Melt Lens Research Articles

Behavior of MORB magmas at uppermost mantle beneath a fast-spreading axis: an example from Wadi Fizh of the northern Oman ophiolite

Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# [Mg/(Mg + Fe2+) atomic ratio] clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as “MORB-saturated harzburgite” for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt/harzburgite ratios. The modified melt left clinopyroxene (not clinopyroxene + plagioclase as in plagioclase-impregnated abyssal harzburgite) to form the MORB-saturated harzburgites in the vicinity (harzburgite) of the fracture, which are left as gabbro-clinopyroxenite bands. This local modification mimics the whole lithological and chemical variation of the MTZ and makes chemical variation of MORB suite at fast-spreading ridge.

Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading

Research Article| April 01, 2012 Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading Andrew M. McCaig; Andrew M. McCaig 1Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Michelle Harris Michelle Harris 2National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (4): 367–370. https://doi.org/10.1130/G32789.1 Article history received: 31 Aug 2011 rev-recd: 19 Nov 2011 accepted: 25 Nov 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Andrew M. McCaig, Michelle Harris; Hydrothermal circulation and the dike-gabbro transition in the detachment mode of slow seafloor spreading. Geology 2012;; 40 (4): 367–370. doi: https://doi.org/10.1130/G32789.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract One of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast-spreading ocean crust. This boundary marks the brittle-ductile transition at the ridge crest, and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi-steady-state melt lens. In contrast, at slow-spreading ridges, the crustal structure appears chaotic, with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor. Recent work suggests that as much as 50% of the Atlantic Ocean crust formed by a detachment mode of seafloor spreading, including the formation of oceanic core complexes capped by long-lived, convex-upward detachment faults. These detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain. Here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall, thus explaining the link between faulting and black smoker systems. We suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike-gabbro transition in the detachment mode of spreading, inevitably leading to exposure of gabbros on the seafloor through continued faulting. This concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges

The genesis of oceanic crust at intermediate to fast spreading ridges occurs by the crystallization of mantle melts accumulated in at least one shallow melt lens situated below the ridge axis. Seismic reflection data suggest that the depth of this melt lens is inversely correlated with spreading rate and thereby magma supply. The heat released in it by crystallization and melt injection is removed by a combination of hydrothermal cooling and diffusion. Due to the different time scales of hydrothermal cooling and crustal accretion, numerical models have so far focused on only one of the two processes. Here we present the results from a coupled model that solves simultaneously for crustal accretion and hydrothermal cooling. Our approach resolves both processes within one 2D finite-element model that self-consistently solves for crustal, mantle, and hydrothermal flow. The formation of new oceanic crust is approximated as a gabbro glacier, in which the entire lower crust crystallizes in one shallow melt lens. We find that the depth of the melt lens and the shape of hot (potentially molten) lower crust are highly dependent on the ridge permeability structure. The predicted depth of the melt lens is primarily controlled by the permeability at the ridge axis, whereas the off-axis permeability determines the width of hot lower crust. A detailed comparison of the modeling results with observed locations of the melt lens at intermediate to fast spreading ridges shows that only a relatively narrow range of crustal permeabilities is consistent with observations. In addition, we find significant deviations between models that resolve or parameterize hydrothermal cooling: the predicted crustal thermal structures show major differences for models that predict the same melt lens location. This illustrates the importance of resolving hydrothermal flow in simulations of crustal accretion.

Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges

The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of ∼80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens–sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to “granoblastic dikes” by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called “varitextured gabbro” unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.

Timescale of open-reservoir evolution beneath the south Cleft segment, Juan de Fuca ridge

Lavas erupted at the southern end of the intermediate Juan de Fuca ridge (Cleft segment) are mostly cogenetic and their chemical diversity results from melt evolution in an open magma system. In the present study, we apply a theoretical model allowing the time evolution of this periodically recharged and tapped magma chamber to be estimated. In our mathematical procedure, the melt quantity supplied to the reservoir varies through time following a sinusoidal function. The rare earth element concentrations in the refilling melt were calculated on the basis of the REE distribution in lavas. This theoretical composition is akin to that previously estimated for a Mg#70 MORB from mineralogical and chemical data. Then, we approached the temporal evolution of the reservoir using a set of suitable parameters deduced from the geometry of the crust and magma system beneath the Cleft segment. Particularly, we considered two end-members scenarios for the melt repartition through the magma reservoir beneath the Cleft segment: the “gabbro glacier” model (crystal nucleation and growth occur within one single melt lens and crystals subside vertically and laterally) and the “sheeted sill” model (crystallization takes place within a network of connected sills located at various depths within the crust). We estimated that the magma chamber is refilled every thousand years and that the melt resides approximately one hundred years within the reservoir.

Hydrothermally-induced melt lens cooling and segmentation along the axis of fast- and intermediate-spreading centers

[1] The heat output and thermal regime of fast and intermediate spreading centers are strongly controlled by boundary layer processes between the hydrothermal system and the underlying crustal magma chamber (AMC), which remain to be fully understood. Here, we model the interactions between a shallow two-dimensional cellular hydrothermal system at temperatures <700°C, and a deeper AMC at temperatures up to 1200°C. We show that hydrothermal cooling can freeze the AMC in years to decades, unless melt injections occur on commensurate timescales. Moreover, the differential cooling between upflow and downflow zones can segment the AMC into mush and melt regions that alternate on sub-kilometric length scales. These predictions are consistent with along-axis variations in AMC roof depth observed in ophiolites and oceanic settings. In this respect, fine-scale geophysical investigations of the structure of AMCs may help constrain hydrothermal recharge locations associated with active hydrothermal sites.

Petrological relationships among lavas, dikes, and gabbros from Integrated Ocean Drilling Program Hole 1256D: Insight into the magma plumbing system beneath the East Pacific Rise

A continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Integrated Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Petrological examination demonstrates that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow (?50–100 MPa) melt lens reasonably explains mineral and whole rock compositions of many lavas and dikes. Elevated concentrations of trace elements in some rocks appear to have resulted from mixing between primitive magma and highly evolved magma. About half of the dike samples have more evolved Fe-rich compositions than the extrusive lava samples. Magma densities of the Fe-rich dikes are a little higher (?30 kg/m3) than those of lavas, suggesting that these dike magmas would not reach the surface. Mineralogical investigations reveal that both lavas and dikes contain oscillatory zoned plagioclase xenocrysts, implying magma mixing caused by successive episodes of fractionation and magma replenishment in the melt lens. The plagioclase xenocrysts contain high-Anorthite sections [An: 100 × Ca/(Ca+Na) in mole percent] whose compositions are not in equilibrium with host liquids. The high-An sections were likely crystallized when primitive magmas with high CaO/Na2O were injected in the melt lens. Since the oscillatory zoned plagioclase generally forms crystal clots, they were probably accumulated in a mush zone. The petrographical examination favors a model suggesting that injection of primitive magma into the melt lens broke the mush zone and pushed out the oscillatory zoned plagioclase.

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.

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite

[1] The Oman ophiolite is regarded as derived from a fast spreading oceanic ridge. As in its present-day marine analogs, expectedly, the gabbro unit crystallized in a large magma chamber underneath a small melt lens. In ophiolites, this melt lens is reduced to a horizon mapped in the field, where the floor and the roof of the melt lens joined. The magmatic activity of the former melt lens has been studied in gabbros located below the melt lens horizon. Beneath a stable melt lens, gabbros subside and drift through the steep walls of the magma chamber as steeply foliated gabbros. In contrast, in unstable melt lenses more specifically considered here, flat-lying gabbros are exposed beneath the lens. These gabbros were settling on the floor while the melt lens was retreating. Their strong magmatic foliations and lineations point to a dynamic deposition on the floor, and lineation trends record the structure of convection rolls within the lens. Time constraints suggest a ∼100 year periodicity of melt recharge by short and powerful melt pulses followed by a shorter time of convection activity after recharge and a longer period of static magma settling, as recently proposed for the East Pacific Rise. At each new melt pulse, the lens nearly doubles its volume by surface expansion, within a short time lapse, before slowly and progressively retreating to feed crustal accretion. Large horizontal surface inflation of the lens better explains the seawater hydration of the accreting crust than melt lens vertical motion through the lid.

Axial melt lenses at oceanic ridges — A case study in the Oman ophiolite

The Oman ophiolite, a fossil fast-spreading oceanic ridge had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing. In ophiolites, this melt lens is reduced to a horizon that has been identified in the field. The gabbros that subsided from the lens horizon and drifted out of the magma chamber, while developing a steep magmatic foliation, provide insights into the active melt lens processes. A gabbro, sampled right below the horizon where the lens closed, has virtually not subsided and it offers a direct glance at the floor of the lens. Below, the deeper the gabbros are sampled, the more they have subsided before drifting out of the magma chamber, and the closer they derive from the center of the lens floor. Cross sections in these gabbros record successive events occurring within the lens. In particular, anorthosite layers and lenses that are interlayered within the foliated gabbros record the intrusion of a new melt where plagioclase is the first mineral to crystallize on liquidus. Detailed mapping of anorthosite swarms suggest that melt intrusions are massive, lasting less than 4 yr and that their frequency is ~ 100 yr. Their large volume controls the stability of melt lenses. A new melt surge induces rapid and large inflation of the lens. Melt lens deflation follows, that is due to the ongoing process of crustal accretion both in the gabbro unit by continuous subsidence of a thick gabbroic mush from the lens floor, and in the lid above the lens roof by sheeted dike intrusions feeding basalt extrusions at the higher frequency of ~ 10 yr.

Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256

ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ∼100 m into plutonic rocks. We use mineralogy, oxygen isotopes, and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (&lt;150°C) to phyllosilicates and iron oxyhydroxides, with a stepwise increase in grade downward to greenschist minerals in the upper dikes. This resulted from generally upwelling hydrothermal fluids in the dikes mixing with cooler seawater solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (&gt;350°C up to ∼600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ∼800°C down to &lt;300°C. Fluid inclusion and oxygen isotope data show that vein quartz formed at ∼300°C to &gt;450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low‐temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture‐controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.

Reactive infiltration of hydrous melt above the mantle transition zone

Seismic studies repeatedly image a low‐velocity layer overlying the mantle transition zone in tectonic settings ranging from subduction zones to continental cratons. This layer has been hypothesized to result from the presence of a dense partial melt formed by dehydration melting as relatively wet transition zone material is advected by convection currents into the low‐water‐solubility upper mantle. Here we examine this hypothesis by considering the dynamic infiltration of a low‐viscosity reactive hydrous melt into a high‐viscosity ambient solid. The thickness of the melt layer is strongly controlled by return flow induced in the surrounding viscous solid, and in steady state a dynamic equilibrium is achieved where the melt lens is restrained from collapse by the exterior mantle. Melt layers with thicknesses in excess of 10 km develop for a wide range of mantle parameters and develop on timescales equivalent to the lifespan of an ocean basin.

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.

Crystal growth during dike injection of MOR basaltic melts: evidence from preservation of local Sr disequilibria in plagioclase

Profiles of a total of 23 plagioclase crystals erupted within the 1982–1991 and 1993 flows of the Coaxial segment of the Juan de Fuca ridge, the 1996 flow of the North Gorda ridge, and from the Western Volcanic Zone of the ultra-slow spreading Gakkel Ridge, have been studied for variations in major and trace element concentrations. We derive equilibration times for the relatively rapidly diffusing Sr in mid-ocean ridge basalt (MORB) plagioclase crystals of the order of months to a few years in each case. All crystals preserve diffusive disequilibria of strontium and barium. Crystal residence times at MORB magmatic temperatures are thus significantly shorter, of the order of days to a few months at most, precluding prolonged crystal storage in axial magma chambers and instead pointing to rapid crystal growth (up to ~10−8 cm s−1) and cooling (up to ~1°C h−1) shortly prior to eruption of these samples. Growth of these crystals is therefore inferred to occur almost entirely within oceanic layer 2 during dike injection. Crystals that grew at lower crustal levels or earlier in the differentiation sequence appear to have been excluded from the erupted magmas, as might occur if most of the gabbroic rocks in oceanic layer 3 formed an interlocking crystal framework, with viscosities that are too high to carry earlier formed crystals with the melt. The vertical extent of eruptible, crystal-poor melt lenses within the gabbroic zone is constrained to ~1 m or less by considering the width of local equilibrium growth zones, equilibration times, and crystal settling velocities. This lengthscale is consistent with field evidence from ophiolites. Finally, crystal aggregates within the Gakkel ridge sample studied here are the result of synneusis within the propagating dike during melt ascent.

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.

Crustal velocity structure of the Lucky Strike segment of the Mid‐Atlantic Ridge at 37°N from seismic refraction measurements

We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge (SISMOMAR) survey. The three‐dimensional velocity model shows an upper crustal low‐velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low‐velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low‐velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low‐velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.

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.

Evidence and implications of crustal magmatism on the flanks of the East Pacific Rise

Formation of oceanic crust along the East Pacific Rise is thought to be complete within a few kilometers of the rise axis. Here, however, we present evidence for magmatism 20 km from the spreading center in 300-ka-old crust. Seismic data reveal an intrusive complex ~ 2 km beneath the seafloor, which is limited in lateral extent (< 5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust. The crustal magma body overlies a region of off-axis delivery of mantle melt that results from a long-lived skew between the axes of mantle upwelling and plate spreading. Magmatism on the flanks of the EPR may be commonplace, thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off-axis hydrothermal activity.

Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge

The oceanic crust extends over two-thirds of the Earth's solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis, but the style of accretion of the lower oceanic crust is actively debated. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens, consistent with geophysical studies that suggest the presence of melt within the lower crust. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses and sills within the MTZ have been described, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5-6 km beneath the sea floor and 850-900 m above the MTZ, and is located 1.4-3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies.

Paired melt lenses at the East Pacific Rise and the pattern of melt flow through the gabbroic layer at a fast-spreading ridge

From geophysics, two thin and narrow sill-like melt lenses occur directly beneath the ridge axis on the East Pacific Rise (EPR) near 9°N. One is at the base of sheeted dikes, and the other at the base of the crust in the mantle transition with largely crystalline material in between. Along-axis continuity indicates that the upper one is a steady-state feature along much of the EPR.What causes melt to pool at two levels in the crust, with little melt in between? Normative densities of gabbro sampled from Hess Deep, eastern Pacific, at solidus temperatures are greater than melt densities calculated from compositions of basalt glass from the EPR. Therefore, abyssal-tholeiite liquids of all compositions are buoyant in a hot gabbroic crystalline matrix with no more than a few percent melt. There is therefore no level in the gabbroic layer at which pure basaltic melt attains neutral buoyancy, and by this means produces sill-like rather than dike-like intrusives. Instead, the upper melt lens appears to collect at a combined compositional and permeability barrier, defined by the coarse-grained base of the dike sequence sampled on the northern rift margin at Hess Deep. About 200m of massive dike rocks with ophitic to subophitic coarse-grained texture occur just above gabbronorite and oxide–gabbro cumulates which are the crystallized relics of highly differentiated ferrobasaltic and ferroandesitic liquids in the melt lens. At temperatures of crystallization of cumulates at the level of the melt lens (1100–1000°C), the basal dike rocks of less differentiated basaltic composition just above this were 90–98% crystalline, and they annealed to the point where dike contacts cannot be seen on outcrop surfaces. These slightly molten rocks need to be fractured by rift dilatancy or overpressure from beneath for magmas to ascend beyond the melt lens. Neutral buoyancy occurs in fine-grained dikes above where densities of escaping magma finally match those of the fractured and porous formation and this is the level that can be laterally intruded.The base of the crust, on the other hand, presents an abrupt reduction in the density contrast between ascending melt and surrounding rock, and a higher concentration of melt pockets and channels. Neutral buoyancy can be attained where primitive gabbros are first encountered if the ascending magmas are picritic, laden with olivine phenocrysts. High olivine concentrations in the narrow bodies of moving magma can be attained by a combination of sinking, relative plagioclase flotation, and flow differentiation. Sill-like intrusions that crystallize to dunite and plagioclase–peridotite cumulates will result. Such rocks are found but their structural orientation is unknown at the base of the intra-rift ridge at Hess Deep; sill-like analogs are found in some ophiolites. Lateral leakage of deep melt lenses into transform faults might account for both primitive picritic tholeiites and strongly plagioclase-phyric basalts found, for example, at Siqueiros Fracture Zone at 8°30′N along the EPR.