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.
Reaction products between hydrothermal fluids and uppermost mantle harzburgite-lowermost crustal gabbro have been reported along Wadi Fizh, northern Oman ophiolite. They are named mantle diopsidite ...(MD) or crustal diopsidite (CD) depending on the stratigraphic level. They construct network-like dikes crosscutting structures of the surrounding harzburgite or gabbro. The MD is mainly composed of diopsidic clinopyroxene, whereas the CD is of diopsidic clinopyroxene and anorthitic plagioclase. Here, we report a new reaction product, crustal anorthosite (CA), from the lowermost crustal section. The CA is always placed in the center of the CD network, and mainly consists of anorthitic plagioclase with minor titanite and chromian minerals such as chromite and uvarovite.
Aqueous fluid inclusions forming negative crystals are evenly distributed in minerals of the CA. The fluid inclusions contain angular-shaped or rounded daughter minerals as calcite or calcite-anhydrite composite, which were identified by Raman spectroscopic analysis. We estimated their captured temperature at 530°C at least by conducting microthermometric analysis of the fluid inclusions. Furthermore, we examined their chemical characteristics by direct laser-shot sampling conducted by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). The results indicate that the trapped aqueous fluids contain an appreciable amount of Na, but no K and Cr.
Hydrothermal fluids involved in the CA formation transported Cr, which was probably taken up from chromite seams in the uppermost mantle section. Cr got soluble by forming complexes with anions as SO42–, CO32– and Cl–. In addition, these hydrothermal fluids transported Fe, Mg and trace elements (Ti, Sr, Y, Zr and rare-earth elements) governing whole-rock chemical compositions of the MDs, CDs and CAs. Our estimation for the condition of CA formation yielded rather low temperatures (530–600°C), which indicates a later stage production of the CA than the MD and CD (~800°C). A series of high-temperature hydrothermal events had been significantly contributed to the chemical flux occurring around the Moho, boundary between the mantle and crustal sections.
•We report hydrothermal fluid/peridotite-gabbro reaction products in the Oman ophiolite.•The reaction products are of great importance for tracing temporal evolution of the fluids.•Aqueous fluid inclusions were characterized in chemistry and entrapment temperature.•Whole-rock and in-situ chemical analyses restrict formation circumstance of the reaction products.•The results delineated chemical evolution of the fluids at high temperature (530–600°C).
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.
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.
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.
Whole-rock chemical compositions are significant for igneous petrology, especially volcanic rock studies. ICP-MS is widely used as a conventional method to determine the trace-element compositions ...using solutions prepared by digestion of rocks. In the past decades, LA-ICP-MS technique has been well developed to measure whole-rock compositions by using pressed powdered pellet or fused glass of rock samples. In this study, trace-element concentrations (Li, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, U and REEs) of basaltic and andesitic rocks were determined by using flux-free fused glasses and LA-ICP-MS. The fused glass was prepared by the direct fusion method where the powdered rock sample is melted with an iridium-strip heater. The heating temperature was manually controlled by monitoring the current and the heating lasted less than 30 sec to achieve melting. We evaluated our routine analytical work performed by measurements of 3 random target positions in each fused glass. The data accuracy was evaluated by measurement of 11 fused glasses created from USGS and GSJ international reference materials (BCR-2, BHVO-2, BIR-1a, JB-2, JA-1 and JGb-1). The homogeneity of our fused glasses is represented by less than 5% relative standard deviation (1σ) in most elements. The mean values of most elements are in agreement with the reference values within 10% deviation. However, Pb concentration cannot be reproduced because of strong volatilization during the melting, and Lu, Hf and Ta are occasionally enriched by the elemental contamination/interference from the iridium-strip heater. 9 fused glasses created from selected petrological samples, such as the Ogasawara volcanic rocks and Oman diopsidite, were also measured to examine our routine measurement work and glass preparation method for a wider range of the trace-element abundance, especially the Cr abundance. The analytical results of 20 fused glasses demonstrate that our method is capable of determining the whole-rock trace-element compositions for petrological study.
The life cycle of the oceanic lithosphere commences in the spreading axis and ends in the subduction zone. To trace the cooling and evolutional history of the Earth, the change in thermochemical ...state during the life cycle of present-day oceanic lithosphere is desired to be elucidated. In terms of the material science, spatial limitation of human-accessible Earth interior is a bottleneck in reconstructing the thermochemical state of the oceanic lithosphere. Yet, by combining active sampling methods using ocean research vessels (ocean drilling, ocean bottom dredging, submersible survey, etc.) and passive sampling methods using Earth’s deep materials exposed to the surface owing to tectonic forces and volcanoes, we can collect samples that cover a considerable dimension. Here, I present efforts toward the elucidation of the thermochemical state of the oceanic lithosphere during its life cycle from the spreading axis to the subduction zone. The Oman ophiolite is presented as an analogue of oceanic lithosphere formed in the vicinity of a fast-spreading axis, whereas the peridotite xenoliths from Tahiti Island are treated as an analogue of thermochemically disturbed oceanic lithosphere by a mantle plume, and those from petit-spots are considered as an analogue less affected by thermochemical disturbance considering the lack of mantle plume beneath the petit-spots. A heterogeneous thermal state corresponding to the segment structure is inferred in the fast-spreading axis. The thermochemical state of the aging oceanic lithosphere is modified by mantle plume and petit-spot magmatism, but pristine state can be reconstructed by using suitable peridotite xenoliths whose heating-cooling and melting history is well characterized. The peridotite xenoliths from the petit-spots can enhance a step toward reconstructing the thermochemical state of the deep oceanic lithosphere because deep-rooted garnet-stable peridotite xenoliths can be founded.
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.
Hydrothermal circulation beneath the spreading axis plays a significant role in the exchange of energy and mass between the solid Earth and the oceans. Deep‐seated hydrothermal circulation down to ...the crust/mantle boundary in the fast‐spreading axis has been introduced by a number of studies regarding geological investigations and numerical models. In order to assess a reaction between hydrothermal fluid and host rock around the crust/mantle boundary, we conducted bulk trace element and Sr isotope analyses with a series of in situ investigations for crustal anorthosite, a reaction product between hydrothermal fluid and gabbro in the lowermost crustal section along Wadi Fizh, northern Oman ophiolite. In addition, we conducted titanite U–Pb isotope analyses to evaluate timing of the crustal anorthosite formation in the framework of the evolutional process of the Oman ophiolite. We estimated the formation age of the crustal anorthosite at 97.5 Ma ± 5.0 Ma, overlapping with the timing of the crust formation in the paleo spreading axis. The crustal anorthosite shows high‐Th/U ratio (~2.5) and high‐initial 87Sr/86Sr ratio (0.7050) due to seawater‐derived hydrothermal fluid ingress into the precursor gabbro. With using analytical technique of micro‐excavation at cryo‐temperature, we detected Cl from a few micrometer‐sized inclusion of aqueous fluid and chromite grains. The solubility of Cr was enhanced by complexation reactions with Cl in the hydrothermal fluid. Regarding reconstructed three‐dimensional mass distribution of the inclusion and chromite composition, maximum Cr content of parental fluid was estimated at ~69 000 μg/g. The exceptionally high‐Cr content was achieved locally by leaking of fluid and synchronous chromite crystallization during fluid entrapment. Presence of the deep‐seated hydrothermal circulation could be assigned to the segment end, where cold seawater penetrates into the lowermost crust and extract heat along widely spaced network‐like fluid channel.
Peculiar dunites, in part wehrlitic, that contain up to 3vol.% sulfides from a thick (~1000m) 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 (0.1–0.3wt.%) 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 (<200°C) from high-temperature mono-sulfide solid solution. Iron released from olivine during serpentinization produced magnetite, which was combined with the sulfides to form the composite grains. In-situ S isotope ratios of the sulfides (δ34S=0.7–2.8) are slightly higher than mantle values but lie within the range for magmas from oceanic island arcs, such as the Marianas. The δ34S are lower than those for sulfate from seawater and MORB-related sulfides, such as TAG (Trans-Atlantic Geotraverse) deposits. One of the Cyprus type massive sulfide deposits (Aarja) from the crustal section of the same area shows similar S isotope ratios to the sulfides in the Thuqbah sulfide-rich dunites/wehrlites, indicating their genetic linkage. The Aarja sulfide deposit was formed within the V2 lavas, which are relatively sulfur-rich and of an off-axis origin, as a result of high-temperature seawater circulation. The Thuqbah sulfide-rich dunite possibly represents an igneous root of the Cyprus-type massive sulfide deposit of Aarja formed in an off-ridge magmatic-hydrothermal system.
► Sulfide-rich dunites were found from a thick MTZ of Oman ophiolite. ► Sulfides are pyrrhotite and pentlandite, and olivines are low in NiO. ► Sulfides are similar in S isotopic ratio to one of Cyprus-type ore deposits upsection. ► S supplied with water from the slab promoted partial melting of the then mantle wedge.
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