Structures of dolomite at ultrahigh pressure and their influence on the deep carbon cycle

Abstract

Carbon-bearing solids, fluids, and melts in the Earth's deep interior may play an important role in the long-term carbon cycle. Here we apply synchrotron X-ray single crystal micro-diffraction techniques to identify and characterize the high-pressure polymorphs of dolomite. Dolomite-II, observed above 17 GPa, is triclinic, and its structure is topologically related to CaCO 3 -II. It transforms above 35 GPa to dolomite-III, also triclinic, which features carbon in [3 + 1] coordination at the highest pressures investigated (60 GPa). The structure is therefore representative of an intermediate between the low-pressure carbonates and the predicted ultra-high pressure carbonates, with carbon in tetrahedral coordination. Dolomite-III does not decompose up to the melting point (2,600 K at 43 GPa) and its thermodynamic stability demonstrates that this complex phase can transport carbon to depths of at least up to 1,700 km. Dolomite-III, therefore, is a likely occurring phase in areas containing recycled crustal slabs, which are more oxidized and Ca-enriched than the primitive lower mantle. Indeed, these phases may play an important role as carbon carriers in the whole mantle carbon cycling. As such, they are expected to participate in the fundamental petrological processes which, through carbon-bearing fluids and carbonate melts, will return carbon back to the Earth’s surface.

A primary natrocarbonatitic association in the Deep Earth

“…This agrees with the experimental data of Mao et al (2011), who demonstrated, that CaMg(CO 3 ) 2 is stable under lower-mantle conditions at pressures of up to 83 GPa and, possibly, higher, and contrasts earlier data on the decomposing of dolomite into magnesite + aragonite, under lower-mantle P-T conditions (Biellmann et al 1993;Luth 2001;Shirasaka et al 2002). Recent experiments confirm that high-pressure polymorphism in dolomite could stabilize CaMg(CO 3 ) 2 ; this composition transforms at ~17 GPa into 'dolomite-II' (with a monoclinic structure, according to Santillán et al 2003, or an orthorhombic structure, according to Mao et al 2011, or a triclinic structure for Fe-dolomite, according to Merlini et al 2012) and then, at ~35-41 GPa, into 'dolomite-III' (with monoclinic structure, according to Mao et al 2011 or a triclinic structure in the case of Fe-dolomite, according to Merlini et al 2012) .…”

Evidence of interspersed co-existing CaCO3-III and CaCO3-IIIb structures in polycrystalline CaCO3 at high pressure

“…Mao et al (2011) reported that dolomite, above 35 GPa, transforms into a highpressure polymorph which does not decompose into a mixture of aragonite and magnesite at high temperature, as it does at lower pressures (Biellmann et al, 1993). The structure of this polymorph, called dolomite-III, was determined on an Fe-rich sample (Merlini et al, 2012a) and it (1971) possesses structural features similar to those of CaCO 3 -III. The polymorphism of carbonates was also investigated theoretically (Oganov et al, 2006(Oganov et al, , 2008(Oganov et al, , 2013.…”

High-pressure compressibility and phase stability of Mn-dolomite (kutnohorite)

“…Dolomite II (triclinic P1) was observed at ambient temperatures and pressures above ~20 GPa (Santillán et al 2003;Mao et al 2011;Merlini et al 2012). A third dolomite phase, Dolomite III, was recently observed in Ca(Mg 0.92 Fe 0.08 )(CO 3 ) 2 by Mao et al (2011) and Ca(Mg 0.6 Fe 0.4 )(CO 3 ) 2 by Merlini et al (2012), by compressing above 35 GPa and laser heating. This dolomite III phase is stable during temperature-quench, allowing Merlini et al to refine a structure (triclinic P1).…”