Seismic velocity is a function of bulk vibrational properties of the media, whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase. In the absence of a minor conducting phase then the two should be inter‐relatable primarily due to their sensitivity to temperature variation. We develop expressions between shear wave velocity and resistivity for varying temperature, composition, and water content based on knowledge from two kimberlite fields: Jagersfontein (Kaapvaal Craton) and Gibeon (Rehoboth Terrane). We test the expressions through comparison between a new high‐resolution regional seismic model, derived from surface wave inversion of earthquake data from Africa and the surrounding regions, and a new electrical image from magnetotelluric (MT) data recorded in SAMTEX (Southern African Magnetotelluric Experiment). The data‐defined robust linear regression between the two is found to be statistically identical to the laboratory‐defined expression for 40 wt ppm water in olivine. Cluster analysis defines five clusters that are all geographically distinct and tectonically relate to (i) fast, cold, and variably wet Kaapvaal Craton, (ii) fast and wet central Botswana, (iii) slow, warm, and wet Rehoboth Terrane, (iv) moderately fast, cold, and very dry southernmost Angola Craton, and (v) slow, warm, and somewhat dry Damara Belt. From the linear regression expression and the MT image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations. The differences between the two demonstrate that the linear relationship between V s and resistivity is appropriate for over 80% of Southern Africa. Finally, using the regressions for varying water content, we infer water content in olivine across Southern Africa.
The determination of the present-day thermal and compositional structure of the lithospheric and sub-lithospheric upper mantle is one of the fundamental goals in modern lithospheric modelling. In this context, a detailed knowledge of the thermophysical properties of mantle minerals, their temperature and pressure dependence, and their equilibrium assemblages within the lithospheric mantle is crucial. The lithosphere-asthenosphere boundary (LAB) can be defined in various ways depending on the proxy used to constrain it: seismic velocity, seismic anisotropy, electrical resistivity, composition or temperature. The extent to which these alternative definitions can be made compatible is still a matter of much debate. In a case-study from southern Africa we examine the Proterozoic Rehoboth Terrane and the Archaean Kaapvaal Craton. Although the presence of many kimberlite pipes within the Rehoboth Terrane has been reported, none of them has yet proved to be diamondiferous, suggesting variations in the LAB depth and/or mantle composition between the two lithospheric domains. The LAB of the two terranes has been investigated using the software package LitMod. This software combines petrological and geophysical modelling of the lithosphere and sub-lithospheric upper mantle within an internally consistent thermodynamic-geophysical framework, where all relevant properties are functions of temperature, pressure and composition. In particular, LitMod is used in this work to define realistic temperature, pressure, density and electrical conductivity distributions within the upper mantle, and to characterize the mineral assemblages given bulk chemical compositions as well as water contents. This allows us to determine the topography (local isostasy), surface heat flow and magnetotelluric responses for different models of lithospheric composition and structure. Critically, we also assess the extent to which the lithospheric and sub-lithospheric mantle might be wet or dry within each terrane and the implications of the (potentially depth-variable) hydration state with respect to the lithospheric evolution of each terrane. Finally, as part of a work in progress, we analyze the study region within the framework of a new multiobservable probabilistic inversion method particularly designed for high-resolution (regional) thermal and compositional mapping of the lithosphere and sublithospheric upper mantle.
A 1400 km-long, 2-D magnetotelluric (MT) profile across the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The profile indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred conductive geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mWm-2), Western Kimberley Block 190 km (44 mWm-2), Rehoboth Terrane 180 km (45 mWm-2) and Ghanzi-Chobe/Damara Belt 160 km (48 mWm-2). Previously published mantle xenolith pressure-temperature (P-T) arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40 – 42 mWm-2 ) very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some (unconstrained) time prior to the Mesozoic eruption of the kimberlites. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138 – 167 km. It appears most likely that the chemical depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is accounted for by refertilisation of the lower lithospheric mantle.
The Southern African Magnetotelluric Experiment (SAMTEX) is the largest ever land-based magnetotelluric (MT) project. The main objective of the project is to define the geo-electric structure across the region in order to gain a better understanding of Archean and Proterozoic tectonic processes. Only the MT profiles crossing the Rehoboth Terrane, the Neoproterozoic Ghanzi-Chobe/Damara belts (collectively termed the DMB) and the southern Angola craton are the focus of this study. One of the ways in which geo-electrical structural information is obtained is by detailed analysis of the measured impedance tensor. The Groom and Bailey decomposition technique was applied to the MT data and indicates significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). The geo-electric strikes are generally parallel to the north-east trending tectonic fabric as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in the lithospheric structure. The Rehoboth terrane, south of the DMB, exhibits a strongly one dimensional (1D) to moderate two dimensional (2D) structure, with preferred strike directions in the range 200-450 20for the crust-mantle period (i.e. depth) range, indicating little crust-mantle decoupling. The DMB appears to be moderately 2D at lower crustal and upper mantle depths (10-100 s) with no consistent/preferred strike direction and significant phase differences between the conductive and resistive directions. North of the DMB and into the Angola craton there are significant variations in geo-electric strike direction and dimensionality at most sites for lower-crustal and upper mantle lithosphere. Our results further indicate that the profiles have to be divided into smaller areas having similar strike directions to allow for 2D modelling and inversion.
The academia-government-industry collaborative IRETHERM project (www.iretherm.ie), funded by Science Foundation Ireland, is developing a strategic understanding of All-Ireland's (north and south) deep geothermal energy potential through integrated modelling of new and existing geophysical and geological data. One aspect of IRETHERM’s research focuses on Ireland’s radiothermal granites, where increased concentrations of radioelements provide elevated heat-production (HP), surface heat-flow (SHF) and subsurface temperatures. An understanding of the contribution of granites to the thermal field of Ireland is important for assessing the geothermal energy potential of this low-enthalpy setting. This study focuses on the Galway granite in western Ireland, and the Leinster and the buried Kentstown granites in eastern Ireland. Shallow (less than 250 m) boreholes were drilled into the exposed Caledonian Leinster and Galway granites as part of a 1980’s geothermal project. These studies yielded HP = 2-3 μWm -3 and HF = 80 mWm -2 at the Sally Gap borehole in the Northern Units of the Leinster granite, located to the SW of Dublin. In the Galway granite batholith, on the west coast of Ireland, the CostelloeMurvey granite returned HP = 7 μWm -3 and HF = 77 mWm -2 , measured at the Rossaveal borehole. The buried Kentstown granite, 35 km NW of Dublin, has an associated negative Bouguer anomaly and was intersected by two mineral exploration boreholes at depths of 660 m and 490 m. Heat production is measured at 2.4 μWm -3 in core samples taken from the weathered top 30 m of the granite. The core of this study consists of analysis, modeling and interpretation from a program of magnetotelluric (MT) and audiomagnetotelluric (AMT) data acquired across the three granite bodies. MT and AMT data were collected at 59 locations along two profiles over the Leinster granite. Over the Galway granite, MT and AMT data were collected at a total of 75 sites (33 consist of only AMT data acquisition, with both MT and AMT recorded at the remaining 42). MT and AMT data have been acquired along a profile at 22 locations over the Kentstown granite. The MT and AMT data will be integrated with gravity and seismic refraction data (in the case of the Leinster granite) to identify deeply-penetrating faults, which may provide conduits for hydrothermal fluids, and to produce a robust estimation of the volumetric extent of the granites, which is crucial for defining their thermal contribution and therefore geothermal energy potential. Thermal conductivity and geochemical data will be incorporated to constrain the heat contribution of granites to the Irish crust.
Significance X-ray computed tomography (CT) imaging has become popular for investigating, nondestructively and three-dimensionally, both external and internal structures of various specimens. However, the limited resolution of conventional laboratory-based CT systems (≥500 nm) still hampers the detailed visualization of features on the low nanometer level. We present a laboratory CT device and data processing pipeline to routinely and efficiently generate high-resolution 3D data (≈100 nm) without requiring synchrotron radiation facilities. Our setup is especially relevant for conducting detailed analysis of very small biological samples, as demonstrated for a walking appendage of a velvet worm. Comparative analyses of our CT data with those obtained from other popular imaging methods highlight the advantages and future applicability of the nanoCT setup.
'/%3e%3cpath d='M200 752.362h200v300H200z' style='fill:%23fff;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3cpath d='M400 752.362h200v300H400z' style='fill:%23ff883e;fill-opacity:1;stroke:none' transform='translate(0 -752.362)'/%3e%3c/svg%3e)
