Chromian Spinel Grains Research Articles

Evidence for suboceanic small-scale convection from a “garnet”-bearing lherzolite xenolith from Aitutaki Island, Cook Islands

Garnet peridotite xenoliths have been rarely reported from suboceanic mantle. Petrographic and geochemical characteristics of garnet-bearing oceanic peridotite xenoliths provide precious information on dynamics of the suboceanic lithosphere and asthenosphere interaction. We examined a lherzolite xenolith included in olivine nephelinite lava from Aitutaki Island, a member of the Cook-Austral volcanic chain. The lherzolite xenolith contains reddish fine-grained (< 5 µm in size) mineral aggregates (FMAs) with size range of 0.5–6 mm, consisting of olivine, calcic and sodic plagioclases, aluminous spinel, native iron, and nepheline. Microstructural observations and chemical data corroborate that the FMA is a decomposed pyrope-rich garnet including chromian spinel grains with an irregular highly indented morphology in the center. The FMA is surrounded by pyroxene-poor and olivine-rich aureole. The spatial and morphological relationships of FMA and chromian spinel with pyroxene-depleted margin suggest a reaction of aluminous spinel + pyroxenes → pyrope-rich garnet + olivine, which requires a compression before decomposition of the garnet to FMA. An orthopyroxene grain shows slight but clear chemical zoning characterized by increase in Al, Ca, and Cr from the grain center to the rim. The zoning patterns of Al and Ca in the orthopyroxene grain can be modeled by diffusion-controlled solid-state reactions induced by pressure and temperature changes, keeping surface concentrations in equilibrium with the other coexisting mineral phases. The results indicate that the mantle, from which the lherzolite xenolith was derived, underwent isothermal decompression followed by a weak heating on a time scale of a few tenths of million years before the xenolith extraction. From the deduced compression and decompression histories, we hypothesize that the mantle beneath Aitutaki Island was once dragged down to a garnet-stable deep mantle region and brought up later by small-scale sublithospheric convection.

Exploratory Study of Separation of Sulphidised Chrome Spinels from Reduced Ilmenite

The removal of chromium-containing impurities, such as chrome spinel (ZCr2O4 where Z = Fe, Mg, Mn) from ilmenite (FeTiO3) concentrates through selective sulphidation, has been investigated by the authors. Laboratory experimental studies using sulphur added to ilmenite concentrates under Becher-type reduction conditions showed it is possible to selectively sulphidise chrome spinels from different ilmenite deposits. In this paper, processes to remove the sulphidised chrome spinels from the bulk ilmenite concentrates were investigated using flotation and magnetic separation techniques. Clustering or fusing of the reduced ilmenite (RI) and sulphidised chrome spinel grains was found to have a detrimental effect on flotation performance and made it difficult to have clear separation. A light wet grind was effective for breaking the clustering, but it caused the sulphide rim to spall off from chrome spinel surfaces, which reduces flotation efficiency. The preliminary results obtained after a magnetic separation (0.7 A) of a demetallised sulphidised RI sample show that the sulphidised chrome spinels preferentially report to the magnetic fraction. Additional magnetic separation of the non-magnetic fraction at a lower current (0.3 A) improved the recovery of sulphidised chrome spinels. The demetallisation process followed by a magnetic separation provided insights into a potential route for the removal of chrome spinels from reduced ilmenite concentrates. These two steps simulate the aeration stage of the Becher process. Further studies are required to optimise the process parameters.

Chromitite deposits of Ufaley ultramafic massif (South Urals)

Data on the morphology, composition, textural and structural features of chromite deposits of the Ufaley ultramafic massif are presented. The mineralogical and compositional features of the host ultramafic rocks allow us to interpret them as depleted restite from partial melting of mantle peridotites. Relatively wide variations in the composition of ore-forming chromian spinel grains (#Cr 0.6–0.8) and noticeable metamorphism of disseminated ores with replacement of chromite by Cr-magnetite are noted. It is assumed that chromitite bodies were initially formed under the conditions of the upper mantle by a rheomorphic mechanism, and then their structural and geochemical transformation took place in the collisional setting of the upper part of the crust. Flattened bodies of disseminated chromitites have been preserved near competent gabbroid blocks, while other deposits have been transformed into lenses and podiform bodies of densely disseminated and massive ores of smaller size. The “cold tectonics” of the crustal stage led to the disintegration of deposits and the simultaneous local enrichment of deformed chromitite bodies.

The mid-Ordovician meteorite flux to Earth shortly before breakup of the L-chondrite parent body

The breakup of the L-chondrite parent body (LCPB) in the asteroid belt at 466 Ma ago is the largest asteroid breakup documented in Earth's geological record for the past ca. three billion years. Recovery of abundant macroscopic fossil L chondrites in mid-Ordovician marine sediments as well as reconstructions of the flux of micrometeoritic chrome spinel through the ages have given much new information on the precise timing of the breakup and its effects on Earth. In the present study, we focus on the flux of large micrometeorites to Earth shortly (in the 2 Ma time interval) before the LCPB breakup (pre-LCPB), which may be crucial for understanding the dynamical evolution of the asteroid belt leading up to the breakup. We present chrome-spinel data (32–355 μm grain size) from two mid-Ordovician limestone sections in Sweden (Kinnekulle and Öland, 300 km apart) and one section in western Russia (Lynna River), ca. 1100 km from Kinnekulle. One aim is also to test the level of reproducibility of chrome-spinel flux reconstructions between different sites.Between 300 and 600 kg of limestone were collected from each section in the stratigraphic interval corresponding to ca. 2 Ma before up to immediately before the LCPB breakup. The relations between H, L and LL meteorites from Kinnekulle (38.7 ± 6.3%, 33.2 ± 6.1% and 28.1 ± 5.8%), Öland (46.0 ± 5.6%, 31.2 ± 3.1% and 24.5 ± 4.8%) and Lynna River (38.2 ± 5.5%, 32.8 ± 5.3% and 29.0 ± 5.1%) sections are indistinguishable from each other within uncertainties, revealing a globally homogeneous influx of H, L and LL meteorites. This gives support for the validity of previous reconstructions for the meteorite flux based on chrome-spinel reconstructions for fifteen time windows through the Phanerozoic.All the pre-LCPB samples from the three regions show a collective dominance of H-chondritic grains (42 ± 3%) over L (31 ± 3%) and LL grains (27 ± 3%), largely similar to the Phanerozoic background flux. Intriguingly, the presence of background concentrations of L-chondritic material also in the pre-LCPB flux demonstrates that the idea of a largely intact LCPB still existing before the final breakup may be far from reality. Apparently, a substantial amount of equilibrated chondritic material from deep levels of an L-chondritic body reached Earth even before the inferred catastrophic disruption at 466 Ma ago. This would concur with a “rubble pile” structure of the L-chondrite parent body and exposure of abundant deep-seated material as a result of earlier disruption and re-accretion events. The LL-chondritic contribution in the pre-LCPB flux is higher than at other Phanerozoic time windows, including the Cambrian. This anomalously enhanced flux thus cannot be ascribed to the Neoproterozoic breakup of the large LL-chondritic Flora asteroid. Previously observed high concentrations of chrome-spinel grains of achondritic origin can be reproduced only in our samples from immediately (< 1 Ma) before the LCPB breakup. We speculate that this, together with the high LL percentage before the breakup, may be explained by dynamic perturbations, possibly in the near-Earth region, leading up to the LCPB event.

Komatiitic Subvolcanic Rocks in the Mount Khanlauta Massif, Serpentinite Belt (Kola Peninsula)

Abstract —For the first time, exposures of ultramafic rocks of subvolcanic origin have been investigated at the foot of Mount Khanlauta (Kola Peninsula) located at a distance of ~2 km from the southwestern margin of the Pados-Tundra layered complex of dunite–harzburgite–orthopyroxenite composition, which hosts zones of chromitite and unconventional PGE mineralization. The ultramafic body is composed of micro- to fine-grained harzburgite and subordinate orthopyroxenite and has a cryptically zoned structure. The body is of E–W strike and has a small size with an apparent thickness of ~0.1 km; the exposed outcrops exhibit a blocky surface as a result of extensive cracking caused by degassing and rapid cooling of a parental komatiitic melt. Elements of columnar parting are recognized, which have a hexagonal shape in cross section and are consistent with the inferred subvolcanic origin. Two zones are identified. Zone I is formed by a more magnesian olivine with Mg# = 86.0–87.9 (0.15–0.21 wt.% MnO). In Zone II, olivine grains are notably less magnesian (Mg# = 81.8–84.1) and invariably have higher contents of manganese (0.19–0.30 wt.% MnO). The presence of this zoning is corroborated by the lateral distribution of accessory chromian spinel grains with maximum Mg# values (&amp;gt;20) in Zone I. The Khanlauta rocks also contain subordinate orthopyroxene (Mg# = 86.3–87.2), amphiboles of the tremolite–actinolite series, and anthophyllite of deuteric (autometasomatic) origin along with accessory minerals: ilmenite, hematite (~15 mol.% escolaite, Cr2O3, in solid solution), and mono- and diphase grains of sulfides in the form of intergrowths of Co-bearing pentlandite (Ni/Fe = 0.9–1.3; 1.00–16.74 wt.% Co; up to 1.7–6.8 wt.% Cu) and heazlewoodite (locally in intergrowth with hematite). The inferred front of crystallization moved in the western direction, causing the formation of Zone II from a more fractionated melt with a notably lower Mg# value. A sharp increase in oxygen fugacity locally caused the formation of anomalous parageneses of chromite and ilmenite, in which the observed Mg# values of ilmenite are considerably greater than those of the coexisting chromite. The geochemical whole-rock characteristics based on major, minor, and trace elements (including LILE, REE, and HFSE), as well as the compositions and trends of chromian spinels, are similar to those in differentiated (zoned) sills of the Chapesvara complex, which are closely associated with the Pados-Tundra layered complex. The obtained data indicate comagmatic relationships among the Khanlauta massif, zoned sills of the Chapesvara complex, and the Pados-Tundra layered complex. All of them crystallized from a primitive highly magnesian Fe- and Cr-enriched komatiitic magma (Al-undepleted). Thus, they belong to a single subvolcano–plutonic association being part of the Serpentinite belt–Tulppio belt (SB–TB) megastructure of presumably Paleoproterozoic age.

Detrital chromian spinels in the Cretaceous Sindong Group, SE Korea: implications for tectonic emplacement of hydrated mantle peridotites

The Sindong Group (Aptian–Albian), distributed in southeastern Korea, is a fluvio-lacustrine sequence deposited in the NE–SW-trending Nakdong Trough formed in an active continental margin. Sediments were derived from the Yeongnam Massif located west and NW of the Nakdong Trough, where Early Jurassic arc magmatism began in the southern part of the Korean Peninsula. We report mineral chemistry of detrital chromian spinels of first-cycle origin occurring as an accessory mineral in arkose to feldspathic litharenites of the Sindong Group. Chromian spinel grains have Cr/(Cr + Al) atomic ratio between 0.36 and 0.94, Mg/(Mg + Fe 2+ ) atomic ratio between 0.12 and 0.74, Fe 3+ /(Fe 3+ + Al + Cr) value mostly &lt;0.1, and &lt;0.2 wt% TiO 2 ; these values are consistent with spinel composition of forearc mantle peridotites, but are different from spinel composition of ultramafic rocks exposed in the southern Korean Peninsula. The results of this study indicate that forearc mantle peridotite or serpentinite was present in the Yeongnam Massif, and its tectonic emplacement is interpreted to have occurred by protrusion through the NE–SW-trending strike-slip fault zone developed in Middle Jurassic–Early Cretaceous time in the Yeongnam Massif and along the border with the adjacent tectonic province to the NW as a result of oblique convergence. Supplementary material : Locations of sandstone samples and compositions of spinels are available at https://doi.org/10.6084/m9.figshare.c.6004354

Microtextural evolution of chrome spinels in dunites from Mayodia ophiolite complex, Arunachal Pradesh, India: Implications for a missing link in the “two-stage” alteration mechanism

Although occurring as an accessory phase in mafic-ultramafic rocks, the universal presence of chrome spinel in ophiolitic settings bears importance in deciphering the tectonic setting of the ophiolite. Compositional and textural modification of chrome spinel via post-magmatic processes (metamorphism and fluid-induced alteration) is widely reported from host chromitites and peridotites, however, well-documented studies from dunite in ophiolitic settings are sparse. Here we present a detailed petrographic and geochemical account of accessory chrome spinels from previously unreported massive dunite bodies of Mayodia ophiolitic complex, Arunachal Pradesh, India, that portrays a microtextural evolution during alteration/metamorphism. Although having massive appearance, the bodies are crudely foliated and deformed. Severe alteration obliterated primary spinel chemistry as well as pre-alteration mineralogical assemblages. From textural and chemical point of view, the disseminated accessory chrome spinel grains are classified into four broad groups: (1) less chloritized spinel, (2) heavily chloritized spinel, (3) patchy-zoned spinel, and (4) homogeneous spinel. The chloritized spinels are relatively coarse grained, compositionally enriched in Cr and Fe2+ (ferrous chromite). They contain Cr-rich chlorite inclusions and are surrounded by a mixed matrix of Cr-rich chlorite and Cr-rich serpentine. Unzoned, homogeneous spinels appear as aggregates and are devoid of chlorite inclusions. They are compositionally different from the chloritized spinels, being enriched in Fe3+ and depleted in Cr (ferrian chromite). The patchy-zoned spinels have a non-uniform zoning with patches of ferrous chromite floating within ferrian chromite, separated from each other by micro fracture networks. Adapting a novel technique, we reconstruct bulk rock composition based on global data compilation, which has been applied to thermodynamic modelling. Comparing our integrated results with global examples we infer that fluid-induced alteration (chloritization) caused the formation of chloritized spinels from their primary counterpart. Later oxidizing fluid interacted with the chloritized spinels and converted them to homogeneous spinel; supporting the two-stage alteration model. We interpret the patchy zoning as an arrested intermediate stage between chloritized spinel and homogeneous spinel, preserving the characters of both stages. From a regional perspective, we postulate that the chrome spinel microtextures evolved between ~780 °C and ~ 550 °C during amphibolite facies retrogression of the dunite bodies, possibly related to exhumation of Mayodia ophiolitic complex.

Origin of spinel-hosted mineral inclusions in mantle peridotite from Setogawa in the Circum-Izu Massif Serpentine Belt, central Japan: Implications for the chromitite genesis

Plagioclase-impregnated mantle harzburgites from Setogawa area in the Circum-Izu Massif Serpentine Belt (of Miocene in age), central Japan, contain chromian spinel (chromite) grains that have minute globular inclusions composed of micas, amphiboles and pyroxenes. They are similar in texture and chemistry to those in chromitites as well as in some plutonic rocks, such as dunites and troctolites, form the Moho transition zone (MTZ). The Setogawa harzburgite host is similar in petrography and geochemistry to the MTZ harzburgite from the Oman ophiolite, and is possibly a low-pressure restite after melt-assisted partial melting or reaction with a percolating magma. The orthopyroxene-melt reaction, which is essentially responsible for producing those rocks from the peridotite (harzburgite), can precipitate chromian spinel as well as generate chemically heterogeneous melts along decomposing orthopyroxenes. The spinel-hosted melt inclusions in podiform chromitites are most frequently trapped at the reaction front between the spinel-oversaturated, spinel-loaden magma and mantle orthopyroxene (=harzburgite). The spinel-loaden magma had been produced via the same reaction at lower levels and moved upward. The spinel quickly overgrows to include chemically heterogenous melt drops, some of which are high in Cr/Al ratio and precipitate high-Cr spinel via reaction with the wall spinel. The chemical character (Cr/Al ratio) of podiform chromitites would be determined by the accumulative contribution from the melt derived from orthopyroxene decomposition during the passage of the melt through the upper mantle. We can speculate that the chromian spinel grains containing similar melt inclusions from the stratiform chromitite are transported as xenocrysts of mantle origin. The parental magma for layered intrusions possibly reacted with the peridotite when it passed through the mantle before arriving at a crustal magma chamber, showing a possible genetic linkage between the podiform and stratiform chromitites.

Asteroid break-ups and meteorite delivery to Earth the past 500 million years

The meteoritic material falling on Earth is believed to derive from large break-up or cratering events in the asteroid belt. The flux of extraterrestrial material would then vary in accordance with the timing of such asteroid family-forming events. In order to validate this, we investigated marine sediments representing 15 time-windows in the Phanerozoic for content of micrometeoritic relict chrome-spinel grains (>32 μm). We compare these data with the timing of the 15 largest break-up events involving chrome-spinel-bearing asteroids (S- and V-types). Unexpectedly, our Phanerozoic time windows show a stable flux dominated by ordinary chondrites similar to today's flux. Only in the mid-Ordovician, in connection with the break-up of the L-chondrite parent body, do we observe an anomalous micrometeorite regime with a two to three orders-of-magnitude increase in the flux of L-chondritic chrome-spinel grains to Earth. This corresponds to a one order-of-magnitude excess in the number of impact craters in the mid-Ordovician following the L-chondrite break-up, the only resolvable peak in Phanerozoic cratering rates indicative of an asteroid shower. We argue that meteorites and small (<1-km-sized) asteroids impacting Earth mainly sample a very small region of orbital space in the asteroid belt. This selectiveness has been remarkably stable over the past 500 Ma.

Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 μm-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 μm; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 °C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures < 400 °C under high H2 fugacity. This latter stage corresponds to the serpentinization of the Sabzevar oceanic peridotite and associated podiform chromitite deposit. The two alteration stages are interpreted as the result of the interaction between seawater and oceanic mantle at two different depth ranges in the course of its exhumation. Our thermodynamic calculations and textural relationships revealed that Cr is immobile and Fe is the main element to be transferred to the magnetite ore during alteration processes. Fe possibly originated from direct transport of the Fe2+ produced during olivine dissolution or from the dissolution of nano-sized magnetite grains initially formed in the host serpentinite during early serpentinization. Mass balance calculation reveals significant iron transport at a scale > 10 m during serpentinization.

Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Abstract —The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

Metallogeny of Serpentinite‐hosted Magnetite Deposits: Hydrothermal Overgrowth on Chromite or Metamorphic Transformation of Chromite?

AbstractPeculiar and rare occurrences of serpentinite‐hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece (Skyros), Iran (Nain and Sabzevar) and Oman (Aniba). These deposits have diverse thickness (from a few centimeters up to 50 m) and length (2 to &gt;500 m). Magnetite ores show variable textures, including massive, nodular and banded ores, veins, net and fine‐grained disseminations in serpentinites. Intriguingly, the investigated magnetite deposits can be mistaken for chromitite pods. Serpentinite‐hosted magnetite deposits show three modes of occurrences including: (i) boulders strewn across the serpentinites (i.e. Skyros Island); (ii) ore bodies along the nonconformity contacts between serpentinites and limestones (i.e. Aniba); (iii) irregular and discontinuous trails of massive and semi‐massive ore bodies within highly sheared serpentinite masses (i.e. Nain; Sabzevar). In all of these magnetite ore bodies, relicts of chromian spinel grains are occasionally enclosed in magnetite crystals. The chemistry of Cr‐spinel relics found in these magnetite bodies are comparable to those of accessory Cr‐spinels in the surrounding serpentinized peridotites. BSE images and elemental mapping revealed that magnetite occurs as a nucleation on chromian spinels but not being involved in reaction either with chromite or ferritchromite. Low‐grade metamorphic transformation of chromite into Fe‐chromite is documented along the cracks and fractures of a few chromite grains. Generally, magnetite has typical hydrothermal compositions, characterized by low Cr, V and Ti and high Mg and Mn. It is crucial to note that a few magnetite grains with metamorphic origin are characterized by high Cr and low Ti and Ni. The potential source of iron is essentially the Fe‐rich olivine. We believe that multi‐episodic serpentinization of peridotite systems at high fluid‐rock ratios is the main process responsible for precipitation of magnetite at ore levels whereas low‐grade metamorphic transformation of chromite to magnetite has minor contribution. Cumulative factors in generation of these deposits are modal volume of mantle olivine, peridotite composition, fluid chemistry, fluid‐rock ratio, mechanisms of transportation and precipitation, structural controls such as cracks and shear zones.

Composition, structure and magnetic properties of ore chrome spinels of the Klyuchevsky massif (Middle Urals)

The overall objective was to create a geological and geophysical field test site for chromite mineralization and detailed works in order to determine and test the main search criteria for disseminated mineralization. To create a field test site, an area was selected in the southern part of the Klyuchevsky massif characterized by abundant development of disseminated mineralization in the banded dunite-clinopyroxenite complex and strong processes of superimposed metamorphism. This paper gives a piece of research on the composition of chrome spinel from disseminated ores that underwent metamorphism of different stages. The composition of chrome spinelide, the evolution of its metamorphism and the related changes in its magnetic properties are examined in detail. It was established that all chrome spinels are of a high-chromium type, to one degree or another, they were subject to secondary changes, leading to the appearance of magnetic phases. The degree and nature of the change in chrome spinels in dunites directly depends on the degree of metamorphism of the rocks. The formation of the magnetic phase begins with the formation of nuclei consisting of single magnetic dipoles, groups of 2-3 dipoles or chains of magnetic dipoles, which was first discovered by us in chrome spinel grains. Then there is an increase in the number of such phases, the appearance of optically diagnosed secondary changes in chrome spinelide. A comparison of the degree of variation of chrome spinel and its reflection in the pattern of recorded anomalies of geomagnetic fields allows us to identify some criteria for the search for disseminated chromite ores of this type.

Morphological and Compositional Features of Chromian Spinel from Mantle Ultramafic Rocks of The Nurali Massif (South Urals)

Accessory chromian spinels of lherzolites and dunites from a mantle section of the Nurali ophiolite massif are described in the paper. Lherzolites typically host anhedral chromian spinel grains associated with olivine, pyroxenes and plagioclase. The compositions of silicates and chromian spinels are typical of those from ophiolite mantle sections. Olivine and orthopyroxene are characterized by high Mg content (forsterite and enstatite); clinopyroxene is diopside. The compositions of chromian spinel on the Al–Cr–Fe+3 plot occur close to the Al–Cr side. The #Cr and #Mg values of chromian spinels increase from lherzolites to dunites. Both vermicular spinels trapping olivine and orthopyroxene fragments (type I) and symplectite-like intergrowths of chromian spinel and plagioclase (type II) are most genetically interesting. Type I formed during synkinematic growth in deformed silicate matrix. Type II possibly formed as a result of decompression breakdown of a high-P mineral phase enriched in Cr, Al and Ca (a knorringite-type garnet?). In dunites, numerous tiny chromian spinel rods (type IV) in plastic deformed olivine are observed along with typical euhedral chromian spinel (type III) with inclusions of olivine and pargasite. Latter ones locally occur closely to fne pargasite grains. The formation of chromian spinel rods is explained as a result deformation-induced segregation of trace elements on the structural defects of the olivine lattice. Figures 7. Tables 4. References 48.

Mineralogy and petrology of Dominion Range 08006: A very primitive CO3 carbonaceous chondrite

Abstract Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach. The interchondrule matrix in DOM 08006 consists of unequilibrated mixtures of silicate, metal, and sulfide minerals and lacks Fe-rich rims on silicates indicating that DOM 08006 has only experienced minimal, if any, thermal metamorphism. This is also reflected by the Co/Ni ratios of Ni-rich and Ni-poor metal, a sensitive indicator of thermal metamorphism, and the presence of euhedral chrome-spinel grains, which typically become subhedral to anhedral during progressive metamorphism. DOM 08006 matrix shows minor evidence for aqueous alteration and while the presence of magnetite surrounding metal in chondrules indicates that there has been some interaction with fluid, much metal remains and none of the sulfides analyzed show evidence of being formed by aqueous alteration. Furthermore, the plagioclase of ∼50% of chondrules analyzed show resolvable excess silica indicating that these chondrules have experienced minimal, if any, reprocessing in the CO parent body. Noble gas data for DOM 08006 show that it contains the highest concentrations of trapped 36Ar and 132Xe of all CO chondrites analyzed to date, further indicating that DOM 08006 is the most primitive CO chondrite known. The cosmic ray exposure age of DOM 08006 is estimated to be ∼19 Ma. The minimally altered nature of DOM 08006 demonstrates that it is an extremely important sample for providing valuable insight into early Solar System conditions. At a total weight of 667 g, a significant amount of material is available for a wide array of future studies.

Diffusion pathways of Fe2+ and Fe3+ during the formation of ferrian chromite: a µXANES study

The alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity (fO2) and Fe2+ ↔ Fe3+ exchange during fluid–rock interaction. Chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid–rock interaction frequently overprints the record of earlier stages of alteration. To shed light on that process we have studied the distribution of Fe2+ and Fe3+ in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron-based microscopic chemical imaging and spatially resolved X-ray absorption near edge structure spectroscopy. Our results show that early stages of alteration do not involve changes in Cr3+ and Fe2+ contents or in Fe speciation but only depletion in Al3+ and Mg2+ resulting in the formation of porous chromite. With ongoing alteration Fe3+ migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries. Sheared-type chromitites record the maximum rates of fluid–rock interaction because in these chromitite-types the accommodation of deformation on porous chromite allowed higher rates of diffusion of Fe3+ and Fe2+ (a magnetite component with Fe3+/Fetotal = 0.66) into the newly formed neoblasts. In porous chromite-type texture (all the original chromite grains fully transformed to porous chromite) the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains. In contrast, in partly altered-type texture (chromite grains with original cores surrounded by porous chromite), such processes are only restricted to the porous rims, giving rise to zoned chromian spinel-ferrian chromite grains.

Composite origin of magnetite deposits hosted in Oman peridotites: Evidence for iron mobility during serpentinization

We describe mineralogy of magnetite deposits concentrated within a nonconformity surface between Aniba serpentinites and overlying limestones in the Southern Oman ophiolite and discuss their origin to understand iron mobility during serpentinization, with special reference to factors controlling the magnetite-orebody sizes. The Aniba magnetite deposits occur in several forms such as fibrous crystals, oval or rounded grains, strings and cluster shapes, in fine-grained iron-rich chlorite and calcite matrices. The chromian spinel (Cr# = 0.38–0.56) within the magnetite deposits is similar in texture, mode of occurrence and chemical composition to that in the underlying serpentinized harzburgites (chromian spinel Cr# = 0.37–0.50), and plots in the space of abyssal peridotites. The mineral chemistry of chromian spinel, olivine and clinopyroxene (Cpx) suggested that the Aniba harzburgites are refractory residue after high-degree (15–25%) partial melting and are similar to abyssal harzburgites from a normal ridge segment. This is confirmed by a Cpx trace-element character with high depletion in light rare earth elements (LREE), Ta and Zr relative to heavy REE. The iron of our magnetite deposits was possibly derived from two sources: one is an essentially internal source of iron from the breakdown of Fe-rich serpentines after olivine during low-T serpentinization (<400 °C) and the external source of iron from hydrous fluids. The essential origin of our magnetite deposits is hydrothermal one due to the mobility and leaching of iron from serpentinites and the in-situ selective precipitation of iron within the nonconformity surface. This process was feasible because the nonconformity zone was a weathering surface, a channel for both hydrothermal-fluid flow providing high fluid/rock ratios that enhance iron mobilization in serpentinites and leach iron, besides chromian spinel grains, from the host ultramafic rocks. Chromian spinel grains (up to 10 vol%) in the magnetite deposits were of a residual detrital origin from the underlying serpentinized peridotites on weathering, and acted as a nucleus for magnetite precipitation. The morphology and colloform shape of some magnetite grains reflect their supergene origin. Factors controlling the size of the magnetite deposits include the compositions of the underlying serpentinized harzburgites as a main source of iron, high fluid/rock ratios and high iron mobility within fluid conduits such as a nonconformity surface, abundance of chromian spinel grains as a nucleus for magnetite concentration, low silica activity, pressure of water and oxygen fugacity.

Significance of compositional zoning in cumulate chromites of the Kabanga chonoliths, Tanzania

ABSTRACTCompositional zoning is observed rarely in chrome-spinel grains from slowly-cooled layered intrusions because diffusion of cations continues within the spinel to low temperatures. However, in certain circumstances, such gradational zoning of both divalent and trivalent cations is observed and may be useful in deciphering the thermal history of the host intrusions. The accessory chrome-spinels of the Kabanga mafic-ultramafic chonolith intrusions of the Kibaran igneous event in north western Tanzania are notable because they have preserved gradational compositional zoning. This zoning is demonstrated to predate and be independent of later hydrous alteration of the silicate assemblage. At Kabanga, most chrome-spinel grains within olivine-rich cumulate rocks are gradationally and cryptically zoned from Fe2+-Cr3+ rich cores to more Mg2+-Al3+ rich rims (normal zoning). A few grains are zoned from Mg2+-Al3+ rich cores to more Fe2+-Cr3+ rich rims (reverse zoned). The zoning of divalent cations is proportional to that of trivalent cations with Mg2+ following Al3+ and Fe2+ following Cr3+ from core to rim. The zoning of trivalent and tetravalent cations is interpreted to be caused by either new growth from an evolving melt or peritectic reactions between evolved or contaminated melt and adjacent Al-Cr-bearing ferromagnesian minerals, which is preserved by relatively rapid initial cooling in the small chonolith intrusions. Divalent cation zoning is controlled by sub-solidus exchange of Fe2+ and Mg with adjacent ferromagnesian minerals and continues to lower temperatures, indicated to be 580 to 630°C by the spinel-olivine geothermometer. Preservation of such zoning is more likely in the smaller chonolith intrusions that typically host magmatic nickel-copper sulfide deposits and can be used as an exploration indicator when interpreting chromite compositions in regional heavy indicator mineral surveys.

Origin and significance of diamonds and other exotic minerals in the Dingqing ophiolite peridotites, eastern Bangong-Nujiang suture zone, Tibet

We have recovered unusual mineral assemblages of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite from the peridotites of the Late Triassic Dingqing ophiolite along the Bangong-Nujiang suture zone in south-central Tibet. Diamond grains are 100–300 μm, pale yellow and reddish-orange to colorless, commonly anhedral in habit, and range in morphology from elongated to octahedral and subhedral crystals. They display a characteristic shift in the Raman spectra between 1325 cm -1 and 1333 cm -1 , mostly at 1331.51 cm -1 or 1326.96 cm -1 . Chromian spinels in the Dingqing harzburgites and dunites are high-Cr varieties with Cr# = 40.1–49.1 and Cr# = 65.3–69.6, respectively. Recovered diamonds, ultrahigh-pressure, and highly reduced minerals appear to have crystallized and then been encapsulated in chromian spinel grains over a wide range of depth in the diamond stability field near the mantle transition zone. These chromian spinels and their host peridotites were then brought up to shallow mantle levels by the convecting asthenosphere, and were subsequently trapped in a mantle wedge of an intraoceanic subduction zone. The peridotites underwent further partial melting processes and reactions with island arc tholeiitic and boninitic melts, significantly modifying their whole-rock and mineral compositions. The preexisting chromian spinel grains with the inclusions of diamonds and other exotic minerals coalesced to form podiform chromitites during this stage in a suprasubduction zone setting.

Late Eocene 3He and Ir anomalies associated with ordinary chondritic spinels

During the late Eocene there was an enigmatic enhancement in the flux of extraterrestrial material to Earth. Evidence comes from sedimentary 3He records indicating an increased flux of interplanetary dust during ca. 2Myr, as well as two very large impact structures, Popigai (100km diameter) and Chesapeake Bay (40–85km), that formed within 10–20kyr at the peak of the 3He delivery. The Massignano section in Italy has one of the best sedimentary records of these events, including a well-defined 3He record, an Ir-rich ejecta bed related to the Popigai impact event, and two smaller Ir anomalies. Recently we showed that the Popigai ejecta is associated with a significant enrichment of chromite grains (>63μm) with an H-chondritic elemental composition (17 grains in 100kg of rock). Most likely these grains are unmelted fragments from the impactor. Slightly higher up (ca. 20cm) in the section, where a small Ir anomaly possibly related to the Chesapeake Bay impact has been measured, we found a weak enrichment in L-chondritic grains (8 grains in 208kg of rock). Here we report an extended data set increasing the total amount of sediment dissolved in acid and searched for extraterrestrial chromite grains from 658 to 1168kg. In altogether 760kg of background sediment from 17 levels over 14m of strata outside the interval corresponding to the Popigai and Chesapeake Bay impacts, we only found 2 extraterrestrial chromite grains. Both grains have L-chondritic compositions and were found in a 100kg sample from the ca. 10.25m level in the section where the second of the smaller Ir anomalies has been reported. A correlation appears to exist between Ir, 3He and chromite from ordinary chondrites. We also report oxygen three-isotope measurements of the extraterrestrial chromite grains associated with the Popigai ejecta and confirm an H-chondritic composition.The new results strengthen our scenario that the upper Eocene 3He and Ir enrichments originate from the asteroid belt rather than the Oort cloud as originally proposed when the 3He anomaly was discovered. The generally low background concentrations of extraterrestrial chromite through the section speak against any major single asteroid breakup event such as in the mid-Ordovician after the break-up of the L-chondrite parent body. Instead the data reconcile with a small, possibly a factor of 2–3, increase in the flux of extraterrestrial material to Earth, but of both H- and L-chondritic composition. We also report the composition of all the 2310 terrestrial chrome spinel grains recovered, and show that their chemical composition indicates a dominantly regional ophiolitic source. Four anomalous chrome spinel grains with high Ti and V concentrations were found in the Popigai ejecta. These grains originate from Siberian Traps basalts in the Popigai crater at the time of impact.