H2OCO2 fluids supplied in alpine-type mantle peridotites: electron petrology of relic fluid inclusions in olivines

Fluids supplied in alpine-type mantle peridotites and trapped as fluid inclusions in olivines have been fixed by low-temperature reactions, and theirCO₂/H₂O ratios can be deduced from the minerals in the inclusions. Relic fluid inclusions were commonly observed by the optical microscope in olivines from almost all examined solid intrusive ultramafic complexes (Papua, Oman, Troodos and eleven alpine-type complexes of Japan). Such complexes were emplaced into the crust in a solid state. Electron microscopic studies of olivines from three complexes, Higashiakaishi, Horoman and Iwanai-dake, showed that relic fluid inclusions in these olivines have distinctive mineral parageneses: serpentine + magnesite + talc, serpentine + magnesite + brucite, and serpentine + brucite, respectively, depending on theCO₂/(H₂O+CO₂) ratio of the trapped fluid.It is deduced that the fluids had been supplied to peridotites, at least partly, but almost wholly in some case, when the peridotites were still hot, probably at the upper mantle for the following reasons: (1) the curved surfaces along which the inclusions are distributed are cut by post-emplacement serpentine veins; (2) for the Higashiakaishi dunite, the relic fluid inclusions are exclusively found in porphyroclast olivines and are totally absent in matrix olivines recrystallized during the Sanbagawa metamorphism.Recent models on the derivation of ophiolitic or some alpine-type peridotites favor the island-arc or fore-arc settings. Dehydration of the descending oceanic slab may supply H₂OCO₂ vapor to the overlying mantle wedge. Fluid inclusions trapped in such mantle wedge may abound in H₂O component. H₂O-bearing fluid inclusions may, therefore, be important H₂O containers in the upper mantle, especially near the edge of the mantle wedge above downgoing oceanic slabs.     

The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids

We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine-orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine-orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine-orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources.Laser ablation ICP-MS analyses of fluid inclusions in the olivine-orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4-2 wt.% NaCl_equiv) as internal standards, yielding maximum average ^fluid/rockD_B∼5 and ^fluid/rockD_Li∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4-8 wt.% NaCl_equiv. The antigorite breakdown produces less saline fluids (0.4-2 wt.% NaCl_equiv) and is characterized by lower partition coefficients for Cl (25-60) and B (12-30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. ^fluid/rockD_B/^fluid/rockD_Cl<1 (∼0.5) indicates that Cl preferentially partitions into the evolved fluids relative to B and that the B/Cl of fluids progressively increases with increasing depths and temperatures.Despite light element release in fluids, appreciable B, Cl and Li are still retained in chlorite, olivine and Ti-clinohumite beyond the antigorite stability field. This permits a bulk storage of about 10 ppm B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in mantle reservoirs. Chlorite is the Cl repository and its stability controls the Cl and H₂O budget beyond the antigorite stability; B and Li are bound in olivine and clinohumite. The subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources, potentially introducing anomalies in the upper mantle.     

Relicts of deformed lithospheric mantle within serpentinites and weathered peridotites from the Godzilla Megamullion,Parece Vela Back‐arc Basin,Philippine Sea

Relicts of deformed lithospheric mantle have been identified within serpentinites and weathered peridotites recovered from nine dredge sites and one submersible dive site from across the Godzilla Megamullion, which was emplaced at the now‐extinct Parece Vela Rift in the Parece Vela Basin, a back‐arc basin in the Philippine Sea. The serpentinites consist dominantly of lizardite ± chrysotile and magnetite with minor relict primary minerals that include pyroxene, spinel, and rare olivine. The weathered peridotites consist of pyroxene, spinel, lizardite ± chrysotile, and magnetite as well as weathering products of olivine. These rocks were classified in hand specimen into three types with different structures: massive, foliated, and mylonitic. In thin‐section the serpentine minerals show no sign of deformation, whereas relict primary minerals show evidence of plastic deformation such as undulose extinction, kink bands, dynamic recrystallization, and weak to moderate crystallographic preferred orientations. Therefore, the serpentinites and weathered peridotites result from the static replacement and weathering of previously ductile‐deformed peridotite. Given their location close to or on the detachment surface that exposed them, the relicts of peridotite provide evidence of deformation in the lithospheric mantle that could be related to the formation and emplacement of the Godzilla Megamullion in the Parece Vela Rift.     

Serpentinite textural evolution related to tectonically controlled solid-state intrusion along the Kurosegawa Belt, northwestern Kanto Mountains, central Japan

Abstract   Serpentinite bodies in the Kurosegawa Belt are mapped along fault boundaries between the Cretaceous Sanchu Group (forearc basin-fill sediments) and the rocks of the Southern Chichibu Belt (Jurassic to Early Cretaceous accretionary prism) in the northwestern Kanto Mountains, central Japan. The serpentinites were divided into three types based on microtextures and combinations of serpentine minerals: massive, antigorite and chrysotile serpentinites. Massive serpentinite retains initial pseudomorphic textures without any deformation after serpentinization. Antigorite serpentinite exhibits shape-preferred orientation of antigorite replacing the original lizardite and/or chrysotile to form pseudomorphs. It has porphyroclasts of chromian spinel, and is characterized by ductile deformation under relatively high-pressure–temperature conditions. Chrysotile serpentinite shows evidence for overprinting of pre-existing serpentinite features under shallow, low-temperature conditions. It exhibits unidirectional development of chrysotile fibers. Foliations in antigorite and chrysotile serpentinites strike parallel to the elongate direction of the serpentinite bodies, suggesting a continuous deformation during solid-state intrusion along the fault zones after undergoing complete serpentinization at deeper levels (lower crust and upper mantle).     

Synthesis of pyrope-knorringite solid solution series

The garnet solid solution series between pyrope Mg₃Al₂Si₃O₁₂ and knorringite Mg₃Cr₂Si₃O₁₂ has been synthesized from oxide mixtures at pressures of 60–80 kbars and 1400–1500°C. Lattice parameters and refractive indices of solid solutions vary linearly with (molecular) composition within the limits of measurement. The lattice parameter of pure knorringite is 11.600Åand its refractive index is 1.83. The genetic significance of mineral inclusions in natural diamonds is discussed, particularly in the light of the very high knorringite contents often found in garnet inclusions. It is suggested that the most common mineral assemblage occurring as inclusions in diamonds (olivine + knorringite-rich garnet + enstatite) might be explained in terms of subduction into the mantle of olivine + chrome-spinel + enstatite cumulates originally formed by crystallization of mafic magmas within the oceanic crust. The cumulate assemblage experienced alteration by circulating hydrothermal solutions, resulting in the introduction of some carbonate and serpentine minerals. During subduction, this assemblage was partially melted at depth below 150 km, accompanied by reduction of carbonate, to form a reconstituted assemblage consisting of olivine + knorringite-rich garnet + enstatite ± diamond.     

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series,Eseka area,SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated(to the less amount) with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO( 40 wt%) content and high Mg#values(88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg#values(40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites( 50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalizedREE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.     

Pseudosection analysis for talc-Na pyroxene-bearing piemontite-quartz schist in the Sanbagawa Belt, Japan

Abstract Various modes of occurrence of talc were identified in piemontite‐quartz schists collected from schist and eclogite units in the Kotsu area of the Sanbagawa Belt, eastern Shikoku, Japan. They can be classified into the following types: (A) matrix and (B) pull‐apart talc. The matrix talc is associated with aegirineaugite or glaucophane in the eclogite unit and with albite or chlorite in the schist unit. The pull‐apart talc is developed at the pull‐apart of microboudin structures of Na‐amphibole, along with albite or chlorite in samples from both units, suggesting that the pull‐apart talc was formed by Na‐amphibole consuming reactions in both units. The talc–aegirineaugite–phengite association is found in a thin layer (a few millimetres thick), with higher Na₂O/(Na₂O + Al₂O₃ + MgO) ratio in the ANM (Al₂O₃–Na₂O–MgO) diagram projected from phengite, epidote and other minerals, in the eclogite unit. Crystals of aegirineaugite have decreased jadeite content [= 100 × Al/(Na + Ca)] and increased aegirine content [= 100 × (Na – Al)/(Na + Ca)] from the core (ca Jd₄₀Aeg₄₀Di₂₀) to the rim (ca Jd₂₃Aeg₅₃Di₂₄), and are replaced by winchite and albite in varying degrees at the crystal margins. Na‐amphibole is glaucophane/crossite, commonly rimmed by Al‐poor crossite or winchite at the margin in the eclogite unit, although it is relatively homogeneous crossite in the schist unit. These textures suggest that the talc‐phengite‐(aegirineaugite or glaucophane) assemblage equilibrated during an early stage of metamorphism and the pull‐apart talc was formed at a later stage in the eclogite unit. A plausible petrogenetic grid in the NCKFe³⁺MASH system with excess piemontite (regarded as epidote), hematite, quartz and water, pseudosection analysis for the aegirineaugite‐bearing layer and the observed mineral assemblages suggest that the talc‐aegirineaugite‐phengite assemblage is stable under high pressure conditions (ca 560–580°C and 18–20 kbar). The pull‐apart talc was formed at ca 565–580°C and 9.5–10.5 kbar by the reaction of glaucophane/crossite + paragonite = talc + albite during the decompression stage, suggesting that the piemontite‐quartz schist in the eclogite unit experienced high‐pressure metamorphism at ca 50–60 km depth and was then exhumed to ca 30 km depth under nearly adiabatic conditions.     

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks,Japan, a possible high-pressure dehydration product of antigorite serpentinite

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high-grade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Al-spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet-cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H₂O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived.     

The system iron-enstatite-water at high pressures and temperatures—formation of iron hydride and some geophysical implications

The system iron-enstatite-water was investigated at pressures around 5 GPa and at temperatures ranging from 1000 to 1200°C, using several different kinds of starting materials. Quenched samples showed the coexistence of iron, olivine and pyroxene. Synthesis of the Fe-containing olivine in the run products proves that a series of reactions, Fe + H₂O → FeH_x + FeO and FeO + MgSiO₃ → (Mg, Fe)₂SiO₄, have taken place. Spherical “balls of iron” were observed in the 1200°C run. This strongly indicates that the melting temperature of iron decreased by ～ 500 K by the possible dissolution of hydrogen. Following geophysical implications are derived from these experimental results. If water was retained in the hydrous minerals in the primordial material, the iron-water reaction is expected to occur throughout the core-formation process. The reaction product FeH_x will melt and then sink to form a proto-core and iron oxide will be dissolved in the Earth's mantle. The dissolution of hydrogen in the Earth's core is a natural consequence of the core-formation process.     

A magnetic study of the serpentinization process at Burro Mountain,California

A study has been made of the magnetic properties of a suite of continental serpentinites from Burro Mountain, California. The chemistry of this set of samples has been previously studied, enabling the magnetic properties to be compared to the chemical changes which occurred during serpentinization. Two distinct magnetic phases have been recognized. The first is extremely stable but does not appear to contribute significantly to the natural remanent magnetization of the most strongly magnetized samples. The second phase is clearly multi-domained magnetite having a well-defined transition in its coercivity near 120°K. However, this second phase is not apparent in either the least serpentinized or the most serpentinized of the samples studied. The magnetic data argue strongly for the existence of two types of serpentinites; the first is magnetized dominantly by a stable component which we suggest may be Ni₃Fe, the second is magnetized Fe₃O₄ with unstable magnetization. There is no clear connection between the appearance of the stable component and the amount of serpentinization.     

Slab-fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites,South Kitakami Belt,northeast Japan

The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto-East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine-grained antigorite and lack of olivine, and the latter have coarse-grained antigorite and inclusion-rich olivine. Ca-amphibole occurs as isolated crystals or vein-like aggregates in the Type 1 serpentinites and as needle-shaped minerals in the Type 2 serpentinites. Ca-amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca-amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca-amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto-East Asian continental margin.     

FeO and H2O and the homogeneous accretion of the earth

We present new shock devolatilization recovery data for brucite (Mg(OH)₂) shocked to 13 and 23 GPa. These data combined with previous data for serpentine (Mg₃Si₂O₅(OH)₄) are used to constrain the minimum size terrestrial planet for which planetesimal infall will result in an impact-generated water atmosphere. Assuming a chondritic abundance of minerals including 3–6%, by mass water, in hydrous phyllosilicates, we carried out model calculations simulating the interaction of metallic iron with impact-released free water on the surface of the accreting Earth. We assume that the reaction of water with iron in the presence of enstatite is the prime source of the terrestrial FeO component of silicates and oxides. Lower and upper bounds on the terrestrial FeO budget are based on mantle FeO content and possible incorporation of FeO in the outer core. We demonstrate that the iron-water reaction would result in the absence of atmospheric/hydrospheric water, if homogeneous accretion is assumed. In order to obtain10²⁵g of atmospheric water by the end of accretion, slightly heterogeneous accretion with initially 36% by mass iron planetesimals, as compared to a homogeneous value of 34% is required. Such models yield final FeO budgets, which either require a higher FeO content of the mantle (17 wt.%) or oxygen as a light element in the outer core of the Earth.     

Hydration effects on crystal structures and equations of state for silicate minerals in the subducting slabs and mantle transition zone

There are potentially huge amounts of water stored in Earth’s mantle, and the water solubilities in the silicate minerals range from tens to thousands of part per minion (ppm, part per million). Exploring water in the mantle has attracted much attention from the societies of mineralogy and geophysics in recent years. In the subducting slab, serpentine breaks down at high temperature, generating a series of dense hydrous magnesium silicate (DHMS) phases, such as phase A, chondrodite, clinohumite, etc. These phases may serve as carriers of water as hydroxyl into the upper mantle and the mantle transition zone (MTZ). On the other hand, wadsleyite and ringwoodite, polymorphs of olivine, are most the abundant minerals in the MTZ, and able to absorb significant amount of water (up to about 3 wt.% H₂O). Hence, the MTZ becomes a very important layer for water storage in the mantle, and hydration plays important roles in physics and chemistry of the MTZ. In this paper, we will discuss two aspects of hydrous silicate minerals: (1) crystal structures and (2) equations of state (EoSs).     

Origin of waters responsible for serpentinization of the Izu-Ogasawara-Mariana forearc seamounts in view of hydrogen and oxygen isotope ratios

Many serpentinite seamounts occur over a region 20–120 km west of the trench axis in the Izu-Ogasawara-Mariana forearc regions. The hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization: seawater and water derived from dehydration of the descending slab. Serpentine from two Mariana and two Torishima samples with microscopically ductile and sheared texture (sheared-type) have lowerδD(−63to−52‰) and slightly higherδ18O values (+6.1 to +8.2‰) than that of other nine Ogasawara samples with mesh texture (mesh-type) (δD= −43to−49‰ andδ¹⁸O= +5.8to+6.7‰). This suggests that the sheared-type serpentine with lowerδD and slightly higherδ¹⁸O values was formed within the wedge mantle by interaction with water derived from a descending slab. The sheared texture is likely to have been produced during diapiric uplift. The unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor, resulting in formation of the mesh-type serpentine with higherδD values.     

Evidence of magma mixing: Petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan

Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group.Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fo₃₀)/quartz, iron-rich hypersthene(En₅₅)/iron-poor augite(Wo_43.5, En_42.5, Fs_14.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000°C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite ± hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in in which dacitic-andesitic magma had been fractionating.     

High-pressure phase equilibria in a garnet lherzolite,with special reference to Mg2+Fe2+ partitioning among constituent minerals

Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg²⁺Fe²⁺, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg₂SiO₄Fe₂SiO₄. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe²⁺/(Mg + Fe²⁺) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe²⁺/(Mg + Fe²⁺) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe²⁺ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg²⁺Fe²⁺ partition relation among coexisting minerals, an increase of about 1% in the Fe₂SiO₄ component in (Mg,Fe)₂SiO₄ modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.     

Chrome spinel in normal MORB‐type greenstones from the Paleozoic–Mesozoic Mino terrane,East Takayama area,central Japan: Crystallization course with a U‐turn

Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe²⁺) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe³⁺ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe²⁺ and Fe³⁺. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe²⁺‐ and Fe³⁺‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe²⁺‐ and Fe³⁺‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe²⁺‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe²⁺) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe²⁺)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.     

Negative pressure effect on the electrical conductivity of San Carlos olivine and its implication to the electrical structure in the upper mantle

Pressure effect on the electrical conductivity of San Carlos olivine was investigated by the newly installed electrical conductivity measurement system at China University of Geosciences. Electrical conductivity of San Carlos olivine aggregates was measured up to 12 GPa and 1475 K using the Walker-type multi-anvil apparatus equipped with eight WC cubes as the second-stage anvils. The pressure generation against applied load for the experimental assemblage was examined by phase transition of Bi,quartz, forsterite under different P-T conditions. To check the data validity of this new system, electrical conductivities of the serpentinites and talc samples were measured. The results are consistent with the published data of the same samples. Electrical conductivity(σ) of the San Carlos olivine aggregates and temperature(T) satisfy the Arrhenian formula: σ=σ0exp[.(ΔE+PΔV)/kT].The pre-exponential factor(σ0), activation energy(ΔE) and activation volume(ΔV) yield value of 7.74 S/m, 0.85 eV and 0.94cm3/mol, respectively. Electrical conductivities of the San Carlos olivine aggregates decline with increasing pressure at same temperatures. The negative pressure effect can be interpreted by strain energy model of defect energy together with the lattice deformation. In addition, the electrical conductivity-depth 1-D profile of the upper mantle was constructed based on our results and some assumptions. The calculated profile is concordant with the geophysical observation at the depth of 180–350 km beneath Europe, which indicates that the upper mantle beneath Europe might be dry.     

Oligocene crustal xenolith‐bearing alkaline basalt from Jandaq area (Central Iran): implications for magma genesis and crustal nature

The Oligocene alkaline basalts of Toveireh area (southwest of Jandaq, Central Iran) exhibit northwest–southeast to west–east exposure in northwest of the central‐east Iranian microcontinent (CEIM). These basalts are composed of olivine (Fo_70–90), clinopyroxene (diopside, augite), plagioclase (labradorite), spinel, and titanomagnetite as primary minerals and serpentine and zeolite as secondary ones. They are enriched in alkalis, TiO₂ and light rare earth elements (La/Yb = 9.64–12.68) and are characterized by enrichment in large ion lithophile elements (Cs, Rb, Ba) and high field strength elements (Nb, Ta). The geochemical features of the rocks suggest that the Toveireh alkaline basalts are derived from a moderate degree partial melting (10–20%) of a previously enriched garnet lherzolite of asthenospheric mantle. Subduction of the CEIM confining oceanic crust from the Triassic to Eocene is the reason of mantle enrichment. The studied basalts contain mafic‐ultramafic and aluminous granulitic xenoliths. The rock‐forming minerals of the mafic‐ultramafic xenoliths are Cr‐free/poor spinel, olivine, Al‐rich pyroxene, and feldspar. The aluminous granulitic xenoliths consist of an assemblage of hercynitic spinel + plagioclase (andesine–labradorite) ± corundum ± sillimanite. They show interstitial texture, which is consistent with granulite facies. They are enriched in high field strength elements (Ti, Nb and Ta), light rare earth elements (La/Yb = 37–193) and exhibit a positive Eu anomaly. These granulitic xenoliths may be Al‐saturated but Si‐undersaturated feldspar bearing restitic materials of the lower crust. The Oligocene Toveireh basaltic magma passed and entrained these xenoliths from the lower crust to the surface.