The Li-content and isotopic composition of altered upper oceanic crust and the hydrothermal carbonate within it
Abstract
Changes in the Li-isotopic composition of seawater over the Cenozoic potentially provide important insight into the working of the global carbon cycle. However, we currently have too poor a constraint on both: (i) the magnitude and isotopic composition of the important fluxes (e.g. river, hydrothermal) and (ii) the controls on the variability of these. Systematic study of the Li content and isotopic composition of four sections through the lava section of the Troodos ophiolite indicate a Li uptake flux of 14-21x10^9 mol yr-1, with an isotopic composition of 10 per mil (Coogan et al submitted). Comparison of these data with published data suggest there is a significant temperature dependence of the bulk Li-isotopic fractionation factor between seawater and altered oceanic crust; i.e. as bottom water temperature decreased over the Cenozoic the fractionation factor between seawater and the alteration products (clays being the main Li repository) in the upper ocean crust increased driving the Li-isotopic composition of seawater higher. The large amount of Li added to the oceanic crust by off-axis hydrothermal systems suggests that off-axis hydrothermal fluids should be strongly depleted in Li relative to seawater, and have Li-isotope ratios much heavier than seawater. Carefully prepared calcite from amygdules have extremely low Li contents (9-33 ppb) but have Li-isotope ratios of 19 to 23 per mil (i.e. not heavier than seawater as predicted), irrespective of their Li content. All data have been duplicated using different splits and different dissolution procedures suggesting the Li-isotope ratio accurately reflects that of the calcite. We hypothesize that the much lighter Li isotopic composition than expected may be due to a large calcite-fluid fractionation factor under the conditions of calcite growth. We are currently undertaking experiments to quantify this fractionation factor under differing conditions and these will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP23C2341C
- 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