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| Titel |
Silicon isotopes fractionation in meteoric chemical weathering and hydrothermal alteration systems of volcanic rocks (Mayotte) |
| VerfasserIn |
Isabelle Basile-Doelsch, Romain-Arnaud Puyraveau, Abel Guihou, Frederic Haurine, Pierre Deschamps, Setareh Rad, Pierre Nehlig |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250142901
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| Publikation (Nr.) |
 EGU/EGU2017-6579.pdf |
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| Zusammenfassung |
| Low temperature chemical weathering fractionates silicon (Si) isotopes while forming
secondary silicates. The Si fractionation ranges of high temperature secondary phyllosilicates
formed in hydrothermal alteration environments have not been investigated to date. Several
parameters, including temperature, reaction rates, pH, ionic concentrations in solution,
precipitation/dissolution series or kinetic versus equilibrium regime are not the same in
hydrothermal alteration and surface weathering systems and may lead to different
fractionation factors. In this work, we analyzed Si isotopes in these two types of alteration
conditions in two profiles sampled on the volcanic island of Mayotte. In both profiles,
Si-bearing secondary mineral was kaolinite.
Both profiles showed 30Si depletion as a function of the degree of alteration but each with a
distinct pattern. In the meteoric weathering profile, from the bottom to the top, a gradual
decrease of the δ30Si from parent rock (-0.29 ± 0.13 ‰) towards the most weathered
product (-2.05 ± 0.13 ‰) was observed. In the hydrothermal alteration profile,
in which meteoric weathering was also superimposed at the top of the profile, an
abrupt transition of the δ30Si was measured at the interface between parent-rock
(-0.21 ± 0.11 ‰) and the altered products, with a minimum value of -3.06 ± 0.16
‰˙ At the scale of Si-bearing secondary minerals, in the chemical weathering system, a
Δ30Sikaol−parentrock of -1.9 ‰ was observed, in agreement with results in the
literature. A low temperature kinetic fractionation 30ɛ of -2.29 ‰ was calculated using
a simple steady state model. However, an unexpected Δ30Sikaol−parentrock of
-2.85 ‰ was measured in the hydrothermal alteration site, pointing to possible
mechanisms linked to dissolution/precipitation series and/or to ionic composition
of the solution as the main controlling factors of fractionation in hydrothermal
conditions.
At the scale of the profiles, both δ30Si bulk rocks showed linear correlations with the
SiO2:Al2O3 ratios, suggesting an alternative alteration index based on Si isotopic
composition. |
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