The temperature and compositional dependence of disordering in Fe-bearing dolomites

Abstract

Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered (R3), but transforms with increasing temperature into a disordered higher symmetry structure (R3c).To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300-1350 K and 2.6-4.2 GPa. The long-range order parameter s, quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20-30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe . Complete disorder is attained in dolomite at ∼1240 K, 100-220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ∼900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ∼470 K.The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ∼800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO3-MgCO3-FeCO3

“…The ordered/disordered configuration in dolomite does not affect significantly the volume behavior, at least as a first approximation and within the experimental accuracy achieved in these experiments. We noticed the disappearance of superstructure peaks of the ordered configuration at high temperature, without volume discontinuity, in agreement with previous determined temperatures (Hammouda et al 2011;Franzolin et al 2012). The minimum discrepancies between measured and calculated volume data indicates that the reported EoS parameters and the current experimental accuracy in monochromatic in-situ experiments, may provide the needed accuracy and precision for planning in-situ kinetic experiments, where change of composition of carbonates is expected, especially concerning the variation of Ca/(Mg+Fe) ratio.…”

Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

“…more easily into the crystal lattice. The high concentrations of Mn and Fe in the type 2 dolomites and the lower concentrations of Mn and Fe in the precursor limestone show that there were no significant sources of these elements, which suggests that the dolomitic fluids precipitated under slightly reducing conditions or that the dolomite formed in a late burial environment (Warren 2000;Franzolin et al 2012) (Fig. 11).…”

Origin of high‐Mg arc volcanism and fate of subducted sedimentary carbonates in the western Pacific: Evidence from partial melting experiments on mixed sediment and peridotite

“…During subduction, carbonate remains stable and enters the deep mantle or breaks down and participates in arc magma/fluid activities, resulting in release of CO 2 to the shallow depths. While it is suggested that carbonates may enter into the deep mantle stably with the subducting slab (Dasgupta & Hirschmann, 2010; Franzolin, Merlini, Poli, & Schmidt, 2012; Tao, Zhang, Fei, & Liu, 2014), some studies indicated that most of the subducted carbonates were decarbonated during metamorphic reactions, or slab melting/dehydration processes, and thus cannot enter into the deep mantle (Frezzotti, Huizenga, Compagnoni, & Selverstone, 2014; Kelemen & Manning, 2015; Kerrick & Connolly, 2001a; Kerrick & Connolly, 2001b). In fact, the stability of subducted carbonates is susceptible to variable factors, for example, the thermal state of subduction zone, age of the subducting slab, and compositional variability of the carriers of the carbonates.…”