Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through ...geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle.
When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean–continent transition.
Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.
Display omitted
•Review of >900 geochemical analyses of serpentinites worldwide•Identification of geochemical criteria to distinguish serpentinite protoliths•Discussion of the possible origins of subducted serpentinites•Serpentinites are one of the major components of the Earth's budget for fluid-mobile elements.
The dynamics of continental subduction is largely controlled by the rheological properties of rocks involved along the subduction channel. Serpentinites have low viscosity at geological strain rates. ...However, compelling geophysical evidence of a serpentinite channel during continental subduction is still lacking. Here we show that anomalously low shear-wave seismic velocities are found beneath the Western Alps, along the plate interface between the European slab and the overlying Adriatic mantle. We propose that these seismic velocities indicate the stacked remnants of a weak fossilised serpentinite channel, which includes both slivers of abyssal serpentinite formed at the ocean floor and mantle-wedge serpentinite formed by fluid release from the subducting slab. Our results suggest that this serpentinized plate interface may have favoured the subduction of continental crust into the upper mantle and the formation/exhumation of ultra-high pressure metamorphic rocks, providing new constraints to develop the conceptual and quantitative understanding of continental-subduction dynamics.
At plate boundaries, where deformation is localized along centimetre- to kilometre-scale shear zones, the influence of serpentinite on tectonic processes is linked to its unique rheological ...properties. In this paper we review the physical properties of serpentinites and their role in tectonic processes. At the ocean–continent transition, serpentinization weakens the upper mantle layer, promoting strain localization and allowing the normal faults in the distal margin to root at low angle. Similarly, at slow to ultra-slow spreading ridges, serpentinite is potentially very abundant at the seafloor and locally associated with domal structures. Extensional deformation is localized in a ~100m thick shear zone at the footwall of detachment zones dominated by serpentine derived minerals. Within subduction zone, the depth of decoupling between the mantle wedge and the subducting slab corresponds to the stability depth of serpentine weak mineral. Dehydration of serpentine has also been hypothesized to play an important role in the origin of double seismic zones, however the exact mechanism through which dehydration promotes seismicity remains a matter of debate. During exhumation of high-pressure or ultrahigh-pressure rocks, the opposite trajectories of exhumation and subduction require a decoupling zone within the subducting slab. A serpentinized layer has the potential to become a decoupling zone between the oceanic crust and underlying lithosphere. The buoyancy of serpentinite also likely contributes to eclogite exhumation. Finally, along major strike-slip faults, serpentinites have been associated with fault creep, as well as low fault strength. The presence of serpentinite blocks along creeping segments of active faults worldwide is therefore likely to originate from fluids deriving from the progressive dehydration of the mantle wedge that move such bodies upward.
•The role of serpentinites in various tectonic settings is reviewed.•Serpentinite is a weak rock that localizes deformation in lithospheric shear zones.•In oceanic domains, mantle is exhumed along serpentinized fault zones.•Serpentinized layers at the subduction interface favour stagnant wedge corner.•Active strike-slip faults may creep when serpentinized.
In order to understand the potential controls on Asian tectonics during the subduction of the Tethys and Indian lithospheres, we reconstruct the coupled subduction-continent deformation history using ...tomographic imaging, kinematics constraints and numerical modeling.
The global P-waves tomographic images of the mantle below the India-Asia collision zone provide constraints on the deep structure of continents and subduction history. Linking the slab positions in the mantle to the Asian tectonics reconstructions and the Indian plate kinematics, we reconstruct the timing and location of successive subduction and breakoff events, showing one major breakoff occurred between India and the Tethys Ocean ~45Ma. In the western syntax, a vertical slab continuous to the continent is shown to override the deeper detached Tethys slab. In the central region similar structure is found with a detached slab, yet closer to the Tethys slab. In the eastern syntax, no slab is imaged. It is inferred that after Tethys slab had broke off, subduction only resumed in the center of the margin, while underthrusting took place at both extremities of the convergent margin. During following convergence, a second breakoff event detached the central Indian slab from the margin ~15Ma ago, which renewed Indian lithosphere underthrusting below Asia. This most probably occurred when the Tibetan Plateau was already uplifted, implying that uplift is not a direct result of underthrusting.
Numerical models of breakoff during subduction illustrate the controls of slab detachment on the complexities of the Indian margin. In these models the subduction of continental lithosphere resumes after breakoff only where this is entrained by the mantle flow associated with the long lasting oceanic slab sinking, that is in the center of the margin, while converging continent edges underthrusts the upper plate. Furthermore, the breakoff during subduction has profound implications on the Asian intra-plate tectonics. In the models, the breakoff is rapidly followed by large stresses in the upper plate interiors, propagating at large distance from the margin, along a belt oriented at ~45° from the trench. The long-term evolution of the Asian continental tectonics shows drastic changes in the fault pattern, with successive strike-slip faulting across the Asian continent, which are in agreement with the mechanisms illustrated by the models. Transient large coupling at the trench caused by the breakoff events during India-Asia convergence offers an explanation for episodic nucleation of lithospheric faults within the Asian continent and their link to deep processes.
Display omitted
•Global tomography evidences Indian continent subduction and breakoff.•After complete breakoff at the OCT, Indian continent subduction resumes.•Mantle flow generates by oceanic subduction drags the Indian continent in the mantle.•Indian slab breakoff generated localized stress pulses in Asian continent.
Serpentinites associated with eclogitic rocks were examined from three areas: the Alps, Cuba, and the Himalayas. Most serpentinites have low Al/Si and high concentrations of Ir‐type platinum group ...elements (PGE) in bulk rock compositions, indicating that they are hydrated mantle peridotites. A few samples contain high Al/Si and low concentrations of Ir‐type PGE, suggesting that they are ultramafic cumulates. Among the hydrated mantle peridotites, we identified two groups, primarily on the basis of Al/Si and Mg/Si ratios: forearc mantle serpentinites and hydrated abyssal peridotites. Forearc serpentinites occur in the Himalayas and along a major deformation zone in Cuba. All serpentinites in the Alps and most serpentinites in Cuba are hydrated abyssal peridotites. Himalayan serpentinites have low Al/Si and high Mg/Si ratios in bulk rock compositions, and high Cr in spinel; they were serpentinized by fluids released from the subducted Indian continent and enriched in fluid‐mobile elements, and show high 87Sr/86Sr, up to 0.730, similar to the values of rocks of the subducted margin of the Indian continent. Although Himalayan serpentinites have a similar refractory geochemical signature as the Mariana forearc serpentinites, the former contain markedly high concentrations of fluid‐mobile elements and high 87Sr/86Sr compared to the latter that were hydrated by subducted Pacific Ocean crust. The data indicate that the enrichment of fluid‐mobile elements in forearc serpentinites depends on the composition of subducted slabs. Alpine serpentinites and most Cuban serpentinites show moderate Al/Si similar to abyssal peridotites. Hydration of peridotites near the seafloor is supported by micro‐Raman spectra of earlier formed lizardite, high δ34S (+11 to +17‰) of sulphides, and elevated 87Sr/86Sr, ranging from 0.7037 to 0.7095. The data support the contribution of S and Sr from seawater and sediments. These serpentinites are not highly enriched in fluid‐mobile elements because serpentinization occurred at a high water/rock ratio. Alkali elements are conspicuously unenriched in all serpentinites. This lack of alkali enrichment is explained by slab retention of alkalis. This is also consistent with the observation of relatively low alkali concentrations in volcanic front magmas, since partial melting related to the volcanic fronts is triggered by dehydration of serpentinites.
Body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle. Focusing on the India/Asia collision zone, we postulate the existence of several ...Asian continental slabs, based on seismic global tomography. We observe a lower mantle positive anomaly between 1100 and 900km depths, that we interpret as the signature of a past subduction process of Asian lithosphere, based on the anomaly position relative to positive anomalies related to Indian continental slab. We propose that this anomaly provides evidence for south dipping subduction of North Tibet lithospheric mantle, occurring along 3000km parallel to the Southern Asian margin, and beginning soon after the 45Ma break-off that detached the Tethys oceanic slab from the Indian continent. We estimate the maximum length of the slab related to the anomaly to be 400km. Adding 200km of presently Asian subducting slab beneath Central Tibet, the amount of Asian lithospheric mantle absorbed by continental subduction during the collision is at most 600km. Using global seismic tomography to resolve the geometry of Asian continent at the onset of collision, we estimate that the convergence absorbed by Asia during the indentation process is ~1300km. We conclude that Asian continental subduction could accommodate at most 45% of the Asian convergence. The rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction/underthrusting processes. Continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like Eurasia.
Display omitted
► Global seismic tomography shows the existence of three Asian continental slabs. ► Lower mantle AS anomaly is the remnant of an ancient Asian subduction process. ► Two shallow narrow Asian subducting slabs are also observed. ► Asian continental subduction accommodates about 45% of the Asian convergence. ► Continental subduction is a major lithospheric process during collision.
Terra Nova, 23, 171–178, 2011
Serpentinization of the oceanic lithosphere contributes significantly to the geochemical cycle from spreading ridges to subduction zones. In situ trace element analysis ...of oceanic serpentinites from the Mid‐Atlantic Ridge and the Greater Antilles (Cuba, Dominican Republic) shows that all serpentine minerals are enriched in fluid‐mobile elements (FME: As, Sb, B, Li, Cs, Pb, U, Ba, Sr). We observe no loss of these elements from abyssal to subduction environments during prograde metamorphism. Moreover, the transition from lizardite/chrysotile to antigorite during subduction is marked by a strong over‐enrichment in As and Sb in antigorite, indicating late contamination by a sedimentary source. This suggests that a second stage of serpentinization occurs in the earlier stages of subduction, when newly formed or reactivated normal faults ease fluid penetration, and/or in the subduction channel. Our study shows that, from spreading ridges to forearc environments, serpentines act as sponges for FME. We posit that, until ultimate antigorite breakdown, serpentinites efficiently transport significant amounts of FME down to great depths in the mantle.
Several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the India–Eurasia collision zone, with steep northward subduction of ...Indian lithosphere beneath the Hindu Kush and southward subduction of Asian lithosphere under the Pamir. We investigate the geometry and the timing of these subduction processes. Seismic tomography images are used to constrain the geometry of the two slabs under the Pamir and the Hindu Kush. By measuring the tomographically inferred length of the Indian slab under the Hindu Kush region and by comparison with paleomagnetic reconstructions, we estimate that the process of Indian slab break-off most likely occurred at the early stage of the collision, at ca. 44–48 Ma. We infer that after slab break-off, western India continued its northward motion until its northern boundary reached the present Hindu Kush region, where it began to subduct. We estimate an age for the initiation of subduction in this region of about 8 Ma. We apply 2D thermo-kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the Hindu Kush and Pamir regions. For the Hindu Kush we introduce parameters reproducing fast and near vertical subduction, whereas in the Pamir region we use the results from published studies to reproduce slower subduction. Modeling results indicate that faster subduction under the Hindu Kush results in a deeper brittle region, which is consistent with observed deeper seismicity than in the Pamir region.
High wavespeed tomographic anomalies shallower than 1100
km beneath the India/Asia collision zone are interpreted as continental slabs subducted during collision. Combining anomaly positions with ...paleogeographic reconstructions of India, we constrain the spatio-temporal evolution of multiple episodes of continental subduction likely related to these anomalies. This study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone. The evolution of the western part is characterized by two episodes of steep subduction of the northern margin of India. The first episode, involving an area with a lateral extent as large as 1500
km, started at about 40–30 and ended by a slab break-off process at ∼
15
Ma, The second episode consists on subduction beneath the Hindu Kush mountains since ∼
8
Ma. To the east of the collision zone, no anomaly related to steep subduction along the northern edge of India is found. We interpret two tomographic anomalies beneath the eastern border of the Indian plate, beneath Burma and beneath the Andaman Sea, as the result of two successive episodes of southeastward extrusion followed by subduction. We suggest that both extruded portions were initially located along the northern margin of India, and that they slid around the eastern syntaxis, then southward along the eastern boundary of Indian plate. Both portions subducted along the eastern border of India, south of the eastern syntaxis.
We provide a rough estimate of the amount of Indian lithosphere consumed during these subduction and extrusion episodes. By comparing this amount with the total amount of Indian lithosphere at the onset of collision, we conclude that these processes accommodated most of the India/Asia convergence during collision.
Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early ...Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks (“durbachites”). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348–335 Ma) and the late Carboniferous (305–299 Ma), with a more limited activity in between. A younger Permian event at ca. 280–275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE–LREE-rich metasomatized lithospheric mantle. Trace elements and Nd–Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO
2
, Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The εNd
i
values of both high-K calc-alkaline granitoids and durbachites decreases from − 3.8; − 2.9 to − 6.4; − 5.2 between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. εNd
i
values then rise to − 3.7; − 0.5 during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage.
Graphic abstract
Possible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.
PDF
