Augite Crystals Research Articles

Preparation of CaO-MgO-Al2O3-SiO2 glass-ceramic with a high content of Cr2O3 using the spark plasma sintering (SPS)

2, 6, or 10 wt% of Cr2O3 was added into the CaO-MgO-Al2O3-SiO2 precursor glasses (PG), aiming to investigate the effects of high contents of Cr2O3 on the crystallization behavior and mechanical properties of glass-ceramics (GCs). The PGs were milled into powder and then sintered by spark plasma sintering (SPS) technology at 750∼850 °C under the pressure of 10 MPa. It was found that Cr2O3 induced the precipitation of spinel nuclei in the PGs, which served as the heterogeneous nucleation sites for the augite crystals. Augite and spinel were the main phases of the GCs, and the crystallinity increased by increasing the Cr2O3 addition amount and sintering temperature. The GC prepared at 850 °C with 6 wt% Cr2O3 addition showed excellent bending strength, with Vickers hardness, and fracture toughness of 214 MPa, 8.2 GPa, and 1.3 MPa·m1/2, respectively.

Utilization of waste glass with natural pozzolan in the production of self-glazed glass-ceramic materials

Abstract Significant self-glazed glass-ceramic was obtained from a natural pozzolan and external glass cullet. Natural pozzolan with/without glass cullet was fused to glass melt that quenched in water to glass frits. The dried glass frits were pulverized (<0.083 mm) and then shaped in a stainless mold. The thermal behavior of the glasses shows a widening of the crystallization temperature with the incorporation of the glass cullet between ∼800 and 950°C. Sintering of the shaped glass powder at 1,050°C/2 h lead to the crystallization of augite, enstatite, cristobalite, and hematite. The microcrystalline structure shows massive texture with pores in-between; however, at high magnification regular euhedral to subhedral crystals in submicron to nearly submicron size was developed in the glassy matrix and their microanalysis indicates the dominant augite. The density of the glass-ceramic samples decreases from 2,706 to 2,539 kg/m3 with the incorporation of glass wastes. The sample surfaces show, through force electron microscopy, the fineness and smoothness of the grains with the incorporation of the glassy portion. The microhardness values were between 6.43 and 6.11 GPa. The coefficient of thermal expansion increased from 32.87 (25–300°C) to 66.89 (25–500°C) × 10−7°C−1. The chemical resistance of samples in water (0.0002–0.0016) is better than in an acidic medium (0.0011–0.0017). These glass-ceramic enjoy good density, hardness, and thermal expansion and can be used in the ceramic industry and cladding walls and floors.

A Case Study on Distribution of Gemstone in Bela Ophiolite, District Khuzdar, Balochistan

Bela Ophiolite and suture zone host several types of gemstones in the underlain and overlain rocks such as garnet (demantoid and almandine), quartz, brucite, malachite, chrysocolla, azurite, calcite, natrolite, green chert, jasper, serpentine, agate, epidote, augite and prehnite. However, the rare gemstones including demantoid garnet are found as dodecahedron green crystals associated with the altered peridotite rocks such as serpentinite. Brucite is found in the botryoidal habit in the ultramafic rocks of Bela Ophiolite associated with the chromite mines in the Baran Lak and Amorzai area of Wadh in the host rock dunite. The serpentine is found in several locations associated with the altered ultramafic rocks formed after the serpentinization of peridotite. Furthermore, one of the rare gemstones such as petroleum and diamond quartz is formed due to the hydrothermal intrusion in the sedimentary rocks such as limestone; calcite also occurs in limestone as dogtooth crystals. While azurite, malachite and chrysocolla are formed as secondary deposits in the oxidized zone of copper and sulphide mines in the Manjawi, Nal and Sonaro associated adjacent to basalt. Moreover, epidote is found as green crystals having porphyritic texture in the host rock granite mainly in the Bakhalo area, Wadh. Agate, jasper, green chert and several other kinds of chert are found associated with the basalt chert unit in the mélange and gossan. In addition to that natrolite is found in the Nal ophiolite mainly in the Hazarghanji town found associated with the cracks and vugs of dolerite dykes as fibrous and cylindrical crystals. In metamorphic sole rocks of Bela Ophiolite in the Sonaro area, marble and quartzite are associated with the metamorphic rocks such as greenschist and amphibolite facies that are formed during the e volution period of the ophiolite. In pegmatite gabbro, the elongated and prismatic black crystals of augite are found in the host rock gabbro near the Ornach cross area, and the prehnite gemstone is found associated with the basalt and gabbroic rocks transition zone in the Ornach area.

Insights into the petrogenetic history of the Northwest Africa 7635 augite‐rich shergottite

AbstractThe petrogenesis of the Northwest Africa (NWA) 7635 Martian meteorite involved the entrainment of xenocrystic olivine grains into a relatively magnesian and oxidized melt, followed by a redox‐dependent reaction between olivine and melt that resulted in the crystallization of orthopyroxene and magnetite. Subsequent crystallization of the melt began with augite, plagioclase, and magnetite phenocrysts, and was followed by crystallization of augite, plagioclase, magnetite, ilmenite, and pyrrhotite in the groundmass, which took place under more rapid conditions of cooling, as reflected in the groundmass grain size. The petrogenetic history of NWA 7635 is similar in many ways to that of NWA 8159; this observation, coupled with similarities in geochemical and isotopic characteristics from other studies, suggests that the parent melts of the two rocks—as represented by all minerals except the xenocrystic olivine—were one and the same. The main distinctions between the two rocks are that their parent melts entrained xenocrystic olivine of different composition, and the cooling rate of the groundmass of NWA 7635 was more rapid than that of NWA 8159. The conclusion that the redox reaction took place between olivine and melt is in contrast to other work that suggests the reaction took place in the subsolidus, and has implications for the nature of the reaction in both NWA 7635 and NWA 8159.

The monotonous intermediate magma system of the Permian Wurzen caldera, Germany: Magma dynamics and petrogenetic constraints for a supereruption

The Lower Permian Wurzen caldera formed during a VEI 8 supereruption that tapped monotonous intermediate magmas in a continental rift. The well-preserved crystal-rich intracaldera ignimbrite (<58 vol%) and its related plutonic facies allow for compositional and textural studies on juvenile fragments and mineral phases, providing a unique opportunity to unravel the caldera's pre-eruptive evolution.The climactic Wurzen ignimbrite contains different fiamme of fine-grained mafic, fine-grained rhyolitic (71.8 to 76.6 wt% SiO2), porphyritic (trachy-) dacitic (64.5 to 67.6 wt% SiO2), and coarse porphyritic rhyodacitic composition (64.7 to 74.3 wt% SiO2), which point to a permanent interplay between rejuvenation, differentiation, and cooling in the Wurzen magma body. The similarity of their trace element concentrations suggests derivation from a shallow magma chamber (Nb and Ta at 22 and 1.6 ppm, respectively). The picture becomes more differentiated when considering mineral composition and thermobarometric estimations. Resorption textures in anorthitic plagioclase (An >45 mol%) are presumably formed by excess heat from underplating magmas. Coarse sieve textures in plagioclase indicate a rapid ascent of magma into the shallow magma body. In contrast, oscillatory zonation and rapakivi texture in quartz, albitic plagioclase, and sanidine indicate convection during crystal growth. The spatially discrete crystallization of augite and pigeonite indicates tapping of anhydrous Ca-saturated and -undersaturated magma. The presence of pigeonite indicates eruption from a superheated anhydrous (trachy-) dacitic magma batch at temperatures of 1010 °C, whereas the occurrence of annite implies simultaneous tapping of hydrous shallow magma chambers at low temperatures (~750 °C). By applied barometers on clinopyroxene and biotite, it suggests a deep-seated magma chamber at depths of 25 to 15 km and a shallow magma chamber at ca. 11 km, respectively.Mineral assemblage of the intracaldera Wurzen ignimbrite (quartz, sanidine, plagioclase, calcic and calcic-sodic clinopyroxene, pigeonite, annite) is more typical for crystal-poor, hot rhyolites like in the Snake River Plain (Idaho, USA) or Messum igneous complex (Namibia), than that of monotonous intermediates at active continental margins.

The formation of three-grain junctions during solidification. Part II: theory

We provide a simple geometric theory of crystal growth which predicts the shape and final dihedral angle of three-grain junctions of an augite crystal with two plagioclase grains. The predicted dihedral angle Delta depends on the initial impingement angle psi formed by the plagioclase grains, and also on the relative growth rates of the augite and the plagioclase, and shows reasonable agreement with data obtained from natural samples. We show that the two augite-plagioclase grain boundaries will normally curve towards each other, which is consistent with the first two types of junction described in the companion paper. However, the third type, the eagle’s beak, is formed by the meeting of grain boundaries which curve in the same direction. Although it is possible to account for this type of junction by invoking the localised dissolution of one of the plagioclase grains, this is unlikely to occur. A more plausible explanation involves the late impingement of the two plagioclase grains, consistent with the observation that eagles’ beaks are common in gabbros and strongly orthocumulate troctolites, in which the plagioclase framework has not been established by the time augite is growing in substantial quantities. An observed flattening of the curve of Delta values at high values of psi can be explained by taking into account the importance of interfacial energy in late-stage crystallisation.

Naryn-Gol Creek Sapphire Placer Deposit, Buryatia, Russia

A new gem corundum occurrence has been discovered in the Naryn-Gol Creek placer of the Dzhida volcanic field (Russia). In this placer deposit, sapphire associates with large crystals of garnet, spinel, augite, olivine, enstatite, ilmenite, Ti-magnetite, and alkali feldspar. Such a combination of minerals is typical for the placer deposits associated with alkali basalts widely distributed in Southeastern Asia and Australia. We have also found sapphire crystals in phonotephrites of the nearby Cenozoic alkali-basalt paleovolcano Barun Khobol Pravyi, and in basalt sample and trachybasalt from the valley flood basalts. The chemical composition of sapphire is generally typical for ‘basalt’ corundum: it is rich in Fe, and depleted in Ti and Cr. The δ18O SMOW values of corundum and related megacrysts range from 4.6 to 6.8 ‰, thus corresponding to the isotopic signature of igneous rocks. Etched and corroded surfaces of sapphire and other megacrysts indicate that they are in non-equilibrium with their host alkali basalts. Volatile components, CO2 in particular, played a significant role during sapphire formation as gas inclusions reveal.

A Case Study on Distribution of Gemstone in Bela Ophiolite, District Khuzdar, Balochistan

Bela Ophiolite and suture zone host several types of gemstones in the underlain and overlain rocks such as garnet (demantoid and almandine), quartz, brucite, malachite, chrysocolla, azurite, calcite, natrolite, green chert, jasper, serpentine, agate, epidote, augite and prehnite. However, the rare gemstones including demantoid garnet are found as dodecahedron green crystals associated with the altered peridotite rocks such as serpentinite. Brucite is found in the botryoidal habit in the ultramafic rocks of Bela Ophiolite associated with the chromite mines in the Baran Lak and Amorzai area of Wadh in the host rock dunite. The serpentine is found in several locations associated with the altered ultramafic rocks formed after the serpentinization of peridotite. Furthermore, one of the rare gemstones such as petroleum and diamond quartz is formed due to the hydrothermal intrusion in the sedimentary rocks such as limestone; calcite also occurs in limestone as dogtooth crystals. While azurite, malachite and chrysocolla are formed as secondary deposits in the oxidized zone of copper and sulphide mines in the Manjawi, Nal and Sonaro associated adjacent to basalt. Moreover, epidote is found as green crystals having porphyritic texture in the host rock granite mainly in the Bakhalo area, Wadh. Agate, jasper, green chert and several other kinds of chert are found associated with the basalt chert unit in the mélange and gossan. In addition to that natrolite is found in the Nal ophiolite mainly in the Hazarghanji town found associated with the cracks and vugs of dolerite dykes as fibrous and cylindrical crystals. In metamorphic sole rocks of Bela Ophiolite in the Sonaro area, marble and quartzite are associated with the metamorphic rocks such as greenschist and amphibolite facies that are formed during the e volution period of the ophiolite. In pegmatite gabbro, the elongated and prismatic black crystals of augite are found in the host rock gabbro near the Ornach cross area, and the prehnite gemstone is found associated with the basalt and gabbroic rocks transition zone in the Ornach area.

Ferrotorryweiserite, Rh5Fe10S16, a New Mineral Species from the Sisim Placer Zone, Eastern Sayans, Russia, and the Torryweiserite–Ferrotorryweiserite Series

Ferrotorryweiserite, Rh5Fe10S16, occurs as small grains (≤20 µm) among droplet-like inclusions (up to 50 μm in diameter) of platinum-group minerals (PGM), in association with oberthürite or Rh-bearing pentlandite, laurite, and a Pt-Pd-Fe alloy (likely isoferroplatinum and Fe-Pd-enriched platinum), hosted by placer grains of Os-Ir alloy (≤0.5 mm) in the River Ko deposit. The latter is a part of the Sisim placer zone, which is likely derived from ultramafic units of the Lysanskiy layered complex, southern Krasnoyarskiy kray, Russia. The mineral is opaque, gray to brownish gray in reflected light, very weakly bireflectant, not pleochroic to weakly pleochroic (grayish to light brown tints), and weakly anisotropic. The calculated density is 5.93 g·cm–3. Mean results (and ranges) of four WDS analyses are: Ir 18.68 (15.55–21.96), Rh 18.34 (16.32–20.32), Pt 0.64 (0.19–1.14), Ru 0.03 (0.00–0.13), Os 0.07 (0.02–0.17), Fe 14.14 (13.63–14.64), Ni 13.63 (12.58–14.66), Cu 4.97 (3.42–6.41), Co 0.09 (0.07–0.11), S 29.06 (28.48–29.44), and total 99.66 wt.%. They correspond to the following formula calculated for a total of 31 atoms per formula unit: (Rh3.16Ir1.72Pt0.06Ru0.01Os0.01)Σ4.95(Fe4.48Ni4.11Cu1.38Co0.03)Σ10.00S16.05. The results of synchrotron micro-Laue diffraction studies indicate that ferrotorryweiserite is trigonal; its probable space group is R3¯m (#166) based on its Ni-analog, torryweiserite. The unit-cell parameters refined from 177 reflections are a = 7.069 (2) Å, c = 34.286 (11) Å, V = 1484 (1) Å3, and Z = 3. The c:a ratio is 4.8502. The strongest eight peaks in the X-ray diffraction pattern derived from results of micro-Laue diffraction study [d in Å(hkil)(I)] are 2.7950 (202¯5) (100); 5.7143 (0006) (60); 1.7671 (224¯0) (44.4); 3.0486 (202¯1) (39.4); 5.7650 (101¯2) (38.6); 2.5956 (202¯7) (37.8); 3.0058 (112¯6) (36.5); and 1.5029 (42¯ 2¯12) (35.3). Ferrotorryweiserite and the associated PGM crystallized from microvolumes of residual melt at late stages of crystallization of grains of Os- and Ir-dominant alloys occurred in lode zones of chromitites of the Lysanskiy layered complex. In a particular case, the residual melt is disposed peripherally around a core containing a disequilibrium association of magnesian olivine (Fo72.9–75.6) and albite (Ab81.6–86.4), with the development of skeletal crystals of titaniferous augite: Wo40.8–43.2En26.5–29.3Fs20.3–22.6Aeg6.9–9.5 (2.82–3.12 wt.% TiO2). Ferrotorryweiserite represents the Fe-dominant analog of torryweiserite. We also report occurrences of ferrotorryweiserite in the Marathon deposit, Coldwell Complex, Ontario, Canada, and infer the existence of the torryweiserite–ferrotorryweiserite solid solution in other deposits and complexes.

Characterizing a weathering profile over the Kuantan Basalt

Three broad morphological zones can be differentiated; the top pedological soil (Zone I) being 3.60 m thick and comprising brown, soft to stiff, clays. The intermediate saprock (Zone II) is 1.12 m thick and consists of brown, very stiff, sandy silt with many lateritic concretions, whilst the bottom bedrock (Zone III) is an outcrop of vesicular olivine basalt with weathering along joints. Constant volume samples show the saprolite (sub-zone IC) to have dry unit weights of 11.78 to 12.80 kN/m3, whilst the solum (sub-zones IA and IB), and saprock, have values ranging from 10.65 to 11.09, and from 11.35 to 11.50, kN/m3, respectively. Porosities are variable; the saprolite with the lowest values of 52 to 56% and the solum and saprock with values of 57 to 60%. Clay and silt contents increase up the profile with a corresponding decrease in sand and gravel contents. Colloid (&lt;1 μm size) contents especially increase up the profile from 10 to 15% in saprock through 30 to 40% in saprolite and exceeding 57% in the solum, These increasing colloid contents point to the increasing effects of pedological processes. Thin-sections of weathered rims (1-2 cm thick) show alteration of basalt to start with formation of micro-cracks (Stage 1) that become stained by secondary iron oxides and hydroxides. Decomposition of the essential minerals then occurs in the order: olivine (Stage 2), augite (Stage 3), and plagioclase feldspar (Stage 4). An increase in apparent porosity, but a decrease in unit weights and specific gravity, reflect these stages of weathering; the boundary between ‘rock’ and ‘soil’ material occurring when all olivine and augite crystals have decomposed. It is concluded that the weathering profile results from in situ alteration of basalt due to lowering of an unconfined groundwater table; pedological processes giving rise to further alteration.

Sapphire of the Naryn-Gol Creek Placer Deposit (Buryatia, Russia)

A new occurrence of gem corundum was discovered in the vicinity of Naryn-Gol Creek, on the Dzhida volcanic field, Russia. Sapphires were found in both alluvial deposits of the Naryn-Gol Creek and pyroclastic rocks of the nearby Cenozoic alkali basalt volcano Barun-Hobol Pravyi. These rocks contain, in addition to sapphire, large crystals of garnet, spinel, augite, olivine, enstatite, ilmenite, and feldspar. Chemical composition of the sapphires overall is typical for corundum from basalt: rich in Fe, low in Ti and Cr. The oxygen isotopic compositions of corundum and associated minerals show δ18Osmow values ranging between 4.6‰ and 6.8‰ that is attributed to magmatic rocks. Study of impurities reveals the great role of volatiles, CO2, in particular, during the formation process.

Idiomorphic oikocrysts of clinopyroxene produced by a peritectic reaction within a solidification front of the Bushveld Complex

Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.

Lanthanum modification of crystalline phases and residual glass in augite glass ceramics produced with industrial solid wastes

The effects of La2O3 on microstructure and properties of the CaO-MgO-SiO2-Al2O3 glass ceramics, made from Bayan Obo West Mine tailing and fly ash, were investigated. Results showed that La2O3 additions up to 2 wt% promoted augite crystallization. Above this concentration, La2O3 suppressed augite crystallization by rather forming lanthanum oxyapatite that acted as a barrier for the growth of augite. Also Cr ions, the key element for the augite nuclei, were enriched in lanthanum oxyapatite. The La3+ cations were found to partially substitute for Ca2+ at the M2 structural sites in clinopyroxene (augite). Nano crystalline clusters, containing La3+, Na+, Ca2+, Mg2+ and Fe3+ in augite-like proportions, were found uniformly dispersed in the residual glassy phase. The glass ceramic with 2 wt% La2O3 shows optimum mechanical properties and chemical durability. The bending strength and Vickers hardness are 238 MPa and 7.57 GPa, acid and alkali-resistance are 97.5% and 98.9%, respectively.

Effect of SiO2/MgO ratio on the crystallization behavior, structure, and properties of wollastonite-augite glass-ceramics derived from stainless steel slag

Wollastonite-augite glass-ceramics of the CaO–SiO2–Al2O3–MgO system were prepared from stainless steel slag as the main raw material. The effects of SiO2/MgO mass ratio on the crystallization behavior, crystalline phases, structure, and properties of the as-prepared stainless steel slag glass-ceramics were investigated by differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, and physicochemical property measurements. The results showed that with decreasing SiO2/MgO ratio, the crystallization activation energies of the samples decreased from 436.48 kJ/mol to 383.84 kJ/mol, and their crystallization indexes increased from 1.67 to 3.21; in addition, the augite crystals were gradually refined and their content increased. On the other hand, the amount of elongated wollastonite crystals decreased and the crystals gradually dispersed in the augite crystals. Furthermore, among all the prepared glass-ceramics, the one with a density of 3.08 g/cm3 exhibited the best mechanical properties with a bending strength of 176.21 MPa and microhardness of 8.81 GPa.

Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites

In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely and quantitatively, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite <c> axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their <c> axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette's is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ∼20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.

Origin and evolution of the early magmatism in the Oslo Rift (Southeast Norway): Evidence from multiple generations of clinopyroxene

This study presents in-situ major and trace element data on zoned clinopyroxenes in the oldest lavas (B1, ca. 300 Ma) in the Vestfold-Jeløya and Skien areas in the southern part of the Oslo Rift (southeast Norway), in addition to Sr-Nd data on clinopyroxene separates. The aim of the study is to throw light on the causal mechanism for the formation of the Oslo Rift, and the alkaline nature of the magmatism. Two different models have been proposed: (i) passive rifting caused by extension north of the Variscan Front, in which MORB-type melts became enriched through interaction with enriched sub-continental lithospheric mantle (SCLM), and (ii) active rifting in which the magmatic products were produced by partial melting of enriched sub-lithospheric plume material. Clinopyroxene in B1 basalts commonly consist of three types: (1) a resorbed, homogeneous Cr-Mg-rich diopside core (CrMgDi) strongly depleted in Zr-Hf and Yb-Lu, but enriched in MREE relative to LREE and HREE; (2) Cr-poor diopside (MgDi) that forms concentrically zoned overgrowth on the CrMgDi cores, and may form reaction rims on the CrMgDi; and (3) outer zones of augite. The concentrations of incompatible trace elements show a general increase from the CrMgDi, through the MgDi to the outer rims of augite (although reverse zoning is common). In the Vestfold-Jeløya area some CrMgDi cores have ɛNd300 that are significantly higher (≤+6.0) than found in any magmatic rock in the Oslo Rift (≤4.4), implying that the resorbed CrMgDi cores represent xenocrysts from a mantle source that was depleted up to the initiation of the Oslo rifting event: the subcontinental lithospheric mantle (SCLM). High LREE and MREE contents in the CrMgDi are caused by interaction with enriched host magma. The enriched trace element signatures of the B1 magmatism in the Vestfold-Jeløya area is thus a primary feature, as implied by the plume model (ii). In the Skien area, all clinopyroxene types, as well as the host B1 lavas, show HIMU Sr-Nd signatures, supporting former suggestions that the underlying SCLM has suffered local metasomatism during emplacement of the 580 Ma old Fen carbonatite complex. The MgDi in both areas formed by reaction between B1 magmas and CrMgDi xenocrysts, followed by crystallization from magnesian B1 melts undergoing fractional crystallization, magma mixing and assimilation of crustal melts. Progressive evolution of the B1 melts by these processes led to the crystallization of augite.

Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) – record of Middle to Late Jurassic arc-forearc system in the Tethyan subduction factory

In the Late Jurassic to Early Cretaceous ophiolite melange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rocks have been documented. They emerge as massive lavas made of augite, spinel, albite and secondary hydrous silicates (e.g., chlorite, epidote, prehnite, and pumpellyite). An established crystallization sequence (spinel→clinopyroxene→plagioclase±Fe-Ti oxides) was found to be typical for the boninite series from the suprasubduction zones (SSZ). Augite crystallization temperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as a plausible source of boninite parental lavas. Their whole-rock geochemistry is characterised by low Ti, P2O5, Zr, Y, high-silica, and high Mg# and Cr# values. Low and U-shaped REE profiles are consistent with the negative Nb-Ta, P and Ti anomalies indicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (eNd(T = 150 Ma) + 0.49 to +1.27) act as vestiges of mantle-wedge metasomatism. The mantle source was likely depleted by the MORB and IAT melt extraction and was contemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. This eventually gave rise to boninite lavas and an ultra-refractory harzburgite residiuum. The genesis of boninites is related to the Tithonian mature forearc setting that evolved from an intra-oceanic, Callovian to Oxfordian, island-arc environment. The Mt. Medvednica boninite rocks stand for the youngest SSZ-related Jurassic oceanic crust from the NW segment of the Dinaric-Vardar Tethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in the ophiolite zones of Serbia, Albania and Greece, the boninites of Mt. Medvednica strongly favours the single Tethyan oceanic basin that existed in this part of Europe during the Late Jurassic.

Ferri-fluoro-katophorite from Bear Lake diggings, Bancroft area, Ontario, Canada: a new species of amphibole, ideally Na(NaCa)(Mg4Fe3+)(Si7Al)O22F2

AbstractFerri-fluoro-katophorite is the second species characterised involving the rootname katophorite in the sodium–calcium subgroup of the amphibole supergroup. The mineral and its name were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification, IMA2015-096. It was found in the Bear Lake diggings, Bancroft area, Ontario, Canada, where coarse euhedral crystals of amphibole, phlogopite, sanidine solid-solution (now coarsely exsolved to microcline perthite), titanite, augite, zircon and fluorapatite crystallised from a low-viscosity silicocarbonatitic magma of crustal origin. Greenish grey prismatic crystals of ferri-fluoro-katophorite generally protrude from the walls into a body of coarsely crystalline calcite, but they also occur away from the walls, completely enclosed by calcite. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is: A(Na0.55K0.32)Σ0.87B(Na0.79Ca1.18Mn2+0.03)Σ2.00C(Mg3.29Mn2+0.02Fe2+1.19Fe3+0.31Al0.09Ti4+0.08Li0.02)Σ5.00T(Si7.39Al0.61)Σ8.00O22W[F1.23 (OH)0.77]Σ2.00. Ferri-fluoro-katophorite is biaxial (–), with α = 1.640(2), β = 1.652(2), γ = 1.658(2), 2Vmeas. = 68.9(2)° and 2Vcalc.. = 70.1°. The unit-cell parameters are a = 9.887(3), b = 18.023(9), c = 5.292(2) Å, β = 104.66(3)°, V = 912.3(6) Å3, Z = 2 and space group C2/m. The strongest ten lines in the powder X-ray pattern [d values (in Å) I (hkl)] are: 2.708, 100, (151); 2.388, 74, (131); 3.139, 72, (310); 8.449, 69, (110); 2.540, 65, ($\bar{2}$02); 2.591, 53, (061); 2.739, 47, ($\bar{3}$31); 2.165, 45, (261); 3.279, 44, ($\bar{2}$40); 2.341, 43, ($\bar{3}$51).

Crystallization and partial melting of rhyolite and felsite rocks at Krafla volcano: A comparative approach based on mineral and glass chemistry of natural and experimental products

Rhyolite and felsite cuttings were collected at Krafla volcano during the perforation of the Iceland Deep Drilling Project Well 1 (IDDP-1). The perforation was stopped at a depth of 2100 m due to intersection with a rhyolite magma that intruded the felsite host rock. Rhyolite cuttings are vitrophiric (glass ~95%, RHL) and exhibit a mineral assemblage made of plagioclase + augite + pigeonite + titanomagnetite. Felsite cuttings display evidences of partial melting, responding to variable degrees of quartz + plagioclase + alkali feldspar + augite + titanomagnetite dissolution. The interstitial glass analyzed close to (i.e., FLS1) and far from (i.e., FLS2) the reaction surface of pyroxene from felsite cuttings shows continuous changes between the two end-members. FLS1 is compositionally similar to RHL, showing Na2O + K2O + REE depletions, counterbalanced by MgO + CaO enrichments. Conversely, FLS2 exhibits opposite chemical features. REE-exchange thermobarometric calculations reveal that plagioclase and augite cores from rhyolite and felsite formed under identical conditions, along a thermal path of 940–960 °C. However, in terms of major and trace element concentrations, plagioclase and augite crystal cores are not in equilibrium with the rhyolite magma, suggesting the incorporation of these minerals directly from the host felsite. To better understand the petrogenetic relationship between rhyolite and felsite, two sets of crystallization and partial melting experiments have been carried out at P = 150 MPa and T = 700–950 °C. Rhyolite crystallization experiments (RCE) reproduce the two-pyroxene assemblage of IDDP-1 rhyolite cuttings only at T ≤ 800 °C, when the crystal content (≥19%) is higher than that observed in the natural rhyolite (~5%). Under such conditions, the RCE glass is much more differentiated (i.e., marked CaO depletion and Eu anomaly) than RHL. On the other hand, felsite partial melting (FPM) experiments show interstitial glass with a bimodal composition (i.e., FPM1 and FPM2) comparable to FLS1 (≈RHL) and to FLS2, only at T = 950 °C. This effect has been quantified by fractional crystallization and batch melting modeling, denoting that FLS1 (≈RHL) and FLS2 reflect high (≥70%) and low (≤8%) degrees of felsite partial melting, respectively. In contrast, modeling RHL by crystal fractionation requires the removal of an amount (~22%) of solid material that is inconsistent with the low crystal content of the natural IDDP-1 rhyolite. It is therefore concluded that natural rhyolite and felsite represent, respectively, the near-liquidus and sub-solidus states of a virtually identical silicic magma, either feeding aphyric to subaphiric rhyolitic eruptions, or solidifying at depth as phaneritic quartzofeldspathic rocks. Felsite lenses from the Krafla substrate may explore variable degrees of remelting and remobilization processes. The intrusion into felsite of a fresh silicic magma from depth may lead to low degrees of partial melting, whereas the persistent heat release from intense basaltic intrusive events at Krafla may be the source of high degrees of felsite partial melting and consequent rejuvenation of the previously solidified silicic magma.

A new approach to laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) using the flexible map interrogation tool ‘Monocle’

A new flexible interrogation tool for 2-dimensional LA-ICP-MS element and isotope maps is presented. Called Monocle, the tool works as an add-on in the software Iolite. Initial inspection of map data is undertaken with a size and shape adjustable loupe that is moved across the map. As the loupe is panned over the map, histograms and density/probability plots of the element concentration (or ratio, or age) contained within the area under the loupe are displayed. The data of any loupe position can be saved as a region of interest (ROI) in a table. For maps with rare earth element (REE) data a live normalised REE graph can be shown. Similarly, live graphs (e.g. Wetherill or Tera-Wasserburg concordiae) can be displayed for U/Pb data.The second step of map interpretation refines the initial data selection, for example by drawing polygons over irregular zones of similar chemistry. Where areas are separated (e.g. by cracks) several polygons can be nested into one ROI to improve counting statistics. Additional means of data pooling exist. Firstly, by growing a ROI from a seed, whereby the data distribution in the seed is used to find adjacent pixels that are within a user-defined statistical boundary (e.g. 1 standard deviation). Secondly, multiple criteria can be set (e.g. element A>0.1ppm and element B<40wt%) to determine which pixels (connected or unconnected) are included in a ROI.This new approach is illustrated with several examples. A Mariana Arc oceanic Mn crust is used to show how elemental maps resolve alternating periods of precipitation from variably intense hydrothermal input and how their REE systematics reflect mixing between two distinct sources. A map of a garnet from a lherzolite illustrates that with modern LA-ICP-MS instruments it is possible to obtain reproducible REE data from strongly depleted mantle minerals (e.g. La≈14ppb) with a 20×20μm2 beam and how mineral inclusions, metasomatism and serpentinisation can be avoided to obtain data from the most pristine parts of a grain. A map of a zoned augite crystal from Mt. Etna was used to examine the distribution of Ni and Cr, whose interrelationship in the strong oscillatory zoning can be modelled with extreme fractional crystallisation. A U/Pb map of a complex zircon was used to separate data from the complex inherited core and the much younger magmatic rim.Extraction of data from 2-dimensional maps (<1μm deep) avoids limitations of conventional LA-ICP-MS analysis that typically ablates much deeper. In combination with fast aerosol transfer systems that permit high repetition rate mapping (50–80Hz), extraction of quantitative data with map analysis tools heralds a new era of in situ chemical and isotopic analysis.