K-Ca-Ar systematics of authigenic sanidine from Waukau, Wisconsin, and the diffusivity of argon

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doi: 10.1130/0091-7613(1986)14<936:KSOASF>2.0.CO;2
Authors:Marshall, B. D.; Woodard, H. H.; DePaolo, D. J.
Author Affiliations:Primary:
Univ. Calif., Dep. Earth and Space Sci., Los Angeles, CA, United States
Beloit Coll., United States
Volume Title:Geology (Boulder)
Source:Geology (Boulder), 14(11), p.936-938. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0091-7613
Publication Date:1986
Note:In English. 22 refs.; illus. incl. 2 tables
Summary:Authigenic sanidine occurs in almost monomineralic beds in the basal St. Peter Sandstone near Waukau, Wisconsin. The high purity of this K-feldspar indicates that it formed at low T either contemporaneously with deposition or during later diagenesis. K-Ca analyses of two samples from the same bed yield discordant ages of 436+-4 and 407+-3 m.y. Because 40Ca loss is unlikely at the low T to which these minerals have been subjected, these ages are interpreted as the times of formation of the sanidine. Both ages are younger than the estimated stratigraphic age of the St. Peter Sandstone. K/Ar analyses of splits of these same two samples give ages of 402 and 377 m.y., respectively. These ages are lower by about 8% than the K/Ca ages because of the diffusive loss of radiogenic argon over geological time. An estimate of the diffusion co-efficient for argon in the sanidine on the basis of the difference between the K/Ca and K/Ar ages suggests that diffusion of argon is about five orders of magnitude faster at low T than an extrapolation of high T experimental data would indicate. Comparison of K/Ca and K/Ar ages may be useful in general for distinguishing diffusive loss from recrystallization events in feldspars. [R.A.H.]
Subjects:Absolute age; Alkali feldspar; Argon; Authigenic minerals; Clastic rocks; Dates; Diffusion; Diffusivity; Feldspar group; Framework silicates; Geochemistry; Geochronology; K-feldspar; K/Ar; Methods; Middle Ordovician; Noble gases; Ordovician; Paleozoic; Processes; Recrystallization; Saint Peter Sandstone; Sandstone; Sanidine; Sedimentary rocks; Silicates; United States; Winnebago County Wisconsin; Wisconsin; East-central Wisconsin; K/Ca; Waukau
Abstract Numbers:90M/1291
Record ID:1987006138
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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