Daniel Bideau

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N

Serpentinized peridotites dredged in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N during theridelente cruise of R.V.Jean Charcot are mantle-derived harzburgites, plastically deformed in low stress/high temperature conditions similar to those inferred for the asthenospheric mantle. This early asthenospheric event was followed by ductile, then brittle, deformation in higher stress and progressively lower temperature conditions. We argue that this deformation occurred during the tectonic uplift of the peridotites throughout the lithosphere of the axial region. The ultramafics studied have mineral compositions similar to those of other mantle-derived Mid-Atlantic Ridge harzburgites. Strongly depleted mineral compositions in samples from the 15°37′N dredge could reflect a high degree of mantle partial melting. Emplacement of peridotites in the seafloor at mid-ocean ridges may not therefore be systematically correlated with low degrees of mantle partial melting. Serpentinized peridotites dredged at 15°37′N were intruded by evolved Na- and Fe-rich apatite and zircon-bearing gabbros. These gabbros were emplaced after the ductile deformation of the peridotites, and prior to their extensive serpentinization. Similar intrusive relationships between extensively fractionated gabbros and mantle peridotites have been observed in samples from the Mid-Atlantic Ridge axial valley south of the Kane transform, in dredged samples from Southwest Indian Ocean fracture zones, and in western Alps ophiolites. We propose that such intrusive relationships are characteristic of magma-starved spreading environments, in which stretching of the axial lithosphere may locally lead to emplacement of peridotites and gabbros in the seafloor.

PETROLOGY OF THE EAST PACIFIC RISE CRUST AND UPPER MANTLE EXPOSED IN HESS DEEP (EASTERN EQUATORIAL PACIFIC)

The Hess Deep, a rifted oval-shaped depression located east of the Galapagos Triple Junction at the tip of the Cocos-Nazca ridge (about 101°W, 2°N), was explored in 1988 during 21 submersible dives. A total of 11 dives were devoted to the exploration of the E-W trending Intrarift ridge (15 km in length, 3000–5400 m in depth) north of the Hess Deep depression. The Intrarift ridge represents an outcrop of recent (1 m.y.) crustal and subcrustal material created at the axis of the East Pacific Rise (EPR), and emplaced during the lithospheric extention responsible for the westward propagation of the Cocos-Nazca rift (Francheteau et al., 1990). The lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites. The samples are metamorphosed to varying degrees, but their protolith textures are generally well preserved. Their relic mineralogy indicates that they consist of harzburgites, dunites, gabbroic cumulates (gabbronorites and olivine gabbros), isotropic gabbros, dolerites, and basalts. Some samples of refractory harzburgites and most dunitic cumulates (with local accumulation of chromite) have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle-crust transition zone of the Samail ophiolite complex (Oman). The mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus-solidus temperature range of 980–1100°C. The dolerites and basalts have been derived from mid-ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the EPR volcanics near 13°N (Hekinian et al., 1989). Most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies (ultramafics and gabbros) to diagenetic alteration (volcanics). The cumulate gabbronorites, the isotropic gabbros, and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization. Several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks. The gabbroic cumulates and the plagioclase-rich melt impregnations were variably rodingitized (presence of various Ca-silicates such as epidote, prehnite, hydrogarnet, and zeolite) in relation to the serpentinization of the peridotites. One dive located on the scarps forming the northern wall of the Hess Deep to the east of the explored area, revealed the presence of in situ outcrops of isotropic gabbros, doleritic dikes, and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros.

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.

Ultramafic and mafic rocks from the Garret Transform Fault near 13°30′S on the East Pacific Rise: Igneous petrology

Serpentinized peridotites, metagabbros and fresh basaltic rocks were dredged from the Garret Transform Fault near 13°S on the East Pacific Rise. Two dredge hauls taken on the northern wall (from about 3100–3600 m) consisted of aphyric and picritic basalts; while peridotites, gabbros and moderately phyric basalts were recovered in a single dredge located near the deepest part (4616–4820 m) of the transform valley. Well preserved igneous textures and mineralogical assemblages enable us to differentiate between tectonites (including harzburgites and clinopyroxene-bearing harzburgites) and cumulates (consisting of plagioclase-dunite, troctolites, olivine-gabbros and ilmenite-gabbros). The harzburgites are likely to represent deep-seated peridotites left after extraction of basaltic melt during upper mantle partial melting of a lherzolite. The ultramafic cumulates underwent some deformation and show textural and mineralogical evidences which suggest formation at a minimum depth corresponding to the very lower crust. It is also inferred that the composition of the coexisting liquid along with early cumulus crystals has a ratio Mg/(Mg + Fe2+) of 0.7, high CaO/Na2O ratios and a low Ni content (about 150 ppm) when compared to similar rock types from Atlantic fracture zones. Subsequent uplift during rifting of the oceanic lithosphere enhanced plastic deformation, subsolidus recrystallization and retrogressive metamorphism of the gabbros and the ultramafics.

Plastic deformation and magmatic impregnation in serpentinized ultramafic rocks from the Garrett transform fault (East Pacific Rise)

The microstructural study of a set of serpentinized ultramafic samples dredged in the Garrett transform fault, shows that these rocks have been deformed plastically at temperatures probably in excess of 1000°C, and under moderate deviatoric stresses (below 1 kbar). Such temperature and stress conditions probably prevail in the asthenosphere, or in the deepest and hottest part of the lithosphere below the ridge. The deformed paragenesis of the studied samples belong to two groups: harzburgites with petrological characteristics typical of residual mantle; and plagioclase-dunites. A melt circulated in these two groups of deformed ultramafic rocks at the end of their deep-seated plastic deformation. This melt crystallized plagioclase, clinopyroxene, and also possibly orthopyroxene in one sample, in dykelets, and in interstices between the minerals of the deformed host rock. The deformation and magmatic impregnation history of the studied ultramafic samples recall the results of on-land studies of the crust-mantle transition zone of ophiolitic massifs. This suggests that our set of samples may represent fragments of the East Pacific Rise crust-mantle transition zone. The deformed ultramafic paragenesis of these samples was chemically modified by interactions with the impregnating melt: the deformed spinel was enriched in TiO2 and Cr2O3, the deformed clinopyroxene and orthopyroxene of the harzburgitic samples were also enriched in TiO2, and the orthopyroxene was significantly depleted in Al2O3. The high mobility of titanium during the impregnation process suggests that the Ti content of a melt, often used as an indicator of its degree of evolution, or of the degree of partial melting undergone by its source rock, may also be modified by melt/rock interactions during the circulation of the magma through the crust and upper mantle.

Mantle compositional control on the extent of mantle melting, crust production, gravity anomaly, ridge morphology, and ridge segmentation: a case study at the Mid-Atlantic Ridge 33^35‡N

Mantle temperature variation and plate spreading rate variation have been considered to be the two fundamental variables that determine the extent of mantle melting and ocean crust production. Along the length of a ~ 200 km portion of the Mid-Atlantic Ridge (MAR) between the Oceanographer (35°N) and Hayes (33°N) transforms, the mantle potential temperature is the same, the plate spreading rate is the same, but the extent of mantle melting and crustal production vary drastically. In addition to the typical crustal thickness variation on ridge segment scales at the MAR, i.e. thicker at segment centers and thinner at segment ends, there exist between-segment differences. For example, the ~ 90 km long segment OH-1 is magmatically robust with a central topographic high, thick crust, and a large negative gravity anomaly whereas the ~ 45 km long segment OH-3 is magmatically starved with a deep rift valley, thin crust and a weak negative gravity anomaly. We demonstrate that the observed differences in the extent of mantle melting, melt production and crustal mass between segments OH-1 and OH-3 are ultimately controlled by their fertile mantle source compositional difference as reflected by the lava compositional differences between the two segments: > 70% of OH-1 samples studied (N = 57) are enriched MORB with [La/Sm] > 1, but > 85% of OH-3 samples studied (N = 42) are depleted MORB with [La/Sm] < 1. Calculations show that the mean OH-1 source is more enriched in incompatible elements, total alkalis (~ 0.36 wt% NaO and ~ 0.09% KO) and HO content (~ 280 ppm) than the mean OH-3 source, which is depleted of incompatible elements, total alkalis (< 0.17% NaO and < 0.01% KO) and HO content (~ 70 ppm). These fertile compositional differences result in significantly reduced solidus temperature of OH-1 source over that of OH-3 source, and allows melting to begin at a significantly greater depth beneath OH-1 (~ 90 km) than beneath OH-3 (< 60 km), leading to a taller melting column, higher degrees of decompression melting, greater melt production, thus thicker crust and more negative gravity anomaly at OH-1 than at OH-3. We emphasize that fertile mantle source compositional variation is as important as mantle temperature variation and plate spreading rate variation in governing the extent of mantle melting, crustal production, and MORB chemistry. The buoyancy-driven focused mantle upwelling model better explains the observations than the subcrustal melt migration model. Future mantle flow models that consider the effect of fertile mantle compositional variation are expected to succeed in producing along-axis wavelengths of buoyant flow comparable to the observed size of ridge segments at the MAR. We propose that the size and fertility of the enriched mantle heterogeneities may actually control the initiation and evolution of ridge segments bounded by non-rigid discontinuities at slow-spreading ridges.

Detailed geological mapping by submersible of the East Pacific Rise axial graben near 13°N

An intensive exploration of the East Pacific Rise spreading center was conducted in two areas of restricted size near 13°N: site A (0.75 km2) and site B (0.15 km2). The two sites were chosen for their different general appearance based on data acquired previously. A large number of submersible dives (“Cyana”) at site A (21) and site B (6), has permitted the construction of morpho-structural maps at a scale of 1/1000. This paper compares the two areas on the basis of their structure, morphology, lava types, hydrothermal activity and mineral associations. The observed variations are interpreted as the expression of different stages of a cyclic evolution of the axial zone of mid-ocean ridges in contrast to the steady-state model. In this evolution, a tectonic phase leads to the creation of the axial graben by progressive collapse of slabs separated by fissuring of the rise. After extension of the graben, the tectonic phase is followed by a volcanic phase which partially or locally fills in the central graben with volcanic effusions. Ultimately, a dome replaces the central graben with lava flowing down the flanks of the rise. Hydrothermal activity does not appear to be preferentially linked to a given stage of this cyclic evolution. This evolution of alternate tectonic and volcanic cycles best explains observations at sites A and B, which are only 5.5 km apart. Possible magmatic processes responsible for these cycles are discussed.

O and H stable isotope compositions of oceanic crust and upper mantle rocks exposed in the Hess Deep near the Galapagos Triple Junction

Abstract A large variety of rocks, consisting of basalts, dolerites, gabbros and ultramafics, are exposed on the Intra-rift ridge in the Hess Deep, representing a section of the oceanic crust formed at the East Pacific Ridge [1]. We conducted a stable isotope study of O and H to assess the nature of the interaction between seawater and the oceanic crust in this region. Fresh basalt and dolerite samples have magmatic MORB values (δ18O ≈ 6.0, −105 ≤ δD ≤ −70) and low H2O+ content ( −57) and H2O+ contents (> 1.3%). Most gabbros have δ18O ≈ 5.6, essentially identical to their initial magmatic values. Their δD values are generally higher than −57, suggesting that they have reacted with some seawater-derived fluids, as is also suggested by their mineralogy. In gabbros, displaying little or no visual evidence of alteration of their plagioclases and pyroxenes, Δ18Oplagioclase-pyroxene is small (≈ 0.3) and consistent with magmatic values. The calcic (an > 50) plagioclases have magmatic δ18O (≈ 5.8). Albitized, prehnitized plagioclases have variable δ18O values (4–8). It also appears that the 18O-modified gabbros are those in which plagioclases are transformed into albite and/or prehnite, which are characteristic of greenschist facies conditions. The lack of calcic (an > 50) plagioclases depleted in 18O and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater-derived fluids (T ≥ 400 °C) or that they were subsequently altered to greenschist facies gabbros. The serpentinized peridotites were depleted in 18O (3.3–4.9). In one sample, the Δ18Oserpentine-magnetite (≈ 5.7) corresponds to a temperature of about 325 ± 50°C. These values are compatible with serpentinization by seawater-derived fluids at temperatures between 200° and 350°C. Another sample has a higher δ18O value of 10, corresponding to a serpentinization temperature of less than 50°C. The 18O mass balance between seawater and the oceanic crust seems to have attained steady-state where enrichment in 18O of metabasalts is balanced by the depletion in 18O of metadolerites. The contribution of the gabbros is minor. The depletion in 18O of the serpentinized peridotites is not taken into account since we believe that the high-temperature serpentinization is characteristic of the Hess Deep and cannot be extrapolated to the normal oceanic crust.

Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13°28′S in the Pacific

The 130 km long Garrett transform fault, which offsets two ultrafast (14.5 cm/yr) EPR spreading segments, is the site of volcanic and tectonic activity. Evidence for recent volcanic events was observed during several dives on oblique ridges (3100–3500 m high) trending 040° (Alpha, Beta and Gamma), and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500–5000 m. The constructional features show a fissured terrain, suggesting a component of lithospheric opening. The strike-slip motion of the transform is inferred at several locations between the two EPR-transform intersection zones, from the presence of abundant sediment-free talus with crushed rock debris, fresh scarps, and well-developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia. The width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western EPR-transform intersection zones, along the E-W trending Median Ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south-facing wall of the transform domain first escarpment. The presence of recent volcanic activity implies that the Garrett transform behaves as a leaky transform fault. The most common volcanics, consisting of plagioclase and olivine phyric lava erupted in the transform, are compositionally more depleted [ K/Ti< 0.08 ] and less evolved (Mg#60–70) than the basalts encountered on the southern EPR segments [Mg#40–66, and K/Ti= 0.05–0.25 ]. This suggests that different magmatic conditions are operating in the transform domain. The transform strike-slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the Median Ridge at the contact with outcrops of metamorphosed basaltic breccia. The ultramafics consist of residual harzburgites, often impregnated with irregular lenses and dykes of basaltic liquids (plagioclase-wehrlite and gabbro), and are found associated with troctolites, dunite and gabbros. The emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the Median Ridge in the central transform basin.