 |
| Titel |
XAS study of Cl and K speciation in glasses quenched from alkalic silicate and carbonate-silicate melts at high-pressure |
| VerfasserIn |
Andrei Shiryaev, Oleg Safonov, Thomas Huthwelker |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035222
|
|
|
|
|
|
| Zusammenfassung |
| Data on microinclusions in kimberlitic diamonds and experimental results indicate that
potassic Cl-bearing silicate and carbonate-silicate melts could be potential media for diamond
nucleation and precursors of carbonatite-kimberlite magmatism in the Earth’s mantle. These
HP melts were presumably formed in equilibrium immiscible chloride or chloride-carbonate
liquids [1, 2]. The immiscibility results from structural properties of the melts, in particular,
from K and Cl speciation in them. We report preliminary results on X-ray absorption
study of K and Cl local environments in the glasses quenched from melts in the
systems NaAlSi2O6-KCl and CaMgSi2O6-CaCO3-Na2CO3-KCl at pressure 5
GPa.
Experimental study of the system NaAlSi2O6-KCl [3] revealed a very strong shift of
equilibrium between immiscible aluminosilicate and (K,Na)Cl melts NaAlSi2O6+ KCl
= KAlSi2O6+ NaCl to the right, resulting in formation of the K–rich (up to 16
wt. % of K2O) aluminosilicate melt with 1.6-1.8 wt.% of Cl. It indicates active
separation of K and Cl, which implies different structural positions of these ions in the
aluminosilicate melt. Cl XAS spectra in most cases are fairly similar to the spectra of
crystalline KCl with minor contribution of NaCl. Thus, chlorine is totally segregated into
K(Na)Cl-like clusters of different sizes. K XAS spectra of the glasses could be
represented as superposition of contributions from KCl and KAlSi3O8-NaAlSi3O8 glass
[4]; the second component is dominant. Thus, in the glasses (and, presumably, in
corresponding melts) K is predominantly bound to silicate units, represented by 4-membered
rings as follows from Raman spectroscopy. Its CN is higher, than in crystalline
leucite (>6). In contrast, Cl is coordinated exclusively by alkali ions in chloride
clusters.
System CaMgSi2O6-CaCO3-Na2CO3-KCl at 5 GPa shows a wide miscibility gap
between Cl-bearing carbonate-silicate and Si-saturated chloride-carbonate melts [1], which
converge with a decrease of the SiO2/carbonate ratio. The K/Cl ratio in the carbonate-silicate
melts is > 1, suggesting different K and Cl speciation in the melts with various carbonate
content. Both K and Cl XAS spectra of glasses show regular variations with the SiO2. The
potassium spectra of the carbonate-rich glass resemble the spectra of the K4Si4O9 glass [5]
mixed with crystalline KCl. The KCl contribution increases with the SiO2/carbonate
ratio in the glasses. Thus, an increase of the silicate content in the melts results in
segregation of potassium ions to KCl-like clusters. The environment of Cl ions becomes
closer to KCl with increase of the silicate content, as well. The clustering of the
individual K-Cl units with the increase of the silica content in the melts could be
considered as a structural manifestation of the immiscibility processes in the studied
system.
XAS spectra obtained for microinclusions in Brazilian fibrous diamonds [6] shows that
local environment of K and Cl included contributions both from KCl and K-Al-Si glass. This
fair similarity supports the applicability of experimental approach by Safonov et al. [1, 2,
3].
The study is supported by the RFBR (10-05-00040), Russian President Grant
(MD-380.2010.5) and Russian Science Support Foundation. The XAS spectra were measured
at LUCIA beamline, Swiss Light Source.
References: [1] Safonov et al. (2007), Earth Planet. Sci. Lett, 123, 112-128; [2] Safonov
et al. (2009), Lithos, 112S, 260-273; [3] Safonov et al. (2007), Dokl. Earth Sci., 415,
105-109; [4] Jackson et al. (1987), J. Non-Cryst. Solids, 93, 311-322; [5] Kamijo et al.
(1996), Material Trans., JIM, 37, 927-931; [6] Shiryaev et al. (2005), Russ. Geol.Geophys.,
46, 1185. |
|
|
|
|
|
|