Spinel Exsolution Research Articles

The effect of the alumina-rich spinel exsolution on the microstructure of CAC-bonded castables

According to the magnesia-alumina phase diagram, the alumina-rich spinel may undergo exsolution of corundum during firing when incorporated into the castable. This research aimed to explore the phenomenon of alumina-rich spinel exsolution and its subsequent impact on the distribution of CaO·6Al2O3 inside the spinel and the strength of the castables when subjected to a firing temperature of 1600 °C. The results indicated that the amount of exsolved corundum escalated as the alumina content within the spinel rose after heat treatment, and this exsolved corundum was predominantly found within the original spinel particle. The CaO·2Al2O3, which formed when calcium aluminate cement was heated above 1200 °C, reacted preferentially with the exsolved corundum rather than with pre-added α-Al2O3 leading to the formation of CaO·6Al2O3 inside the spinel after firing at temperatures exceeding 1400 °C. This reaction formed an interpenetrated "spinel-CaO·6Al2O3" structure during firing, which enhanced the strength of the castables.

Whole-rock geochemistry and mineral compositions of gabbroic rocks and the associated Fe–Ti (–V) oxide deposit in the Gonamsan intrusion, South Korea

The NS trending sill-like Gonamsan gabbroic intrusion in the central region of the Korean Peninsula intruded Neoproterozoic metasedimentary rocks composed of schists intercalated with gneisses, marbles, and quartzites. The intrusion has an east zone (EZ) composed of diorite–monzodiorite, diorite–gabbro, and oxide gabbro–monzodiorite, and a west zone (WZ) composed of quartz monzodiorite. These silicate rocks have similar primitive-mantle normalized trace elements and chondrite-normalized REE patterns, suggesting that they are co-magmatic. Plagioclase anorthite (An) contents (1.9–54.0), pyroxene MgO/ (MgO + FeO) (Mg#) (35.4–73.7), and olivine forsterite (Fo) values (14.5–20.9) show a wide range through the intrusive silicate suite, but each unit has narrow ranges, and reverse fractionation is absent. The Fe–Ti (–V) oxide mineralization only occurs in the EZ and is characterized by variations in morphology, size, mineral assemblages, and chemical compositions of the Fe–Ti oxide minerals. Stratiform Fe–Ti (–V) oxide mineralization interlayered with diorite–monzodiorite is restricted to metasedimentary host rocks in the central region of Gonamsan and comprises magnetite and ilmenite with lesser amount of olivine, tschermakite, apatite, spinel, pyroxene, and plagioclase. Lenticular mineralization occurs along the western margin of the diorite–gabbro and consists of magnetite and ilmenite with minor hornblende, apatite, chlorite, and spinel. Oxide gabbro interlayered with relatively thin monzodiorite bands forms cyclical units irregularly distributed throughout the EZ and consists of olivine and pyroxene with minor interstitial magnetite, ilmenite, and apatite. Magnetite is the principal Fe–Ti oxide mineral, and varies in chemical composition and microtextures in different ore zones. Overall, the compositions of magnetite from all mineralized zones fall in the compositional fields for magmatic Fe–Ti, V deposits defined by Dupuis and Beaudoin (2011). Magnetite from the stratiform Fe–Ti oxide mineralization is compositionally consistent with those from high-temperature silicate melts and displays complex ilmenite and spinel exsolution textures that resulted from re-equilibration of ulvöspinel–magnetite solid solution during cooling. In contrast, magnetite from the lenticular ore and oxide gabbro are compositionally consistent with being derived from evolved low-temperature melt. Based on the whole-rock geochemistry, mineral composition, and mineralization distribution, the Fe–Ti (–V) oxide mineralization in the Gonamsan intrusion seems to be related to Fe–Ti-rich melt formed from magmatic fractionation and migration along a relatively permeable zone, rather than other magmatic and post-magmatic processes.

An enigma in rock magnetism: can microstructures in magnetite cause a threefold increase in the efficiency of NRM acquisition in the Stardalur Basalts?

SUMMARY Quaternary lavas of the Stardalur Caldera, 20 km northeast of Reykjavik, Iceland, create a 27 300 nT magnetic anomaly visible in both ground and aeromagnetic surveys. Here, we provide a comprehensive mineralogical and rock magnetic data set to analyse NRM intensities and Koenigsberger ratios of 57 drill-core samples from the critical zone (CZ) of the anomaly high at depths between 41 and 131 m. This extends previous studies and verifies that the anomaly is due to an unusually high intensity of remanent magnetization carried by magnetite. The NRM of the CZ samples was acquired during the Olduvai subchron in a field of at most today’s strength. NRM intensities range from 20 to 128 A m–1 with a median of 55 A m–1, and an average of 61 A m–1, respectively, approximately 13–15 times higher than in typical Icelandic basalts (AIB) with an NRM intensity of 4 A m–1. Our new data set shows that the magnetite concentration throughout the CZ basalts is at most twofold higher than in AIB lavas. New data on domain state and TRM efficiency prove that these properties account for an additional factor of at most 2.3. Because magnetite is the most abundant remanence carrier in rocks on Earth, and its remanence acquisition is considered to be extremely well understood, we assert that the remaining discrepancy is a critical enigma in rock magnetism. Results from scanning electron microscopy show that a significant fraction of all CZ magnetite particles have dendritic shapes with grain sizes <1 μm, indicating rapid crystallization. Most large magnetite grains are heavily subdivided by very fine oxidation-exsolution lamellae of ilmenite, and subordinate amount of exsolved spinel as needles, blebs and blades. These common microstructures found throughout the CZ subdivide the initially homogeneous mineral into separate cubicles, here denoted as compartments. The magnetite compartments then have sizes below 1 μm. Hysteresis data, Preisach maps and FORC data consistently confirm that the coercivity distribution is dominated by values above 10 mT, such that multidomain behaviour is of little relevance in the CZ. Between 5 and 20 per cent of the IRM is carried by coercivities above 100 mT, which for magnetite indicates unusually high anisotropy effects in the individual particles. Based on the quantitative analysis of all magnetic contributions to the NRM, we can demonstrate that the average efficiency of NRM acquisition in the CZ Stardalur basalts must be at least a factor 3 higher than in typical basalts. We speculate that this is related to the observed focused compartment size distribution <1 μm, and indicates thermochemical remanence acquisition below the Curie temperature of magnetite. Yet, a detailed physical mechanism for the extreme overefficiency of NRM acquisition remains enigmatic.

An exsolution constructed FeNi/NiFe2O4 composite: preferential breaking of octahedral metal-oxygen bonds in a spinel oxide.

Exsolution is an ingenious strategy for the in situ construction of metal- or alloy-decorated oxides and, due to its promising energy related catalysis applications, has advanced from use in perovskites to use in spinels. Despite its great importance for designing target composites, the ability to identify whether active metal ions at octahedral or tetrahedral sites will preferentially exsolve in a spinel remains unexplored. Here, an inverse spinel NiFe2O4 (NFO) was employed as a prototype and FeNi/NFO composites were successfully constructed via exsolution. The preferential breaking of octahedral metal–oxygen bonds in the spinel oxide was directly observed using Mössbauer and X-ray absorption spectroscopy. This was further verified from the negative segregation energies calculated based on density-functional theory. One exsolved FeNi/NFO composite exhibits enhanced electrochemical activity with an overpotential of 283 mV at 10 mA cm−2 and a long stability time for the oxygen evolution reaction. This work offers a unique insight into spinel exsolution based on the preferential breaking of chemical bonds and may be an effective guide for the design of new composite catalysts where the desired metal ions are deliberately introduced to octahedral and/or tetrahedral sites.

Nano-Micron Exsolved Spinels in Titanomagnetite and Their Implications for the Formation of the Panzhihua Fe-Ti-V Oxide Deposit, Southwest China.

The nano-micron exsolved spinels with various mineralogical characteristics in titanomagnetite from Fe-Ti oxide gabbros in the Panzhihua Fe-Ti-V oxide deposit, SW China, have been studied by field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) based on comparisons of physical and chemical conditions at different stratigraphic heights to investigate the compositional inheritance between titanomagnetite and exsolved spinel and further explore the relationship between the morphology and growth of exsolved spinels. Restored chemical data for titanomagnetite combined with evidence from petrography and whole-rock geochemistry imply fractional crystallization of the Panzhihua Fe-Ti-V oxide deposit, where the titanomagnetite of thick massive oxides at the bottom of the No. VIII orebody represents the early crystallizing phase characterized by high temperature and oxygen fugacity. The chemical variation in the exsolved spinel, which has the same trend as the restored composition of titanomagnetite, represents inheritance from the parent rock within the Panzhihua deposit. Exsolved spinel continuously adjusts morphology and grain size to decrease the total energy of the manganate-spinel system from fine-grained spinels parallel to the {100} plane of titanomagnetite to spinels with complex stellate morphology to bulky granular spinels with high degrees of idiomorphism. The unusual multiple magma replenishment during the mineralizing process and at different stratigraphic heights in the Panzhihua intrusion had an important influence on the thermal evolution history of the orebody, resulting in the identifiable spatial distribution patterns of spinel morphology and grain size. Using spinel exsolution as a discriminator for the provenance of magmatic ore deposits may provide intuitive and easy mineralogical evidence to qualitatively discuss the evolution of the metallogenetic environment and the ore-forming conditions for similar large mafic intrusions.

P-T-melt/fluid evolution of abyssal mantle peridotites from the Nagaland Ophiolite Complex, NE India: Geodynamic significance

Spinel lherzolite from ophiolitic mélange in the Nagaland Ophiolite Complex, Indo-Myanmar ranges is used to reconstruct the P-T and chemical evolution of slab mantle in a Neo-Tethyan subduction channel. Five sequential high-T stages are identified for lherzolite based on petrography, mineral chemistry (major and trace elements), P-T pseudosection modelling and thermobarometry. (1) Large aluminous orthopyroxene + aluminous clinopyroxene + high magnesian olivine + chromiferous primary spinel grains formed during dry melting of source abyssal spinel lherzolite at T > 1150–1335 °C and P = 12–16 kbar. (2) Clinopyroxene and spinel exsolution in orthopyroxene, clinopyroxene + spinel coronae around megacrystic orthopyroxene and olivine formed during sub-solidus cooling from ~1100 to ~800 °C. (3) Neoblastic low-Al ortho- and clinopyroxene at ~800 °C formed during high-T mylonitisation, as the subducted lherzolite body accreted to the overlying wedge mantle as a metamorphic sole. Tectonism in this stage switched from being divergent to convergent. (4) Light REE-enriched, high-Mg clinopyroxene veins developed locally within the orthopyroxene megacrysts reflect the transit of a hybridised-mantle wedge melt. (5) A transient phase of high-T (T > 800 °C) hydration of clinopyroxene and locally orthopyroxene stabilised pargasite. Stages (4) and (5) relate to slab-derived melt/fluid migration through the accreted mantle peridotite. These findings provide new insights into the geodynamic evolution of the north-eastern segment of the Neo-Tethyan lithosphere as it evolved from a mid-oceanic ridge to a supra-subduction zone setting.

Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence

The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2–M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and ≈850–930 oC. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5–0.8 GPa and ≈770–795 oC. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4–0.6 GPa and 350 km) to the lower crust depth (≈40–20 km), resulting in an average exhumation rate of 9.11–9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at ≈500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.

Mineralogical Characteristics of Exsolved Spinel in the Panzhihua V‐Ti Magnetite Deposit, Sichuan: Implications for the Mineralization Process

AbstractSpinel exsolution is widespread in titanomagnetite from the Fe‐Ti oxide gabbro of the Panzhihua intrusion, Emeishan Large Igneous Province, SW China. However, little research has been conducted into the implications of patterns in the mineralogical characteristics of the spinel for spatial variation in the controls on the exsolution mechanism and, hence, the formation process of the ore deposit. This study selected the Lanjiahuoshan Ore Block in the Panzhihua V‐Ti magnetite deposit to explore this issue, systematically studying exsolution textures in the titanomagnetite through petrographic observation and the integrated use of in‐situ microanalysis. The results show that the exsolved spinel gradually becomes finer‐grained and less abundant from the center to edge and the bottom to top of the ore bodies. Compositionally, there is an inverse correlation between the size of exsolved spinel grains and their Mg# value. In addition, there is compositional zonation in the spinel interiors, with a gradual increase in the Mg content and decrease in Fe content from the core to the rim. The analysis suggests that fractional crystallization of ferrotitanium magma with a high oxygen fugacity in a shallow magma chamber caused compositional differences in the primary magnetite solid solution in different parts of the Panzhihua intrusion. Additionally, the thermal evolution of the magnetite solid solution differed in different parts of orebody, bringing about variations in spinel development. Together, these effects resulted in spatial variation in the abundance, grain size, and morphology of spinel in different parts of the orebody and intrusion that follows an identifiable distribution law. Furthermore, the compositional zonation of exsolved spinels reflects the rapid growth of exsolution features in a high‐temperature environment. Thus, the size, morphology, abundance, and composition of spinel exsolution features in titanomagnetite provide a valuable petrogenetic tool for estimating the maturity and formational environment of the deposit.

Nanoscale Diopside and Spinel Exsolution in Olivine from Dunite of the Tethyan Ophiolites, Southwestern Turkey: Implications for the Multi-Stage Process

Nanoscale Diopside and Spinel Exsolution in Olivine from Dunite of the Tethyan Ophiolites, Southwestern Turkey: Implications for the Multi-Stage Process

Imprints of modal metasomatism in the post-Deccan subcontinental lithospheric mantle: petrological evidence from an ultramafic xenolith in an Eocene lamprophyre, NW India

Abstract We report here on the occurrence of an interesting mantle-derived ultramafic xenolith entrained in an Eocene ( c. 55 Ma) lamprophyre dyke from the Dongargaon area of the Chhotaudepur alkaline subprovince located within the Narmada Rift Zone, NW India. The mineralogy of the xenolith comprises olivine, clinopyroxene and mica (phlogopite), with the latter occurring essentially as rims around the clinopyroxene. Inclusions of apatite, interstitial sulphide (pyrite) and micron-scale exsolved spinel are widespread. Olivine is forsteritic (Fo 85.34 ), displays little compositional variation and overlaps with that reported from worldwide mantle peridotite xenoliths. Clinopyroxene is a diopside with a compositional range of Wo 48.36 En 43.83 , Fs 6.53 and Ac 1.27 , and is conspicuous by its high CaO (up to 24.4 wt%) and TiO 2 (up to 1.6 wt%) content. Clinopyroxene is also compositionally similar to that reported from ‘enriched’ (metasomatized) peridotite xenoliths rather than those that occur in the ‘normal’ (depleted) peridotitic xenoliths. Phlogopites have a high concentration of fluorine (up to 1 wt%), whereas the apatites show an anomalous enrichment of F (up to 5 wt%), as well as enrichment in Sr (SrO up to 1.9 wt%). Our study provides the first direct petrographical evidence for the modal metasomatism in the post-Deccan subcontinental lithospheric mantle (SCLM) from this domain. From the textural and mineralogical assemblage of the xenolith, we infer that a possible olivine + garnet + orthopyroxene assemblage, in the presence of a metasomatic fluid, has given rise to clinopyroxene + phlogopite + spinel. The paragenesis of apatite essentially as inclusions suggests that it to be the earliest crystallized phase during the metasomatic event. Geothermobarometry of the clinopyroxene in the xenolith reveal temperatures of approximately 1200°C and pressures of approximately 12 kb, which are comparable with such data reported from other Deccan-related xenoliths. Preservation of phlogopite and apatite in the ultramafic xenolith imply that some of the readily fusible metasomatized portions in this domain escaped wholesale melting during the eruption of the Deccan Traps, possibly due to the variable thickness of the underlying SCLM.

Host rock geochemistry, texture and chemical composition of magnetite in iron ore in the Neoarchaean Nyong unit in southern Cameroon

ABSTRACTA combination of petrography, whole-rock geochemistry, geochronology and compositional variation in magnetite is employed in this study to elucidate the nature and origin of enigmatic magnetite mineralisation hosted within gneissic rocks in the Nyong Unit in southern Cameroon. The mineralisation occurs as magnetite-bearing calc–silicate gneisses. The host rock mineral assemblage comprises quartz–plagioclase–biotite–amphibole–chlorite–clinopyroxene–garnet that provides evidence of medium-grade metamorphism and retrograde alteration. Textural and chemical analyses of the magnetite grains show variable textural and trace element chemical characteristics attributed to metamorphic-hydrothermal overprint and weathering. Magnetite occurs as disseminations and stringers commonly intergrown with amphiboles. It is also observed to show vermiforms wrapped around quartz and clinopyroxenes within a biotite–chlorite- plagioclase groundmass. Massive-granoblastic magnetite is rare and mainly observed within vein-like domains. On backscattered secondary electron images the magnetite grains are anhedral, with minor spinel exsolution lamellae. Electron microprobe analysis on magnetite suggests both a hydrothermal skarn and banded iron formation (BIF) affinity. The lack of negative Ce anomalies excludes a Proterozoic BIF setting, but it is in agreement with Archaean BIF. Sensitive high resolution ion microprobe U-Pb isotope data on zircon in the magnetite gneiss define an identical Wetherill concordia and Tera–Wasserburg Neoarchaean age of 2699 ± 7 Ma (1σ; MSWD (mean square weighted deviation) = 1.3; n = 13), and Pan-African disturbance at about 500 ± 200 Ma. The Neoarchaean age is in accordance with the known onset of BIF deposition at the northern edge of the Congo Craton and therefore constrains the maximum age of formation of the Nyong magnetite gneisses.

RETRACTED: Sapphirine and fluid inclusions in Tel Thanoun mantle xenoliths,Syria

Volcanoes along the Syrian rift, which extend a distance of about 1000 km, brought to the surface mantle xenoliths within erupted basalts, during multiples periods of volcanic activity. Xenoliths in early Cretaceous volcanoes originate in the garnet peridotite field of the subcontinental mantle, whereas those in recent Cenozoic volcanoes, the prime object of this study, are issued from shallower levels (spinel peridotite field). The recent discovery of sapphirine-bearing websterite in Tel Thanoun, a small volcanic diatreme inside the larger Quaternary volcanic field (Djebel Al Arab), allows us to estimate the P-T evolution and fluid–rock interaction at the volcanic source. Harzburgites and lherzolites are equilibrated at a temperature of about 1000 °C at a depth of 35–40 km. Sapphirine appears to have formed during cooling, at depth at a temperature of about 900 °C, at a time where spinel exsolution occurred in harzburgite and lherzolite pyroxenes. This occurred in the presence of a high-density pure CO2 fluid phase, still present in primary fluid inclusions. The highly-aluminous sapphirine-bearing protolith might be former garnet websterite (possibly uplifted during cretaceous magmatism), which resided and cooled in the spinel peridotite stability field, and was then dragged and brought to the surface by quaternary basalts.

Peridotitic minerals of the Nidar Ophiolite in the NW Himalaya: sourced from the depth of the mantle transition zone and above

Abstract The Nidar Ophiolite Complex (NOC) consists of a c. 10 km thick ophiolite suite in the NW Himalaya, India. The c. 7 km thick lower ultramafic part of the ophiolite body terminates against the Tso Morari Crystallines, which represent the leading edge of the Indian continental margin. Mineral inclusions from the peridotites in the lower ultramafic part of the NOC were studied, including C 2 /c clinoenstatite, disordered coesite and high-pressure Mg 2 SiO 4 (probably β-Mg 2 SiO 4 ). These minerals, found in two lherzolite bodies from the ophiolite's mantle section, were characterized by laser Raman spectroscopy and electron micro-probe analysis. Textural evidence supporting decompression from an ultra-high-pressure condition was also observed, such as Cr spinel exsolution needles in olivine crystals. The systematic mineral phase transitions of coesite→quartz, high-pressure clinoenstatite→orthoenstatite and β-Mg 2 SiO 4 →Cr spinel exsolution needles in olivine suggest that the mantle section of the Nidar Ophiolite evolved from the deep mantle beneath a palaeo-spreading centre. The phase stabilities of these high-pressure minerals require derivation from the depth of the mantle transition zone (410–660 km). A transport mechanism for these minerals is suggested via dunite channels along a mantle adiabat in the focused convective flow below the spreading centre. This mechanism brought these deep mantle phases into the ultramafic part of the NOC. These observations suggest that some part of the mantle section of the NOC in the NW Himalaya originated in a mid-ocean ridge setting. Supplementary material: Representative mineral chemical compositions of the lherzolites (1M1 and 1NU27) and the host channelized dunites are available at http://www.geolsoc.org.uk/SUP18836 .

Sulfide Saturation and Magma Emplacement in the Formation of the Permian Huangshandong Ni-Cu Sulfide Deposit, Xinjiang, Northwestern China

The eastern Tianshan orogenic belt, northwestern China, hosts ~120 mafic-ultramafic intrusions and at least six of them host significant Ni-Cu sulfide mineralization of which the Permian Huangshandong intrusion is the largest. The mafic-ultramafic rocks of the Huangshandong intrusion comprise lherzolite, gabbronorite, gabbro, and diorite. Individual orebodies are composed of pyrrhotite, pentlandite, and chalcopyrite as disseminated to net-textured or massive to semimassive sulfide mineralization. Oxide-bearing sulfide mineralization contains 1 to 5% magmatic magnetite that has ilmenite and spinel exsolution lamellae. Silicate rocks and sulfide mineralization have variable whole-rock platinum group elements (PGE; 0.38–362 ppb), Cu (42–97,000 ppm), and Ni (26–33,000 ppm) concentrations. The mineralization is broken out into two types termed oxide rich and oxide poor. The oxide-rich sulfide mineralization has high PGE concentrations with low 187Re/188Os ratios (38.9–151), whereas oxide-poor sulfide mineralization has low PGE concentrations with high 187Re/188Os (159–781). Both oxide-rich and -poor sulfide mineralization have highly variable γ Os values (49–189 and 30–278, respectively). Oxide-rich sulfide mineralization contains magnetite with variable amounts of MgO, TiO2, Al2O3, FeOtotal, Cr, V, Zn, and Sn, indicating that they crystallized from different stages during magma differentiation. We propose that the host intrusion and associated sulfide mineralization were derived from high Mg basaltic magmas from a mantle source that was previously modified by subducted oceanic slab. Oxide-poor sulfide mineralization of the Huangshandong intrusion formed from a magma that underwent <0.03% sulfide removal before emplacement and was thus PGE depleted. Oxide-rich sulfide mineralization formed from magma that had much lower amounts of sulfide (<0.003%) removed in the early stage before entering the Huangshandong magma chamber. An early sulfide saturation event was likely associated with fractionation of silicate minerals in a deep-staging magma chamber. The second sulfide saturation event that formed the sulfide mineralization was probably triggered by selective crustal contamination and fractionation in the high-level chamber where mixing of magmas and accumulation of immiscible sulfide occurred.

Les enclaves du volcanisme recent du rift syrien

Abstract In direct continuity with the Dead Sea fault, the Syrian rift, which links the rigid Arabian plate to the mobile ophiolite belt of Cyprus-southern Turkey, plays a very important role in the regional geodynamic structure. Its exact position, as well as the related fracture system, has been documented from the analysis of a complete aerial photo coverage of the whole Syrian territory. The rift corresponds to a transform fault, with lateral displacements decreasing from more than 100 km, to the south, to less than 30 km to the north. Several major episodes of volcanic activity have occurred since early Mesozoic times, with eruptive centers located in three major domains, southern (S), center (M) and northern (N), respectively. The recent (Neogene-Quaternary) volcanism is compared to the Cretaceous one (Bhannes-Tayasir episode). Erupted lavas are in general very basic (picrobasalts, basanites), with rather primitive magmas, except for one occurrence at El Kafr (southern Syria) which corresponds to a silica-undersaturated, strongly differentiated phonolite. Major and partial (Rb, Ba, Nb, Sr, Y, Lu) trace-element data show overall similarities between recent and Cretaceous volcanisms, with however a more distinct alkaline trend and stronger variations of LILE-elements for recent lavas. Few volcanoes contain a number of ultrabasic xenoliths, notably lherzolites, harzburgites and pyroxenites. Rare garnet-bearing varieties have also been observed in M and S-domains, including few grenatites. Xenolith texture is protogranular or granular, with a variable (mostly limited) degree of local melting by the enclosing basalt. Olivines are Mg-rich (mg (super *) = 0.93-0.83), as are ortho-and clinopyroxene. This last mineral, which may show spectacular Opx and spinel exsolution lamellae, is relatively abundant, as shown by the frequent occurrence of pyroxenite (Cpx-Opx) and Cpx-rich lherzolite (wehrlite). Because of the possible occurrence of Cretaceous rocks with kimberlitic affinities [Nabi Mata, Sharkov et al., 1993], garnet has been studied in detail. On the garnet triangle, most analyses plot well away from high-pressure mantle rocks, notably kimberlites, but close to infracrustal garnet-bearing rocks (granulites). Very few analytical points (3 out of a total of 20) could correspond to garnet peridotite. All other rocks are equilibrated in the field of spinel peridotite or infracrustal granulites. Pure CO 2 -bearing fluid inclusions have been found in olivine and pyroxenes from xenoliths and in phenocrysts from enclosing basalts. Highest density fluids (up to 1.15 g/cm 3 ), are observed in pyroxenites, especially from M-domain. They occur in primary, tubular inclusions adjacent to or even containing minute spinel grains, oriented along the exsolution lamellae of the clinopyroxene-host. P-T conditions of mineral equilibration in the xenoliths have been estimated from the pyroxene thermometer [Bertrand and Mercier, 1986; Brey and Kohler, 1990] and maximum density of fluid trapped in primary inclusions. They correspond to about 1 100-1 300 degrees C for the temperature, 10-13 kb for the pressure. These P-T conditions do not show any significant variation between different regional occurrences, but well between various petrographical types, the maximum conditions being recorded in pyroxenites. These results suggest that some clinopyroxene at least has been formed by mantle metasomatism caused by ephemeral carbonate magmas, in a mantle plume located under the Arabic plate.

A closer look at remanence‐dominated aeromagnetic anomalies: Rock magnetic properties and magnetic mineralogy of the Russell Belt microcline‐sillimanite gneiss, northwest Adirondack Mountains, New York

A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline‐sillimanite‐quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence‐dominated, although the carriers of remanence were not well documented. Russell Belt gneisses have a strong natural remanent magnetization with steep remanence directions, D = 263°, I = −58°, an average intensity of 3.6 A/m, and typical susceptibilities of 10−4 SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650°–750°C during the Ottawan Orogen (1050–1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self‐reversed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susceptibility values and highly stable remanence. Optical observations show titanohematite grains with multiple generations of ilmenite, pyrophanite, rutile, and spinel exsolution lamellae. Microprobe analyses confirm titanohematite compositions ranging from 72 to 97% Fe2O3 with hematite83 being most typical. In rare samples, inclusions of magnetite were identified. The ubiquitous presence of titanohematite, and the rare occurrence of magnetite, is supported by thermal and alternating field demagnetization studies, saturation magnetization measurements, hysteresis properties, temperature‐hysteresis studies, and low‐temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this may contribute a strong rémanent component to what have previously been considered to be solely induced anomalies.

Petrological constraints on the cooling history of high-temperature garnet peridotite massifs in lower Austria

High-temperature peridotite massifs occur as lensoid bodies with high-pressure granulites in the southern Bohemian massif. In lower Austria the peridotites comprise garnet lherzolites lacking primary spinel, rare garnet and garnet-spinel harzburgites, and harzburgites containing Cr-rich primary spinel instead of garnet. These phase assemblages suggest initial high-pressure equilibration and are consistent with results from garnet-orthopyroxene geobarometry indicating equilibration at around 3–3.5 GPa. Maximum temperature estimates obtained on core compositions of coexisting minerals from the peridotites are not higher than ca. 1100 °C. In contrast, pyroxene megacryst compositions, garnet exsolution textures in the garnet pyroxenites, and results from geothermometry indicate much higher original equilibration temperatures in most of the pyroxenites (up to 1400 °C). High temperatures, modal zoning, the occasional presence of Mg-rich garnetites and chemical evidence suggest that the pyroxenites are cumulates which crystallized from low-degree melts derived from the sub-lithospheric mantle. Isothermal interpolation of the high temperatures to an upper mantle adiabat suggests that the melts were derived from a minimum depth of 180–200 km. The formation of small garnet II grains and garnet exsolution lamellae in the pyroxenites and pyroxene megacrysts may reflect isobaric cooling of the cumulates from temperatures above 1400 °C to ca. 1100–1200 °C (at 3–3.5 GPa) to approach the ambient lithospheric isotherm. This model differs from other models in which the formation of garnet II was explained by an increase in pressure during cooling in a subduction zone. Isobaric cooling was followed by near-isothermal decompression from 3–3.5 GPa to 1.5–2 GPa at 1000–1200 °C, as indicated by the increase of Al in pyroxenes near garnet. Further cooling in the spinel lherzolite stability field is indicated by spinel exsolution lamellae in pyroxenes from lherzolites. The formation of symplectites and kelyphites indicate sub-millimetre scale re-equilibration during exhumation in the course of the Carboniferous collision in the Bohemian massif. The peridotite massifs represent fragments of normal (non-cratonic) lithospheric mantle from a Paleozoic convergent plate margin.

Chromian spinels in metamorphosed ultramafic rocks from Mangabal I and II complexes, Goi�s, Brazil

Chromian spinel occurs in feldspathic metaperidotites and feldspathic metaharzburgites, respectively, at Mangabal I and II mafic-ultramafic layered complexes, Brazil. Both complexes were metamorphosed (700-750°C and 6-7 Kbars) under variable PH2O conditions. Wherever PH2O = Ptot an almost complete metamorphic recrystallization occurred, but where PH2O < Ptot relict igneous cumulate textures are well preserved. Chromian spinel occurs as opaque grains included in cumulus olivine and as brown translucent crystals enclosed by intercumulus orthopyroxene, clinopyroxene, and plagioclase. Subsolidus reactions resulted in exsolution of opaque spinel into two phases: a Fe+2Fe2+3-rich and a MgAl2-rich one. Fe2TiO4-rich lamellae and patches may also occur. Coronas developed between olivine and plagioclase including orthopyroxene, pargasite and MgAl2-rich green spinel. Whenever complete metamorphic reequilibration occurred (PH2O = Ptot) the brown and green spinels reacted with silicate phase to form clinochlore, leaving the Fe+2Fe2+3-rich spinel phase, magnetite.

Formation of opaque minerals in CK chondrites

The opaque minerals of 19 CK chondrites from Antarctica and the Nullarbor Region, Australia were studied mineralogically by optical and analytical methods. The most striking feature of the CK chondrites—indicating the oxidized nature of these meteorites—is the presence of magnetite as the most abundant opaque phase. It occurs in modal abundances between 1.2 and 8.1 vol.%, averaging about 4 vol.%. Magnetite occurs in two different populations: as tiny micronsized grains dispersed throughout the meteorite; and as rounded aggregates up to 1 mm in size. In the latter type exsolution lamellae of ilmenite and spinel were observed. Chemically, the magnetites contain small amounts of MgO, Al 2O 3 and TiO 2 (usually below 1 wt.%), and an average of about 4 wt.% Cr 2O 3. Sulphide phases occur in abundances below 1 vol.% (except Karoonda 1.3 vol.%). However, six different phases were found: pentlandite ((Fe,Ni) 9S 8), pyrite (FeS 2), pyrrhotite (Fe 1− x S), monosulphide solid solution (MSS; (Fe,Ni) 1− x S), millerite (NiS), and chalcopyrite (CuFeS 2). Usually, the parageneses pentlandite-pyrrhotite/MSS-pyrite and pentlandite-pyrite were found. The chemical compositions of the observed pentlandites vary substantially regarding the Fe Ni ratio. In all phases various amounts of Co and, in some cases Cr were detected. Associated with magnetite and/or sulphides, micron-sized noble metal-rich sulphides, tellurides and arsenides were found. Among the sulphides the most frequently occurring phase is (Os,Ru,Ir)S 2. In addition, PtS, (Au,Fe,Ag) 2S, and (Fe,Au,Co) 2S 3 were identified. Among the tellurides and arsenides, chengbolite (PtTe 2), in some cases containing dissolved Fe and Au, and a phase (Ir,Pt,Fe,Os)(As,S) 2 were discovered. For the formation of the observed opaque phases and assemblages a multistage process under increased oxygen fugacities is proposed. Magnetite is formed from Fe,Ni-metal during a metamorphic event under oxygen fugacities on the order of the NiNiO oxygen buffer. Ni became disengaged, oxidized and diffused into olivine. The observed exsolution of ilmenite and spinel was established during cooling. Based on stability conditions the observed sulphide parageneses must have been formed by subsolidus reactions under low temperatures from MSS. The noble metal elements which were either dissolved in the Fe,Ni-metal or in the MSS, formed noble metal-rich phases by exsolution during the oxidation of the metal phases or during the subsolidus breakdown of MSS.

Heterogeneous Nucleation of Superparamagnetic Phases in “Fe-Bearing Cordierite-Glasses

ABSTRACTGlasses of stoichiometric cordierite composition Mg2(Al1−xFex)4Si5O18, containing very low iron contents (x=0.015 and 0.005) have been investigated by 57Fe Mössbauer spectroscopy. At higher Fe concentrations spinel exsolution has been observed in X-ray powder patterns. To allow Mössbauer spectroscopy at very low Fe-concentrations (&lt;0.8 weight* Fe2O), starting materials were doped with 100% 57Fe. Glasses prepared in air at 1560°C were also oxidized In water saturated O2 stream. Glasses were crystallized to cordierite by heating In air at 1100 to 1400°C. 57Fe-Mössbauer spectra of all samples are governed by a doublet typical for Fe2+ in octahedral coordination (IS: 1.09–1.15, QS: 2.00–2.31 in MM/s relative to metallic iron). X-ray powder patterns exhibit no additional phases. But, the 295 K and 5 K Mössbauer spectra of treated and untreated glasses and of cordierltes exhibit broad lines, which have been fitted by applying internal magnetic hyperfine fields H of ca. 500 kG. These lines are attributed to heterogeneously nucleated, superparamagnetic (MgFe) (AlFe)2O4 spinels of complex compositions. From the calculated subspec-tra one may conclude that 35 to 65% of the total iron, depending on preparation conditions, is incorporated in the spinels.