Sumio Miyashita

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Evolution of the Hidaka metamorphic belt, northern Japan

The Hidaka metamorphic belt is a tilted island-arc assembly of crustal layers developed during early to middle Tertiary age. The P-T estimates of metamorphic rocks in the amphibolite and granulite facies reveal a relatively high geothermal gradient (40°C km−1), possibly caused by the large amount of gabbroic intrusions, at an early evolutional stage of the crust. The crust was subsequently displaced subhorizontally from north to south, giving rise to detachment of lower to upper crustal layers from the lowest crust, and to the formation of a duplex of crustal metamorphic layers. This was followed by dextral transpression in which the crustal layers were uplifted and tilted steeply eastward. Geological setting In Hokkaido, northern Japan, there are two Cretaceous arc-trench systems; one occupies the western part of Hokkaido and the other occupies the eastern part (Fig. 1). The western system consists of the Oshima granitic-volcanic terrane and the Sorachi-Yezo subduction-accretionary terrane on the eastern side where the Kamuikotan serpentinite melange with high-P/T schists occurs. Westward subduction occurred during early to late Cretaceous time (Okada 1982, Kiminami & Kontani 1983, Ishizuka et al. 1983). The eastern system is composed of the Tokoro subduction-accretion terrane which is situated on the western side of the Nemuro volcanic terrane. This northeast-ward subduction was active in the late Cretaceous (Sakakibara 1986, Sakakibara et al. 1986). Late Cretaceous accretionary complexes of both arc-trench systems are juxtaposed in the central part of Hokkaido where the Hidaka metamorphic belt occurs (Fig. 1). A collision between the two arc-trench

Petrology of local concentration of chromian spinel in dunite from the slow-spreading Southwest Indian Ridge

This is the first detailed report on local concentration of chromian spinel in a dunite from an ultraslow spreading ridge, the Southwest Indian Ridge (SWIR). The sample was collected from an outcrop with detailed observations using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. The dunite occurs as a tabular-shaped layer in a lherzolite host outcrop. Spinel is found as a string of small micropods 2–3 centimeters in size. These spinel micropods make a layer in the middle part of a spinel-poor dunite ( 0.4) compared with other peridotite samples in the studied area (lherzolites to harzburgite with low-Cr# spinel, typically Cr# ≤ 0.3). The occurrence and chemical compositions of clinopyroxene in the enstatite-poor harzburgite suggest that some clinopyroxenes crystallized from infiltrated interstitial melts. The host peridotites are interpreted as a residue of relatively low degrees of partial melting consistent with a location along the SWIR far from a mantle hot spot. This was then followed by crystallization of clinopyroxene from interstitial melt in the dunite. Irrespective of their small size, the lithological relationships between the spinel micropods and the host peridotites are the same as those for podiform chromitite in ophiolites and orogenic peridotites. The spinel Cr# in the micropods (0.3) is compatible with the lower range of those in basalts from SWIR far from hot spot. The spinel micropods were mainly formed by interaction between relatively depleted peridotite and a locally significant volume of basaltic melt traversing the upper mantle. This study coupled with the previous works on chromitites suggest that podiform chromites occur in every geodynamic setting, though economic concentrations of chromite (Cr-rich spinel) are unlikely to occur in the mantle at ultraslow spreading ridges.

Magmatic srilankite (Ti2ZrO6) in gabbroic vein cutting oceanic peridotites: An unusual product of peridotite-melt interactions beneath slow-spreading ridges

Abstract We report srilankite in a gabbroic vein cutting a serpentinized peridotite collected from the Atlantis II Fracture Zone, the slow-spreading Southwest Indian Ridge, using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. Srilankite occurs in small patches, <30 μm across, always coexisting with ilmenite and rutile. Zircon, apatite, and phlogopite also occur as accessory minerals in the vein. The Zr/Ti ratio of the srilankite is close to the stoichiometric value of one-half (Ti2.00Zr0.98Hf0.01Fe0.01O6). Based on petrography, the srilankite appears to have co-crystallized with ilmenite and rutile from melts rather than through metamorphic recrystallization. Mineral assemblages and mineral compositions in the vein indicate that melts that produced the vein have high concentrations of compatible elements (MgO and Cr2O3) as well as incompatible elements (high-field strength elements, K2O, and H2O). On the other hand, TiO2-enrichment of minerals in the peridotite host on the periphery of the gabbroic vein may have resulted from interaction with the melts. Geochemical interactions between peridotite and melt in the upper mantle may effectively concentrate incompatible elements in a modified melt, which may precipitate srilankite directly. Physical conditions under slow-spreading ridges, characterized by a highly attenuated magma supply and high rock/melt ratio, favor peridotite-melt interactions

Initiation of ophiolite emplacement: a modern example from Okushiri Ridge, Northeast Japan arc

Abstract Submersible dives by Shinkai 2000 discovered thickly piled back-arc basin basalts and an overlying sequence of siliceous hemipelagites and turbidites on the western fault escarpment of the Okushiri Ridge, off western Hokkaido. The Okushiri Ridge is an allochthonous tectonic block marked by an east-dipping main boundary thrust and a west-dipping back thrust. The block is interpreted to have formed as a result of the plate convergence between the Northeast Japan and Eurasian plates and all available data, including seismic profiles and geochemical analyses, suggest that the Okushiri Ridge constitutes a piece of oceanic crust of the Japan Basin. Ultimately, this piece of oceanic crust will be incorporated into the Japan magmatic arc. The Okushiri Ridge therefore demonstrates that tectonic inversion, i.e. the change from extension to compression, plays an important role in ophiolite emplacement.

Evolution of volcanism and magmatism during initial arc stage: constraints on the tectonic setting of the Oman Ophiolite

Abstract Based on detailed stratigraphy, petrology and geochemistry, the initial arc magmatism of the Oman Ophiolite consisting of tholeiitic lavas followed by boninite flows and tephras is studied in the Wadi Bidi area, northern Oman Mountains. An 1110-m-thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequences by a 1.0-m-thick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2, while the upper part consists mainly of sheet flows with sparse interbedded pelagic sediments and a cylindrical plug. In addition to the presence of a feeder conduit, the flow-dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates that the study area was the centre of a volcano grown in a short period. The UV2 is composed of boninite sheet flows overlain by a 2.0-m-thick pyroclastic fall deposit. A small amount of boninite lavas at the end of the V2 sequence overlain by thick pelagic sediments suggests that the subduction-related arc volcanism was short lived and terminated long before the ophiolite obduction. Supplementary material: Locations, mode of occurrence, phenocryst assemblages and bulk-rock major and trace element compositions of lavas in the Wadi Bidi area are available at http://www.geolsoc.org.uk/SUP18684.

Origin of large wehrlitic intrusions from the Wadi Barghah to Salahi area in the northern Oman Ophiolite

Abstract Wehrlitic intrusions constitute an important element of oceanic lower crust of the Oman Ophiolite, and numerous such dunite and plagioclase wehrlitic intrusions cut gabbro units in the northern Salahi Block of this ophiolite. For the first time, we describe a large wehrlitic intrusion (the Barghah complex) that has disturbed and folded surrounding gabbro units around this complex. The Barghah complex contains many gabbro blocks that record evidence of magma mingling. The crystallization sequence of the wehrlitic intrusions is olivine followed by co-crystallization of plagioclase and clinopyroxene. The forsterite content of olivine and the Mg# of clinopyroxene are more evolved than in rocks from the Moho Transition Zone (MTZ). TiO2 and Na2O contents of clinopyroxene are similar to those of the MTZ and are characterized by wide compositional variability over a narrow range of Mg#, which is indicative of melt–mantle interaction. Compositional co-variation of plagioclase and olivine in the wehrlitic intrusions has a similar signature to that of V1 magmatism (ocean ridge stage). In light of these observations, we conclude that the wehrlitic intrusions are the product of off-axis magmatism. Inversion tectonics that led to a transition from spreading to compression may have accelerated the emplacement of the wehrlitic intrusions.