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| Titel |
Geochemistry of calcite grown from seawater on a substrate at sub-micron spatial resolution. |
| VerfasserIn |
Rinat Gabitov, Aleksey Sadekov, Vasiliy Yapaskurt, Andrey Bychkov, Kaitlyn Sabourin |
| 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 |
250146177
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| Publikation (Nr.) |
 EGU/EGU2017-10185.pdf |
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| Zusammenfassung |
| Precipitation of calcium carbonate minerals and their geochemistry was extensively studied
for the last few decades because of importance of naturally occurring carbonates as
paleorecorders. The most of those studies employed bulk experimental approach where
batches of small crystals were examined for their morphological, crystallochemical, and
geochemical properties. However, in case of precipitation of multiphase CaCO3 missing one
or more of non-abundant (<1%) a CaCO3 phase is possible, for example if bulk X-ray
diffraction is used. Therefore, geochemical analyses of CaCO3 mineral mixture can yield
significant offset from geochemistry of an individual phase. For example presence of 1% of
aragonite in 99% calcite causes elevation of Sr content to about factor of two compare to Sr in
100% calcite bulk. In the present work experiments on crystals growth were conducted on
calcite substrate in order to eliminate nucleation effect on crystal structure and geochemistry
of the precipitated carbonate mineral. Unlike other in situ studies we anticipate to present
not only element:calcium ratios but also partition coefficients between calcite and
seawater.
Experiments proceeded for 1 and 8 months where high magnesium (high-Mg) calcite was
grown on low magnesium calcite substrate and in situ techniques were applied to evaluate
elemental partitioning between calcite and artificial seawater. Seawater saturation states with
respect to calcite were elevated by addition of Na2CO3 aliquots and varied from 5.6 to 19.2 in
initial fluids (before crystallization started). After a month of experimental duration one
cleavage fragment was collected from each of the five runs, mounted into epoxy to expose
sections parallel to calcite growth direction, and examined with micro-Raman spectroscopy
and scanning electron microscopy (SEM). Experiments were continued for the next 7
months and rare earth elemental spikes were introduced to mark growth zones and
determine calcite growth (extension) rate. Overgrowth layers on calcite surface
perpendicular to growth direction were examined with LA-ICP-MS using depth profiling
technique.
In four experiments micro-Raman spectroscopy yielded that the rims of calcite substrates
contains both calcite and aragonite minerals. SEM examination with energy dispersive
spectroscopy (EDS) and electron backscattered diffraction (EBSD) yielded thin (< 5
microns) layer of high-Mg calcite (Mg=1.35±0.13 wt%, Sr=0.07±0.03 wt%) coating the
substrate and chunks of aragonite (up to 20 micron in length) (Mg=0.09±0.02 wt%,
Sr=0.98±0.05 wt%) adjacent to calcite substrate or high-Mg calcite overgrowth.
Morphology of aragonite changed with decreasing of seawater saturation state from fibrous
to monocrystalline. Presentation of successfully collected LA-ICP-MS data (Li,
B, Mg, Al, Ca, Mn, Cu, Zn, Sr, Y, La, Ce, Nd, Sm, and U), partition coefficients
between calcite overgrowth and seawater, and calcite growth rates are anticipated. |
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