Degree Of Serpentinization Research Articles

Shear velocity structure across the Sumatran Forearc-Arc

We present a series of 1-D shear velocity models for the Sumatran Forearc and Arc derived from Rayleigh wave group dispersion in noise correlation functions from vertical and pressure records from an onshore–offshore seismic deployment. The 1-D models represent the crustal structure of the downgoing Indian Plate, the accretionary prism and the arc. There is a progression in shear velocity across the forearc to the arc associated with thickening of the accretionary prism and the development of an arc crust. The velocity structure inferred for the upper 20 km based on path averages between stations on the accretionary prism has velocities consistent with a thick sediment package in agreement with estimates of depth to the plate boundary determined from active source experiments. We also find low Indian Plate shear velocities, <4 km s-’1 to 25 km depth beneath our station locations on the downgoing plate. These low seismic velocities are consistent with at least 14–24 per cent serpentinization of the oceanic crust and upper mantle of the downgoing plate. This high degree of serpentinization, may weaken the plate interface and explain the segmentation observed in the great Sumatran thrust earthquakes if the serpentinization is localized. The success of this study suggests that future onshore–offshore seismic deployments will be able to utilize this method.

Serpentinization of oceanic peridotites: 1. A high‐sensitivity method to monitor magnetite production in hydrothermal experiments

A new method using the magnetic properties of magnetite, Fe3O4, was developed to monitor experimental serpentinization. The saturation remanent magnetization signal (Jrs) was measured during the course of experiments designed to react San Carlos olivine, (Mg0.91, Fe0.09)2SiO4, with water at 250 to 350°C and 500 bars. At the end of the experiments, the ratio with saturation magnetization (Jrs/Js ratio) allowed to convert each successive Jrs measurement into an in situ amount of magnetite produced by the serpentinization reaction. Water weight loss was also measured on the end product to determine the final degree of serpentinization. The application of this procedure to a series of experiments performed at 300°C/500 bars for various run duration (9 to 514 days) and starting olivine grain size (1 to 150 μm) shows a linear relationship between magnetite production and reaction progress. This relationship can be safely transposed to other experimental conditions using thermochemical modeling and/or the Fe content of the product phases. We show that this high‐sensitivity magnetic method is a powerful tool to precisely monitor serpentinization kinetics in Fe‐bearing systems. It represents, in addition, a new indirect mean for monitoring the production of hydrogen which is bound to magnetite production rate.

Seismic anisotropy produced by serpentine in mantle wedge

Trench-parallel seismic anisotropy has been observed in many subduction zones in the upper mantle. In this study, the crystal-preferred orientation (CPO) and seismic anisotropy of natural serpentine from Val Malenco in northern Italy and Punta Bettolina in western Italy are studied. It is found that the [001] axes are aligned subnormal to the foliation but the [010] axes of the serpentine are aligned subparallel to the lineation, which is significantly different from that produced in a recent high-pressure experiment. CPOs of serpentine found herein can be used to explain the trench-parallel seismic anisotropy in a cold mantle wedge for serpentine deformed not only at angles greater than 45° but also at low angles such as in horizontal shear from the surface, indicating the broader implications of the CPO of serpentine for interpreting seismic anisotropy than previously thought. It is also found that seismic anisotropy caused by the CPO of serpentine depends on the degree of serpentinization and flow geometry. The seismic anisotropy increases with antigorite volume fraction. The seismic anisotropy of shear waves is large, i.e., 23–36% for samples with an antigorite volume fraction of 40–93%. Current data suggest that the trench-parallel seismic anisotropy with a delay time of 1–2 s in the forearc mantle wedge can be attributed to the CPO of antigorite having [010] axes aligned subparallel to the flow direction and [001] axes subnormal to the flow plane.

Feedbacks between mantle hydration and hydrothermal convection at ocean spreading centers

Hydration of the oceanic lithosphere is an important and ubiquitous process which alters both the chemical and physical properties of the affected lithologies. One of the most important reactions that affect the mantle is serpentinization. The process of serpentinization results in a drastic decrease in the density (up to 40%), seismic velocity and brittle strength as well as water uptake of up to 13 wt.% of the ultramafic rock. In this paper, we use numerical models to study the amount and extent of serpentinization that may occur at mid-ocean ridges and its effects on fluid flow within the lithosphere. The two dimensional, FEM model solves three coupled, time-dependent equations: (i) mass-conserving Darcy flow equation, (ii) energy conserving heat transport equation and (iii) serpentinization rate of olivine with feedbacks to temperature (exothermic reaction), fluid consumption and variations in porosity and permeability (volume changes). The thermal structure of the ridge is strongly influenced by rock permeability in addition to the spreading velocity of the ridge. Increased rock permeability enhances hydrothermal convection and results in efficient heat mining from the lithosphere whereas higher spreading velocities result in a higher thermal gradient. Serpentinization of the oceanic mantle, in turn, depends on the aforementioned, competing processes. However, serpentinization of mantle rocks is itself likely to result in strong variations of rock porosity and permeability. Here we explore the coupled feedbacks. Increasing rates of serpentinization lead to large volume changes and therefore, rock fracturing thereby increasing rock porosity/permeability while as serpentinization reaches completion, the open pore space in the rock is reduced due to the relative dominance of mineral precipitation. Although, variations in the relation between porosity and permeability and serpentinization before the reaction reaches completion do not significantly affect the degree of serpentinization, we find that unreasonably large portions of the mantle would be serpentinized if rock closure does not occur at the final reaction stage. The amount of water trapped as hydrous phases within the mantle shows a strong dependency on the spreading velocity of the ridge with water content ranging from 0.18 × 10 5 kg/m 2 to 2.52 × 10 5 kg/m 2. Additionally, two distinct trends are observed where the water content in the mantle at slow-spreading ridges drops dramatically with an increase in spreading velocity. The amount of water trapped in the mantle at fast-spreading ridges, on the other hand, is lower and does not significantly depend on spreading velocity.

Lamé parameters of common rocks in the Earth's crust and upper mantle

Lamé parameter (λ) and shear modulus (μ) are the most important, intrinsic, elastic properties of rocks. The Lamé parameter λ, which relates stresses and strains in perpendicular directions, is closely related to the incompressibility and contains a high proportion of information about the resistance to a change in volume caused by a change in pressure. Recent studies have emphasized the roles played by λ in the discrimination of gas sands from carbonates and shale in sedimentary basins and in the seismic reflection of crustal fault zones. Here we analyze the equivalent isotropic elastic data of 475 natural rocks in order to characterize λ values for common types of crystalline rocks in the Earth's crust and upper mantle and their variations with pressure (P), temperature (T), and mineralogical composition. When no partial melting, metamorphic reaction, dehydration, or phase transformation occurs, λ of a crystalline rock as a function of P and T can be described by λ = a + (dλ/dP)P − c exp(− kP) − (dλ/dT)T, where a is the projected λ value at zero pressure if microcracks were fully closed; dλ/dP is the pressure derivative in the linear elastic regime; c is the initial λ drop caused by the presence of microcracks at zero pressure; k is a decay constant of the λ drop in the nonlinear poroelastic regime; and dλ/dT is the temperature derivative. The parameter λ increases nonlinearly and linearly with increasing pressure at low (&lt;∼300 MPa) and high (&gt;∼300 MPa) pressures, respectively. In the regime of high pressures, λ decreases quasi‐linearly with increasing temperature with dλ/dT values in the range of 1–10 × 10−3 GPa/°C. Approaching the α‐β quartz transition temperature, quartzite displays negative λ values. In the λ‐ρ (density) and μ‐λ plots, the main categories of lithology can be clearly distinguished. The ultramafic rocks display systematic decreases in both μ and λ with increasing the degree of serpentinization. Eclogites, mafic rocks (gabbro, diabase, mafic granulite, and mafic gneiss), and felsic rocks (granite, diorite, felsic gneiss, intermediate gneiss, and metasediments) are characterized by high, moderate, and low μ and λ values, respectively. For pyroxene and olivine, both λ and ρ increase, but μ decreases with increasing the Fe/Mg ratios. In the plagioclase series, both λ and μ increases with increasing the anorthite content. Increases in the contents of garnets, sillimanite, rutile, zircon, ilmenite, and spinel result systematically in an increase in rock's λ and μ values. The present results provide improved constraints on the discrimination of composition for crustal and upper mantle rocks in terms of λ and μ.

Magnetic properties of serpentinized garnet peridotites from the CCSD main hole in the Sulu ultrahigh‐pressure metamorphic belt, eastern China

Magnetic properties of 14 serpentinized garnet peridotites from the interval of 603.20–683.53 m of the Chinese Continental Scientific Drilling (CCSD) hole in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China, were investigated and compared with those of ultramafic rocks from other tectonic settings, to relate them to the UHP (post‐)metamorphic evolution. Serpentinized garnet peridotites are characterized by high susceptibility (χ), natural remanent magnetization (NRM), and Q (Köenigsberger ratio = (NRM/χ × H), where H is the intensity of the local geomagnetic field) values. The average χ, NRM, and Q values are 299.65 × 10−7 m3 kg−1, 19.15 × 10−3 Am2 kg−1, and 16.59, respectively, which are generally higher than those of other serpentinized peridotites in the world. The serpentine concentration for most samples ranges from 40% to 60%, suggesting that the peridotites are of obvious homogeneous serpentinization degree. Micro‐Raman and X‐ray diffraction spectra of serpentine for representative samples indicate that lizardite is the only serpentine mineral. Opaque minerals in samples are mainly nonstoichiometric magnetite and Cr‐spinel with or without minor ilmenite and Fe‐sulfide. Magnetite occurs as rims around olivine and garnet as well as in clusters. A chromian garnet further overgrew on the rims of the garnet. The grain size of magnetite in rims is in the pseudo‐single‐domain region and favorable for development of high NRM. High pressure‐UHP metamorphic processes possibly lead to enhanced concentrations of Fe‐Ti oxides in garnet peridotites and, thus, increase the magnetism of serpentinized garnet peridotites in the CCSD main hole. Integrated lizardite formation temperature (&lt;450°C) and its relationships with the pressure‐temperature‐time evolution of the associated UHP rocks permit us to infer that serpentinization of garnet peridotites in the CCSD main hole occurred during late exhumation stages, ca. 200–215 Ma ago, under temperatures of 425°C. However, the exact relationship between magnetic properties of garnet peridotite and the UHP metamorphic process will require further studies.

Spinel compositions and tectonic relevance of the Bibong ultramafic bodies in the Hongseong collision belt, South Korea

The Hongseong area in the western Gyeonggi Massif, South Korea, is characterized by the occurrence of several isolated ultramafic lenses in close association with metabasites within the granitic gneiss. Most of these bodies suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. The Bibong ultramafic rock is assumed to be a mantle section of a probable Neoproterozoic supra-subduction zone (SSZ) and its evolution involved two igneous stages overprinted by later stage metamorphism. The compositions of the chromian spinel cores and olivines representative of the igneous stages were used to deduce the petrogenesis and tectonic environments for the formation of the Bibong ultramafic rocks. Spinel Cr# (100 ⁎ Cr/(Cr + Al)) for the igneous stages in lherzolite, harzburgite and chromitite are 24.5–33.3, 37.4–61.2 and 37.3–50.9, respectively. During the first stage, magma formed in the nascent back-arc basin spreading center by a small degree of partial melting of wedge mantle intruded the preexisting lithospheric mantle causing the melt/rock interaction that resulted in lherzolite with Al-rich chromian spinel. During the second stage more evolved magma formed in the mature back-arc basin by high degrees of partial melting due to the increased supply of fluid or melt from the subducting oceanic crust and intruded the lherzolitic mantle forming harzburgite with chromian spinel (with high Cr#) through the coupled process of olivine precipitation and dissolution of pyroxene. The Bibong ultramafic rocks have later undergone three metamorphic episodes. The first stage is evidenced by development of medium to fine grained olivine, orthopyroxene, clinopyroxene and amphibole around orthopyroxene porphyroblasts. The second stage is serpentinization which is followed by the third stage amphibolite facies metamorphism as evidenced by Fe-rich rims around Al-rich chromian spinel due to the diffusional exchange of Al, Mg, Cr, and Fe between chromian spinel and adjacent silicate minerals. The spinel composition shows that the transition from the nascent back-arc basin to mature back-arc basin tectonic setting may have been caused by trench roll back during the Neoproterozoic in the Hongseong area, and the serpentinization and amphibolite facies metamorphism could have occurred during uplift after Triassic collision between the North and South China blocks.

Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite

We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001)liz, corresponding to the pseudofibres observed optically. The plane (001)liz is locally parallel to the narrow boundary ol–liz; thus column orientations register the interface of serpentinization. The ol–liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010)ol. In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010]atg show domains of very regular modulation with a 43·5 Å wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP–HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100]atg//[010]ol and <100>atg//<100>ol; the planes in contact are (001)atg//(100)ol and (001)atg//(010)ol, respectively. The [010]atg//[001]ol and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine–serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.

Boron, lithium and strontium isotopes as tracers of seawater–serpentinite interaction at Mid-Atlantic ridge, ODP Leg 209

Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50–100%), but still preserve relics of primary phases (olivine ≥ orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on δ 11B and δ 7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest δ 11B (+ 29.6 to + 40.52‰) and lightest δ 7Li (− 28.46 to + 7.17‰) found so far in oceanic mantle. High 87Sr/ 86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed. Applying the known fractionation factors for 11B/ 10B and 7Li/ 6Li between seawater and silicates, serpentinized peridotite in equilibrium with seawater at conditions corresponding to those of the studied drill holes (pH: 8.2; temperature: 200 °C) should have δ 11B of + 21.52‰ and δ 7Li of + 9.7‰. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater–rock interaction where δ 11B and δ 7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with δ 11B and δ 7Li of ODP Leg 209 samples, we obtain δ 11B and δ 7Li values of + 50 to + 60‰, − 2 to + 12‰, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high δ 11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the δ 7Li and δ 11B evolution in the fluid, shows that modification of δ 7Li during serpentinization requires higher water/rock ratios than modification of δ 11B. Extremely heavy δ 11B in serpentinized oceanic mantle can potentially be transported into subduction zones, as the B budget of the oceanic plate is dominated by serpentinites. Extremely light δ 7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.

Geologic and Edaphic Controls on a Serpentine Forest Community

This study examined woody vegetation, edaphic factors, bedrock geochemistry, petrography, and outcrop structure to evaluate some of the community-structuring factors in an ultramafic terrain of Maryland. Analyzing the dynamic nature of combined geological and ecological processes can detect correlative relationships between factors that are typically considered as independent such as tectonically driven bedrock fracturing and ecological community interaction. This study provides evidence for structural variation in fracture density of bedrock as a partial control on tree species distribution in an ultramafic woodland/forest ecosystem. Increases in the number of bedrock fractures correlates negatively with plot-level volumetric soil moisture. Additionally, the degree of serpentinization of the ultramafic parent material results in compositional variation in Ca, Mg, and Ni of parent materials and soils. The combination of these factors provides a significant level of control on the distribution of xeric tree species.

Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone, Southern Tibet: Evidence for an arc–back-arc origin

The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a western extention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massif comprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite mélange (± amphibolite), metamorphosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermost crustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolite mélange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro. Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and 0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/Yb N ratios close to 1, probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for the Saga (Cr# ~ 0.10–0.22) and Sangsang (Cr# ~ 0.30–0.55) peridotites suggest that the Saga peridotites have a deeper mantle provenance (> 20 kbar) and have undergone lower degrees of partial melting (5–12%) than the Sangsang peridotites (< 15 kbar; 17–30%). The composition of the Saga peridotites is similar to the composition of pre-oceanic peridotites while peridotites from the Sangsang massif resemble abyssal and subduction-related peridotites. Mafic rocks from both ophiolites have basalt and basaltic andesite compositions. They are slightly depleted in light REE with respect to the heavy REE, with La/Yb N between 0.5 and 0.8 and with a small negative Ta-Nb anomaly suggesting the presence of a subduction component. The abundances of incompatible elements in these mafic rocks are similar to N-MORB (mid-ocean ridge basalt) or back-arc-basin basalts (BABB). Our data suggest that the Saga and Sangsang ophiolites belong to an intraoceanic suprasubduction zone segment as postulated for other ophiolitic massifs in the eastern portion of the YZSZ. However, the geochemistry of mantle rocks from these two massifs is different compared to each other (fertile vs. refractory) and compared to other YZSZ ophiolites suggesting different petrogenetic histories. Field relationships and geochemical data furthermore suggest that the Saga and Sangsang ophiolites were formed in a complex arc–back-arc setting where at least two subducting slabs must have been active. This study places one more piece in the YZSZ puzzle and lead to a better understanding of the morphology of the convergence zone before the final stage of collision which led to the present configuration of the suture zone.

Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment

The Pindos ophiolite represents oceanic lithosphere obducted during theJurassic.The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (47508C) forming pargasite and edenitic hornblende. During further cooling (from 350^4008C to 51008C), talcþ tremolite serpentine olivine, serpentineþ magnetite, and finally plagioclase alteration phases formed.To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma^mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3 7 g/g) are slightly elevated. The bulk Li contents (0 5^1 1 g/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (50 04^1 1 g/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (50 005 g/g), except for pargasite, where a maximum Be content of 0 012 g/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reactionmodified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0 1^28 g/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples.The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of highand low-B serpentine can be explained by low fluid^rock ratios, decreasingTduring serpentinization and lack of equilibrium as a result of fast obduction^exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone.We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.

Stability and dynamics of serpentinite layer in subduction zone

The hydrous phyllosilicate serpentines have a strong influence on subduction zone dynamics because of their high water content and low strength at shallow and intermediate depths. In the absence of data, Newtonian rheology of serpentinites has been assumed in numerical models yet experimental data show that serpentine rheology is best described by a power law rheology recently determined in subduction zone conditions [Hilairet, N., et al., 2007. High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. Science, 318(5858): 1910–1913]. Using a simple 1D model of a serpentinized channel and – as opposed to previous models – in this power law rheology, we examine the influence of channel thickness, temperature and subduction angle on serpentine flow driven by density contrast (serpentinization degree) with the surroundings. At temperatures of 200–500 °C relevant to intermediate depths a fully serpentinized channel is unlikely to be thicker than 2–3 km. For channel thicknesses of 2 km upward velocities are comparable to those using a constant viscosity of 10 18 Pa s. The velocity profile using power law rheology shows shear zones at the edges of the channel and a low strain rate region at its centre consistent with the frequent observation of weakly deformed HP-rocks. Upward velocities estimated for channels 1 to 3 km thick are comparable to the serpentinization rates for maximum estimates of fluid velocities within shear zones in the literature. Competition between the upward flow and serpentinization may lead to intermittent behavior with alternating growth periods and thinning by exhumation. At shallower levels the thickness allowed for a channel may be up to ~ 8–10 km if the rheology has a higher dependence on stress. We therefore propose that the exhumation of HP oceanic units in serpentinite channels is organized in two levels, the deepest and fastest motion being driven by density contrast with the surrounding mantle and the shallowest circulation being driven by forced return flow. The thicknesses estimated here for serpentinized layers at intermediated depths are similar to the precision of seismic studies. The deepest serpentinite channel may thus be difficult to detect by seismic methods, but it will have a strong influence on the mechanical coupling between the slab and mantle wedge.

Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization

The shallow part of the interface between the subducting slab and the overriding mantle wedge is evidently weakened by the presence of hydrous minerals and high fluid pressure. We use a two‐dimensional finite element model, with a thin layer of uniform viscosity along the slab surface to represent the strength of the interface and a dislocation‐creep rheology for the mantle wedge, to investigate the effect of this interface “decoupling.” Decoupling occurs when the temperature‐dependent viscous strength of the mantle wedge is greater than that of the interface layer. We find that the maximum depth of decoupling is the key to most primary thermal and petrological processes in subduction zone forearcs. The forearc mantle wedge above a weakened subduction interface always becomes stagnant (&lt;0.2% slab velocity), providing a stable thermal environment for the formation of serpentinite. The degree of mantle wedge serpentinization depends on the availability of aqueous fluids from slab dehydration. A very young and warm slab releases most of its bound H2O in the forearc, leading to a high degree of mantle wedge serpentinization. A very old and cold slab retains most of its H2O until farther landward, leading to a lower degree of serpentinization. Our preferred model for northern Cascadia has a maximum decoupling depth of about 70–80 km, which provides a good fit to surface heat flow data, predicts conditions for a high degree of serpentinization of the forearc mantle wedge, and is consistent with the observed shallow intraslab seismicity and low volume of arc volcanism.

Implications of present‐day abiogenic methane fluxes for the early Archean atmosphere

During Earth's early history, greenhouse warming by atmospheric methane helped to maintain elevated surface temperatures. Here, we estimate the present‐day abiogenic CH4 flux generated by mineral alteration (serpentinization) at mid‐ocean ridges, volcanic emissions, and geothermal sources; in addition, we assess the impact that abiogenic methane may have had on greenhouse warming during the early prebiotic Archean. Based on estimates of the rate of seafloor spreading and the degree of serpentinization within the oceanic crust, the flux of methane generated by serpentinized lithosphere is calculated to be ∼1.35 Mt CH4 y−1, while volcanic and geothermal sources are estimated to contribute ∼0.1 and ∼0.9 Mt CH4 y−1, respectively. Furthermore, it is shown that if atmospheric CO2 partial pressures were above 0.01 bar, the present‐day level of abiogenic methane production could have been sufficient to maintain above‐freezing surface temperatures during the Archean. The very high temperatures (∼70°C) that have been suggested for the early Archean, however, would have required extremely high methane fluxes or, more likely, greatly elevated atmospheric CO2 levels.

Seismic and magnetic anisotropy of serpentinized ophiolite: Implications for oceanic spreading rate dependent anisotropy

Compressional and shear wave anisotropy, shear wave birefringence, and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the Dinardic–Hellenic ophiolites. The densities of these materials ranged from 3330 kg/m 3 to 2620 kg/m 3 and indicate degrees of serpentinization from 2.3% to 87.9%, respectively. Magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers. In all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation, but the magnetic anisotropy is not related to the degree of serpentinization. The compressional wave anisotropy ε monotonically decays from 12% for nearly pure olivine dunite to less than 2% for the most serpentinized sample; this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines. Typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion. This loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere. The more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy.

Elasticity of serpentines and extensive serpentinization in subduction zones

Elastic constants of lizardite [Mg3Si2O5(OH)4] were computed using first‐principles quantum mechanical calculations within the density functional theory. The predicted c‐axis compressibility is much larger than measured. Modeling of the weak O‐H⋯O interactions between layers must be improved in order to better predict layered hydrated mineral elastic properties. The large computed bulk modulus range is consistent with equation of state and seismic velocities in chrysotile‐lizardite serpentinites, but shear wave velocities are lower than the lowest theoretical estimate. The low seismic velocities measured for chrysotile serpentinites can be attributed to specific contribution of the nanotube‐textured chrysotile. For antigorite, the data from acoustic and EoS measurements are consistent. If used for interpreting seismic velocities, the inferred degrees of serpentinization of the mantle wedge are higher than with the commonly used calibrations using chrysotile‐serpentinite properties. Serpentine is likely a dominant phase in low velocity areas of the mantle wedge at 30–50 km depths.

Serpentine polymorphs andP-Tevolution of metaperidotites and serpentinites in the Takab area, NW Iran

AbstractThe Takab meta-ultramafic rocks of northwestern Iran crop out in association with a variety of metamorphic rocks and they can be divided into three types on the basis of dominant mineral assemblage and degree of serpentinization: metaperidotites, talc-serpentine rocks and serpentinites. The peridotite protoliths were mainly spinel-harzburgites and -dunites. The talc-serpentine rocks formed under low-grade conditions and their dominant mineral assemblage is lizardite/chrysotile + talc + calcite/dolomite. Pseudomorphic textures after olivine and pyroxene are typical. Later, lower amphibolite-facies metamorphism produced antigorite, clinochlore and tremolite. Late alteration, assisted by local shearing, produced more antigorite and late lizardite in the metaperidotites. The mineral chemistry of the serpentine polymorphs is variable. Compositions of coexisting spinel and olivine point to an ophiolitic origin. Thermometric estimates from co-existing relict olivine, pyroxenes and spinel are in the range 1000–1200ºC; clinopyroxene barometry yields a pressure of 24± 2.7 kbar for the mantle conditions. Exhumation and hydration followed, and the stability of lizardite/chrysotile constrains the T and XCO2to &lt;280ºC and &lt;0.002, respectively. Subsequent prograde regional metamorphism took place in an orogenic setting, reaching peak temperatures in the range 410–540ºC at XCO2&gt;0.1. A Neoproterozoic–Early Cambrian age for the Takab protoliths is suggested on the basis of correlations with the Central Iran Zone. The strips of ultramafic rock in the Takab complex can be attributed to lithospheric remnants of the Proto-Tethyan ocean which closed during the Pan-African orogeny.

Compressional and shear wave velocities of serpentinized peridotites up to 200 MPa

AbstractCompressional and shear wave velocities of serpentinized peridotites were measured at room temperature and high confining pressures of up to 200 MPa. Rock samples were collected from the Hida outer belt, Central Japan, and classified into High-T (containing antigorite) and Low-T (containing lizardite and/or chrysotile) types. Antigorite is stable up to 600∼700°C, while lizardite and chrysotile are stable below 300°C. High-T type samples have distinctly higher velocities than their Low-T type counterparts with the same density. The High-T type with strong foliation shows significant velocity anisotropy, and the azimuthal anisotropy of the compressional wave velocity reaches 30%. These properties can be explained by the crystallographic structure of antigorite. Poisson’s ratio increases with serpentinization in both types. The High-T type shows a lower Poisson’s ratio than the Low-T type with the same density. The High-T type requires a higher degree of serpentinization than the Low-T type to give a certain value of Poisson’s ratio. Observations of high Poisson’s ratio have been interpreted using Low-T type properties. However, High-T type serpentinized peridotite is expected in warm subduction zones. The use of Low-T type properties will lead to a significant underestimation of serpentinization. For good interpretations, it is essential to use the properties of the appropriate type of serpentinized peridotite.

Ancient melt extraction from the oceanic upper mantle revealed by Re–Os isotopes in abyssal peridotites from the Mid-Atlantic ridge

Comprehensive major-, trace-element and rhenium–osmium (Re–Os) isotope data are presented for abyssal peridotites from Ocean Drilling Program (ODP) Leg 209, in the North Atlantic. The samples are from a single core (Site 1274A) located on the western wall of the axial rift valley of the Mid-Atlantic ridge, and their study allows elemental and isotope information to be precisely related to spatial variations in primary lithology, serpentinisation and seafloor weathering. The harzburgites and dunites at this site are highly serpentinised (with the degree of serpentinisation increasing with depth below the sea floor). Petrographic observations and variations of fluid mobile elements (such as Ba, Sr, U, and Re) are consistent with seawater interaction in the upper part of the core. Nevertheless, major and trace element indicators of the extent of melt depletion indicate extreme melt loss, and suggest that these are amongst the most depleted abyssal peridotites recovered thus far. Despite the evidence for extensive serpentinisation and sea floor weathering all of the samples possess 187Os/ 188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle (PUM), lower than any yet reported for abyssal peridotites, and consistent with melt loss over, at least, the past 1.5 Ga. Single sulphide Re–Os data show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic ( 187Os/ 188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re–Os chemistry of mid-ocean ridge basalts.