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.
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.
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.
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.
Carbon isotope data is desired to be increased to promote the understanding of carbon cycle throughout in the Earth. Diamond is a key carbonaceous tool to study deep carbon cycle, but most diamond ...occurrences are limited from kimberlite pipes in the continental region. Recently, micron-sized diamonds have been discovered from the oceanic region and investigated to understand deep carbon cycle in the oceanic mantle. However, some fundamental cautions have been issued on the oceanic diamonds because some of them could be of artificial origin. Hence, alternative oceanic mantle-derived carbonaceous material is needed to increase oceanic carbon isotope data. We report micron-sized calcite vein in a lherzolite xenolith hosted by enriched mantle I (EM1)-type olivine nephelinite from Aitutaki Island, Cook Islands in the southern Pacific. With employing various techniques to determine carbon and oxygen isotope compositions from sub-micrograms of calcite, we demonstrate that carbonaceous fluid originated from EM1-type mantle source exhibited organic carbon signature based on its light carbon isotope composition along with petrographic characteristics of the calcite vein. The oceanic mantle hosts organic carbon in places due to the recycling of surface materials.
•Calcite in oceanic lherzolite xenolith exhibited light carbon isotope composition.•Carbonaceous fluid was originated form enriched mantle I-type mantle source.•Oceanic mantle hosts organic carbon due to the recycling of surface materials.
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.
Peridotitic, dunitic, and wehrlitic rocks drilled at Atlantis Massif (Mid-Atlantic Ridge), where mantle-derived rocks were exhumed to the sea floor via detachment faulting, were investigated ...employing in situ geochemical analyses. Since dunitic and wehrlitic rocks are commonly interpreted as fossil melt channels, they likely transported parental melts of mid-ocean ridge basalt (MORB) beneath paleo-ridge axis. Although the rocks are severely serpentinized, primary olivines, chromian spinels, orthopyroxenes, and clinopyroxenes were found in several samples. The Cr/(Cr + Al) ratios (Cr#) of the chromian spinel show bimodal distribution, where one group records higher Cr# (0.35–0.5) and the other group is represented by lower Cr# (0.2–0.3). The extent of mantle melting is strongly correlated to the Cr# of the chromian spinel, thus the mantle-derived materials of the Atlantis Massif bear at least two end-members with regard to the extent of melting: refractory mantle material with higher Cr#, and less refractory mantle material with lower Cr#. To quantitatively evaluate mantle melting and melt transport mechanisms in the suboceanic mantle beneath the section of the Mid-Atlantic Ridge, a one-dimensional, steady-state decompressional mantle melting model was carried out employing rare-earth element concentrations of clinopyroxene. Our modeling results demonstrate the presence of refractory peridotites molten in the presence of garnet, but crustal materials are limited in the Atlantis Massif. We advocate that the refractory mantle materials underwent little recent melting beneath the paleo-ridge axis, and alternatively had been subjected to ancient partial melting. The refractory mantle materials probably contributed to the initiation of the detachment fault and affected the extreme variations in MORB chemistry in the Mid-Atlantic Ridge.
•Peridotites are divided into at least two end members in terms of melt depletion.•Refractory peridotites underwent little recent melting beneath paleo-ridge axis.•Wehrlite was crystallized from a depleted melt.•Detachment fault was initiated under melt-limited condition.•Refractory peridotites affect the extreme variation in MORB chemistry.
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