Improving the Hydrogeologic Conceptualization of a Remote Semi-Arid Palaeovalley Groundwater System using Airborne Electromagnetics, Seismic Refraction and Reflection, and Downhole Nuclear Magnetic Resonance

A hydrogeologic conceptualization is critical to understand, manage, protect, and sustain groundwater resources, especially in regions where data are sparse, and accessibility is difficult. We used airborne electromagnetic (AEM), shallow seismic reflection and refraction data, and downhole nuclear magnetic resonance (NMR) logs to improve our understanding of an arid groundwater system influenced by palaeovalleys. In the current hydrogeologic conceptualization it is unknown if the palaeovalley and underlying bedrock aquifers are connected. We focused on defining the spatial distribution of saprolite, which is the layer of chemically altered rock separating the palaeovalley and bedrock aquifers. The AEM data provided an estimate of the top of saprolite but failed to effectively image the bottom. In contrast, the seismic data provided an estimate of the bottom of saprolite but failed to image the top. This unique geophysical combination of electrical and elastic data allowed us to map saprolite thickness in detail along a 1.7 km long transect that runs perpendicular the main trunk of a well-defined palaeovalley. We show that the palaeovalley is lined with a heterogenous layer of saprolite (3-120 m thick) that is thickest near the palaeovalley edges. Despite the variability, only a small percentage of the bedrock aquifer (8-17%) is in contact with the palaeovalley aquifer. Furthermore, the lack of an elastic boundary at the top of saprolite suggests that the porosity of the saprolite is similar to the palaeovalley sediments. An observation that is supported by the downhole NMR water contents. The electrical change at the top of saprolite is caused by a change in pore structure associated with the difference of weathering in situ versus transported materials. Our geophysical data suggest that the saprolite acts as an aquitard limiting groundwater exchange between the palaeovalley and bedrock aquifers.

Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

The occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.

Ramsauer–Townsend minimum in electron scattering from CF$$_4$$: modified effective range analysis

Elastic cross sections for electron scattering on tetrafluoromethane (CF$$_4$$ 4 ) from 0 up to 5 eV energy are analyzed using semi-analytical approach to the modified effective range theory (MERT). It is shown that energy and angular variations of differential, integral and momentum transfer cross sections can be parameterized accurately by six MERT coefficients up to the energy region of the resonant scattering. In particular, the model is used to determine the depth and the position of the Ramsauer–Townsend minimum as well as the s-wave scattering length. Moreover, we investigate the influence of the dipole polarizability value on the predictions of present model. To further validate our approach, the elastic data are combined with the Born-dipole cross sections for vibrational excitations as the input data for Monte Carlo simulation of electron swarm coefficients. Graphic Abstract

Linking elastic and electrical properties of rocks using cross-property DEM

Summary Combining electrical and elastic measurements can be instrumental in lowering the uncertainty of subsurface characterisation. Many commonly used rock physics relations for joint electrical-elastic properties are at least partly empirical, and often rely on the estimation of porosity as an intermediate step. We combine differential effective medium schemes which relate respectively elastic and electrical properties to porosity and pore shape. The resulting expressions are independent of porosity, depending only on pore aspect ratio. Analysis of published joint electrical-elastic data shows that a single aspect ratio model performs well for clean sandstones, allowing us to model Vp/Vs ratios as a function of resistivity. Clay-bearing sandstones are more complex, but our modelling can still identify the correct trends. We speculate about the potential to extend our approach to produce additional cross-property relations.