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Mafic and ultramafic rocks have become a promising approach for atmospheric carbon dioxide (CO[sub.2]) reduction, as they are major sources of CO[sub.2]-reactive minerals, i.e., olivine, pyroxene, plagioclase, and serpentine. The minerals potentially sequester CO[sub.2] by turning it into a stable solid phase through carbon mineralization in the rock weathering process. However, detailed descriptions and evaluations of the target formations are lacking. This study investigates the mineralogical composition and microtextural characteristics of representative mafic and ultramafic rocks observed in northern Thailand, using a petrographic analysis. The results show that variations in CO[sub.2]-reactive mineral assemblages of rocks certainly affect their theoretical CO[sub.2] uptake potential. Ultramafic rocks tend to sequester larger amounts of CO[sub.2] than mafic rocks. The microtextural observation reveals the mineral size ranges of 0.05–5 mm for ultramafic and mafic intrusive rocks and 0.01–2 mm for mafic extrusive and metamorphosed rocks. Reducing the rock size to be equal to the average size of the reactive minerals could be considered one of the practical designs in enhanced rock weathering activities. Understanding the mineralogical and textural characteristics of target rocks thus plays a crucial role in further georesource exploration and engineering designs, supporting climate action strategies on various scales.

Germanate-pyroxenes often are used as model systems to study the stability and phase relationships of analog silicate systems. Based on such analyses, it is assumed that silicates and germanates behave ideally in terms of mixing. A systematic study was performed to monitor in detail the changes introduced by a Si[sup.4+] through Ge[sup.4+] replacement in the important rock-forming pyroxene enstatite MgSiO[sub.3]. Well-shaped, idiomorphic singe crystals of a MgSi[sub.1−x]Ge[sub.x]O[sub.3] pyroxene solid solution were grown at ambient pressure from a high-temperature flux-assisted synthesis. Structural analysis using single-crystal X-ray diffraction methods revealed orthorhombic symmetry, Pbca, Z = 8, for the complete solid-solution series. Long-term storage over a period of 8 years at ambient conditions or annealing at 525 °C over a period of 10 weeks did not change the symmetry of the proposed thermodynamically stable monoclinic polymorph. Within the solid-solution series, lattice parameters increased almost linearly with increasing Si[sup.4+] by Ge[sup.4+] substitution. The main changes occurred on the tetrahedral sites, which showed an almost linear increase in individual and average bond lengths but also in distortion parameters. The refined site occupancy of Si[sup.4+] and Ge[sup.4+] showed a distinct preference of Ge[sup.4+] for the TB site. The altered topology and kinking state in the tetrahedral chains also imposed significant changes to the bonding topology and geometry of the neighboring M1 and M2 sites.

Within the Ca-Al-silicate system, dense, layered hexagonal phases occur at high temperatures and pressures between 20 and 23 GPa. They have been observed both in nature and in experiments. In this study, we describe the endmember with a dominant sixfold coordinated Si as a mineral zagamiite (IMA 2015-022a). This new mineral identified in Martian meteorites has a general formula of (Ca,Na)(Al,Fe,Mg)[sub.2](Si,Al,□)[sub.4]O[sub.11], thus defining CaAl[sub.2]Si[sub.3.5]O[sub.11] as a previously unknown endmember of the hexagonal CAS phases. Zagamiite assumes space group P6[sub.3]/mmc with a unit cell of a = 5.403(2) Å, c = 12.77(3) Å, V = 322.9(11) Å[sup.3], and Z = 2. Zagamiite contains significant Fe and Mg and a substantial deficit of Na relative to plagioclase of an equivalent Al/Si, suggesting that it was formed through crystallization from a melt that was derived from a plagioclase-dominant mixture of plagioclase and clinopyroxene above the solidus beyond 20 GPa.

Phase relations in the system [Mg.sub.4][Si.sub.4][O.sub.12]-[Mg.sub.3][Cr.sub.2][Si.sub.3][O.sub.12] were studied at 10-24 GPa and 1,600°C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the knorringite-majorite system to derive a P-X phase diagram and synthesize garnets of a wide compositional range. Samples synthesized in the pressure range 10-14 GPa contain knorringite-majorite garnet and Cr-bearing pyroxene. With increasing Cr content in the starting materials, an association of knorringite-majorite garnet and eskolaite is formed. Garnets contain a significant portion of majorite (> 10 mol%) even for a pure [Mg.sub.3][Cr.sub.2][Si.sub.3][O.sub.12] starting composition. Knorringite-majorite garnets were obtained in the pressure range from 10 to 20 GPa. With increasing pressure, the phase assemblages include Cr-bearing MgSi[O.sub.3] akimotoite and MgSiO3 bridgmanite, as well as Mg[Cr.sub.2][O.sub.4] with calcium titanate structure, and stishovite. Single-crystal X-ray diffraction shows that the incorporation of Cr into the structure of garnet, as well as MgSiO3 akimotoite, and bridgmanite results in an increase in their unit cell parameters. Results of the experimental high-pressure investigation of the pseudo-binary system [Mg.sub.4][Si.sub.4][O.sub.12]-[Mg.sub.3][Cr.sub.2][Si.sub.3][O.sub.12] (Si[O.sub.2]-MgO-[Cr.sub.2][O.sub.3]) may be applied to the origin of high chromium phases (mostly garnet) found as inclusions in peridotitic diamonds and formed in bulk rock compositions with high Cr/Al ratios in relation to the primitive mantle.

Two Archaean komatiitic flows, Fred's Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred's Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred's Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30-38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2-5 vol% amygdales, a 110-120 m intermediate layer of olivine porphyry and a 20-30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38-40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% [H.sub.2]O. This liquid degassed during the eruption, creating a supercooled liquid that solidified quickly and crystallized olivine with non-equilibrium textures and compositions.

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite-eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid-rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H.sub.2O in the form of OH groups. H.sub.2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 [micro]g/g H.sub.2O, pyroxene from ~ 50 to ~ 310 [micro]g/g H.sub.2O, and granulitic plagioclase from ~ 10 to ~ 140 [micro]g/g H.sub.2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H.sub.2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H.sub.2O content range from ~ 150 to 425 [micro]g/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H.sub.2O increase from ~ 450 to ~ 2500 [micro]g/g H.sub.2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

Over 60 years of spacecraft exploration has revealed that the Earth’s Moon is characterized by a lunar crust 1 dominated by the mineral plagioclase, overlying a more mafic (richer in iron and magnesium) mantle of uncertain composition. Both crust and mantle formed during the earliest stages of lunar evolution when late-stage accretional energy caused a molten rock (magma) ocean, flotation of the light plagioclase, sinking of the denser iron-rich minerals, such as olivine and pyroxene, and eventually solidification 2 . Very large impact craters can potentially penetrate through the crust and sample the lunar mantle. The largest of these craters is the approximately 2,500-kilometre-diameter South Pole-Aitken (SPA) basin 3 on the lunar far side. Evidence obtained from orbiting spacecraft shows that the floor of the SPA basin is rich in mafic minerals 4 , but their mantle origin is controversial and their in situ geologic settings are poorly known. China’s Chang’E-4 lunar far-side lander recently touched down in the Von Kármán crater 5 , 6 to explore the floor of the huge SPA basin and deployed its rover, Yutu-2. Here we report on the initial spectral observations of the Visible and Near Infrared Spectrometer (VNIS) 7 onboard Yutu-2, which we interpret to represent the presence of low-calcium (ortho)pyroxene and olivine, materials that may originate from the lunar mantle. Geological context 6 suggests that these materials were excavated from below the SPA floor by the nearby 72-km-diameter Finsen impact crater event, and transported to the landing site. Continued exploration by Yutu-2 will target these materials on the floor of the Von Kármán crater to understand their geologic context, origin and abundance, and to assess the possibility of sample-return scenarios. Initial spectral observations by China’s Chang’E-4 far-side lunar rover suggest the presence of materials that may originate from the Moon’s mantle.

Fotouni-Kékem shear zone (FKSZ) and the Nyakong-Manyi shear zone (NMSZ) are respectively located to southwest and northwest of the N50E branch of the central Cameroon shear zone (CCSZ). Three deformation phases are recorded in these shear zones including, D 1 , D 2 and D 3 . The D 1 phase, with σ 1 applied in the NE-SW direction, is remnant and poorly represented, whose structures (NW–SE S 1 foliation) were transposed by the late D 2 and D 3 phases related structures. The D 2 phase is an early sinistral shear phase, with σ 1 applied in the WNW-ESE direction, which developed NNW-SSE to NNE-SSW S 2 foliation, B 2 shear band boudins, F 2 knee-like folds and asymmetric fish-like structures. The D 3 phase is a NE-SW dextral mylonitic shear phase, with σ 1 applied in the NW–SE direction, responsible the development of S 3 foliation, P 3 recumbent and overturn folds, B 3 shear band boudins, σ-type sigmoids and asymmetric amphibole fishes. Pyroxene amphibolite (PA) occurs as slab stones, banded to lens-like, egg-like enclaves, folded bands, sheared and/or boudinaged green to dark green rocks displaying NE-SW preferred orientation. It displays heterogranular nematoblastic texture marked by amphibole (60%, hornblende), plagioclase (≈20%) and clinopyroxene (15 à 20%) porphyroblasts dispersed in between medium-grained mineral showing preferred orientation. Under microscope, PA evidenced a polyphasic prograde-peak-retrograde high-grade regional metamorphism. Prograde-peak phase is evidenced by primary mineral paragenesis (stable amphibole + pyroxene + plagioclase + K-feldspar) and microstructures, which indicate granulite facies. This occurs during the D 1 deformation phase. retromorphic relic-like pyroxene crystals displaying amphibole-plagioclase-quartz-opaque minerals assemblage, which follows the peak metamorphism, related to relaxation during D 2 - D 3 , evidence retrograde phase. Early sinistral syn- D 2 and late dextral syn- D 3 mylonitic events, whose microstructures evidence high-grade deformation setting, overprinted this regional metamorphism. This polyphasic activation of the CCSZ during these mylonitic events ( D 2 - D 3 ) stretched, sheared, folded dismembered and scattered amphibolites along the Pan-African mobile zone during late phases of the Pan-African orogeny. Geochemical data indicate that PA derives from mafic protoliths originating from a garnet lherzolite reservoir melting that was contaminated by both subducted sediment and slab-derived fluids as evidenced by the slightly positive ƐNd 600Ma (+ 1.27). The model age (T DM  = 1.25 Ga) with initial 87 Sr/ 86 Sr ratios of 0.70488 suggest an ancient Mesoproterozoic crust that underwent metamorphic transformation during the collisional (burial) and post collisional (exhumation) stapes of the Pan-African orogeny.