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1.
A wide variety of unusual mantle has been reported from podiform chromitite orebodies Cr-31 and Cr-74 in the Luobusa (罗布莎) ophiolite, Tibet. A detailed investigation of chromitite ore-body Cr-11, located in the Kangjinla (康金拉) district at the eastern end of the ophiolite, has revealed many of the same minerals, including diamond, moissanite, and some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. This orebody is particularly rich in diamonds, with over 1 000 grains recovered from about 1 100 kg sample of chromitite. More detailed studies and experi-ments are needed to understand the origin and significance of these unusual minerals because they have not been found in situ. It is a great breakthrough in mineralogical research that we have picked up more than 40 kinds of minerals from the Kangjinla chromite deposit in Luobusa. It is notable that a large amount of diamonds were firstly discovered from the Kangjinla chromite deposit as well as many other unusual minerals, such as moissanites, rutiles, native irons, and metal alloys. Especially, that diamond was found again in different chromitites In the same ophiolite belt provided new key evidence for discussing the origin of the diamond and the hosted ehromitite and ophiolite. The mantle mineral group in Tibet has great significance in mineralogy and geodynamics.  相似文献   

2.
In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high-Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province,western China,which is characterized by having high-Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds,moissanite and other highly reduced minerals,as well as common crustal minerals. Thus far,more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100-200 μm in size and range in color from pale yellow to reddish-orange to colorless. Most of the grains are anhedral to subhedral octahedra,commonly with elongate forms exhibiting well-developed striations. They all display characteristic Raman spectra with shifts between 1325 cm-1 and 1333 cm-1,mostly 1331.51 cm-1 or 1326.96 cm-1. The moissanite grains are light blue to dark blue,broken crystals,50-150 μm across,commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm-1,785 cm-1,and 966 cm-1. This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high-Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.  相似文献   

3.
Summary ?We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGE-sulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly laurite-erlichmanite) and alloys, are typical only of the unaltered podiform chromitite (assemblage I). Euhedral mono- and polyphase PGM grains in the submicron to 100 μm range show features of primary magmatic assemblages. The diversity of PGM in these assemblages is unusual for ophiolitic environments. In assemblage II, laurite-erlichmanite is intergrown with and overgrown by PGE-sulpharsenides; other minerals of assemblage I are missing. Potarite, stibiopalladinite, mertieite II, native gold and Ag-Au alloys, as well as PGE-sulpharsenides, sperrylite and base metal arsenides and sulphides are characteristic for the highly altered chromitite (assemblage III). They occur either interstitial to chromite in association with metamorphic silicates, in chromite rims or along cracks, and are thus interpreted as having formed by remobilization of PGE by hydrothermal processes during polyphase regional metamorphism. Received August 3, 2000;/revised version accepted December 28, 2000  相似文献   

4.
In recent years diamonds and other unusual minerals(carbides,nitrides,metal alloys and native elements) have been recovered from mantle peridotites and chromitites(both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings.Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite,west Albania.So far,more than 20 grains of microdiamonds and 30 grains of moissanites(SiC) have been separated from the podiform chromitite.The diamonds are mostly light yellow,transparent,euhedral crystals,200~300 μm across,with a range of morphologies;some are octahedral and cuboctahedron and others are elongate and irregular.Secondary electron images show that some grains have well-developed striatums.All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ~1325 cm~(-1).The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue,but some are yellow to light yeUow.All the analyzed grains have typical Raman spectra with shifts at 766 cm~(-1),787 cm~(-1),and 967 cm~(-1).The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon.This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys.Our new findings suggest that diamonds and moissanites are present,and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.  相似文献   

5.
A New Intergrowth Consisting of FeO and SiO_2 Phases from Lower Mantle   总被引：2，自引：0，他引：2  
1. IntroductionIn order to confirm the presence or absence of diamonds in the Luobusa ophiolitic chromitite in Tibet, about 1500 kg chromitite sample was collected for separating its associated minerals, in which diamond, moissanite, octahedral pseudomorphic serpentine and chlorite, native silicon, native chromium, some alloys and other minerals were found. A wide variety of minerals, including about 70-80 types of mantle minerals from the Luobusa ophiolite, were identified by energy dispersi…  相似文献   

6.
The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by spinel harzburgites with minor dunites.The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition,suggesting high degrees of partial melting.Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns,implying the crystallization of interstitial sulphides from sulphur-saturated melts(e.g.MORB-like forearc basalt).U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone.Based on the mineral and whole-rock chemical compositions,the PKO peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon(MTZ).Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element(PGE)contents than those of the MTZ chromitites.However,both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt(e.g.boninitic or island arc tholeiitic melt).Dunite enveloping chromitite lenses in the ho*s ting harzburgite resulted from melt-rock reaction.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than200 grains of microdiamond and more than 100 grains of moissanite(Si C)have been separated from podiform chromitites.These minerals have been identified by EDX and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam.In total,61δ13CPDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.31δ13CPDB results for Moissanite vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.Diamond has relatively large variation in nitrogen isotopic composition with 40δ15NAIR results ranging from-19.1‰to 16.6‰.The discovery of diamond,moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PKO chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.  相似文献   

7.
Deeply subducted lithospheric slabs may reach to the mantle transition zone(MTZ,410-660 km depth)or even to the core–mantle boundary(CMB)at depths of~2900km.Our knowledge of the fate of subducted surface material at the MTZ or near the CMB is poor and based mainly on the tomography data and laboratory experiments through indirect methods.Limited data come from the samples of deep mantle diamonds and their mineral inclusions obtained from kimberlites and associated rock assemblages in old cratons.We report in this presentation new data and observations from diamonds and other UHP minerals recovered from ophiolites that we consider as a new window into the life cycle of deeply subducted oceanic and continental crust.Ophiolites are fragments of ancient oceanic lithosphere tectonically accreted into continental margins,and many contain significant podiform chromitites.Our research team has investigated over the last 10 years ultrahigh-pressure and super-reducing mineral groups discovered in peridotites and/or chromitites of ophiolites around the world,including the Luobusa(Tibet),Ray-Iz(Polar Urals-Russia),and 12 other ophiolites from 8orogenic belts in 5 different countries(Albania,China,Myanmar,Russia,and Turkey).High-pressure minerals include diamond,coesite,pseudomorphic stishovite,qingsongite(BN)and Ca-Si perovskite,and the most important native and highly reduced minerals recovered to date include moissanite(Si C),Ni-Mn-Co alloys,Fe-Si and Fe-C phases.These mineral groups collectively confirm extremely high?pressures(300 km to≥660 km)and super-reducing conditions in their environment of formation in the mantle.All of the analyzed diamonds have unusually light carbon isotope compositions(δ13C=-28.7 to-18.3‰)and variable trace element contents that*d i stinguish them from most kimberlitic and UHPmetamorphic varieties.The presence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around380 km,near the mantle transition zone.The carbon isotopes and other features of the high-pressure and super-reduced mineral groups point to previously subducted surface material as their source of origin.Recycling of subducted crust in the deep mantle may proceed in three stages:Stage 1–Carbon-bearing fluids and melts may have been formed in the MTZ,in the lower mantle or even near the CMB.Stage 2–Fluids or melts may rise along with deep plumes through the lower mantle and reach the MTZ.Some minerals,such as diamond,stishovite,qingsongite and Ca-silicate perovskite can precipitate from these fluids or melts in the lower mantle during their ascent.Material transported to the MTZ would be mixed with highly reduced and UHP phases,presumably derived from zones with extremely low f O2,as required for the formation of moissanite and other native elements.Stage 3–Continued ascent above the transition of peridotites containing chromite and ultrahigh-pressure minerals transports them to shallow mantle depths,where they participate in decompressional partial melting and oceanic lithosphere formation.The widespread occurrence of ophiolite-hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in-situ oceanic mantle.Because mid-ocean ridge spreading environments are plate boundaries widely distributed around the globe,and because the magmatic accretion of oceanic plates occurs mainly along these ridges,the on-land remnants of ancient oceanic lithosphere produced at former mid-ocean ridges provide an important window into the Earth’s recycling system and a great opportunity to probe the nature of deeply recycled crustal material residing in the deep mantle  相似文献   

8.
<正>There is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites.The presence of these minerals suggests that grains of amphibolite(plagioclase,amphibole and zircon)and eclogite(coesite,kyanite and garnet)were present in the magmas from which chromite crystallized.Multiphase  相似文献   

9.
We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite, Tibet. The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 μm× 10 μm and 20 μm× 7 μm, respectively. The moissanite is green with parallel extinction. The absorption peaks in its Raman spectra are at 967-971 cm-1, 787-788 cm-1, and 766 cm-1. The absorption peaks in the infrared spectra are at 696 cm-1, 767 cm-1, 1450 cm-1, and 1551 cm-1, which are distinctly different from the peaks for synthetic silicon carbide. Moissanites have been documented to form in ultra-high pressure, high temperature, and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin. Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite, the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite. Further, we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment. Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.  相似文献   

10.
Voluminous platinum-group mineral(PGM) inclusions including erlichmanite(Os,Ru)S_2, laurite(Ru,Os)S_2, and irarsite(Ir,Os,Ru,Rh)As S, as well as native osmium Os(Ir) and inclusions of base metal sulphides(BMS), including millerite(NiS), heazlewoodite(Ni_3S_2), covellite(CuS) and digenite(Cu_3S_2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite(biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides(such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S_2 and ?O_2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S_2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S_2 increased. The abundance of primary BMS in the chromitites suggests that ?S_2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.  相似文献   

11.
Yarlongite: A New Metallic Carbide Mineral   总被引：1，自引：0，他引：1  
Yarlongite occurs in ophiolitic chromitite at the Luobusha mine (29°5′N 92°5′E, about 200 km ESE of Lhasa), Qusum County, Shannan Prefecture, Tibet Autonomous Region, People’s Republic of China. Associated minerals are: diamond, moissanite, wüstite, iridium (“osmiridium”), osmium (“iridosmine”), periclase, chromite, native iron, native nickel, native chromium, forsterite, Cr-rich diopside, intermetallic compounds Ni-Fe-Cr, Ni-Cr, Cr-C, etc. Yarlongite and its associated minerals were handpicked from a large heavy mineral sample of chromitite. The metallic carbides associated with yarlongite are cohenite, tongbaite, khamrabaevite and qusongite (IMA2007-034). Yarlongite occurs as irregular grains, with a size between 0.02 and 0.06 mm, steel-grey colour, H Mohs: 5?-6. Tenacity: brittle. Cleavage: {0 0 1} perfect. Fracture: conchoidal. Chemical formula: (Cr4Fe4Ni)Σ9C4, or (Cr,Fe,Ni)Σ9C4, Crystal system: Hexagonal, Space Group: P63/mc, a = 18.839(2) ?, c = 4.4960 (9) ?, V = 745.7(2) ?3, Z = 6, Density (calc.) = 7.19 g/cm3 (with simplified formula). Yarlongite has been approved as a new mineral by the CNMNC (IMA2007-035). Holotype material is deposited at the Geological Museum of China (No. M11650).  相似文献   

12.
A new mineral species, named naquite(FeSi), is found in the podiform chromitites of the Luobusha ophiolite in Qusong County, Tibet, China. The detailed composition is Fe 65.65, Si 32.57 and Al 1.78 wt%. The mineral is cubic, space group P213. The irregular crystals range from 15 to 50 μm in diameter and form an intergrowth with luobusaite. Naquite is steel grey in color, opaque, with a metallic lustre and gives a grayish-black streak. The mineral is brittle, has a conchoidal fracture and no apparent cleavage. The estimated Mohs hardness is 6.5, and the calculated density is 6.128 g/cm3. Unit-cell parameters are a 4.486 (4) ?, V 90.28 (6) ?3, Z=4. The five strongest powder diffraction lines [d in ? (hkl) (I/I0)] are: 3.1742 (110) (40), 2.5917(111) (43), 2.0076 (210) (100), 1.8307 (211) (65), and 1.1990 (321) (36). Originally called ‘fersilicite’, the species and new name have now been approved by the CNMNC (IMA 2010–010).  相似文献   

13.
The origin of native Si-Fe alloy mineral is thought to be related with mantle and aerolite. The native Si-Fe alloy minerals from podiform chromites of the Luobusha ophiolite in the Yarlong Zangbo suture zone were examined by a new method for powder-like diffractograms of small single crystals, using an SMART APEX-CCD area-detector X-ray diffractometer. The powder diffraction pattern shows that the minerals are composed of FeSi, FeSi2, β-FeSi2 and native silicon. The association of these minerals suggests that the crystallization order of the mineral may be from early to late FeSi→FeSi2→native silicon, accompanied by gradually increasing deoxidization. Translated from Acta Petrologica et Mineralogica, 2005, 24(5): 453–456 [译自: 岩石矿物学杂志]  相似文献   

14.
西藏蛇绿岩地幔中的主要自然金属矿物   总被引：9，自引：0，他引：9  
在西藏雅鲁藏布江蛇绿岩带的罗布莎蛇绿岩块的豆荚状铬铁矿床中 ,揭示出一个由 70～ 80种矿物组成的地幔矿物群 ,包括自然金属、合金、硫 (砷 )化物、氧化物和硅酸盐等。这些矿物呈包裹体或脉石产于铬铁矿石中 ,经人工重砂分析 ,自然元素矿物有自然硅、自然铁、自然锌、自然铅、自然铝、自然铬、自然锡、自然镍、自然钨、自然钛、自然锇、自然铱、自然钌、自然钯、石墨、金刚石、自然金和自然银等。文中选择一些自然元素矿物 ,探索这些地幔矿物特点以及蛇绿岩和铬铁矿的形成机制。根据共生矿物群以及罗布莎地幔橄榄岩为新鲜的未蛇纹石化的岩石 ,认为罗布莎自然元素矿物与蛇纹石化作用无关。它们可能是在地核形成时期滞留于地幔中的成核物质 ,抑或是核幔之间化学反应的产物 ,后来被铬铁矿矿浆捕获 ,并同铬铁矿一起由地幔柱作用和板块作用侵位于浅部并仰冲出露于地表。  相似文献   

15.
We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-Pd-Bi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh)2S4, braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.  相似文献   

16.
Mössbauer spectroscopy was applied to study the valence state of iron in chromite from massive, nodular and disseminated podiform chromitite ores of the Luobasa ophiolite massif of Tibet. The results show that Fe3+/ΣFe = 0.42 in chromite from massive ore, and Fe3+/ΣFe = 0.22 in chromite from nodular and disseminated ores. The massive ore records traces of ultra high pressure mineralogical assemblages, such as diamond inclusions in OsIr alloys, exsolution lamellae of coesite and diopside in chromite, inclusions of metal‐nitrides, native iron and others, which suggests a strongly reducing environment. In contrast, chromite from nodular and disseminated ore contains abundant low‐pressure OH‐bearing mineral inclusions whose formation requires a more oxidizing environment. The high value of Fe3+/ΣFe in the ‘reduced’ massive ore is explained by crystallographic stabilization of Fe3+ in a high‐pressure polymorph of chromite deep in the upper mantle despite low ambient fO2 conditions. The presence of high‐pressure phases within the massive chromitite ore requires that the latter, together with its host peridotite, was transported in the solid state from a highly reduced deep mantle environment to shallow depths beneath an ocean spreading centre. It is suggested that in the low‐pressure environment of the spreading centre, the deep‐seated, reduced, massive chromitites partially reacted with their host peridotite in the presence of hydrous melt, yielding the nodular and disseminated chromitite ores. The preponderance of evidence suggests that the latter interaction involved boninitic melts in a supra‐subduction zone environment as proposed previously.  相似文献   

17.
陈艳虹  杨经绥 《地球科学》2018,43(4):991-1010
豆荚状铬铁矿是蛇绿岩的特征性矿产,对其成因的认识还存在较大的分歧,包括:（1）早期岩浆熔离;（2）地幔熔融残余;（3）熔体-岩石反应.豆荚状铬铁矿及其围岩地幔橄榄岩中大量异常地幔矿物群的发现,引起了地质学家对其形成过程的重新思考.回顾了铬铁矿的研究,借助pMELTS热力学软件模拟浅部地幔过程,使用定量化的方法限定这些过程对豆荚状铬铁矿形成的贡献,通过一个新的角度讨论其形成.初步模拟结果显示,单独的地幔部分熔融、熔体分离结晶以及拉斑质熔体与亏损地幔的反应等过程形成的铬铁矿,无论在数量还是品位上都难以达到矿床水平,暗示豆荚状铬铁矿的形成可能为多种作用耦合的结果,或与深部地幔作用有关.   相似文献   

18.
Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr2O3 increased on the expense mainly of Al2O3 and to less extent Fe2O3 to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe2+ rather than Cr, Al, and Fe3+ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.  相似文献   

19.
A new type of podiform chromitite was found at Wadi Hilti in the northern Oman ophiolite. It is within a late-intrusive dunite body, possibly derived from olivine-rich crystal mush, between the sheeted dike complex and upper gabbro. This chromitite forms small (<30 cm in thickness) pods with irregular to lenticular shapes. Neither layering nor graded bedding is observed within the pods. The chromitite is in the upper crust, by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the Moho transition zone to the upper mantle. It is distributed along a small felsic to gabbroic melt pool within the dunite body, which was formed by melting of gabbroic blocks captured by the mush. Chromian spinel was precipitated due to mixing of two kinds of melt, a basaltic interstitial melt from the mush and an evolved, possibly felsic, melt formed by the melting of gabbro blocks. The podiform chromitite reported here is strikingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions. The former contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions. Chromian spinel of the upper crustal podiform chromitite from Oman has relatively low content of (Cr2O3 + Al2O3), the Cr/(Cr + Al) atomic ratio of around 0.6, and the relatively high TiO2 content ranging from 1 to 3 wt%. We conclude that assimilation of relatively Si-rich materials (crustal rocks or mantle orthopyroxene) by olivine-spinel saturated melts can explain the genesis of any type of chromitite.Editorial responsibility: V. Trommsdorff  相似文献   

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