Multiple magnetization events in the Red River carbonates, Williston Basin, Canada: Evidence for fluid-flow?

Samples collected from the Upper Ordovician Red River carbonates in a well at the centre of the Williston Basin revealed two paleomagnetic components with different inclinations, 60.3 ± 3.9° (k = 70.7, N = 12) and 20.4 ± 3.3° (k = 141.2, N = 8), but similar declination values in individual specimens. Inclination-only analysis indicates two possible scenarios for the age of these two magnetizations: in scenario (a) the timing of magnetization happened sometime between Late Ordovician to Devonian; and in scenario (b) there are two different remagnetizations, one that overlaps Pennsylvanian to Permian time while the other can have either a Late Jurassic or a Tertiary age. Whereas dolomitization and some isotopic data tend to support scenario (a), previous paleomagnetic data from the Williston Basin and from younger units in the same well, the tectonic evolution of the basin, and the hydrocarbon maturation pattern in the Red River carbonates all favour chemical remagnetization(s) driven by orogenic fluids during the Alleghenian and Laramide orogenies.     

Environmental magnetic and magnetic fabric studies in LakeWaynewood, northeastern Pennsylvania, USA: Evidence for changes inwatershed dynamics

An environmental magnetic and magnetic fabric study of sedimentscollected from Lake Waynewood, a post-glacial lake in the PoconoMountains of Pennsylvania, USA, provided a history of the lake's watersheddynamics over the past 3500 years. Two 5 m long, Mackereth coresof lake sediments and three watershed soil profiles were analyzed magnetically.Paleosecular variation and ¹⁴C measurements allowed timing ofchanges in the lake's watershed which are documented by changes in ARM,SIRM,, S-ratio, SIRM/, ARM/ and ARM/SIRMdowncore. Prior to 2900 years BP, there is little evidence for allogenicinflux. Dramatic changes in mineral magnetic properties and a strong AARmagnetic fabric appear approximately 2900 years BP, suggesting major changes inwatershed conditions, either in the hydrologic regime or in vegetative cover.Between 2900 and 1800 yrs BP, large variations in magnetic mineralogy areapparent, whereas about 1800 years BP, a single sediment source began todominate the magnetic mineralogy. About 100–200 years ago, conditionsagain changed, probably due to clearcutting and settlement of the watershed.Topsoil erosion appears to have dominated the magnetic signal. S and Mnconcentration downcore indicate that there is little evidence for reductiondiagenesis having caused the changes observed in magnetic mineral type andconcentration, except in the top 10 centimeters of the sediment column.     

SD-1型数字大地电磁记录系统

SD-1型数字大地电磁测深仪是一种用途较广的地球物理勘探仪器,可用于研究地壳上地幔结构,普查石油、天然气田构造,寻找热田及监视地震前地壳电阻率随时间的变化。该仪器已于1982年8月通过国家鉴定。鉴定以后又在华南地区作了大地电磁测深工作。本文只是简述一下仪器的基本结构、几个主要的技术问题及鉴定前后用该仪器所     

黑龙江省多宝山斑岩型铜(钼)矿床成矿流体特征及演化

黑龙江省多宝山斑岩铜(钼)矿床位于小兴安岭西北部,是中亚-兴蒙造山带北东段最大的斑岩型铜(钼)矿床,矿体产于加里东期花岗闪长岩和中奥陶世多宝山组安山岩、凝灰岩中。铜矿化与绢英岩化关系密切,而钼矿化主要产于钾硅化带中。矿区内脉体广泛发育,从早到晚依次为:石英+钾长石脉、早阶段石英+辉钼矿脉、晚阶段石英+辉钼矿脉、石英+黄铜矿+黄铁矿脉、石英+黄铁矿脉和方解石+石英脉。脉石英中广泛发育流体包裹体,包括气液两相水溶液包裹体(W型)、纯气相包裹体(G型)、含CO2三相包裹体(C型)及含子矿物多相包裹体(S型)。石英+钾长石脉中仅发育气液两相包裹体,均一温度峰值﹥550℃、盐度为16.2%～18.1%NaCleqv;早阶段石英+辉钼矿脉中发育大量气液两相包裹体和含子矿物多相包裹体,并见少量含CO2三相包裹体,均一温度集中在350～450℃、盐度变化于1.1%～﹥65.3%NaCleqv;晚阶段石英+辉钼矿脉体发育大量含CO2三相包裹体和含子矿物多相包裹体,另有少量气液两相包裹体,均一温度集中在270～350℃、盐度为0.8%～42.4%NaCleqv;石英+黄铜矿+黄铁矿脉中发育丰富的气液两相包裹体,见少量含子矿物多相包裹体、含CO2三相包裹体和纯气相包裹体,均一温度峰值在230～330℃、盐度为0.8%～42.4%NaCleqv;石英+黄铁矿脉和方解石+石英脉中仅发育气液两相包裹体,均一温度变化于110～200℃、盐度为3.9%～8.4%NaCleqv。成矿流体在古深度4.1km左右,温度在230～450℃之间、压力在10～41MPa之间,发生了强烈的流体沸腾作用,大量CO2等气体从流体中释放出来,黄铜矿、斑铜矿和辉钼矿等巨量沉淀下来,形成了铜(钼)矿体。成矿流体总体上属H2O-CO2-NaCl体系,多期次的流体沸腾作用是该矿床的主要成矿机制。     

Investigation of the changes in magnetic and chemical properties of soil during plant growth in a controlled environment

Changes in magnetic and chemical properties of soil during the growth of tomato plants (Lycopersicon esculentum) are examined in this study. The synthetic soils, prepared from sand, topsoil and organic material, were treated with magnetite powder (<5 μm) in order to simulate metal contamination. Six soil treatments were prepared from two soil types: controls, low-contamination and high-contamination treatments (0.01 and 0.05 g of magnetite powder/kg soil, respectively). Overall, the contaminated soils had a greater decrease in magnetic susceptibility (MS) than the controls, and the difference in MS decrease between the treatments was found to be statistically significant for both soil types. Potential reasons for the overall MS decrease were explored, and among them, trace element uptake by plants probably had a minor contribution as the concentration differences of Fe and other trace elements (Ni, Mn) between treatments were not statistically significant. In soils, oxidized and weakly magnetic minerals (maghemite, goethite and hematite) were common after plant growth, when compared with the untreated (background) soil. Such mineral transformations could have contributed to the overall MS decrease. The results show that exposure to Fe contaminants can affect plant growth and suggest that plant growth can measurably change the magnetic properties of their growth media. While the potential variables affecting plant growth were controlled as much as possible, there still remains the potential that biotic and abiotic chemical reactions could have affected the results. Thus, continuous monitoring of the changes in soil magnetic and chemical properties in more complex soil–plant systems is needed.     

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

A paleomagnetic study of subsurface core samples from dolomitized carbonates of two producing reservoirs in the Upper Ordovician Trenton Formation, collected from four wells in southwestern Ontario yielded a paleomagnetic direction of D = 152.3°, I = − 12.3° (N = 49, α₉₅ = 8.7). This characteristic remanent magnetization (ChRM) direction was azimuth-corrected by aligning the viscous remanence magnetization (VRM) with the present Earth's magnetic field direction. A drilling-induced magnetization (VRMdi) was present in less than half the specimens sampled in this study. In addition, where the VRM correction could not be made, a paleolatitudinal arc calculated from the inclination-only mean of I = − 9.0° (N = 34, α₉₅ = 3.0°) intersected the apparent polar wander path in the Late Permian–Early Triassic. These paleodirections are similar to the paleomagnetic directions observed in Ordovician Trenton carbonates from the Michigan Basin and New York State, U.S.A., suggesting a related regional late Paleozoic remagnetization.     

Magnetic survey of topsoils in Windsor–Essex County, Canada

Windsor–Essex County is a major cross-border truck and transportation route, with significant localized industrialization as well as rural and farming areas. Magnetic property measurements (in-field and laboratory susceptibility, frequency-dependent susceptibility, hysteresis properties, thermomagnetic and thermosusceptibility curves, anhysteretic and isothermal magnetizations) were made in order to determine the potential for using such variables to distinguish between natural and anthropogenic pollutants. In-field magnetic susceptibility measured on 324 soil sampling sites on a 0.5–2 km grid spacing through Windsor–Essex County ranged from 3.7 × 10^− 6 to 305.2 × 10^− 6 SI (average 36.2 ± 35.8 × 10^− 6 SI), and showed that high magnetic susceptibility values were obtained on soil sampling sites in and around the cities/towns of Windsor, Harrow, Olinda and Oakland and near the beaches of Point Pelee National Park (PPNP) and Deerbrook, whereas lower susceptibility values were observed in near the towns of Lakeshore and Essex. On this grid spacing, Highway 401 (the major truck route) did not show anomalous susceptibility values; however, closer (1–3 m) sampling on other roads did show anomalously high values, suggesting that the coarser grid spacing may have missed anomalies. Laboratory measurements indicated that the dominant magnetic mineral in the Windsor–Essex County soils is magnetite; however, the grain size is variable. Pseudo-single domain (PSD)–multidomain (MD) magnetite is generally found on beaches and in PPNP, whereas single domain (SD)–PSD magnetite has been found near the City of Windsor and other towns. While certain correlations exist between some anthropogenic activities and the measured magnetic susceptibility and magnetic property values, no overall correlation can be made. A variety of geologic and anthropogenic factors must be considered when interpreting the origin of the magnetic signal in a particular area.     

Preliminary magnetostratigraphic dating of Paleogene lacustrine sediments from the Shandong Province, East China

A detailed magnetostratigraphic and rock magnetic study was conducted on Tertiary lacustrine/fluvial sediments from Jiyang, Shandong, East China. High temperature susceptibility variation and thermal demagnetization show that magnetite is the main magnetic remanence carrier. The characteristic remanent magnetization (ChRM) was isolated above 150 °C by thermal demagnetization, and a detailed magnetic polarity sequence was established. The lacustrine/fluvial sediments were dated from polarity chron C18n.1r to polarity chron C15n, i.e., within the interval 39.631–34.655 Ma. The results of this study indicate the likelihood that either no hydrocarbon-bearing fluids were present after 34.655 Ma or, if there was any such fluid flow, the fluids must have gone through the section very quickly, without having any significant influence on the magnetic signal.