Isotopic constraints on the formation of carbonates during low-temperature hydrothermal oceanic crust alteration
Abstract
Low temperature (<100°C) water-rock reactions in oceanic crust have a potentially large influence on seawater chemical compositions and atmospheric pCO2. Quantification of the conditions (e.g., temperature) of oceanic crust alteration is needed to evaluate its importance for global silicate weathering fluxes. The isotopic and chemical compositions of secondary carbonates in oceanic crust reflect the temperature and chemistry of the circulating fluid and thus are used to reconstruct past conditions of crustal alteration. For example, temperatures are calculated via carbonate δ18O thermometry using measured δ18Ocarb vs. assumed δ18Ofluid. δ18Ofluid is usually assumed to be the seawater value at the time of carbonate formation. We present measured clumped-isotope temperatures (Tclump) and δ18O, δ13C, δ44Ca, and 87Sr/86Sr values of Jurassic carbonates from altered oceanic crust (ODP Site 801). Tclump measured at Caltech ranges from 24-51°C. Calculated δ18Ofluid (based on Tclump and δ18Ocarb) ranges from -0.4‰ (±0.4, 1σ) to -3.5‰ (±0.6). Higher temperatures correlate with lower δ18Ofluid (R2 = 0.75). This suggests that at elevated temperatures, δ18Ofluid was modified away from seawater values, likely via the preferential incorporation of 18O vs. 16O into secondary minerals relative to water. This indicates that δ18Ofluid values of circulating fluids are not necessarily identical to seawater δ18O. Tclump measurements are being replicated at Harvard for further verification. Carbonates with δ13C indicating a seawater C source (δ13C > 0‰) have average δ44Ca (relative to modern seawater) of -0.84‰ (±0.08). This is indistinguishable from igneous rock δ44Ca and suggests that carbonate Ca is derived from igneous Ca released during crustal alteration. Carbonates with δ13C indicating an organic C source (δ13C < -2.5‰) have lower δ44Cacarb (< -1‰). Carbonate 87Sr/86Sr ranges from 0.70742 to 0.70656. Based on the seawater 87Sr/86Sr curve, this range requires the release of low 87Sr/86Sr strontium from igneous rocks (87Sr/86Sr ≈ 0.7025) during alteration. Together these results support the presence of substantial water-rock interactions and fluid modification during alteration and carbonate precipitation. They will be discussed in the context of models of fluid flow coupled to alteration reaction kinetics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP22B..04S
- Keywords:
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- 0325 Evolution of the atmosphere;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 5220 Hydrothermal systems and weathering on other planets;
- PLANETARY SCIENCES: ASTROBIOLOGY