Search

Akarem Intrusion in the southern part of the Eastern Desert of Egypt is one of Alaskan-type ultramafic-mafic complexes of the Neoproterozoic age. The olivine and pyroxene-rich ultramafic core is surrounded by hornblende gabbro and hornblendite margin with gradational contacts. The pyroxene chemistry indicates that the intrusion was crystallized from tholeiitic arc magmas. The calculated liquids in equilibrium with clinopyroxene show enrichment in Th, U and LREE over the HREE, similar to arc magma formed from the mantle metasomatized by fluids from subducted sediments. Both olivine and pyroxene compositions indicate fractional crystallization from the ultramafic core to the mafic margin. However, amphiboles show systematic increase in concentrations of compatible elements, such as V, and shift in REE patterns from a bowed-up shape to bowed-down shape from the mafic margin to the ultramafic core. Such variation in trace elements of amphiboles indicates progressive crystallization of amphiboles from the mafic margin to the ultramafic core. The variation in amphibole chemistry has implication on both development of Alaskan-type intrusion and subduction zone geochemical signature of the evolved magma. The formation of the Akarem Intrusion started with an early fractional crystallization stage which led to the formation of a stratified magma chamber. The circulation in the magma chamber resulted in the solidification of the mafic magma along the margin where the early-stage amphiboles crystallized along with plagioclase. Subsequently, an evolved liquid from the crystallizing mafic margin percolated into the liquid-rich ultramafic core to form the later generation of interstitial amphiboles. The calculated trace element composition of the liquids in equilibrium with amphibole shows a progressive development of Nb and Ti negative anomalies and increase in La/Yb ratios from the margin to the core of the intrusion. Such variations manifest the role of amphibole fractionation in the evolution of the geochemical signature of arc magma.

Micro-inclusions of methane and propane were examined in olivine and titanoclinohumite in dunite from Fujiwara, Sanbagawa high- P / T metamorphic belt, Japan, in order to understand the behavior of hydrocarbons in the subduction zone and mantle wedge. In the Fujiwara dunite, olivines coexist with magnetite and exhibit a wide range of chemical compositions (Fo 88–96 and 0.2–0.6 wt% NiO), possibly indicating a deserpentinization origin for the dunite. The primary chromian spinel shows an intermediate Cr/(Cr+Al) atomic ratio, 0.5–0.6, and 1–3 wt% TiO 2 , and are enclosed by its alteration phases (ferritechromite and magnetite) that contain less than 0.8 wt% TiO 2 . Titanoclinohumite is frequently accompanied with the altered spinels, suggesting its Ti was provided by spinel alteration. Thin (

Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-Ōshima) in the Japan Sea suggests that the Oshima-Ōshima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-Ōshima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-Ōshima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

The crust–mantle boundary zone of the oceanic lithosphere is composed mainly of olivine-rich rocks represented by dunite and troctolite. However, we still do not fully understand the global variations in the boundary zone, and an effective classification of the boundary rocks, in terms of their petrographical features and origin, is an essential step in achieving such an understanding. In this paper, to highlight variations in olivine-rich rocks from the crust–mantle boundary, we describe the compositional variations in spinel-hosted hydrous silicate mineral inclusions in rock samples from the ocean floor near a mid-ocean ridge and trench. Pargasite is the dominant mineral among the inclusions, and all of them are exceptionally rich in incompatible elements. The host spinel grains are considered to be products of melt–peridotite reactions, because their origin cannot be ascribed to simple fractional crystallization of a melt. Trace-element compositions of pargasite inclusions are characteristically different between olivine-rich rock samples, in terms of the degree of Eu and Zr anomalies in the trace-element pattern. When considering the nature of the reaction that produced the inclusion-hosting spinel, the compositional differences between samples were found to reflect a diversity in the origin of the olivine-rich rocks, as for example in whether or not a reaction was accompanied by the fractional crystallization of plagioclase. The differences also reflect the fact that the melt flow system (porous or focused flow) controlled the melt/rock ratios during reaction. The pargasite inclusions provide useful data for constraining the history and origin of the olivine-rich rocks and therefore assist in our understanding of the crust–mantle boundary of the oceanic lithosphere.

Two types of podiform chromitite, concordant and discordant, were examined in the mantle section of northern Oman ophiolite along Wadi Hilti, to revisit the structural classification of podiform chromitite. They are contrasted in mineral chemical characteristics, in addition to the difference in attitude; the Cr/(Cr + Al) atomic ratio of spinel is around 0.6 for the concordant chromitite and surrounding peridotites, but is around 0.7 for the discordant one and surrounding peridotites. Chromian spinel grains contain pargasite-rich inclusions of primary origin from the both types, but they are far less abundant and smaller in size in the concordant chromitite than in the discordant one. Thin lamellae of pyroxenes in chromian spinel, similar to those in ultrahigh-pressure (UHP) chromitites from Tibet, are available only from the concordant chromitite. The dunite enveloping the concordant chromitite is extraordinarily high in NiO (up to >0.5 wt.%), suggesting subsolidus Ni diffusion from the chromitite. The involved melt was quite different between the two types of chromitite; the melt to precipitate the discordant one was more hydrous than that for the concordant one because of far more abundance of hydrous minerals in the former. The difference in duration of subsolidus cooling, and probably decompression, is prominent between the two types of chromitite. The concordant chromitite cannot be formed from the discordant one simply by metamorphic conversion: the former is of deep magmatic origin whereas the latter, of shallow magmatic origin.

Group II xenoliths, corresponding to the lithology of dunite, wehrlite to olivine clinopyroxenite and olivine websterite to websterite, occur in Pleisto-Holocene alkali basalts from Jeju Island, South Korea. The large grain size (up to 5 mm), moderate mg# [=100 × Mg/(Mg + Fetotal) atomic ratio] of olivine (79–82) and pyroxenes (77–83), and absence of metamorphic textural features indicate that they are cumulates of igneous origin. Based on textural features, mineral equilibria and major and trace element variations, it can be inferred that the studied xenoliths were crystallized from basaltic melts enriched in incompatible trace elements and belong to the Jeju Pleisto-Holocene magma system. They appear to have been emplaced near the present Moho, an estimated 5–8 kbars beneath Jeju Island. Consolidation of cumulates was followed by infiltration of silica-enriched metasomatic melt, producing secondary orthopyroxenes at the expense of olivine. The metasomatic agent appears to have been a silica-enriched residual melt evolved from an initially slightly silica-undersaturated alkali basalt to silica-saturated compositions by fractional crystallization under relatively high pressure conditions. The result of this study indicates that relatively young olivine-bearing cumulates could have been metasomatized by a silica-enriched melt within underplates, suggesting that silica enrichment can occur in intraplate Moho-related rocks as well as in the upper mantle of the subarc area.

Based on pressure and temperature (P–T) information of mantle-derived xenoliths, this report describes a geotherm in Far Eastern Russia, which is situated in the backarc of volcanic chains in eastern Asia. The mantle xenoliths have abundant CO 2 fluid inclusions. Accurate determination of the internal pressure of the CO 2 fluid inclusions enables estimation of the depth provenance of the mantle xenoliths. The depth provenances obtained from five mantle xenoliths are correlated with equilibrium temperatures estimated using a geothermometer. Assuming that the correlation reflects the geotherm in this region, it corresponds to heat flow of around 100 mW/m 2 , which is comparable to the surface heat flow in the volcanic arc. Such high heat flow implies a thin lithosphere and high temperature of the shallower upper mantle. Backarc areas are universally characterized by both high surface heat flow and the occurrence of recent volcanic activities. Furthermore low seismic velocity anomaly is widely observed in the shallower upper mantle in the backarc, especially in eastern Asia. The present xenolith geotherm suggests that the seismic anomaly arises from high temperature of the shallower upper mantle. It would be direct evidence for warming of the lithospheric mantle and subsequent lithospheric thinning, which is likely attributable to asthenospheric upwelling.

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93−94.5) than that in dunites (Fo94.3−Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46−0.81 and Mg#s, 0.34−0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82−0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25−30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine−spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ƒO2 = 2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (

Gabbroic xenoliths and diverse megacrysts (e.g., clinopyroxenes, amphiboles and plagioclases), which correspond to the lithology ranging from gabbro-norite to gabbro, occur in the Pleisto-Holocene alkali basalts from Jeju Island, South Korea. The gabbroic xenoliths consist primarily of moderate-K 2 O plagioclase, Ti-Al-rich clinopyroxene and CaO-rich orthopyroxene; additionally, TiO 2 -rich amphibole (kaersutite) and Ti-Fe oxides might or might not be present. The plagioclase is the most dominant phase (approx. 60–70 vol.%). The xenoliths and megacrysts provide evidence for the modal metasomatism of the lower continental crust by the mafic magmas during the Pleistocene. The coarse grain size (up to 5 mm), moderate Mg# [= 100 × Mg/(Mg + Fe total ) atomic ratio] of pyroxenes (70–77) and textural features (e.g., poikilitic) indicate that the gabbroic xenoliths are consistent with a cumulus origin. The clinopyroxenes from these xenoliths are enriched in REE with smooth convex-upward MREE patterns, which are expected for cumulus minerals formed from a melt enriched in incompatible trace elements. The strikingly similar major and trace element variations and the patterns of constituent minerals clearly indicate a genetic link between the gabbroic xenoliths (plus megacrysts) and the host basalt, indicating that the xenoliths belong to the Jeju Pleisto-Holocene magma system. On the basis of the textural features, the mineral equilibria and the major and trace element variations, the xenoliths appear to have crystallized from basaltic melts at the reservoir-roof environment within the lower crust (4–7 kbars) above the present Moho estimates beneath Jeju Island, where the xenoliths represent wall rocks. Following the consolidation of the xenolith lithologies, volatile- and incompatible element-enriched melt/fluid, as metasomatic agents, infiltrated through the grain boundaries and/or cracks and reacted with the preexisting anhydrous phases, which produced the metasomatic amphiboles. This volatile-enriched melt/fluid could have evolved from the initially anhydrous compositions to the volatile-saturated compositions by the active fractional crystallization in the Jeju Pleisto-Holocene magma system. This process was significant in that it was a relatively young event and played an important role in the formation of the hydrous minerals and the metasomatization of the lower continental crust, which is a plume-impacted area along the Asian continental margin.

A suite of mantle–crust transition zone (Moho transition zone = MTZ) rocks are exceptionally well exposed in Gebel Mudarjaj area, southeastern desert of Egypt. The MTZ rocks were thermally metamorphosed by younger granitic intrusion, forming mafic–ultramafic hornfels with characteristic metamorphic mineral assemblages. The MTZ rocks are remarkably thin (30–50 m thick) and are composed mainly of dunites, troctolites, gabbroic rocks and pyroxenite masses overlying a basal serpentinized mantle harzburgite section. The Cr# of spinels of the basal serpentinized harzburgites and the MTZ dunites are on average 0.76 and 0.74, respectively, which is consistent with the range for arc peridotite spinels. The melt in equilibrium with these MTZ rocks is compositionally similar to boninitic magmas produced by high degrees of partial melting. The basal harzburgites and MTZ dunites have been produced by 19–23% mantle melting, and are compositionally similar to supra-subduction zone (SSZ) peridotites. The mantle melt in equilibrium with pyroxenites was formed after 16–17% partial melting, which subsequently reacted with the lower crustal gabbroic rocks to produce pyroxenites. The occurrence of pyroxenite masses at the crust–mantle boundary suggests a medium- to high-pressure accumulation of pyroxenes in mid- to lower crustal magma chambers. The original MTZ rocks were partially or fully hydrated, prior to the granitic intrusion, during the regional metamorphism, tectonic disruption and emplacement as various fragments of a dismembered ophiolite, to form antigorite-bearing serpentinized mafic–ultramafic rocks. Progressive metamorphic assemblages then overprinted the primary features due to the contact metamorphism of the MTZ rocks. The resultant metamorphic mineral assemblages are: (1) olivine + anthophyllite + tremolite ± chlorite ± talc (in the basal serpentinites), (2) olivine + enstatite ± chlorite (in the MTZ dunites), and (3) olivine + aluminous spinel (pleonaste) + chlorite + magnetite ± enstatite (in the troctolites). The peak of thermal metamorphism was about 650°–700 °C and

To assess the ability of densimetry for CO2 fluid in CO2 inclusions, we compare two methods, microthermometry and Raman microspectroscopic densimetry for CO2. The comparative experiment was performed for nine CO2 inclusions in three mantle xenoliths. The results are as follows: (1) microthermometry precisely determines CO2 density with the range of 0.65 to 1.18g/cm 3 compared with Raman microspectroscopic densimetry; (2) CO2 density obtained by Raman microspectroscopic densimetry is fairly consistent with that by microthermometry; (3) it is hard to determine CO2 density in CO2 inclusion with diameter of less than around 3 mm using microthermometry; and (4) microthermometry can be applied only to the CO2 inclusion whose CO2 density ranges from around 0.65 to 1.18g/cm 3 , whereas the Raman microspectroscopic densimetry is applicable to CO2 density ranging from 0.1 to 1.24g/cm 3 . The above features carry the potential for estimation of depth origin of mantle-derived rocks. The depth where the rocks were trapped by host magma can be estimated using both geothermometric data and CO2 fluid density in CO2 inclusions in the rocks. Typical precisions of density of CO2 in CO2 inclusions obtained by the Raman microspectroscopic densimetry (~0.01g/cm 3 ) and by the microthermometry (< 0.001g/cm 3 ) correspond to uncertainties in the depth origin of 2.4km and

Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# [Mg/(Mg + Fe2+) atomic ratio] clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as “MORB-saturated harzburgite” for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt/harzburgite ratios. The modified melt left clinopyroxene (not clinopyroxene + plagioclase as in plagioclase-impregnated abyssal harzburgite) to form the MORB-saturated harzburgites in the vicinity (harzburgite) of the fracture, which are left as gabbro-clinopyroxenite bands. This local modification mimics the whole lithological and chemical variation of the MTZ and makes chemical variation of MORB suite at fast-spreading ridge.

Fission-track (FT) and (U–Th–Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault-block mountain range which has been uplifted since ca 0.8 Ma. We obtained nine zircon FT ages ranging 59.3–42.1 Ma, 18 apatite FT ages ranging 81.9–2.3 Ma, and 13 apatite He ages ranging 36.7–2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of

The prograde deserpentinized peridotites from the talc zone in the Happo-O’ne complex, central Japan, show differences in their field relation and mineral assemblage with the high-P retrograde peridotites of the other part of the complex. They show a mineral assemblage, olivine + talc + antigorite ± prograde tremolite ± chlorite, formed by thermal metamorphism around the granitic intrusion at T , 500–650 °C and P 2 O 3 ( 2 O 3 ( 2 O ( 2 (up to 59.9 wt.%), is different in size, shape and chemistry from the retrograde tremolite. The prograde peridotites display a U-shaped REE pattern (0.02–0.5 times PM), similar to diopside-zone retrograde metaperidotites, possible protoliths. They are enriched in LILE (e.g., Cs, Pb, Sr, Rb) relative to HFSE (e.g., Ta, Hf, Zr, Nb), like their protoliths, because of their local re-equilibration with the fluid released during dehydration of the protoliths. They have high contents of REE and some trace elements (e.g., Cs, Th, U, Ta) relative to their protoliths because of an external-element addition from the granitic magma. In-situ analyses of peridotitic silicates confirmed that the prograde tremolite and talc display a spoon-shaped primitive mantle (PM)-normalized REE pattern (0.1–3 times PM) in which LREE are higher than HREE contents. The prograde tremolite is depleted in Al, Na, Cr, Sc, V, Ti, B, HREE and Li, but is enriched in Si, Cs, U, Th, HFSE (Hf, Zr, Nb, Ta), Rb and Ba relative to the retrograde tremolite; the immobile-element depletion in this tremolite is inherited from its source (antigorite + secondary diopside), whereas the depletion of mobile elements (e.g., Li, B, Na, Al) is ascribed to their mobility during the deserpentinization and/or the depleted character of the source of tremolite. The enrichment of HFSE and LILE in the prograde tremolite is related to an external addition of these elements from fluid/melt of the surrounding granitic magma and/or in situ equilibrium with LILE-bearing fluid released during dehydration of serpentinized retrograde metaperidotites and olivine-bearing serpentinites (protoliths). The prograde olivine is higher in REE and most trace-element contents than the retrograde one due to the external addition of these elements; it is enriched in B, Co and Ni, but depleted in Li that was liberated during deserpentinization by prograde metamorphism.

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle–crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit ophiolite rocks. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without chilled margins or any visible deformations. The Ras Salatit wehrlites are orthopyroxene-free and composed mainly of olivine and clinopyroxene. They are texturally equilibrated and show a characteristic poikilitic texture. Crystallization order of the Ras Salatit wehrlites is olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Ras Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, mostly around 0.93 and 0.92, respectively. Moreover, the clinopyroxene shows low Ti and Al contents coupled with marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites is largely similar to lavas from the Izu-Bonin forearc. Given the above characteristics, the Ras Salatit wehrlites were produced by crystal accumulation from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous tholeiitic melt has been probably formed by fluid-assisted partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.

We examined peridotite massifs in the eastern part of the Mirdita ophiolite (EMO), Albania, where arc-related magmas are abundant in the upper volcanic sequences. Structurally, clinopyroxene porphyroclast-bearing harzburgites (Cpx-harzburgite hereafter) occur in the lower parts of the peridotite massifs, whereas harzburgites and dunites are more abundant towards the upper parts. Dunite is commonly associated with chromitite layers. Orthopyroxenite occurs as dikes and/or networks at all structural levels, although it is more abundant in the uppermost sections. Orthopyroxenite commonly crosscuts the foliation of peridotites and the lithological boundaries between dunites (chromitite) and harzburgites, suggesting that it was formed in the late stage. Major and trace element compositions of minerals in the Cpx-harzburgites indicate that they were formed as the residue of less-flux partial melting, and are similar to those in abyssal peridotites from mid-ocean ridge systems. Harzburgites have more depleted major element compositions than the Cpx-harzburgites. Light rare earth element (LREE)-enrichment in clinopyroxene coupled with hydrous silicate mineral inclusions in spinels in harzburgites indicate that harzburgites were produced as a result of enhanced partial melting of depleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. Based on olivine and spinel chemistries, dunites are classified into two types: high-Cr# (= Cr/(Cr + Al) atomic ratio) spinel-bearing dunite and medium-Cr# spinel-bearing dunite. Orthopyroxenites formed at the expense of the pre-existing peridotite by reaction with hydrous orthopyroxene-saturated melts, which were produced by assimilation of dissolved pyroxene during the formation of the dunite. Refractory harzburgite, high-Cr# spinel-bearing dunite, and orthopyroxenite may have a genetic link to the late stage boninitic magmas in the crustal section of the EMO. In contrast, the Cpx-harzburgite was a residue related to mid-ocean ridge basalts (MORBs) or the “MORB-like” fore-arc basalt recently proposed by Reagan et al. (2010) from the Izu–Bonin–Mariana fore-arc. The medium-Cr# spinel-bearing dunite can be caused by interaction with a melt transitional between MORB-like and boninitic melts. The lithological variations and their relationships in the upper mantle section of the EMO were caused by changes in magmatic compositions from mid-ocean ridge signatures to boninitic magmas, due to an increasing contribution of slab-derived fluids in an island arc setting.

Thin discordant gabbronorite veins (∼ 0.5 mm to 3 cm in width) occur within remarkably fresh harzburgite boulders of the Sibuyan Ultramafics. The harzburgite host rock displays protogranular to porphyroclastic textures and is dominantly composed of olivine, orthopyroxene with minor amounts of clinopyroxene, spinel and amphibole. The mineral chemistry of the harzburgite is comparable to depleted abyssal peridotites as shown by the Fo content of the olivine (= 90-91) and Cr# of the spinel (= 0.40-0.52). The gabbronorites are coarse-grained adcumulates and comprised of orthopyroxene, plagioclase and amphibole. Plagioclase in the gabbronorites shows high An content whereas both orthopyroxene and amphibole show variable Mg# almost similar to reported values from arc gabbros. The contact between the harzburgite and the gabbronorite veins is demarcated by the formation of secondary orthopyroxene with low Cr2O3 and CaO contents. Clinopyroxene in the harzburgite shows strong enrichment in light rare earth elements (LREEs). Amphibole rimming the clinopyroxene in the harzburgite has similar patterns as the clinopyroxene but with much higher REE abundance. The amphibole in the gabbronorites also shows enrichment in LREEs, Rb, Ba and Ti. We propose that the harzburgite-gabbronorite occurrence in the Sibuyan Ultramafics is a product of mantle-melt reaction. The metasomatic agent is a silicate melt enriched in Si, Cr, Fe and LREEs. At a bigger scale, the harzburgite-gabbronorite connection observed in the Sibuyan Ultramafics possibly documents early stage modification and conversion of abyssal peridotites to ophiolitic peridotites by SSZ-related melts.

The Ise area is located in the western part of the Hida Marginal Belt (central Japan), which includes several sporadic exposures of ultramafic rocks, sometimes forming a serpentinite melange of the Paleozoic age. Ultramafic rocks in the Ise area, enveloped by Paleozoic-Mesozoic sedimentary rocks, are completely serpentinized; however, the abundance of bastite after orthopyroxene suggests harzburgite protoliths. The bastite- and mesh-textured serpentines are distinguished from each other in Al2O3, Cr2O3 and NiO contents. The bastite-textured serpentine is high in Al2O3 (up to 4.0 wt%) and Cr2O3 (up to 1.2 wt%), but low in NiO (

Radioisotope concentrations, such as 238 U and 232 Th, of bulk sediments are good environmental indicators and useful for radiometric age determinations. Pressed pellets of sediment can be analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with little preparation time. However, the matrix effects caused by the different properties between the external standard and measured sample may yield unreliable results. This study examines the preparation method of pressed powder pellets using geological rock reference materials, and tests them as external standards. Powders of the rock reference materials (JA-1, JA-2, JB-1 and JB-2 from the Geological Survey of Japan), a lacustrine sediment (4A-18 from Lake Baikal, Russia), and solid glass reference standards (SRM 610 and 612 from the National Institute of Standard and Technology) were analyzed. In calculating 238 U or 232 Th concentrations, one sample was selected as an external standard and other six samples were treated as unknown samples. 29 Si was chosen as an internal standard. When the grain size is small enough (about

Secondary microinclusions in olivine and orthopyroxene in a harzburgite sample obtained from northern Oman ophiolite were examined by micro-Raman spectroscopy. The microinclusions were a consequence of fluid inclusions trapped by the minerals during the healing of fluid-filled cracks. We can expect an almost closed-system reaction to have occurred between the fluid and minerals. We attempted to distinguish the individual contributions of olivine and orthopyroxene to the serpentinization of peridotite. The inclusions in olivine mainly consist of lizardite and brucite with small amounts of magnetite, H2, and CH4, while those in orthopyroxene mainly consist of talc and chromian spinel with or without CH4 and graphite. The fluid involved was most probably a mixture of H2O and a smaller amount of CO2. We examined whether reduced gases were produced through reactions of the fluid with olivine and orthopyroxene. The role of magnetite (or magnetite component in chromian spinel) is of vital importance in the generation of these gases. Orthopyroxene facilitates the formation of talc in the serpentinization of peridotite.

It was reported by Meyer and Boyd (1972) that zincian chromite, having 2 wt% to 3 wt% ZnO and high (>0.9) Cr# (= Cr/(Cr + Al) atomic ratio), appears as inclusions in diamonds obtained from kimberlite. Zincian chromite is characterized by a low ( 0.8); moreover, the major-element chemistry of these chromites is similar to that of the magnesiochromite inclusions in diamonds. Spinels obtained from altered rocks show a chemical range that comprises the zincian chromite inclusions found in diamonds. The origin of these inclusions possibly lies in deep recycling. They were initially formed at the Earth's surface in altered/metamorphosed peridotites; they then sank deep down into the mantle and were entrained to the surface again by kimberlite magmatism.

This paper details a new discovery of anorthite-bearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high-Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and interpreted as an interaction product between high-temperature Ca-rich hydrothermal fluids and peridotites within the mantle section. The anorthite diopsidites were probably formed by interaction between the fluids involved in diopsidization within the mantle section and the layered gabbro. The fluids responsible for the formation of the anorthite diopsidite were also Ca-rich and carried Cr to form uvarovite from the underlying mantle section. High-temperature hydrothermal circulation plays an important role in the transportation of elements, e.g., Ca and Cr, from the upper mantle to the lower crust across the Moho in the ocean floor.

Enigmatic dunitic veins and veinlets crosscutting a podiform chromitite ore body discovered in Wadi Rajmi, northern Oman ophiolite, display a peculiar characteristic of being almost or completely spinel-free. Olivines show an evolution trend from the spinel-free dunites (Fo93-94 and 0.4 wt% NiO) to the spinel-bearing dunites (Fo91-93 and 0.2-0.3 wt% NiO). The Cr/(Cr + Al) of chromian spinel grains increases from 0.6 to 0.8, accompanied by a slight increase in Fe3+ ratio, and the Fo content of olivine decreases in the spinel-bearing dunites. A high-Mg magma, initially undersaturated with chromian spinel, precipitated the spinel-free dunites. Once the spinel saturation of the melt was attained by fractionation, the spinel-bearing dunites precipitated. The initial magma was possibly komatiitic in nature, which was produced at an early stage of detachment of the oceanic lithosphere that formed the Oman ophiolite.

金沢大学理工研究域自然システム学系, We summarize chemical characteristics of chromian spinels from ultramafic to mafic plutonic rocks (lherzolites, harzburgites, dunites, wehrlites, troctolites, olivine gabbros) with regard to three tectonic settings (mid-ocean ridge, arc, oceanic hotspot). The chemical range of spinels is distinguishable between the three settings in terms of Cr# (=Cr/(Cr+Al) atomic ratio) and Ti content. The relationships are almost parallel with those of chromian spinels in volcanic rocks, but the Ti content is slightly lower in plutonics than in volcanics at a given tectonic environment. The Cr# of spinels in plutonic rocks is highly diverse; its ranges overlap between the three settings, but extend to higher values (up to 0.8) in arc and oceanic hotspot environments. The Ti content of spinels in plutonics increases, for a given lithology, from the arc to oceanic hotspot settings by mid-ocean ridge on average. This chemical diversity is consistent with that of erupted magmas from the three settings. If we systematically know the chemistry of chromian spinels from a series of plutonic rocks, we can estimate their tectonic environments of formation. The spinel chemistry is especially useful in dunitic rocks, in which chromian spinel is the only discriminating mineral. Applying this, discordant dunites cutting mantle peridotites were possibly precipitated from arc-related magmas in the Oman ophiolite, and from an intraplate tholeiite in the Lizard ophiolite, Cornwall. © 2010 Blackwell Publishing Asia Pty Ltd.

The metaperidotites found in the Happo-O'ne region of central Japan are mostly lherzolites to harzburgites with subordinate dunites. Their protoliths originated as a series of refractory residues by near-fractional melting, 15–25% melting in the spinel field, which is confirmed by a HREE melting model of the apparently primary clinopyroxene. These metaperidotites display U-shaped primitive mantle (PM)-normalized REE patterns (0.02–0.5 times PM) and are highly enriched in LILE (0.2–20 times PM) relative to HFSE (

We report the occurrence and characteristics of diopsidite dykes and veins from the Palghat-Cauvery Suture Zone (PCSZ) marking the boundary between the Archaean Dharwar craton to the north and the Proterozoic Madurai Block to the south, which is considered as a trace of the Cambrian Gondwana suture zone in southern India. The diopsidites are composed predominantly of coarse crystals of diopside [Mg no. (100 Mg/(Mg+Fetot)) up to 89] surrounded by retrograde calcic amphibole, plagioclase and phlogopite with accessory titanite and calcite. The major, trace and rare earth element characteristics of the diopside crystals suggest their formation in a subduction zone setting. We correlate the petrogenesis of the diopsidites with the tectonics associated with the subduction and closure of the Neoproterozoic Mozambique Ocean prior to the final collisional assembly of the Gondwana supercontinent in Cambrian.

We describe a rare terrestrial occurrence of hibonite (CaAl 12 TiO 19 ) from the ultrapotassic rocks which occur as veinlets in graphite-bearing spinellite in the Achankovil Shear Zone (ACSZ), southern India. The ultrapotassic rocks are characterized by an unusual mineral assemblage, kalsilite + leucite + corundum + spinel + perovskite + apatite + hibonite ± phlogopite, along with mantle-derived graphite and Fe-sulfides. Three types of hibonite were identified in terms of chemical zoning as observed in back scattered images: (i) unzoned hibonite, (ii) zoned hibonite with brighter core and dark rim, and (iii) zoned hibonite with brighter core, a less bright intermediate zone and a dark outer rim corresponding to the variations in the distribution of LREEs (light rare earth elements), Ca, Al and Ti in the hibonite structure. The core regions of hibonite are highly enriched in LREEs and Sr, and depleted in Ca, Ti and Al, compared to the rim areas. Chondrite-normalized REE distribution patterns for hibonite exhibit extreme LREE enrichment (La N /Yb N = 763 to 374,494) with a negative Eu anomaly. Apatite displays MREE-enriched patterns. Partitioning of LREEs in hibonite and apatite crystallized from the melts is chiefly determined by their preference for hibonite > apatite. We propose a model that considers the formation of the LREE-rich Ti-depleted hibonite from ultrapotassic silica-undersaturated melts. We envisage that the melts were generated by low degree partial melting of a phlogopite-bearing peridotite that was fertilized in LREEs. The ultrapotassic rock that occurs as veins along the ACSZ represents a melt influx associated with deep-seated extension after the final amalgamation of the Gondwana supercontinent.

Various characteristics of podiform chromitites, an enigmatic mantle rock member, are reviewed in this article. Chromitites are composed of chromian spinel, with the general formula (Mg, Fe2+)(Cr, Al, Fe3+)2O4, and silicates (mainly olivine). The Fe3+ content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg/(Mg + Fe2+) ratio (= Mg#) changes inversely with the Cr/(Cr + Al) ratio (= Cr#), which increases with an increase of degree of partial melting of mantle peridotites. The Cr# of chromian spinel is generally higher than 0.4 (generally 0.6 to 0.8) in podiform chromitites, varying widely from 0.1 to 0.9 in the mantle peridotite. The podiform chromitite forms pod-like bodies (dimensions of up to 1.5 km × 150 m for an individual pod) with a dunite envelope, totally set within mantle harzburgite. In well-preserved ophiolites, they occur in the uppermost mantle, especially in and beneath the Moho transition zone, which is dominated by dunite. The Cr# of chromian spinel is relatively low (0.4 to 0.6) around the Moho transition zone, and high (>0.6) at deeper levels in the mantle section. Chromitites are denser and less anisotropic in Vp than peridotites, and the Vp is 8.5 to 9 km/sec depending on the proportion of chromian spinel, and higher in the former than in the latter. The podiform chromitite has been interpreted to be one of melt/rock interaction products within the uppermost mantle harzburgite; hybridization of relatively Si-rich melt formed by the breakdown of orthopyroxenes of the wall harzburgite and subsequently supplied primitive melt cause oversaturation in chromian spinel, giving rise to formation of chromitite with a dunite envelope. The fractionated melt leaving high-Cr# podiform chromitite is possibly of arc-magma affinity. Chromitites with low-Cr# (0.4 to 0.6) chromian spinel can be in equilibrium with MORB. Recently found ultra-high pressure minerals, such as diamond, moissanite, Fe-silicides and Ni-Fe-Cr-C alloys, within chromian spinel of podiform chromitites make the genetical history of chromitites highly enigmatic. A new story, which incorporates the genesis and involvement of these highly reducing, ultra-high pressure minerals, is required.

Podiform chromitites have been interpreted as cumulates by harzburgite/melt interaction and related melt mixing at the upper mantle to the Moho transition zone. Recent discovery of diamond and other ultrahigh-pressure (UHP) minerals from some podiform chromitites, especially those from Tibet, however, has raised a question about the depth of their formation. These UHP chromitites are possibly of deep recycling origin; they had been originally formed at the upper mantle before sinking down to deeper mantle, and upwelling again to the shallowest mantle by convection. Diamond is formed by reduction/oxidation of fluidal carbon species (e.g., CO2 or CH4) obtained during the convection history, and has survived oxidation because of strong encapsulation in metal alloys further included by chromian spinel. Exsolved silicates (diopside and coesite) in chromian spinel from some UHP chromitite are possibly derived from hydrous mineral inclusions in chromian spinel formed at low pressures, which have been decomposed/molten during sinking and solved in the UHP chromian spinel phase. Both the UHP chromitites and ordinary low-pressure ones could be present in the upper mantle derived from the mid-oceanic ridge, one of the main ends of the mantle upwelling flow. The mantle recycling issue unraveled through chromitite thus can be one of the targets of deep oceanic mantle drilling including the Mohole.

Ultramafic xenoliths from Kurose, Southwest Japan, are fragments of the lithospheric mantle beneath the back arc of the active Southwest Japan volcano-tectonic arc. Trace element and Sr-Nd isotopic compositions of the clinopyroxenes in the ultramafic xenoliths suggest that several enrichment processes are responsible for the geochemical variation. These xenoliths would have witnessed the following events: (1) open-system partial melting of the continental lithosphere with an influx of an asthenospheric melt with MORB-like isotopic composition during the formation of the back-arc basin that resulted from the Japan Sea opening, (2) open-system melting under an influx of a fluid/melt derived from a subducted slab, and (3) metasomatism after these partial melting events by an island arc tholeiitic basalt melt in the Neogene period. Moreover, variations in the depletion levels of Zr and Ti in the clinopyroxenes suggest that source depletion preceded the melting and metasomatic events.

We report here for the first time the occurrence of a high-temperature metamorphic/plutonic complex (amphibolites, metagabbros, hornblende pyroxenites and hornblendites) in Calaton Hill, Tablas island, Romblon, Central Philippines. The mineral assemblages and relic magmatic textures in these rocks imply apparent derivation from arc-related protoliths. Major element and trace element data are also comparable to those of gabbroic rocks in arc-related setting. Subsolidus re-equilibration under granulite to amphibolite facies is documented by the triple junctions between mineral phases in the different lithologies, the recrystallization of plagioclase and the presence of coronas around olivine with mineral assemblage of orthopyroxene + amphibole ± green spinel. The formation of hornblendite and the pervasive occurrence of amphiboles in the different lithologies are being attributed to the infiltration of a younger hydrous arc magma which also caused metamorphism and hybridization on the surrounding rocks. The characteristics of the Calaton Hill samples are comparable with those of the well-studied xenoliths from Ichinomegata, NE Honshu arc, Japan. We therefore interpret the Calaton Hill metamorphic/plutonic complex as representative of the lower crust underlying the Philippine island arc.

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO2 content is extremely low in chromian spinels

The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite melange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo88–Fo91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al2O3 (up to 0.25 wt%), Cr2O3 (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO2 (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the orthopyroxenes, have high TiO2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe3+ [=Fe3+/(Cr + Al + Fe3+) atomic ratio

Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) is applied to determining the concentrations of radioactive isotopes (87Rb, 232Th, 234U and 238U) in soil samples from Lake Baikal, Russia, for an annual dose estimation for luminescence dating. After an examination of pellet making procedures, the dried sediments were pressed into pellets. The concentrations of radioisotopes were successfully measured taking the National Institute of Standards and Technology standard glass (SRM 610 from NIST) as an external standard (reference material) to convert the signal intensity to a concentration. When the laser ablation setting is chosen to send a significant amount of material to the mass spectrometer (e.g., the repetition frequency of 20 Hz and spot diameter of 140 µm), a very small quantity of 234U (~ 10 ppb order) could be measured, and the resultant concentration agreed with the result obtained by conventional alpha-spectrometry analyses. Radioisotopes, along with other major elements measurable by LA-ICP-MS, can be measured quickly on fine-grained sediments (e.g., lake and deep-sea sediments) of small quantity by the simple procedure proposed here.

The Co-Ni arsenide ore deposits at Bou Azzer, Morocco, represent an uncommon type of Co mineralization. Several Co-Ni-As-Au-Ag mines are active in Bou Azzer and adjacent areas. The Co-Ni arsenide ores are structurally controlled and concentrated mainly within quartz-carbonate veins along the boundaries of Neoproterozoic serpentinized mantle peridotites, quartz diorite, and Precambrian volcanic rocks. They have varying shapes and sizes; however, all deposits have the same general mineralogy in decreasing order of abundance: arsenides, sulfarsenides, and sulfides. Arsenide minerals include skutterudite (CoAs3), safflorite (CoAs2), loellingite (FeAs2), nickeline (NiAs), rammelsbergite (NiAs2), and S-rich Ni-Co diarsenide. Co-rich ores are predominant over Ni-rich ones in almost all deposits. Arsenopyrite and, to a lesser extent, gersdorffite are the main sulfarsenide minerals in almost all samples studied. Bornite, an unidentified Cu-Zn sulfide, chalcocite, chalcopyrite, pyrite, pyrrhotite, sphalerite, and tetrahedrite, in decreasing order of abundance, are the main sulfide minerals present in Bou Azzer ores. Gold-silver alloys, with about 15 wt percent Ag on average, are commonly associated with Ni-Co arsenides and/or disseminated in quartz veins. In the Ni-rich assemblage pure nickeline with high Ni content (~48 wt %) and very low Co content (≤ 0.1 wt %) grades outward to compositions with less Ni (~20 wt %) and very high Co (up to 18 wt %). This suggests a depositional sequence from Ni monoarsenide (nickeline) to Ni-Co diarsenide to cobalt triarsenides (i.e., a progressive increase in Co and As). The ore-forming elements (Co, Ni, Fe, and As) were originally leached from serpentinites by acidic magmatic fluids under moderately reducing conditions at high fluid/rock ratios. Deposition of ore minerals occurred in response to increasing pH and decreasing oxygen fugacity of the mineralizing fluids caused by mixing between the magmatic brines and meteoric water. The predominance of Co over Ni arsenide minerals in the Bou Azzer mineralization despite the high Ni/Co ratio of the serpentinites may be attributable to the different solubilities of Ni and Co in the hydrothermal system.

金沢大学理工研究域自然システム学系, Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma-mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle. © 2009 The Authors Journal compilation © 2009 Blackwell Publishing Asia Pty Ltd.

Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

We examined vertical variations of the petrological characteristics of a 33-m-thick peridotite section under the layered gabbro section along Wadi Fizh of the northern Oman ophiolite to understand the formation mechanism of the Mohorovicic discontinuity (Moho) beneath a spreading center. Here, we refer to the base of the layered gabbro section as “L-Moho” for the sake of simplicity. Network-like gabbro sills in peridotites increase in frequency upward to the L-Moho. The L-Moho is underlain by a 1-m-thick wehrlite layer, under which exists a 10-m-thick dunite layer, overlying a harzburgite layer where total pyroxenes slightly increase downward. Wehrlite is also found as screens between gabbro layers above the L-Moho. The mineral chemistry indicates systematic variations toward the L-Moho within the peridotite section; the Fo content (91 to 85) and NiO (0.4 to 0.2 wt%) of olivine decrease; the TiO2 content of clinopyroxene (0.1 to 0.6 wt%) and spinel (nil to 1.4 wt%) and atomic ratios of Cr/(Cr + Al) (0.5 to 0.6) and Fe3+/(Cr + Al + Fe3+) (0.05 to >0.1) in spinel increase upsection from the base (harzburgite) to the around L-Moho wehrlite via dunite. These variations are essentially similar to those observed in harzburgite/MORB reaction products from Hess Deep, East Pacific Rise, and possibly indicate that the lithological and mineral chemical variations within the examined peridotite layer resulted from the reaction between a harzburgite and a melt that produced the layered gabbros.

Ophiolitic rocks (chromitites and serpentinized peridotites) were petrologically examined in detail for the first time from Rayat, in the Iraqi part of the Zagros thrust zone, an ophiolitic belt. Almost all the primary silicates have been altered out, but chromian spinel has survived from alteration and gives information about the primary petrological characteristics. The protolith of the serpentinite was clinopyroxene-free harzburgite with chromian spinel of intermediate Cr# (=Cr/[Cr+Al]atomic ratio) of 0.5 to 0.6. The harzburgite with that signature is the most common in the mantle section of the Tethyan ophiolites such as the Oman ophiolite, and is the most suitable host for chromitite genesis. Except for one sample, which has Cr#=0.6 for spinel, the Cr# of spinel is high, around 0.7, in chromitite. The variation in Cr# of spinel in chromitite observed here has been also reported in the Oman ophiolite. The peridotite with chromitite pods exposed at Rayat was derived from an ophiolite similar in petrological character to the Oman ophiolite, one of the typical Tethyan ophiolites (fragments of Tethyan oceanic lithosphere). This result is consistent with the previous interpretation based on geological analysis. © 2009 The Authors Journal compilation © 2009 Blackwell Publishing Asia Pty Ltd.

Silicate glasses in peridotite xenoliths from Avacha volcano have high SiO2 (up to 72 wt.%) and highly SiO2-oversaturated characteristics; normative quartz content is up to 50 wt.%. The glasses represent secondary melts solidified after interaction with mantle peridotite, i.e. crystallization of secondary orthopyroxene at the expense of olivine. We identified two kinds of silicate glasses in Avacha peridotites; one is higher in K2O and enriched in Rb, Ba, U, and Pb than the other. The glasses show basically similar chemical characteristics to the host basaltic andesite to andesite of the Avacha volcano. These chemical characteristics are inherited from slab-derived fluids/melts, which metasomatize the mantle wedge and induce partial melting. The differences of chemical features among the Avacha glasses are attributed to chemical difference of the slab-derived fluids/melts, possibly due to the difference of sediments/basalt ratio of the relevant slab. The low-degree partial melt of peridotite assisted by these fluids/melts, is primarily SiO2-oversaturated, and can conduct silicate metasomatism, evolving through interaction with surrounding mantle peridotite, i.e. formation of orthopyroxene at the expense of olivine. Highly silicic glasses, also reported from peridotite xenoliths from oceanic hotspots and continental rift zones, mostly result from assimilation of orthopyroxene by SiO2-undersaturated melts, which crystallize clinopyroxene and olivine. The glasses also show similar trace-element patterns to their host alkali basaltic magmas, as in the case of arc glasses/calc-alkali magmas. If the glasses in peridotite xenoliths are of silicate metasomatism origin, they are similar in chemistry to host magmas. Reaction between carbonatite melts and peridotites shows the same petrographical feature as that of SiO2-undersaturated silicate melts with peridotites. The glasses originated from carbonatite metasomatism, however, exhibit clearly different trace-element patterns from their host alkali basaltic magmas.

LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238U, has upon LA-ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples - three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238U concentrations measured on the etched and unetched mineral surfaces. The 238U concentrations determined by the LA-ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA-ICP-MS apatite FT results, promotes the use of the LA-ICP-MS for FT chronometry. © Geological Society of London 2009.

We examined dike-like chlorite rocks that replaced isotropic gabbro and dolerite in northern Oman ophiolite in order to understand the chemical budget of hydrothermal alteration of the oceanic lithosphere. During chloritization, the concentrations of Si, Ca, Na, and K decreased, while those of Fe increased. REE (rare-earth elements), except Eu, which showed a strong depletion in the chlorite rocks, were immobile during chloritization, which was caused by the downward (recharge) flow of circulated seawater. A portion of Fe was supplied from the overlying mafic extrusives, possibly through the alteration of their plagioclases. We found Ti-rich minerals such as rutile and titanite to be the reservoirs of most REE in the chlorite rocks. If the residual fluid, after chloritization, moves upward, it can realize the positive Eu anomaly of the seafloor vent fluids. And, if the fluid is transported to deeper parts of the oceanic lithosphere, rodingites, serpentinites (antigorite rocks), and diposidites with a positive Eu anomaly are formed within gabbros and mantle peridotites.

Hornblende peridotite xenolith in the spessartite dike in the Tamba belt, northern Kyoto city, is mainly composed of olivine with high Fo and NiO contents (Fo87−94 and 0.22-0.51 wt%). The olivine encloses chromian spinels with a high Cr/(Cr + Al) atomic ratio, 0.66-0.80, partly replaced and digested by hornblendes with a low Mg/(Mg + total Fe) atomic ratio, 0.68-0.81. Microscopic and geochemical features of the hornblende peridotite xenolith indicate a reaction between a dunitic cumulate from a high-Mg and -Cr melt and an evolved hydrous high-Mg melt to form hornblendes. Olivines are lower in Fo when in contact with hornblendes. Rare earth element (REE) patterns of the hornblendes, common to the xenolith and host spessartite, indicate that all the involved melts are similar but different in the degree of differentiation. The strong hydration of olivine in the xenolith suggests a wide distribution of hornblende-rich rocks, such as hornblendites, formed by the hydration of olivine-rich rocks, such as dunites, around the Moho beneath the arc. This may give rise to a layer with a Vp intermediate between typical lower crustal and mantle values.