068.651.7772.23.10.54221.36.8750.90247.114.5
    R-3-50.1491.50.3501.90.00912.67.9542.32.90.42167.34.8640.88189.610.9
    R-3-60.8181.41.6241.90.00613.246.5142.02.60.50206.65.4910.92225.312.7

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. U-Pb Concordia diagrams.
    • fig. S2. Ablation pits.
    • table S1. He and eU data used for maps.
    • table S2. U-Pb analytical data.

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    Files in this Data Supplement:

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