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The continental margin is of profound importance as it records continental growth by accretion of orogenic magmas and following continental rifting. A high degree of mantle melting due to hydrous fluid input is expected to simultaneously stimulate continental growth and lower the intrinsic density of the mantle than more fertile mantle, which in turn isolates the continental lithosphere from the convective mantle. The mantle peridotites from Gibbs Island (South Shetland Islands) and Bruce Bank in the Drake Passage provide us an insight into the tectonic history in the circum-Antarctic region. To elucidate the continental growth of Antarctica, we present geochemical data of eight dunites from Gibbs Island and one dunite from Bruce Bank, including Re–Os isotope and highly siderophile element compositions. The dunites are severely affected by serpentinization as evidenced by antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt.%) but contain primary euhedral to subhedral chromites with or without spherical inclusions. The chromites rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture in the chromite with low LOI (3 wt.%), indicating a deserpentinization origin from a precursor serpentinized dunite. The dunites show highly depleted bulk-rock major element compositions (Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for Gibbs Island dunites, Mg/Si = 0.66 and Al/Si = 0.008 for Bruce Bank dunite), overlapping a compositional field defined by forearc peridotites. The positive correlation in Re/Ir–LOI space corroborates Re input during the later serpentinization process. The 187Os/188Os ratios of the dunites range from 0.11907 to 0.14493. Phanerozoic Re-depletion (melt depletion) ages of ca. 535–129 Ma are recorded in the Gibbs Island dunites, except for one with a Mesoproterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, the ages provide minimum time estimates for melt depletion events. The early Paleozoic melt depletion is inferred to have occurred at a very early stage of Antarctic Peninsula formation in response to plate convergence along the margin of Gondwana, whereas the Mesoproterozoic Re-depletion age reflects convecting mantle heterogeneity unrelated to any nearby crust-forming events. The petrographic characteristics of the chromites and highly depleted nature of the dunites are attributed to melt–peridotite reaction in a subduction zone setting. A feasible interpretation for the dunite formation is that the mantle had experienced two stages of melting with the final stage occurring along the Gondwana continental margin in the subduction zone setting. Resultant highly refractory lithospheric mantle was later displaced and dispersed during the Gondwana breakup. Widespread existence of the dunite may be attributed to multi-stage melt depletion along the continental margin.

Base-metal sulfides (BMSs) are minerals that host platinum-group elements (PGE) in mantle peridotites and significantly control the bulk PGE content. They have been investigated in detail down to the sub-micrometer scale to elucidate PGE behavior in the Earth's interior. Base-metal sulfides are supposedly subjected to supergene and seawater weathering, leading to the redistribution of PGEs at low temperatures. Careful and thorough measurements of BMSs are thus required to elucidate PGE behavior in the Earth's interior. In the present study, a sub-micrometer-sized PGE-bearing sulfide inclusion in a clinopyroxene crystal in a harzburgite xenolith from Tahiti (Society Islands, French Polynesia) was investigated in detail (down to the sub-micrometer scale) using transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDS). The sulfide inclusion is of carbonatitic metasomatic origin, as it is enveloped by carbonaceous glass, and forms a planar inclusion array with other PGE-bearing sulfide inclusions. The following sulfide phases were identified using TEM-EDS: Fe- and Ni-rich monosulfide solid solutions (MSSs), Fe- and Ni-rich pentlandite, sugakiite, heazlewoodite, chalcopyrite, and Cu-Ir-Pt-Rh-thiospinel (cuproiridsite–malanite–cuprorhodsite). We established the formation process of the metasomatic PGE-bearing sulfide inclusion by considering morphological and mineral characteristics in addition to the chemical composition. A primary MSS first crystallized from metasomatic sulfide melt at ca. 1000 °C, followed by the crystallization of an intermediate solid solution (ISS) below 900 °C. A high-form (high-temperature origin) Fe-rich pentlandite simultaneously crystallized with the primary MSS below ca. 850 °C and recrystallized into a low-form (low-temperature origin) Fe-rich pentlandite below ca. 600 °C. The primary MSS decomposed to Fe- and Ni-rich MSSs, low-form Ni-rich pentlandite, sugakiite, and heazlewoodite. The ISS decomposed to chalcopyrite below ca. 600 °C. Meanwhile, a Cu-Ir-Pt-Rh-thiospinel crystallized directly from the evolved Cu-rich sulfide melt below ca. 760 °C. Thus, Ir, Pt, and Rh preferentially partitioned into the melt phase during the crystallization process of the metasomatic sulfide melt. Metasomatic sulfide melts could be a significant medium for the transport and condensation of Pt together with Ir and Rh during the fractionation process in the Earth's interior. We hypothesize that the compositional variability of PGEs in carbonatites is due to the separation of sulfide melt leading to the loss of PGEs in the carbonatitic melts.

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km. ISSN:0009-2541 ISSN:1872-6836

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 11th Symposium on Polar Science/Interdisciplinary sessions: [IN] New insights of ship-based interdisciplinary study in Soutnern Ocean, Thu. 3 Dec.

Rare nickel hydroxide-hydroxyl carbonate, theophrastite (Ni(OH)2)-zaratite (Ni3(CO3)(OH)4&middot, 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.

Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale ( i.e. scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded. Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8 wt% Na 2 O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides.

The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.

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 SO 4 2– , CO 3 2– 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.

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.

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.

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 (

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.

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 (

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

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.

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.

International audience; Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies1 and ancient analogues (ophiolites)2, 3, 4, 5 that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges3, 6. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle7 and mid-ocean-ridge basalt differentiation8, 9. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.