Earth and Planetary Sciences Faculty and Staff Publications

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Chlorine stable isotope compositions of two Swiss sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n=25) were collected from a 50 m section of Triassic deltaic and lagoonal strata and Liassic marine black shales in a well-characterized quarry. Borehole samples through the same sequence (n=12) were acquired from the collection of M. Frey. Low greenschist to middle amphibolite facies equivalents (n>80) were collected from the Glarner Alps, Urseren Zone, and Lukmanier region. \u03b437Cl values of silicate-bound chloride (SBC) are constant within individual sedimentary layers, but vary from \u20132.0 to +2.5\u2030 throughout the Triassic sequence and from \u20133.0 to 0\u2030 in the black shales. All dolomitic and gypsiferous samples have positive \u03b437Cl values. Colored marls and shales from the base of the Triassic sequence are isotopically negative, whereas those from the top are isotopically positive. Water-soluble \u03b437Cl values are 0.5-3.0\u2030 lower than SBC values in Triassic samples, but are 0.4-2.4\u2030 higher than SBC in black shales. Cl\u2013 contents range from 5-100 ppm in SBC fractions in both protolith series, and from 5-70 ppm in WSC fractions. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in \u03b437Cl as their protoliths. Samples with similar bulk composition but different metamorphic grades differ in \u03b437Cl by \u22641\u2030. These data lead to the following conclusions: (1) Both continental and marine sedimentary rocks display large heterogeneities in \u03b437Cl. (2) Negative \u039437Cl (SBC-WSC) fractionation in black shales may reflect partitioning fluid and insoluble organohalogen compounds, in addition to silicate-fluid partitioning. (3) Alpine-style metamorphism has little to no effect on rock Cl isotopic compositions up to mid-amphibolite facies, despite significant devolatilization and major changes in Cl-hosting mineralogy. (4) Cl is largely retained in the rocks during devolatilization, contrary to the normally hydrophilic behavior of chlorine. This likely reflects the fact that the fluids in all lithologies were CO2-rich. This study shows that metamorphic rocks can preserve their protolith \u03b437Cl values, with initial cm to m scale sedimentary heterogeneities, up to amphibolite-grade conditions

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NSF grant EAR-1144369


metamorphism, stable isotopes, fluid-rock interaction, devolatilization


Poster presented at American Geophysical Union Fall Meeting 2013;V51A-2638