Study of Polyhalite from the Wipp Site, New Mexico
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Inc.
FOR NUCLEAR WASTE MANAGEMENT
257
Stephen V. Topp, editor STUDY OF POLYHALITE FROM THE WIPP SITE, DOUGLAS G. BROOKINS Department of Geology,
NEW MEXICO
University of New Mexico,
Albuquerque, NM,
USA
ABSTRACT Polyhalite
(K 2 Ca 2 Mg(SO4).2H 2 0)
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 because of 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
*4OAr.
Polyhalites
mixed
with
sylvite, halite, and (Na,K)CI yield lower ages due to loss of *4OAr 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.
INTRODUCTION Many evaporite minerals yield K-Ar ages which are anomalously young,
a fact
which has been known for many years (5), due to loss of *4OAr from the mineral structure. The halides, sylvite, carnallite, and K-bearing halite are all known to yield young ages for this reason. It has also been assumed (5) that many other evaporite minerals, including polyhalite (K 2 Ca 2 Mg(S0 4 ) 4 2H 2 0), will also yield young ages due to *4OAr loss. This assumption has been shown to be erroneous (4) for some polyhalites from the WIPP site area and, for these initial samples, K-Ar ages fell into two groups: (a) those with ages in excess of 200 MYBP (millions of years before the present) and (b) those with apparent ages between 150 - 190 MYBP. No polyhalites with K-Ar ages younger than 150 MYBP have been found to date. One exception to this is the secondary polyhalite formed penecontemporaneously with or shortly after a 34 MYBP dike was emplaced some 70 km. from the WIPP site (4). Polyhalite is of special importance as it has been assumed as a secondary mineral in evaporites (6) in general and it has been proposed that this mineral has formed more or less continuously throughout the time interval from 230 MYBP to the Recent (1,2). If po
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