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The thermoelastic properties and thermal expansion of natural single-crystal zircon without detectable radiation damage were determined in the temperature range between 100 and 1650 K on five samples from Cambodia, Sri Lanka, and Tanzania. Up to about 1500 K, the results are in excellent agreement and fully reversible. The average adiabatic elastic stiffness coefficients at room temperature in GPa are c11 = 426.8(6), c12 = 68.1(4), c13 = 152.0(4), c33 = 490.8(9), c44 = 113.1(2), and c66 = 49.0(1). Polycrystalline averaging for the aggregate bulk and shear modulus (Voigt-Reuss-Hill average) yields K = 229.3(4) GPa and G = 109.3(1) GPa. From 293 to 1573 K, the elastic stiffnesses soften almost linearly with the temperature coefficients in MPa/K: dc11/dT = –45.36(7), dc12/dT = –2.03(7), dc13/dT= –9.59(7), dc33/dT = –42.85(12), dc44/dT = –9.60(3), and dc66/dT = –2.19(1). Below room temperature, the behavior of the cij is nonlinear, with decreasing absolute values of the temperature coefficients. Above about 1500 K, a temperature- and time-dependent irreversible elastic stiffening accompanied by an increasing ultrasound dissipation was observed in all samples. These anomalies are interpreted to be the consequence of increased dislocation densities. [ABSTRACT FROM AUTHOR]

The difference in radiation damage retained in unknown and reference zircons, caused by the α-dose matrix effect, has been considered as a great influence on the accuracy and precision of zircon U–Pb dating as found from LA-ICP-MS and SIMS analysis. This study aims to explore the effect of high-temperature thermal annealing (>1300 °C) on the optimization of the U–Pb dating of standard zircons, such as Sri Lanka, Plešovice, and Qinghu zircons, with heterogeneous radiation damage. Raman spectroscopy was employed to investigate the structural state of Sri Lanka zircons that were annealed at 100 °C to 1300 °C at various times (isochronal and isothermal annealing), indicating that annealing at 1400 °C for 96 h can result in a nearly complete crystalline state for zircons with a low-moderate degree of radiation damage. SIMS U–Pb isotopic analyses of unannealed and annealed Sri Lanka zircon grains revealed that the homogeneity of sample SL-3 annealed at 1300 °C with a 206Pb/238U age of 572.4 ± 4.4 Ma (2σ, MSWD = 0.08) is greater than those of the sample SL-2 annealed at 850 °C with 206Pb/238U age of 575.0 ± 4.5 Ma (2σ, MSWD = 0.57) and sample SL-1 unannealed with 206Pb/238U age of 575.3 ± 4.3 Ma (2σ, MSWD = 0.30). Furthermore, the results of LA-ICP-MS U–Pb ages and trace element concentrations for Plešovice and Qinghu zircons indicate that high-temperature thermal annealing can reduce the distribution of trace element composition and the scatters of the 206Pb/238U ages with MSWD decreasing by 82% and 71%, respectively, despite the annealed zircons yielding slightly younger 206Pb/238U ages than unannealed zircons. This approach provides new insights into acquiring homogeneous U–Pb ages of standard zircons with heterogeneous radiation damage, which would be useful to calibrate and obtain unknown zircon U–Pb dating with better precision and accuracy. [ABSTRACT FROM AUTHOR]

We present a new methodology for laser-induced steady-state photoluminescence (PL) spectroscopy of Dy3+ that aims at a direct quantification of the amorphous fraction f a present in zircon (ZrSiO4), which undergoes a transition from a crystalline to a metamict state due to cumulative self-irradiation damage caused by the radioactive decay of substituted U and Th. Using state-of-the-art confocal spectrometers attached to optical microscopes, measurements may be performed non-destructively on the micrometre length-scale with the option to visualize radiation-damage patterns as revealed by hyperspectral PL maps. Zircon from the Ratnapura district (Sri Lanka, ~520 Ma), was used as reference material to substantiate the applicability of the proposed method. The accumulation of radiation damage in this material was investigated in detail and obtained f a values correlate with calculated ff-doses in accordance to the direct impact model reported variously in the literature. The impact of chemically-induced, heterogeneous broadening of Raman and Dy3+ emission spectral bands is discussed on two examples from Mt. Malosa district, Malawi. A mean weighted U-Pb isotope age of 111 ± 1 Ma (pegmatitic-type) and a discordia age of 112 ± 1.6 Ma (hydrothermal-type) as obtained by LA-ICP-MS confirm their close genetic and temporal relationship. Studied zircon examples demonstrate that the amount of radiation damage present may have a substantial effect on the precision of LA-ICP-MS ages, but cannot be considered an exclusive cause for bias of obtained isotope ages. [ABSTRACT FROM AUTHOR]

Structural reconstitution upon dry thermal annealing of mildly to strongly radiation-damaged, gem-quality zircon from Sri Lanka has been studied by single-crystal X-ray diffraction and Raman spectroscopy. Results of structure refinement of a strongly radiation-damaged zircon (sample GZ5, calculated alpha dose ~4 × 1018 g−1) indicate the existence of an interstitial oxygen site that is sparsely occupied (about 4% of all O atoms). Annealing of this sample at T a (annealing temperature) = 700 °C has resulted in nearly complete recrystallization of its amorphous volume fraction and significant decrease in the occupation of O-interstitial sites. For all samples studied, annealing up to T a ≤ 650–700 °C is characterised by preferred recovery of Raman shifts (compared to Raman FWHMs; full width at half band maximum) and extensive contraction of the unit-cell volume, in particular along unit-cell dimension a. This low- T annealing is dominated by epitaxial growth of the crystalline volume fraction at the expense of the amorphous volume fraction, and general recovery of low-energy defects. During annealing at T a = 700–1400 °C there is preferred recovery of Raman FWHMs (compared to Raman shifts) and only mild unit-cell contraction. High- T annealing is dominated by the recovery of high-energy defects such as recombination of cation Frenkel pairs. Here, unit-cell parameter a shows a remarkable behaviour (namely, mild re-increase at T a = 700–1150 °C and mild final shrinking at T a = 1000–1400 °C), which is attributed to enhanced contortion of ZrO 8 polyhedrons due to cation repulsion. The combined data set of Raman band and unit-cell parameter presented herein will help analysts to assign Raman spectra of annealed unknowns to certain recovery stages. • Structural recovery of radiation-damaged zircon upon dry annealing is evaluated. • At moderate damage, there exists an interstitial oxygen site. • Low-T annealing involves recrystallization of the amorphous volume fraction. • This process is driven by epitaxial growth; random nucleation is subordinate. • High-T annealing involves recovery of high-energy point defects. [ABSTRACT FROM AUTHOR]