Dipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italy.
Proc Natl Acad Sci U S A. 2012 Aug 21;109(34):13509-14. doi: 10.1073/pnas.1201336109. Epub 2012 Aug 6.
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.
地球深部的含碳固体、流体和熔体可能在长期碳循环中发挥重要作用。在这里，我们应用同步辐射 X 射线单晶微衍射技术来识别和表征白云石的高压多型体。在 17 GPa 以上观察到的白云石-II 是三斜晶系的，其结构在拓扑上与 CaCO(3)-II 有关。它在 35 GPa 以上转变为白云石-III，也是三斜晶系的，在研究的最高压力（60 GPa）下，碳具有[3 + 1]配位。因此，该结构代表了低压碳酸盐和预测的超高压碳酸盐之间的中间相，具有四面体配位的碳。白云石-III 直至熔点（43 GPa 时为 2,600 K）都不会分解，其热力学稳定性表明这种复杂相可以将碳输送到至少 1700 km 的深度。因此，白云石-III 是富含再循环地壳板块区域中可能存在的相，这些区域比原始下地幔更氧化和富含钙。事实上，这些相可能作为整个地幔碳循环中的碳载体发挥重要作用。因此，它们有望参与通过含碳流体和碳酸盐熔体将碳返回地球表面的基本岩石学过程。
