[S-GL22] Geochronology and Isotope Geology

[SGL22-03] Investigation of Sr isotopic signatures of clay minerals during deformation experiments

*Horst Zwingmann¹, Dawn Kellett², Kyle Larson³, Mark Button⁴, Alfons Berger⁵, Masakazu Niwa⁶, Meinert Rahn⁷ (1.Kyoto University, 2.NRC, Dartmouth, NS, Canada, 3.UBC, EEGS, Kelowna, BC, Canada, 4.UBC, FiLTER, Kelowna, BC, Canada, 5.Bern University, CH, 6.Tono Geoscience Center, JAEA, Japan, 7.ENSI, 5200 Brugg, CH)

Keywords:deformation experiments, Sr isotopes, clay minerals, Rochester shale, Opalinus Clay

The effects on clay minerals of physical deformation processes like shearing and grinding, including the possibility of porewater release, fluid exchange, and its impact on clay isotopic signatures and, therefore reliable clay isotopic ages, remain generally unknown [1]. Recent deformation experiments involving the Silurian Rochester Shale (RS), US, and the Jurassic Opalinus clay (OC), CH, two natural clays with different mineralogy, were investigated by K-Ar dating [2]. The RS is illite rich (~60%), and also contains quartz (23%) and minor kaolinite (~9%). In comparison, the OC contains less illite (~30%), more kaolinite (~ 35%), as well as chlorite (~10%), carbonates (~6%) and quartz (~13%) [2].

In this pilot study, the influence of physical deformation on the Sr isotope signatures of clay minerals is investigated in a set of well-defined laboratory experiments combining isotopically distinct fluids (Bern water, Fiji water) with RS and OC in a McCrone mill. Centrifugation and filtration were applied to minimize potential fluid-solid contamination prior to Sr measurements by ICP-MS/MS of the solid clay and fluid milling splits. For the RS, the solid control sample yields an ⁸⁷Sr/⁸⁶Sr value of 0.77160 ± 0.0020 (2SE) and ranges between 0.77130 to 0.77460 among the deformation experiments. For the OC, the solid control sample yields an ⁸⁷Sr/⁸⁶Sr value of 0.71590 ± 0.0016 (2SE) and deformation experiments ranges between 0.71460 to 0.71780. The corresponding water data for the RS experiments range from 0.6968 ± 0.001 (2SE) to 0.7106 ± 0.0018 (2SE) ⁸⁷Sr/⁸⁶Sr and for the OC they are within error from 0.6842 to 0.6843 ± 0.001 (2SE).

These data allow an exploration of the relationships of isotopic values between clay phases and fluids , and how rock composition might affect isotopic exchange.

References
[1] Zwingmann et al., 2019. Chemical Geology, 514, doi.org/10.1016/j.chemgeo.2019.03.034
[2] Zwingmann et al., 2024. 61st Annual CMS meeting, University of Hawai’i, 3–6 June 2024, abstract vol. A224-225.