Susumu Umino

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.

Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc

The discovery of boninite, a typical high-MgO andesite, in the Oman ophiolite is reported. The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence that overlie or crosscut the spreading-ridge-derived lavas (Geotimes volcanic sequence) and sheeted dikes. The phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the Izu-Mariana forearc boninites, indicating that the Alley boninites represent primitive melt generated by partial melting of hydrous peridotite. The occurrence of boninite provides strong thermal and chemical constraints on the formation of the Oman ophiolite that require hot, hydrous shallow mantle (>1250 °C at <30 km depth) to have underlain the proto-Oman ophiolite at the time of boninite generation. The initiation of subduction of the young, hot oceanic lithosphere (and obduction of the future Oman ophiolite) near the spreading ridge and the resultant melting of the highly depleted, shallow-mantle wedge metasomatized by slab-derived fluid represent the most favorable mechanism for the genesis of the Alley boninites.

Magma mixing in boninite sequence of Chichijima, Bonin Islands

Abstract Boninite from Chichijima can be divided into two major types: (A) porphyritic type (phenocryst mode exceeds 1%) and (B) aphyric type (phenocryst mode is less than 1%). Both types are further subdivided by means of microphenocryst assemblages into five subtypes: (I) Olivine (Ol) + Clinoenstatite (Cen) + Bronzite (Brz); (II) Ol + Brz; (III) Brz; (IV) Brz + Pigeonite (Pig) + Augite (Aug); (V) Cen + Brz. There is a systematic correlation between these rock types and the bulk composition of aphyric boninites. The most magnesian boninites are of type I, and the least magnesian ones are of types III and V. Type II is slightly higher in CaO than type I at a given FeO ∗ /MgO ratio. Type IV is distinct from the others for its high CaO and low incompatible element contents at a given FeO ∗ /MgO ratio. Type IV cannot be derived from the others by crystallization differentiation. Compositional data for the other types of boninite define rectilinear compositional variations which lie on a tie-line connecting Fe-rich bronzite and primitive boninite. Microphenocrysts in equilibrium with boninite magma are more magnesian, so it would appear that the Fe-rich bronzite was in equilibrium not with boninite magma but with some evolved descendant (e.g. bronzite andesite magma). The variations of aphyric boninites are then best explained by a combination of mixing of primitive boninite and evolved descendants together with equilibrium crystallization processes. In order to account for this combination of processes, a density-stratified, open-system magma chamber of the type proposed for oceanic ridge systems seems appropriate. The upper part of the magma chamber is composed of differentiated (bronzite andesite) magma and the lower part consists of hot, dense and primitive boninite magma. Crystallization and sporadic overturn and mixing give rise to the spectrum of magma types erupted at the surface.

Subaqueous lava flow lobes, observed on ROV KAIKO dives off Hawaii

Remotely operated vehicle (ROV) KAIKO dives north of Oahu Island, Hawaii, and on the lower south rift zone of Loihi Seamount revealed diverse flow morphologies of submarine lava that correlate with slope and rate of lava delivery. Steep to moderate (>10°) slopes are covered with elongate pillows and narrow pahoehoe streams; bulbous pillows and smooth pahoehoe lobes occur on flat areas and gentle slopes. Some gentle slopes are covered by lobate sheet flows that supply pillow flows. Smooth pahoehoe lobes change upslope into lobate sheets, indicating that the sheets form by coalescence and inflation of successively emplaced flow lobes. Many pahoehoe flows contain hollow, tumuli-like lobes that have inflated and collapsed. Thin crusts (4–20 cm) and large volumes (0.7–1050 m 3 ) of such inflated lobes suggest lava supply rates of 0.01–8 m 3 /min. These calculated supply rates are more than one order of magnitude larger than those for subaerial tumuli in Iceland. Thinner viscoelastic layers of subaqueous lobes at the time of inflation allowed higher excess pressures and expansion rates.

Tectono-magmatic processes investigated at deep-water flanks of Hawaiian volcanoes

Hawaiian volcanoes are exceptional examples of intraplate hotspot volcanism. Hotspot volcanoes, which frequently host large eruptions and related earthquakes, flank-failure landslides, and associated tsunamis, can present severe hazards to populated regions. Many studies have focused on subaerial parts of Hawaiian volcanoes, but the deep-water flanks of the edifices, which can reach 5700 m below sea level, remain poorly understood because they are so inaccessible. In 1998 a collaborative program between Japan and the United States was initiated to explore the evolution of Hawaiian volcanoes, including their growth and degradation.

Lava deposition history in ODP Hole 1256D : insights from log-based volcanostratigraphy

A log-based volcanic stratigraphy of Ocean Drilling Program Hole 1256D provides a vertical cross-section view of in situ upper crust formed at the East Pacific Rise (EPR) with unprecedented resolution. This stratigraphy model comprises ten electrofacies, principally identified from formation microscanner images. In this study, we build a lava flow stratigraphy model for the extrusive section in Hole 1256D by correlating these electrofacies with observations of flow types from the modern EPR, such as sheet flows and breccias, and pillow lavas and their distribution. The resulting flow stratigraphy model for the Hole 1256D extrusive section represents the first realization of detailed in situ EPR upper oceanic crust construction processes that have been detected only indirectly from remote geophysical data. We correlated the flow stratigraphy model with surface geology observed from the southern EPR (14°S) by Shinkai 6500 dives in order to obtain the relationship between lava flow types and ridge axis-ridge slope morphology. This dive information was also used to give a spatial-time reference frame for modeling lava deposition history in Hole 1256D. In reconstructing the lava deposition history, we interpreted that the origins of the ∼100 m thick intervals with abundant pillow lavas in Hole 1256D are within the axial slope where pillow lavas were observed during the Shinkai 6500 dives and previous EPR surveys. This correlation could constrain the lava deposition history in Hole 1256D crust. Using the lateral scale of ridge axis–ridge slope topography from the Shinkai 6500 observations and assuming the paleospreading rate was constant, 50% of the extrusive rocks in Hole 1256D crust were formed within ∼2000 m of the ridge axis, whereas nearly all of the remaining extrusive section was formed within ∼3000 m of the ridge axis. These results are consistent with the upper crustal construction model previously suggested by seismic studies.

Transition from seamount chain to intraplate volcanic ridge at the East Pacific Rise

A number of large submarine intraplate volcanic ridges have been discovered throughout the South Pacific Basin, but their origins are enigmatic. Recent shipboard geophysical surveys reveal that the Sojourn Ridge, one of these large intraplate ridges, becomes a chain of discrete seamount volcanoes that intersects the ridge axis. This seamount chain exhibits several features that suggest that it is directly related to the Sojourn Ridge. The Sojourn Seamount Chain grows continuously in both volume and number of seamounts with distance from the spreading axis; several loci of recent volcanic activity along the chain are evident in the side-scan imagery, and a mantle Bouguer anomaly low underlies the entire length of the chain. This evidence provides new constraints on the origin of intraplate volcanic ridges. The continuation of the Sojourn Ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism, such as flexural stresses induced by the mass of the volcanic pile, as the probable mechanism for developing volcanic ridges and long seamount chains.

Thermal and chemical evolution of the subarc mantle revealed by spinel-hosted melt inclusions in boninite from the Ogasawara (Bonin) Archipelago, Japan

Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago (Japan) compose two discrete groups of high-SiO2, high-MgO (high-Si) and low-SiO2, low-MgO (low-Si) boninitic suites, with ultra-depleted dish- and V-shaped, and less-depleted flat, rare earth element patterns. The most magnesian melt inclusions of each geochemical type were used to estimate the temperature-pressure conditions for primary boninites, which range from 1345 °C at 0.56 GPa to 1421 °C at 0.85 GPa for the 48–46 Ma low-Si and high-Si boninites, and 1381 °C at 0.85 GPa for the 45 Ma low-Si boninite. The onset of the Pacific slab subduction at 52 Ma forced upwelling of depleted mid-oceanic ridge basalt mantle (DMM) to yield proto-arc basalt (PAB). With the rise of DMM, refractory harzburgite ascended without melting. At 48–46 Ma, introduction of slab fluids induced melting of the PAB residue and high-temperature harzburgite, resulting in the low-Si and high-Si boninites, respectively. Meanwhile, convection within the mantle wedge brought the less-depleted residue of PAB and DMM into the region fluxed by slab fluids, which melted to yield the less-depleted low-Si boninite at 45 Ma, and fertile arc basalts, respectively.

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.

Drilling to Earth’s mantle

Half a century after the first efforts to drill through oceanic crust failed, geoscientists are ready to try again.