Deep drill holes bored into in situ oceanic crust were reviewed. Since the beginning of the Deep Sea Drilling Project, 45 drill holes have penetrated over ca. 50 m into normal oceanic crust, however, most are concentrated in the north Atlantic and east Pacific Oceans. Basement ages of most holes are younger than 20 Ma, and are rather biased toward younger crust compared to the average age of the oceanic crust (61 Ma). Only five deep drill holes have penetrated over 500 m into the basement, three of which drilled into slow spread crust formed at < 4 cm/yr, with only one hole is fast spread crust formed at >8 cm/yr and one in intermediately spread crust. Three deep holes (332B, 395A, 418A) drilled into slow spread crust formed at Mid-Atlantic Ridge gave the first evidence of magnetic reversals through the vertical oceanic crust, and showed that the slow spread upper oceanic crust away from hot spots is dominantly composed of pillow lavas with a normal MORB-like affinity. Younger 332B and 395A holes (3.5 and 7.3 Ma) gave poor core recovery (18-21%), while the oldest Hole 418A (110 Ma) yielded a fairly high recovery of 72%. Hole 504B is the only hole to penetrate the extrusive rocks and most of the way through the sheeted dike complex (1836.5 m sub-basement). Average core recovery dropped from 29.8-25.3% in the lava and transition zone down to 14.3% in the sheeted dike complex. Unfortunately, the 504B lava is the depleted extremity of MORBs from intermediate-fast spread ridges. One of the most important findings of Hole 504B is a discrepancy between the seismic velocity structure and the downhole lithology in that the Layer 2/3 boundary resides in the middle of the sheeted dikes, as interpreted by the difference in porosity and bulk density. Hole 1256D is dedicated to coring typical oceanic crust and ultimately penetrates the entire crust into the upper mantle. The site is located on the 15-Ma Cocos plate generated at a superfast rate (22 cm/yr). 502-m-long cores of basement (48% recovery) are lavas showing moderately evolved MORB-like compositions similar to those from the present fast spread ridges. The hole has been cleaned and left ready for future drilling, possibly into Layer 3. The above examples of deep drill holes show that the major obstacles to ultradeep drilling are hole collapse and poor core recovery. Riser drilling is expected to overcome these obstacles for “the 21-century Mohole”.
Volumetrically, magmatism at spreading axes occupies more than 60% of present day volcanism, and oceanic crust generated there covers more than two thirds of earth surface. Therefore, studies of the oceanic crust and magmatism at spreading axes yield significant insights to understanding earth evolution. Recent studies on oceanic crust through drilling, submersible surveys, dredging and geophysical exploration indicates that there is considerable variation in lithologic composition and stratigraphic sequences in oceanic crust that is dependent on the spreading rate. Typical ophiolitic stratigraphic sequences are expected to occur at fast-spreading ridges, while complicated and variable stratigraphic successions are encountered at slow-spreading ridges. Mid-Ocean Ridge Basalts (MORBs) are generally regarded as having comparatively uniform compositions. Major element compositions of MORB corrected for low-pressure fractionation show a global systematic variation correlated to axial highs, which can be ascribed to the degree of partial melting. However, the composition of MORB should represent a final product of magmas whose compositions have been greatly modified during ascent through mantle peridotite, by crustal fractionation that produced lower crust (ultramafic to gabbroic cumulates), and by mixing in a melt lens in shallow crust. Therefore, to fully understand MORB petrogenesis it is necessary to study the total magmatic system, which includes surface lavas, dike complexes, plutonic cumulates extracted from melts, and residual mantle. Recent results from the MARK area of the Mid-Atlantic Ridge, the Atlantis bank of the Southwest Indian Ridge and Hess Deep near the East Pacific Rise strongly suggest that source mantle is heterogeneous and that the magmatic system differs significantly depending on the spreading rate.
A comparative study of deep mantle structures beneath spreading centres revealed asymmetry of lithosphere and mantle. Multidisciplinary marine expeditions were conducted at two survey sites : one around a super-fast spreading ridge in the East Pacific Rise, and the other around a slow and stable ridge segment of the continental breakup region, the Gulf of Aden. At the former site, a Mantle ELectromagnetic and Tomography experiment (MELT) was conducted with a large number of seismometers and electromagnetometers deployed at the seafloor to image the shape and the geometry of partial melt bodies below the fast spreading centre. At the latter, a seafloor magnetotelluric array was deployed to obtain a twodimensional electrical conductivity structure beneath the continental breakup region, and the structure was compared with precise swath mapping and sea surface gravimetry. At both sites, the deep electrical conductivity structures showed remarkable asymmetry with respect to the ridge crest. Partial melt bodies that correspond to the seafloor-producing factory at the surface could not be observed right below the ridge crest, but they significantly shifted by several tens of kilometers from the ridge crest. The asymmetry also appeared in bathymetry, gravity and seismic structures. This implies that the mid-ocean ridges are created at the contacts of different oceanic mantles rather than right above the symmetric partial melt bodies.
Petological constitution of the upper mantle beneath the ocean floor has been poorly known except for oceanic fracture zones of slow-spreading ridges. Information from ophiolites may supplement the paucity of data to some extent ; however, the ophiolites should be treated carefully because of their polygenetic nature. The abyssal peridotite varies from lherzolite with Cr# of spinel of 0.1 to harzburgite with Cr# of spinel of 0.6. Dunite is relatively rare from the ocean floor. An exotic lherzolite with continental mantle signatures appears in midoceanic areas. The refractoriness of the abyssal peridotie has been proposed to correlate with the spreading rate of the ridge system, but this is false. The upper mantle beneath the ocean floor changes downwards from dunite to lherzolite via harzburgite, being independent of spreading rate. The lithological change is more abrupt in a slowspreading system than in a fast-spreading one, so it is around ridge segment boundaries rather than around the segment center on the same spreading ridge. The thin harzburite layer in slow-spreading ridges has resulted in its rarity there, and the deep seat of lherzolite in fast-spreading ridges has caused its apparent absence. The primitive MORB can be in equilibrium with dunite, which is formed along the melt conduit beneath the ridge via peridotie/melt reaction, and the dunite part is laid down by the corner flow of the mantle just below the lowermost gabbro layer as it leaves the ridge axis. We proposed the following deep ocean-floor drilling to explore scientific proglems concerning the abyssal upper mantle : (1) non-riser drilling on the “continental peridotite” to know the relationship with abyssal peridotite, and (2) non-riser or riser drilling on the ocean floor where deep-seated rocks have already been exposed to examine the deep constitution of the upper mantle. The “21st Century Mohole” drilling through the oceanic Moho should primarily be directed to the segment center of a fast-spreading ridge system. The back-arc basin such as the Sea of Japan will be the alternate for Mohole drilling because we have had relatively little information on the petrological nature of the back-arc basin lithosphere despite its importance. We can solve the “ophiolite problem” simultaneously if we are careful in choosing the drilling sites. We also propose a close linkage between the ophiolite study and ocean drilling in the coming IODP.
To characterise the crust-mantle boundary (petrological Moho) and to find evidence of ophiolite model, we investigated the lithology and the development process of the oceanic crust. We carried out geological and geophysical studies of Atlantis Bank core complex located at the eastern margin of the Atlantis-II active transform in the Southwest Indian Ridge (SWIR) using deep sea submersibles and remotely operated vehicles. Unaltered lower crust and uppermost mantle rocks were observed at the southwestern slope of Atlantis Bank. The lower crust of this part of Atlantis Bank is similar to the ophiolite exposed ashore. On the other hand, a large number of dike intrusions into gabbroic massifs were observed at the eastern wall and at the southern slope of the bank. This corresponds to the dike-gabbro transition in the ophiolite model. Dike intrusions were also observed in the mantle peridotite domains. This may, however, suggest melt intrusions into the bank near the spreading axis posterior to the mantle peridotite that was dragged out along the detachment faults, or may suggest possible horizontal melt intrusion from the segment centre to the segment edge characterised by a thin plutonic layer. The northern ridge-transform intersection RTI of the Atlantis-II active transform presents an L-shaped nodal basin, while the southern RTI presents a V-shaped one. The difference between northern and southern RTI types suggests differences in the structure and basement rock types. A fossil transform fault and RTI relics at the northern side of the spreading axis west of the Atlantis-II active transform were observed, suggesting a sudden change of the spreading direction in SWIR from 20 Ma
Paleomagnetic results of mantle peridotites from Wadi al Hilti section, Oman Samail Ophiolite indicate that the magnetic layer of the upper mantle is about 2 km from Moho, while the lower portion is much less magnetized. This finding, together with the revised magnetization model for ocean crust and upper mantle, can explain the “missing magnetization of the upper oceanic lithosphere” which has been a long-standing problem brought by MAGSAT magnetic anomalies over the Pacific Cretaceous Quiet Zone.
We report major and trace element compositions for 45 fresh basalts from 13 dredge sites between 45.5° E and 49° E along the Gulf of Aden obtained during the Aden New Century cruise (KH-005 : RN Hakuho-maru, ORI, University of Tokyo). The basalts from Aden New Century (ANC) Seamount at around 45.5° E, discovered by bathymetric mapping during the cruise, have the highest Nb/Zr (0.22-0.27) and La/SmN (3.1-3.7) and show the strongest OIB signature among those in the Gulf of Aden. The longitudinal variation of Nb/Zr and La/SmN of the dredged basalts shows two enrichment peaks, i.e., first “keen” enrichment peak with OIB signature near ANC Seamount (45.5°E) and second “broad” enrichment peak with E-MORB signature along the ridge axis from 46° E to 48° E, and at 49° E, N-MORB is dominant. In spite of variable Nb/Zr and La/SmN, the estimated pressure of magma segregation and degree of partial melting from the dredged basalts, using reported results of experimental melting on spinel lherzolite, are relatively constant and almost similar to those of N- and T-MORBs in Pacific, Atlantic, and Indian Oceans. This suggests that the longitudinal variation of the dredged basalts reflects mantle compositions equilibrated with their primitive magmas. Nb/Zr, Nb/Y, and La/SmN systematics for the dredged basalts suggest binary magma mixing of the NMORB source with the OIB source, composed of FOZO (or C) mantle component with the incorporation of minor, if any, HIMU components. Recent studies (George et al., 1998; Orihashi et al., 1998) have suggested that two mantle plumes might be impinged beneath the Afar province in Eocene to Oligocene (45-30 Ma) and Miocene (19-15 Ma) times. Considering the above, two enrichment peaks of the mantle in the Gulf of Aden must be produced by two mantle plumes impinged in the Afar province and spread sideways sequentially, with the front of spreading first plume head corresponding to the broad enrichment peak. This broad peak is formed because the plume head emplaced beneath the lithosphere, producing a Large Igneous Province (30-26Ma), was progressively defused into and diluted by the upper mantle component (N-MORB source) over time.
We examine the petrological nature of the mantle-crust transition zone (MCTZ) based on detailed field observations of the northern Oman ophiolite. Two kinds of MCTZ, early-gabbroin-dunite and late-dunite-in-gabbro transition zones, can be recognized between the residual peridotite and the layered gabbro sequence. They are distinguished by an intrusive relationship between gabbro and dunite. In the early-gabbro-in-dunite transition zone, gabbro forms network-like sills and has intrusive contact with dunite. The frequency of the gabbro sills gradually increases from the top of residual peridotite to the base of layered gabbro, which itself has a sharp boundary with the underlying dunite. All constituents of the earlygabbro-in-dunite transition zone are deformed, and lithological boundaries are parallel to foliation of the rocks. On the other hand, in the late-dunite-in-gabbro transition zone, dunite has intrusive contact with gabbro sills and layered gabbro. Clinopyroxenite produced by reaction/partial melting occurs frequently along the intrusive contact. The late-dunite-ingabbro transition zone is of secondary origin, being modified from the primary layered gabbro to the early-gabbro-in-dunite transition zone by later dunite intrusion. Degree of serpentinization is irregularly distributed, and antigorite, a high-temperature serpentine species, is not found in the peridotite portion. This indicates the Hess model that the oceanic Moho is placed within peridotite as a serpentinization front is not deduced from observations of the Oman ophiolite. We propose a model for the formation of two kinds of MCTZ. The gabbro sills in dunite were originally formed beneath a mid-oceanic ridge as a network of upward-moving melt within residual harzburgite. Dunite was produced by a reaction between melt and harzburgite. The network-like gabbro and dunite were deformed to become elongated by the horizontal mantle flow with leaving the spreading ridge. The boundary between the melt-rich part (center of paleo-melt flow) and melt-poorer part later became the layered gabbro/dunite boundary. The early-gabbro-in-dunite transition zone is the part between the layered gabbro/dunite boundary and the residual harzburgite. An off-ridge magmatism formed socalled late-intrusive plutonic bodies including dunite, cutting the primary rocks formed at the spreading ridge. The dunite formed intrusive contacts throughout the pre-existing crustal sequence, that is, the secondary late-dunite-in-gabbro transition zone. The intrusive bodies have island-arc geochemical signatures and are genetically linked to effusive rocks of islandarc type including picrite. The late-dunite-in-gabbro transition zone probably formed at an island-arc setting during detachment and obduction of a slice of oceanic lithosphere as an ophiolite suite. The early-gabbro-in-dunite transition zone may occur frequently beneath the ocean floor, especially that of the fast-spreading ridge system. We predict a common latedunite-in-gabbro transition zone beneath some oceanic island arcs and back-arc basins.
The mid-Cretaceous Oku-Niikappu Dam accretionary complex (ONDC) in the Idonnappu Zone of Hokkaido contains volcanic, plutonic, and ultramafic rocks with island-arc chemical features. The volcanic rocks are overlain by radiolarian chert. This stratigraphic succession represents an ancient island arc, in which activity ceased within a pelagic environment prior to accretion to the continental margin. The ophiolitic rocks of the ONDC, which may have comprised upper mantle, arc crust, and cover sediments, are thus regarded as accreted fragments of an intraoceanic remnant arc. Basaltic to andesitic volcanics, gabbroic to tonalitic plutonics, and serpentinite are constituents common to the ONDC remnant arc and equivalent settings in the modern Philippine Sea. They also have a common cover sedimentary sequence, from volcanic basement through volcaniclastic debrites to pelagic sediments. This sequence may record the history from arc activity through back-arc rifting to intraoceanic isolation. Clasts of serpentinite, plutonic, and metamorphic rocks in the volcaniclastic debrites in the ONDC and those from the Daito Ridge in the Philippine Sea imply extensional exhumation of lower crustal and upper mantle rocks during back-arc rifting.
Mantle melting and production of magmas in NE Japan may be controlled by locally developed hot regions within the mantle wedge that form inclined, 50 km-wide fingers. In this case, are these hot fingers chemically and/or isotopically different from the host mantle wedge? Forty-four Quaternary volcanoes in NE Japan have been reviewed to evaluate twodimensional strontium isotopic variations, and to infer 87Sr/86Sr contours of the source mantle. The isotopic composition of magma source materials at depth is found to have little relationship with slab depth, suggesting that mantle heterogeneity was established before the flux of fluid released from the subducting slab reached the magma source regions. On the other hand, Miocene Japan Sea back-arc Yamato basin basalts have the same isotopic variation as the Quaternary volcanic arc. Cousens et al. (1994) suggested the possibility that partial melts of sediments, forming at a depth of >200 km may mix with mantle wedge material (87Sr/86Sr0.703), resulting in a magma source component with enriched 87Sr/86Sr of0.705. I suggest that after the cessation of Yamato basin rifting, a MORB-like mantle source (87Sr/86Sr0.703) in the mantle wedge below the Quaternary NE Japan arc was replenished by a fertile mantle material (87Sr/86Sr0.705) through convection induced by the subducting lithosphere. On its way to the shallower mantle wedge (<150 km), the fertile mantle material changes shape from a hot sheet to hot fingers, for reasons not yet fully understood. Thus, the hot fingers, with 87Sr/86Sr of0.705, extend from150 km below the back-arc region towards the shallower mantle (50 km) beneath the volcanic front. A conveyor-like return flow is interpreted to carry the remnants of these fingers to depth, resulting in greater amounts of fertile material being incorporated in diapirs beneath the volcanic front, and smaller amounts incorporated in areas behind the front.