Reappraisal of the GL‐O Reference Material for K‐Ar Dating: New Insight from Microanalysis, Single‐Grain and Milligram Ar Measurements

Abstract

The homogeneity and Ar-dating suitability of the GL-O reference material were re-evaluated to determine whether this material is sufficiently homogeneous to be suitable for the calibration of modern high sensitivity instruments. Based on new micro-analyses and noble gas determinations, our contribution reveals several kinds of inhomogeneity at the grain scale: disparity in the glauconitisation among and within the pellets, variable occurrence of a phosphatic component within pellets (1% m/m on average), and rare occurrences of calcite and detrital grains. Measurements on test portions of ≤ 1 mg reflect such heterogeneity with variability in 40 Ar* content that exceeds analytical uncertainty, including a few highly anomalous values. The lesser evolved glauconite population yielded 40 Ar* contents ~15% lower than the value of 24.8 nl g -1 recommended by Odin et al. (1982, Numerical dating in stratigraphy. Wiley (Chichester, UK), 123-148). But the measured concentrations of 40 Ar* converge towards the aforementioned value as test portion mass increased to > 3 mg. A few rare 3 mg experiments still yielded 40 Ar* contents lower than the recommended value (down to 24.0 nl g -1 ), and we recommend using more conservative minimum masses of 5-10 mg. A further purification step for GL-O or the intercalibration of its powder version could be considered to diminish the size of the test portions and the intensity of the measured signals.

An Automated System for Measuring In Situ K–Ar Ages

“…A plan for finding and preparing age reference materials homogeneous at the scale of 50–100 µm is also essential – see for example Boulesteix et al . (2020). It is relevant to note here – for the sceptical reader – that in the 1980s when ICP‐MS was developed, the opinion was expressed that the system seemed impossible to calibrate for quantitative analysis; two decades of development by numerous scientists has decisively repudiated this view.…”

“…The reliability and analytical uncertainty of the method, even if already remarkable, could be much improved if other laboratories build similar systems and contribute their own experiences and ideas. A plan for finding and preparing age reference materials homogeneous at the scale of 50-100 µm is also essentialsee for example Boulesteix et al (2020). It is relevant to note herefor the sceptical readerthat in the 1980s when ICP-MS was developed, the opinion was expressed that the system seemed impossible to calibrate for quantitative analysis; two decades of development by numerous scientists has decisively repudiated this view.…”

Characterisation of Reference Materials forIn SituRb‐SrDating byLA‐ICP‐MS/MS

“…The results are consistent with a previous study by Boulesteix et al . (2020) who noted the presence of apatite and carbonate in GL‐O grains. These authors described phosphate phases in GL‐O grains as massive or disseminated apatite, as observed in the present work.…”

Revisiting Glauconite Geochronology: Lessons Learned from In Situ Radiometric Dating of a Glauconite-Rich Cretaceous Shelfal Sequence

“…From the obtained glauconite bulk ages it becomes clear that any potential post-depositional modifications and/or significant resetting of the Rb-Sr systematics of the studied glauconite are negligible. These include possible phenomena such as (i) decomposition or diagenetic dissolution of Rb-and K-rich detrital minerals (e.g., feldspar and mica) in the sediment substrate and the subsequent (ii) uptake of inherited (non-marine) Rb, K, and Sr isotope signals by authigenic or late diagenetic glauconite [67], and also (iii) a possible loss or gain of Sr during diagenetic processes or (iv) isotopic exchange with non-marine burial/diagenetic fluids [68,69]. Results also indicate that resetting of the isotopic signatures of glauconite is insignificant during comparatively shallow burial diagenesis, as is the case for the Langenstein profile (≤1500 m burial depth; ≤65 °C [16]).…”

“…However, analytical challenges of glauconite dating, such as the incorporation of inherited non-marine and typically more radiogenic Sr due to substrate interaction [81], presence of detrital Rb-and K-bearing feldspars and mica within glauconite grains [67], post-depositional alteration into Fe-illite or nontronite [69], and subsequent post-depositional resetting of the glauconite Rb-Sr isotope system [82,83], have until now prevented a good assessment of glauconitization rates for deep-water and shallow-marine settings.…”