Composition Of Chromian Spinel Research Articles

Provenance of Miocene sandstones in northern Iraq: constraints from framework petrography, bulk-rock geochemistry and mineral chemistry

Modal analysis, bulk-rock geochemistry and phase chemistry of sandstones of the Miocene Fat'ha and Injana formations, northern Iraq, show that the clastics were derived from heterogeneous sources that include basic igneous and metamorphic rocks as well as older sedimentary rocks. The sandstones are generally carbonate-rich lithic arenites. Their geochemistry supports the petrographic results and indicates that they are all Fe-rich, lithic or quartz arkosic sandstones. According to geochemical data, garnets are derived from metamorphic sources, hornblende is of igneous origin, and clinopyroxenes, are produced by basic igneous rocks. Epidote is most probably to be a product of disintegration of metamorphic rocks, essentially, metamorphosed igneous rocks. Rutile geochemistry implies low-grade metamorphic and basic to ultrabasic igneous sources. Chemical composition of chromian spinels indicates that they are derived from Alpine-type peridotite. The ophiolitic-radiolarite belts of Taurus-Zagros as well as the uplifted Cretaceous and Paleocene strata of north and northeastern Iraq are likely to be the major source of clastics to the Fat'ha–Injana basin, a foreland basin formed as a result of the continental Arabian and Turkish/Iranian plates collision.

CHROMIAN SPINEL IN THE TURNAGAIN ALASKAN-TYPE ULTRAMAFIC INTRUSION, NORTHERN BRITISH COLUMBIA, CANADA

The Early Jurassic Turnagain intrusion (24 km2) in northern British Columbia, Canada, is one of a number of Alaskan-type intrusions that occur within the Mesozoic Quesnel accreted arc terrane. It is distinguished by the presence of magmatic Ni sulfide mineralization hosted by ultramafic cumulate rocks. Chromian spinel is a common accessory mineral ( 0.85, Fe2+/(Fe2+ + Mg) < 0.4] are preserved in magnesiochromite grains from the chromitites, which were isolated from nearby olivine and interstitial melt and thus did not significantly re-equilibrate during cooling. Extensive re-equilibration with evolved interstitial melt, defined by broadly linear trends of increasing Al, Fe3+, Ti, V and Fe2+/(Fe2+ + Mg) with decreasing Cr content, is recorded in chromian spinel grains from dunite, wehrlite, and olivine clinopyroxenite. These trends converge toward the compositional field of grains from the chromitites. Rims of Fe3+-rich chromite and Cr-bearing magnetite, up to 30 μm thick, formed around chromian spinel grains during subsolidus infiltration of oxygenated fluids and serpentinization. Relative to the global database of spinel compositions from Alaskan-type intrusions, the majority of the chromian spinel compositions from the Turnagain intrusion are characterized by lower Fe3+/(Fe3+ + Cr + Al) and higher Cr/ (Cr + Al), which is consistent with a relatively low oxygen fugacity for the parental magmas to the ultramafic cumulates. The Turnagain parental magmas were likely reduced by assimilation of graphite- and pyrite-bearing phyllite host-rocks, which led to local saturation in a sulfide liquid.

Sources of placer platinum in Yukon: provenance study from detrital minerals

Source rocks for the platinum group minerals (PGM), historically reported in a number of Yukon placers, remain either unknown or poorly understood. A study of heavy-mineral samples from five creeks draining bedrock in west and central Yukon was undertaken to confirm the presence of placer platinum, to determine which mafic–ultramafic rock is the source of PGM in Kluane area, southern Yukon, and to explain platinum occurrences in Canadian and Florence creeks, central Yukon, where no known mafic–ultramafic rocks are present. Diverse composition of chromian spinel and clinopyroxenes from three creeks in the Kluane area indicate several sources of ultramafic rocks, including fragments of Alpine-type peridotites formed in back-arc basin and mid-ocean-ridge settings, and a source rock for zoned zinc-rich chromites of unknown origin. The Kluane ultramafic sills are the most likely source of PGM in this area. The heavy-mineral sample from Canadian Creek returned one PGM grain, no chromite, and abundant ilmenite and titanomagnetite. A group of chromium-rich magnesian ilmenites (∼4 wt.% MgO) closely match the composition of ilmenites from continental mafic intrusions produced during continental rift magmatism. This supports the continental rifting event recently proposed for this part of Yukon and indicates the economic potential of the Canadian Creek platinum occurrence. Composition of spinel from Florence Creek sample indicates an Alaskan-type intrusion as the source of PGM.

Chromian spinels in mafic–ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite

This paper describes a comprehensive study of the chromian spinels present in mafic–ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-Mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO 2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45 < Cr# < 0.63; 0.3 < Mg# < 0.6; TiO 2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic–andesitic melts that preferentially form in subduction zone settings (0.35 < Cr# < 0.80; 0.1 < Mg# < 0.7; TiO 2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent–daughter relationships. The Cr# is on average higher, and the TiO 2 lower in the NW than in the SE, consistent with an increasing influence of “marginal” magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an “open” ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the spreading history: the distribution of the Cr# around a former mantle diapir cropping out in the Maqsad area is concentric, with the highest values in the centre of this structure, consistent with higher degrees of partial melting of the mantle. On the other hand, unusually low Cr# – diagnostic of a low melting degree – in a ridge-parallel band of about 20 km to the northeast of the Maqsad area can be related either to temporal variations in the partial melting degree or to off-axis magmatic activity.

房総半島上総層群長浜層中の蛇紋岩礫

Petrological features of gravels of serpentinized peridotites in the Nagahama Formation of the upper Kazusa Group (middle Pleistocene), distributed in the central Boso Peninsula, are examined in detail. Studied peridotites are serpentinized to various degrees, but their protoliths were inferred from textures and relic minerals; harzburgites frequently containing plagioclase are dominant, and lherzolites (with plagioclase) and dunites are subordinate. Forsterite (Fo) contents of olivine are high, around 91, and Cr/(Cr + Al) ratios of chromian spinel are intermediate, mostly from 0.4 to 0.6. The geological and petrological characteristics of the serpentinite gravels indicate that they were derived from the Mineoka Belt (Circum-Izu Massif Serpentine Belt), southern Boso Peninsula. Serpentinite gravels in several strata, including the Nagahama Formation, from the early Miocene to Recent in the Boso Peninsula, show very similar petrological features (particularly chemical composition of chromian spinels), so the gravels were possibly derived from mantle peridotites of a common origin. Almost homogeneous peridotite bodies have continued to be uplifted at the fore-arc region of the Honshu arc (the Mineoka Belt) throughout the time after protrusion near/at the trench in the early Miocene. Northward streams were active in the central part of the Boso Peninsula due to uplift of the southern part at the time of sedimentation of the Nagahama Formation. The ‘Kamogawa Graben’, which topographically divides the Mineoka Belt and the northern area now, had not been developed in the middle Pleistocene.

Mineral chemistry of Al Haruj low-volcanicity rift basalts, Libya: Implications for petrogenetic and geotectonic evolution

Al Haruj basalts represent the largest volcanic province in Libya (∼16,000 km 2). Field, petrographic and mineral compositional studies of the NW corner of this province have been undertaken in order to better understand their petrogenetic and geotectonic evolution. These basalts have been distinguished into two phases of different field and mineralogical characteristics. Clinopyroxene compositions classified the earlier phase (AHB-I) as subalkaline, tholeiitic basalts and the later, more voluminous, phase (AHB-II) as alkaline ones. Clinopyroxene and chromian spinel compositions suggest anorogenic within plate affinity for basalts of the two phases. Chromian spinel of the Al Haruj basalts shows compositional similarities to those of the ocean island basalts (OIB). Field and mineralogical features of the Al Haruj and Libyan volcanics in general are typical of the low-volcanicity rifts. The Libyan continental rift zone is probably related to the larger Afro-Arabian rift system. The low Mg# of clinopyroxene phenocrysts (59–81 and 50–73 for AHB-I and AHB-II, respectively) and olivine groundmass crystals (60–71 and 53–66 for AHB-I and AHB-II, respectively), implying the evolved nature of these rocks with the AHB-II are relatively more fractionated. Clinopyroxene, olivine and chromian spinel compositions suggest that the AHB-II were crystallized at higher temperature and pressure, probably at greater depth, compared to the AHB-I. It is proposed that the tholeiitic basalts (AHB-I) represent pre-rift stage magma, probably generated by higher degree of partial melting of asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts (AHB-II) represent the rift stage magma formed by low degree of partial melting of the same source, at greater depth.

Secondary and backscattered electron imaging of weathered chromian spinel

Secondary (SE) and backscattered electron (BSE) imaging as well as x-ray microanalysis have demonstrated that the weathering of chromian spinel occurs as a progressive form of alteration. Numerous chemical discriminant analysis methods based on the composition of chromian spinel are used to locate valuable deposits of minerals. These methods will be misleading if the correct interpretation of the weathering of chromian spinel and the subsequent pattern of changes in its mineral chemistry are not properly assessed using scanning electron microscopy. This assessment is vital in understanding the geological processes involved and the economic potential of any indicated deposit. Minerals such as chromian spinel, pyrope garnet, and picroilmenite are considered to be highly resistant to weathering and abrasion and are therefore useful in the search for associated valuable deposits of diamond, nickel, platinum, and gold. Known as indicator minerals, they are usually present in relatively large concentrations compared with the target mineral (e.g., diamond) and form large and often subtle dispersion anomalies adjacent to the deposit. Chromian spinel has long been regarded as a stable indicator mineral; however, detailed SE and BSE imaging indicates that many of the chromian spinels that are routinely examined using scanning electron microscopes (SEM) and microprobes are extensively altered. Secondary electron and BSE imaging of weathered chromian spinel in a normal SEM provides valuable data on the form and chemical style of the alteration. Secondary electron imaging of weathered chromian spinel in the environmental SEM (ESEM) not only enhances the difference in atomic number between unaltered and altered areas but also allows high-resolution imaging of these very fine replacement textures.

On genetic classification of chromian spinels in deep-seated rocks from continental structures

A large data base of 1821 chromian spinel compositions (Cr2O3 > 2 wt %) in ultramafic nodules from basalts, kimberlites, lamproites, ultramafic massifs, and from the diamond assemblage has been analyzed using mathematical statistics methods. For each spinel component under study (TiO2, Al2O3, Cr2O3, FeO , MnO, MgO), a considerable overlap is observed, spinels with the highest Cr-number (#Cr) being found in the diamond assemblage and those with the highest Mg-number (#Mg), in nodules from basalts. The lowest oxidation degree is detected in spinels from nodules in basalts and from the diamond assemblage. Spinels from ultramafic massifs have the highest oxidation degrees and Fe- numbers. In ultramafic massifs from kimberlite to lamproite through to the diamond type, spinel compositions follow the same variation trends. Spinels from the basaltic group fall away from these trends. Compositional variability of spinels is due chiefly to isomorphic replacement of Fe +2 for Mg and of Al and Mg for Cr. Kimberlite-type spinels have high and broadly variable Ti contents. Igneous and metamorphic/metasomatic origins for spinels of the five groups under study are discussed. Spinel compositions from ultramafic rocks found in continental structures are shown to indicate that at depth beneath these structures, there exist physicochemical conditions for crystallization inherent to all the principal tectonic features of the earth-continents, oceans, and transition zones (island arcs).

Numerical modeling of major element distribution between chromian spinel and basaltic melt, with application to chromian spinel in MORBs

A set of empirical equations is developed which allows calculation of chromian spinel composition in equilibrium with a basaltic melt under a known set of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 °C and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spinel from the experiments, which crystallizes from basaltic, boninitic, and komatiitic melt and varies from high-Al to high-Cr, and high-Fe3+ can be successfully reproduced using the suggested model. The composition of chromian spinel has been calculated using the glass composition for a set of primitive basaltic lavas using the suggested set of empirical equations. Good agreement between the calculated composition and composition of rims of chromian spinel included in glass is achieved for compositionally diverse spinel from a Hawaiian sample and 15 MORB samples. The Fe2O3 content in the chromian spinels is, however, often variable and higher than the calculated Fe2O3 content. Additional calculations using 76 published MORB glass analyses reproduced most of the MORB chromian spinel range, except for the most Cr- and Mg-rich. The crystallization of these Cr-rich chromian spinels likely occurred from a more primitive Al-poor melt than that of the 76 MORB glasses. An example of a more primitive glass can occasionally be found as glass inclusions in these Cr- and Mg-rich chromian spinel microphenocrysts.

Primitive intratransform volcanism at Garrett Transform Fault (East Pacific Rise)

The paper presents the results of a petrological study of samples collected by submersible from volcanic features that floor the intratransform domain of the Garrett Transform Fault. Most intratransform volcanics are typically highly porphyritic and primitive mid-ocean ridge basalt (MORB) (glasses have Mg# higher than 0.65), whereas most volcanics close to the East Pacific Rise – transform intersection zone are nearly aphyric evolved MORB (glasses have Mg# lower than 0.54). In the intratransform volcanics, phenocrysts and megacrysts are plagioclase and olivine and accessory spinel microphenocrysts in the magnesian lavas, and clinopyroxene and plagioclase in ferrobasalts. Variable mineral chemistry of plagioclase and spinel, chemical disequilibria between these phases and surrounding glass, and resorption features and oscillatory zoning in plagioclase suggest that limited magma mixing occurred during genesis of intratransform MORB. Aluminous- to chromian-spinel compositions are in agreement with melt interactions in small magma pockets. These processes are superimposed on complex partial melting events of a heterogeneous source region underlying the intratransform domain. Generation of most ferrobasalts can be explained by crystal fractionation of primitive MORB. Some ferrobasalts appear to derive from discrete magma bodies, since they are not simply connected to the magnesian tholeiites through fractional crystallization processes.

Detrital Chromian Spinel Compositions Used to Reconstruct the Tectonic Setting of Provenance: Implications for Orogeny in the Canadian Cordillera

ABSTRACT Detrital chromian spinel is an important accessory mineral in chert and volcanic lithic sandstones from the northern Bowser Basin. Microprobe analyses show that Bowser Basin detrital chromian spinels have Cr/(Cr + Al) between 0.21 and 0. 89, Mg/(Mg + Fe2+) between 0.24 and 0.70, and Fe3+/(Fe3+ + Al + Cr) values below 0.12. Comparison with spinels from literature on ultramafic rocks indicates that the compositional range of the detrital spinels closely matches that of spinels from Alpine-type peridotites emplaced by obduction of marginal-basin crust and island-arc complexes. Furthermore, Bowser Basin spinels are compositionally distinct from spinels derived from either mid-ocean ridge, stratiform ultramafic complex, or Alaskan-type peridotite. The composition of the detrital spinel demonstrates that the source for Bowser Basin sediments included extensive Alpine-type peridotite, which probably originated as marginal-sea lithosphere. In addition, there is no evidence for material derived from mid-ocean ridge, Alaskan-type peridotites, or stratiform complexes. This provenance interpretation is in agreement with earlier interpretations that related chert pebbles in the Bowser Basin to chert-rich strata of the Cache Creek terrane and is consistent with the detrital modal analysis of sandstones, which calls for a source terrane consisting of obducted oceanic crust and island arc. The chemistry of the detrital spinels further suggests the source area was eroded prior to emplacement of Alaskan-type peridotites or any regional gree schist or higher-grade metamorphic events. Thus, microprobe compositional analysis of detrital spinels adds important detail to the provenance interpretation that is not available through analysis of detrital modes alone. The inference that marginal-basin lithosphere rather than material derived from mid-ocean ridges was the sediment source suggests that the obducted source terranes were closely associated with North America.

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr# 0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.

Spinel minerals in transitional and alkali basaltic glasses from Iceland

Crystallization of spinel minerals in transitional and alkali basalts from Iceland can be related to the FeO, MgO, TiO2 and Cr contents of the coexisting melt. Chromian spinel occurs in glasses in which TiO2 is less than 2.8 wt.% and the weight ratio FeO/MgO is less than 2.0, whereas titanomagnetite occurs when the same parameters are greater than 4 wt.% and 2.7, respectively. In addition, chromian spinel only occurs in basalts with Cr greater than 200 ppm. It is suggested that chromian spinel crystallizes, together with olivine, from liquids with olivine liquidus temperatures ranging from above 1,200° C to approximately 1,150° C. A discontinuity in spinel crystallization follows until below 1,100° C, where titanomagnetite starts to crystallize. Compositional variations in chromian spinel attached to, or included, in homogeneous olivine phenocrysts, however, cannot be related to equilibrium relations. Textural relations suggest homogeneous nucleation for titanomagnetite, whereas chromian spinel nucleates heterogeneously, dependent on growth of olivine phenocrysts. The composition of chromian spinels cannot in detail be related to physical and compositional parameters of the average melt, but may be related to local compositional relations in the melt adjacent to growing crystals. Such compositional variation around growing olivine crystals may be the prime reason for the non-equilibrium precipitation of included chromian spinels.

21.Oxidation state of iron in sanukite magma estimated from the composition of chromian spinel

21.Oxidation state of iron in sanukite magma estimated from the composition of chromian spinel