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Abstract: The possible presence of the high-density carbon polymorph with hexagonal symmetry known as “lonsdaleite” provides an important marker for shock impact events. It is typically considered to form as a metastable phase produced from graphite or other carbonaceous precursors. However, its existence has recently been called into question. Here we collected high-resolution synchrotron X-ray diffraction data for laboratory-shocked and natural impact diamonds that both show evidence for deviations from cubic symmetry, that would be consistent with the appearance of hexagonal stacking sequences. These results show that hexagonality can be achieved by shocking diamond as well as from graphite precursors. The diffraction results are analyzed in terms of a general model that describes intermediate stacking sequences between pure diamond (fully cubic) and “lonsdaleite” (fully hexagonal) phases, with provision made for ordered vs disordered stacking arrangements. This approach provides a “hexagonality index” that can be used to characterize and distinguish among samples that have experienced different degrees of shock or static high pressure-high temperature treatments. We have also examined the relative energetics of diamond and “lonsdaleite” structures using density functional theoretical (DFT) methods. The results set limits on the conditions under which a transformation between diamond and “lonsdaleite” structures can be achieved. Calculated Raman spectra provide an indicator for the presence of extended hexagonal stacking sequences within natural and laboratory-prepared samples. Our results show that comparable crystallographic structures may be developed by impact-generated shockwaves starting from ambient conditions using either of the two different allotropes of carbon (diamond, graphite). This broadens the scope for its occurrence in terrestrial and planetary systems.

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Abstract: Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary multiphase inclusions in UHP minerals. Saline aqueous inclusions with variable solute load prevail in high pressure (HP) rocks (e.g. [1,2]), whereas multiphase solid inclusions in some UHP rocks have been attributed to silicate-rich fluids or hydrous melts at supercritical conditions (e.g. [3-7]). These inclusions are frequently hosted by minerals stable at mantle depths, such as garnet, and show the same textural features as fluid inclusions (Fig. 1). The mineral infillings of the solid multiphase inclusions are generally assumed to have crystallized by precipitation from the solute load of dense supercritical fluids equilibrating with the host rock. Notwithstanding the validity of this assumption, the mode of crystallization of daughter minerals during precipitation within the inclusion and/or the mechanism of interaction between the fluid at supercritical conditions and the host mineral are still poorly understood from a crystallographic point of view. A case study is represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatised at ∼ 4 GPa, 750 °C by a silica- and incompatible trace element- rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary inclusions in garnet display a size within a few tens of micrometers and negative crystal shapes. Infilling minerals (spinel: 10–20 vol.%; amphibole, chlorite, talc, mica: 80– 90 vol.%) occur with constant volume ratios and derive from trapped solute-rich aqueous fluids [5]. To constrain the possible mode of precipitation of daughter minerals, we performed for the first time a single-crystal X-ray diffraction experiment by means of Synchrotron Radiation at Diamond Light Source. In combination with electron probe microanalyses, this measurement allowed the unique identification of each mineral phase and, most importantly, of their reciprocal orientation with uncertainties typically smaller than 2° (see [8] for further details). Applying this methodology for the first time to multiphase solid microinclusions, we have been able, for example, to infer possible epitaxy and to quantify preferred crystallographic orientation. We demonstrated the epitaxial relationship between spinel and garnet (Fig. 1) and between some hydrous minerals. Epitaxy drives a first-stage nucleation of spinel under near-to-equilibrium conditions, likely promoted by a dissolution and precipitation mechanism between the UHP fluid and the host garnet. A second-stage nucleation involved hydrous phases (amphiboles, chlorite and phlogopite), which nucleate in a non-registered manner and under far-from-equilibrium conditions. From the mineral chemistry of the mineral infillings and the crystallization sequence, nucleation and subsequent precipitation of the mineral phases occurred as a consequence of a fluid/garnet interaction. This conclusion is in agreement with previous studies on multiphase solid inclusions from UHP rocks which claimed that the precipitation process is due to post-entrapment modifications, such as dissolution and precipitation of the host mineral walls [9] and/or passive water diffusion from the inclusion to the host [10]. Such information is discussed in relation to physico-chemical aspects of nucleation and growth shedding light on the mode of mineral crystallization from a fluid phase at supercritical conditions.

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Abstract: Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary multiphase inclusions in UHP minerals. The mode of crystallization of daughter minerals during precipitation within the inclusion and/or the mechanism of interaction between the fluid at supercritical conditions and the host mineral are still poorly understood from a crystallographic point of view. A case study is represented by garnet–orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatized at ~4 GPa, 750 °C by a silica- and incompatible trace element-rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary inclusions in garnet display a size within a few tens of micrometres and negative crystal shapes. Infilling minerals (spinel: 10–20 vol%; amphibole, chlorite, talc, mica: 80–90 vol%) occur with constant volume proportions and derive from trapped solute-rich aqueous fluids. To constrain the possible mode of precipitation of daughter minerals, we performed for the first time a single-crystal X-ray diffraction experiment by synchrotron radiation at Diamond Light Source. In combination with electron probe microanalyses, this measurement allowed the unique identification of each mineral phase and reciprocal orientations. We demonstrated the epitaxial relationship between spinel and garnet and between some hydrous minerals. Such information is discussed in relation to the physico-chemical aspects of nucleation and growth, shedding light on the mode of mineral crystallization from a fluid phase trapped at supercritical conditions.

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Abstract: Clinopyroxenes are mainly Ca-Na-Fe-Mg-silicates constituting a significant portion of the Earth’s upper mantle up to 20% of such shell of our planet. They could be found as typical mineral inclusions in diamonds being diopsidic and omphacitic in composition and, together with garnets, cover a key role in providing indications concerning the source rock in which the diamond crystallize. In detail, it is well known that eclogitic diamonds are characterized by clinopyroxenes with omphacitic compositions (about Ca0.5Na0.5Mg0.5Al0.5Si2O6) whereas peridotitic diamonds show clinopyroxenes very rich in the diopside end-member (CaMgSi2O6). In order to get direct chemical composition on the inclusions, and therefore on the diamond origin source, it is obviously necessary to extract them breaking and/or polishing the diamond host. However, a non-destructive investigation of an inclusion still trapped in a diamond is useful and important for different reasons: (1) the inclusions could be under pressure and their crystal structure can be modified if the pressure is released by the extraction; (2) the residual pressure on the inclusion can provide information about the formation pressure of the diamond (e.g. Nestola et al. 2011 and references therein); (3) the morphology and growth relationships of the inclusion with the host diamond can provide indications about its protogenetic vs. syngenetic and/or epigenetic nature; and (4) preservation of the diamond surface growth features can maintain crucial information on late oxidation processes (Fedortchouk et al. 2011). However the available methods to measure the composition of the inclusions implies to destroy the sample. The aim of this work is to obtain chemical information on the inclusions still trapped in their diamond host and therefore to indicate the diamond origin without extracting the inclusions. The work was carried out by single crystal X-ray diffraction using a new experimental approach by high energy synchrotron radiation at I15 extreme-conditions beamline, Diamond Light Source Ltd. Such approach makes the absorption from the large diamond host nearly negligible (i.e. with a decrease in the mass attenuation coefficient of more than 75% with respect to a conventional laboratory X-ray source for single crystal diffraction) and allows to collect extremely high quality data on the inclusions. The details will be discussed.

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Abstract: About 200 km above the core-mantle boundary, the D '' seismic discontinuity marks the depth where magnesium silicate perovskite-the main mantle mineral-is transformed into its high-pressure phase of post-perovskite(1,2). Observations of seismic anisotropy within the D '' region are inferred to arise fromtextures within post-perovskite(3-5) that are created by flow in the deep mantle. Specifically, mantle flowis thought to cause post-perovskite to deform, creating a lattice-preferred orientation within the post-perovskite(6-10). However, it is difficult to explain all of the observed patterns of seismic anisotropy in the D '' region from this deformation mechanism alone. Here we use a low-pressure fluoride analogue system(11) to study the transformation from perovskite to post-perovskite in laboratory experiments. We find that post-perovskite can inherit texture from the perovskite phase. If a similar transformation mechanism operates in the Earth, post-perovskite will inherit textures from deformed perovskite and vice versa, as lowermantle material passes into and out of the D '' region. We find that this textural inheritance, combined with lattice-preferred orientation in post-perovskite, can explain the observed patterns of anisotropy in the lowermost mantle.