Search

Peridotite xenoliths from kimberlites are useful tools for exploring the architecture and composition of the thick sub-cratonic lithosphere, and thus understanding the long-term evolution of the Earth’s mantle. However, the continuous infiltration of kimberlite-related melts and fluids prior to - and during - the transport of mantle-derived fragments towards the surface makes it difficult to extract information about the original texture and chemistry of the mantle rocks and the deep-seated metasomatic processes.In this study, fresh spinel- to garnet-bearing peridotite xenoliths from Udachnaya-East were studied to unveil the nature and composition of the lithospheric mantle beneath the Siberian craton. The studied samples have mostly harzburgitic to dunitic composition, even though lherzolites and rare wehrlites are also found. Occasionally, harzburgites are orthopyroxene-rich (up to 40 vol.%) or garnet-rich (up to 30 vol.%). The texture of the peridotites is extremely variable, ranging from protogranular to highly recrystallized and/or sheared. In spinel-bearing rocks, primary olivine is Mg- and Ni-rich (Fo90-93; NiO = 0.34-0.46 wt%), orthopyroxene has Mg# of 92-94 and Al2O3 in the range of 0.3-3.0 wt%, while clinopyroxene is Mg-rich (Mg# 94-96), with Al2O3 comprised between 1.0 and 3.5 wt%. In garnet-bearing peridotites, olivine ranges from Mg- and Ni-rich (Fo92; NiO = 0.45 wt%) to Fe-rich and Ni-poor (Fo87; NiO = 0.25 wt%), while pyroxenes have Mg# from 93 to 87-88 and comparatively low Al2O3 contents (orthopyroxene: 0.5-1.1 wt%; clinopyroxene: 0.8-2.2 wt%). High-precision electron microprobe analyses complemented by thermo- and oxy-barometric models were used to reconstruct the thermo-chemical log of the Siberian sub-cratonic mantle, in comparison to what proposed by Liu et al. (2022). Textural-compositional studies of the reaction zones enabled to discriminate the secondary-formed minerals with composition ascribable to the liquid line of descent of kimberlite-related melts at Udachnaya (Casetta et al. 2023) from those formed during melt/fluid-rock reactions taking place in the mantle before xenoliths’ entrainment by the host kimberlites. Altogether, our results enable to trace the P-T-X evolution experienced by the Siberian mantle, opening a window onto the comprehension of the interactions between kimberlitic-related fluid/melts and the sub-cratonic lithosphere. Casetta, F., Asenbaum, R., Ashchepkov, I., Abart, R., & Ntaflos, T. (2023). Mantle-Derived Cargo vs Liquid Line of Descent: Reconstructing the P–T–fO2–X Path of the Udachnaya–East Kimberlite Melts during Ascent in the Siberian Sub-Cratonic Lithosphere. Journal of Petrology, 64(1), egac122. Liu, Z., Ionov, D. A., Nimis, P., Xu, Y., He, P., & Golovin, A. V. (2022). Thermal and compositional anomalies in a detailed xenolith-based lithospheric mantle profile of the Siberian craton and the origin of seismic midlithosphere discontinuities. Geology.

Dendritic crystallisation is a key geological pattern-forming mechanism, typical of, and recording information about, rapid crystallization events. In this contribution we report on clinopyroxene (Cpx) dendrites in a basaltic rock fulgurite, which formed due to electrical discharge impacting a basaltic rock. Unusually, these dendrites exhibit a curved morphology. The curved, tapering main dendrite branches are up to 50 µm long, range from 3 µm to 100 nm thickness, and are surrounded by several higher orders of branches, which are also curved. The morphological curvature corresponds to lattice rotation, so branches have consistent elongation directions in crystal coordinates. Total rotation exceeds 180° for some branches, with the highest curvature found being 7° per µm. Such “bent” Cpx dendrites have been observed in experiments (e.g. Hammer et al. 2010), but the mechanism of bending was not previously understood.By combining microstructural observations with crystallographic orientation maps from electron backscatter diffraction analyses of multiple Cpx dendrites, their three-dimensional morphological and crystallographic configuration was reconstructed. Dendrites feature a planar latticework of branches parallel to the Cpx (010) plane. Branches in this plane are elongated either parallel to {001}* (i.e. normal to the (001) plane) or , and exhibit strong and weak lattice curvature, respectively. Sprouting out of this plane are branches parallel to {021}* (originating from {001}* branches) and (originating from branches), both types being weakly curved. Regardless of the crystallographic direction parallel to elongation, all branches exhibit a crystallographic rotation axis parallel to [010] of Cpx. Furthermore, the rotation sense is consistent regardless of elongation direction in crystal or sample coordinates.The crystallographic control on the sense of bending and on the rotation axis indicates that bending is not caused by sample-scale compositional, thermal, or mechanical gradients. Instead, asymmetric compositional and thermal fields around branch tips are responsible for bending, supported by the fact that compositional gradients exist in the glass surrounding dendritic crystals. The specific cause of bending is inferred to be asymmetric distribution of melt supersaturation at branch tips, resulting from unequal growth rates of different facets. Branch-tip morphology alone poorly explains the constant sense of rotation of all branches, as the sense of morphological asymmetry is unlikely to be consistent for all branch types. The [010] rotation axis implies that lattice rotation is accomplished by incorporation of a single sign of [001](100) edge dislocations, with a maximum inferred density of 2*1014 m-2.This work provides new insights into fundamental processes occurring during rapid crystallization of Cpx and other minerals. Furthermore, microstructural observations suggest that higher degree of undercooling correlates with greater lattice curvature. Bent dendrites may thus encode information about spatial variations in the cooling rate and/or undercooling of samples. Finally, the consistent [010] rotation axis is expected to be preserved during recrystallization, offering a potential way to identify curved dendritic growth stages even after recrystallization.References:Hammer et al. (2010), Geology 38:367-370. https://doi.org/10.1130/G30601.1Griffiths et al. (2022), J Petrol, egac125. https://doi.org/10.1093/petrology/egac125This contribution was funded by the Austrian Science Fund (FWF): P 33227-N

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered to be mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m sized mafic lens mainly formed by garnet pyroxenite. The primary mineral assemblage comprises calcium-rich garnet (XGrs = 0.4), kyanite, and sodium-rich clinopyroxene (XNa_M2 = 0.29) (± quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 °C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine–spinel–plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase–orthopyroxene–spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine–spinel–plagioclase symplectites and the plagioclase–clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine–spinel–plagioclase symplectite, the garnet composition changes from Alm14Prp42Grs44 in the pristine garnet to Alm22Prp63Grs15 at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm19Prp40Grs41 to Alm30Prp54Grs16. The secondary zoning towards the plagioclase–orthopyroxene–spinel symplectites is characterized by an increase of XAlm from 0.19 to 0.27 and a concomitant decrease of XPrp from 0.55 to 0.49 at constant XGrs of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase–orthopyroxene–spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 °C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.

Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.

Climatic variability and silicate weathering are remarkable features throughout the Late Cretaceous period. Late Campanian black shale is considered the most significant silicate source rock in the southern Tethys. Here, we used mineralogical and geochemical data to evaluate the continental weathering intensity and climatic changes as well as their impact on the deposition of the Late Campanian black shale in the Western Desert of Egypt. The studied black shale has a relatively high concentration of Al, Fe, Mg, Ca, Sr, Ga, Co, Cr, and V when compared to the average Post-Archean Australian Shales (PAAS). The studied samples have elevated values of Ga/Rb, and low values of Rb/Sr, Sr/Cu, and K2O/Al2O3, supporting the deposition of Late Campanian shale under warm/humid conditions. Furthermore, the average chemical index of alteration (CIA, 78.6%), chemical index of weathering (CIW; 83.8%), C-value (1.26), Fe/Mn (408), and Mg/Ca (1.54) reveal the predominance of warm/humid climate. The chemical weathering proxies (CIA, CIW, PIA, LnAl2O3/Na2O) and ACNK diagram imply that the Late Campanian samples were exposed to a moderate grade of chemical alteration. The deposition of black shale occurred under high seawater salinity conditions based on Sr/Ba (Avg = 3.6). Additionally, the weathering indices are well correlated with paleoclimatic proxies, suggesting that weathering intensity is strongly affected by paleoclimate. However, chemical weathering during the Late Campanian has a weak influence on oceanic nutrient fluxes. No substantial impact of the paleoclimate during the deposition of Late Campanian black shale on water salinity was reported.

The microstructural and textural characteristics of a spinel corona that formed around a faceted corundum xenocrystal by reaction with the hosting basaltic melt in the Siebengebirge volcanic field demonstrate that the crystallographic and shape preferred orientation of spinel is influenced by the orientation of the reaction interface with respect to the corundum crystal lattice. The spinel roughly shows the common topotactic orientation relationships with corundum, where one of the $\{111\}_{Spl}$ planes is parallel to the (0001)$_{Crn}$ plane, and three of the $\{110\}_{Spl}$ planes are parallel to the $\{10\overline {1}0\}_{Crn}$ planes. In detail, there are subtle but systematic deviations from this topotactic relationship due to small rotations about the c-axis and/or an a-axis of corundum. The former is observed when the corundum c-axis is closely parallel to the interface plane, while the latter require a corundum a-axis orientation perpendicular to the interface. In this case, the preferred sense of rotation depends on the sign of the a-axis direction, irrespective of the spinel growth direction being parallel or antiparallel to this axis. Additionally, the selection of either one or both of two spinel twin variants that equally fulfill the topotactic orientation relationship depends on the orientation of the corundum-spinel interface with respect to the lattices of both the corundum and the spinel. Finally, also the grain boundary character is controlled by the interface orientation and the corundum lattice. Despite the differences between corona segments, the nature of these textures are persistent along and across each segment. We emphasize that all these microstructural and textural features are ascribed to the period of spinel growth in magmatic environment. The extent to which prominent slip planes in spinel are aligned parallel with the corundum-spinel interface seems to be of crucial importance for the nature of the spinel texture and microstructure, indicating that the activity of dislocations pertaining to these slip systems ease the accommodation of lattice misfit across the corundum-spinel interface. By comparison with experimentally grown spinel layers, we infer predominantly interface reaction controlled growth of the studied spinel corona.

Oriented needle-, lath- and plate-shaped magnetite micro-inclusions in rock forming plagioclase from mafic intrusive rocks, were investigated using correlated optical microscopy and scanning transmission electron microscopy. The magnetite micro-inclusions were analysed on cuts parallel and perpendicular to the inclusion–elongation directions. The crystal structures of the two phases are in direct contact along the interfaces. The shape, shape orientation and crystallographic orientation relationships between the magnetite micro-inclusions and the plagioclase host appear to be controlled by the tendency of the system to optimise lattice match along the interfaces. The elongation direction of the inclusions ensures good match between prominent oxygen layers in the magnetite and plagioclase crystal structures across the interfaces bounding the inclusions parallel to their elongation direction. In cross-section, additional modes of lattice match, such as the commensurate impingement of magnetite and plagioclase lattice planes along the interfaces, the parallel alignment of the interfaces to low-index lattice planes of magnetite or plagioclase, or the parallel alignment to low index lattice planes of both phases are observed, which appear to control the selection of interface facets, as well as the shape and crystallographic orientation relationships between magnetite micro-inclusions and plagioclase host. The systematics of the inclusion cross-sectional shapes and crystallographic orientation relationships indicate recrystallisation of magnetite with potential implications for natural remanent magnetisation of magnetite-bearing plagioclase grains.

The late Neoproterozoic gabbroic intrusion of the Wadi El-Faliq area in the central Eastern Desert of Egypt (north Arabian–Nubian Shield; henceforth, ANS) is a fresh, undeformed elliptical body elongated in a NW–SE trend following the main sinistral strike-slip faults of the Najd fault system. Mineralogical and geochemical evidence suggest that they were derived from hydrous tholeiitic mafic magmas with arc-like geochemical fingerprints resembling the post-collisional gabbroic intrusions in Saudi Arabia. Despite the arc-like signatures, their fresh and undeformed nature, together with the field relationships, indicates that the studied gabbroic intrusion post-dates the main collisional phase, supporting its emplacement after subduction ceased and during the post-collisional stage. As a result, the arc-like signatures were possibly transmitted from the earlier ANS subduction episode. Indeed, the high (La/Sm)N, and negative-Nb and positive-Pb anomalies suggest contributions from subduction components. Lithospheric delamination was possibly facilitated by the Najd faults and shear zones formed during the post-orogenic crustal extension associated with the Pan-African orogenic collapse. The delamination process could have generated a rapid upwelling and melting of the asthenosphere mantle. The melt-rock reaction process likely played an important role in the genesis of the studied rocks through the interaction of the asthenosphere melts with lithosphere mantle rocks during ascent. The HREE fractionation suggests a probable mixing between melts from both spinel- and garnet-bearing peridotites. We suggest that the Wadi El-Faliq gabbroic intrusion was likely emplaced due to the stretching and thinning of the lithosphere during the extensional tectonism following the Pan-African orogeny.

Understanding dendritic crystallization is key to obtaining petrological information about rapid crystallization events. Clinopyroxene dendrites from a basaltic rock fulgurite from Nagpur, India, exhibit curved branches with corresponding lattice rotation that exceeds 180° for some branches. This paper combines crystallographic orientation mapping with microstructural observations and compositional information to determine the dendrites’ 3-D morphology and their bending mechanism. Dendrites exhibit a network of branches in the (010) plane, following either {001}* (normal to {001} planes, strong lattice curvature) or (weak lattice curvature). Three or more orders of branches are observed in the (010) plane, alternating between {001}* and . Side branches with weak lattice curvature extend sub-perpendicular to the (010) plane, following either {021}* (sprouting from {001}* branches) or (from branches) and defining curved ‘ribbons’ containing their respective central branch. All branches rotate about [010], with a consistent rotation sense regardless of elongation direction in sample or crystal coordinates. Bending must therefore be caused by local asymmetric thermal and compositional fields in the melt, generated by dendritic growth itself, not by sample-scale compositional, thermal or mechanical gradients. The most likely cause of bending is asymmetric distribution of melt supersaturation around branch tips, related to unequal growth rates perpendicular to different facets. Lattice rotation is inferred to occur via preferential incorporation of high densities of [001] (100) edge dislocations of one sign. High inferred dislocation densities imply that the preservation of bent dendrites requires rapid quenching. Higher inferred degree of undercooling (based on microstructural observations) correlates with greater lattice curvature. Bent dendrites can thus potentially be used to deliver information about spatial variations in degree of undercooling and place limits on the history of a sample after dendritic crystallization. Finally, finding lattice rotation exclusively about [010] is a new criterion to identify cryptic dendritic growth stages in euhedral crystals.

The evolution of essentially periodic crack patterns appears in many natural and technical processes. For example, typical cases of periodic crack patterns are known from drying soils, from the formation of basalt columns, from high temperature loaded ceramics, from fissure formation in coke, and from drying protein solution drops. The appearance of more or less periodic crack patterns is the result of cascades of bifurcation-instabilities of the propagation process. There exist established, mathematically rigorous criteria for the bifurcation of the crack growth process of two initially equally long parallel cracks (Nemat-Nasser et al., 1978; Bazant et al., 1979). These criteria require the determination of the mutual influence of the neighboring cracks on their growth by calculation of partial derivatives. If discretization methods are used for simulating such a growth process, the calculation of these partial derivatives may become very sensitive with respect to the density of the spatial discretization. This is especially true if strong gradients of a crack driving eigenstress field appear as, for instance, in chemical diffusion processes. In the present paper, an alternative approach is introduced, which circumvents estimations of partial derivatives and, in addition, provides a more realistic picture of the generation of crack patterns. This approach is inspired by a computationally efficient method for investigating the buckling and post-buckling behavior in structural mechanics by introducing small imperfections. As a typical example, in which the approach introduced here is especially beneficial, the simulation of self-organized crack pattern evolution in alkali feldspar, a very common brittle crystal, is presented. There, the cracks are driven by eigenstrains caused by cation exchange between the crystal and a salt melt. Comparisons between the character of the pattern formation in the simulation and in experiments show excellent agreement. Furthermore, by combining simulations and experiments the J c -value can be determined at considerably high accuracy.

Unravelling the processes taking place during the genesis of kimberlites, their ascent through the sub-cratonic mantle and their emplacement in the crust is challenging, as kimberlites are mixtures of mantle-derived and magmatic components, rarely preserving pristine evidence of their original nature. Furthermore, their intense state of alteration makes it difficult to access the textural-compositional record of information engraved in the phase constituents. In this study, fresh samples of kimberlites and related mantle-derived xenocrysts-xenoliths from the Udachnaya pipe (Siberia) were investigated to reconstruct the pressure-temperature-time-composition (P-T-t-X) framework of the sub-cratonic lithosphere at the time of kimberlite emplacement. Routine and high-precision electron microprobe analyses of olivine, phlogopite and spinel from different facies of the Udachnaya pipe (intrusive coherent, hypabyssal and pyroclastic, sensu Scott Smith et al., 2013) showed that specific phase assemblages are associated with each evolutionary stage of the kimberlite. Olivine composition, in particular, is extremely variable, ranging from high-Fo and high-Ni (Fo93; NiO = 0.45 wt%) to low-Fo and low-Ni (Fo85; NiO = 0.10 wt%), but also to high-Fo and low-Ni (Fo>93; NiO A comparison between the main constituents of the Udachnaya kimberlite and those of the mantle xenoliths sampled during ascent, complemented by detailed major-trace element profiles on olivine crystals, was put forward to: (i) discriminate between the mantle-derived xenocryst cargo and the magmatic assemblage; (ii) model the P-T-fO2 path of kimberlites; (iii) speculate about their ascent rate; (iv) model the interactions between kimberlite-related fluid/melts and the Siberian sub-cratonic lithosphere.REFERENCESScott Smith, B.H., Nowicki, T.E., Russell, J.K., Webb, K.J., Mitchell, R.H., Hetman, C.M., ... & Robey, J.A. (2013). Kimberlite terminology and classification. In Proceedings of 10th international kimberlite conference (pp. 1-17). Springer, New Delhi.

Oriented needle shaped magnetite inclusions in plagioclase may give rise to magnetic anisotropy of individual plagioclase grains and, in case of preferred orientation of the plagioclase grains, contribute to magnetic anisotropy of the bulk-rock. Understanding how oriented magnetite inclusions generate magnetic fabrics in single grains of plagioclase is important for interpreting rock magnetic fabrics and for correcting paleomagnetic data. Plagioclase grains from oceanic gabbro dredged at the Mid Atlantic Ridge (11-17°N) were analyzed using optical microscopy, electron backscatter diffraction (EBSD), as well as alternating field demagnetization and anisotropy of magnetic remanence (AMR) measurements to investigate the influence of the shape orientation distribution of acicular magnetite inclusions on the magnetic properties of magnetite bearing plagioclase grains.In pristine magmatic plagioclase, the needle elongation directions form a 30° wide girdle distribution parallel to the pl(010) plane. This girdle distribution is insensitive to twinning after the Albite, Pericline, Carlsbad and Manebach laws. The statistical maximum in the inclusion orientation lays in the pl(010) plane, closely parallel to the pl[001] direction. The overall shape orientation distribution of the magnetite inclusions produces a triaxial magnetic anisotropy ellipsoid with the minimum axis direction sub-perpendicular to the pl(010) plane and the maximum axis sub-parallel to the pl[001] direction.The vector of natural remanent magnetization parallels the maximum AMR axis direction indicating that the magnetic anisotropy caused by the magnetite inclusion fabric controls the paleomagnetic signature. In hydrothermally modified plagioclase, most or all magnetite needles are oriented parallel to the pl[001] direction and a prolate rotational ellipsoid of remanent magnetization with the maximum remanent magnetization parallel to pl[001] should occur.Plagioclase-hosted magnetite inclusions are particularly stable recorders of the paleomagnetic filed. The magnetic anisotropy arising from the anisotropic shape orientation distribution of the magnetite inclusions may, however, bias the magnetic record, an effect that needs to be accounted for in paleomagnetic reconstructions or for absolute paleointensity experiments.This work was supported by the Austrian Science Fund (FWF): I 3998-N29, and by the Russian Foundation for Basic Research, Grant no. 18-55-14003.

Uncommon Ba-Cl-rich phases including Ba-Cl micas and Cl-phosphates have been found in garnet pyroxenites as a part of the matrix or in polyphase inclusions in garnets. Polyphase inclusions are rich in carbonates (dolomite, magnesite, norshetite), phosphates (Cl-apatite, goryainovite (Ca2PO4Cl), monazite) and other silicates (spinel, amphibole, orthopyroxene, clinopyroxene, margarite, aspidolite, scapolite, cordierite). The inclusions appear as chains crosscutting garnet crystals and their presence is not linked with any chemical zoning in the host garnet.The Ba-Cl-rich mica has composition ranging from Ba-rich phlogopite to chloroferrokinoshitalite and to oxykinoshitalite. The mica present in the matrix correspond to Ba-rich phlogopite with low Cl contents and occur together with celsian and low-Cl hydroxyl apatite. The mica in the polyphase inclusions ranges to almost pure chloroferrokinoshitalite and oxykinoshitalite endmembers and coexists either with Cl-apatite (Cl = 1.2 apfu) or rarely goryainovite containing up to 2.5 wt% of SrO. This is second world occurrence of goryainovite and first evidence that Ca can be partially replaced by Sr in this mineral.Special attention was paid to the composition trends of the Ba-Cl-rich micas. These are mainly related to the XFe ratio, which correlates positively with Cl, Ba, and Al and negatively with Si and Na. Positive correlation of Cl with Ba and XFe leads to the formation of mica with composition Ba0.95K0.03Fe2.69Mg0.37Al1.91Si2.02Cl1.98, XFe0.88, which is the most Cl-rich mica so far described from natural samples (10.98 wt% Cl) and is very close to the theoretical formula of chloroferrokinoshitalite BaFe3Al2Si2O10Cl2. The positive correlation of Ba with Al and their negative correlation with Si and K is corresponding to the coupled substitution Ba1Al1K-1Si-1 linking the composition of phlogopite and kinoshitalite. Composition trend related with the Ti-content shows that Ti correlates positively with Ba but negatively with Cl, XFe, and with the sum of Mg and Fe. It implies that Ti is incorporated into mica in coordination with O (Ti1O2(Mg,Fe2+)-1(OH)-2) and it leads to the formation of oxykinoshitalite (BaMg2TiSi2Al2O12). Since the incorporation of either Cl or Ti + O correlates with XFe content of mica, XFe ratio can be the crucial factor controlling the ability of mica to incorporate Cl into its crystal lattice. In some cases, two micas with contrasting composition corresponding closer to chloroferrokinoshitalite or oxykinoshitalite coexist in one polyphase inclusion, demonstrated by distinct content of XFe, Ti and Cl (for example: XFe0.20:0.77, Ba0.48:0.63, Ti0.35:0.02, Cl0.27:1.45). This could imply the existence of an immiscibility between the composition trends of chloroferrokinoshitalite and oxykinoshitalite .Such Ba, Cl and K-rich phases are atypical for garnet pyroxenite. Their presence may be caused by the injection of fluid/melt of crustal source during subduction and subsequent exhumation processes or may be related to earlier mantle metasomatism. The presence of Cl-rich phases together with carbonates indicates extremely high activity of Cl and CO2 in the metasomatizing fluid/melt that interacted with garnet pyroxenites.

Corona microstructures comprised of garnet (grt) and clinopyroxene (cpx) were observed at the contacts between plagioclase (pl) and Fe-rich orthopyroxene (opx) in meta-gabbroic rocks in a several 100 m sized (ultra-)mafic lens embedded in felsic granulite of the Gföhl unit (Moldanubian zone, Lower Austria).The corona microstructures are formed around monomineralic aggregates of opx and they are comprised of two layers, an inner about 100 μm thicklayer of polycrystalline cpx and an outer, about 800 μm thick layer of polycrystalline garnet. The corona structures are surrounded by the pl-rich rock matrix. The cpx layer shows a weak but systematic chemical zoning characterized by increasing Mg and decreasing Na and Al contents from the contact with grt towards the contact with opx. The grt layer shows a pronounced and complex chemical zoning. There is a consistent trend of decreasing Mg and increasing Ca contents from the contact with the cpx layer, where the composition is Alm22 Prp67 Grs11 towards the contact with the rock matrix, where we observe Alm25 Prp48 Grs28. This pattern is interpreted as a primary growth zoning. Superimposed on the growth zoning there is a secondary zoning, which is evident from a decrease of the Ca content and a concomitant increase of the Mg content from the interior of the individual grainsof the grt polycrystal forming the grt layer towards the grt grain boundaries. The secondary zoning is most pronounced in the outermost portions of the garnet layer, where the primary growth zoning shows the highest Ca and the lowest Mg contents. Locally the garnet grains contain abundant primary melt inclusions. In most segments of the corona, secondary opx and pl form layers along the contact between the primary cpx and grt layer, where the opx partially replaces the cpx layer and the pl partially replaces grt. The secondary opx has higher Mg and lower Na, Al, and Ca contents than the opxin the core of the corona structure. The secondary pl has the same composition as the matrix pl. At its outer edge, the garnet layer is locally replaced by spinel bearing cpx-pl symplectites. The primary compositional zoning of the garnet layer could be reproduced in equilibrium assemblage diagrams (pseudosections). Calculated equilibrium phase relations indicate that the grt-cpx corona formed at the contacts between opx and pl at supersolidus HP − HT conditions of P > 1.8 GPa and T > 900 °C and low H2O content. Growth of coronal grt and cpx requires the diffusive transport of Fe and Mg from the opx to the pl and concomitant transport of Ca and Al in the opposite direction. The secondary zoning of garnet, the back reaction forming secondary opx and pl at the contact between the primary grt and cpx layer and the spinel bearing pl-cpx symplectites locally replacing garnet at the outer edge of the grt layer are related to different decompression stages. Preservation of the secondary garnet zoning indicates relatively rapid cooling during latestages of or immediately after decompression.

Gabal El-Ineigi fluorite-bearing rare-metal granite with A-type affinity, located in the Central Eastern Desert of Egypt, is distinguished by its abundance of large fluorite-quartz veins and mafic enclaves. Plagioclase (labradorite to oligoclase), Mg-rich biotite, and Mg-rich hornblende are the main components of mafic enclaves, with significant amounts of fluorite as essential phases, and titanite and Fe-Ti oxides (Nb-free rutile and ilmenite-rutile solid solution) as the main accessories. These enclaves are monzodioritic in composition, Si-poor, and highly enriched in Ca, Fe, Mg, and F compared to the host alkali feldspar F-poor Si-rich granites. Given the conflicting evidence for a restitic, xenolithic, magma mixing/mingling, cumulate, or bimodal origin for these enclaves, we propose that the mafic enclaves and felsic host granites are two conjugate liquids, with contrasting compositions, of a single parental melt. This is inferred by the normalized REE patterns that are similar. As a result, liquid immiscibility is proposed as a probable explanation for this mafic–felsic rock association. These enclaves can be interpreted as transient melt phases between pure silicate and calcium-fluoride melts that are preserved from the early stages of separation before evolving into a pure fluoride (Ca-F) melt during magma evolution. Due to element partitioning related to melt unmixing, the enclaves are preferentially enriched in Ca, F, Li, Y, and REE and depleted in HFSE (such as Zr, U, Th, Ta, Nb, Hf, and Ga) in comparison to the host granites. Furthermore, mafic enclaves exhibit W-type tetrad effects, while host granites exhibit M-type tetrad effects, implying that the REE partitioning, caused by liquid immiscibility, is complementary.

The investigation of the role played by CO2 circulating within the mantle during partial melting and metasomatic/refertilization processes, together with a re-consideration of its storage capability and re-cycling in the lithospheric mantle, is crucial to unravel the Earth's main geodynamic processes. In this study, the combination of petrology, CO2 content trapped in bulk rock- and mineral-hosted fluid inclusions (FI), and 3D textural and volumetric characterization of intra- and inter-granular microstructures was used to investigate the extent and modality of CO2 storage in depleted and fertile (or refertilized) Sub-Continental Lithospheric Mantle (SCLM) beneath northern Victoria Land (NVL, Antarctica). Prior to xenoliths entrainment by the host basalt, the Antarctic SCLM may have stored 0.2 vol% melt and 1.1 vol% fluids, mostly as FI trails inside mineral phases but also as inter-granular fluids. The amount of CO2 stored in FI varies from 0.1 μg(CO2)/g(sample) in olivine from the anhydrous mantle xenoliths at Greene Point and Handler Ridge, up to 187.3 μg/g in orthopyroxene from the highly metasomatized amphibole-bearing lherzolites at Baker Rocks, while the corresponding bulk CO2 contents range from 0.3 to 57.2 μg/g. Irrespective of the lithology, CO2 partitioning is favoured in orthopyroxene and clinopyroxene-hosted FI (olivine: orthopyroxene = 0.10 ± 0.06 to 0.26 ± 0.09; olivine: clinopyroxene = 0.10 ± 0.05 to 0.27 ± 0.14). The H2O/(H2O + CO2) molar ratios obtained by comparing the CO2 contents of FI to the H2O amount retained in pyroxene lattices vary between 0.72 ± 0.17 and 0.97 ± 0.03, which is well comparable with the values measured in olivine-hosted melt inclusions from Antarctic primary lavas and assumed as representative of the partition of volatiles at the local mantle conditions. From the relationships between mineral chemistry, thermo-, oxy-barometric results and CO2 contents in mantle xenoliths, we speculate that relicts of CO2-depleted mantle are present at Greene Point, representing memory of a CO2-poor tholeiitic refertilization related to the development of the Jurassic Ferrar large magmatic event. On the other hand, a massive mobilization of CO2 took place before the (melt-related) formation of amphibole veins during the alkaline metasomatic event associated with the Cenozoic rift-related magmatism, in response to the storage and recycling of CO2-bearing materials into the Antarctica mantle likely induced by the prolonged Ross subduction.

A system of edge cracks was applied to polished (010) surfaces of K-rich gem-quality alkali feldspar by diffusion-mediated cation exchange between oriented feldspar plates and a Na-rich NaCl-KCl salt melt. The cation exchange produced a Na-rich layer at and beneath the specimen surface, and the associated strongly anisotropic lattice contraction lead to a tensile stress state at the specimen surface, which induced fracturing. Cation exchange along the newly formed crack flanks produced Na-enriched diffusion halos around the cracks, and the associated lattice contraction and tensile stress state caused continuous crack growth. The cracks nucleated with non-uniform spacing on the sample surface and quickly attained nearly uniform spacing below the surface by systematic turning along their early propagation paths. In places, conspicuous wavy cracks oscillating several times before attaining their final position between the neighboring cracks were produced. It is shown that the evolution of irregularly spaced towards regularly spaced cracks including the systematic turning and wavyness along the early propagation paths maximizes the rate of free energy dissipation in every evolutionary stage of the system. Maximization of the dissipation rate is suggested as a criterion for selection of the most probable evolution path for a system undergoing chemically induced diffusion mediated fracturing in an anisotropic homogeneous brittle material.The online version contains supplementary material available at 10.1007/s00269-022-01183-9.

Shear wave splitting measurements in the Qiangtang terrane of central Tibet revealed extraordinarily large delay times of >2.0 s for SKS splitting and an abrupt change of the fast polarization direction from a NNE-SSW direction into an E-W direction in an about 100 km wide zone south of the Banggong-Nujiang Suture. Peridotite xenoliths brought to the surface from a depth of 55 km to 70 km by Cenozoic potassic volcanism in Sailipu in the north-western Lhasa terrane show lattice preferred orientation of their olivine with a concentration of [010] directions and a large circle girdle of the [100] and [001] directions defining an important fabric plane with a polarization anisotropy of 4.6% on average in directions parallel to the plane. In contrast, the polarization anisotropy is very weak in the direction perpendicular to this fabric plane. According to the seismic anisotropy of the peridotite xenoliths, to match the delay times ranging from 1.5 s to more than 2.0 s in the Lhasa and Qiangtang terranes would require an anisotropic layer at least 150 km to 200 km thick even if the fabric plane with maximum seismic anisotropy were oriented vertically and the effect of the intrinsic elastic anisotropy of the peridotites on SKS splitting were maximized. This is unrealistic, and we propose to attribute the large delay times to an extraordinarily strong seismic anisotropy of the lithospheric mantle underneath central Tibet. We propose that this anisotropy is due to the presence of partially molten domains with high aspect ratio that are aligned sub-vertically enhancing the effect of the intrinsic elastic anisotropy of the peridotites on SKS splitting. The abrupt change of the fast polarization direction south of the Banggong-Nujiang Suture may reflect the change of the fabric of olivine which is fundamentally different in the northward subducting Indian lithospheric mantle and in the lithospheric mantle beneath northern Tibet.

Determining the origin of mineral inclusions is a key problem in petrology. Assuming different inclusion formation mechanisms can lead to dramatically different petrological interpretations. Crystallographic orientation relationships (CORs), systematic relationships between the crystallographic orientations of crystals sharing boundary segments, are sensitive to the mechanisms of inclusion formation. Electron backscatter diffraction (EBSD) in the scanning electron microscope yields highly spatially resolved information about host-inclusion CORs. EBSD point analyses allow collection of large COR datasets, while retaining a link to the location of every measured inclusion and any shape preferred orientation (SPO) relative to host crystallography and microstructures. Based on combined COR, SPO and location information, we can differentiate between multiple origin hypotheses where COR formation is predicted, and the large number of measurements achievable allows observation of the relative frequency of different CORs.Acicular rutile inclusions in garnet with SPOs parallel to garnet crystal directions are often interpreted based on microstructures alone as products of exsolution, implying the existence of precursor Ti-bearing garnet. We studied rutile needles from metapegmatite garnets from two localities with separate geological histories. Rutile needles occur in zones that extend parallel to garnet {112} (both localities) and {110} (one locality) crystal planes. Needles are elongated parallel to (both localities) and (one locality) directions in the garnet hosts. The majority of needles show a “specific” (completely fixed) COR to the garnet host. Several different CORs can be found within a single garnet domain and the frequency of different CORs varies both between domains from the same locality and between localities. Despite the existence of several CORs, there is a systematic link between the rutile-garnet COR exhibited by a given needle inclusion and its elongation direction relative to the crystallography of both garnet and rutile.A comparison with literature datasets of CORs from garnets with acicular rutile inclusions reveals that both the type and frequencies of rutile-garnet CORs found in metapegmatite garnets differ strongly from those found in garnets of purely metamorphic origin. CORs judged to result in a poor alignment between rutile and garnet structures are considerably more frequent in the metapegmatite samples.In garnets from one locality, the SPO of rutile needles does not favour all crystallographically equivalent garnet directions equally. Instead, needles are preferentially elongated parallel to garnet directions at high angles to the garnet facets defined by inclusion zoning. SPO and COR of the rutile needles thus depend on the orientation of the growing garnet interface, which is incompatible with an exsolution origin for these inclusions. Oriented nucleation of rutile at the garnet interface and subsequent simultaneous growth of both phases can account for these observations.These results show the power of combining spatially resolved COR data with SPO information. An exsolution origin for rutile needles cannot be proposed based on needle SPO alone, and specific CORs are not necessarily indicative of an exsolution origin for rutile needles even if they occur together with an SPO relative to the garnet host.We acknowledge funding by the Austrian Science Fund (FWF): I4285-N37.

Magnetite micro-inclusions in silicate minerals are important carriers of the remanent magnetization of rocks. Their shape orientation relationships (SOR) and crystallographic orientation relationships (COR) to the host crystal are of interest in the context of the bulk magnetic properties of the inclusion-host assemblage. We investigated the SOR and COR of magnetite (MT) micro-inclusions in plagioclase (PL) from oceanic gabbro using correlated optical microscopy, scanning electron microscopy, Electron backscatter diffraction analysis and Transmission electron microscopy.In the mm-sized PL crystals of the investigated gabbros MT is present as equant, needle- and lath-shaped (sub)micrometer sized inclusions. More than 95% of the needle-shaped inclusions show SOR and specific COR to the plagioclase host. Most of the needles are elongated perpendicular to one of the MT{111} planes, which is aligned parallel to one of the (112), (1-12), (-312), (-3-12), (150), (1-50) or (100) planes of plagioclase. These inclusions are classified as “plane-normal type”. The needle elongation parallel to MT, which is the easy direction of magnetization, ensures high magnetic susceptibility of these inclusions. The underlying formation mechanism is related to the parallel alignment of oxygen layers with similar lattice spacing across the MT-PL interfaces that are parallel to the elongation direction [1].Apart from the SOR and the alignment of a MT{111} with one of the PL low index planes, the MT crystals rotate about the needle elongation direction. The rotation angles are statistically distributed with several maxima representing specific orientation relationships. In some cases one of the MT axes is aligned with PL[14 10 7] or PL[-14 10 -7], which ensures that FeO6 octahedra of MT well fit into channels // [001] of PL, which are formed by six membered rings of SiO4 and AlO4 tetrahedra [2]. This COR is referred to as the “nucleation orientation” of magnetite with respect to PL. There are several other possibilities to fit FeO6 octahedra into the [001] channels of PL, but the alignment stated above allows for the additional parallel alignment of one of the MT{111} with one of the above mentioned low index lattice planes of PL. MT crystals with one of these nucleation orientations can undergo directional growth to develop laths and needles. MT crystals with other nucleation orientations that do not allow for the parallel alignment of MT{111} with the above mentioned PL lattice planes, do not significantly grow and form the equant inclusions.For some needles one or more of the MT{011} planes that are parallel to the needle elongation direction, are aligned with low-index planes of plagioclase such as PL (112), PL(150), PL(1-50) etc., and form MT facets. This situation corresponds to achievement of the best possible match between the two crystal lattices. This can either be generated during primary growth or during re-equilibration of the micro-inclusions and the plagioclase host.Funding by RFBR project 18-55-14003 and Austrian Science fund (FWF): I 3998-N29 is acknowledged.Reference[1] Ageeva et al (2020) Contrib. Mineral. Petrol. 175(10), 1-16.[2] Wenk et al (2011) Am. Min. 96, 1316-1324

Magnetite (Mt) is the foremost carrier of rock natural remanent magnetization (NRM). Needle- and lath shaped Mt micro-inclusions in plagioclase (Pl) from gabbro often have systematic crystallographic- and shape orientation relationships (CORs, SORs) with the Pl host. The SORs of Mt leads to magnetic anisotropy which may bias the NRM of the Mt-Pl inclusion-host assemblage. Thus, the origin of the CORs and SORs between Mt and Pl is important for paleomagnetic reconstructions. In this context, the atomic structures of Mt-Pl interfaces are of particular interest.The CORs and SORs between Mt and Pl were reported earlier and the underlying systematics was revealed from correlated optical and scanning electron microscopy (SEM) including electron back scattered diffraction (EBSD) analyses [1] (and references therein). The so-called plane normal type Mt micro-inclusions extend parallel to the Mt direction, which is perpendicular to the densely packed Mt{222} oxygen layers that are parallel to one of seven Pl lattice planes with nearly identical d-spacings, namely Pl(112), Pl(-312), Pl(1-50), Pl(150), Pl(100), Pl(31-2) and Pl(1-12). Direct imaging of Mt-Pl interfaces has rarely been reported due to the beam sensitivity of Pl. Here we present the microscopic structure of a Mt-Pl interface along the inclusion elongation direction using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast STEM (iDPC-STEM) techniques.The TEM foil was prepared using a focused Ga-ion beam (Ga-FIB) from a lath-shaped Mt micro-inclusion of 23 μm x 17 μm x 0.1 μm extending perpendicular to Mt{111}/Pl(-312). The foil is oriented so that the Mt/Pl(-312)-pole are parallel and Mt{110}/Pl(150) planes are perpendicular to the foil.The STEM images show that the Mt-Pl interface is perfectly straight and parallel to Mt{110}/Pl(150) and that it is devoid of steps. Electron diffraction patterns confirm that the elongation direction of the micro-inclusions is determined by the good fit of oxygen layers across the Pl-Mt interface. A 2.4% difference in the d-spacings between Pl(-312) and Mt{222} is likely accommodated by every about 42'nd Mt{222} plane forming an edge dislocation at the Mt-Pl interface. In addition, elastic strain is indicated by a deviation of d111/d110 of Mt from the strain free reference lattice. Moreover, lattice fringes in iDPC-STEM images reveal coherence between Pl(22-1) and Mt{111} planes without misfit dislocations. This additional coherence may explain the particularly strong alignment of Mt{111} and Pl(-312) reflected by the EBSD data.In summary, the elongation directions of the Mt inclusions are determined by the alignment of important oxygen layers of both phases across the Mt-Pl interface, which is parallel to oxygen-rich lattice planes in both phases. Misfit dislocations are presumably introduced to compensate the 2.4% lattice misfit along the elongation direction. The well-organized interface structure ensures a low interfacial energy and is a viable explanation for the observed Mt-Pl CORs and SORs. AcknowledgementFunding by FWF project I 3998-N29 and RFBR project 18-55-14003 is acknowledged.Reference[1] Ageeva et al (2020) Contrib. Mineral. Petrol. 175(10), 1-16.

Feldspar is an important constituent of airborne mineral dust. Some alkali feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (101‾0) prism planes and the (100) planes of alkali feldspar. Here the effect of generating feldspar surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of alkali feldspar was investigated experimentally. To this end, gem-quality K-rich alkali feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl–KCl salt melt at 850 ∘C. By this procedure, a system of parallel cracks with an orientation close to the (100) plane of the feldspar was induced. Droplet-freezing assay experiments performed on grain mounts of the cation-exchanged alkali feldspars revealed an increase in the overall density of ice-nucleating active site (INAS) density with respect to the untreated feldspar. In addition, annealing at 550 ∘C subsequent to primary cation exchange further enhanced the INAS density and led to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange with an alkali halide salt is unlikely to be of relevance in the conditioning of alkali feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous alkali feldspar, a widespread phenomenon in natural alkali feldspar known as perthite formation. Perthitic alkali feldspars indeed show the highest IN activity. We tentatively ascribe this phenomenon to the preferential exposure of feldspar crystal surfaces oriented sub-parallel to (100).

Plagioclase hosted needle- and lath-shaped magnetite micro-inclusions from oceanic gabbro dredged at the mid-Atlantic ridge at 13° 01–02′ N, 44° 52′ W were investigated to constrain their formation pathway. Their genesis is discussed in the light of petrography, mineral chemistry, and new data from transmission electron microscopy (TEM). The magnetite micro-inclusions show systematic crystallographic and shape orientation relationships with the plagioclase host. Direct TEM observation and selected area electron diffraction (SAED) confirm that the systematic orientation relations are due to the alignment of important oxygen layers between the magnetite micro-inclusions and the plagioclase host, a hypothesis made earlier based on electron backscatter diffraction data. Precipitation from Fe-bearing plagioclase, which became supersaturated with respect to magnetite due to interaction with a reducing fluid, is inferred to be the most likely formation pathway. This process probably occurred without the supply of Fe from an external source but required the out-diffusion of oxygen from the plagioclase to facilitate partial reduction of the ferric iron originally contained in the plagioclase. The magnetite micro-inclusions contain oriented lamellae of ilmenite, the abundance, shape and size of which indicate high-temperature exsolution from Ti-rich magnetite constraining the precipitation of the magnetite micro-inclusions to temperatures in excess of ~ 600 °C. This is above the Curie temperature of magnetite, and the magnetic signature of the magnetite-bearing plagioclase grains must, therefore, be considered as the thermoremanent magnetization.

Plagioclase hosted, oriented magnetite micro-inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks. Understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements. The systematics of the shape and crystallographic orientation relationships between needle- and lath-shaped magnetite micro-inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements, scanning electron microscopy, and crystal orientation analysis by electron backscatter diffraction. The magnetite inclusions show preferred shape orientations following six well-defined directions and with specific crystallographic orientation relationships to the plagioclase host. These relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase, which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases. The parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite, which occurs by the accommodation of FeO6 octahedra in six-membered rings of SiO4 and AlO4 tetrahedra of the plagioclase structure. The orientation systematics of the magnetite micro-inclusions is related to four orientation variants for placing the FeO6 octahedra into the plagioclase structure.

We present a model for multicomponent diffusion in ionic crystals. The model accounts for vacancy-mediated diffusion on a sub-lattice and for diffusion due to binary exchange of different ionic species without involvement of vacancies on the same sub-lattice. The diffusive flux of a specific ionic species depends on the self-diffusion coefficients, on the diffusion coefficients related to the binary exchanges, and on the site fractions of all ionic species. The model delivers explicit expressions for these dependencies, which lead to a set of coupled non-linear diffusion equations. We applied the model to diffusion of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{23}$$\end{document}23Na, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{39}$$\end{document}39K, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{41}$$\end{document}41K in alkali feldspar. To this end, gem-quality crystals of alkali feldspar were used together with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{41}$$\end{document}41K doped KCl salt as diffusion couples, which were annealed at temperatures between 800\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document}∘ and 950\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^\circ$$\end{document}∘C. Concentration-distance data for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{23}$$\end{document}23Na, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{39}$$\end{document}39K, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{41}$$\end{document}41K were obtained by Time of Flight Secondary Ion Mass Spectrometry. Over the entire investigated temperature range the Na self-diffusion coefficient is by a factor of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge 500$$\end{document}≥500 higher than the K self-diffusion coefficient. Diffusion mediated by binary \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{39}$$\end{document}39K–\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{41}$$\end{document}41K exchange is required for obtaining satisfactory fits of the model curves to the experimental data, and the respective kinetic coefficient is well constrained.

Mt. Etna is one of the most protrusive features of the eastern coastline of Sicily, Italy. As Europe’s most active volcano it has been studied extensively to reveal its geodynamic setting, plumbing system and due to the constant monitoring of the volcano edifice the prediction of the risk future events is sophisticated at Mt. Etna. The eruptive activity has been divided according to the age into 6 stages: (1) “Tholeiitic Stage”, was active between 600-320 ka ago, (2) the “Timpe Stage” between 220 and 110 ka ago, (3) the “Ancient Alcaline Volcanism” between 110 and 65 ka ago and (4) the “Ellittico Stage” between 57 and 15 ka ago (5) the “Mongibello Stage” from 15 ka ago until 1971 and (6) the “post -1971 Stage” active since 1971 (Casetta et al., 2019). The lava propagating through the Etnean plumbing system generated a complex network consisting of sills and dykes responsible for the formation of the summit craters and a plethora of eccentric cones that cover the flanks of the volcano. We studied using whole rock and mineral analyses the lavas from three eccentric cones (Monte Maletto, Monte Nuovo and Monte Frumento) and the 2001 eruption on the south flank from the main crater. All lavas are characterized by trachytic texture with variable modal composition of olivine, clinopyroxene and plagioclase phenocrysts. The Monte Maletto whole rock composition with an Mg# ranging between 56-58 and a CaO content of 12.0 wt% are the most primitive lavas among the sampled outcrops whereas the Monte Frumento lavas are the most evolved since the Mg# ranges from 43 to 46 and the CaO content from 9.5 to 10.8 wt%. Both, Monte Nuovo and 2001 eruption are more evolved than the Monte Maletto since they have Mg# ~ 50 and 51.5-52.9 respectively. The CaO concentration in both outcrops is relatively constant ranging from 9.8 to 10.7 wt%. The olivine compositions follow the same trend as their whole rocks. The most MgO-rich olivine (Fo=87.5 %) found in the Monte Maletto lavas. This olivine is of magmatic origin and cannot be considered as mantle derived xenocryst since the NiO content is low (NiO=0.16 wt%) and the CaO-content high (CaO=0.22 wt%). The most evolved lavas from Monte Frumente have the lowest Fo-content (Fo=64-68 %). Olivine from both, Monte Nuovo and 2001 eruption have a characteristic inverse zonation with Fo-content in the core ranging from 69.9 to 75 and in the rim from 78.2 to 81.7 respectively. In conclusion, the Monte Maletto lavas represent the most primitive magma formed at high temperatures (skeletal growing of the olivine) and the Monte Frumento lavas the most evolved magma. The Monte Nuovo and 2001 eruption experienced magma mixing as inferred from the olivine inverse zonation. Monte Nuovo can be considered a flank eruption of lava deviated from the central conduit rather than an eccentric cone. Casetta, Federico, et al. "The evolution of the mantle source beneath Mt. Etna (Sicily, Italy): from the 600 ka tholeiites to the recent trachybasaltic magmas." International Geology Review (2019): 1-22.

Calcite powder aggregates were deformed to shear strains up to 80 by high-pressure torsion at temperatures of 235 and 450 °C and confining pressures of 1–4 GPa. At 235 °C the deformation occurs by twinning and limited dislocation creep as well as by brittle processes. In contrast, subgrain rotation recrystallization is the predominant deformation mechanism at 450 °C. The high confining pressures affect the microstructural evolution by inhibiting crack nucleation and propagation and by constraining the mobility of point defects. While at 1.2 GPa confining pressure transient core-mantle microstructures are produced for all strain-rates, the recrystallization is strain-rate dependent at 2–4 GPa confining pressure. Core-mantle microstructures are produced at high strain-rates, whereas the porphyroclasts deform more homogeneously at low strain rates. This change in microstructural evolution is ascribed to the phase transition from calcite to CaCO 3 -II at 1.6 GPa. Within the stability field of CaCO 3 -II an increase in the confining pressure leads to reduction of the recrystallized grain size. Irrespective of the confining pressure a strong crystallographic preferred orientation is developed during deformation at 450 °C, which is compatible with dislocation glide on the r { 10 1 ¯ 4 } 〈 2 ¯ 021 〉 slip systems.

Mt. Etna is one of the most protrusive features of the eastern coastline of Sicily, Italy. As Europe’s most active volcano it has been studied extensively to reveal its geodynamic setting, plumbing system and due to the constant monitoring of the volcano edifice the prediction of the risk future events is sophisticated at Mt. Etna.The eruptive activity has been divided according to the age into 6 stages: (1) “Tholeiitic Stage”, was active between 600-320 ka ago, (2) the “Timpe Stage” between 220 and 110 ka ago, (3) the “Ancient Alcaline Volcanism” between 110 and 65 ka ago and (4) the “Ellittico Stage” between 57 and 15 ka ago (5) the “Mongibello Stage” from 15 ka ago until 1971 and (6) the “post -1971 Stage” active since 1971 (Casetta et al., 2019).The lava propagating through the Etnean plumbing system generated a complex network consisting of sills and dykes responsible for the formation of the summit craters and a plethora of eccentric cones that cover the flanks of the volcano.We studied whole rock and mineral chemistry of the lavas from three eccentric cones (Monte Spagnolo, Monte Fiori and Monte Rossi) and the 2002/2003 southern flank lava flow. All lavas are characterized by trachytic texture with variable modal composition of olivine, clinopyroxene and plagioclase phenocrysts. Euhedral and skeletal olivine phenocrysts can be distinguished into three main groups; a) normal zoning, b) inverse zoning, and c) patchy appearance with melt inclusions of andesitic and trachytic composition. The Monte Spagnolo whole rock composition has an Mg# ranging between 52-54 and 10.7 wt% CaO , being are the most primitive lavas among the sampled outcrops whereas the Monte De Fiore lavas are the most evolved since the Mg# ranges from 48.6 to 49.2 and the CaO content from 11 to 11.2 wt%. Both, Monti Rossi and the 2002/2003 lava flow are more evolved than the Monte Spagnolo since they have Mg# ~ 50 and 49-49.3 respectively. The CaO concentration in both outcrops is relatively constant ranging around 10.5 wt%.The olivine compositions follow the same trend as their whole rocks. The most MgO-rich olivine (Fo=88.9 %) was found in the Monte Spagnolo lavas. This olivine is of magmatic origin and cannot be considered as mantle derived xenocryst since the NiO content is low (NiO=0.17 – 0.2 wt%) and the CaO-content high (CaO=0.24 – 0.26 wt%). The most evolved lavas from Monte De Fiore have the lowest Fo-content (Fo=75 - 78 %). Olivine from all samples has a characteristic inverse zonation with, at Monti Rossi and 2002/2003 lava flow, Fo-content in the core ranging from 69% to 75% and in the rim from 77% to 80% respectively.In conclusion, the studied eccentric cones show extensive magma mixing as can be inferred from the olivine inverse zoning. Monte Spagnolo lavas represent the most primitive magma formed at high temperatures (olivine skeletal growing) and the Monte De Fiore lavas the most evolved magma. Casetta et al., 2019. International Geology Review, DOI: 10.1080/00206814.2019.1610979

Microinclusions of Fe-Ti-oxides in rock-forming plagioclase are protected from subsea-floor alterations by the silicate matrix and are stable carriers of the paleomagnetic record of oceanic gabbros. We studied plagioclase-hosted microinclusions in oceanic gabbro (“gabbro 1241” from the Vema Lithospheric section, Mid-Atlantic ridge, Pertsev et al., 2015) with a complex petrogenetic history. Important events in the gabbro evolution caused consecutive transformations of the micro-inclusions and presumably affected their paleomagnetic records.The earliest generation of the microinclusions was present as ulvospinel and presumably titanomagnetite which were probably formed by sub-solidus oxidation exsolution of early-magmatic plagioclase (An#42-45). An increase of the anorthite component around the early generation of microinclusions to typical values of late-magmatic plagioclase (An#53) suggest involvement of the late-magmatic fluid accompanying residual melt at 800-900°С (Pertsev et al., 2015). Subsequently, the gabbro was locally affected by hydrothermal alteration at about 600°С as a result of interaction of the gabbro with reduced brine containing 20-21% NaCl. The reducing conditions of this process ensured a “non-oxidative” character and primarily cooling driven exsolution of the microinclusions and transformation of the titanomagnetite microinclusions into ulvospinel-magnetite (“Usp-Mt1”) intergrowths at about 500°С, which is close or higher than the Curie temperature (Tc) of the exsolving titanomagnetite but lower than the Tc of the newly forming Mt1, and the acquired magnetisation may be referred to chemical remanence. The further evolution of the micro-inclusions correlates with low-temperature hydrothermal alteration induced by inflow of seawater-derived fluids during tectonic unroofing of the lithospheric section (Pertsev et al., 2015). The homogeneous ulvospinel inclusions and ulvospinel of the “Usp-Mt1”-inclusions were replaced by “Ilm-Mt2” -aggregates under more oxidizing conditions: 3 Fe2TiO4(Ulv) + 0.5 O2= 3 FeTiO3(Ilm) + Fe3O4(Mt2). The Mt1 was more stable to increase of fO2 that resulted in simultaneous presence of the “Ilm-Mt2”-, “Mt1/Ilm-Mt2“- and “Usp-Mt1” -inclusions with two generations of magnetic phases (magnetite) within a single plagioclase grains.Thus, despite of protection by silicate matrix the microinclusions of Fe-Ti-oxides in rock-forming plagioclases evolve under the influence of petrogenetic processes. It is important to note that bulk-rock AF demagnetization of 14 specimens (extracted from “gabbro-1241“) in the interval 100-250 Oersted (Oe) and 300-700 Oe revealed both moderately-grouped (k=27.4) and variable (deviated with angle 40-104°) directional components of magnetisation, which may have resulted from the presence of different generations of magnetite. Further magnetic investigation of separates of plagioclase single grains will allow to evaluate capacities of plagioclase-hosted Fe-Ti-micro-inclusions to save initial and “newfound” paleomagnetic information and to serve as stable sources of paleomagnetic record in regions of mid-oceanic ridges.Funding by RFBR project 18-55-14003 and FWF project I 3998-N29 is acknowledged.Pertsev, A. N., Aranovich, L. Y., Prokofiev, V. Y., Bortnikov, N. S., Cipriani, A., Simakin, S. S., & Borisovskiy, S. E. (2015). Signatures of residual melts, magmatic and seawater-derived fluids in oceanic lower-crust gabbro from the Vema lithospheric section, Central Atlantic. Journal of Petrology, 56(6), 1069-1088.

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al2O3/MgAl2O4/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl2O4layers have been grown between Al2O3and MgO, and the atomic structure of Al2O3/MgAl2O4interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al2O3to cubic close-packed (c.c.p.) stacking in MgAl2O4. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al2O3/MgAl2O4interface into Al2O3is accomplished by the glide of partial dislocations accompanied by the exchange of Al3+and Mg2+cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl2O4grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl2O4grains compete with each other, leading to a growth selection and successive coarsening of the MgAl2O4grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.

The chemically driven propagation of interacting parallel cracks in monoclinic alkali feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl–KCl salt melt at a temperature of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$850\,^{\circ }\hbox {C}$$\end{document}850∘C and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium–potassium interdiffusion within the feldspar. A chemical shift of potassium-rich alkali feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8–2.2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$~ \hbox {J}\,\hbox {m}^{-2}$$\end{document}Jm-2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature feldspar alteration.

Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbekalla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting. The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism. The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration. The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from the host basalt. The estimated time for diffusive Ca exchange between matrix olivine and olivine overgrowth in contact with the melt pockets is very short, ranging between 21 and 200 days, indicating that amphibole breakdown took place immediately before or during the xenolith entrainment in the alkali basalt.

We used atom probe tomography to complement electron microscopy for the investigation of spinodal decomposition in alkali feldspar. To this end, gem-quality alkali feldspar of intermediate composition with a mole fraction of $$a_{\text {K}}=0.43$$ a K = 0.43 of the K end-member was prepared from Madagascar orthoclase by ion-exchange with (NaK)Cl molten salt. During subsequent annealing at $$550\,^\circ \hbox {C}$$ 550 ∘ C and close to ambient pressure the ion-exchanged orthoclase unmixed producing a coherent lamellar intergrowth of Na-rich and K-rich lamellae. The chemical separation was completed, and equilibrium Na–K partitioning between the different lamellae was attained within four days, which was followed by microstructural coarsening. After annealing for 4 days, the wavelength of the lamellar microstructure was $$\approx 17\,\hbox {nm}$$ ≈ 17 nm and it increased to $$\approx 30\,\hbox {nm}$$ ≈ 30 nm after annealing for 16 days. The observed equilibrium compositions of the Na-rich and K-rich lamellae are in reasonable agreement with an earlier experimental determination of the coherent solvus. The excess energy associated with compositional gradients at the lamellar interfaces was quantified from the initial wavelength of the lamellar microstructure and the lamellar compositions as obtained from atom probe tomography using the Cahn–Hilliard theory. The capability of atom probe tomography to deliver quantitative chemical compositions at nm resolution opens new perspectives for studying the early stages of exsolution. In particular, it helps to shed light on the phase relations in nm scaled coherent intergrowth.

The chemo-mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally. Dense calcite-periclase aggregates with

To understand how a new phase forms between two reactant layers, MgAl2O4 (spinel) has been grown between MgO (periclase) and Al2O3 (corundum) single crystals under defined temperature and load. Electron backscatter diffraction data show a topotaxial relationship between the MgO reactant and the MgAl2O4 reaction product. These MgAl2O4 grains are misoriented from perfect alignment with the MgO substrate by similar to 2-4 degrees, with misorientation axes concentrated in the interface plane. Further study using atomic resolution scanning transmission electron microscopy shows that in 2D the MgAl2O4/MgO interface has a periodic configuration consisting of curved segments (convex towards MgO) joined by regularly spaced misfit dislocations occurring every similar to 4.5nm (similar to 23 atomic planes). This configuration is observed along the two equivalent [100] directions parallel to the MgAl2O4/MgO interface, indicating that the 3D geometry of the interface is a grid of convex protrusions of MgAl2O4 into MgO. At each minimum between the protrusions is a misfit dislocation. This geometry results from the coupling between long-range diffusion, which supplies Al3+ to and removes Mg2+ from the reaction interface, and interface reaction, in which climb of the misfit dislocations is the rate-limiting process. The extra oxygen atoms required for dislocation climb were likely derived from the reactant MgO, leaving behind oxygen vacancies that eventually form pores at the interface. The pores are dragged along by the propagating reaction interface, providing additional resistance to interface motion. The pinning effect of the pores leads to doming of the interface on the scale of individual grains.

A thermodynamic model has been developed for interlayer growth in a binary system between two phases of fixed composition producing an intermediate solid-solution phase. Thereby long-range diffusion, interface migration and generation/annihilation of vacancies at the reaction interfaces have been considered as potentially rate limiting. The coupling among these processes governs overall growth rate, position of the Kirkendall plane and the compositions of the solid-solution phase at the reaction interfaces. Model calculations illustrating the relations between the corresponding kinetic parameters and system evolution are presented. In particular, the systematics of non-equilibrium element partitioning across moving reaction interfaces is addressed. It is found that the deviation from equilibrium element partitioning at a moving reaction interface is a more sensitive monitor for the departure from local equilibrium than the deviation from parabolic growth behavior. Finally, the model is applied to interlayer growth of magnesio-aluminate spinel.

Crystallographic orientation relationships (CORs) between mineral inclusions and their hosts could potentially deliver information about inclusion formation processes and conditions. Most previous studies are based on small numbers of analyses. This paper uses EBSD to study host–inclusion CORs in an inclusion-rich Permian metapegmatite garnet (Koralpe region, Eastern Alps, Austria), demonstrating the importance of large data sets and of EBSD in particular for the analysis of CORs. The distribution of measured orientations reflects host garnet point group symmetry for 89% of inclusions analyzed (total N = 530). Each inclusion phase (rutile, corundum, and ilmenite) shows at least three different CORs to host garnet. “Statistical” CORs are introduced to describe distributions of inclusion orientations that have one or two degrees of freedom with respect to the host, but still reflect host crystal symmetry. Two end-member characteristics of statistical CORs are distinguished: rotation and dispersion. Most statistical CORs observed show a mixture of both. Each inclusion phase shows at least one statistical COR. Multiple coexisting CORs and statistical CORs are not restricted to rutile. Re-examination of previous garnet–rutile COR studies in light of the new results indicates that COR information may have been overlooked when using small data sets. Variation in COR parameters correlates with broad differences in assumed metamorphic conditions for new and literature samples, suggesting that petrogenetic information may be available if COR formation can be understood. The favorability of the detected CORs cannot be explained by a simple model involving minimization of misfit between lattice planes, implying that other interface properties or the inclusion formation mechanism are important controls on COR development.

The use of iron ores with high gangue content in industrial sintering leads to varying sinter quality; this also applies when formation of calcium-ferrites is optimized to enhance sinter strength. In this study, pot sinters containing calcium-aluminosilicoferrites as a matrix phase were tested for their mechanical properties. It was found that the presence of the ferrite matrix had a positive effect on sinter strength, which was also influenced by the chemical compositions of the ferrites. Nanoindentation measurements on custom-synthesized calcium-aluminosilicoferrite crystals revealed a correlation between the alumina content of the ferrite phase and its mechanical properties, which ultimately also controls sinter strength. Both the modulus and the hardness of the SFCA phase increased with further incorporation of Al2O3. However, with increasing hardness the SFCA phase became more brittle, and this is a cause of bulk sinters becoming weaker. There was however no detectable effect of silica or other minor components on the mechanical properties of the ferrites.