MOBILITY OF TRACE-ELEMENTS AND LEACHING RATES OF RHYOLITIC GLASS SHARDS FROM SOME NEW-ZEALAND TEPHRA DEPOSITS

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.