Low Cr2O3 Research Articles

Linking kimberlite magmatism in the Brazilian Platform with Pangea break-up events using in situ Rb-Sr in phlogopite

Kimberlite magmatism provides insights into periodic tectonothermal processes linked to the evolution of supercontinents. Paleozoic and Mesozoic kimberlites overlapping with the Pangea cycle are exposed in the Amazonian Craton (Pimenta Bueno field) and the Neoproterozoic Brasília Belt. Phlogopite in mantle xenoliths in kimberlites from the Amazonian Craton are characterized by low Cr2O3 (< 1 wt%) and an enrichment in TiO2 (> 2 wt%), akin to secondary phlogopite formed by kimberlite magma infiltration in the mantle. Low Ti-Cr (< 1 wt%) and high #Mg (91–92) grains are linked to extensive metasomatism of garnet peridotites resulting in clinopyroxene-phlogopite rocks. In situ phlogopite Rb-Sr isochron ages of 250–220 Ma constrain the emplacement of pipes in the Amazonian Craton with no age difference between distinct phlogopite compositions (higher and lower TiO2 phlogopite) and microstructures (porphyroclasts, neoblasts, reaction rims). Mantle xenoliths in kimberlite pipes from the Brasília Belt present high TiO2 (> 2 wt%) akin to secondary phlogopite formed by magma infiltration, and low Ti-Cr (1 wt%), and high #Mg (> 92) phlogopite akin to primary grains in equilibrium with garnet. In situ phlogopite Rb-Sr isochron ages from all Brasília Belt phlogopite types span 90–80 Ma, constraining the emplacement of diamondiferous and barren pipes, with no relic of older metasomatic events. The kimberlites from the Pimenta Bueno field (Amazonian Craton) are coeval with the beginning of the extension within Pangea that culminated with the Central Atlantic rifting. In fact, the Permian-Triassic kimberlites occur close to intracratonic basins that are crosscut by ca. 200 Ma dykes and sills of the Central Atlantic Magmatic Province. In situ phlogopite Rb-Sr isochron ages spanning 90–80 Ma in the pipes from central and southeastern Brazil (Brasília Belt) may be associated with propagation of far-field stresses linked to opening of the South Atlantic Ocean during the Pangea break-up.

Some Mineralogical Characteristics of the Egyptian Black Sand Beach Ilmenite Part I: Homogeneous Ilmenite and Titanhematite-Ferriilmenite Grains

The high-grade Egyptian beach ilmenite concentrate contains various mineral textures in addition to the main homogeneous ilmenite grains (63%), which may contain solid solutions of geikielite (MgTiO3) and pyrophanite (MnTiO3) mineral components. Few homogeneous ferrilmenite grains (2%) associated with the concentrated ilmenite grains are detected. The contents of Fe2O3, MgO, Al2O3, and Cr2O3 in the ferrilmenite grains range between 7.3% and 22.8%, 3.4% and 6.6%, 0.2% and 0.7%, and 0% and 1.2% respectively. The detected hematite-ilmenite exsolved intergrowths (21.4%) have titanhematite exsolutions of different shapes, sizes, and orientations. They occupy 5%-40% of the whole intergrowth and may show one or two distinct generations. In some ferrilmenite components, MnO ranges between 1.5% and 8.6%. The Cr2O3 and Al2O3 contents range between 0% and 1.2% and 0% and 3.2% respectively. They are mostly between 0% and 0.1% for either of the ferrilmenite components, while relatively greater content is present in the titanhematite components. In some grains, the titanhematite exsolution bodies are replaced by goethite or hydrated iron oxides. In others, the ferrilmenite intergrowth may be partially or completely altered into leucoxene. Some minor composite grains are detected in the concentrate, where each grain consists of two parts, one part is titanhematite-ferrimenite and the other is ferrilmenite-titanhematite. The titanhematite exsolved components have relatively lower TiO2 content (5.8%-23.8%). Both MgO and MnO are positively correlated with FeO rather than Fe2O3. The presence of sphenes in the obtained ilmenite concentrate may be responsible for the recorded amounts of SiO2 (30.1%-30.8%) and CaO (28.1% and 28.8%). The Cr2O3 content is relatively much higher in sphene spots than in ilmenite spots, ensuring that Cr2O3 neither follows TiO2 nor FeO. The nature of the problem of the relatively lower Ti content and the relatively higher Fe and Cr contents of the obtained ilmenite concentrates is the target of the article. The problem is related to the mineralogy of ilmenite or to the used physical concentration flowsheet of the separated concentrate and the ability to improve the ilmenite concentrate’s specifications. It is concluded that although the homogeneous ilmenite is characterized by low Cr2O3 content, some of the other exsolved texture components, e.g. titanhematite and sphenes, have relatively higher Cr2O3, in addition to Fe2O3, SiO2, or CaO. They can negatively affect the marketable specifications of the separated Egyptian black sand ilmenite concentrate.

Multi‐Stage Evolution of the South Australian Craton: Petrological Constraints on the Architecture, Lithology, and Geochemistry of the Lithospheric Mantle

AbstractTo improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on &gt;3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (&gt;160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere.

Microstructure and corrosion behavior of Al2O3–Cr2O3 composites with various Cr2O3 content in Al2O3–SiO2–CaO–FeO slag

The corrosion of refractory materials by high-temperature slag has caused severe technical challenges and economic losses to the metallurgical industry. In this study, the microstructure and mechanical properties of Al2O3–Cr2O3 composites were investigated. The static crucible method was used to investigate the corrosion and infiltration behavior of a typical smelting reduction ironmaking slag for the Al2O3–Cr2O3 refractory. Corrosion test results indicate that the corrosion region of Cr2O3–Al2O3 composites is mainly composed of a corrosion and infiltration layer, and the thickness of the corrosion layer decreases significantly with increasing Cr2O3 content in the sample, whereas the thickness of the infiltration layer increases gradually. The dissolution of Al2O3 into the slag is the main reason for the severe erosion of Al2O3–Cr2O3 composites with a low Cr2O3 content. Increasing the amount of Cr2O3 can significantly improve the slag corrosion resistance of Cr2O3–Al2O3 composites, however will have a negative effect on the slag infiltration resistance, which is mainly owing to the increase in porosity of the composite refractory. The formation of the (Al, Cr)2O3 solid solution effectively inhibited the dissolution of alumina in the slag. In addition, excessive Cr2O3 content can also consume Al2O3 in the slag and form an (Al, Cr)2O3 solid solution, which improves the slag corrosion resistance of the aluminum chromium composite refractory.

In-situ O-isotope analysis of relict spinel and forsterite in small (

We report high-precision secondary ion mass spectrometer triple oxygen isotope systematics (95 individual analyses) from 37 micrometeorites (MMs) collected from South Pole Water Well (SPWW), Antarctica. The study population focuses on unmelted coarse-grained (Cg) MMs (n = 23) with both multiple (n = 14) and single-mineral (n = 9) varieties investigated. We also analysed relict minerals in porphyritic cosmic spherules (n = 13) and the relict matrix in a single scoriaceous fine-grained (Fg) MM. The target minerals investigated are primarily olivine (Fo ∼ 43–99%) and spinel. Textural, chemical and isotopic data confirm that both olivine and spinel grains have retained their pre-atmospheric O-isotope compositions, allowing inferences to be drawn about their formation and parent body affinities. We separate the study population into three groups: spinel-free particles (consisting of the CgMMs and PO cosmic spherules), spinel-bearing MMs and the single FgMM.Olivine grains in spinel-free MMs vary between δ17O: −12.6‰ and +3.5‰, δ18O: −9.6‰ and +7.5‰, and Δ17O: −9.5‰ and +1.3‰ and define a slope-1 profile in δ18O–δ17O isotope space. They are most likely fragmented chondrules, with both type I and type II varieties represented. Their observed Mg#-Δ17O distribution is best explained by a mixture of CM chondrules and either CR chondrules, Tagish Lake chondrules or WILD2 cometary silicates. One of these chondrule-like MMs has an isotopically heterogeneous composition, characterised by a single olivine grain with a markedly 16O-rich composition (Δ17O: −16.3‰), suggesting it is a relict silicate fragment of AOA material that was incorporated into the chondrule precursor.We analysed 11 spinel grains in five spinel-bearing MMs. In all instances spinels are nearly pure MgAl2O4 with isotopically light (16O-rich) compositions (ranging from δ17O: −34.4‰ to −0.9‰, δ18O: −30.8‰ to +11.0‰, and Δ17O: −18.3‰ to −4.4‰). They are therefore 16O-poor relative to spinel found in unaltered CAIs, indicating a different origin. Grains with high Cr2O3 contents (>0.5 wt.%) are interpreted originating from Al-rich chondrule precursors, while low Cr2O3 spinels (<0.5 wt.%) are interpreted as CAI-derived material affected by parent body aqueous alteration.Finally, we report a single FgMM with a 16O-poor composition (Δ17O > 0‰ and δ18O > +15.0‰). This particle adds to our growing inventory of water-rich C-type asteroid samples united by their formation history which is characterised by accretion of abundant heavy water.Our work strongly supports findings from earlier in-situ O-isotope studies, concluding that small MMs overwhelmingly sample material from CC parent bodies and that CgMMs largely sample chondrules and, to a lesser extent, CAI material. The analysis of CgMMs therefore provides insights into the primitive O-isotope reservoirs that were present in the early solar system and how they interacted.

Metasomatic Modification of the Mesoarchaean Ulamertoq Ultramafic Body, Southern West Greenland

AbstractNumerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650°C–850°C and &amp;lt;1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ~800°C–900°C and &amp;lt;2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.

Cryptic zoning in primitive olivine as an archive of mush fluidization at mid-ocean ridges

Processes operating within magmatic crystal mushes exert a fundamental control on the physical and chemical evolution of mid-ocean ridge systems. It is widely recognized that mush fluidization in response to magma recharge could produce variable compositional zoning in crystals. However, olivines generally show relatively smooth MgFe zoning in comparison with complex and short-length scale zoning in plagioclases. It is still unclear about the proceedings of the olivine before the mush disaggregation immediately prior to eruption and unknown whether plagioclase zoning is genuinely more complex than olivine zoning. Here, using slow-diffusing elements X-ray maps, we identify complex compositional zoning in olivines from MORBs of the Southwest South China Sea (SCS) and thus provide mineralogical evidence for the re-organization within a mush that takes place over an extended period of time. Primitive olivines have homogeneous forsterite concentrations (Fo = 89.5 mol%), but preserve zoning in Al2O3 and Cr2O3. Multiple cores have anomalously low Al2O3 and Cr2O3 contents, with or without high P2O5, Al2O3, and Cr2O3 dendrites, and are surrounded by Al–Cr-rich but P-depleted mantles, in which boundaries between cores and mantles are sharply defined. This compositional and textural zoning indicates dissolution, re-equilibration, and re-growth of evolved olivine within primitive magmas, demonstrating a redistribution of crystals within a mush zone, consistent with mush fluidization. Therefore, primitive olivines, widely-used monitors of mantle processes, may also have undergone mush fluidization, such that apparently primitive olivines may result from the re-equilibration of evolved olivines with primitive melts. PAl zoning patterns in olivine can be used to evaluate the influence of mush processes on olivine mantle probes.

Diamondiferous and barren eclogites and pyroxenites from the western Kaapvaal craton record subduction processes and mantle metasomatism, respectively

Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO = 7.3 to 14.5 wt%; n = 43) with accessory diamond ± corundum and kyanite, high garnet grossular content (median Ca# = 0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29 ppm) and have low median Cr2O3 (0.06 wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO = 14.0 to 20.0 wt%; n = 29), which are barren and enriched in LREE (median La 1.39 ppm), Cr2O3 (0.25 wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr > 0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3 Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton.Based on the proportions of MgO-poor and –rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135 ± 15 km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ≤20 km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenites, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.

Geochemistry of the chromitite stringer at the contact of the mafic sequence and the ultramafic sequence in the Unki Mine area, Shurugwi Subchamber of the Great Dyke, Zimbabwe

ABSTRACTSeveral models have been proposed to explain the origin of a chromitite stringer located at the contact between the Mafic and Ultramafic Sequences in the Unki Mine area of the Shurugwi Subchamber of the Great Dyke, Zimbabwe. A petrographic and geochemical study of this chromitite stringer was undertaken with the aim of constraining its origin. Forty-three chromite compositions were obtained from the studied chromitite stringer, which is characterized by a chromium number between 59.9 and 62.8 and a magnesium number which ranges from 37.8 to 46.4. The chromites at the contact zone in the Unki Mine commonly contains inclusions of sulfides, orthopyroxene, plagioclase, and/or amphiboles. The chromites likely formed early in the crystallization history of the Mafic Sequence, as they are commonly partially rimmed by sulfides and they occur as inclusions in plagioclase crystals. Unlike chromites from underlying Ultramafic Sequence chromitite layers, chromites at the contact zone contain low Cr2O3 contents which range from 39.4 to 42.6 wt.%. Furthermore, these chromites are enriched in Fe compared to most Great Dyke chromitites, which is interpreted to be a consequence of subsolidus exchange of Mg into orthopyroxene and Fe into the chromite. The absence of zoning in the chromites at this contact zone, and their low Mn, Fe contents, is consistent with attainment of equilibrium because the altered chromites often contain Cr-bearing magnetite rims. Two possible models for the formation of this chromitite stringer are mixing of relatively primitive and evolved magmas (i.e., ultramafic and anorthositic magma), possibly of different oxygen fugacities, and chemical diffusion across the contact between the Mafic and the Ultramafic sequences which resulted in melting at and below this boundary. The latter would have caused preferential loss of orthopyroxene from the underlying P1 Pyroxenite Layer, accompanied by re-precipitation of chromite at this contact.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N &gt; 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

The Role of Siliceous High-Magnesium Basalts during the Formation of a Neoproterozoic Mafic-Ultramafic Intrusion in the Tarim Craton (China)

The Xingdi No.2 intrusion in the Kuluktag Block of northeastern Tarim Craton, which intrudes into the Palaeoproterozoic basement with an exposed area of ca. 12 km2, is an orthopyroxene-rich mafic-ultramafic intrusion in a continental rift setting. It consists of gabbros, pyroxenites, and peridotites, and exhibits a crystallization sequence of the principal rock-forming minerals from olivine, orthopyroxene, clinopyroxene, to plagioclase. The gabbros show a concordant SHRIMP U-Pb zircon age of 752 ± 5.4 Ma. In addition, the olivine grains have forsterite content values of 78–85 mole% and mostly contain low NiO, MnO, CaO, and Cr2O3. The rocks are relatively enriched in large ion lithophile elements and LREE, and depleted in HSFE, have non-radiogenic Pb, low εNd (t) values (−2.8 to −23), initial 87Sr/86Sr ratios (0.7059–0.7130). It could be concluded that the rocks represent an analogue of siliceous high magnesium basaltic magma originated by the partial melting of a hydrous and enriched subcontinental lithospheric mantle and contaminated by the continental crust.

Compositional variations of chromian spinels from peridotites of the Spontang ophiolite complex, Ladakh, NW Himalayas, India: petrogenetic implications

The Spontang ophiolite complex exposed along the Indus Tsangpo Suture Zone (ITSZ) represents a fragment of oceanic lithosphere emplaced after the closure of the Neo-Tethyan Ocean. The complex lying south of the ITSZ forms the highest tectonic thrust slice above the Mesozoic–Early Tertiary continental margin in the Ladakh-Zanskar Himalaya. The complex consists of a well-preserved ophiolite sequence dominated by peridotites, gabbros and ultramafic cumulates along with highly tectonized sheeted dykes and pillow lavas. The mantle suite of rocks is represented by minor lherzolites, harzburgites and dunites. Chromian spinel is brown to reddish, and its morphology and textural relationship with coexisting silicates varies with strain. Spinel occurs as blebs and vermicular exsolutions within orthopyroxene to spherical inclusions within olivine, characteristic of which is the elongate holly leaf shape. Chrome spinels are characterized by low TiO2 and high Cr2O3 indicative of their depleted nature. Cr# [= atomic ratio Cr/(Cr + Al)] in the studied spinels are characterized by a small decrease in TiO2 for a larger increase in Cr# consistent with observations for spinels aligned along the Luobusa trend of the Yarlung Zangpo Suture Zone (YZSZ) ophiolites. Variations in Cr-spinel Cr# and Mg# observed in the investigated peridotites may have resulted from a wide range of degrees of mantle melting during their evolution. Mineral and whole-rock chemistry of the Spontang peridotites is characterized by interaction between depleted magma and pre-existing oceanic lithosphere, typical of supra-subduction zone settings. The Spontang peridotites have olivine, clinopyroxene and orthopyroxene compositions similar to those from both abyssal and fore-arc peridotites and display spoon shaped REE profiles characteristic of interaction between LREE-enriched melt, derived from the subducting slab and LREEdepleted mantle residues. Equilibration temperatures calculated for the above rocks indicate that the studied samples represent typical mantle peridotites formed within the spinel stability field.

Chromian spinel compositions from Madawara ultramafics, Bundelkhand Craton: Implications on petrogenesis and tectonic evolution of the southern part of Bundelkhand Craton, Central India

Madawara ultramafic complex (MUC) in the southern part of Bundelkhand Craton, Central India comprises peridotite, olivine pyroxenite, pyroxenite, gabbro, and diorite. Coarse‐grained olivine, clinopyroxene (Cpx), amphibole (Amp), Al‐chromite, Fe‐chromite, and magnetite with rare orthopyroxene (Opx) are common minerals in peridotite. Chromites are usually coarse‐grained euhedral found as disseminated crystals in the olivine matrix showing both homogeneous and zoned texture. Al‐chromite, primarily characterizes Cr‐spinels and its subsequent fluid activity and alteration can result in the formation of Fe‐chromite, chrome magnetite, and magnetite. Mineral chemistry data suggest that Al‐chromite is characterized by moderately high Cr2O3 (38.16–51.52 wt.%) and Fe2O3 (3.22–14.51 wt.%) and low Al2O3 (10.63–21.87 wt.%), MgO (1.71–4.92 wt.%), and TiO2 (0.22–0.67 wt.%), whereas the homogeneous Fe‐chromite type is characterized by high Fe2O3 (25.54–47.60 wt.%), moderately low Cr2O3 (19.56–37.90 wt.%), and very low Al2O3 (0.06–1.53 wt.%). Subsequent alteration of Al‐chromite and Fe‐chromite leads to formation of Cr‐magnetite and magnetite. The Cr# of Al‐chromite varies from 55.12 to 76.48 and γFe3+# from 8 to 19, whereas the ferrian chromite has high Cr# varying from 94.27 to 99.53 while its γFe3+# varies from 38 to 70. As a whole, the primary Al‐chromite shows low Al2O3, TiO2 contents, and high Fe#, Cr# values. Olivines have forsterite ranging from 75.96% to 77.59%. The bulk‐rock geochemistry shows continental arc geochemical affinities indicated by the high concentration of large‐ion lithophile elements and U, Th relative to the low concentration of high‐field strength elements. These petrological and mineralogical as well as primary Al‐chromite compositions plotted in different discrimination diagrams suggest an arc environment that is similar to Alaskan‐type intrusion.

Thermobarometry of Whangarei volcanic field lavas, New Zealand: Constraints on plumbing systems of small monogenetic basalt volcanoes

The intra-plate, basaltic Whangarei volcanic field (WVF) is a little-studied cluster of Quaternary monogenetic volcanoes in northern New Zealand. Clinopyroxene-melt equilibria provides an insight to the ascent and storage of the magmas that is not evident from whole-rock-scale geochemistry. Basalts from two of the younger volcanoes contain a population of equilibrium and disequilibrium clinopyroxene phenocrysts. Many of the crystals are resorbed, and are characterised by diffuse, patchy zoning, and low MgO (Mg#70–80) and Cr2O3 contents. Such crystals also occur as relic cores in other phenocrysts. These grew in a magma that was more evolved than that of the host rock composition. Equilibrium clinopyroxenes are enriched in MgO (Mg#83–88) and Cr2O3 (~0.4–0.9wt%), and occur as reverse-zoned crystals, and rim/mantle overgrowths on relic cores of other crystals. These crystals and rim/mantles zones nucleated in magma with a composition similar to that of the host rock. The textural relationships demonstrate that a mafic magma intruded a more silicic resident magma, resulting in crystal-exchange and entrainment of antecrysts. Clinopyroxene-melt equilibria indicate that the crystallisation occurred at temperatures in the range 1135–1195°C, and pressures in the range 290–680MPa. The dominant pressure mode (400–550MPa) equates to depths of about 15–19km which coincides with a present-day body of partial melt in the crust. Higher pressures indicated by subordinate crystal populations indicate staged ascent and crystallisation above the Moho (~26km depth). Thus, the magmatic system is envisaged as a crystal mush column through the lower and mid crust. Such crystallisation histories are perhaps not expected in low flux, monogenetic magma systems, and reflect the importance of the crustal density structure beneath the volcanoes. Future activity could be preceded by seismic events in the lower crust as the magmas intrude localised crystal mush bodies.

Mylonitized peridotites of Songshugou in the Qinling orogen, central China: A fragment of fossil oceanic lithosphere mantle

The Songshugou mylonitized peridotites within the Qinling Group metamorphic rocks in Central China are distributed in the northern part of the Shang-Dan Suture Zone (SDSZ) and contain abundant dunites and harzburgites. The dunites were intensely deformed and mylonitized converting the coarse-grained type to medium- and fine-grained types which contain prominent lenticular structure and relict olivine (Ol) porphyroclasts. Mineralogical and geochemical compositions suggest that the protoliths of the mylonitized peridotites were coarse-grained peridotites of lithospheric mantle origin. The harzburgites occur as enclaves within mylonitic peridotites in the form of lenses or veins. The orthopyroxenes in harzburgites were formed at the expense of Ol and have similar compositions to those of metasomatized harzburgites, characterized by low Al2O3, CaO and Cr2O3 contents. The harzburgites exhibit the gently U-type REE patterns with enriched incompatible elements (Rb, Ba, Sr, Zr and Hf), suggesting the metasomatic origin. The obvious ductile deformation of the large porphyroclastic orthopyroxene (Opx) suggests that the metasomatism occurred before the deformation. Ductile shearing deformation is indicated by the small fold structures and net-style ductile shearing zones within the Songshugou peridotite massif. The process is also result in the alignment of elongated Ol grains from initially coarse-granular via porphyroclastic to fine-granular texture. The relatively low Fo olivine, together with high Al2O3, and CaO contents and the abnormally low total PGE abundance in the fine-grained dunites suggest the ingress of melt/fluid during the mylonitization. The presences of significant amount of amphibole in the peridotites indicate the ingress of hydrous fluids. In general, the Songshugou peridotites have similar compositional characteristics with peridotites of Oman and Troodos ophiolites which are fragments of oceanic lithosphere mantle. One coarse-grained dunite has a TRD age of 875Ma. Additionally two stages Paleozoic TRD ages are obtained from medium-grained and fine-grained dunites (491Ma and 550Ma; 446Ma and 476Ma). The broadly coeval nature of mylonitization with progressive metamorphism of surrounding amphibolites suggested that the Songshugou peridotites were generated before the early Paleozoic deformation. Our data, combined with the previous work on the surrounding HP/UHP metamorphic rocks, demonstrate that the Songshugou mylonitized peridotites represent fragments of the Neoproterozoic fossil oceanic lithospheric mantle that experienced extensive deformation during the Early Paleozoic subduction processes.

Mineralogy of the TK1 and TK4 ‘kimberlites’ in the Timmasamudram cluster, Wajrakarur Kimberlite Field, India: Implications for lamproite magmatism in a field of kimberlites and ultramafic lamprophyres

A mineralogical study of the hypabyssal facies, late Cretaceous macrocrystic pulse of TK1 intrusion and the Mesoproterozoic aphanitic pulse of TK4 intrusion in the Wajrakarur Kimberlite Field of southern India shows that the rocks contain macrocrysts of forsteritic olivine, phenocrysts and microphenocrysts of Al–Na-poor diopside and phlogopite set in a groundmass mainly of Al–Na-poor diopside and phlogopite. Other groundmass minerals are spinel, perovskite and fluorapatite in TK1, and spinel, titanite, chlorite, calcite and gittinsite in TK4. K-richterite and perovskite occur only as inclusions in phlogopite and titanite, respectively in TK4. Late-stage deuteric phases include pyrite and barite in TK1, and strontianite, chalcopyrite, galena and pentlandite in TK4. Diopside microphenocrysts in TK4 exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth. TK1 spinels show magmatic trend 2 that evolves from magnesiochromite and culminates in titaniferous magnetite, whereas TK4 spinels are less evolved with magnesiochromite composition only. TK1 phlogopites show a simple compositional trend that is typical of lamproite micas, while four distinct growth zones are observed in TK4 phlogopites with the following compositional characteristics: zone I: high Cr2O3 and TiO2 and low BaO; zone II: low Cr2O3; zone III: low TiO2 and high BaO; zone IV: low BaO. Forsterite contents and trace element concentrations reveal two xenocrystic core populations and one magmatic rim population for TK1 olivines. Mineralogically, both TK1 and TK4 are classified as diopside–phlogopite lamproites rather than archetypal kimberlites. The two lamproites are considered to have formed from the same parent magma but crystallised under distinct oxygen fugacity conditions. With elevated content of Fe3+ in phlogopite, spinel and perovskite, TK1 appears to have crystallised in a relatively high oxygen fugacity environment. Multiple growth generations of phlogopite, spinel and fluorapatite in TK4 indicate a complex evolutionary history of the magma. Close spatial and temporal associations of Mesoproterozoic kimberlites and lamproites in southern India can possibly be explained by a unifying model which accounts for the generation of diverse magmas from a range of geochemical resevoirs in a continental rift setting.

Cr-rich rutile: A powerful tool for diamond exploration

Mineralogical studies and U–Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile+forsterite+enstatite+Cr-diopside+Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3>1.7wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin.New analytical developments and availability of standards have made rutile accessible to in situ U–Pb dating by laser ablation ICP-MS. A U–Pb age of 369±10Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between ~725 and 1030°C, corresponding to a depth range of ca ~100–165km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U–Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites.The synthesis of distinctive rutile compositions and U–Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.

Magnetite with anomalously high Cr2O3 as a fingerprint to trace upper Yangtze sediments to the sea

This paper examines geochemical properties of detrital magnetite, in order to link sediments in a Plio-Quaternary core taken in the delta area to their sources in the Yangtze River basin. A total of 40 sediment samples were collected from both the main river channel/tributaries and a sediment core from the Yangtze delta. The geochemical compositions of detrital magnetite in these sediments were analyzed by electron microprobe, including FeO, TiO2, CoO, MgO, Cr2O3, MnO, ZnO, Al2O3 and V2O3. The results revealed that the detrital magnetite grains with anomalously high Cr2O3 occurred exclusively in the upper reaches of the Yangtze (upstream of the Three Gorges Dam), where the E'mei Basalt block is located. This type of magnetite could therefore be considered a unique sediment proxy of the upper river basin to help identify sediment source in the delta area. Our analysis found such magnetite grains with high Cr2O3 occurring throughout the core depth above 186.5m, in contrast to the extremely low Cr2O3 below this depth. The boundary between high and low Cr2O3 in magnetite grains of the core sediments was dated by paleomagnetism at ca. ~1.2–1.0Ma, signifying that the linkage between the Yangtze River course and the sea was before ~1.2–1.0Ma. This demonstrates that the sediment provenance of the Yangtze delta has experienced a change from local to distal Yangtze River, which took place with the uplift of the Tibetan plateau and coastal subsidence during the Plio-Quaternary.

Genesis and geodynamic significance of chromitites from the Orhaneli and Harmancık ophiolites (Bursa, NW Turkey) as evidenced by mineralogical and compositional data

Chromitites from Orhaneli and Harmancık ophiolites (Bursa, NW Turkey) are mainly associated with dunites. They occur as banded and lenticular bodies showing massive and disseminated textures and are composed of magnesiochromite grains characterized by the following composition: Cr2O3=58.30–61.19wt.%, Al2O3=8.32–10.03wt.% and TiO2=0.14–0.21wt.%. The Cr# [Cr/(Cr+Al)] values of magnesiochromite from both ophiolites range between 0.80 and 0.83 and their Mg# [Mg/(Mg+Fe2+)] values vary from 0.52 to 0.69. However, one chromitite sample (BO13B) from the Orhaneli ophiolite contains magnesiochromite with relatively low Cr2O3 (48.38wt.%), high Al2O3 (15.16wt.%) and TiO2 (0.30wt.%) contents, and average Cr# and Mg# values of 0.68 and 0.56, respectively. Although the total content of platinum-group elements (PGE) in most chromitite samples varies between 83 and 169ppb, two samples from the Orhaneli ophiolite show enrichments in total PGE (up to 500ppb). Several platinum-group mineral (PGM) grains occur as minute inclusions in magnesiochromite grains (1–15μm across). Laurite is dominant PGM phase, followed by erlichmanite, unidentified Ru–Ni–Fe sulfide, irarsite, hollingworthite, native osmium and iridium. Millerite and pentlandite represent the most abundant primary base-metal mineral inclusions in magnesiochromite, whereas bornite, awaruite, heazlewoodite, violarite and orcelite are mostly observed in the secondary silicate groundmass. Olivine, clinopyroxene and amphibole are also identified as silicate inclusions in the magnesiochromite. The composition of the magnesiochromite and the parental melts in equilibrium indicate that these magnesiochromite grains were most likely crystallized from boninitic melts in an arc setting. However, BO13B chromitite sample from the Orhaneli chromitites may have crystallized from arc-related melt of probably an island arc tholeiite affinity.

New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

The Hongge magmatic Fe-Ti-V oxide deposit in the Panxi region, SW China, is hosted in a layered mafic–ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr2O3 and 3.20 to 16.2 wt% TiO2, whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr2O3 (<0.4 wt%) but more variable TiO2 (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the net-textured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.