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Abstract Garnet peridotite xenoliths have been rarely reported from suboceanic mantle. Petrographic and geochemical characteristics of garnet-bearing oceanic peridotite xenoliths provide precious information on dynamics of the suboceanic lithosphere and asthenosphere interaction. We examined a lherzolite xenolith included in olivine nephelinite lava from Aitutaki Island, a member of the Cook-Austral volcanic chain. The lherzolite xenolith contains reddish fine-grained (

Abstract Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift.

Abstract The lithospheric mantle, formed at the mid-ocean ridge as a residue of crustal production, comprises theoretically depleted peridotite, but more fertile components (e.g., lherzolite and pyroxenite) have been reported, creating an enigmatic picture of the lithosphere. The oceanic lithosphere has also been found to be locally modified by intraplate magmatism as proposed from geochemistry of mantle xenolith. Petit-spot xenoliths are particularly notable as direct evidence of old lithospheric mantle and expected to retain essential information about oceanic lithosphere prior to its subduction. In this study, we report on the lithological structure of Pacific lithosphere aged at 160 Ma, just subducting into Mariana Trench, based on petrology and chemistry of ultramafic xenoliths from a petit-spot knoll, and then, we suggest the occurrence of petit-spot melt infiltration resulting in mantle metasomatism and formation of pyroxene-rich vein. Our petit-spot ultramafic xenoliths can be divided into three main types: a depleted peridotite as a residue of crust production, an enriched peridotite, and fertile pyroxenites as the product of melt–rock interactions prior to entrapment. Geothermobarometry also suggests that the depleted peridotite was derived from the uppermost lithospheric mantle, whereas the enriched peridotite and Al-augite pyroxenites were obtained from deeper layers of the lithosphere. Moreover, thermal gradient of the lithosphere estimated from these data is considerably hotter than pristine geotherm estimated on the basis of plate age. Hence, we could illustrate that the oldest portion of the Pacific lithosphere (160 Ma), which was not observed before, was locally fertilized and heated by prior multiple petit-spot magmatic events, and pyroxene-rich metasomatic veins penetrated from the base to the middle/upper lithosphere. Such local lithospheric fertilization is plausible at the plate-bending field, and the nature of Pacific Plate subducting into Mariana Trench may be partly different from what has been assumed so far.

Sagittal otoliths, typically composed of aragonite, are frequently laid down rather as vaterite during growth in hatchery-reared fish populations. Sagittal vateritization is believed to impair individual hearing/balancing abilities, but the causal mechanism remains unclear. Here we experimentally demonstrated that rearing in Sr-rich water induces sagittal vateritization in the HdrR-II1 inbred strain of the Japanese rice fish, Oryzias latipes. Both sagittae were partly vateritized in 70% of individuals subjected to the Sr2+ treatment (n = 10), whereas fish reared in normal tap water showed no sagittal vateritization (n = 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr2+ concentration in solution increases. A vateritic layer develops surrounding the original aragonitic sagitta in vateritized otoliths, some of which take on a comma-like shape. Electron probe microanalysis demonstrates that the vateritized phase is characterized by lower Sr2+ and higher Mg2+ concentrations than the aragonitic phase. It is unlikely that increased environmental Sr2+ is responsible for the sagittal vateritization in farmed fish. However, our findings likely help to establish an in vivo assay using O. latipes to understand the physiological process underlying the sagittal vateritization in farmed fish.

Abstract This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Rock and sediment samples were collected from petit-spots in the northwestern Pacific. The sampling was conducted using deep-submergence vehicle (DSV) Shinkai 6500 and its mother ship, research vessel (RV) Yokosuka during YK20-14S and YK21-07S cruises. The collected rock samples are basalt and peperite. Some of the basalts include small mantle xenoliths (∼3 cm in diameter). The dataset of rock and sediment samples from the petit-spots located on >130 Ma northwestern Pacific plate are presented herein. The peperites are a reaction product between petit-spot magma and wet sediment, and the mantle xenoliths are fragmented mantle materials transported by the petit-spot magmas. Therefore, the petit-spot samples are of significant importance to elucidate modification process of the surface condition by petit-spot magma and to characterize the deep lithospheric mantle. The dataset presented herein provides in a sense a unique insight into the whole Pacific plate just before its subduction beneath the Japan arc.

Rare nickel hydroxide-hydroxyl carbonate, theophrastite (Ni(OH)2)-zaratite (Ni3(CO3)(OH)4·4H2O) aggregates were found from a partially serpentinized dunite from Fujiwara, the Sanbagawa metamorphic belt of high-pressure intermediate type, Japan. The dunite was regionally metamorphosed within the Sanbagawa subduction complex of Cretaceous age. The theophrastite-zaratite aggregate from Fujiwara most typically occurs in association with nickel sulfides, which form a composite grain with awaruite and magnetite within an antigorite-rich part of the rock. The theophraste-zaratite formed possibly together with millerite (NiS) from heazlewoodite (Ni3S2). This represents a partial desulfurization of heazlewoodite, which contains or interlocks with laths of antigorite, suggesting their cogenesis. The desulfurization occurred at an early stage of, or during, exhumation of the subduction complex toward the surface, where sulfur was oxidized and removed as sulfate ions. Serpentinization of olivine has not been associated with the formation of theophrastite-zaratite, and an oxidized condition has been kept at this post-serpentinization stage. The sulfate ions liberated in part precipitated anhydrite where calcium was available in the surrounding rocks. This shows one of the shallow migration pathways of sulfur in the subduction zone, especially to the forearc area.

The role of water-rich solutions in the formation of chromitites has been the matter of controversy. We found small chromite concentrations (chromitites) in diopsidites, precipitated from high-temperature hydrothermal fluids, in the mantle to the crust of the Oman ophiolite. Here, we present petrologic characteristics of the hydrothermal chromitites to understand their genesis. In the chromitites, the chromite is associated with uvarovite in the crust and diopside + grossular in the mantle. They are discriminated from the magmatic podiform chromitite by dominance of the Ca-Al silicates in the matrix. The fluids responsible for chromite precipitation are possibly saline, being derived from the seawater circulated into the mantle through the crust. The saline fluids precipitate chromite to form chromite upon decompression and cooling, and transport platinum-group elements (especially Pt and Pd). The fluids obtain Ca and Al from the crustal rocks and Cr from the mantle rocks during circulation. Saline fluids are also supplied from the slab to the mantle wedge, and can metasomatically precipitate chromite and pyroxenes within peridotites. They re-distribute Cr and chromite in peridotites along with circulation of saline fluids in the mantle wedge.

The lithosphere--asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations and sampling of concordant dunite bands (CDB) and their host harzburgite at selected outcrops covering a wide depth range in the mantle section along an inferred paleo-ridge segment in the northern to central part of the Oman ophiolite. The CDB increase in thickness and decrease in frequency upward. They are thicker and more frequent in the centre of the segment than near the segment ends when compared at the same stratigraphic level. The CDB consist mostly of olivine with minor spinel and very rare amounts of pyroxene. Clinopyroxene has a small grain size and an interstitial position relative to olivine. The constituent minerals in the CDB and their host harzburgite were analyzed by electron microprobe for major elements and by laser ablation inductively coupled plasma mass spectrometry for trace elements. Most of the CDB have refractory major element mineral compositions, such as high Fo [100 x Mg/(Mg / Fe)] in olivine (>90.5), high Cr# [Cr/(Cr / Al)] in chromian spinel (>0.50), and low Al2O3 (<3.5 wt %) in clinopyroxene. Chondrite-normalized trace element patterns of clinopyroxene in the host harzburgites consistently show a gentle decrease from heavy REE (HREE) to middle REE (MREE) and a sharp decrease from MREE to light REE (LREE) (= highly depleted), but those in the CDB show weaker LREE depletion, which is more variable depending on the stratigraphic level and position along the paleo-ridge segment. In contrast, the HREE concentrations in clinopyroxene in the CDB are higher than or similar to those of the host harzburgites. Trace element compositions of clinopyroxene in the CDB and their host harzburgites are evaluated with a one-dimensional, steady-state, opensystem decompressional melting-reaction model. The modeling results suggest that an LREE-enriched melt generated at high pressure was transported upwards through melt channels to the shallow mantle (up to the Moho transition zone), where it mingled with highly depleted melts accumulated from fractionally melted peridotites to generate normal (N)-MORB-like melts. The mantle started upwelling (= melting) in the garnet stability field in the segment centre, but either in the garnet or in the spinel stability field near the segment ends. This suggests a variation of geothermal gradient along the paleo-ridge segment: higher in the segment centre and lower near the segment ends. This inference is supported by the presence of thicker (up to 250 cm) CDB as well as more frequent occurrence of CDB in the segment centre than near the segment end and by the geochemical evidence for chromatographic N-MORB-like melt percolation into the host peridotite only in the uppermost horizons near the segment ends. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]