Carbonado revisited: Insights from neutron diffraction, high resolution orientation mapping and numerical simulations

One of the most controversial diamond types is carbonado, as its origin and geological history are still under debate. Here, we investigate selected carbonado samples using neutron diffraction and high resolution orientation mapping in combination with numerical simulations. Neutron diffraction analyses show that fine grained carbonado samples exhibit a distinct lack of crystallographic preferred orientation. Quantitative crystallographic orientation analyses performed on transmission electron microscope (TEM) sections reveal that the 2–10 μm grains exhibit locally significant internal deformation. Such features are consistent with crystal plastic deformation of a grain aggregate that initially formed by rapid nucleation, characterized by a high number of nucleation sites and no crystallographic preferred orientation. Crystal plastic deformation resulted in high stress heterogeneities close to grain boundaries, even at low bulk strains, inducing a high degree of lattice distortion without significant grain size reduction and the development of a crystallographic preferred orientation. Observed differences in the character of the grain boundary network and internal deformation structures can be explained by significant post-deformation annealing occurring to variable degrees in the carbonado samples. Differences in intensity of crystal bending and subgrain boundary sharpness can be explained by dislocation annihilation and rearrangement, respectively. During annealing grain energy is reduced resulting in distinct changes to the grain boundary geometry. Grain scale numerical modelling shows that anisotropic grain growth, where grain boundary energy is determined by the orientation of a boundary segment relative to the crystallographic orientation of adjacent grains results in straight boundary segments with abrupt changes in orientation even if the boundary is occurring between two triple junctions forming a “zigzag” pattern. In addition, in diamond anisotropic grain growth results in triple junctions that rarely show 120° angles. Our results support the interpretation that carbonados may have undergone at least 2 or 3 stages of development with rapid nucleation, crystal plastic deformation to low strains and variable degrees of post-deformation annealing. Such a history is commonly observed in Earth's crustal or mantle rocks. Hence, for carbonados it is not necessary to invoke an extraordinary and/or extraterrestrial origin and history. The combination of methods utilized here, promises to help advance our understanding of diamond and diamond aggregates in the future

The Earth’s core as a reservoir of water

Current estimates of the budget and distribution of water in the Earth have large uncertainties, most of which are due to the lack of information about the deep Earth. Recent studies suggest that the Earth could have gained a considerable amount of water during the early stages of its evolution from the hydrogen-rich solar nebula, and that a large amount of the water in the Earth may have partitioned into the core. Here we calculate the partitioning of water between iron and silicate melts at 20–135 GPa and 2,800–5,000 K, using ab initio molecular dynamics and thermodynamic integration techniques. Our results indicate a siderophile nature of water at core–mantle differentiation and core–mantle boundary conditions, which weakens with increasing temperature; nevertheless, we found that water always partitions strongly into the iron liquid under core-formation conditions for both reducing and oxidizing scenarios. The siderophile nature of water was also verified by an empirical-counting method that calculates the distribution of hydrogen in an equilibrated iron and silicate melt. We therefore conclude that the Earth’s core may act as a large reservoir that contains most of the Earth’s water. In addition to constraining the accretion models of volatile delivery, the findings may partially account for the low density of the Earth’s core implied by measured seismic velocities