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| Titel |
Experimental study of the Mg and Sr isotopic evolution of seawater interacting with basalt between 150 and 300 ˚ C. |
| VerfasserIn |
Martin Voigt, Christopher R. Pearce, Eric H. Oelkers |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250126044
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| Publikation (Nr.) |
 EGU/EGU2016-5718.pdf |
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| Zusammenfassung |
| The chemical exchange of material between seawater and the oceanic crust plays a major
role in marine geochemical cycles [1]. Isotopic signatures provide an important
means of tracing elemental transfer in hydrothermal environments, yet only a limited
amount of experimental data on the extent of isotopic fractionation under these
conditions is currently available. This study consequently investigated the extent
of δ26∕24Mg and 87Sr/86Sr isotopic variation during a seawater-basalt interaction
experiments at 150, 250 and 290 ˚ C. A suite of closed system experiments were run for
several months at each temperature under saturated water pressure, using either
crystalline or glassy basalt as the starting material and a water/rock ratio of 10 or
25.
Our results demonstrate that the dissolution of basaltic material in hydrothermal
environments occurs at the same time as the precipitation of alteration minerals (mainly
smectite and anhydrite), which is consistent with results from similar studies in the past (e.g.
[2]). As expected, the rate of reaction using crystalline basalt was slower than with basaltic
glass, and both sample types reacted faster at higher temperatures. The 87Sr/86Sr composition
of the experimental fluids decreased from the initial seawater value (0.70916) towards
the lower basaltic signature during the experiments (0.70317), demonstrating the
progressive release of Sr during basalt dissolution. Magnesium was steadily removed
from the fluid via the precipitation of clay minerals, with the residual fluids having
progressively lighter δ26∕24Mg compositions. The mean Mg isotope fractionation factor
(αsolid−solution) observed at 250 oC was 1.0005±0.0002, supporting low-temperature
evidence that clay minerals preferentially incorporate isotopically heavy magnesium
[3].
These experiments provide quantitative information on the extent of Mg isotopic
fractionation between fluids and secondary silicate minerals in hydrothermal systems, and
demonstrate the potential for combined radiogenic and stable isotope analysis to track
solid-fluid reactions in the oceanic crust. Further characterisation of the extent of isotopic
fractionation in these systems will help establish how such processes have affected the
long-term chemical evolution of the oceans.
[1] H. Elderfield and A. Schultz, “Mid-Ocean Ridge Hydrothermal Fluxes and the
Chemical Composition of the Ocean,” Annu Rev Earth Planet Sci, vol. 24, pp. 191–224,
1996.
[2] W. E. Seyfried Jr and J. L. Bischoff, “Experimental seawater-basalt interaction at 300˚
C, 500 bars, chemical exchange, secondary mineral formation and implications for the
transport of heavy metals,” Geochim. Cosmochim. Acta, vol. 45, no. 2, pp. 135–147,
1981.
[3] J. A. Higgins and D. P. Schrag, “The Mg isotopic composition of Cenozoic seawater –
evidence for a link between Mg-clays, seawater Mg/Ca, and climate,” Earth Planet. Sci. Lett.,
vol. 416, pp. 73–81, 2015. |
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