Primary Spinel Research Articles

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

A petrographic study of carbonate phases in the Ulten Zone ultramafic rocks: insights into carbonation in the mantle wedge and exhumation-related decarbonation

We present a comprehensive petrographic study on carbonates in orogenic mantle wedge-derived peridotites from the Ulten Zone (UZ) in the Eastern Italian Alps representing a fragment of the Variscan belt. These peridotites are found in ultramafic bodies incorporated in high-grade crustal rocks from a former continental slab and are characterized by highly variable mineral assemblages and microstructures, which reflect their evolution from spinel peridotite in a hot mantle wedge, to garnet peridotite after incorporation into the subducting slab and finally exhumation accompanied by retrogression. Carbonate phases in UZ peridotites are observed in diverse textural sites and can be related to particular petrographic peridotite types. Inclusions of dolomite and dolomite-breakdown products in primary spinel from coarse-grained protogranular peridotites indicate that carbon-bearing liquids were introduced into the mantle wedge at a time before achieving garnet stability. Discrete dolomite grains occur in fine-grained deformed garnet-bearing peridotite and are suggested to have formed simultaneously with hydrous phases (amphibole, apatite) from a carbon-bearing aqueous crustal slab-derived fluid during garnet stability. Intergrowths of calcite and brucite occur mainly in serpentinized fine-grained garnet-amphibole peridotites and are interpreted to be products of dedolomitization (CaMg(CO 3 ) 2 + H 2 O → CaCO 3 + Mg(OH) 2 + CO 2 ), thus decarbonation, during exhumation. Veins of dolomite and calcite-brucite indicate secondary dolomite formation from carbon-bearing fluids due to interaction with slab-derived crustal fluids during retrogression on the exhumation path of the UZ peridotite. Magnesite veins and calcite veins that are texturally linked to alteration features are probably of low-temperature origin. In summary, this petrographic study reveals multi-stage carbonation of the UZ peridotites during residence in the mantle wedge and decarbonation with the release of a carbon species during low-temperature peridotite reaction with aqueous fluids leading to serpentinization accompanied by dolomite breakdown, thus providing important constraints on the carbon budget and carbon cycling in collisional settings. Overall, the ubiquity of carbonates in a variety of textural settings in the UZ peridotites suggests that the supra-subduction zone mantle in continental settings represents an efficient carbon trap.

PETROGENETIC SIGNIFICANCE OF SPINELS FROM SERPENTINISED PERIDOTITES FROM THE VERIANAOUSA OPHIOLITE

The Veria-Naousa ophiolitic complex represents a dismembered ophiolite unit, which is superimposed on a basement consisting of rocks belonging to the Pelagonian and Axios (Almopias subzone) isopic zones in northern Greece. Mantle peridotites are composed of variably serpentinised lherzolite and harzburgite intruded by a sparse network of pyroxenitic dykes. The serpentinised lherzolite and harzburgite contain Alspinels (Cr#=38.83-42.52 and Mg#=58.94-64.77), Cr-spinels (Cr#=43.37-64.92 and Mg#=49.20-58.66) and magnesiochromites (Cr#=53.93 57.13 and Mg#=55.73- 61.71). All of them display commonly richer-in-Cr cores rimmed by secondary ferrian chromite and magnetite. Whole-rock geochemicall compositions and primary spinel chemical composition of these peridotites are analogous to peridotites that formed in a suprasubduction zone. Ιt is supported that the Mantle peridotites of the VeriaNaousa ophiolitic complex formed in a back-arc basin

RELICTS OF THE PRIMARY MAGMATIC ACCESSORY SPINELS IN THE ARCHEAN KOMATIITES OF THE EASTERN FENNOSCANDIA

The article presents the results of a complex study of the relicts of the primary magmatic spinels from the Archean (3.0-2.7 Ga) komatiitic series of the greenstone structures of the eastern Fennoscandian Shield. The accessory minerals of the massive and mesocumulate zones of differentiated komatiite flows (containing 24-30 wt. % MgO) are analyzed. The coexistence of several generations of spinels including relicts of the primary magmatic chromite-alumochromite (Cr 2 O 3 44.64-51.80, AI 2 O 3 9.39-13.74, #Cr 68.97-78.52, #Mg 3.98-12.14, #Fe 5 kbar and temperatures > 500 °C leads to a complete recrystallization of the primary spinels.

Review of Innovative Energy Savings Technology for the Electric Arc Furnace

A review of the energy innovations for the electric arc furnace (EAF) steelmaking route is discussed. Preheating of scrap using vertical and horizontal shafts that have been commercially successful in lowering the energy consumption to as much as 90 kWh/t reaching almost the operational limit to heating input scrap materials into the EAF is discussed. Bucket-type and twin-shell preheaters have also shown to be effective in lowering the overall power consumption by 60 kWh/t, but these have been less effective than the vertical shaft-type preheaters. Beyond the scrap preheating technologies, the utilization of waste heat of the slags from the laboratory scale to the pilot scale has shown possible implementation of a granulation and subsequent heat exchange with forced air for energy recovery from the hot slags. Novel techniques to increase metal recovery have shown that laboratory-scale testing of localized Fe concentration into the primary spinel crystals was possible allowing the separation of an Fe-rich crystal from an Fe-depleted amorphous phase. A possible future process for converting the thermal energy of the CO/CO2 off-gases from the EAF into chemical energy was introduced.

Evolution of microstructure during annealing of Mn1.5Co1.5O4 spinel coatings deposited by atmospheric plasma spray

Plasma spraying is a promising cost-effective method to manufacture different functional layers, especially on the sophisticated surface. In this work, atmospheric plasma spray (APS) was used to manufacture Mn1.5Co1.5O4 (MCO) coatings on T441 substrates. To further enhance the bond strength with the substrate and densify the protective coatings, reducing and subsequent oxidation were performed on MCO coatings. The redox behavior of spinel oxide during heat-treatment was investigated. Moreover, the evolution of vertical crack was in-situ observed by SEM. The results showed that dramatic changes had occurred on the microstructure of MCO coatings during heat-treatments. After reduced for 5 h in a 20%H2/Ar environment at 800 °C, the continuous nanoscale vertical cracks converted to be discontinuous and even extended in local area. However, after oxidation treatment for 10 and 20 h, all the vertical cracks had healed. Primary spinel layer that consisted of MnCo2O4, Co3O4 and Mn3O4 were obtained. With the increasing of heat-treating time up to 95 h and 250 h, cubic Co3O4 and tetragonal Mn3O4 were converted into CoMn2O4, dense spinel coatings were finally produced. The cross-sectional image indicates that the coating is still well-bonded with the substrate and free from cracks after oxidation in air for 250 h. The SEM image shows an oxide scale of less than 1 μm.

SERPENTINITE-WATER INTERACTION AND CHROMIUM(VI) RELEASE IN SPRING WATERS: EXAMPLES FROM TUSCAN OPHIOLITES

Since 2006 the Regional Environmental Agency (ARPA) of Tuscany has found Cr(VI) concentration in groundwater above the maximum acceptable con- centration (5 ppb) according to the Italian regulation (D.M. 25 October 1999, n. 471). Recently, hydrogeological investigations (Grassi, 2009) confirmed high total Cr and Cr(VI) contents in soils and waters related to ultramafic rocks from coastal Tuscany. To study the release of Cr(VI) to water from geogenic sources we examined the spring waters, soils and bedrocks compositions within two serpentinized bodies of the coastal Tuscany, namely Santa Luce and Querceto. These selected sites are characterized by Cr(VI)-free and Cr(VI)-bearing spring waters, respec- tively. The prevalence of Mg-HCO3 waters at these sites indicates that waters are primarily interacting with serpentinites. Samples from Querceto are enriched in Mg suggesting the leaching of brucite. Soil profiles and bedrocks were analysed for major and trace elements by XRF to assess the fate of Cr during weather- ing. Detailed petrographic and chemical analyses of both soil and rock samples revealed that Cr is mainly hosted within chromites. Chemical data and alteration features indicate a metasomatism coeval to the oceanic serpentinization leading to a Mn-enrichment of the alteration products of primary spinel. The co-occur- rence of Mn-bearing and Fe-rich oxides with Cr-rich spinels creates a chemical system potentially able to promote the Cr(III) to Cr(VI) oxidation. However, the occurrence of Mn-rich minerals seems to be not correlated to the oxidation state of chromium in the spring waters. In fact, they are ubiquitous in both the study localities, while Cr(VI) has been detected only at Querceto. Unlike Santa Luce, serpentinites from Querceto contain a significant amount of Fe-rich brucite and show evidence for ongoing brucite dissolution coupled with precipitation of hydromagnesite crusts and veinlets. Formation of complex hydroxy-carbonate min- erals could be an alternative way for producing the proper environment for oxidative leaching of chromium from serpentinites to spring waters.

Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over <0.5km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges of Al2O3, Na2O, Cr2O3 and TiO2 contents. Primary spinels show a large variation of Cr# [=Cr/(Cr+Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr)+LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3– <10% melting for type I lherzolites and ~15% for harzburgites. Hornblendes are enriched in fluid-mobile elements relative to HFSE+U inherited from their precursor Cpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (<5cm) mineralogical and chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incremental melt that formed during decompression melting of a normal MORB mantle source. The studied peridotites possibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.

Mineral chemistry and geothermometry using relict primary minerals in the La Cocha ultramafic body: A slice of the upper mantle in the Sierra Chica of Córdoba, Sierras Pampeanas, Argentina

Abstract The La Cocha ultramafic body, in the Sierra Chica of Cordoba (Sierras Pampeanas, Argentina), is formed by serpentinized spinel harzburgites, with lenses of spinel pyroxenites and hornblendites. The associated metamorphic rocks are garnet sillimanite gneisses, intercalated with tabular bodies of pyroxene amphibolites and forsterite marbles. Mineral chemistry of relict primary phases (olivine, orthopyroxene and spinel) from samples of the spinel harzburgites and pyroxenites was determined, and several geothermometers were applied to estimate the temperature conditions under which these rocks may have been equilibrated. In the spinel harzburgites, the primary spinel is Al–chromite [Cr# = Cr/(Cr + Al) = 0.48–0.57], which is replaced by ferrichromite and chlinoclore by metamorphism. Orthopyroxene is enstatite (En 92 ) and olivine is classified as forsterite (Fo 92 ); this last one shows a homogeneous and constant composition along the ultramafic body. Using geothermometric calibrations of the pair olivine–spinel, the highest temperature of 1157 °C would correspond to the primary conditions of formation of the harzburgites. The spinel pyroxenites show a mineral composition defined by orthopyroxene (En 85 , enstatite), olivine (Fo 86 , chrysolite), spinel ( s. s. ) and magnetite. Serpentine and clinochlore were produced by metamorphism. Spinel has high concentrations in Al and very low in Cr, and is classified as spinel sensu stricto ; magnetite replacement was produced by metamorphism. Orthopyroxene and olivine are depleted in MgO regarding these minerals in the harzburgites. Temperatures of 785–734 °C calculated using geothermometers with orthopyroxene are interpreted to be produced by metamorphism in amphibolite to granulite facies conditions. Cumular textures were not observed in outcrops and thin sections of the studied rocks. The narrow composicional range and high forsterite content in olivine, the high Cr# in spinel, and the low concentrations of Ni and Cr in whole rock analyses indicate a mantle residual origin for these peridotites, which would exclude a cumular origin. The association of peridotites with mafic bodies formed in an N-type MORB environment and a relict mantle fabric showed by the elongated crystals/aggregates of olivine (preserved in pseudomorphic replacements), indicating a high temperature flow, allow to interpret the La Cocha ultramafic body as a slice of oceanic mantle, belonging to basal tectonites of an ophiolite complex. This body shows similar petrological, geochemical and structural features than the other ultramafic bodies in the Sierras de Cordoba, therefore the origin proposed here could be applied to the other bodies.

Post-entrainment mineral-magma interaction in mantle xenoliths from inner Mongolia, western North China craton

In order to distinguish the primary microstructures developed under mantle conditions from the secondary phenomena after xenolith entrainment in the host magma, this study intends to discuss the genesis of spongy, sieve-textured, and reaction rims on mineral grains of mantle xenoliths in the Cenozoic basalts from the western North China craton. The spongy rims on primary clinopyroxene show neither obvious compositional zoning nor preferential development towards the host basalt and probably suggest an origin via partial melting within the lithospheric mantle or pressure release as the xenoliths were carried upwards. The sieve-textured rims on primary spinel show clear chemical zoning with increasing Cr# and decreasing Al towards the host basalt. They are interpreted as the result of partial melting due to heating of the host basaltic magma and decreasing pressure during ascent proc- ess. Post-entrainment reaction mainly generated secondary minerals at contacts between the host ba- saltic melt and xenoliths. The secondary clinopyroxene in reaction rims develops on primary clinopy- roxene and has higher Ti, Ca, and Fe contents and lower Mg# and Si contents than primary one, while the secondary spinel on primary Cr-Al spinel is titanomagnetite. The secondary olivine and clinopy- roxene in the reaction rims on primary orthopyroxene are enriched in Fe, Al, and Ti. The occurrence of reaction rims in mantle xenoliths reflects disequilibrium processes after xenolith entrainment in the basaltic melt. The spongy rims on primary clinopyroxene may not be related to the interaction with the

MINERALOGY AND GEOCHEMISTRY OF PLATINUM-RICH CHROMITITES FROM THE MANTLE-CRUST TRANSITION ZONE AT OUEN ISLAND, NEW CALEDONIA OPHIOLITE

Micropods, irregular bodies and schlieren of chromitite occur within a unit of layered plagioclase-bearing wehrlite on Ouen Island, southern New Caledonia. The primary chromian spinel is Al-rich, with Cr# [atomic (Cr/Cr + Al)] between 0.52 and 0.62, and Mg# [Mg/(Mg + Fe 2+ )] between 0.57 and 0.72. Contents of minor and trace elements (Ga, Ni, Zn, Co, Mn, V, Sc) in primary spinel, obtained by LA–ICP–MS, are comparable to those reported in chromian spinel from podiform chromitites in other ophiolite complexes. The chromitites are strongly enriched in Pt and Pd (up to 11.5 ppm Pt + Pd), reflecting the presence of abundant platinum-group minerals (PGM), including alloys, sulfides, oxides and compounds containing Te, Sb or Hg. Also present are Ru–(Os–Ir) phases, in accessory amounts: sulfides (mainly laurite, two unidentified Os–S and Ir–S compounds) and sulfarsenides of Ir and Rh. Abundant base-metal minerals (BMM) occur together with the PGM; they include Ni–Cu–Fe sulfides (pentlandite, millerite, heazlewoodite, chalcopyrite and bornite), awaruite (Ni 3 Fe), and a phase with a composition close to NiAsS (probably gersdorffite). The geochemistry and mineralogy of the chromitite and the PGE mineralization show that the Ouen Island chromitites have a different origin than those found in the mantle and crust sequences of the New Caledonia ophiolite complex. The distribution, morphology and composition of the PGM and BMM in the chromitites define two distinct paragenetic assemblages: i) primary or magmatic, consisting of PGM and BMS grains enclosed in the unaltered core of chromian spinel grains and in primary anhydrous silicates (olivine and pyroxene) of the matrix; ii) secondary or postmagmatic, consisting of PGM and BMM grains found along fractures, enclosed or in contact with high-temperature secondary hydrous silicates (amphibole) and low-temperature silicates (serpentine, chlorite). Microtextural and compositional data for the secondary PGM and BMM mineral assemblages suggest that after the crystallization of chromian spinel, an excess of H 2 O generated a residual high-temperature hydrothermal fluid that modified the magmatic PGM mineralogy, producing small-scale remobilization of the PGE. Late serpentinization overprinted this hydrothermal alteration, and contributed to the degradation of the primary PGM and BMM. At this stage, PGE-bearing alloys and sulfides were replaced by new stable alloys and oxides of the PGE.

Enhancement of MgAl2O4 spinel formation from coprecipitated precursor by powder processing

Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO-nAl2O3, n = 1) MgAl2O4 spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl2O4 — γ-Al2O3 solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures, due to large agglomerate size, MgO can not properly interact with the exsolved α-Al2O3 from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary) is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel, can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited precursor is most effective in terms of nano size (< 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C.

An alternative model for ultra-high pressure in the Svartberget Fe-Ti garnet-peridotite, Western Gneiss Region, Norway

Thepreviouslyreported''Fe-Titype''garnet-peridotiteislocatedinthenorthernpartofthewellknownultra-highpressure (UHP) area of the Western Gneiss Region (WGR) in Norway. Primary spinel stable up to only 2.0 GPa at 800 ! C coexists with Caledonian ultra-high pressure (4.0 GPa at 800 ! C) grt-cpx-opx-ol assemblages in the Svartberget garnet-peridotite. The body is cut byaconjugatesetofmetasomaticfracturesfilleddominantlywithdiamond-bearinggarnet-phlogopite-websterite (5.5GPaat800 ! C) and garnetite. Single zircon U-Pb dating suggests metamorphic growth of zircon in the garnetite at 397.2 1.2 Ma, either coinciding or predating an initial phase of leucosomes formation at 397-391 Ma. Field observations, major and trace elements, mineral- chemistry, polyphase inclusions including microdiamond, coupled with 87 Sr/ 86 Sr ratios in clinopyroxene and whole rock ranging from 0.73 to 0.74, suggest that the Svartberget garnet-peridotite was infiltrated by melts/fluids from the host-rock gneiss during the CaledonianUHPevent.PresentobservationsintheWGR documentaregionalmetamorphicgradientincreasingtowardstheNW,and structures in the field can account for the exhumation of the (U) HP rocks from # 2.5 to 3 GPa. Assuming lithostatic pressures the diamond-bearing Svartberget peridotite body must have come from a burial depth of more than 150 km. However, there is a lack of observable structures in the field to explain exhumation from extreme UHP conditions (5.5 GPa or more) to normal HP-UHP conditions (2.5-3 GPa), which are common pressures calculated from eclogites in western parts of the WGR. Because of the regional and mostly coherent metamorphic gradient across the WGR terrain, it is difficult to account for local extreme pressure excursions such as documented from within the Svartberget peridotite. We introduce here a conceptual model to explain the main features of the Svartberget body. During burial and heating, rocks surrounding the peridotite start to melt but surrounding non-molten rocks confine the space and pressure builds up. When pressure is high enough, conjugate brittle shear fractures develop in the peridotite. Melt (or supercritical fluid) that has the same pressure (5.5 GPa) as the surrounding gneiss can flow in as soon as fractures propagate into the peridotite. This supercritical fluid is now highly reactive and metasomatism takes place at UHP conditions along the fractures capturing micro inclusions of diamond while growing. Finally the lithosphere holding the overpressurised gneiss con- strained breaks duetoformationoflarge-scale fracturesinthe crustanddecompression meltingstarts.Modelling usingfinite-element method (FEM) shows that melting of the gneiss results in pressure variations when gneiss is ten to hundred times weaker than surroundings and peridotite enclave. These pressure variations can be up to several GPa and are qualitatively similar to observations in the field.

Spinel–quartz high temperature paragenesis in Al–Fe granulites from the Ihouhaouene area (NW Hoggar, Algeria)

The spinel–quartz-bearing Al–Fe granulites from Ihouhaouene (In Ouzzal, West Hoggar) have a migmatitic appearance with quartzo-feldspathic layers intercalated with restitic layers. These granulites are characterized by a hercynitic spinel–quartz assemblage typical of high grade terranes. The stability of the spinel–quartz assemblage is attributed to an elevation of temperature (from 800 to >1100 °C) at high pressures (10–11 kbar), followed by an isothermal decompression from 9 to 5 kbar, an evolution typical of the In Ouzzal clockwise P– T path. The Al–Fe granulites’ history can be subdivided into different successive crystallisation stages. During the first stage, the spinel–quartz assemblage formed, probably following a prograde event that also produced partial melting. During a second stage, the primary spinel–garnet–sillimanite–quartz paragenesis broke-down to give rise to the secondary assemblage. The metamorphic evolution and phase relations during this stage are shown in P– T– X pseudosections calculated for the simple FMASH system. These pseudosections show that the orthopyroxene–cordierite–spinel symplectite appeared during a high temperature decompression, as a product of destabilisation of garnet in sillimanite-free microdomains with high X Mg values. At the same time, the spinel–quartz association broke-down into cordierite in Fe-rich microdomains. Average pressure and temperature estimates for the orthopyroxene–spinel–garnet–cordierite–quartz association are close to the thermal peak of metamorphism (1000 ± 116 °C at 6.3 ± 0.5 kbar). With decreasing temperatures garnet–sillimanite corona developed from the breakdown of the primary spinel–quartz assemblage in the Fe-rich microdomains, whereas cordierite–spinel formed at the expense of primary sillimanite and garnet in the Mg-rich microdomains.

Oxygen solubility and speciation in sulphide-rich mattes

Sulphide-rich liquids are common in magmatic environments forming over a wide range of temperature, pressure, fO 2 and fS 2. They are economically important because they sequester valuable metals such as Cu, Ni, Au and Pt from silicate melts. The presence of accessory amounts of primary oxides associated with sulphide mineralisations is often ignored or unexplained. Experimental work has shown that large amounts of oxygen can dissolve into mattes at fO 2 typical of terrestrial environments. At the quartz–fayalite–magnetite fO 2 buffer, the molar fraction of O in the matte exceeds that of S, placing the composition of the matte to the magnetite side of the mss (monosulphide solid solution)–magnetite join in the Fe–S–O system. However, sulphides crystallise before magnetite in most sulphide mineralisations and are much more abundant. Moreover, the speciation of O in a matte is not well known. Here we report the results of an experimental study of the solubility of O in mattes as a function of fS 2, fO 2, temperature, and composition. We confirm previous observations that Ni and Cu have a negative effect on the solubility of O in mattes. We show evidence for the existence of FeSO as a structural constituent of mattes in the Fe–S–O system. We present a simple parameterisation of the amount of O dissolved in mattes under relevant geological conditions, and use this parameterisation to discuss mechanisms for the crystallisation of primary spinels associated with sulphides in the Kambalda massive sulphide deposit (Western Australia) and the Sudbury Igneous Complex (Ontario, Canada).

Retrograde Reactions of an Ultrahigh-Pressure Metamorphic Spinel Pyroxenite Lens, Northeast Sulu UHP Terrane, Eastern China

A ~5 cm thick garnet-rich layer, with relict spinel grains in its center, is intercalated within garnet pyroxenite at Rongcheng. Both the garnetite layer and the host garnet pyroxenite underwent the same P-T evolution, but exhibit quite different mineral assemblages, microtextures, and associated mineral reactions. Microtextures and mineral compositions confirm our previous suggestion that the Rongcheng garnet pyroxenite recrystallized from a spinel pyroxenite cumulate derived from a gabbroic magma. The present study concludes that: (1) The garnet-rich layer resulted from a net transfer reaction between primary spinel and clinopyroxene, Ca1.5(FeMg)0.5 Si2O6 + (FeMg)Al2O4 + TiO2 = Ca1.5(FeMg)1.5Al2(Si2Ti)O12, and mass-balance reactions between clinopyroxene end-members, 2[Ca0.75(FeMg)0.25][(FeMg)0.5Al0.5] (Si1.5Al0.5)O6 = Ca1.5(FeMg)1.5Al2Si3O12, under UHP conditions that produced garnet-I. (2) Garnet-I decomposed during exhumation of the UHP terrane, leading to the assemblage of Grt-II + Ilm + Cpx-II + Amp; ilmenites were partially released to the relict spinel along micro-fractures to form intergrowths of Spl-I + Ilm. A corona assemblage of Cpx-II + Spl-II + minor Amp developed along contacts between garnet and relict Spl-I at ~890°C and ~9 kbar. And (3), during a later stage of retrogression, aqueous fluid infiltrated heterogeneously through the garnet-rich layer, leading to the formation of hydrous minerals. Parageneses and mineral compositional maps suggest that Ti may have substituted for Si and entered the T site in the garnet structure, represented by a garnet component such as M3Al2[Si(3-x)Tix]O12, in addition to the commonly suggested M2+Ti4+ → 2Al3+ substitution (forming the garnet component M3-0.5x[Al2-xTix]Si3O12). Heterogeneous microtextures and mineral compositions indicate that the domain equilibrium was controlled by mass transfer at least at the mineral-grain scale due to various degrees of fluid infiltration.

Petrological and geochemical constraints on the composition of the lithospheric mantle beneath the Syrian rift, northern part of the Arabian plate

Abstract A suite of mantle xenoliths from the Neogene–Quaternary volcanic province of Jabel El Arab (Syria) is dominated by spinel±amphibole harzburgites, with rare lherzolites and wehrlites that were equilibrated at temperatures of 900–1100 °C. The major elements of pristine minerals and trace element compositions of clinopyroxene and amphibole indicate that the lithospheric mantle experienced various degrees of melt extraction (olivine Mg-number=89.4–91.8, spinel Cr-number=10.4–46.4), followed by a multistage metasomatic history. The primary clinopyroxene has variable and high Mg-number (89.1–93.6) and highly variable major element concentrations (Al 2 O 3 2.7–7.6 wt%, Na 2 O 0.5–2.5 wt% and Cr 2 O 3 0.4–2.5 wt%). Three groups of harzburgites were identified on the basis of petrographical, mineralogical and geochemical data. Group I harzburgites have compositions indicating a residual origin after polybaric partial melting with F &lt;20%, which started in the garnet stability field and continued in the spinel stability field. Group II harzburgites are interpreted as a result of a percolation mechanism involving the infiltration of large volumes of undifferentiated basaltic melts through the residual lithosphere. Finally, the mineral major element compositions and the selectively enriched trace element contents of clinopyroxenes in group III harzburgites (high (La/Sm) N and Th, U, Sr and low high field strength element contents) are attributed to a percolation mechanism involving small volume melt fractions. Such small melt fractions correspond to CO 2 -bearing alkaline silicate magmas that have evolved to CO 2 -rich melts during repeated percolation-reaction within the Syrian lithospheric mantle. Shortly before eruption, some xenoliths were infiltrated by small silicate melt fractions, which produced discrete reaction zones composed of cpx±ol±sp±glass surrounding reacting primary spinels. The glass in the melt pockets has a trachytic to trachy-andesitic composition and its composition suggests that glass is derived from melting of pre-existing amphibole in the lithospheric mantle, triggered by infiltration of a Na-rich metasomatic agent.

Effect of in situ spinel seeding on synthesis of MgO-rich MgAl2O4 composite

MgO-rich MgAl2O4 spinel was prepared by adopting solid-state reaction of commercially available sintered seawater magnesia and α-alumina. Starting materials were mixed in weight ratio (Al2O3: MgO) of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, where the developed MgAl2O4 spinel crystal seed in calcined powder varied (5–50%) with respect to addition of MgO content and temperature. Around 70% spinellisation could be noticed in Al2O3: MgO (1:1) batch through double stage sintering. Densification and grain size of the sintered specimen was found to be greatly dependent on initial calcination temperature and content of primary spinel seed. The diametrical compressive strength of sintered specimen was ∼115 MPa for highest amount of spinel content. The hardness of MgO-rich spinel composite was varied with spinel addition.

Preparation of MgO–MgAl2O4 composite for refractory application

Solid-state reaction was adopted to prepare MgO-rich MgAl 2 O 4 spinel from commercially available sintered seawater magnesia and α-alumina. The starting materials were mixed in weight ratio (Al 2 O 3 :MgO) of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4. Calcination led to the development of MgAl 2 O 4 spinel crystal seed, which was varied (5–50%) with respect to MgO content and temperature. Spinellisation of 70 vol.% was observed for equal weight proportionate calcined powder, when calcined at 1000 °C for 2 h and subsequent sintered at 1600 °C for 4 h. The initial calcination temperature and hence the primary spinel seed content was found to influence the densification and microstructure of sintered specimen. Finally attempts were made to correlate the effect of spinel seed content on the crystallization behavior and microstructure.

Low-T formation of high-Cr spinel with apparently primary chemical characteristics within podiform chromitite from Rayat, northeastern Iraq

AbstractChemical modification of chromian spinel at low-T alteration was examined in detail for a podiform chromitite from a Tethyan ophiolitic mélange belt at Rayat, northeastern Iraq. The chromitite is highly brecciated and the matrix has been completely altered, producing chlorite and carbonate (dolomite and calcite). High-Cr, low-Fe3+ spinel has formed along the margins and cracks of chromian spinel grains throughout the alteration, associated with unaltered primary spinel and magnetite without ferritchromite. In associated harzburgites, only ferritchromite is found instead of the high-Cr, low-Fe3+ spinel. The high-Cr, low-Fe3+ secondary spinel apparently has chemical properties of mantle origin, plotted at the extension of ordinary mantle spinels on compositional spaces. The character is due to subtraction of Al as chlorite with the addition of an amount of magnetite component from the silicate matrix, which is small in volume relative to peridotite and composed of highly magnesian olivine (up to Fo97). We should treat high-Cr chromian spinels with caution in highly altered mantle-derived rocks, especially chromitite and other rocks with highly magnesian olivine, as well as in detrital particles for provenance study.