This study presents a method for measuring the electrical conductivity of porous rock materials in situ under a shallow Earth crust environment simulated according to temperature, confining pressure, and liquid water saturation in a high-temperature autoclave. The sample was first encased within a poly tetra fluoroethylene container with two Pt wires leading out and was then placed into the high-temperature autoclave. The lead wires were connected to an external measurement system after passing through the autoclave sealing plug. The electrical conductivity of sandstone was measured under different temperatures (30, 60, 90, 120, 150 °C), liquid water saturation levels (36%, 51%, 100%), and 2 MPa by using this method. The electrical conductivity of the sandstone samples increased with increasing temperature and also increased as the level of water saturation increased. All the results agreed well with the Arrhenius relationship, Archie’s law, and previous experimental study. This method can be used to measure other kinds of porous water-containing rocks, and the results can be applied in geothermal/oil research.
Accurate interpretation of field electromagnetic sounding signals requires laboratory conductivity measurements of rocks under simulated physicochemical conditions at specific underground depths. Current laboratory measurements of conductivity corresponding to geophysical exploration depth of resource and energy still face challenges, including the lack of a proper high-temperature and high pressure simulation platform, or an accurate in situ electrical conductivity measurement circuit adaptable to the former. In this work, we developed a new system, including an autoclave device capable of simulating temperatures up to 350°C and pressures up to 350 MPa, simultaneously (approximately 13 km depth), and an accurate electrical conductivity measurement circuit embedded in the autoclave device. Feasibility tests were performed on saturated sandstones. The observed temperature dependence of electrical conductivity and the formation factor of our samples were consistent with previous studies, demonstrating the effectiveness of our system and it can be widely used in future geophysical exploration of shallow crust.
The Xuanwei Formation is widely distributed in western Guizhou Province, NW China, the lower section of which is primarily composed of gray-white kaolinitic claystone interbedded with thin layers of grayish black carbonaceous mudstone that are extremely enriched with rare earth elements. In order to determine the distribution patterns and existing status of ore-forming elements in these rocks, careful field investigations were performed along a selected geological profile and rock samples were collected and studied in terms of mineralogical and geochemical characteristics. The results show that: 1) REEs are primarily enriched in the grayish white kaolinitic clay sediments and grayish black carbonaceous mudstone. Mineralogical analyses revealed kaolinite as the major mineral in rocks along with smaller amounts of smectite, illite, boehmite, hornblende, pyrophyllite, calcite, dolomite and/or iron-bearing minerals, as well as a certain proportion of feldspar, quartz crystal debris and non-crystal debris. 2) ∑ REE contents are 89.0 to 9965 ppm with an average of 1312 ppm. The thickness of the host rock with ∑ REE higher than 1300 ppm is more than 4 m, which is referred to as the "REE-enriched layer". 3) The REE contents of bulk rocks exhibit a negative correlation with kaolinite, positive correlations with boehmite, hornblende and iron-bearing minerals, and weak positive correlations with smectite, illite and pyrophyllite, indicating that the REE might exist in an ion adsorption state in the space between the layers of clay minerals. 4) Compared with the underlying Emeishan Basalts, the REE patterns of samples are quite similar but are enriched in both LREE and HREE. The degree of enrichment of HREE is relatively high. Based on these results, a model is suggested where the REE-enriched layers originated from the Emeishan Basalts and were controlled by the transportation and deposition of detritus from a paleo-weathering crust. The hard clay rocks have a significant resource potential, as the contents of REE, Ga, Nb and Zr are considerably higher than those in the weathering crust type of REE deposit.
Abstract During the early Ediacaran, there was a large influx of phosphorus into the oceans and a resultant high phosphorus concentration in seawater, where multicellular eukaryotes may have been the primary type of marine productivity. The eukaryotes could play a critical role in regulating Zn cycling and isotopes. To establish Zn geochemical cycling patterns in the phosphorus‐rich ocean, this study investigates Zn isotopic signatures of shallow water phosphorite that contains phosphatized microfossils (Weng'an biota) and deep‐water shale from the Doushantuo Formation. Our results indicate that phosphorite commonly preserves heavier Zn isotope composition, with an average of 0.80‰. The positive δ 66 Zn values in phosphorites may be ascribed to Zn isotope fractionation associated with the complexation of Zn with phosphate and the adsorption of isotopically heavy Zn onto Fe‐Mn oxides and organism's surfaces. We argue that phosphorite may represent an important sink of isotopically heavy Zn in a phosphorus‐rich ocean during Earth history. Meanwhile, deep‐water organic‐rich shale shows an enrichment of isotopically light Zn with an average of 0.23‰, which may be attributed to sulfide precipitation in mid‐depth environment. The organic‐rich shale may represent an isotopically light Zn sink. In addition, the highest δ 66 Zn value (0.45‰) in a euxinic black shale may indicate that Zn isotope signal of anoxic deep water is similar to that of modern deep seawater. If that is the case, it suggests that Zn geochemical cycling in the early Ediacaran oceans was similar to that of modern oceans.
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