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Plagioclase from plutonic, hypabyssal and volcanic regimes have been analyzed using the "high-resolution" (ie. fine temperature increments) 40Ar-39Ar method with particular emphasis being placed at high (> 1000^oC) temperatures. While undisturbed or even completely over-printed volcanic/hypabyssal materials display flat reliable age spectra, those of undisturbed plutonic plagioclases tend to be discordant, generally yielding anomalously low apparent ages. Total-gas ages for the latter also tend to be lower than control values. The above results essentially parallel conclusions based upon conventional K-Ar data.

Disturbed hypabyssal or volcanic plagioclases, however, yield anomalous patterns similar to certain lunar plagioclases. Two major sources of argon, each differing in ⁴⁰Ar*/³⁹Ar ratio and in diffusion characteristics are suggested. Plateau-like features in apparent age (the HTP) are frequently observed at high temperatures and appear, in at least some cases, to yield valid crystallization ages. Correlation of Ca/K microprobe data with the relatively high, often constant ³⁷Ar/³⁹Ar values in this interval indicates that the HTP is a "pure" plagioclase feature, and that no significant redistribution of ³⁷Ar or ³⁹Ar has occurred. This phenomenon appears to coincide with the first appearance of Al-Si ordering in the framework structure. It should be emphasized, however, that the appearance of a HTP does not necessarily rule out the presence of excess argon.

At intermediate temperatures (ie. ~720-880^oC), a characteristic minimum in the release spectra appears to approximate the age of the overprinting event. Ca/K - microprobe data when compared with ³⁷Ar/³⁹Ar values in this temperature interval suggest i) the domination by an altered, K-rich component, one that is possibly occluded within the plagioclase as a "contaminant", and ii) minor redistribution and/or loss of ³⁹Ar.

Some plagioclase may therefore yield chronologically significant information under partially overprinting conditions, a property not found in other minerals dated by the K-Ar method. Great care, however, must be exercised in interpreting the discordant age spectra.

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Supervisors: Peter Reynolds

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I'm more of a chemist or chemical engineer now as I run an ultrapure acid distillation plant ( Seastar Chemicals. I designed and built most of the equipment ). We actually sell to a lot of geochemists and to chemical oceanographers, so there's still a tenuous thread with the old Dal days. We produce and package for Fisher Scientific and Anachemia, as well as sending some acid to Europe or Australia. It's hard to think that there could be an acid plant in the paradise of Vancouver Island, but there you have it! Somebody has to provide alternate jobs to the pot farms, forestry, fisheries and the government.

Seastar Chemicals Inc.
10005 McDonald Park Road
Sidney, BC, Canada V8L 3S8
ph: (250) 655-5880 extn 1312
toll free: 1 (800) 663-2330
fax: (250) 655-5888
web:

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