Ordering in double carbonates and implications for processes at subduction zones |
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| Authors: | T Hammouda D Andrault K Koga T Katsura " target="_blank">A M Martin |
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| Institution: | 1.Laboratoire Magmas et Volcans,Clermont Université, Université Blaise Pascal,Clermont-Ferrand,France;2.CNRS, UMR 6524, LMV,Clermont-Ferrand,France;3.IRD, R 163, LMV,Clermont-Ferrand,France;4.Institute for the Study of the Earth Interior,Okayama University,Misasa,Japan;5.Bayerisches Geoinstitut,Universit?t Bayreuth,Bayreuth,Deutschland;6.NASA Johnson Space Center,Houston,USA |
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| Abstract: | We have studied cation ordering in dolomite in situ as a function of pressure, temperature, and experimental time using the
multi-anvil apparatus and synchrotron radiation. Starting with ordered dolomite, we observe the onset of disordering taking
place at 950°C, while complete disordering is achieved at 1,070 (±20)°C, for pressures ranging between 3.37 and 4.05 GPa.
Pressure does not appear to have significant effect on the order/disorder transition over the investigated range. We find
that dolomite can reach its equilibrium ordering state above 900°C within duration of laboratory experiment (few hours), both
from disordered state and from ordered state. In addition, we have reversed the dolomite breakdown reaction magnesite + aragonite = dolomite]
between 4.5 and 5.5 GPa, by monitoring diffraction peak intensity. We also have determined that dolomite is stable up to 7.4 GPa
at 1,100°C. We confirm some earlier studies where a change in slope (dP/dT) has been observed, but we find a non-zero slope in the low pressure range. Combining the values of entropy obtained from
dolomite degree of ordering with enthalpy values deduced from our bracketing of magnesite + aragonite = dolomite] equilibrium,
we model the location of dolomite breakdown in the P–T space as a function of cation ordering. By comparing previous conflicting
studies, we show that, although kinetics of order/disorder is fast, disequilibrium dolomite breakdown is possible. Our modeling
shows that subducted disordered dolomite present in carbonated sediments could be decomposed to magnesite + aragonite] at
lower pressure (3.5 GPa) than usually considered (>5 GPa). This 2-GPa (60 km) difference is valid on a fast subduction path
and is possible if disorder inherited from sedimentation is preserved. On a slow subduction path, however, dolomite breakdown
is encountered at about 250 km depth, which is 100 km deeper than currently considered. |
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