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Reply to discussion on 'A high-precision U-Pb age constraint on the Rhynie Chert Konservat-Lagerstatte: time scale and other implications': Journal, 168, 863-872

Parry, S.F.; Noble, S.R.; Crowley, Q.G.; Wellman, C.H.. 2013 Reply to discussion on 'A high-precision U-Pb age constraint on the Rhynie Chert Konservat-Lagerstatte: time scale and other implications': Journal, 168, 863-872. Journal of the Geological Society, 170 (4). 703-706. 10.1144/jgs2012-089

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Abstract/Summary

We welcome the opportunity to address the points raised by Mark et al. in their discussion of the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA–ID–TIMS) U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte presented by Parry et al. (2011) and also to make some further observations of our own. We begin by briefly providing some context for the benefit of the wider readership. Two radio-isotopic age constraints on the Rhynie Chert Konservat-Lagerstätte and, by corollary, its parental hydrothermal (hot-spring) system have recently been published. The first of these is a weighted mean 40Ar/39Ar plateau age of 403.9 ± 2.1 Ma (2σ) derived from the analysis of two samples of vein-hosted hydrothermal K-feldspar and a single sample of hydrothermally altered andesite (Mark et al. 2011). In order to account for systematic uncertainties associated with the 40Ar/39Ar geochronometer, Mark et al. (2011) recalculated their individual sample ages with reference to the Fish Canyon Tuff sanidine (FCs) age of 28.201 Ma (Kuiper et al. 2008), thereby producing a ‘U–Pb comparable’ mean age of 407.1 ± 2.2 Ma (2σ). An alternative ‘preferred age’ for the Rhynie hot-spring activity (407.6 ± 2.2 Ma (2σ)) has now been produced from the ‘raw’ data using the optimization model of Renne et al. (2010, 2011) (this discussion). The 40Ar/39Ar system calibrations on which these various ages are based are summarized in Table 1. The second radio-isotopic age constraint in question is a weighted mean 206Pb/238U zircon age of 411.5 ± 1.3 Ma (2σ, including decay constant- and tracer calibration-related uncertainties; MSWD = 0.12, n = 4) yielded by the Milton of Noth Andesite, a moderately altered basaltic andesite lava flow (cum near-surface sill?) that lies along the northwestern margin of the Rhynie Outlier (Parry et al. 2011). U–Pb titanite data corroborate the zircon data, and c. 411.5 Ma is interpreted as the crystallization-eruption age of the Milton of Noth Andesite. Lavas and tuffs of andesitic composition occur elsewhere within the northern half of the Rhynie Outlier (Rice & Ashcroft 2004) and a holistic view of the available evidence would suggest that these volcanic rocks represent the surficial expression of the thermal drive for the Rhynie hot-spring system. Parry et al. (2011) therefore concluded that that the U–Pb zircon age yielded by the Milton of Noth Andesite dates the Rhynie hydrothermal activity within error [our italics].

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1144/jgs2012-089
ISSN: 0016-7649
Date made live: 07 Jan 2014 13:48 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/504426

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