『Abstract
We present some of the first analyses of the stable isotopic
composition of dissolved silicon (Si) in groundwater. The groundwater
samples were from the Navajo sandstone aquifer at Black Mesa,
Arizona, USA, and the Si isotope composition of detrital feldspars
and secondary clay coatings in the aquifer were also analyzed.
Silicon isotope compositions were measured using high-resolution
multi-collector inductively coupled mass spectrometry (HR-MC-IPC-MS)
(Nu1700 & NuPlasma HR). The quartz dominated bulk rock and
feldspar separates have similar δ30Si of -0.09±0.04‰
and -0.15±0.04‰ (±95% SEM), respectively, and clay separates are
isotopically lighter by up to 0.4‰ compared to the feldspars.
From isotopic mass-balance considerations, co-existing aqueous
fluids should have δ30Si values heavier than the primary
silicates. Positive δ30Si values were only found in
the shallow aquifer, where Si isotopes are most likely fractionated
during the dissolution of feldspars and subsequent formation of
clay minerals. However, δ30Si decreases along the flow
path from 0.56‰ to -1.42‰, representing the most negative dissolved
Si isotope composition so far found for natural waters. We speculate
that the enrichment in 28Si is due to the dissolution
of partly secondary clay minerals and low-temperature silcretes
in the Navajo Sandstone. The discovery of the large range and
systematic shifts of δ30Si values along a groundwater
flow path illustrates the potential utility of stable Si isotopes
for deciphering the Si cycling in sedimentary basins, tracing
fluid flow, and evaluating global Si cycle.』
1. Introduction
2. Geology, hydrogeology, and mineralogy
3. Sampling and analytical methods
3.1. Sampling and analyzing groundwater
3.2. Solid sampling and size fractionation
3.3. Silicon isotope determination - mass spectrometry
4. Results
5. Discussion
5.1. Clay precipitation induced Si isotope fractionation
5.2. Silicon isotopic evolution of groundwater
6. Conclusions
Acknowledgments
Appendix A. Hydrochemistry data and discussion
References