Contribution to the discussion on processing and measurement methodology of apatite fission-track analysis

<p>Apatite fission track dating and T,t-modelling are now a well-established thermochronological instruments for investigating geological problems (Malusà and Fitzgerald, 2019). In the course of their development, complicating factors that affect the track counts and confined track lengths in geological samples were corrected for, foremost among them the crystallographic orientation of the confined track and the chemical composition of the apatite (Green et al., 1986, and subsequent papers). Methods have also been proposed to improve the confined track statistics, using <sup>252</sup>Cf irradiation, ion irradiation, fracturing, and re-etching (Yamada et al., 1998). However, there is to date no adequate correction for the protocol used to reveal the tracks, which differs from lab to lab although all are based on nitric acid.</p><p>Recent step-etch experiments with the most used etchants show that both the duration of the etch and the temperature and concentration of the solution have non-negligible effects on the measured lengths (Sobel and Seward, 2010; Jonckheere et al., 2017 and references therein; Tamer et al., 2019). Earlier attempt to overcome these problems investigated etching for such a time that the track openings conform to a pre-determined size (Ravenhurst et al., 2003) or measuring confined tracks of a given minimum width (Yamada et al., 1993). The first method has the drawback that the widths of the host tracks and confined tracks are not directly related, whereas the second fails to consider the anisotropic width of confined tracks.</p><p>In our geological investigation of the German Naab area, we adopt a step-etch approach, measuring the <strong>c</strong>-axis angle, length, width and dip of each individual confined track after 20s and 30s immersion in 5.5 M HNO<sub>3</sub>. From the width increase we calculate the rate of widening of the track (apatite etch rate; Aslanian et al., 2021), and from that the effective etch time t<sub>E</sub>, i.e., the true duration that the confined track has been etched, equal to the immersion time minus the time needed for the etchant to reach the specific confined track. Our results show that the confined track lengths are correlated with their effective etch times. This information is used to account for etch-protocol-related differences between the induced and fossil track lengths entered in the T,t-modelling software. We envisage this will improve the accurateness and resolution of the resulting T,t-paths. We will check this against the excellent independent geological constraints that exist for the Naab region.</p><p>The research was funded by the EU/MEYS (CZ.02.2.69/0.0/0.0/19_074/0014756).</p><p> </p><p>References</p><p>Aslanian et al., 2021. American Mineralogist. In press.</p><p>Green et al., 1986. Chemical Geology 59, 237-253.</p><p>Jonckheere et al., 2017. American Mineralogist 102, 987-996.</p><p>Malusà and Fitzgerald, 2019.  Fission-Track Thermochronology and its Application to Geology. Pp 393.</p><p>Ravenhurst et al., 2003. Canadian Journal of Earth Sciences 40, 995-1007.</p><p>Sobel and Seward, 2010. Chemical Geology 271, 59-69.</p><p>Tamer et al., 2019. American Mineralogist 104(10), 1421-1435.</p><p>Yamada et al., 1993. Chemical Geology 122, 249-258</p><p>Yamada et al., 1998. Chemical Geology 149, 99–107.</p>

Spilitization of Early-Permian Volcanics from Głuszyca Górna (the Intra-Sudetic Basin, Poland)—Constraints from Chlorite Thermometry Coupled with Apatite Fission-Track Dating (AFT)

The Intra-Sudetic Basin, a Late-Paleozoic intramontane trough located on the NE flank of the Bohemian Massif, is comprised of numerous outcrops of continental (extension-related) Early-Permian volcanogenic rocks that are commonly altered to spilites. In this contribution, we provide insights into the formation of spilitized (albite- and chlorite-rich) trachyandesites from the Głuszyca quarry (Lower Silesia, Poland), based on mineralogical and micro-textural investigations supported by apatite fission-track dating (AFT). Our results indicate that the trachyandesites, emplaced as a shallow-level laccolith-type body, have been strongly affected by chloritization of both aegirine and augite, combined with an occasional celadonitization of volcanic glass. Furthermore, chlortitization of sodic pyroxenes must have released notable amounts of Na+, which could be involved during later pervasive albitzation of primary andesine-labradorite. According to various chemical and semi-empirical thermometers, the replacive chlorites formed in the range of 124–170 °C. Trachyandesites from Głuszyca contain abundant fluorapatites, marked by the occurrence of swallow-type terminations, which are indicative of rapid-cooling formation conditions. Central AFT ages of the samples vary between 161–182 Ma and correspond to the Middle-Jurassic period. Meanwhile, these ages are significantly younger than the emplacement of igneous rocks during the Middle-Rotliegendes period (~299–271 Ma). The discrepancy between the stratigraphic age of the rocks and the AFT results cannot be, however, explained by, for example, slow cooling rates of magmatic body, compositional variations of apatite, or burial under Late-Mesozoic sediments. Hence, it may be assumed that the obtained AFT ages (161–182 Ma) reflect the timing of spilitization and associated partial reheating of volcanic rocks from the Intra-Sudetic Basin above the apatite partial annealing zone (70–110 °C).

Thermal history of the Guli pluton (north of the Siberian platform) according to apatite fission-track dating and computer modeling

We present the first results of fission-track dating of apatite monofractions from two rock samples taken from the Southern carbonatite massif of the world’s largest alkaline ultrabasic Guli pluton (~250 Ma), located within the Maymecha-Kotuy region of the Siberain Traps. Based on the apatite fission-track data and computer modeling, we propose two alternative model of the Guli pluton's tectonothermal history. The models suggest (1) rapid post-magmatic cooling of the studied rocks in hypabyssal conditions at depth about 1.5 km, or (2) their burial under a 2-3 km thick volcano-sedimentary cover and reheating above 110°C, followed by uplift and exhumation ca. 218 Ma.