Wedge Interface Research Articles

Beyond the Wedge: Impact of Tidal Streams on Salinization of Groundwater in a Coastal Aquifer Stressed by Pumping and Sea‐Level Rise

AbstractSaltwater intrusion (SWI) is a well‐studied phenomenon that threatens the freshwater supplies of coastal communities around the world. The development and advancement of numerical models has led to improved assessment of the risk of salinization. However, these studies often fail to include the impact of surface waters as potential sources of aquifer salinity and how they may impact SWI. Based on field‐collected data, we developed a regional, variable‐density groundwater model using SEAWAT for east Dover, Delaware. In this location, major users of groundwater from the surficial aquifer are the City of Dover and irrigation for agriculture. Our model includes salinized marshland and tidal streams, along with irrigation and municipal pumping wells. Model scenarios were run for 100 years and included changes in pumping rates and sea‐level rise (SLR). We examined how these drivers of SWI affect the extent and location of salinization in the surficial aquifer by evaluating differences in chloride concentration near surface waters and the subsurface freshwater‐saltwater interface. We found the presence of the marsh inverts the typical freshwater‐saltwater wedge interface and that the edge of the interface did not migrate farther inland. Additionally, we found that tidal streams are the dominant pathways of SWI at our site with salinization from streams being exacerbated by SLR. Our results also show that spatial distribution of pumping affects both the magnitude and extent of salinization, with an increase in concentrated pumping leading to more intensive salinization than a more widely distributed increase of the same total pumping volume.

Two-wedge slope stability analysis considering a nonvertical wedge interface

Two-wedge slope stability analysis considering a nonvertical wedge interface

Electrical conductivity of siderite and its implication for high conductivity anomaly in the slab-mantle wedge interface

Carbonate minerals as a dominant carbon host can be transported to the Earth’s deep interior via subduction of the oceanic lithosphere, and their physicochemical behavior potentially has a significant influence on the compositional heterogeneity and physical properties in the deep mantle. In this study, we measured the electrical conductivity of natural siderite at 1–3 GPa and 100–700°C using a complex impedance analyzer in a large volume multi-anvil high-pressure apparatus. A sharp increase in conductivity was observed at ∼400°C under various pressures, and subsequently, the electrical conductivity keeps anomalously high values in the whole temperature range owing to a small quantity of interconnected highly conductive phases (graphite and magnetite) produced from the low degree decarbonation of siderite. The change in electrical conductivity and activation enthalpy suggest that the conduction mechanisms before and after low degree decarbonation of siderite are the small polaron (electron hopping in Fe2+–Fe3+) and highly conductive phases, respectively. Our results indicate the incipient decarbonation temperatures at 1–3 GPa are considerably lower than the decomposition boundary of siderite determined by phase equilibrium experiments, implying the initial decarbonation reaction of Fe-bearing carbonates in the subducting oceanic crust occurs at a shallower depth. The 30 vol.% of siderite is required to enhance the electrical conductivity of (Mg, Fe)CO3 solid solutions. Magnetite and graphite generated from the decarbonation reaction of the siderite component of Fe-bearing carbonate make a significant contribution to the high conductivity anomaly observed in the slab-mantle wedge interface.

The Subducting Slab as a Chromatographic Column: Regimes of Sub‐Solidus Mass Transport as a Function of Lithospheric Hydration State, With Special Reference to the Fate of Carbonate

AbstractWe investigate the chemical budget of subduction zones at sub‐solidus conditions using a thermodynamic‐numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid‐rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central‐eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub‐arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long‐term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.

Mixing, fluid infiltration, leaching, and deformation (MILD) processes on the slab-mantle wedge interface at high T and P conditions: Records from the Dalrymple Amphibolite, Philippines

The whole rock compositions of the blocks and the surrounding matrix of the Dalrymple Amphibolite are investigated in this study to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite. Similarities in their rare earth and trace element patterns indicate the genetic relationship between the mafic lavas of the Palawan Ophiolite and the metamafic blocks of the Dalrymple Amphibolite. This confirms that the metamafic blocks represent the basalt to gabbro section of the oceanic lithosphere of the subducting slab.The matrix surrounding the blocks exhibit highly variable phase assemblages. In order to determine its petrogenesis, we distinguished groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light rare earth elements; Group 2 Cr, Ni and MgO) based on statistical (correlation coefficient) analyses. These groups indicate mixing of metasedimentary (Group 1) and metamafic (Group 2) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration by changing the end-member and their relative proportions. The end-members and mixing ratio with the highest regression value (r2) was selected to obtain the modelled composition of the matrix. The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile. This assumption is supported by the prevalence of kyanite, ilmenite and zircon in the matrix mineral assemblage.This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. In particular, the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones. Mass balance calculations further revealed that an early fluid infiltration event likely occurred following the mixing process. This preferentially leached out elements which are either fluid-mobile (e.g. CaO and SiO2) or are not incorporated into the growing minerals in the matrix. The strong control of mineral assemblage of the matrix in its chemistry is exhibited by a number of samples which showed variable degrees of losses and gains in elements traditionally interpreted to be fluid immobile (e.g. heavy rare earth elements and Y). A later hydration event linked to retrograde metamorphic stage imprinted gains of K2O, Rb, and Ba in the matrix samples with the growth of replacement minerals (e.g. muscovite on kyanite). This later fluid infiltration event possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.

The slab–mantle wedge interface of an incipient subduction zone: Insights from the P–T–D evolution and petrological characteristics of the Dalrymple Amphibolite, Palawan Ophiolite, Philippines

AbstractIn this contribution, we present the petrological characteristics and pressure–temperature–deformation (P–T–D) history of the Dalrymple Amphibolite in Palawan Island, Philippines. This unit occurs below the residual mantle peridotites of the Palawan Ophiolite composed of interlayered dunite and harzburgite. The Dalrymple Amphibolite is predominantly composed of high‐grade metamafic blocks with minor metasedimentary units surrounded by a highly sheared hybridized matrix (kyanite + Ca‐amphibole + biotite + ilmenite ± garnet). Zr‐in‐rutile geothermometry and quartz‐in‐garnet geobarometry of the garnet amphibolite block reveal the P–T conditions of the prograde metamorphism from ~625°C, 11.5 kbar to ~700°C, 13 kbar. Zr‐in‐rutile geothermometer and the TZARS geobarometer (clinozoisite + rutile + quartz = anorthite + titanite + H2O) further indicate comparable peak metamorphic conditions for the amphibolite and epidote amphibolite (~605–710°C, 10–13 kbar) blocks. Geothermobarometry of the matrix surrounding the blocks reveals similar peak metamorphic conditions of ~700°C and 13 kbar to the garnet amphibolite blocks. The paleogeothermal gradient preserved in the Dalrymple Amphibolite (~16°C/km) and its block‐in‐matrix structure are atypical of metamorphic soles and are more akin to high‐T mélange complexes interpreted to have formed during incipient subduction. These P–T estimates and its block‐in‐matrix structure suggest that the Dalrymple Amphibolite likely represents the slab–mantle wedge interface of a young arc complex already transitioning from the much warmer paleogeothermal gradients of the slab–mantle wedge interface when subduction was first initiated. The similar peak metamorphic conditions constrained in the matrix and the metamafic blocks in the Dalrymple Amphibolite indicate that the matrix‐forming deformation started before the sequence reached peak metamorphic conditions at depths of around 45 km. Furthermore, the petrological and geochemical characteristics of this block‐in‐matrix sequence and the multiple generation of cross‐cutting veins reveal the complex processes that occur in the slab–mantle wedge interface of an arc in its infancy. Our results highlight the prevalence of Ca‐amphibole and kyanite over phyllosilicates (e.g., chlorite and serpentine) in the mélange matrix, mixing of crustal components along the slab–mantle wedge interface promoted by multiple deformation events and the ubiquity of fluids throughout its history. Subduction zones with similar geothermal gradients (e.g., warm subduction zones) possibly share these petrological characteristics.

Analytical Solutions for General Two-Wedge Stability

A force-based limit equilibrium analysis is presented for the stability of a general two-dimensional, two-wedge sliding mass of soil, including a vertical or nonvertical wedge interface. The analysis is conducted using three failure planes and can accommodate variable conditions for wedge geometry, pore pressure, shear strength parameters, reinforcement, applied loads, and pseudostatic seismic coefficients. A constant factor of safety is assumed for each failure plane and reinforcement element, although this assumption can be relaxed through selection of strength parameters. The factor of safety is obtained analytically and requires solution for the roots of a polynomial equation. Verification checks show exact agreement with existing solutions for simplified conditions, including Mononobe–Okabe dynamic active force. Numeric examples are provided to demonstrate the method and illustrate the importance of several parameters for stability of a landfill bottom liner system and reinforced soil retaining wall. The analytical solutions take compact form, provide insight for the two-wedge method, and offer good capability to tailor conditions for applications that can be suitably characterized by wedge failure.

Semi-brittle deformation of antigorite serpentinite under forearc mantle wedge conditions

Serpentinization of the mantle wedge immediately above a subducting slab is a key process controlling the strength contrast between the slab and the overriding nominally anhydrous (i.e., olivine-rich) mantle. To assess the influence of serpentinite rheology on the degree of decoupling between the slab and mantle wedge, we conducted axial compression tests on partially foliated antigorite serpentinite using a modified Griggs-type apparatus at confining pressures (Pc) of 1.0–2.5 GPa, temperatures (T) of 300–600 °C, and a strain rate of 1.5 × 10−5 s−1. With increasing temperature, the dependence of maximum differential stress (i.e., strength) on Pc changes from positive to flat, suggesting an increasingly important contribution from plastic deformation. Microstructural observations show that increasing temperature promotes a transition in deformation mechanisms from distributed inter- and intra-granular microcracking to comminution and kinking localized within a through-going shear zone. The ductile-to-brittle transition can be attributed to the operation of thermally activated processes such as dislocation glide and grain boundary sliding in aligned antigorite grains. Comparison with strength data for dunite demonstrates that in the semi-brittle regime, the strength contrast between antigorite and olivine is < 2, which suggests that a thin antigorite layer will have only a weak decoupling effect at the slab–mantle wedge interface.

Variant across-forearc compositions of slab-fluids recorded by serpentinites: Implications on the mobilization of FMEs from an active subduction zone (Mariana forearc)

Serpentinite mud volcanism in the Mariana forearc provides a window into the shallow portions of an active subduction zone. Fluid–rock interactions and related mass transfers into the mantle wedge can be assessed by studying the trace element compositions of slab-derived fluids and serpentinized mantle wedge materials brought to the seafloor by the serpentinite mud volcanoes. We investigated variably serpentinized ultramafic clasts from the Yinazao, Fantangisña, and Asùt Tesoru mud volcanoes recovered on International Ocean Discovery Program Expedition 366 to examine the transfer of fluid-mobile elements (FMEs) from the slab to the wedge. These mud volcanoes sample the slab–wedge interface at depths of ~13–18 km and estimated temperatures of 80–250 °C. Our samples represent the serpentinized forearc and exhibit a multi-phase serpentinization history, as apparent from microfabrics, mineralogy, and in situ major and trace elemental analyses of distinct generations of serpentine. Initial hydration of the forearc mantle occurred under reducing conditions by Si-rich fluids. Early serpentine is characterized by generally high concentrations of Li, Sr, Rb, Cs, and Ba. Subsequent fluid–rock interactions were driven by Si-rich and FME-poor fluids and at later stages by Si- and FME-poor fluids in the mud volcano conduits, the latter of which resulted in the abundant formation of Fe-rich brucite. Iowaite and hematite indicate that less reducing conditions prevailed during the alteration of clasts after their emplacement at the seafloor. Concentrations of B are generally high but our dataset does not allow distinguishing slab- from seawater-derived B.Serpentine from the shallow-sourced Yinazao exhibits high Rb/Cs ratios of ≤37, highest concentrations of Li, but lowest Rb, Sr, Ba, and Cs contents. The serpentinizing fluids were derived from expulsion of sedimentary pore waters and by the breakdown of opal in the subducted sediments. Serpentine at the intermediate-sourced Fantangisña has Rb/Cs ratios of <10, similar Li, Sr, and Ba concentrations as Yinazao, but higher Rb and Cs contents. These patterns likely reflect dewatering and FME-release from clays in the subducted sediments. Fluids at the deeply sourced Asùt Tesoru as well originate from clay-breakdown, but increased concentrations of Rb, Sr, Cs, and Ba are further indicative of beginning dehydration of altered oceanic crust.Including data from the South Chamorro serpentinite mud volcano (18 km slab depth; Kahl et al., 2015, Lithos), we provide a detailed record of slab dehydration reactions at shallow forearc depths and the related mobilization of FMEs as well as their transport into the mantle wedge. Our study demonstrates that slab-derived fluids undergo extensive alteration during the interaction with mantle wedge peridotite. Pore waters from the serpentinite mud volcanoes hence provide incomplete insight into the processes at depth; fluid signatures at the slab–wedge interface as well as their across-forearc changes are best recorded in early hydration products such as serpentine.

Wedge prism approach for simultaneous multichannel microscopy

Multichannel (multicolor) imaging has become a powerful technique in biology research for performing in vivo neuronal calcium imaging, colocalization of fluorescent labels, non-invasive pH measurement, and other procedures. We describe a novel add-on approach for simultaneous multichannel optical microscopy based on simple wedge prisms. Our device requires no alignment and is simple, robust, user-friendly, and less expensive than current commercial instruments based on switchable filters or dual-view strategies. Point spread function measurements and simulations in Zemax indicate a reduction in resolution in the direction orthogonal to the wedge interface and in the axial direction, without introducing aberration. These effects depend on the objective utilized and are most significant near the periphery of the field of view. We tested a two-channel device on C. elegans neurons in vivo and demonstrated comparable signals to a conventional dual-view instrument. We also tested a four-channel device on fixed chick embryo Brainbow samples and identified individual neurons by their spectra without extensive image postprocessing. Therefore, we believe that this technology has the potential for broad use in microscopy.

Combined Effect of Tides and Varying Inland Groundwater Input on Flow and Salinity Distribution in Unconfined Coastal Aquifers

AbstractTides and seasonally varying inland freshwater input, with different fluctuation periods, are important factors affecting flow and salt transport in coastal unconfined aquifers. These processes affect submarine groundwater discharge (SGD) and associated chemical transport to the sea. While the individual effects of these forcings have previously been studied, here we conducted physical experiments and numerical simulations to evaluate the interactions between varying inland freshwater input and tidal oscillations. Varying inland freshwater input was shown to induce significant water exchange across the aquifer‐sea interface as the saltwater wedge shifted landward and seaward over the fluctuation cycle. Tidal oscillations led to seawater circulations through the intertidal zone that also enhanced the density‐driven circulation, resulting in a significant increase in the total SGD. The combination of the tide and varying inland freshwater input, however, decreased the SGD components driven by the separate forcings (e.g., tides and density). Tides restricted the landward and seaward movement of the saltwater wedge in response to the varying inland freshwater input in addition to reducing the time delay between the varying freshwater input signal and landward‐seaward movement in the saltwater wedge interface. This study revealed the nonlinear interaction between tidal fluctuations and varying inland freshwater input will help to improve our understanding of SGD, seawater intrusion, and chemical transport in coastal unconfined aquifers.

H2O-rich mantle melting near the slab–wedge interface

To investigate the first melts of the mantle wedge in subduction zones and their relationship to primitive magmas erupted at arcs, the compositions of low degree melts of hydrous garnet lherzolite have been experimentally determined at 3.2 GPa over the temperature range of 925–1150 °C. Two starting compositions with variable H2O contents were studied; a subduction-enriched peridotite containing 0.61% Na2O, 0.16 K2O% (wt%) with 4.2 wt% H2O added (Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015) and an undepleted mantle peridotite (Hart and Zindler in Chem Geol 57:247–267, 1986) with 14.5% H2O added (Till et al. in Contrib Mineral Petrol 163:669–688, 2012). Saturating phases include olivine, orthopyroxene, clinopyroxene, garnet and rutile. Melting extent is tracked from near solidus (~ 5 wt%) to 25 wt%, which is close to or beyond the point where clinopyroxene and garnet are exhausted. The beginning of melting is a peritectic reaction where 0.54 orthopyroxene + 0.17 clinopyroxene + 0.13 garnet react to produce 1.0 liquid + 0.88 olivine. The melt production rate near the solidus is 0.1 wt% °C−1 and increases to 0.3 wt% °C−1 over the experimentally studied interval. These values are significantly lower than that observed for anhydrous lherzolite (~ 1 wt% °C−1). When melting through this reaction is calculated for a metasomatized lherzolite source, the rare earth element characteristics of the melt are similar to melts of an eclogite, as well as those observed in many subduction zone magmas. Moreover, since rutile is stable up to ~ 8 wt% melting, the first melts of a hydrous lherzolite source could also show strong high field strength element depletions as is observed in many subduction zone lavas. The silicate melts measured at the lowest temperatures and melting extents (< 10 wt%) are high silica andesites (56–60 wt% SiO2) and contain very low Ca/Al and high alkalis. These deep low degree andesitic melts, if added to experimentally produced hydrous liquids from melting (20–25 wt%) of harzburgite residues at shallow pressures (1.0–1.2 GPa, Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015), can match the compositional characteristics of primitive natural basaltic andesite and magnesian andesite lavas found globally in arcs. In addition to a silicate melt phase, there is a small amount of silicate dissolved in the H2O supercritical fluid that coexists with the silicate liquid and solids in our experiments. The composition of this fluid is in equilibrium with the Mg-rich minerals and it is granitic. The results presented here are used to develop a model for producing hydrous arc magmas. We hypothesize that mantle wedge melting produced by the flux of hydrous fluid from the slab occurs over a range of depths that begins at the base of the mantle wedge and ends at shallow mantle depths. These melts ascend and remain isolated until they mix in the shallow, hottest part of the mantle wedge. In this melting scenario, the metasomatic “slab melt” contributions to arc magmas is small (~ 5 wt%), but its effect on the alkali, REE and incompatible trace element budget of the derivative magmas is large and able to reproduce the trace elemental characteristics of the primitive andesites. Higher proportions of slab or sediment melt do not resemble primitive high magnesian arc andesites and basaltic andesites.

Electron diffraction determination of 11.5 Å and HySo structures: Candidate water carriers to the Upper Mantle

The dehydration reactions of minerals in subduction zones strongly control geological processes, such as arc volcanism, earthquakes, serpentinization, or geochemical transport of incompatible elements. In aluminum-bearing systems, chlorite is considered the most important hydrous phase at the top of the subducting plate, and significant amount of water is released after its decomposition. However, the dehydration mechanism is not fully understood, and additional hydrates are stabilized by the presence of Al beyond the stability field of chlorite. We applied here a cutting-edge analytical approach to characterize the experimental rocks synthesized at the high pressures and temperatures matching with deep subduction conditions in the upper mantle. Fast electron diffraction tomography and high-resolution synchrotron X-ray diffraction allowed the identification and the successful structure solution of two new hydrous phases formed as dehydration product of chlorite. The 11.5 A phase, Mg 6 Al(OH) 7 (SiO 4 ) 2 , is a hydrous layer structure. It presents incomplete tetrahedral sheets and face-sharing magnesium and aluminum octahedra. The structure has a higher Mg/Si ratio compared to chlorite, and a significantly higher density (ρ 0 = 2.93 g/cm 3 ) and bulk modulus [ K 0 = 108.3(8) GPa], and it incorporates 13 wt% of water. The HySo phase, Mg 3 Al(OH) 3 (Si 2 O 7 ), is a dense layered sorosilicate, [ρ 0 = 3.13 g/cm 3 and K 0 = 120.6(6) GPa] with an average water content of 8.5 wt%. These phases indicate that water release process is highly complex, and may proceed with multistep dehydration, involving these layer structures whose features well match the high-shear zones present at the slab-mantle wedge interface.

Controls on the stability and composition of amphibole in the Earth’s mantle

Presented here is a suite of new experiments aimed at quantifying the effects of pressure, temperature, bulk composition, and H2O content on the stability and composition of amphibole in the Earth’s mantle. Experiments have been performed from 2 to 4 GPa and 950 to 1100 °C on fertile and depleted mantle compositions. H2O contents of most experiments are 0.65 wt%. In the fertile mantle composition, pargasitic amphibole is stable up to ~3.8 GPa at 1000 °C, approximately 0.5 GPa higher than any previous study. The upper stability limit of amphibole in depleted mantle is 0.7 GPa and 40 °C lower than in fertile mantle. The addition of 3 wt% H2O to fertile mantle destabilizes amphibole by 0.5 GPa and 40 °C relative to the 0.65 wt% H2O experiments. Compared to existing experiments on amphibole stability, these experiments indicate that pargasitic amphibole may be stable in mantle lithosphere to almost 4 GPa (0.5 GPa higher (15 km deeper) than previously thought). The extremely strong destabilizing effect of H2O suggests that deeper portions of the strongly fluid-fluxed mantle wedge may be amphibole-free even at low temperatures near the slab–wedge interface. The molar alkali content of amphibole is shown to be a linear function (R 2 = 0.98) of pressure and temperature and is relatively insensitive to bulk compositional differences between fertile and depleted mantle. This relationship is used to produce an empirical thermobarometer for pargasite-bearing spinel and garnet lherzolites. Comparison to existing experimental data shows that this thermobarometer has predictive ability over the pressure range of 1–4 GPa. Comparisons with pressure–temperature estimates of garnet + amphibole peridotites further corroborate the applicability of this thermobarometer for natural samples. Pressure estimates are presented for four examples of metasomatized spinel peridotites otherwise lacking pressure information, and future avenues for refinement of the thermobarometer are discussed.

Boundary layer transition mechanisms behind a micro-ramp

The early stage of three-dimensional laminar-to-turbulent transition behind a micro-ramp is studied in the incompressible regime using tomographic particle image velocimetry. Experiments are conducted at supercritical micro-ramp height $h$ based Reynolds number $Re_{h}=1170$. The measurement domain encompasses 6 ramp widths spanwise and 73 ramp heights streamwise. The mean flow topology reveals the underlying vortex structure of the wake flow with multiple pairs of streamwise counter-rotating vortices visualized by streamwise vorticity. The primary pair generates a vigorous upwash motion in the symmetry plane with a pronounced momentum deficit. A secondary vortex pair is induced closer to the wall. The tertiary and even further vortices maintain a streamwise orientation, but are produced progressively outwards of the secondary pair and follow a wedge-type pattern. The instantaneous flow pattern reveals that the earliest unstable mode of the wake features arc-like Kelvin–Helmholtz (K–H) vortices in the separated shear layer. Under the influence of the K–H vortices, the wake exhibits a high level of fluctuations with a pulsatile mode for the streamwise momentum deficit. The K–H vortices are lifted up due to the upwash induced by the quasi-streamwise vortex pair, while they appear to undergo pairing, distortion and finally breakdown. Immediately downstream, a streamwise interval of relatively low vortical activity separates the end of the K–H region from the formation of new hairpin vortices close to the wall. The latter vortex structures originate from the region of maximum wall shear, induced by the secondary vortex pair causing strong ejection events which transport low-speed flow upwards. The whole pattern features a cascade of hairpin vortices along a turbulent/non-turbulent interface. The wedge-shaped cascade signifies the formation of a turbulent wedge. The turbulent properties of the wake are inspected with the spatial distribution of the velocity fluctuations and turbulence production in the developing boundary layer. Inside the wedge region, the velocity fluctuations approach quasi-spanwise homogeneity, indicating the development towards a turbulent boundary layer. The wedge interface is characterized by a localized higher level of velocity fluctuations and turbulence production, associated to the deflection of the shear layer close to the wall and the onset of coherent hairpin vortices inducing localized large-scale ejections.

Orthopyroxene-rich Rocks from the Sanbagawa Belt (SW Japan): Fluid–Rock Interaction in the Forearc Slab–Mantle Wedge Interface

The Western Iratsu body of the Sanbagawa belt (SW Japan) is a mafic–ultramafic complex that underwent an initial metamorphism in the amphibolite facies and a subsequent metamorphism in the eclogite facies, and represents a fossil forearc slab–mantle wedge interface in a developing subduction zone. Two generations of orthopyroxene (Opx1 and Opx2) that were formed during the amphibolite-facies (antigorite unstable) and eclogite-facies (antigorite stable) stages can be recognized in the ultramafic domain. Opx1-rich rocks contain Ni-rich relict olivine (up to 0 7 wt % NiO) and grade into dunite, suggesting that they represent metasomatic rocks derived from dunite. Opx1 can be subdivided into two types: one (Opx1L) constitutes replacive harzburgite to orthopyroxenite layers and the other (Opx1V) occurs in metasomatic reaction veins in dunite. Relatively high formation temperatures ( 750 C) of Opx1L imply that the relevant metasomatism in the ultramafic domain took place before the juxtaposition with the mafic domain preserved in the Western Iratsu body. Textural relationships and mineral trace element data suggest that Opx1L-rich rocks were formed by reactive porous infiltration of a slab-derived hydrous melt or solute-rich fluid into dunite. Subsequently, Opx1V-rich veins were formed by a prolonged flux of a Si-rich aqueous fluid (sourced from the mafic domain) through brittle fractures in dunite during the amphibolite-facies metamorphism ( 660 C and 1 2 GPa). The initial formation of Opx1V-chlorite-rich selvages along the fluid conduits is likely to have limited the reaction between a Si-rich crustal fluid and host dunite, and this process can be important during the early transportation of slab-derived components into the mantle wedge. Lastly, Opx1L crystals locally show a textural replacement by Opx2 together with antigorite, indicating recrystallization in the eclogite facies ( 620 C and 1 6–1 8 GPa). The Opx2-forming reaction is mainly localized in ductile shear zones, which correspond to major fluid pathways in the partially serpentinized forearc mantle.

Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths

We image the field enhancement at Ag nanostructures using femtosecond laser pulses with a center wavelength of 1.55 μm. Imaging is based on non-linear photoemission observed in a photoemission electron microscope (PEEM). The images are directly compared to ultra violet PEEM and scanning electron microscopy (SEM) imaging of the same structures. Further, we have carried out atomic scale scanning tunneling microscopy on the same type of Ag nanostructures and on the Au substrate. Measuring the photoelectron spectrum from individual Ag particles shows a larger contribution from higher order photoemission processes above the work function threshold than would be predicted by a fully perturbative model, consistent with recent results using shorter wavelengths. Investigating a wide selection of both Ag nanoparticles and nanowires, field enhancement is observed from 30% of the Ag nanoparticles and from none of the nanowires. No laser-induced damage is observed of the nanostructures neither during the PEEM experiments nor in subsequent SEM analysis. By direct comparison of SEM and PEEM images of the same nanostructures, we can conclude that the field enhancement is independent of the average nanostructure size and shape. Instead, we propose that the variations in observed field enhancement could originate from the wedge interface between the substrate and particles electrically connected to the substrate.

Numerical simulations of steady-state salinity distribution and submarine groundwater discharges in homogeneous anisotropic coastal aquifers

Steady-state seawater–groundwater interactions are simulated for homogeneous, anisotropic unconfined coastal aquifers using the two-dimensional numerical model MARUN. The spatial salinity distributions are approximately independent of the horizontal hydraulic conductivity Kx when αLKxKz⩾10−6m2/s. Here Kz is vertical hydraulic conductivity and αL is longitudinal dispersivity. Both the fresh groundwater discharge rate Qf and seawater recirculation rate Qs depend linearly on Kx. These conclusions are validated by a semi-analytical method. The increase of anisotropy ratio pushes the saltwater wedge interface seaward and reduces Qs. The increment of the longitudinal/transverse dispersivity ratio with fixed longitudinal dispersivity decreases the slope of isosalines and Qs. When the seabed slope angle θ increases from 0.01 to π/2, the freshwater–seawater interface moves landward. The increment of θ increases Qs slightly when θ<π/4 and reduces Qs when θ>π/4. The inland recharge rate Qf is independent of the seabed slope.

Introduction to the special issue on SFB 574 “Volatiles and fluids in subduction zones: climate feedback and trigger mechanisms for natural disasters”

combine field work on land with marine cruises probing the seafloor. During the course of the SFB 574, many different aspects of subduction zone processes were investigated. Geophysical investigations identified and quantified the input of water through hydration of the bend-faulted subducting plate as far as mantle depths. This process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling. These studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle. Subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone. Compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated. The fluids, largely generated by clay mineral transformation, are expelled through the forearc by splay faults. At the seafloor, the sites of cold seeps, often associated with mud volcanoes, are populated by biota that control the carbon transfer to the ocean. The most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes. The largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts. Petrological analyses of ancient, exhumed subduction zone metamorphic rocks revealed the important role of pervasive, typically channelized fluids in high-pressure metamorphic reactions of dewatering subducted igneous crust and mantle. Slab-derived fluids hydrate the mantle wedge along the slab–wedge interface and form a subduction channel in which mixing of different fluids and fluid–rock interaction causes metasomatic overprinting. The element redistributions associated with the liberation of fluids from Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in subduction zone tectonic, hydrological, metamorphic and magmatic processes, and resulting hazards. The main areas of research were the subduction zones of Central America and southern Chile. Both extend across shorelines from deepsea trenches to arc-volcano summits and thus required to

Rheological contrast between serpentine species and implications for slab–mantle wedge decoupling

Serpentine is considered the main hydrous mineral to cause significant weakening at the slab–mantle interface; however, despite the possible presence of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine species, respectively) at the base of the mantle wedge, the differences in rheological properties between these two serpentine types are poorly understood. To investigate the effect of serpentine speciation on slab–mantle decoupling, we performed a series of two-layer shear deformation experiments on liz/ctl, antigorite, and olivine at P = 1 GPa and T = 250–300 °C, using a Griggs-type solid-medium apparatus. With increasing shear strain (γ up to ~ 5), the liz/ctl and antigorite samples evolved to deform predominantly by slip or glide on the (001) basal plane, whereas the olivine sample deformed by a combination of brittle and crystal-plastic mechanisms. The contrasting amount of rotation of strain markers between layers demonstrates that the shear strain of liz/ctl is significantly lower than that of antigorite, possibly arising from differences in interlayer bond strength. The strain contrasts between serpentines and olivine were γliz/ctl/γol = 9–12 and γatg/γol = 1–2. Our results indicate that assuming a thin, serpentinized layer upon a subducting slab, significant decoupling can occur only where liz/ctl is stable in the layer. Thus, the distribution of serpentine species in the hydrated mantle wedge is a critical factor that controls the degree of decoupling along the slab–wedge interface, at depths below the seismogenic zone.