XRD-based 40Ar/39Ar age correction for fine-grained illite, with application to folded carbonates in the Monterrey Salient (northern Mexico)

Abstract

Due to their minute size, 40 Ar/ 39 Ar analysis of illite faces significant analytical challenges, including mineral characterization and, especially, effects of grain size and crystallography on 39 Ar recoil. Quantifying the effects of 39 Ar recoil requires the use of sample vacuum encapsulation during irradiation, which permits the measurement of the fraction of recoiled 39 Ar as well as the 39 Ar and 40 Ar* retained within illite crystals that are released during step heating. Total-Gas Ages (TGA) are calculated by using both recoiled and retained argon, which is functionally equivalent to K-Ar ages, while Retention Ages (RA) only involve retained Ar in the crystal. Natural applications have shown that TGA fits stratigraphic constraints of geological processes when the average illite crystallite thickness (ICT) is smaller than 10 nm, and that RA matches these constraints for ICTs larger than 50 nm. We propose a new age correction method that takes into account the average ICT and corresponding recoiled 39 Ar for a sample, with X-Ray Corrected Ages (XCA) lying between Total-Gas and Retention ages depending on ICT. This

Timing of Alpine Orogeny and Postorogenic Extension in the Alboran Domain, Inner Rif Chain, Morocco

“…For each spectrum, the first step with a zero age represents the amount of 39 Ar lost by recoil during neutron irradiation and measured before the step-heating degassing of the sample. As usually observed in fine-grained samples (e.g., Abd Elmola et al, 2018;Clauer, 2013;Dong et al, 1995;Fitz-Díaz et al, 2016;van der Pluijm et al, 2001), the percentage of recoiled 39 Ar increases with decreasing grain size, from about 4.1% for the fraction 2-50 μm to 21.8% for the fraction <0.2 μm (Table 4). The shape of the age spectra is quite different, with the larger grain sizes showing the greatest age discrepancy between the beginning and end of degassing.…”

“…Since the recoil distance for the neutron reaction producing 39 Ar is ∼0.162 μm (Onstott et al., 1995), 39 Ar recoil loss can be expected to be significant in the two finer grain‐size fractions and higher in the fraction <0.2 μm which is in agreement with our data (Table 4). The amount of 39 Ar recoil depends not only on the grain size but also on the distribution of the K atoms in the TOT structure of the crystallites and on their thickness (see Fitz‐Díaz et al., 2016 for a review). Moreover, it is likely that radiogenic argon produced by the natural decay of 40 K in poorly retentive sites of the crystallites (surfaces, vacancies, dislocations) may have escaped in the intergranular space.…”

“…In some circumstances, TGAs fit regional geological constraints; in others, RAs appear to give a better estimate of the geological event to be dated. Fitz-Díaz et al (2016) proposed a method of correction of TGAs based on XRD data to consider the effect of illite crystallite thickness (ICT) on argon retention. This method can be applied to anchizonal-epizonal micas with thickness values between 10 and 50 nm, which is the range observed in the Wm fractions <0.5 μm of Tizgarine unit, showing a ratio length/thickness of the crystallites close to 10 (SEM and XRD measurements; see Figure S1).…”

Estimación de desplazamiento mínimo en fallas inversas de alto ángulo: Caso de estudio en la Falla de San Marcos, Coahuila

“…Las observaciones realizadas en este estudio sugieren que el evento de reactivación con cinemática inversa más importante del sistema de fallas de San Marcos fue posterior al depósito de las Capas Cerro La Bruja, es decir, fue post-Turoniano. Muy probablemente la edad de este evento de deformación es Paleógeno, como lo sugiere la edad del plegamiento en el frente de la curvatura de Monterrey (Fitz-Díaz et al, 2016), y de capas del Maestrichtiano y del Paleógeno en la cuenca de la Popa (Gray y Lawton, 2011), algunas decenas de kilómetros al SE del área de estudio.…”

New geochronology constraints on timing and depth of the ancient earthquakes along the Longmen Shan fault belt, eastern Tibet

“…These fault‐related products are commonly mixed together with two distinct illite polytypes: the 1M d and 2M 1 phases. The 2M 1 phase is commonly rich in detrital illite from the surrounding wall rocks or with authigenic origin during deformation in anchizonal conditions, whereas the 1M d polytype usually grows at diagenetic conditions [ van der Pluijm et al ., ; Solum et al ., ; Hains and van der Pluijm , ; Fitz‐Díaz et al ., , ]. It is almost impossible to isolate pure authigenic illite for isotopic analysis.…”