Eclogite thermobarometry: The consistency between conventional thermobarometry and forward phase-equilibrium modelling

Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature (P–T) conditions of eclogites is complex as they commonly display high-variance mineral assemblages, and the mineral compositions only vary slightly with P–T. In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase-equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges of P–T conditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigated P–T conditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for deriving P–T from eclogites. Although thermobarometry is known to be sensitive to the assumed X_Fe³⁺ of a rock (or mineral), the relative differences between methods are not particularly sensitive to the choice of bulk-rock X_Fe³⁺, except at high temperatures (>650°C, amphibole absent) and for very large differences in assumed X_Fe³⁺ (0–0.5). We find that the most important difference between approaches is the activity–composition (a–x) relations, as opposed to the end-member thermodynamic data or other aspects of experimental calibration. When equivalent a–x relations are used in the conventional barometer, P calculations are nearly identical to phase-equilibrium models (ΔP < 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximum P–T with conventional thermobarometric approaches (e.g. using the highest aGrs² × aPrp in garnet and Si content in phengite, and the lowest aDi in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically bias P–T calculations. Assuming method accuracy, geological meaningful P_max at a typical eclogite facies temperature of ~660°C will be obtained by using the greatest aDi, aCel, and aPrp and lowest aGrs and aMs; garnet and clinopyroxene with the lowest Fe²⁺/Mg ratios may yield geological meaningful T_max at a typical eclogite facies pressure of 2.5 GPa.     

边坡离心模型试验中的降雨模拟研究

为了在离心机试验中模拟降雨引起的滑坡,设计和使用了一套降雨模拟系统。这套系统的构成主要是针对离心试验的特殊要求进行设计的。所采用的特殊技术可以均匀分配雨水,将雨滴的尺寸效益和科氏加速度引起的雨滴偏移最小化,实现了不同强度和历时的降雨模拟。通过一个降雨条件下砂土边坡的离心模型试验,成功应用了该套降雨模拟系统,研究了边坡中的降雨入渗过程。     

Water balance components estimation under scenarios of land cover change in the Vea catchment,West Africa

ABSTRACT

The need for a detailed investigation of the Vea catchment water balance components cannot be overemphasized due to its accelerated land-cover dynamics and the associated impacts on the hydrological processes. This study assessed the possible consequences of land-use change scenarios (i.e. business as usual, BAU, and afforestation for the year 2025) compared to the 2016 baseline on the Vea catchment’s water balance components using the Soil and Water Assessment Tool (SWAT) model. The data used include daily climate and discharge, soil and land use/land cover maps. The results indicate that the mean annual water yield may increase by 9.1% under the BAU scenario but decrease by 2.7% under the afforestation scenario; actual evapotranspiration would decrease under BAU but increase under afforestation; and groundwater recharge may increase under both scenarios but would be more pronounced under the afforestation scenario. These outcomes highlight the significance of land-cover dynamics in water resource management and planning at the catchment.     

Severe soil erosion during a 3‐day exceptional rainfall event: combining modelling and field data for a fallow cereal field

Exceptional rainfall events cause significant losses of soil, although few studies have addressed the validation of model predictions at field scale during severe erosive episodes. In this study, we evaluate the predictive ability of the enhanced Soil Erosion and Redistribution Tool (SERT‐2014) model for mapping and quantifying soil erosion during the exceptional rainfall event (~235 mm) that affected the Central Spanish Pyrenees in October 2012. The capacity of the simulation model is evaluated in a fallow cereal field (1.9 ha) at a high spatial scale (1 × 1 m). Validation was performed with field‐quantified rates of soil loss in the rills and ephemeral gullies and also with a detailed map of soil redistribution. The SERT‐2014 model was run for the six rainfall sub‐events that made up the exceptional event, simulating the different hydrological responses of soils with maximum runoff depths ranging between 40 and 1017 mm. Predicted average and maximum soil erosion was 11 and 117 Mg ha^?1 event^?1, respectively. Total soil loss and sediment yield to the La Reina gully amounted to 16.3 and 9.0 Mg event^?1. These rates are in agreement with field estimations of soil loss of 20.0 Mg event^?1. Most soil loss (86%) occurred during the first sub‐event. Although soil accumulation was overestimated in the first sub‐event because of the large amount of detached soil, the enhanced SERT‐2014 model successfully predicted the different spatial patterns and values of soil redistribution for each sub‐event. Further research should focus on stream transport capacity. Copyright © 2014 John Wiley & Sons, Ltd.     

An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

地下RC结构物地震响应特征土工离心试验的模拟

地下结构物抗震稳定性的研究具有重要实际意义。基于动态土工离心试验,应用有效应力法分析了不同水平地震作用下土层和地下RC结构物的最大水平位移、不同埋深的土层加速度和超孔隙水压力变化规律,以及结构物的破坏发展特征,并进一步与试验结果进行了相互验证和对比,取得了较好效果,为重要地下结构物的抗震设计提供了依据和参考。     

Anomalous lithospheric structure of Northern Juan de Fuca plate — a consequence of oceanic rift propagation?

Anomalous crustal and upper mantle structure of northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the North American continental margin. The velocity model is constructed via ray tracing and conforms to first-arrival amplitude observations and travel time picks of direct, converted and reflected phases. Between sub-sediment depths of 3 to 11 km, depths normally associated with the lower crust and upper oceanic mantle, the final model shows that compressional-wave velocities decrease significantly from southeast to northwest along the profile. At sub-sediment depths of 11 km at the northwestern end of the profile, P-wave velocities are as low as 7.2 km/s. A complementary 2-D gravity model using the geometry of the velocity model and velocity–density relationships characteristic of oceanic crust is produced. The high densities required to match the gravity field indicate the presence of peridotites containing 25–30% serpentine by volume, rather than excess gabbroic crust, within the deep low velocity zone. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault. We propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift.     

Modelling of Ocean Shallow Convection under Fast Ice in the Arctic

Ocean convection in the Antarctic has been studied many times and has been revealed to be responsible for ice-cover reduction. In the Arctic, proof of that phenomenon has not been documented. It is believed that this phenomenon happens on a smaller scale in the Arctic when local circulation of deep warmer water melts and slows ice production. An example of this is the North Water (NOW) polynya in northern Baffin Bay. A polynya is an area of open water in an otherwise ice-covered area. As ice forms under the fast ice near the boundary of the polynya, ocean salts (brine) are ejected from the newly formed ice. This water, which has an increased concentration of salt, sinks and is replaced by warmer water from below, and this slows ice formation. In our study a coupled one-dimensional thermodynamic snow–fast ice model incorporating ocean heat flux input via a shallow convection model was used. Ice thickness was calculated using a thermodynamic model that included a current-induced entrainment model and a convection model to account for brine rejection during ice growth. Atmospheric observations from Grise Fiord and Thule and ocean profiles around the NOW polynya near these sites were used as input to the model. This purely thermodynamic study enables us to obtain ice thickness values that can be compared with qualitative observations. This modelling study compares two sites related to the NOW polynya. The results indicate that the shallow convection model simulates the reduction of fast ice near Thule but not near Grise Fiord.