Journey to the mantle of the Earth Teagle, Damon; Ildefonse, Benoît
Nature (London),
03/2011, Letnik:
471, Številka:
7339
Journal Article
Recenzirano
Odprti dostop
The idea came primarily from Harry Hess, one of the founding fathers of the theory of plate tectonics, and Walter Munk, who pioneered studies of how winds drive ocean currents and explained why one ...side of the Moon is locked towards Earth. (The hole itself is not the deepest ever drilled into the sea floor; that was Hole 504B, which reached 2,111 metres below the bottom of the eastern Pacific off Colombia.) The mission should help to settle many debates: how crust is formed at mid-ocean ridges; how magma from the mantle is intruded into the lower crust; the geometry and vigour of how sea water can pull heat from the lower oceanic crust; and the contribution of the lower crust to marine magnetic anomalies.
The building of oceanic crust at Oceanic Core Complexes (OCC) has been described as a complex process involving multiple intrusions of magma over a protracted period of time. The migration of ...primitive magmas (i.e., Mid Ocean Ridge Basalts, MORBs) can lead to melt-rock interactions during reactive porous flow processes through the lithosphere. The timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained. Diffusion modelling has been widely used to retrieve timescales of magmatic processes. In this study, we use diffusion models to constrain (i) the minimum timescales of melt-rock interactions and (ii) the cooling rates of the gabbroic sequence forming the oceanic crust at an OCC. We chose samples of the most primitive olivine-rich troctolites from the gabbroic sequence sampled in IODP Hole U1309D (Atlantis Massif OCC, Mid-Atlantic Ridge 30°N). Olivine-rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust, and thus allowed to understand the dynamics of mantle assimilation and the formation of slow-spreading oceanic crust at the Atlantis Massif OCC. Olivines in olivine-rich troctolites represent relicts of pre-existing mantle olivine, while clinopyroxenes and plagioclases are crystallized during reactive percolation. Olivine chemical compositions show that olivine-rich troctolites inherit chemical heterogeneity from the mantle precursor. Flat geochemical profiles in olivine indicate complete chemical re-equilibration of olivine crystals with the locally modified percolating melt. Exception is made for most Ca profiles that show lower Ca contents at the olivine rim compared to the relative crystal core, as the result of subsolidus cooling. Three-dimensional (3D) diffusion models at magmatic conditions (T = 1210–1300 °C and P = 2 kbar) reveal that complete chemical re-equilibration of 4 mm-size mantle-derived olivine with percolating MORB-type melts can be attained within durations of less than 300 yr. The Ca-in-olivine geospeedometer reveal that cooling rates from ~1200 °C to ~1050 °C are constant downhole and on average 0.004 °C/yr; they are comparable with lower temperature cooling rates (850 °C–250 °C) estimated at the Atlantis Massif OCC. The minimum timescales from 3D models point to rather fast re-equilibration of olivine. The downhole chemical heterogeneity inherited from the precursor mantle, coupled with the timescales of diffusive re-equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time-frame of a single melt input. Our cooling data are consistent with building of the oceanic crust at OCCs controlled by continuous uplift, in turn governed by long-lived detachment faults. The latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies. Diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros, and the cooling history of the oceanic crust at OCC from magmatic conditions to hydrothermalism.
•Diffusion modelling is powerful to unravel the history of oceanic crust formation.•Mantle olivine composition can be completely reset in short minimum timescales.•Mantle assimilation occurs in the time-frame of a single melt input every ~630 yr.•Downhole cooling rates are constant from magmatic (1200 °C) to low temperatures (250 °C).•Uplift contributes substantially to conductive cooling of the crustal sequence.
This study reports on feedback mechanisms between fluid migration, ductile deformation, and strain localization processes in an incipiently forming mantle wedge: the basal banded unit of the Semail ...ophiolite. These peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction. During this stage, they were affected by coeval ductile deformation, forming (proto)mylonites at ~900–800 °C to ultramylonites at ~700 °C, and interaction with subduction fluids. From the petrological and microstructural study of these hydrated peridotites and their protolith (preserved lenses of porphyroclastic tectonites), we show that peridotite interaction with hydrous fluids triggered dissolution/precipitation processes. Dissolution of coarser grains and precipitation of new and smaller ones (mainly pyroxenes, spinel, and amphibole) resulted in a drastic grain size reduction, phase mixing and a switch from olivine dislocation to grain size sensitive creep in (proto)mylonites. Data also evidence a feedback process of fluid focusing in actively deforming shear zones. This rapid switch in olivine deformation mechanism, driven by subduction fluid‐peridotite interaction, triggered an intense weakening of the peridotites at ~900–800 °C and a transition from a mechanically decoupled to coupled plate interface (as witnessed by the detachment and underplating of a high‐temperature metamorphic sole to the base of the ophiolite). It also explains the intense strain localization (<1‐km‐thick shear zone) along this ductile portion of the plate interface. Considering that similar mechanisms take place in mature subduction zones, they may explain plate interface coupling at subarc depths in worldwide subduction zones.
Key Points
The base of the Semail ophiolite is a proxy for a mantle wedge deforming and interacting with subduction fluids right above a hot plate interface
Fluid/peridotite interaction leads to dissolution/precipitation processes, grain size reduction, and peridotite weakening at short time scales
Such peridotite weakening explains intense strain localization within the ductile mantle wedge and the observed switch in the subduction interface regime
Exhumed faults in granitoids along the Lanterman Fault-Rennick Graben Fault system (northern Victoria Land, Antarctica) show superposed ductile to brittle deformation and pervasive hydrothermal ...fluid-rock interaction. These processes triggered multiple brittle slip events producing crosscutting epidote and prehnite-rich fault veins, ultracataclasites and pseudotachylytes of crushing origin. Combined microstructural and minerochemical investigations on fault damage zones show three types of alteration: (i) albitization of K-feldspar and Ca-plagioclase; (ii) crystallization of prehnite and calcite in veins; (iii) alteration of magmatic phases by secondary hydrous minerals (e.g. chlorite, white mica, epidote and prehnite). The fault experienced various episodes of strain weakening and hardening, due to alteration of minerals and precipitation of epidote and prehnite within ultracataclastic intervals, at decreasing temperature conditions (200 < T°C < 450) and varying CO2 fugacity of the fluids. Cyclic crystallization of epidote/prehnite within the fault cores caused cementation and locking of faults, concentration of deformation at weaker horizons and a progressive broadening of the fault zone. Our results indicate that multiple co-seismic slip and syntectonic fluid flow very likely occurred prior to the Cenozoic brittle reactivation of inherited anisotropies in the northern Victoria Land crust along the Lanterman Fault-Rennick Graben Fault system and underlines its high potential for polyphasicity.
•Exhumed faults in granitoids from Antarctica show multiple ductile to brittle shears.•Syntectonic hydrothermal fluid-rock interaction processes affect slip history.•Epidote and prehnite crystallization in fault rocks influence rheology.
In this study, we describe the characteristics of tectonic stylolites and related veins affecting a low-porosity micritic limestone (Jurassic carbonates, Les Matelles, South of France) in order to ...unravel the conditions of initiation and interaction between pressure-solution and fracturing in such rock. Field description, various petrographic and microstructural investigations (cathodoluminescence, SEM imaging, EBSD analysis), and petrophysical/geochemical analyses (Hg porosimetry, XRD, EPMA) are used. We document that pressure-solution initiates at micropores and propagates along calcite grain contacts, connecting surrounding stylolite micro-segments, and progressively concentrates insoluble material such as clays and siliceous particles. The dissolved material is evacuated to the veins where the newly-formed porous space is progressively filled by calcite cement. These deformation processes are strictly restricted to the stylolitic interface and veins, as no modification of porosity or grain deformation is detected in the neighboring host rock. This is due to the low-permeability of the surrounding host rock impeding the evacuation of dissolved material and fluids through interstitial porosity around the pressure-solution zone, leading to overpressure and veins formation. The water release and microporosity caused by diagenesis of the clay fraction (smectite-illite transformation) are discussed as key diagenetic processes instigating the conditions of pressure-solution initiation, then tectonic stylolite formation in low-porosity limestones.
•The pressure-solution process is restricted to the stylolitic interface.•Stylolites propagate along grain contact and concentrate insoluble material.•Soluble material is evacuated to the vein formed by hydraulic fracturing.•The material transfer from stylolite to vein is a closed system.•Clay diagenesis releasing water and porosity allows the pressure-solution initiation.
Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower ...oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580–2730ppm), intermediate in troctolites (2050–2230ppm) and lowest in gabbros (1355–1420ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the origin of olivine-rich rocks in the lower oceanic crust.
Microbathymetry data, in situ observations, and sampling along the 13°20′N and 13°20′N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between ...tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high‐angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along‐extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension‐parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20′N OCC, and gabbro and peridotite at 13°30′N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30′N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20′N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.
Plain Language Summary
Oceanic detachments are long‐lived faults that are exposed at the seafloor, often showing prominent corrugations. Detailed mapping and inset observations and sampling, using remotely operated and autonomous robots, demonstrate as these fault surfaces are exposed at the seafloor, erosion and deposition of debris cover these faults systematically with a thin layer of rubble, demonstrating that their evolutions intimately linked to surface processes. Locally, the fault itself is exposed, displaying striated fault planes on the flanks of corrugations. A complex, three‐dimensional network of fault planes within the detachment zone defines elongated bodies of rock (corrugations).These detachments also confirm that there is a direct link between detachment formation and hydrothermal activity, as they host both active and inactive hydrothermal fields.
Key Points
Oceanic detachment fault corrugations correspond to anastomosing fault slip planes localizing brittle deformation in shallow lithosphere
Mass wasting is associated with detachment denudation and blankets its surface though shedding of material at termination
Faulting, associated with magmatism and hydrothermalis, terminates detachments, and hydrothermalism linked systematically to detachments
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.
The 1415 m deep IODP Hole U1309D (Mid-Atlantic Ridge, 30 degrees N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic ...rocks, with a large range of compositions. The most primitive end-members of the gabbroic sequence, herein referred to as olivine-rich troctolites (ol > similar to 70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine-rich troctolites, olivines range from coarse-grained subhedral crystals, commonly containing well-developed subgrains, to medium-grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high-temperature slip systems, dominantly (010)100. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon 001 concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high-temperature plastic fabric is modified in a liquid-dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt-rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.
Observations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) ...Expedition 335 ("Superfast Spreading Rate Crust 4") returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m) into cumulate gabbros. Three earlier cruises to Hole 1256D (ODP 206, IODP 309/312) have drilled through the sediments, lavas, and dikes and 100 m into a complex dike-gabbro transition zone. Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes. Hole 1256D is now clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth. doi:10.2204/iodp.sd.13.04.2011
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