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| Titel |
Effect of organic ligands on Mg partitioning and Mg isotope fractionation
during low-temperature precipitation of calcite |
| VerfasserIn |
Vasileios Mavromatis, Adrian Immenhauser, Dieter Buhl, Bettina Purgstaller, Andre Baldermann, Martin Dietzel |
| 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 |
250128988
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| Publikation (Nr.) |
 EGU/EGU2016-9042.pdf |
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| Zusammenfassung |
| Calcite growth experiments have been performed at 25 oC and 1 bar pCO2 in the presence of
aqueous Mg and six organic ligands in the concentration range from 10−5 to 10−3 M. These
experiments were performed in order to quantify the effect of distinct organic ligands on the
Mg partitioning and Mg stable isotope fractionation during its incorporation in calcite at
similar growth rates normalized to total surface area. The organic ligands used in this study
comprise of (i) acetate acid, (ii) citrate, (iii) glutamate, (iv) salicylate, (v) glycine and (vi)
ethylenediaminetetraacetic acid (EDTA), containing carboxyl- and amino-groups.
These fuctional groups are required for bacterial activity and growth as well as
related to biotic and abiotic mineralization processes occurring in sedimentary and
earliest diagenetic aquatic environments (e.g. soil, cave, lacustrine, marine). The
results obtained in this study indicate that the presence of organic ligands promotes
an increase in the partition coefficient of Mg in calcite (DMg = (Mg∕Ca)calcite
(Mg∕Ca)fluid).
This behaviour can be explained by the temporal formation of aqueous Mg-ligand
complexes that are subsequently adsorbed on the calcite surfaces and thereby reducing
the active growth sites of calcite. The increase of DMg values as a function of the
supersaturation degree of calcite in the fluid phase can be described by the linear
equation
LogDMg =0.3694 (±0.0329)×SIcalcite – 1.9066 (±0.0147); R2=0.92
In contrast, the presence of organic ligands, with exception of citrate, does not
significantly affect the Mg isotope fractionation factor between calcite and reactive fluid
(Δ26Mgcalcite−fluid = -2.5 ±0.1 ). Citrate likely exhibits larger fractionation between the
Mg-ligand complexes and free aqueous Mg2+, compared to the other organic ligands studied
in this work, as evidenced by the smaller Δ26Mgcalcite−fluid values. These results indicate
that in Earth’s surface calcite precipitating environments that are characterized
by low dissolved organic carbon levels, the presence of organic ligands is rather
unlikely to significantly affect the Mg isotope composition of precipitated calcite. |
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