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1.
Megacrysts of titanian pyrope, high-Ti bronzite, sub-calcic diopside to augite, and phlogopite in an alnöitic breccia, Malaita, Solomon Islands, have compositional features which distinguish them from silicates in kimberlites. Particularly important is the high content of Al2O3 in the Malaita silicates, which may reflect the higher Al2O3 content of alnöite than kimberlite. The phlogopite megacrysts fall into two populations, one richer in Mg and Cr, and lower in Ti, than the other. The pyroxenes may also fall into two groups characterized by high and low Mg, Cr and Ni. Two stages of crystallization may be responsible.  相似文献   

2.
The ~1.74 Ga Damiao anorthosite complex, North China, is composed of anorthosite and leuconorite with subordinate melanorite, mangerite, oxide-apatite gabbronorite, perthite noritic (i.e., jotunitic) and ferrodioritic dykes. The complex hosts abundant vein-, pod- and lens-like Fe–Ti–P ores containing variable amounts of apatite (10–60 modal%) and Fe–Ti oxides. In addition to Fe–Ti–P ores, there are also abundant Fe–Ti ores which are closely associated with Fe–Ti–P ores in the deposit. Most of Fe–Ti–P ores are dominated by Fe–Ti oxides and apatite, devoid of silicate minerals, mineralogically similar to the common nelsonites elsewhere. In contrast, Fe–Ti ores are dominated by Fe–Ti oxides with minor apatite (<5 modal %). The parental magma of these ores, estimated from olivine and apatite compositions using mineral-melt partition coefficients, has composition similar to the ferrodioritic dykes. Fe–Ti–P ores have variable Fe–Ti oxides and apatite proportions, indicating that they are cumulates. Their simple assemblage of Fe–Ti oxides and apatite and local net-texture suggest that the Fe–Ti–P ores in Damiao have formed from nelsonitic melts immiscibly separated from the ferrodioritic magma during late-stage differentiation. Fe–Ti ores are also cumulates and have mineral compositions similar to Fe–Ti–P ores. The close association between Fe–Ti and Fe–Ti–P ores indicates that the Fe–Ti ores may have also formed from the nelsonitic melts. We proposed that differentiation of nelsonitic melts accompanied by gravity settling is responsible for the formation of Fe–Ti and Fe–Ti–P ores. Such a differentiation process in nelsonitic melts is well supported by variations of Sr, Y, Th, U, REE and Eu/Eu* of apatite in Fe–Ti–P ores. Using oxides/apatite ratio of 2:1 and compositions of apatite and calculated primary oxides, we estimate the composition of the nelsonitic melt as ~52.0 wt% Fe2O3t, ~18.5 wt% CaO, ~14.2 wt% P2O5, ~8.7 wt% TiO2, ~4.0 wt% Al2O3 and ~1.1 wt% MgO with minor SiO2, K2O, Na2O and F. Such a nelsonitic melt is suggested to be possibly conjugated with Si-rich melts compositionally similar to the Damiao jotunitic dykes (~50 wt% SiO2 and ~15 wt% Fe2O3t) which may subsequently evolve to mangeritic rocks in Damiao. Our modeling also indicates that the onset of immiscibility occurs at a time when the evolved melt has ~44 wt% SiO2, ~21 wt% Fe2O3t, ~3.0 wt% TiO2 and ~2.6 wt% P2O5. High oxygen fugacity and phosphorous content in magmas may play important roles in the immiscibility of nelsonitic magmas, including promoting iron enrichments and widening the two-liquid field.  相似文献   

3.
Nelsonite and Fe–Ti oxides ore are common in Proterozoic massif-type anorthosites and layered intrusions. Their geneses have long been controversial, with existing hypotheses including liquid immiscibility between Si-rich and Fe–Ti–P-rich melts and gravitational fractionation among apatite, magnetite, ilmenite and silicates. In this paper, we report detailed field geology and mineral geochemical studies of the nelsonite and Fe–Ti oxides ore from the Damiao anorthosite complex, NE China. Geological observations indicate that the nelsonite and Fe–Ti oxides ore occur as irregularly inclined stratiform-like or lensoid or veins, and are in sharp contact with the anorthosite and gabbronorite. The widespread veins and lenses structure of the Damiao nelsonite and Fe–Ti oxides ore in the anorthosite indicates their immiscibility-derived origin. The apatite in the nelsonite and gabbronorite shows evolution trends different from that in the gabbronorite in the diagrams of Sr versus REEs and Eu/Eu*, suggesting that petrogenesis of the nelsonite and gabbronorite is different from the gabbronorite. Compared with the gabbronorite, the nelsonite and Fe–Ti oxides ore have magnetite high in Cr, plagioclase high in Sr and low in An, and apatite high in Sr, low in REEs with negative Eu anomaly. The evidence permits us to propose that the Damiao Fe–Ti oxides ore/nelsonite and gabbronorite were derived from different parental magmas. The gabbronorite was formed by solidification of the interstitial ferrodioritic magma in the anorthosite, which was the residual magma after extensive plagioclase and pyroxene crystallization and was carried upward by the plagioclase crystal mesh. In contrast, the Fe–Ti oxides ore and nelsonites and mangerite were produced by crystallization of the Fe–Ti–P-rich and SiO2-rich magmas, respectively, due to the liquid immiscibility that occurred when the highly evolved ferrodioritic magma mixed with newly replenished magmas. The variation from Fe–Ti oxides ore to nelsonite and gabbro-nelsonite upwards (as apatite content increases with height) in the steeply inclined Fe–Ti oxides orebodies suggest that gravity fractionation may have played important roles during the crystallization of the Fe–Ti–P-rich magma.  相似文献   

4.
Magnetite is a common mineral in many ore deposits and their host rocks, and contains a wide range of trace elements (e.g., Ti, V, Mg, Cr, Mn, Ca, Al, Ni, Ga, Sn) that can be used for deposit type fingerprinting. In this study, we present new magnetite geochemical data for the Longqiao Fe deposit (Luzong ore district) and Tieshan Fe–(Cu) deposit (Edong ore district), which are important magmatic-hydrothermal deposits in eastern China.Textural features, mineral assemblages and paragenesis of the Longqiao and Tieshan ore samples have suggested the presence of two main mineralization periods (sedimentary and hydrothermal) at Longqiao, among which the hydrothermal period comprises four stages (skarn, magnetite, sulfide and carbonate); whilst the Tieshan Fe–(Cu) deposit comprises four mineralization stages (skarn, magnetite, quartz-sulfide and carbonate).Magnetite from the Longqiao and Tieshan deposits has different geochemistry, and can be clearly discriminated by the Sn vs. Ga, Ni vs. Cr, Ga vs. Al, Ni vs. Al, V vs. Ti, and Al vs. Mg diagrams. Such difference may be applied to distinguish other typical skarn (Tieshan) and multi-origin hydrothermal (Longqiao) deposits in the MLYRB. The fluid–rock interactions, influence of the co-crystallizing minerals and other physicochemical parameters, such as temperature and fO2, may have altogether controlled the magnetite trace element contents of both deposits. The Tieshan deposit may have had higher degree of fO2, but lower fluid–rock interactions and ore-forming temperature than the Longqiao deposit. The TiO2–Al2O3–(MgO + MnO) and (Ca + Al + Mn) vs. (Ti + V) magnetite discrimination diagrams show that the Longqiao Fe deposit has both sedimentary and hydrothermal features, whereas the Tieshan Fe–(Cu) deposit is skarn-type and was likely formed via hydrothermal metasomatism, consistent with the ore characteristics observed.  相似文献   

5.
The >3·0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although the cherts have undergone postdepositional silica enrichment, such as cementation and metasomatic silicification, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other primary to early diagenetic components were carbonates, sulphates (gypsum and anhydrite) and organic matter. Although these mineral associations, on the whole, correspond to those of modern marine evaporites, they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). This feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated; compared with modern ocean waters, the concentrations of Fe and Si were significantly higher and the pH value might have been lower. These conditions could be obtained by contributions of Fe- and Si-enriched hydrothermal solutions and continental run-off to the site of deposition. Grey cherts contain detrital quartz and altered Fe–Ti oxides and were formed in a period of input of terrigenous detrital materials. They are characterized by higher concentrations of TiO2, Al2O3, Cr, Ni, Zn, Rb and Zr compared with the other types of chert and by very low (< 4) Al2O3/TiO2 values. These features are attributed to the supply of terrigenous detrital materials that contain abundant Fe–Ti oxides (ilmenite and titanomagnetite) and fine TiO2 particles. Such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of Ti relative to Al.  相似文献   

6.
Geochemistry of magnetite and maghemite in soils in European Russia   总被引：1，自引：0，他引：1  
A method is proposed for determining the proportions of soluble Fe oxides (magnetite, FeOFe2O3, and maghemite, γ-Fe2O3) based on the measured magnetic susceptibility before and after treatment of soil with the Tamm or Mehra-Jackson (DCB) reagents. The development of hydromorphism in steppe soils in Ciscausiaia is associated with an increase in the magnetite fraction and, consequently, the average magnetite: maghemite ratio increases from 0.8–0.9 to 1.1. In these soils, smectites facilitate magnetite oxidation to maghemite. Soddy-podzolic and dark humic soils in the Cis-Ural region are noted for low values of the magnetite: maghemite ratio (0.5 on average) due to maghemite predominance. Soils in the Cis-Ural region on cover red-earth clays inherit lithogenic Fe oxides: hematite and maghemite. Hydromorphism in humid environments in northern taiga is accompanied by a significant increase in the magnetite: maghemite ratio to 4–9. Some issues of Fe geochemistry in magnetite and maghemite are considered.  相似文献   

7.
The content of major element oxides (Al2O3, SiO2, CaO, MgO, Fe2O3, P2O5, K2O, TiO2, and MnO) in soil surrounding the light metal factory (TLM), which was producing aluminium from 1937 till 1991, was investigated by EDXRF and XRD. The evaluation of a possible aluminium pollution linked to the industrial activity was studied by a comparison of soil around the TLM with control soil samples. Taking into account natural variations caused by bedrock lithology, control samples were taken in the soil developed upon the Upper Cretaceous limestones and Middle Eocene flysch deposits, while the TLM soil was sampled over the Upper Cretaceous and Early to Middle Eocene limestones; that way the samples constituted four groups, each containing five to seven samples. Control soil overlying flysch deposits differed strikingly from the other three groups by its increased CaO values, and decreased content of Al, Fe, Mn, and Ti-oxides, confirming the overall geochemical imprint of bedrock lithology on the analysed soils. However, a specific mineral composition (presence of alumina) of the TLM soil, together with peculiar correlations among Al2O3 and other oxides indicates a certain extent of aluminium pollution of the investigated area.  相似文献   

8.
FeO*‐Al2O3‐TiO2‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al2O3‐TiO2‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al2O3‐TiO2‐rich rocks are characterized by low concentrations of SiO2 (25–54.2 wt%), Na2O + K2O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al2O3 (20–42%), TiO2 (3–9.2%), and P2O5 (0.26–1.30%). TiO2 is positively correlated with Al2O3 and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al2O3)?(CaO + Na2O + K2O)?(FeO*+ MnO + MgO) and (SiO2)?(FeO*)?(Al2O3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al2O3‐TiO2‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.  相似文献   

9.
Relatively strongly magnetic fine components (< 30μm, XS-4J and DS-4J) which are most environmentally sensitive were separated from layer S5-1 in the Xifeng and Duanjiapo loess sections and analyzed by MPV-3 for their morphometric characteristics and reflectance, SEM-ESD for their element contents and XRD for their mineral phases, respectively. The results showed that minerals in both samples are dominated by detrial Fe-Ti oxides of aeolian origin. In sample XS-4J the reflectance and iron contents of magnetic minerals are usually high. In addition to magnetite (Fe3O4), maghemite (γFe2O3) and hematite (Fe2O3), some Fe-high oxide (72.25 wt%–86.67 wt%), ilmenite (FeTiO3), and magnetite-ulvöspinel [Fe(FeCr)O4, Fe (FeNi)O4] were also detected. In sample DS-4J obvious negative linear correlations were found between Ti and Fe, and the contents of Mn, Si, Al and Ca are usually high and the minerals are dominated by magnetite (maghemite), goethite (FeOOH) and limonite (containing Si and OH). In addition, the signs of corrosion of magnetic minerals and newly crystallized magnetite (maghemite) were recognized. Differences in the composition and assemblage characteristics of magnetite minerals between XS and DS reflect significant differences in source rocks and preserving conditions.  相似文献   

10.
Textural and mineral–chemical characteristics in the Bangriposi wehrlites (Eastern India) provide insight into metamorphic processes that morphologically and chemically modified magmatic spinel during serpentinization of wehrlite. Aluminous chromite included in unaltered magmatic olivine is chemically homogenous. In sub-cm to 10s-of-micron-wide veins, magnetite associated with antigorite and clinochlore comprising the serpentine matrix is near-stoichiometric. But Al–Cr–Fe3+ spinels in the chlorite–magnetite veins are invariably zoned, e.g., chemically homogenous Al-rich chromite interior successively mantled by ferritchromite/Cr-rich magnetite zone and magnetite continuous with vein magnetite in the serpentine matrix. In aluminous chromite, ferritchromite/Cr-rich magnetite zones are symmetrically disposed adjacent to fracture-controlled magnetite veins that are physically continuous with magnetite rim. The morphology of ferritchromite–Cr-rich magnetite mimics the morphology of aluminous chromite interior but is incongruous with the exterior margin of magnetite mantle. Micropores are abundant in magnetite veins, but are fewer in and do not appear to be integral to the adjacent ferritchromite–Cr-rich magnetite zones. Sandwiched between chemically homogenous aluminous chromite interior and magnetite mantle, ferritchromite–Cr-rich magnetite zones show rim-ward decrease in Cr2O3, Al2O3 and MgO and complementary increase in Fe2O3 at constant FeO. In diffusion profiles, Fe2O3–Cr2O3 crossover coincides with Al2O3 decrease to values <0.5 wt% in ferritchromite zone, with Cr2O3 continuing to decrease within magnetite mantle. Following fluid-mediated (hydrous) dissolution of magmatic olivine and olivine + Al–chromite aggregates, antigorite + magnetite and chlorite + magnetite were transported in 10s-of-microns to sub-cm-wide veins and precipitated along porosity networks during serpentinization (T: 550–600 °C, f(O2): ?19 to ?22 log units). These veins acted as conduits for precipitation of magnetite as mantles and veins apophytic in chemically/morphologically modified magmatic Al-rich chromite. Inter-crystalline diffusion induced by chemical gradient at interfaces separating aluminous chromite interiors and magnetite mantles/veins led to the growth of ferritchromite/Cr-rich magnetite zones, mimicking the morphology of chemically modified Al–Cr–Fe–Mg spinel interiors. Inter-crystalline diffusion outlasted fluid-mediated aluminous chromite dissolution, mass transfer and magnetite precipitation.  相似文献   

11.
Spinels, Fe–Ti oxide minerals, apatites, and carbonates hosted in ophiolitic serpentinites and metagabbros of Gabal Garf (southern ED) and Wadi Hammariya (central ED) of Egypt are discussed. Microscopic and electron probe studies on these minerals are made to evaluate their textural and compositional variations. Alteration of chromites led to form ferritchromite and magnetite; rutile–magnetite intergrowths and martite are common in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded. Composite intergrowth grains of titanomagnetite–ilmenite trellis lamellae are common in metagabbros. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicate an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the high P H2O and oxygen fugacity of the parental magma. The sulfides minerals include pyrrhotite, pyrite and chalcopyrite. Based on the chemical characteristics, the Fe–Ti oxide from the ophiolitic metagabbros in both areas corresponds to ilmenite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpentinites and calcite in metagabbros. Spinel crystals in serpentinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the altered rims. Compared to cores, the metamorphic rims are enriched in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respectively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67) due to Mg–Fe and Al (Cr)–Fe3+ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. The Fe–Ti oxides have been formed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the altered spinel rims correspond to metamorphism around 500–600 °C. Geochemical evidence of the podiform Al-spinels suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the host rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, suggesting relic magmatic composition unaffected by metamorphism. The composition of Al-spinel grains suggest an ophiolitic origin and derivation by crystallization of boninitic magma that belonging to a supra-subduction setting could form either in forearcs during an incipient stage of subduction initiation or in back-arc basins.  相似文献   

12.
The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton(NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite(TiO_2 ~0.1 wt%) and serpentine(Mg#=92.42–96.55), as well as residual olivine(Fo=89–90), orthopyroxene(En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine(Fo=83–87), orthopyroxene(En=82–86), humite(Mg#=82–84) and hornblende [XMg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite(Th-rich oxide) and monazite(LREE-rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO_2(4.77–25.23 wt%), Fe_2O_3 T(32.9–80.39 wt%) and MgO(5.72–27.17 wt%) and uniformly, those of the wall rocks are also SiO_2(16.34–48.72 wt%), Mg O(16.71–33.97 wt%) and Fe_2O_3 T(6.98–30.92 wt%). The striking high Fe-Mg-Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high-Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron-rich chemical precipitates(BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe-Mg-Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.  相似文献   

13.
Hoegbomite occurs sparingly in minute (mostly 0.1 mm) grains with fine-grained hercynite, magnetite, and rutile in two coarse-grained kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India. The hoegbomite is Ti-poor (2.5 wt% TiO2), Fe-rich (25–26% Fe as FeO), and contains 6.2–6.8% MgO, 59.8–60.1% Al2O3, 1.0–1.3% ZnO, 0.3–0.7% Cr2O3 and 0.02% Li2O. Minor amounts (estimated not to exceed 0.2 wt% oxide) of V, Co, Ni, Ga, and Sn were detected on the electron microprobe, but Be, Nb, and Zr were not detected with the ion microprobe mass analyser. Assuming the crystal structure refined by Gatehouse and Grey (1982) to be applicable to the Ellammankovilpatti hoegbomite, the analyses were recalculated on a basis of 22 cations, 30 oxygens, and two hydroxyls, resulting in 49 to 53% of the iron being ferric. Identification of hoegbomite was confirmed by X-ray powder diffraction. Associated cordierite (Fe/(Fe+Mg)=0.14) and kornerupine (Fe/(Fe+Mg)= 0.27) contain 0.02 weight % Li2O and 0.05–0.07% BeO, while only the kornerupine contains B2O3 — 1.57% (ion microprobe analyses). Hoegbomite and the other oxides may have crystallized at temperatures between 680 and 720° C (P6.5 kbar) following attainment of peak conditions by the reaction: kornerupine+sillimanite±rutile+ZnO+H2O+O2 =cordierite+chlorite+hercynite+hoegbomite +magnetite+B2O3.The conditions for hoegbomite formation at Ellammankovilpatti appear to be characteristic of many hoegbomite parageneses. Critical for hoegbomite are silica undersaturation and relatively high oxygen and water activities at fairly high temperatures, conditions which are most commonly attained in later phases of a metamorphic cycle in upper amphibolite- and granulite-facies terrains.  相似文献   

14.

Shephards Discordant Zone is a 500–600 m thick interlayered sequence of deformed, altered and metamorphosed magnetite metagabbro and about 50 layers or lenses of magnetitite (> 80–90% magnetite). The sequence shows progressive magmatic fractionation upwards: Ti and Ti/Fe increase, and V, V/Ti and Cr decrease upwards in magnetite and in whole‐rock compositions. The main magnetite‐rich sequence (about 400 m thick) is deeply weathered, with 40 m of saprolite showing vertical zonation of weathering minerals due to progressive weathering. Magnetitites (average 1% V2O3) are resistant to weathering and show little chemical change, but magnetite gabbros (average 0.27% V2O3) are extensively weathered and show progressive loss of Ca, Na, Mg and S. Plagioclase, magnetite (1.37% V2O3), chlorite (up to 0.35% V2O3), actinolite, epidote and minor sulfides in unweathered rocks weather to kaolinite, hematite, goethite and minor vermiculite, ilmenite remaining largely unaffected. Vanadium is essentially immobile during weathering and is unaffected during weathering of magnetitites (1% V2O3), but is slightly depleted during weathering of magnetite gabbros (0.23% V2O3).  相似文献   

15.
The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have εNd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr2O3 contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO2 and a volatile-rich condition.  相似文献   

16.
Ti-andradite (melanite) has been found in a metapyroxenite layer in the upper part of the Malenco ultramafics(Italy), coexisting with clinochlore, diopside and magnetite. Field observations, as well as major and trace elementbulk-rock composition, strongly suggest a cumulate origin for the layer. Textural relationships indicate thatTi-andradite formed during two different metamorphic stages. Under peak metamorphic conditions (400–450°C, 5±2 kbar)Ti-andradite grew in an assemblage of diopside, clinochlore, magnetite and rare ilmenite and perovskite. Later, retrograde brittle deformationinduced formation of veins containing the paragenesis Ti-andradite, vesuvianite, diopside, chlinochlore, magnetite and accessory perovskite.The Ti-andradite varies considerably in TiO2 (0.11–9.62 wt%), Fe2O3(14.3–30.5 wt%), Al2O3 (0.65–3.90 wt%), Cr2O3(>0.18–0.98 wt%) and SiO2 (32.1–36.1 wt%); this is mostly, but not entirely, due to distinct zoning.Ti-andradite contains 0.32 to 0.66 wt% H2O as determined by infrared spectroscopy and 0.83 to 1.76 wt% FeO. The CaO shows almost no variation (34.1±0.7 wt%) and Ca completely fills the dodecahedral site. Single crystal site refinements indicate that no tetrahedral Ti or Fe replaces Si. Titanium incorporation is attributed to similar degrees of substitution along the exchange vectors Ti3+ Fe3+, Ti4+ AlIV Al -1 VI Si-1 and (Fe2+, Mn2+, Mg2+)Ti4+ 2Fe -1 3+ . The presence of mixed valence states of both Fe and Ti suggests a low oxygen fugacity during crystallization of Ti-andradite. Mass balance calculations indicate an isochemical origin of the first generation of Ti-andradite in the clinopyroxenite layer. Its occurrence is restricted to antigorite-free mineral assemblages containing clinochlore of 0.95X Al>1.1. The hydrothermal crystallization of Ti-rich andradite in veins demonstrates Ti mobility in aqueous fluids under moderate P-T conditions. The zonation patterns indicate disequilibrium conditions during vein crystallization. As no fluorine-, carbonate- and phosphate-bearing minerals were found, OH- is most probably the ligand complexing Ti.  相似文献   

17.
Nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav massif, Kazakhstan, have been examined utilizing electron microscopy and focused ion beam techniques. Host garnet and zircon contain numerous pockets of multiple inclusions, which consist of 1–3 diamond crystals intergrown with quartz, phengite, phlogopite, albite, K‐feldspar, rutile, apatite, titanite, biotite, chlorite and graphite in various combinations. Recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt% of SiO2, suggesting a relatively low‐temperature fluid rather than a melt. Transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides, rare carbonates and silicates. Within the 15 diamond crystals studied, abundant inclusions were found of SiO2, TiO2, FexOy, Cr2O3, ZrSiO4, and single grains of ThxOy, BaSO4, MgCO3, FeCr2O4 and a stoichiometric Fe‐rich pyroxene. The diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the Kokchetav diamond crystallized from a fluid containing variable amounts of Si, Fe, Ti, Cr, Zr, Ba, Mg and Th and other minor components such as K, Na, P, S, Pb, Zn, Nb, Al, Ca, Cl. Most of the components in crystals included in diamond appear to have their origin in the subducted metasediments, but some of them probably originate from the mantle. It is concluded that Kokchetav diamond most likely crystallized from a COH‐rich multicomponent supercritical fluid at a relatively low temperature (hence the apparently low content of rock‐forming elements), and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components.  相似文献   

18.
Principal components analysis is used to study the chemical compositions of pyroxenes of five Apollo 12 specimens. Important correlations recognized in the variation of oxide weight per cent are: MGO, Al2O3, SiO2| CaO, TiO2, FeO MgO, Al2O3, SiO2| FeO MgO, SiO2, FeO | Al2O3, CaO, TiO2 where the oxides on one side of the bar are correlated positively with each other and negatively with the oxides on the other side. Several other similarly distinct relationships with significantly less variance could be noted. These correlations indicating substitutional relationships can be interpreted as representative of stable and metastable trends of crystallization by using crystal-chemical and thermodynamic information. The per cent variance of pyroxene groups with characteristic trends in each specimen can be evaluated and interpreted in terms of history of crystallization. Distribution of Fe and Mg in certain pairs of olivine and pyroxene, which are found in contact in the rock and which may have crystallized simultaneously, is useful in recognizing the tendency towards chemical equilibrium in FeMg distribution during a limited interval in the liquidus or subsolidus stages.  相似文献   

19.
The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.  相似文献   

20.
Spinel-lherzolite xenoliths in alkali basalts from eastern China have porphyroclastic to equigranular textures displaying varying degrees of deformation and subsolidus re-equilibration. The proportions of minerals in these xenoliths vary from 52 to 72% homogeneous olivine (Fo88-91); 11 to 26% orthopyroxene (Wo0.9.1.6; En88-90; Fs8.7.10.7), with minor discontinuous variations of Al2O3, FeO, and CaO; 6 to 19% clinopyroxene (Wo43.47; En49.51; Fs3.7.6.7); and 1 to 5% spinel, with similar Mg# (79.6 to 82.6), but wider variations of Al2O3 and Cr2O3 (100Cr/(Cr + Al + Fe3+) = 8.1 to 23.6). Although previous trace-element and isotopic studies have shown that at least two distinctly different mantle sources were sampled by Cenozoic basalts, mineralogical heterogeneities seem to be minor within the spinel-peridotite-facies lithosphere beneath eastern China.

These xenoliths experienced limited interaction with the host basaltic magma during eruption. Symplectites of secondary, minute silicates, titanomagnetite, and sulfide have replaced orthopyroxene—and to a lesser extent olivine—at the contact with the basalt. The spinel in the margin of the xenolith is continuously zoned by substitutions of Fe3O4 (magnetite) and Fe2TiO4 (ulvospinel) for MgAl2O3 (spinel), and is rimmed by titanomagnetite with a sharp boundary. However, the compositions of the interior clinopyroxenes were commonly modified by metasomatic partial melting, which resulted in “spongy-textured” rinds on primary clinopyroxene. This secondary assemblage is composed mainly of a refractory, jadeite-poor clinopyroxene, which is largely in optica! continuity with the primary clinopyroxene in addition to interstitial feldspars, with minor titanomagnetite and Fe-Ni sulfides. This assemblage was produced by the introduction of K-rich fluids from the enclosing basaltic magma. The intensity of these secondary reactions appears to have been a function of the residence time of the xenolith in the host basalt. Therefore, all secondary alteration of both external and internal primary minerals in these xenoliths are the result of near-surface metasomatic processes, rather than of mantle phenomena.  相似文献   

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