Amini, Marghaleray

Bock, Barbara

Böhm, Florian

Eisenhauer, Anton

Fietzke, Jan

Garbe-Schönberg, C.-Dieter

Hauff, Folkmar

Lackschewitz, Klas

English

OceanRep

Geophysical Research Abstracts, Vol. 8, 08864, 2006SRef-ID: 1607-7962/gra/EGU06-A-08864© European Geosciences Union 20061 Calcium Isotope Variations along the HydrothermalPathwayM. Amini (1), A. Eisenhauer (1), F. Böhm (1), J. Fietzke (1), B. Bock (1), D.Garbe-Schönberg (2), K. S. Lackschewitz (1), and F. Hauff (1)(1) Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Kiel(2) Institut für Geowissenschaften, Christian-Albrechts-Universität zu KielHydrothermal fluid geochemistry provides insight into water-rock-interactions dur-ing hydrothermal activity. Seawater underlies progressive chemical changes along itspathway through oceanic crust and is modified to a hydrothermal fluid under excep-tional temperature and pressure conditions. In this study, we investigate the evolutionof seawater within hydrothermal systems with respect to calcium.The Ca isotope compositions of 21 fluid samples from the Logatchev HydrothermalField at the Mid-Atlantic Ridge (15˚N) were determined. The δ44/40Ca ratios of thefluids are inversely correlated to their corresponding Ca and Mg contents and thus tothe fluid amounts as deduced by Mg endmember calculations. Binary mixing calcula-tions lead to a Ca concentration of 32 mM and a δ44/40Ca value of -0.97 ± 0.11 permill (2σm) relative to seawater for the hydrothermal endmember. Similar to Ca, the Srconcentrationsand87Sr/86Sr ratios are inversely correlated. A plot of δ44/40Ca against87Sr/86Sr displays a two-component mixing curvature that yields endmember valuesof 87Sr/86Sr = 0.7037 ± (8) and δ44/40Ca = -0.92 ± 0.15 per mill. Both approachescombined result in a value of -0.95 ± 0.09 per mill (95 % confidence level) for theCa isotope composition of the hydrothermal endmember. With regard to Sr, a two-endmember mixing with seawater and MORB as endmembers can be assumed. TheCa isotope ratios in contrast exhibit a significant deviation of about 0.2 per mill be-tween the Ca isotope signature of MORB (δ44/40Ca = -1.17 ± 0.05 per mill (2σm))(Richter et al., 2003) and that of the hydrothermal endmember as determined in thisstudy. The difference might be based on either a different Ca isotope composition ofthe bed-rock, in particular with regard to the ultramafic-hosted Logatchev Hydrother-mal Field or fractionation processes along the hydrothermal pathway. Currently, noCa isotope data exist for ultramafic rocks. But it is assumed that the bulk silicate Earthdisplays a value of about –1.1 per mill relative to seawater.The second suggestion, namely a potential fractionation during the formation of hy-drothermal precipitates such as anhydrite and calcium carbonate would serve as anexplanation when assuming a Rayleigh fractionation law. This approach would pre-dict a Ca isotope composition of hydrothermal precipitates that are around -0.5 permill lighter than the original solution from which they precipitated. Nonetheless host-rocks from the Logatchev Hydrothermal Field have to be analysed to verify one ofthese explanations.

Calcium isotope variations along the hydrothermal pathway

Calcium isotope variations along the hydrothermal pathway

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