U–Pb and Th–Pb dating of apatite by LA-ICPMS

Abstract

Apatite is a common U-and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but widespread detrital component in clastic sedimentary rocks. U-Pb and Th-Pb dating of apatite has potential application in sedimentary provenance studies, as it likely represents first cycle detritus compared to the polycyclic behaviour of zircon. However, low U, Th and radiogenic Pb concentrations, elevated common Pb and the lack of a U-Th-Pb apatite standard remain significant challenges in dating apatite by LA-ICPMS, and consequently in developing the chronometer as a provenance tool.This study has determined U-Pb and Th-Pb ages for seven well known apatite occurrences (Durango, Emerald Lake, Kovdor, Mineville, Mudtank, Otter Lake and Slyudyanka) by LA-ICPMS. Analytical procedures involved rastering a 10μm spot over a 40×40μm square to a depth of 10μm using a Geolas 193nm ArF excimer laser coupled to a Thermo ElementXR single-collector ICPMS. These raster conditions minimized laser-induced inter-element fractionation which was corrected for using the back-calculated intercept of the time-resolved signal. A Tl-U-Bi-Np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias.External standards (Plešovice and 91500 zircon, NIST SRM 610 and 612 silicate glasses and STDP5 phosphate glass) along with Kovdor apatite were analysed to monitor U-Pb, Th-Pb and Pb-Pb ratios. Age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common Pb-corrected ages, a Tera-Wasserburg Concordia intercept age and a Tera-Wasserburg Concordia intercept age anchored through

LA-ICP-MS U-Pb apatite dating of Lower Cretaceous rocks from teschenite-picrite association in the Silesian Unit (southern Poland)

“…Chew et al 2011). The anchored concordia lower intercept ages are 103± 20 Ma (MSDW= 3.5) for Boguszowice (Fig.…”

Apatite: a U-Pb thermochronometer or geochronometer?

“…Moreover, a significant grain size component in these, and likely many apatite geochronology samples, will not be characterizable given current analytical size constraints. Some authors have cautioned that fluid driven elemental mobility may be important in apatite geochronology and thus thermally activated volume diffusion a secondary process affecting U-Pb dates (Bingen et al, 1996;Chen and Simonetti, 2013;Chew et al, 2011;Chew and Spikings, 2015;Cochrane et al, 2014;Corfu and Stone, 1998). For the Greenland samples investigated in this work, fluid driven disturbance and growth processes are implicated on the basis of grain internal textures, petrography, and the relationship between apparent ages and chemistry, where new rim growth appears to be coeval with core U-Pb resetting.…”

“…Unlike zircon, apatite can be readily found in silica-undersaturated mafic lithologies, however its U content is frequently much lower than that of zircon and apatite usually contains moderate to significant amounts of non-radiogenic (common) Pb. The high ratio of common to radiogenic Pb presents challenges for apatite U-Pb geochronology and calculated dates rarely have the precision of those obtained fromzircon (Chew et al, 2011;Glorie et al, 2017;Krogstad and Walker, 1994;LaFlamme et al, 2017). Nonetheless, apatite is still a very useful U-Pb chronometer as its typical closure temperature of around 375-600°C (Cochrane et al, 2014;Schoene and Bowring, 2007) potentially can provide additional time-temperature constraints on retrograde or lowtemperature prograde processes.…”

Evidence for the Multi-Stage Petrogenetic History of the Oka Carbonatite Complex (Québec, Canada) as Recorded by Perovskite and Apatite

“…The NIST SRM 610 international glass standard [35] was used for external calibration and 43 Ca ion signal intensities were employed as the internal standard with the CaO content (wt %) obtained by EMP analysis. The sample grains and standard were ablated using a 25 μm spot size, 4-5 Hz repetition rate, and corresponding energy density of ~10-12 J/cm 2 .…”

“…Fragments of Emerald Lake and Durango apatites were used as secondary apatite standards and both are well characterized with ages of 90.5 ± 3.1 and 30.6 ± 2.3 Ma, respectively [43]. Repeated analyses of these two standards obtained during the course of this study yielded weighted mean 206 Pb/ 238 U ages of 92.6 ± 1.6 Ma (n = 17) and 31.9 ± 1.3 Ma (n = 10), respectively, and both are identical (given their associated uncertainties) to the ages reported by Chew et al (Figure 3A,B) [41].…”