Solid Earth Sciences最新文献

Metasomatic zircons in ultramafic rocks are crucial in tracking orogenic processes. Metasomatic zircons are conceived to form by local zircon saturation resulting from effective Zr transport during metamorphic events and incomplete Zr accommodation in mantle minerals. However, the accommodation capacity of Zr in mantle minerals remains unknown, hampering our understanding of zircon genesis in ultramafic rocks. Here we performed experiments at 2.0−6.0 GPa and 1050−1200 °C and determined Zr content (solubility) in the mantle minerals at zircon saturation (ZCZS). The results show that ZCZS values for the main mantle minerals are olivine (ol, 3.68 ± 0.27 ppm) < orthopyroxene (opx, 4.67 ± 0.23 ppm) < clinopyroxene (cpx, 28−1403 ppm) ≤ garnet (grt, 294−2222 ppm) with ZCZS for amphibole (amp, 189−546 ppm) and magnetite (mag, 263−630 ppm) overlapping that for cpx and grt under the experimental conditions. The significant variations of ZCZS for cpx and grt depends on mineral composition and P−T conditions. In detail, the ZCZS for cpx increases with temperature, Al₂O₃ content of cpx, and decreases with pressure; that for grt increases with temperature and X_Alm (molar fraction of almandine) in garnet. Important implications of this study include: 1) Considering the very low ZCZS for ol and opx, dunite and harzburgite are prone to zircon saturation during the infiltration of Zr-bearing metamorphic agents; 2) Because garnet is the main reservoir of Zr in garnet peridotite, metamorphic decomposition of garnets may produce inter-granular zircons; 3) The ZCZS for garnet excellently correlates with X_Alm and T, providing a potential geo-thermometer for garnet-bearing zircon saturated rocks.

Bayan Obo, located in the north margin of the Yinshan Block, North China Craton (NCC), contains the Archean to Paleoproterozoic basement rocks which may shed light on the early crustal evolution of the NCC. In this paper, to reveal the formation and modification of the basements, we report Hf isotope of zircons from the biotite plagiogneisses in Bayan Obo. Combined with the published U–Pb ages and Hf isotope compositions of the zircons from the Bayan Obo basement rocks, a comprehensive review was employed to provide new insights into the formation and modification of the basement in this area. The oldest formation age of the basement rocks was found to be approximately 2.6 Ga, with positive ε_Hf(t) values indicating the contribution from the depleted mantle to the rocks. The age of ∼2.0 Ga marks the major formation period of the basement rocks. The varied ε_Hf(t) values (−17.2 to +8.9) of the magmatic zircons suggest a mixture origin between the depleted mantle and crustal rocks for the gneiss. The basement went through extensive metamorphism during 1.95–1.9 Ga. In addition, zircon U–Pb–Hf isotope data of the Precambrian units in Guyang and Wuchuan, which are adjacent to Bayan Obo, were compiled for comparison. The comprehensive review on the data of the concordant magmatic zircons indicates the existence of 3.5 Ga crustal fragments in the Yinshan Block. The melting of the depleted mantle and the addition of juvenile materials into the crust at 2.6–2.9 Ga and 2.45–2.55 Ga were confirmed by the high positive ε_Hf(t) values of the magmatic and inherited zircons. An increase in remelting of the crust at 2.0 Ga is deduced from the extremely unradiogenic Hf isotope of the magmatic zircons.

Most W mineralization in the world is genetically related with highly fractionated granites, but little is known about ore-forming fluid evolution of W mineralization associated with weakly fractionated granites. To reveal the ore-forming fluid evolution of W mineralization related to the weakly fractionated granites, a combined study of field and petrographic observations, mineralogical, morphological, and in-situ geochemical data of different-type quartz from Dongyuan porphyry W deposit in the world-class Jiangnan W belt, China, was carried out. The petrographic observation and cathodoluminescence (CL) imaging revealed the quartz in the Dongyuan W deposit can be divided into hydrothermal quartz (Qz1 with core-edge structure, and Qz2 with oscillating zone) in the mineralized alteration zone, and magmatic quartz (Qz3 with inherited core) in granodiorite porphyry. The LA-ICP-MS results of the Dongyuan quartz samples show that Al may enter the quartz structure with trivalent Al³⁺ instead of Si⁴⁺, monovalent alkali metals and bivalent (Ge²⁺, Sr²⁺) cations is mainly of charge compensators in quartz, while Ti possibly is in the form of microinclusions of titanium-containing minerals in quartz. The integrated interpretation of the trace element compositions (e.g., Al and Ti contents, Al/Na ratios) and CL texture of quartz indicate high temperature (>400 °C) and uniform Al-rich acidic fluid are conducive to W precipitation of the Dongyuan deposit. Furthermore, trace element compositions (e.g., Ge, Al) and micro-textures of quartz have the potential for distinguishing magmatic and hydrothermal quartz in the Dongyuan W deposit.

The study utilized the fuzzy analytical hierarchical process (F-AHP) in the multicriteria decision analysis of seven important hydrogeologic parameters associated with groundwater potentiality/yield. The purpose was to develop groundwater potential map using groundwater potential index values (GWPIV) for Ondo metropolis, Southwestern Nigeria. The hydrogeologic parameters were prioritize and assigned different weights hierarchically according to their level of importance using F-AHP, as aquifer layer thickness (0.05), aquifer layer resistivity (0.04), overburden thickness (0.12), transverse resistance (0.18), transmissivity (0.32), coefficient of anisotropy (0.25), and formation factor (0.03). From regional perspective, based on drainage basin, and river network, the region was categorized to be of low/moderate groundwater yield, with moderate to high flow connectivity. The higher and lower elevations are generally remarkable in the northern and southern areas respectively, hence possibility of movement of water towards the southern part with the northern forming the watershed. Furthermore, the longitudinal unit conductance recorded regional average value of 1.168, while 1.20, 1.16, and 1.10 were recorded for granite, gneiss and migmatite, which suggests moderate vulnerability to contamination. The average values recorded for overburden thickness, hydraulic conductivity, transmissivity, transverse resistance, formation factor, coefficient of anisotropy, fracture contrast, and reflection coefficient are 22.5 m, 0.33 m/d, 5.56 m²/d, 4966 ohm-m², 2.42, 1.17, 15.3, and 0.66 respectively. The water table aquifer and the fractured basement are the major water bearing units in the area; while the geological units showed overlapping hydrogeologic properties. The obtained GWPIV ranged from 1.21 to 3.55 with regional average of 1.86, suggesting a moderate potential. Nevertheless, the southern part showed more propensity/yield than the northern area. The study showed the importance of geospatial mapping technique, involving fuzzy analytical hierarchical process, in groundwater evaluation, as it was able to solve and obtain different hydrogeological parameters, which were integrated to prioritize and define the area's groundwater prolificacy for the purpose of improving, expanding, and management of the existing water scheme in the study area.

Fine and coarse typical sand are among the most vital raw materials in building construction. A lot of drilling has been done without appropriate subsurface information resulting to a waste of resources and time. Geophysical methods give information on subsurface lithologies to locate areas with huge success. Thus, a geophysical investigation of subsurface deposits was carried out in this research to ascertain the quantity of fine/Coarse sand or areas with viable fine/Coarse sand utilizing Schlumberger configuration, Dipole–Dipole (2D) and well logging in parts of Okpe and Ughelli North LGA of Delta State, Nigeria, before extraction (dredging) for the benefit of exploitation and development. This was done by assessing the geo-electric formation of fine/coarse sand in the studied areas concerning their depths and thicknesses. Seven Vertical Electrical Sounding (VES) were obtained with the application of Schlumberger array, seven 2D and three well logging to estimate the viability of sand deposits in the study areas. These techniques have the efficacy of detecting near-bed formation with vital resolution. The data obtained from the field were illustrated by partial curve matching coupled with computer iteration using the WIN RESIST and Dipro Software to obtain sounding curves which revealed four to six layers. The layers consist of topsoil, lateritic clay and clayey sand, fine sand deposit, medium to coarse grain sand and coarse sand. For Agbarho, thicknesses of fine/coarse sand in the VES stations ranges from 18.0 to 55.6 m with resistivity varying from 223.7 to 572.9 Ωm, Osubi Fine/Coarse sand thickness ranges from 13.5 to 59.9 m with resistivity values within 211.9–891.0 Ωm and Oha town locations have fine/coarse sand thickness varied from 46.6 to 83.7 m with resistivity interval computed between 145.9 Ω-m and 466.4 Ω-m. Thus, the best VES stations that are more viable for sand mining are VES 1, 5 and 7 which contain relatively huge viable sand deposits in the study areas to a depth above 80 m.

Serpentines are geologically important minerals, and antigorite (Atg), lizardite (Lz) and chrysotile (Ctl) are the three key varieties. Their quick and accurate identification with micro-Raman spectroscopy requires to consider the effects of different crystallographic orientations and different chemical compositions. By collecting from existing literatures all Raman spectroscopic data and compositional data acquired from the same or identical Atg, Lz and Ctl samples, we critically examined the compositional effects for the first time, and found that some compositional parameters like the Al₂O₃ and Cr₂O₃ contents have significant impacts on the Raman features. Taking into account the effects of both compositional difference and crystallographic orientation difference, we propose two identification schemes for Atg, Lz and Ctl: the first one uses those weak but characteristic Raman peaks at 1200–1000 cm⁻¹, and the second one uses those intense and unanimously-observed Raman peaks at ∼688, 378 and 229 cm⁻¹. As for the first identification scheme, no peak at 1200–1000 cm⁻¹ suggests the presence of Lz; a single peak at ∼1045, at ∼1070, or at ∼1106 cm⁻¹ indicates the presence of Atg, Lz, or Ctl, respectively; two Raman peaks at ∼1040 and 1070 cm⁻¹ implies the presence of Lz; the occasionally observed one single peak at ∼1040 cm⁻¹ may imply the presence of either Atg or Lz, which can be sought out by resorting to the peak position ratio R_∼1045/688 (Atg having R_∼1045/688 > ∼1.521 whereas Lz attaining R_∼1045/688 < ∼1.521). As for the second identification scheme, Atg can be readily separated from Lz and Ctl by using the exact wavenumbers of the Raman peaks at ∼688 and ∼378 cm⁻¹, and Lz and Ctl can be further discriminated by using the exact wavenumbers of the Raman peaks at ∼378 and ∼229 cm⁻¹. Under most circumstances, both identification schemes do not require the information of crystallographic orientation or composition, and can be conveniently applied to identify the serpentines of Atg, Lz and Ctl.