HR: 0800h
AN: V41G-1542 [Abstracts]
TI: Fractionation of Stable Si Isotopes During in-situ Dissolution of Feldspars and Formation of Secondary
Clay Minerals
AU: * Georg, R B
EM: georg@erdw.ethz.ch
AF: ETH Zrich, Institute for Isotope Geochemistry and Mineral Resources, Sonneggstrasse 5, Zrich, 8092
Switzerland
AU: Reynolds, B C
EM: reynolds@erdw.ethz.ch
AF: ETH Zrich, Institute for Isotope Geochemistry and Mineral Resources, Sonneggstrasse 5, Zrich, 8092
Switzerland
AU: Halliday, A N
EM: Alex.Halliday@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR
United Kingdom
AU: Zhu, C
EM: chenzhu@indiana.edu
AF: Indiana University, Department of Geological Science, 1001 East 10th St., Bloomington, IN 47405
United States
AB:
It has been proposed that weathering of igneous silicate minerals may fractionate Si isotopes (Douthitt 1982, de la Rocha et
al. 2000). This is supported by the observation that clays yield δ30Si compositions between +0.5‰ and
-2.5‰ compared to the igneous range for δ30Si between +0.1‰ and -1‰ respectively (Douthitt
1982). The difference may relate to a discrimination against heavier Si isotopes during clay mineral formation. However, no
study has yet shown a direct Si isotope fractionation between coexisting primary igneous and secondary clay mineral phases.
We have measured the stable Si isotope fractionation during in-situ feldspar dissolution and formation of secondary clay
minerals in the Navajo Sandstone, Black Mesa, Arizona. The Jurassic Navajo Sandstone is composed of about 94% quartz and
2-4% K-feldspar. The K-feldspar grains are covered with kaolinite, and both quartz and feldspars are covered with a mantle
of smectite coating. Petrographic studies demonstrate that the clay minerals formed in situ as alteration products of
feldspar, and the smectite is of a low-temperature variety (Zhu, 2005). Therefore, the Si isotope fractionation at low
temperature (15-35°C) can be evaluated - something that is difficult to replicate in the laboratory. For the Si isotope
analyses we used 20-30 mg of 5 separated clay samples, and 0.36 mg of hand picked feldspars. The silicates were fused with
an alkaline flux and dissolved in a weak HCl acid. The dissolved Si was then separated by ion-exchange chromatography. The
relative Si isotope compositions were measured using a high-resolution MC-ICP-MS (The Nu1700 at ETH Zrich) and are reported
in δ notation relative to the international Si standard NBS 28.
The bulk rock and separated feldspar fraction have Si isotope compositions are -0.09 ± 0.03‰ and -0.15 ±0.03
‰ (±2σSEM) δ30Si, respectively. The clay samples have δ30Si values of -0.24
±0.05‰, -0.16 ±0.03‰, -0.30 ±0.03‰, -0.42 ±0.03‰ and -0.52 ±0.04‰
(±2σSEM). These Si isotope analyses reveal that the majority of the clay separates are isotopically lighter by up
to 0.4% compared to precursor feldspars. The results demonstrate for the first time that Si isotopes are indeed
fractionated during the breakdown of feldspar minerals and the subsequent formation of isotopically lighter Si in clays.
From mass-balance considerations, circulating fluids should have higher δ30Si values, however the analysis of
groundwater samples show variations of δ30Si values between +0.43‰ and -1.43‰ representing the most
negative dissolved Si isotope composition so far found. As groundwater constitutes 98% of the global fresh water (excluding
ice), the global Si biogeochemical cycle must include groundwater as an important component that is often overlooked.
References: De La Rocha, Brzezinski, and DeNiro (2000), A first look at the distribution of the stable isotopes of silicon
in natural waters, GCA 64, 2467-2477; Douthitt (1982), The geochemistry of the stable isotopes of silicon, GCA 46, 1449 -
1458; Zhu (2005), In situ feldspar dissolution rates in an aquifer, GCA 69, 1435-1453.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1886 Weathering (0790, 1625)
DE: 3617 Alteration and weathering processes (1039)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005