Disturbed 40Ar-39Ar spectra from hornblendes; thermal loss or contamination?

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doi: 10.1016/0009-2541(93)90306-4
Authors:Rex, D. C.; Guise, P. G.; Wartho, J. A.
Author Affiliations:Primary:
University of Leeds, Department of Earth Sciences, Leeds, LS2 9JT, United Kingdom
Volume Title:Chemical Geology
Source:Chemical Geology, 103(1-4), p.271-281. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1993
Note:In English. 25 refs.; illus. incl. 4 tables
Summary:Accurately weighed aliquots of the ∼500-Ma-old interlaboratory standard hornblende MMHb-1 were mixed with known quantities of a ∼100 Ma-old Himalayan biotite and subjected to normal 40Ar/39Ar incremental heating analysis. The addition of the biotite created monotonically increasing apparent ages, similar to profiles interpreted as showing Ar diffusive loss in the literature. The release of Ar from biotite dominates the low-temperature portion of the spectra and lowers the observed age. Lower Ca/K ratios in the first increments are consistent with contamination, but do not distinguish the composition of the contaminant phase. A large number of published amphibole "diffusive loss" profiles may, in fact, be due to contamination by biotite or other phases which release Ar at low extraction temperatures.
Subjects:Absolute age; Alkali metals; Alkaline earth metals; Amphibole group; Ar/Ar; Biotite; Calcium; Chain silicates; Clinoamphibole; Dates; Diffusion; Errors; Hornblende; Metals; Methods; Mica group; Pollution; Potassium; Sheet silicates; Silicates; Standard materials; Thermal analysis; Ca/K
Abstract Numbers:93M/2511
Record ID:1993002237
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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