Sheeted Dike Complex Research Articles

Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite

Boron content and the isotopic composition of boron and oxygen were determined for a complete 5.3-km-thick oceanic crust in the Wadi Fizh area of the Oman ophiolite, a fragment of Cretaceous oceanic lithosphere obducted onto the Arabian shield. The depth profiles of the δ18O values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth. The upper pillow basalts underwent low temperature alteration (<60°C) or seafloor weathering (δ18O>+10‰); the lower pillow basalts and upper sheeted dikes (<2000m of stratigraphic depth) were altered at 250–350°C at the spreading axis and subsequently experienced retrograde alteration (<200°C) in the flank provinces (δ18O=+6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300–450°C (δ18O<+6‰); the lower gabbros were altered at very high temperatures of >450°C (δ18O<+6‰). Plagiogranites from the uppermost gabbro section show exceptionally high δ18O values compared with the adjacent rocks, suggesting the production of 18O-enriched melt.The boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth; 1.4–29.1μg/g in pillow lava (7.9μg/g on average), 1.5–11.6μg/g in sheeted dike complex (5.3μg/g on average), 1.6–5.0μg/g in dolerite dike in gabbro (2.9μg/g on average), 0.25–3.8μg/g in gabbro (1.3μg/g on average). Considering an original boron content of 0.72±0.47μg/g for basalt and 0.06±0.09μg/g for gabbro, boron from seawater was incorporated into the rocks through hydrothermal alteration, even at temperatures higher than 300°C. The δ11B values systematically increase with stratigraphic depth; −1.1‰ to +11.9‰ in pillow lava (+5.5‰ on average), +1.1‰ to +17.5‰ in sheeted dike complex (+6.3‰ on average), +8.3‰ to +18.6‰ in dolerite dike in gabbro (+13.9‰ on average), +7.3‰ to +17.7‰ in gabbro (+12.0‰ on average). The whole-rock δ11B values negatively correlate with the δ18O values, suggesting that the δ11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the δ11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The δ11B values estimated for hydrothermal fluids from rocks completely altered at 300–450°C range from +28‰ to +33‰, values indistinguishable from those of vent fluids observed at modern mid-ocean ridges.The boron content of the bulk oceanic crust of the Oman ophiolite is estimated to be 3.6μg/g, and the δ11B value is estimated at +7.9‰. In contrast to previous views, the hydrothermally altered gabbro section is a large boron sink, accounting for ∼30% of the total boron in the oceanic crust with a high δ11B value of +13‰. This boron-enriched, high-δ11B lower oceanic crust may impact estimates of the δ11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the δ11B value of arc magma generated in the mantle wedge.

Partial melting and assimilation of sheeted dike complex at Hole 1256D inferred from bulk rock composition.

Partial melting and assimilation of sheeted dike complex at Hole 1256D inferred from bulk rock composition.

Microstructures and crystallographic preferred orientation of anorthosites from Oman ophiolite and the dynamics of melt lenses

Microstructures and crystallographic preferred orientation (CPO) of anorthosite samples interlayered in the upper and lower gabbro sections in the Oman ophiolite were analyzed in this paper. In the anorthosites registering the dynamics of the melt lenses, foliation is flat lying and starts to develop a few meters below the root zone of the sheeted dike complex (RZSDC). Microstructures and CPO of these rocks were developed in response to four different mechanisms: (1) density‐controlled settling of plagioclase on the lens floor, (2) deposition of anorthosites related to convection currents, (3) melt compaction, and (4) uncompacted melt accumulation. In these anorthosites, the poles to (010) of plagioclase are parallel to the flow plane of convection, whereas the [100] axes and poles to (001) express the convection flow direction and the axis of convection rolls, respectively. The effect of subsidence of melt lens floor is recorded immediately below the RZSDC and is characterized by the rapid (but progressive) development of dipping foliation and lineation, reflecting the increase of deformation downsection. The degree of foliation and CPO development in the anorthosites is directly related to the distance of the center of the melt lenses before the subsidence starts. Despite the uncertain origin of the anorthosites from the lower gabbro section, all the samples lost the magmatic microstructural characteristics and presently are reequilibrated aggregates. However, they still preserve plagioclase CPO, where some of these patterns present similarities with the anorthosites from the upper gabbro section, but no evidence of intracrystalline deformation under high temperatures.

Formation and fragmentation of the Palaeoproterozoic supercontinent Columbia: evidence from the Eastern Ghats Granulite Belt, southeast India

Recognition of Palaeoproterozoic subduction-induced continental collision and Mesoproterozoic rifting within the Great Proterozoic Fold Belt (GIPFOB) of India supports the existence of Columbia. The Eastern Ghats Granulite Belt (EGGB) is an important component of the GIPFOB and a key player in models of the configuration of the supercontinent. The southern segment of the EGGB records widespread 1.85–1.7 Ga subduction-related arc magmatism and 1.5–1.35 Ga rift-zone igneous activity. The Kondapalli Layered Complex and associated felsic and mafic granulites show unambiguous subduction-related bulk-rock geochemistries and appear to represent deeply eroded remnants of a Palaeoproterozoic continental-margin magmatic arc. The Kandra Ophiolite Complex (KOC), containing a well-developed sheeted-dike complex and plagiogranite, characterizes new oceanic crust formed in a suprasubduction-zone environment. Internal structures, lithological associations, and bulk-rock compositions indicate that the KOC represents a Rocas Verdes-type continental backarc basin. Mesoproterozoic extension, initiated between 1.5 and 1.35 Ga, involved the intrusion of alkaline complexes + variegated mafic dikes and the development of large sedimentary basins. This rifting produced an outboard transitional-to-true oceanic crust paralleling the Palaeoproterozoic arc axis. Voluminous Palaeoproterozoic subduction-related and Mesoproterozoic rift-related magmatism within the EGGB provide strong evidence for global events that correlate with the assembly and break-up of Columbia.

Troodos: A Giant Serpentinite Diapir

Troodos is a classical ophiolite complex. It is proposed that the serpentinized harzburgites that now form the top of the mountain and represent the originally lowest part of the ophiolite sequence rose as a diapir. This diapiric rise is caused by the pervasive serpentinization of a suboceanic harzburgite, due to rock-sea water interaction. The serpentinization caused a 44% expansion of the rocks. Contrary to salt diapirism, the driving force for this diapiric rise is not so much the difference in density, but the volume increase asscociated with the transformation of harzburgite into serpentinite. The overlying gabbros, sheeted dike complex and pillow lavas were pierced by this serpentinite diapir but barely deformed. Their interaction with sea water was li- mited to some pyroxenes in the gabbros being transformed to amphiboles, and epidotisation of some of the dikes in the sheeted dike complex. The location of steep faults in the Troodos massif is determined by the contrasting expansion behavior of different rock-types on both sides of the fault.

Petrophysical properties of the root zone of sheeted dikes in the ocean crust: A case study from Hole ODP/IODP 1256D, Eastern Equatorial Pacific

ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15 Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (> 200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100 m thick boundary layer between the magmatic system (i.e., the axial melt lens, ~ 1100 °C), and the overlying high temperature hydrothermal system (≤ 450 °C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges. This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist facies (~>250 °C) with relatively high and interconnected (cementation index m ~ 1.72, electrical tortuosity τ ~ 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite (~>450 °C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium ( m ~ 1.58, τ ~ 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated.

Early Devonian suprasubduction ophiolites of the southern Urals

The composition of ophiolites widespread in the southern Urals shows that they were formed in a suprasubduction setting. Low-Ti and high-Mg sheeted dikes and volcanic rocks vary from basalt to andesite, and many varieties belong to boninite series. The rocks of this type extend as a 600-km tract. The volcanic rocks contain chert interbeds with Emsian conodonts. Plagiogranites localized at the level of the sheeted dike complex and related to this complex genetically are dated at 400 Ma. The ophiolites make up a base of thick islandarc volcanic sequence. The composition of the igneous rocks and the parameters of their metamorphism indicate that subduction and ascent of a mantle plume participated in their formation. The nonstationary subduction at the intraoceanic convergent plate boundary developed, at least, from the Middle Ordovician.

Direct observation of a fossil high-temperature, fault-hosted, hydrothermal upflow zone in crust formed at the East Pacific Rise

Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Here we describe the geology and geochemistry of an ~40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeastern Easter microplate). Titanium in quartz thermometry gives temperatures of 392 ± 33 °C for quartz precipitation, indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black-smoker vent fl uids. Correlated enrichment in 87 Sr/ 86 Sr and MgO in fault breccias, along with 87 Sr/ 86 Sr ratios higher than in average vent fl uids, provide evidence for mixing between high-temperature upwelling fl uids and a seawater-like fl uid within the fault zone. Large high-temperature fl uid fl uxes are required to maintain high temperatures during mixing. If this fault zone is representative of upfl ow zones beneath hydrothermal vents on the East Pacifi c Rise, then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system.

Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations

In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites; however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850°C. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated.

Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate spreading crust exposed near the Blanco Transform

We present results from a study mapping the seismic layer 2A/2B boundary in young ocean crust located adjacent to the north wall of the Blanco Transform Fault (BTF). We review seafloor features, geochemical and petrologic data, seismic imaging, and seismic structure and conclude that the BTF seismic study region is representative of crust formed at the intermediate spreading Juan de Fuca Ridge. The mean layer 2A two‐way travel time beneath the seafloor is 0.37 ± 0.10 s for the 42 × 12 km seismic survey area. Several regions are observed where layer 2A is consistently thin or thick over lateral distances of 5–10 km, both in a ridge‐parallel and ridge‐perpendicular direction. Layer 2A thicknesses appear more variable in the ridge‐parallel (isochron) direction than the ridge‐perpendicular (flow line) direction. There is no systematic pattern of layer 2A thickness variability with distance from the BTF, nor is there a correlation between seafloor topography and layer 2A thickness. Calculated mean layer 2A thickness for the seismic study region is 485 ± 135 m assuming an interval velocity of 2.65 km/s. The imaged layer 2A/2B boundary projects on average ∼600–650 m above the top of the sheeted dike complex mapped on the adjacent BTF north wall. We argue that the geologic context of the layer 2A/2B boundary will vary as competing processes of magmatic construction and fracturing (increasing porosity) and crustal thickening, compaction, dike intrusion, and hydrothermal sealing (decreasing porosity) vary as the crust spreads laterally from the ridge axis to ridge flanks.

Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and sub-marine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ 7Li signatures for each of these reservoirs were significantly different. The first low temperature (25–250 °C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ ( Δ solution–solid) at 25 °C to +6.7‰ at 250 °C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ solution–solid = 7847/ T − 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250–260 °C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of 6Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90–120 °C and 180 °C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks.

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges

The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D. We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.

Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros

In the Oman ophiolite, the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified. Starting in the Root Zone of the Sheeted Dike Complex (RZSDC) located above this horizon, 18 sections down to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs of this ophiolite. They are complemented by 133 sites, located throughout the entire ophiolite, where the transition from the RZSDC to the uppermost foliated gabbros is well exposed. Altogether, half the sites and 11 cross sections display, within a few tens of meters beneath the RZSDC, a magmatic foliation which is parallel to the overlying sheeted dikes. In the other sites and cross sections, the gabbro foliation is either flat-lying or steep but not parallel to the sheeted dikes. Compared to the RZSDC isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths, in the topmost steeply foliated gabbros, clinopyroxene is idiomorphic, becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende. Moving down from these uppermost gabbros and over one hundred meters, the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized, granular gabbros of the gabbro unit. These repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying, main magma chamber. The topmost gabbros beneath RZSDC, which were expelled from the melt lens by drifting very soon after settling on the melt lens floor, display in plagioclase a spectacular zoning pointing to a fast cooling. Moving downwards, stronger foliation, increased compaction and recovery–recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude (~ 50 to 500 yr) over this vertical distance. This field-based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis.

Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction rollback systems

Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening, producing a collage of ‘protoarc and forearc oceanic lithosphere’ in suprasubduction zone settings. The Jurassic–Cretaceous SSZ Tethyan ophiolites in the eastern Mediterranean region (i.e. Mirdita, Pindos, Troodos, Kizildag, Oman) generally have Penrose-type oceanic crust and contain well-developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT (island arc tholeiite) to boninitic (extremely refractory) protoarc assemblages. However, there are some distinct differences in the geochemical evolution of these Tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics. Whereas a major part of the Kizildag and Troodos lavas shows island arc affinity similar to their counterparts in the Pindos and Mirdita ophiolites, a significant component of the Oman lavas indicates MORB affinity and the majority of the Kizildag and Oman data plot within the mantle array between N-MORB and E-MORB on the Nb/Yb–Th/Yb discriminant diagram. Furthermore, the Troodos and Oman lavas do not show any particular Th-enrichment in their multi-element patterns, suggesting that fluid/melt input from subducted sediments was not that significant in generation of their magmas. Although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions, as evidenced by their negative ɛ Nd values, their overall characteristic trace-element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed.

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

A complete “in situ” section of upper oceanic crust, from extrusive lavas, through dikes into gabbros has been recently drilled for the first time in 15 Ma old crust created at the superfast spreading East Pacific Rise (EPR). The upper igneous basement cored at Ocean Drilling Program (ODP)–Integrated Ocean Drilling Program (IODP) Holes 1256C and 1256D consists of sheet flows and massive flows, including a ponded lava flow unit (“lava pond”) interpreted as a thick lava flow delivered off‐axis and accumulated in a topographic depression. Fine‐scale structural analysis shows that the lava pond is much less fractured than the deeper lavas, probably as the effect of structure spacing that is larger than in the underlying lava flows. In Hole 1256D, structures are mainly subhorizontal in the lava pond and become steeper downhole, toward the sheeted dike complex. This reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth, inducing mostly subvertical eruptive fractures and fracture network, and the distance of the site from active rifting at the EPR. Contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns, as obtained by shipboard and downhole logging data. Electrical resistivity, compressional velocity, and bulk density are generally higher, whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole. The occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and, consequently, its permeability regime which, in turn, controls the geometry and distribution of hydrothermal circulation and basalt alteration.

Altered oceanic crust as an inorganic record of paleoseawater Sr concentration

The minimum Sr content of seawater at 90–95 Ma is determined from the Sr content and Sr isotopic composition of the sheeted dike complex of oceanic crust that was hydrothermally altered at this time on the basis of simple mass and heat balances. When this approach is applied to sheeted dike complexes that were hydrothermally altered by modern seawater a minimum fluid Sr content of &lt;3 μg g−1 is calculated. This is substantially less than the modern ocean Sr content of ∼7.8 μg g−1 supporting the arguments that the approach provides a robust minimum Sr content of the ocean at the time of crust formation. Applying the same approach to the sheeted dike complexes in the Cretaceous Troodos and Oman ophiolites gives minimum Sr contents for the ocean at this time of 27 and 38 μg g−1, respectively. These values are consistent with previous studies of biomineralized carbonate that suggest higher Sr/Ca in seawater at this time and fluid inclusion analyses that suggest higher Ca contents in the ocean at this time. The Ca content of the ocean must have been at the upper end of the range suggested by fluid inclusion analyses (≥30 mmol kg−1). The high Ca and Sr content of seawater at 90–95 Ma was, at least partially, due to enhanced submarine hydrothermal circulation which leaches Sr from the basaltic crust because it coincides with a time of low seawater 87Sr/86Sr. Elevated Sr contents at this time may have impacted the evolution and/or biogeochemical role of the Acantharians which secrete a SrSO4 skeleton.

Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

The escarpments that bound the Pito Deep Rift (northeastern Easter microplate) expose in situ upper oceanic crust that was accreted ∼3 Ma ago at the superfast spreading (∼142 mm/a, full rate) southeast Pacific Rise (SEPR). Samples and images of these escarpments were taken during transects utilizing the human‐occupied vehicle Alvin and remotely operated vehicle Jason II. The dive areas were mapped with a “deformation intensity scale” revealing that the sheeted dike complex and the base of the lavas contain approximately meter‐wide fault zones surrounded by fractured “damage zones.” Fault zones are spaced several hundred meters apart, in places offset the base of the lavas, separate areas with differently oriented dikes, and are locally crosscut by (younger) dikes. Fault rocks are rich in interstitial amphibole, matrix and vein chlorite, prominent veins of quartz, and accessory grains of sulfides, oxides, and sphene. These phases form the fine‐grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt. Bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins. When compositions are normalized to compensate for the excess silica, the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration, including the loss of mobile elements and gain of some trace metals and magnesium. Microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the SEPR; faults related to the opening of the Pito Deep Rift can be distinguished by their orientation and fault rock microstructure. Some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones. Other increments were linked with the influx of relatively fresh seawater. The spacing of the faults is consistent with fault localization occurring every 7000 to 14,000 years, with long‐term slip rates of &lt;3 mm/a. Once spread from the ridge axis, the faults were probably not active, and damage zones likely played a more significant role in axial flank and off‐axis crustal permeability.

Structural architecture of the sheeted dike complex and extensional tectonics of the Jurassic Mirdita ophiolite, Albania

The Jurassic Mirdita ophiolite in northern Albania occurs in a ∼ 40 km-wide zone bounded by the passive margins of Apulia (W) and Pelagonia (E). The ∼ 3 km-thick Western Mirdita ophiolite (WMO) contains lherzolitic peridotites and gabbros overlain by basaltic pillow lavas, whereas the ∼ 12 km-thick Eastern Mirdita ophiolite (EMO) represents a Penrose-type oceanic crust with suprasubduction zone affinities. The 1 km-thick sheeted dike complex (SDC) in the EMO shows mutually intrusive relations with the underlying gabbros, is overlain by a 1.1-km-thick extrusive sequence, and records a history of complex seafloor spreading and rift tectonics in a suprasubduction zone environment. Crosscutting relations within the SDC indicate four generations of dike intrusions becoming progressively younger to the east. Early D1 and D2 dikes have basalt and basaltic andesite compositions and NNE and NNW attitudes, respectively, with moderate to gentle dips to the east. They are cut by mineralized, dike-parallel normal faults defining local grabens. Younger D3 dikes display andesitic and boninitic compositions and have WNW strikes with steep dips. These dikes are cut by both dike-parallel and dike-orthogonal faults that form locally well-developed graben structures with extensive epidosite and chalcopyrite mineralization. The youngest D4 dikes with 240°–290°orientations occur as isolated swarms intruding earlier dike generations in the eastern part of the SDC, range in composition from quartz–microdiorite to rhyodacite and rhyolite, and are cut by ∼ NW-oriented faults and shear zones. Changes in dike compositions from basalt and basaltic andesite to andesite, boninite, rhyodacite and rhyolite through time in the SDC are consistent with changes in the lava chemistry stratigraphically upward and eastward in the extrusive sequence, indicating significant chemical variations in melt compositions as the EMO evolved. Development of the SDC and its extensive normal faulting were a result of upper plate extension and rifting in the protoarc–forearc crust caused by rapid slab rollback in the Mirdita basin, reminiscent of the late Cenozoic extensional tectonics of the Izu–Bonin–Mariana system in the Western Pacific. Shifting of dike attitudes to more easterly orientations in later stages of SSZ magmatism signals a gradual change in the spreading direction likely caused by impingement of the clockwise rotating Pelagonia on the arc–trench rollback system in the south. The SSZ Mirdita basin evolved similarly to the last 100 km of recorded spreading in the West Philippine Basin, where the rotation of the seafloor spreading axis was caused by the incipient collision between the northern part of the Philippine arc with rifted fragments of Eurasia.

Direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers

Rare, fault‐bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise (EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of ∼61° related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well‐defined magmatic flow directions that are dominantly dike‐parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near‐horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along‐axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Sr‐isotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter‐scale hydrothermal veins, show a range of 87Sr/86Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87Sr/86Sr of ∼0.7024. There is no systematic variation in 87Sr/86Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr‐isotope exchange is similar in all of these locations. Models that assume that fluid‐rock reaction occurs during one‐dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid‐rock exchange. In this scenario the fluid at the base of the crust is little modified in 87Sr/86Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh‐rock 87Sr/86Sr, but this is not observed. Alternatively, models in which fluid‐rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87Sr/86Sr with depth in the crust. Minimum time‐integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 × 106 kg m−2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87Sr/86Sr without changing the rock 87Sr/86Sr, these mass balance models must underestimate the time‐integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.