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A peculiar peridotite xenolith with cataclastic texture was found from Ichinomegata crater, Megata volcano, the Northeast Japan arc. This peridotite xenolith is the same in mineral assemblage and mineral chemistry (olivine, Fo90; spinel, Cr/(Cr + Al) atomic ratio, 0.2) to some fertile mantle lherzolites, but quite different in texture from all the documented mantle peridotite xenoliths from Ichinomegata and other localities on Earth. The peridotite is a mixture of coarse and fine grains of olivine, orthopyroxene, clinopyroxene, chromian spinel. The coarse mineral grains are angular but never elongated like those in mylonite. All minerals have been fragmented, and the fragmented pyroxenes and chromian spinel form thin streaks in fine-grained olivine-rich matrix. These features indicate that brittle fracturing does occur even in the upper mantle possibly along pre-existing deep-seated faults that have been frequent in the Northeast Japan arc. Some of the fine grains (

Podiform chromitites are common within the mantle section of the Late Cretaceous Sabzevar ophiolite in NE Iran. We studied chromitite pods and related ultramafic rocks from three Sabzevar massifs: Baghjar-Kuh Siah, Gaft Chromitite Mine and Forumad peridotite–chromitite. These represent an upper mantle sequence just below the Sabzevar Moho. The Baghjar-Kuh Siah mantle sequence contains plagioclase lherzolites, enriched in bulk REEs, with low Cr# spinels and MORB-like clinopyroxenes. These lherzolites formed due to the impregnation of MORB-like melts. The Gaft and Forumad harzburgites are depleted in trace and rare earth elements and thus are residues after high degree of partial melting (more than exhaustion of Cpx). The Gaft Chromitite Mine includes two types of podiform chromitites, high Cr# and low Cr#. The melt precipitating high Cr# spinel was boninitic whereas the melt forming the low Cr# chromitites was tholeiitic. Most Forumad massif chromitites have high Cr# spinels, although those rich in silicate inclusions are aluminous. Trace and REE element patterns of Forumad harzburgite clinopyroxene are similar to those in supra-subduction zone (SSZ) peridotites while those of Baghjar-Kuh Siah lherzolites are similar to MOR peridotite clinopyroxenes. These mineral data are also consistent with bulk rock trace and rare earth elements composition of their host peridotites. Field observations indicate that early tholeiitic magmas were followed by late boninites, as revealed in chromitite compositions as well as mantle rocks and dikes. We suggest a time-integrated model for the evolution of the Sabzevar mantle sequence during an early stage of subduction initiation associated with formation of an incipient arc. In this scenario, MORB-like melts (forearc basalts) formed first, causing low Cr# chromitites and plagioclase–clinopyroxene impregnations. Subsequent arc-like or boninitic melts with increasing contribution of slab-derived fluids were responsible for the formation of replacive dunites and high Cr# chromitites.

Article history:Received 11 July 2014Accepted 31 December 2014Available online 16 January 2015Keywords:Oman ophioliteSpinel-free dunitesKomatiitic meltCumulate dunitesHigh-Ca boninites High-Mg dunites, which are spinel-free or spinel-poor, occur as veins that cut across a pod of chromitite withinthe mantle section deep below the Moho transition zone of the northern Oman ophiolite (Wadi Rajmi). Onlythick (N50 cm) veins were considered in order to avoid the effects of subsolidus Mg–Fe 2+ exchanges with thechromitite wall rock on the compositions of olivine. The spinel-free dunites contain especially high-Mg (Fo N93 )olivines,whereasolivinesinthespinel-poorduniteshavelowerFocontents(Fo 91–93 ).Inaddition,chromianspi-nels inthe spinel-poor dunites display highvalues of Cr# up to 0.8 [Cr# = Cr/(Cr + Al) atomic ratio].We foundthatthesekomatiiticdunitesformedviaanolivinefractionalcrystallizationprocess,andnotbyreplacement,andthatthechromititesservedasisolatingcapsuleswheremeltswerepreservedfrommelt–rockreactionswithpe-ridotite minerals. The olivines show a continuous trend in their chemistry from the spinel-free dunites, throughthe spinel-bearing dunites, to the high-Ca boninites described from the northern Oman ophiolite (Fizh Block).This strongly suggests a genetic linkage between the high-Mg spinel-free or spinel-poor dunites and the high-Caboninites,whichrespectivelyrepresentcumulatesandtheresultantfractionatedmeltsofaprimarykomatiiticmelt.Inthispaper,wechallengetheprevalentmodelsthatruleoutthegenesisofkomatiitesinsupra-subductionzones and shed light on the conditions of the mantle at an early stage of arc evolution.© 2015 Elsevier B.V. All rights reserved.

Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago (Japan) compose two discrete groups of high-SiO2, high-MgO (high-Si) and low-SiO2, low-MgO (low-Si) boninitic suites, with ultra-depleted dish- and V-shaped, and less-depleted flat, rare earth element patterns. The most magnesian melt inclusions of each geochemical type were used to estimate the temperature-pressure conditions for primary boninites, which range from 1345 °C at 0.56 GPa to 1421 °C at 0.85 GPa for the 48–46 Ma low-Si and high-Si boninites, and 1381 °C at 0.85 GPa for the 45 Ma low-Si boninite. The onset of the Pacific slab subduction at 52 Ma forced upwelling of depleted mid-oceanic ridge basalt mantle (DMM) to yield proto-arc basalt (PAB). With the rise of DMM, refractory harzburgite ascended without melting. At 48–46 Ma, introduction of slab fluids induced melting of the PAB residue and high-temperature harzburgite, resulting in the low-Si and high-Si boninites, respectively. Meanwhile, convection within the mantle wedge brought the less-depleted residue of PAB and DMM into the region fluxed by slab fluids, which melted to yield the less-depleted low-Si boninite at 45 Ma, and fertile arc basalts, respectively.

Among a set of peculiar meta-ultramafics, carbonate-orthopyroxenites are observed for the first time in the Heiani ophiolite belt, South Eastern Desert, Egypt. They form massive lensoidal masses up to 50 m long and 20 m wide. The lenses show a marked structural concordance with their neighboring country rocks. The typical country rocks are represented by the following high-grade metamorphic rocks: kyanite–muscovite schists, amphibolites, kyanite-bearing biotite gneisses, migmatites, granite gneisses and mobilizates. The studied carbonate-orthopyroxenites consist mainly of metamorphic orthopyroxene + magnesite, among other metamorphic, relict primary and retrograde secondary minerals. According to primary chromian spinel (Cr#, 0.7–0.84) chemistry and morphology, absence of clinopyroxene and presence of primary mantle olivine (Fo 89–91 ) as relicts in the metamorphic orthopyroxene, the Heiani carbonate-orthopyroxenites seem to have formed from a highly depleted mantle peridotite precursor. At a late collisional stage during the Pan-Africa terrane accretion and the E–W crustal shortening (ca. 650–620 Ma), high-grade (upper amphibolite facies) low- P /high- T regional metamorphism (ca. 660 Ma) accompanied by CO 2 -metasomatism resulted in formation of the Heiani carbonate-orthopyroxenites. Mostly the carbonate-bearing shelf sediments beneath and/or in juxtaposition with the Heiani ophiolite are considered to be the proven source of the CO 2 -rich fluids. Although, a mixed sedimentary-mantle C source is not unlikely. A mineral paragenetic correlation with experimental data for the system MgO–SiO 2 –H 2 O–CO 2 suggests metamorphic conditions consistent with those of the high-grade country rocks; i.e. 630–650 °C, 6–7 kbar (20–23 km depth) and high- X CO2 (0.6–0.7). The CO 2 -bearing fluids discharging along faults gave rise to regionally widespread carbonate-bearing assemblages. Accordingly, the Heiani carbonate-orthopyroxenites are considered the by-products of natural carbon sequestration by an ultramafic rock.

*Department of Earth and Environmental Sciences, Graduate School of Science and Technology, Kumamoto University, 2–39–1 Kurokami, Chuo–ku, Kumamoto 860–8555, Japan **Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920–1192, Japan ***ODS, JAMSTEC, 2–15 Natsushima, Yokosuka 237–0061, Japan Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Russia

Diopsidite, mainly composed of diopsidic clinopyroxene (diopside for simplicity), was precipitated from a high-temperature hydrothermal solution, rich in silicate components mainly in the mantle peridotite. We present here a new type of diopsidite in the uppermost mantle section, which contains Cr-rich minerals such as chromite, Cr-rich diopside (

α-Lipoic acid (ALA) has a chiral center at the C6 position, and exists as two enantiomers, R(+)-ALA (RALA) and S(−)-ALA (SALA). RALA is naturally occurring, and is a cofactor for mitochondrial enzymes, therefore playing a major role in energy metabolism. However, RALA cannot be used for pharmaceuticals or nutraceuticals because it readily polymerizes via a 1,2-dithiolane ring-opening when exposed to light or heat. So, it is highly desired to find out the method to stabilize RALA. The purpose of this study is to provide the spectroscopic information of stabilized RALA and SALA through complexation with cyclodextrins (CDs), α-CD, β-CD and γ-CD and to examine the physical characteristics of the resultant complexes in the solid state. The RALA-CD structures were elucidated based on the micro fourier transform infrared (FT-IR) and Raman analyses. The FT-IR results showed that the C=O stretching vibration of RALA appeared at 1717 cm−1 and then shifted on formation of the RALA-CD complexes. The Raman spectra showed that the S–S and C–S stretching vibrations for RALA at 511 cm−1 (S–S), 631 cm−1 (C–S) and 675 cm−1 (C–S) drastically weakened and almost disappeared upon complexation with CDs. Several peaks indicative of O–H vibrations also shifted or changed in intensity. These results indicate that RALA and CDs form host-guest complexes by interacting with one another.

Spinel harzburgite to lherzolite xenoliths are entrapped in Quaternary intraplate alkali basalts on Jeju Island, South Korea. These xenoliths are unusual in containing late-stage secondary orthopyroxene, free of deformation and exsolution that is replacing olivine as the main pervasive metasomatic mineral. These xenoliths are characterized by high Mg# in olivine, orthopyroxene, and clinopyroxene (89–93) and variable Cr# of spinel (9–53), representing residues left after variable degrees of melt extraction (~ 25%). In contrast to their depleted major-element compositions, clinopyroxenes in the xenoliths are enriched in most incompatible trace elements. Clinopyroxenes display enrichment in light rare earth elements (LREE) or spoon-shaped REE with a general enrichment in La over Ce, and depletion in high field strength elements (HFSE; e.g., Nb-Ta, Zr-Hf, Ti). Orthopyroxenes (either primary or secondary) are characterized by low TiO2, high Al2O3, and moderate CaO contents, and resemble those of sub-continental arc peridotites from the eastern Pacific. The geochemical evidence, in addition to the formation of secondary orthopyroxene, indicates that Jeju peridotite xenoliths have been subjected to different degrees of metasomatism by subduction-related silica- and LREE-enriched fluids (or melts). However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. We therefore propose that the Jeju peridotite xenoliths went through a two-stage evolution, with their composition primarily controlled by early fractional melt extraction, which was subsequently modified by residual slab-derived fluids (or melts). Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of the subcontinental lithospheric mantle, resulting in the enriched mantle domains, such as EMI or EMII.

International audience; Belonging to the huge Ouarzazate volcanic Group that covered the whole Anti-Atlas during the late Ediacaran (580-545 Ma), the Tifnoute valley volcanic formations are mainly pyroclastic and show a large composition, from trachybasalt to rhyolite and are crosscut by dolerite dykes. The Tifnoute valley volcanic rocks are located within a rigid salient of the Anti-Atlas that gives them special extreme characteristics. Due to the heavy greenschist alteration that affects this volcanic group, we focused the more immobile elements, but as REE can also be affected, we used the composition of unaltered clinopyroxene crystals to determine the nature of these volcanic rocks. The clinopyroxene is an augite diopside in the basalt, an augite in the andesite and an augite-salite in the dolerite. Petrography of the Tifnoute mafic volcanic rocks and clinopyroxene compositions indicate the presence of two magmatic series: (i) older high-K calc-alkaline (alkali-calcic) andesite and basalt characterized by the early crystallization of Fe-Ti oxides and of the late fractionation of plagioclase, the modal proportion of the latter increasing from the basalt to the andesite and (ii) younger alkalic dolerite dykes. With clinopyroxene trace element compositions obtained using laser ablation ICP-MS, we calculated the composition of the melts in equilibrium with the pyroxenes.The volcanic rocks of the Tifnoute Valley have positive εNd570 (+1.7 to +5.0), low Sri (

Chromitites in ophiolites are frequently used as a marker of sub-arc mantle magmatism. This has, however, rarely been verified by modern sub-arc mantle materials. Chromitite xenoliths from Takashima, southwest Japan, derived from the current sub-arc mantle, are here described with comparison to ophiolitic chromitites. The chromitite xenoliths from Takashima from the southwest Japan arc show various textures, including nodular, anti-nodular, and layered varieties. Laurite is commonly found as minute (

It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries. © 2014 Elsevier B.V.

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al2O3, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high- Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87Sr/86Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207Pb/204Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.

The Qalander ophiolite is a small, poorly preserved and incomplete melange-type complex situated within the Eocene–Oligocene Walash–Naopurdan Group in the Iraqi Zagros Suture Zone (IZSZ). It is one of six fragmented ophiolite complexes emplaced in the IZSZ during the Cretaceous and Tertiary. Within the Qalander ophiolite, serpentinized dunite and harzburgite hosts small lens-shaped podiform high-Al (North Qalander) and high-Cr (North Shitna) chromitite bodies. The average range of Cr-, Mg-, and Fe 3 -numbers for North Qalander chromitites is 39–53, 75–77, and 3–5, respectively; meanwhile those for North Shitna chromitites are 78–80, 65–73, and 6–8, respectively. The ranges of Al 2 O 3 wt.% and FeO/MgO for the North Qalander and North Shitna chromitites are 15–15.5 and 0.8–1.0, and 9.0–10.5 and 0.4–1.0, respectively. In addition to pyroxene and olivine, inclusions of laurite, millerite and galena were detected within the chromitites of Qalander ophiolite. The matrix minerals between chromite grains are serpentinized olivine and pyroxene, chlorite, and calcite; grains of magnetite, pyrite and ilmenite are also common accessories. The concentrations of platinum-group elements (PGE) in both varieties are typical for the ophiolitic chromitites, but they show two distinct patterns of PGE enrichment. The high-Cr chromitites have relatively uniform mantle-normalized PGE distribution patterns with a steep slope, positive Ru and negative Pt anomalies, and they show enrichment of PGE and depletion in Pt relative to the average upper mantle. The high-Al chromitites show relatively gently sloping patterns with slight positive Ru and negative Pt anomalies, high iridium-group PGE (IPGE) abundances relative to the high platinum-group PGE (PPGE), and are distinctly enriched in Pt and Pd relative to the upper mantle and the average abundances in the high-Cr chromitites. The differences in the PGE content, geochemistry, rare earth element (REE), mineral chemistry and petrographical characteristics of the chromitites suggests two stages of magmatic activity which were responsible for the chromitite genesis in the region. The first stage involved low degree partial melting of S-saturated peridotite melt that produced high-Al chromitites of MORB affinity, which crystallized in the upper mantle close to the Moho. The second stage involved a high degree partial melting of S-undersaturated boninitic melt that produced high-Cr chromitites of arc affinity and crystallized in a deeper mantle section. The presence of low-Cr MORB-type chromitites with high-Cr boninitic-type chromitites in the same mantle section suggests their formation in aback-arc supra-subduction zone tectonic setting.

The deep levels of Neoproterozoic island arcs are poorly known due to limited accessibility. The Genina Gharbia Alaskan-type complex (south Eastern Desert, Egypt) is the remains of a magma chamber that crystallized at the base (crust–mantle boundary) of a mature Neoproterozoic island arc. The rock assemblage comprises hornblende-bearing harzburgite, lherzolite, pyroxenite, norite and gabbro. All lithologies show cumulus texture with evidence of extensive cumulus mineral–melt interactions. Clinopyroxenes from all lithologies have similar rare earth element (REE) patterns with slight medium-rare earth element (MREE) enrichment. Hornblendes are slightly enriched in MREE and light rare earth elements (LREE). Island arc signatures are indicated by high contents of large ion-lithophile elements and low concentration of high field-strength elements. Positive initial e Nd (+ 5.7 to + 7.0) and Nd model ages (963 ± 81 Ma) are consistent with the Genina Gharbia magma being extracted from a depleted mantle source. Modeling of estimated parental magma indicates 10% partial melting of a 90% depleted mantle source with a 10% (MORB + sediments)-derived fluid, commencing in the spinel stability field (

We examined the composition and lithological variability from a road section in south-Andaman which represents a pathway to the crustal section of the Cretaceous Andaman ophiolite. Like other well-studied ophiolites worldwide this transition zone is marked by association of olivine-rich troctolite, wehrlite, pyroxenite and gabbroic rocks. The mineral chemical variations document the evolution of this zone by melt–mantle interaction and fractional crystallization. Petrographic evidence suggests that water was introduced during the evolution of this transition zone. The olivine-rich troctolites record impregnation of MORB melt into a residual olivine-rich lithology (replacive dunite) that formed by an earlier episode of melt–peridotite interaction at a slow spreading ridge. The clinopyroxenites indicate formation from an extreme clinopyroxene saturated melt that might be genetically linked with the formation of olivine-rich protolith of the troctolitic rocks prior to melt impregnation. The wehrlite crystallized from the melt residual after the formation of clinopyroxenite. The composition of the impregnating melt that transformed the replacive dunite to olivine-rich troctolite is identical to the gabbroic rocks. We conclude that the association of these rock types from south-Andaman provides us with a snapshot of the switch over of geodynamic setting of the Andaman ophiolite (MOR to arc) as preserved presently between north-Andaman in the north and Rutland Island in the south.

Melt impregnated plagioclase lherzolites from the Nain melange, central Iran, contain pyroxenes enriched and chemically zoned with Sr. Pyroxenes from the lherzolite and the clinopyroxenite seams, which have been precipitated from the impregnating melt, show similar trace element geochemical characteristics, including a similar Sr anomaly. The associated plagioclase, precipitated from the impregnating melt, has been selectively altered to isotropic saussurite. Strontium concentration increases in the pyroxenes from the core to the rim and toward crosscutting saussurite trails in orthopyroxene porphyroclast cores. The highest Sr content (up to 10.8 ppm in clinopyroxene and 3.8 ppm in orthopyroxene) is found in the finer pyroxenes surrounded by thicker saussurite layers. The Sr enrichment within pyroxenes is neither caused by metasomatism nor modified by fluids involved in hydrothermal alteration because pyroxenes are extremely depleted in fluid-mobile and light rare earth elements. Also, Sr enrichment cannot be related to the melt impregnation, since the Sr supply from the impregnating melt was consumed by plagioclase crystallization. The Sr enrichment in the pyroxene postdates the melt impregnation and is due to the relatively high-temperature (375 °C–850 °C) of saussuritization, that is, the breakdown of plagioclase. Plagioclase decomposition has released appreciable amounts of Sr to enrich adjacent pyroxenes. Saussurite shows significantly lower Sr contents than the plagioclase. Sr enrichment in peridotite pyroxenes, which is ascribed primarily to the metasomatism of slab-derived fluids, should be treated carefully, particularly when altered plagioclase is present.

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over 2 O 3 , Na 2 O, Cr 2 O 3 and TiO 2 contents. Primary spinels show a large variation of Cr# [= Cr/(Cr + Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr) + LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3–

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 (Fo70–90), 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.

Structural changes in silica gel after compression to 5 GPa and 100 °C for one hour were investigated using Raman and IR spectroscopies and X-ray diffraction measurements. The non-powdered silica gel sample was entirely crystallized to coesite under these experimental conditions, showing that the crystallization of silica gel to coesite can be readily achieved in spite of the low temperature conditions and short duration. This behavior may result because the average structure of silica gel is dominated by four-membered rings of SiO 4 tetrahedra, there is a less-polymerized network structure due to the presence of silanol (Si-OH), and larger ring structures, such as 6- and 8-membered rings, of SiO 4 tetrahedra are disrupted by water molecules under pressure. These properties may lead to successive polymerization and reconstruction of the SiO 4 tetrahedra in the bulk silica-gel sample, enabling the easy formation of the coesite-like network structure. This shows the importance of the contribution of silanol and water molecules for coesite formation.

Chromite is a typical refractory igneous mineral, precipitated from mafic magmas at relatively high temperatures. Chromites commonly occur in sedimentary, metamorphic, and metasomatic rocks, where they are interpreted as relics of an igneous phase and serve as the source of Cr for low-temperature Cr-bearing minerals. We present evidence for the nucleation of chromite within hydrothermal solutions. We have found minute euhedal chromite grains enclosed by uvarovite (Ca-Cr garnet) in a diopsidite, metasomatically replacing the layered gabbro of the Oman ophiolite. The uvarovite shows oscillatory concentric zoning in terms of Cr no. [Cr/(Cr+Al)], and the chromite is embedded only in the high-Cr-no. zones of the uvarovite. Another diopsidite, replacing peridotite in the underlying upper mantle section, contains xenocrystic chromite, which is partly dissolved. This suggests that a hydrothermal solution collected Cr by partial to total dissolution of chromite within the upper mantle and precipitated chromite, along with high-Cr-no. uvarovite, within the lower crust upsection. The metasomatic agent involved was a CO2-, SO2-, and Cl-bearing hydrothermal solution containing appreciable silicate components that could carry Cr, possibly as a complex. The hydrothermal chromite is similar in chemistry to that commonly found in igneous rocks [e.g., Cr no. = 0.8, Mg/(Mg+Fe2+) = 0.1–0.2, TiO2 < 0.3 wt% and Fe3+/(Cr+Al+Fe3+), up to 0.3], but its Cr no. is clearly different from that of mantle chromite (0.6–0.7) in peridotites and chromitites from the Oman ophiolite. The results from this study suggest that a hydrothermal origin is possible for chromites in ultramafic rocks that have experienced fluid activity assuming that there is sufficient chromite at the fluid source.

We applied apatite fission-track (AFT) thermochronometry to the Cretaceous-Paleogene granitic rocks across the southern part of the Northeast (NE) Japan Arc in order to elucidate the denudation history. As the results, AFT ages were estimated at 79.5–66.0 Ma on the fore-arc side, 29.8–5.5 Ma in the Ou Backbone range (OBR) and 19.1–4.6 Ma on the back-arc side. Thermal inverse modeling with HeFTy offered high-resolution thermal histories, which were converted into denudation rates under the assumption of appropriate geothermal gradients. On the fore-arc side, stable and slow denudation setting (denudation rates

Recent reassessment of abyssal peridotites obtained during the dredging of the oblique supersegment and the easternmost subsection of the Southwest Indian Ridge by the R/V Knorr Cruise 162 and the R/V Yokosuka YK98-07 revealed the occurrence of dunites containing podiform chromitites and dunites with variable chromite concentration closely associated with lherzolite and harzburgite. The size of the chromitite pods varies from a few mm to 2 cm in width. Chromites in the podifom chromitites have very low Cr# (=0.22–0.23) and low TiO2 (

Detrital chromian spinels from adjacent sediment (recent sands and a Tertiary sandstone) were used to obtain a general view of the lithological and petrological characteristics of the Nain ophiolitic melange, Iran. They display a wide chemical range in terms of Cr# (= atomic ratio), from 0.10 to 0.92. Except for some grains (11% of the total), the spinels show low TiO2 contents ( 0.25 wt%, up to 1.26 wt%) are relatively low in Mg# (= atomic ratio), from 0.15 to 0.65, and were possibly derived from mantle peridotite that reacted with impregnating melt (e.g., dunite or plagioclase-bearing peridotites). The high-Cr# (0.55–0.92) spinels, which are dominant in the Tertiary sandstone, are chemically homogenous and clearly different from high-Cr# altered spinels (ferritchromite and Cr-magnetite), which are formed by low-T alteration, and are low in Mg# and high in Fe3+. The higher abundance of the high-Cr#...

Ultrahigh-pressure (UHP) chromitites containing UHP minerals such as coesite and diamond have been reported from some ophiolites in Tibet and the Polar Urals. Their nature, i.e., origin, P-T path and abundance, however, are still controversial and left unclear. Here we describe chromitites in the Higashi-akaishi (HA) ultramafic complex in the Cretaceous Sanbagawa metamorphic belt, Japan, which experienced UHP condition (up to 3.8 GPa) at the peak metamorphism via subduction, in order to understand the nature of UHP chromitites. The HA peridotites typically contain garnets and are associated with eclogites, and their associated chromitites are expected to have experienced the UHP metamorphism. The Higashi-akaishi (HA) chromitites show banded to massive structures and are concordant to foliation of the surrounding peridotite. Chromian spinels in the chromitite and surrounding peridotites were sometimes fractured by deformation, and contain various inclusions, i.e., blade- and needle-like diopside lamellae, and minute inclusions of pyroxenes, olivine, and pargasite. The peculiar UHP minerals, such as coesite and diamond, have not been found under the microscope and the Raman spectrometer. Spinels in the HA chromitites show high Cr#s (0.7 to 0.85), and low Ti contents (&lt, 0.1 wt %), suggesting a genetic linkage to an arc magma. The HA chromitites share the basic petrographic and chemical features (i.e., diopside lamellae and arc-related spinel chemistry) with the UHP chromitites from Tibet and the Polar Urals. This suggests that some of the characteristics of the UHP chromitite can be obtained by compression, possibly via deep subduction, of low-P chromitite.

The Shiribeshi Seamount off northwestern Hokkaido, the Sea of Japan, is a rear-arc volcano in the Northeast Japan arc. This seamount is composed of calc-alkaline and high-K basaltic to andesitic lavas containing magnesian olivine phenocrysts and mantle peridotite xenoliths. Petrographic and geochemical characteristics of the andesite lavas indicate evidence for the reaction with the mantle peridotite xenoliths and magma mixing between mafic and felsic magmas. Geochemical modelling shows that the felsic end-member was possibly derived from melting of an amphibolitic mafic crust. Chemical compositions of the olivine phenocrysts and their chromian spinel inclusions indicate that the Shiribeshi Seamount basalts in this study was derived from a primary magma in equilibrium with relatively fertile mantle peridotites, which possibly represents the mafic end-member of the magma mixing. Trace-element and REE data indicate that the basalts were produced by low degree of partial melting of garnet-bearing lherzolitic source. Preliminary results from the mantle peridotite xenoliths indicate that they were probably originated from the mantle beneath the Sea of Japan rather than beneath the Northeast Japan arc.

In order to constrain the timing of plutonic magma emplacements relevant to current geothermal activity in the Hachimantai geothermal area, northeast Japan, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Th–Pb analyses were performed on zircons from two different origins. One is from the Quaternary Kakkonda Granite in the Kakkonda geothermal field, whose present temperature at ~ 3700 m depth is 500 °C, and the other is from a clastic dike in the Matsukawa geothermal field. Zircons from the Kakkonda Granite were obtained at a depth of 1998 m and their 238U–206Pb and U–Th ages showed ~ 0.1 Ma, which probably indicates that the Kakkonda Granite is the world's youngest emplaced granitic magma body ever confirmed. The clastic dike in the Matsukawa geothermal field contains dioritic to tonalitic xenoliths; therefore zircons extracted from the dike should also be of plutonic origin. 238U–206Pb age of 1.08 ± 0.08 Ma (after correction for initial 230Th disequilibrium; error shown as 95% confidence level) was obtained from the zircons. Therefore it was found that there were at least two Quaternary plutonic magma activities (~ 1.1 Ma and ~ 0.1 Ma) in the Hachimantai geothermal area. Through this study, it was also found that U–Th–Pb dating using a quadrupole type ICP-MS coupled with excimer laser is applicable to Quaternary zircons as young as 0.1 Ma and Fish Canyon Tuff zircon is usable as a standard material for U–Th–Pb dating method.

The use of laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP-MS) in the measurement of uranium concentration for fission-track (FT) dating can provide additional geochronological information by simultaneously acquiring data for U–Pb dating. To circumvent the use of several calibration factors with uncertainties in the FT age equation (i.e. the decay constant, values related to neutron irradiation, and observation efficiencies), the zeta calibration, which is based on the analysis of age-known standard samples, is recommended internationally. In this report, procedures to acquire LA-ICP-MS data for simultaneous dating by FT and U–Pb methods, the age equation with zeta-equivalent calibration, and the statistical treatment of FT age data, are proposed. An excimer laser beam with 20 μm diameter was used for sampling. We empirically determined zeta calibration constants against NIST SRM610 for FT zircon and apatite dating by analyzing several age standards. The data agreed with the calculated values, which can be obtained using reasonable constants in the equation. The correction factor in 238U–206Pb age determination with NIST SRM610 as an external standard was defined as 0.920 ± 0.034 by analyzing several standards (FCT, Plesovice, Duluth, GJ-1). Based on this calibration system, zircons from the Nisatai Dacite, Japan, which is a secondary age standard commonly used in the Japanese FT community, were simultaneously dated to be 22 Ma by FT and U–Pb methods on the same grains.

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO4 tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO4 tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.

Well-preserved oval-shaped dunite clots occur within the exceptionally fresh massive podiform chromitites from the Coto Block of the Zambales Ophiolite Complex, the Philippines. The dunite/chromitite boundary shows an interlocking texture; olivine inclusions in chromites in the podiform chromitites show the same optical extinction with larger adjacent olivines in the dunite clots. This texture was formed by the reaction between chromite-oversaturated melt and its dunite inclusions. The existence of such type of melt was previously only hypothesized to explain the origin of layered and podiform chromitites but is now confirmed by this discovery.

We report petrologic, geochemical, fluid inclusion and stable isotopic data from an unusual spinel dunite occurring within metasedimentary units of the Late Neoproterozoic–Cambrian accretionary belt in the southernmost part of the Southern Granulite Terrain in India. The volatile-rich dunite is mainly composed of cumulus olivine with intercumulus ilmenite, phlogopite, spinel, graphite, Ni-bearing pyrrhotite and calcite. The olivine is Mg-rich (Fo ∼ 96) and unusually poor in Ni. Spinel, even though a minor component, is Mg–Al type (Mg# ∼ 0.92) and is depleted in Cr (Cr#

Peculiar dunites, in part wehrlitic, that contain up to 3 vol.% sulfides from a thick (~ 1000 m) Moho transition zone (MTZ) are found along Wadi Thuqbah in the northern Oman ophiolite. We discuss their relevance to the formation of Cyprus-type massive sulfide deposits near the surface. Field observations suggest that the sulfide-rich MTZ dunites are of late-intrusive origin. The sulfides form composite grains with magnetite and form angular clasts, which are enclosed or cut by magnetite. The sulfide part is composed of homogeneous pyrrhotite and vermicular intergrowth of pyrrhotite and pentlandite. Sulfide inclusions in clinopyroxene comprise pyrrhotite with pentlandite blebs, free of magnetite. Olivines in the sulfide-rich dunite characteristically show low NiO contents (0.1–0.3 wt.%) relative to a high Fo value (~ 91), and as such they do not lie on a Fo–NiO trend of ordinary sulfide-free MTZ dunites–wehrlites. This low-Ni olivine was precipitated from a high-Mg magma that had segregated Ni-rich sulfide melts. The pentlandite–pyrrhotite intergrowth was formed by subsolidus exsolution at low temperatures (

Peculiar dunitic veins almost or totally free of spinels crosscut a podiform chromitite ore body in the Wadi Rajmi, northern Oman ophiolite. They probably originated from a komatiitic melt which was oversaturated in Fo≤94 olivines and which evolved to precipitate simultaneously both chromian spinels, with Cr# ranging from 0.6 to 0.8, and Fo91-93 olivines. The absence or the low modal amounts of spinels are possibly governed by a Cr-undersaturation state of the involved melt which crystallized under relatively low cooling rates to generate the spinel-free and the spinel-poor dunites. A shallow and highly depleted mantle source for this komatiitic melt was envisaged during a converging tectonic regime, initiated earlier in the dynamic history of the Oman ophiolite.

Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks consist of glaucophane, actinolite, actinolitic hornblende, plagioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300–400°C and 7–11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeiitic basalts. Evident negative anomalies of Nb, Ta and Ti relative to Th, La and Ce, in the primitive mantle normalized spider-gram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relative depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8–13% melting of a spinel lherzolite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio suggests that the subduction zone was young and immature during the volcanism.

We studied basal lherzolites that are exposed along the metamorphic sole at the base of the central Oman ophiolite (Wadi Sarami). We recognized two types of lherzolites (Types I and II) based on field occurrences, textures, and mineral compositions. Type I lherzolites are massive and transition into harzburgites, whereas Type II lherzolites are strongly foliated with mylonitic to porphyroclastic textures. Type II lherzolites only crop out within a direct contact with the amphibolitic sole to few meters above this sole and are overlain and/or surrounded by Type I. The clinopyroxenes [Mg# = Mg/(Mg + Fe) = 0.89-0.94] of Type II lherzolites show higher contents of Al2O3 (4.5-7.3 wt%), Na2O (0.5-1.2 wt%), Cr2O3 (0.6-1.4 wt%), and TiO2 (0.2-0.4 wt%) than those of Type I lherzolites. Positive correlations among the Al2O3, Na2O, and TiO2 contents of clinopyroxenes show a pronounced residual trend from Type I lherzolites to depleted harzburgite, giving rise to chemical heterogeneities at the base of the mantle section. Clinopyroxenes in lherzolites and harzburgites show compositional trends that are similar to those in abyssal peridotites from normal ridge segments. Olivines (Fo89.4-Fo91.5) show a residual character of the Sarami peridotites. Primary spinels show a wide range of Cr# [= Cr/(Cr + Al) from 0.04 to 0.53] and low YFe [Fe3+/(Cr + Al + Fe3+)