The Open UniversitySkip to content
 

Recycling Argon through Metamorphic Reactions: the Record in Symplectites

McDonald, C. S.; Regis, D.; Warren, C. J.; Kelley, S. P. and Sherlock, S. C. (2018). Recycling Argon through Metamorphic Reactions: the Record in Symplectites. Lithos, 300-301 pp. 200–211.

Full text available as:
[img]
Preview
PDF (Version of Record) - Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader
Download (1MB) | Preview
DOI (Digital Object Identifier) Link: https://doi.org/10.1016/j.lithos.2017.11.028
Google Scholar: Look up in Google Scholar

Abstract

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12 kbar at ~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of 40Ar in the neo-crystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history.

Item Type: Journal Item
ISSN: 0024-4937
Project Funding Details:
Funded Project NameProject IDFunding Body
From Subduction to sand: Quantifying the balance between tectonic and surface processes during early continental collision and UHP rock exhumation. (SE-09-133-CW)NE/H016279/1NERC (Natural Environment Research Council)
Keywords: 40Ar/39Ar Thermochronology; Phengite; Biotite; Symplectite; Metamorphism
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Learning and Teaching Innovation (LTI) > Institute of Educational Technology (IET)
Learning and Teaching Innovation (LTI)
Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Item ID: 52649
Depositing User: Clare Warren
Date Deposited: 18 Dec 2017 10:30
Last Modified: 04 Jan 2018 15:53
URI: http://oro.open.ac.uk/id/eprint/52649
Share this page:

Metrics

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