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Study of Polyhalite from the Wipp Site, New Mexico

Published online by Cambridge University Press:  15 February 2011

Douglas G. Brookins
Douglas G. Brookins*
Affiliation:
Department of Geology, University of New Mexico, Albuquerque, NM, USA
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Abstract

Polyhalite (K2Ca2Mg(SO4).2H2o) is an important mineral present in the bedded evaporites of the WIPP site, southeastern New Mexico. Polyhalite contains two structurally bonded water molecules, and it is thus important to know if this mineral formed at or close to the time of sedimentation or, as proposed by others (1,2) more or less continuously throughout the Mesozoic and Cenozoic. If formed much later than original evaporite formation, then this implies a new source of water in order for polyhalite to form in turn raising questions about water abundance and mobility in the bedded evaporites. Polyhalite is not well suited for Rb-Sr geochronologic study becauseof the large amounts of common Sr (3), but it has been shown to be suitable for study by the K-Ar geochronologic methods(4). Pure polyhalites and those mixed with very small amounts of impurities yield K-Ar dates in the range of 200 – 220 MYBP and indicate little if any, loss of *40Ar. Polyhalites mixed with sylvite, halite, and (Na,K)Cl yield lower ages due to loss of *40Ar from the halides. Apparent age lowering is proportional to halide content and an extrapolated age of formation of near 225 MYBP results. Collectively, the K-Ar data convincingly show that the polyhalites formed at the time of sedimentation (except for isolated occurrences near dike intrusions) in Late Permian time. Finally, thermodynamic calculations, supported by petrographic study, show that polyhalite is often stable with respect to carnallite, sylvite and other minerals formerly thought to be older than polyhalite.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 6: Symposium D – Scientific Basis for Nuclear Waste Management IV , 1981 , 257
Copyright
Copyright © Materials Research Society 1982

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References

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