Abstract
Abstract. Zircon Raman dating based on irradiation damage is a debated concept but not an established geo-/thermochronological method. One issue is the temperature range of radiation-damage annealing over geological timescales. We conducted isochronal and isothermal annealing experiments on radiation-damaged zircons between 500 and 1000 ∘C for durations between 10 min and 5 d to describe the annealing kinetics. We measured the widths (Γ) and positions (ω) of the ν1(SiO4), ν2(SiO4), and ν3(SiO4) internal Raman bands, and the external rotation Raman band at ∼974, 438, 1008, and 356 cm−1 after each annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed activation energy model to the fractional annealing data, calculated from the widths of the ν2(SiO4), ν3(SiO4), and external rotation bands. From the kinetic models, we determined closure temperatures Tc for damage accumulation for each Raman band. Tc ranges from 330 to 370 ∘C for the internal ν2(SiO4) and ν3(SiO4) bands; the external rotation band is more sensitive to thermal annealing (Tc∼260 to 310 ∘C). Our estimates are in general agreement with previous ones, but more geological evidence is needed to validate the results. The Tc difference for the different Raman bands offers the prospect of a multi-closure-temperature zircon Raman thermochronometer.
Highlights
Zircon (ZrSiO4) is used with several geochronometers because of the substitution of U and Th for Zr in its lattice
The zircon Raman spectrum is sensitive to lattice damage: the downshift and broadening of the Raman bands provide a quantitative measure for the radiation damage (Nasdala et al, 1995, 1998, 2001; Palenik et al, 2003; Váczi and Nasdala, 2017)
We aim to investigate the change of the major Raman bands due to annealing and to estimate Tc
Summary
Zircon (ZrSiO4) is used with several geochronometers because of the substitution of U and Th for Zr in its lattice. The zircon Raman spectrum is sensitive to lattice damage: the downshift and broadening of the Raman bands provide a quantitative measure for the radiation damage (Nasdala et al, 1995, 1998, 2001; Palenik et al, 2003; Váczi and Nasdala, 2017). Pidgeon et al (1998) analogously suggested zircon dating based on Raman measurements of the radiation damage. The advantages of the determination of radiation damage with Raman over XRD or IR spectroscopy are the simple sample preparation and standardization, short measurement time, high spatial resolution, and the sensitive damage response of the Raman bands over a broad range of damage densities (Nasdala et al, 1995; Pidgeon et al, 1998). One of the key issues for the geological application of a zircon Raman geo-/thermochronometer is the quantification of radiationdamage annealing over geological timescales (Nasdala et al, 2001, 2002; Geisler and Pidgeon, 2002)
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