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Reappraisal of the GL‐O Reference Material for K‐Ar Dating: New Insight from Microanalysis, Single‐Grain and Milligram Ar Measurements
Author(s) -
Boulesteix Thomas,
Solé Jesús,
Pi Teresa,
Cathelineau Michel
Publication year - 2020
Publication title -
geostandards and geoanalytical research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.037
H-Index - 73
eISSN - 1751-908X
pISSN - 1639-4488
DOI - 10.1111/ggr.12306
Subject(s) - calcite , pellets , homogeneity (statistics) , mineralogy , analytical chemistry (journal) , homogeneous , chemistry , glauconite , population , geology , mathematics , statistics , environmental chemistry , demography , combinatorics , sociology , oceanography
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.