The Open UniversitySkip to content

Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar ages in perlitised obsidian

Flude, Stephanie; Tuffen, Hugh and Sherlock, Sarah C. (2018). Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar ages in perlitised obsidian. Chemical Geology, 480 pp. 44–57.

Full text available as:
PDF (Version of Record) - Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader
Download (1MB) | Preview
DOI (Digital Object Identifier) Link:
Google Scholar: Look up in Google Scholar


In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the ~ 27 Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of 36Ar over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of 36Ar in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near-atmospheric composition, resulting in elevated 36Ar and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high 36Ar yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples.

Item Type: Journal Item
Copyright Holders: 2017 The Authors
ISSN: 0009-2541
Keywords: 40Ar/39Ar-dating; Obsidian; Diffusion; Ar-isotopes
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Item ID: 53875
Depositing User: ORO Import
Date Deposited: 16 Mar 2018 10:13
Last Modified: 26 Jan 2020 05:27
Share this page:


Altmetrics from Altmetric

Citations from Dimensions

Download history for this item

These details should be considered as only a guide to the number of downloads performed manually. Algorithmic methods have been applied in an attempt to remove automated downloads from the displayed statistics but no guarantee can be made as to the accuracy of the figures.

Actions (login may be required)

Policies | Disclaimer

© The Open University   contact the OU