Ne. Whitehead et al., MOBILITY OF TRACE-ELEMENTS AND LEACHING RATES OF RHYOLITIC GLASS SHARDS FROM SOME NEW-ZEALAND TEPHRA DEPOSITS, Applied geochemistry, 8(3), 1993, pp. 235-244
It is shown that glass shards from volcanic eruptions of known age are
useful in natural analogue studies connected with nuclear waste dispo
sal. They may be sequentially leached in bulk with HF and the hydratio
n layer thickness determined. If they have also been irradiated with n
eutrons in a reactor, some elemental profile information is available
at the same time. Applying this to New Zealand volcanic glass shards g
ave as many as 15 different fractions, and information about the distr
ibution of up to 16 trace elements. Those shards from a 22 ka eruption
yielded hydration rates of approximately 2.5 x 10(-11) g/cm2/d while
rates from a 340 ka eruption were 1. 5 x 10(-12)-1.2 x 10(-11) g/cm2/d
. The percentage of the total mass hydrated ranged between 5 and 35%.
As found in some other studies, many elements accumulate on the surfac
e of the shards, particularly Cr, Co and Fe, but it is shown here that
they mostly originate in surrounding groundwater rather than the bulk
glass. Rubidium and Cs tend to be removed instead. There was no obvio
us correlation between degree of hydration and the environment of depo
sition. The use of glass shards from volcanic eruptions is recommended
for such studies because they are ubiquitous, and occurrence is less
dependent on local geology than for some sample types.