Magma migration and differentiation processes are key to understanding the development and evolution of oceanic magma reservoirs. To provide new quantitative geochemical constraints on these ...processes, we applied a high‐resolution approach to study an interlayered section of the lower oceanic crust sampled at Atlantis Bank, on the (ultra)slow‐spreading Southwest Indian Ridge. The section is characterized by sharp grain‐size layering between fine‐ and coarse‐grained olivine gabbros that is representative of other layered structures described at the Atlantis Bank oceanic core complex. The textures and fabrics of the layers and the nature of their contacts indicate formation by intrusion of a magma (i.e., crystal‐bearing) into an almost solidified coarse‐grained mush. Petrographic observations and in situ incompatible trace element signatures indicate that the fine‐ and coarse‐grained layers record reactive porous migration of melts. Widespread reactive porous flow occurred prior to intrusion within the coarse‐grained gabbro, producing mineral compositions enriched in incompatible elements. The intrusive fine‐grained lithology records a late stage event of localized reactive melt percolation in cm‐scale structures, which lead to strong light rare earth elements depletion relative to heavy rare earth elements. In addition, we highlight the occurrence of interactions at the contacts between layers and partial modification in compositions of the intruded lithology. This layered section likely represents a contact between two larger magma bodies emplaced within the lower crust during accretion, where the type of melt migration (intrusion or porous flow) and the modalities of melt percolation (widespread or localized) strongly govern the composition of the crustal lithologies.
Plain Language Summary
We present the study of a 1m35 borehole section of oceanic crust exhumed at Atlantis Bank on the Southwest Indian Ridge. This section presents characteristic layering defined by successive fine‐ and coarse‐grained olivine gabbro layers, which recorded the conditions and modalities of magma emplacement and evolution during oceanic crust accretion. We conducted a high‐resolution study of samples collected continuously along the section, including a description of the petrography, analysis of microstructures, and geochemistry of both rocks and minerals. This high‐resolution approach allowed us to reconstruct the intricate formation steps of the section, which involves the two main melt migrations processes at depth: an intrusion, represented by the fine‐grained layers, and porous melt flow, which occurs at all steps of formation of the lithologies. The latter process strongly impacted the textures and composition of the lithologies. These compositions provide information on the modalities of the porous melt migration, which can be pervasive or focused in discrete structures. The geometry of the layer contacts together with the emplacement model suggests that this layering likely represents an irregular contact between two larger intrusions or magma reservoirs emplaced within the oceanic crust.
Key Points
At slow‐spreading ridges, igneous layering in the lower crust records intrusive events of crystal‐bearing magmas
At lower crustal levels, melt migration proceeds by intrusion and reactive porous flow
Melt migration modes strongly influence the chemical compositions of melts, crystal matrices, and crustal lithologies
Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and ...crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.
Our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from ...one stage to another. Within the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures: The outer part of the veins comprises coarse-grained olivine that forms an unusual, brick-like microstructure. The inner part of the veins, surrounding a central fault, is composed of fine-grained olivine. Where the fault movement included a dilational component, optically clear, equant olivine occurs in the centre. Electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation (CPO) from the olivine in the porphyroclastic host rock; however, misorientation is weaker and associated to different rotation axes. We propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its CPO. Growth towards pre-existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick-like microstructure in the veins. When dehydration embrittlement induced slip, a strong strain localisation on discrete fault planes prevented distortion of the CPO due to cataclastic deformation; grain size reduction did not significantly modify the olivine CPO. This illustrates how a CPO can be preserved though an entire metamorphic cycle, including hydration, dehydration, and deformation processes, and that the CPO and the microstructures (e.g. grain shape) of one phase do not necessarily record the same event.
The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and ...crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe
3+ and Fe
2+ between both minerals and a melt phase: plagioclase can incorporate more Fe
3+ than Fe
2+, while clinopyroxene can incorporate more Fe
2+ than Fe
3+. For example, the effect of oxidizing a partly molten basaltic system (Fe
3+ is stabilized with respect to Fe
2+) results in an increase of FeO
total in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average <
1 log unit of the prevailing oxygen fugacity.
In order to reduce the number of different variables influencing the Fe
2+/Fe
3+ mineral/melt equilibrium, our model is restricted to basaltic series with SiO
2
<
60% that have crystallized at intermediate to low pressure (<
0.5
GPa) and under relatively oxidizing conditions (∆FMQ
>
0; where FMQ is the fayalite–magnetite–quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis.
Piton des Neiges basaltic volcano (La Réunion) has been deeply dissected by erosion, exposing large volumes of debris avalanche deposits. To shed light on the factors that led to volcano flank ...destabilizations, we studied the structure, the crystallographic and magnetic fabrics of the substratum of a debris avalanche unit. This substratum is a complex of >50 seaward-dipping sills that has been exposed by the avalanche. Structural observations show that the sill plane in contact with the avalanche is one of the latest intrusions in the sill complex. In this uppermost sill, the anisotropy of magnetic susceptibility (AMS) is correlated to the crystallographic preferred orientation of magmatic silicate minerals, allowing us to use AMS as a proxy to infer the magmatic flow. The AMS fabric across the intrusion is strongly asymmetric, which reveals that the contact sill was emplaced with a normal shear displacement of its hanging wall. The shear displacement and the magma flow in the intrusion are both directed toward the NNE, i.e. toward the sea, which is also the direction of the slope and of the debris avalanche runout. Because all the sills in the intrusion complex have a similar dip and dip direction, it is likely that several of them also underwent a cointrusive slip toward the NNE. We conclude that this cointrusive normal slip, repeated over many intrusions of the sill complex, increased the flank instability of the volcano. This incremental instability may have ended up into the observed debris avalanche deposit. At Piton de la Fournaise, the active volcano of La Réunion, sill intrusion and cointrusive flank displacement have been inferred from geophysical studies for the April 2007 eruption. By providing direct evidence of sheared sills, our study substantiates the idea that repeated sill intrusions may eventually trigger flank destabilizations in basaltic volcanoes.
•A zone of multiple sills localizes shear deformation at Piton des Neiges volcano.•Sill magnetic fabric reveals cointrusive normal slip of the intrusion hanging wall.•The sill zone acted as a sliding surface for a debris avalanche.•Shear deformation, cointrusive sill slip and avalanche are directed toward the sea.•Repeated sill injections may increase volcano instability, leading to flank failure.
The Hirabayashi borehole (Awaji Island, Japan) was drilled by the Geological Survey of Japan (GSJ) 1 year after the Hyogo-ken Nanbu (Kobe) earthquake (1995,
M
JMA=7.2). This has enabled scientists to ...study the complete sequence of deformation across the active Nojima fault, from undeformed granodiorite to the fault core. In the fault core, different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation. Above the fault core and within the hanging wall, compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5–50 μm euhedral and zoned siderite and ankerite crystals. These carbonate-filled fractures have been observed within a 5.5-m-wide zone above the fault, but are especially abundant in the vicinity (1 m) of the fault. The log-normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying-rate nucleation accompanied by surface-controlled growth in a fluid saturated with respect to these carbonates. These carbonate-filled fractures are interpreted as the result of co-seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault, with the fast nucleation of carbonates attributed to a sudden fluid or CO
2 partial pressure drop due to fracturing. The fractures cut almost all visible structures at a thin section scale, although in some places, the original idiomorphic shape of carbonates is modified by a pressure-solution mechanism or the carbonate-filled fractures are cut and brecciated by very thin gouge zones; these features are attributed to low and high strain-rate mechanisms, respectively. The composition of the present-day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite, calcite, dolomite and rhodochrosite. Taken together, this suggests that these fractures formed very late in the evolution of the fault zone, and may be induced by co-seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present-day groundwater. They are therefore potentially related to recent earthquake activity (<1.2 Ma) on the Nojima fault.
As the largest and best exposed example of paleo fast‐spreading oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating deep ...crustal processes at fast‐spreading mid‐ocean ridges. We studied two layered gabbro sequences from different stratigraphic depths: one from the middle of the plutonic crust showing decimeter‐scale modal layering (i.e., varying phase proportions) with olivine abundances gradually decreasing from layer bases to tops (Wadi Somerah, Sumail block) and one located near the crust‐mantle boundary showing millimeter‐scale olivine‐rich layers (Wadi Wariyah, Wadi Tayin block). Our multimethod approach of field, petrographic, geochemical, and microstructural observations focuses on documenting layered textures that are widely observed within the lower oceanic crust as well as understanding their formation mechanisms within the context of small scale crustal accretion processes beneath fast‐spreading mid‐ocean ridges. Results from the mid‐crustal sequence indicate moderate cooling rates (Ca‐in‐olivine: logdT/dt; °C yr−1 = −2.21 ± 0.7) and correlated variations in mineral compositions and microstructures. We infer that decimeter‐scale layers in Wadi Somerah were deposited by density currents of crystal‐laden magma within a sill environment that potentially experienced occasional magma replenishment. The millimeter layering in Wadi Wariyah is best explained by Ostwald ripening emphasizing initial heterogeneities possibly being provoked by cyclical nucleation of olivine through the competing effects of element diffusion and rapid cooling. Fast cooling is recorded for the crustal base (Ca‐in‐olivine: logdT/dt; °C yr−1 = −1.19 ± 0.5, Mg‐in‐plagioclase: logdT/dt; °C yr−1 = −1.35°C ± 0.6) demonstrating that heat locally can be lost very efficiently from the lowermost crust.
Key Points
Gravitational sorting in density currents may be a key process in the formation of decimeter‐scale modal layers
Cyclical nucleation and growth of large crystals at the expense of smaller ones can create millimeter‐scale olivine‐rich bands
Shear strain can create and emphasize primary modal layering in cumulus
This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation ...regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of 100; Axial-A, a strong point maximum concentration of 100 with parallel girdle distributions of (010) and (001); and P-type, point maxima of 100, (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high 100 pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.
Seismic faulting causes wall rock damage, which is driven by both mechanical
and thermal stress. In the lower crust, co-seismic damage increases wall
rock permeability, permits fluid infiltration and ...triggers metamorphic
reactions that transform rock rheology. Wall rock microstructures reveal
high-stress conditions near earthquake faults; however, there is limited
documentation on the effects of a thermal pulse coupled with fluid
infiltration. Here, we present a transmission electron microscopy study of
co-seismic microfractures in plagioclase feldspar from lower crustal
granulites from the Bergen Arcs, Western Norway. Focused ion beam foils are
collected 1.25 mm and 1.8 cm from a 1.3 mm thick eclogite facies
pseudotachylyte vein. Dislocation-free plagioclase and K-feldspar aggregates
in the microfractures record a history of fluid introduction and recovery
from a short-lived high-stress state caused by slip along the nearby fault.
The feldspar aggregates retain the crystallographic orientation of their
host and are elongated subparallel to the pseudotachylyte. We propose that
plagioclase partially amorphized along the microfractures at peak stress
conditions followed by repolymerization to form dislocation-free grain
aggregates. Repolymerization and recrystallization were enhanced by the
infiltration of fluids that transported Ca and K into the microfractures.
Subsequent cooling led to exsolution of intermediate plagioclase
compositions and the formation of the Bøggild–Huttenlocher intergrowth in
the grains from the fracture closest to the pseudotachylyte. Our findings
provide unequivocal evidence that the introduction of fluids in the
microfractures occurred within the timescale of the thermal perturbation,
prompting rapid annealing of damaged wall rock soon after earthquake
rupture.
Hydrothermal circulation at ultramafic‐hosted sites supports a large variety of high‐ and low‐temperature hydrothermal vents and associated ecosystems. The discovery of abundant fossil vesicomyid and ...thyasirid shell accumulations at the ridge crest, approximately 2.5 km east of the active Rainbow vent field on the Mid‐Atlantic Ridge (MAR, 36°13′N), increased our knowledge regarding the diversity of vent communities at slow spreading ridges. Bivalve molluscs of the family Vesicomyidae were represented by the genus Phreagena. Here we present the first record of this genus in the Atlantic Ocean. This second vesicomyid species known from the MAR, Phreagena sp., was found to be associated with a Thyasira species that is affiliated with T. southwardae (at the Logatchev vent field on the MAR) and with T. vulcolutre (in the Gulf of Cadiz). These two clams have close relationships with seep taxa along the continental margin, and were likely associated with sedimented vent fields. δ18O and δ13C analyses of the shells suggested that the burrowing bivalve Thyasira could incorporate isotopically light carbon, derived from the oxidation of methane in the sediment, while the signature of Phreagena sp. shells denoted a different carbonate source. 14C dating of the shells denoted that the hydrothermal activity in the Rainbow area began at least ∼25.5 kyr BP, which is similar to the model of the hydrothermal vent field distribution that was proposed for the Logatchev hydrothermal site. The results provide new insight regarding the diversity of chemosynthetic fauna on the MAR over geologic time. Ultramafic‐hosted, on‐axis sedimented vent fields extend the range of habitats for chemosynthetic communities, underlying the need to further explore the geology of these types of environments on slow‐spreading ridges and to determine their role in the ecology of deep‐sea vent communities.
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