『Abstract
Two different field-based methods are used here to calculate feldspar
dissolution rates in the Topopah Spring Tuff, the host rock for
the proposed nuclear waste repository at Yucca Mountain, Nevada.
The center of the tuff is a high silica rhyolite, consisting largely
of alkali feldspar (～60 wt％) and quartz polymorphs (～35 wt％) that
formed by devitrification of rhyolitic glass as the tuff cooled.
First, the abundance of secondary aluminosilicates is used to
estimate the cumulative amount of feldspar dissolution over the
history of the tuff, and an ambient dissolution rate is calculated
by using the estimated thermal history. Second, the feldspar dissolution
rate is calculated by using measured Sr isotope compositions for
the pore water and rock. Pore waters display systematic changes
in Sr isotopic composition with depth that are caused by feldspar
dissolution. The range in dissolution rates determined from secondary
mineral abundances varies from 10-16 to 10-17
mol s-1 kg tuff-1 with the largest uncertainty
being the effect of the early thermal history of the tuff. Dissolution
rates based on pore water Sr isotopic data were calculated by
treating percolation flux parametrically, and vary from 10-15
to 10-16 mol s-1 kg tuff-1 for
percolation fluxes of 15 mm a-1 and 1 mm a-1,
respectively. Reconciling the rates from the two methods requires
that percolation fluxes at the sampled locations be a few mm a-1
or less. The calculated feldspar dissolution rates are low relative
to other measured field-based feldspar dissolution rates, possibly
due to the age (12.8 Ma) of the unsaturated system at Yucca Mountain;
because oxidizing and organic-poor conditions limit biological
activity; and/or because elevated silica concentrations in the
pore waters (～50 mg L-1) may inhibit feldspar dissolution.』
1. Introduction
2. Yucca mountain geology
3. Feldspar dissolution rates based on alteration mineral abundances
4. Feldspar dissolution rates based on Sr isotopic data
5. Discussion
6. Conclusions
Acknowledgements
References