Environmental mechanism of magnetic susceptibility changes of lacustrine sediments from Lake Hulun, China

The changes of magnetic susceptibility(κ) are correlated with those of corresponding sedimentological, geochemical, mineralogical and biological results, which verifies thatκ can be taken as one of the environmental proxies. However, usually the exact origin of magnetic signal is poorly understood, and is difficult to relate with the environmental evolution. Magnetic properties of material derived from the catchment and sedimentary environment may affect the accumulation, preservation, or authigenesis and diagenesis of magnetic minerals. In the Lake Hulun region in Inner Mongolia, it is found that muddy sediments, deposited during high water level period (corresponding to humid climate), have comparatively highκ values. In contrast, the sandy sediments, deposited during low water level period (corresponding to arid climate), have lowκ values. Detailed rock magnetic investigation confirms that detrital magnetite derived from volcanic rocks in the catchment exists in both muddy and sandy sediments. During high water level period, secondary ferrimagnetic iron sulphide was produced in muddy sediments under relatively reductive conditions. Ferrimagnetic iron sulphide, coexisting with detrital magnetite, predominates the magnetic properties of muddy sediments, resulting in increasingκ. This paper reveals the significance of authigenic ferrimagnetic iron sulphide produced after sediment deposition.     

Magnetic properties of sediments from the Pearl River Delta,South China: Paleoenvironmental implications

Magnetic parameters and their environmental implications of sediments in a core (PD) from the Pearl River Delta, South China, indicate that ferrimagnetic minerals with low coercivity, such as magnetite, dominate the magnetic properties although small amounts of Fe-sulphides occur. The fraction of Fe-sulphides increases and becomes the dominant minerals determining the magnetic characteristics in grey-black organic-rich clay horizons, indicating an anoxic, sulphate-reducing swamp environment resulting from a marine regression. In the "Huaban clay" , hard magnetic minerals, such as hematite and goethite, largely control the magnetic properties of the sediments and imply a long period of exposure and weathering. Where magnetite is the main magnetic mineral, its fraction and grain size determine properties such as magnetic susceptibility (κ ) and saturation isothermal remanent magnetization (SIRM). Ratios of SIRM/κ and χarm/SIRM reflect changes in sea level with high SIRM/κ and χarm/SIRM correlating with a smaller magnetic mineral grain size and rising sea level. Based on downcore variations of these environmental magnetic parameters along with sediment characteristics and microfauna, the sedimentary environment of the Pearl River Delta area can be divided into two main cycles of transgression and regression during the late Pleistocene and Holocene with more sub-cycles of sea level fluctuation during each transgression.     

Origin of the magnetization of Permo-Carboniferous sediments of Spitsbergen, Svalbard Archipelago

Rock magnetic investigations of Permo-Carboniferous carbonate sediments from two areas on Spitsbergen are described, conducted to identify the carriers of the NRM in these rocks. Since microscopic and magnetic separation techniques could not profitably be applied, the nature of magnetic minerals was investigated by thermal demagnetization of the NRM and decay of saturation isothermal remanence (I_rs) during heating to 600°C, as well as by the distribution of the median destructive fields of the NRM and observation of magnetic susceptibility after subsequent heatings. The results show that the NRM of these limestones resides mainly in magnetite, but creation of magnetic pyrrhotite and of fresh magnetite is observed during heating to 600°C. Presence of sulphides indicates that magnetite is an oxidation product of pyrite or of non-magnetic pyrrhotite. Examination of rock magnetic properties of limestones leads to the conclusion that most of the magnetite in the rocks of the Bellsund area is of detrital origin, whereas the rocks at Festningen contain magnetite derived from pyrite probably during an early stage of the diagenetic process.     

某些油气田地表土壤的磁性与烃运移相互关系研究

根据鄂尔多斯盆地靖边气田上方的ＭＩ０２线、江苏储家楼油田与江汉盆地花园油田地表十字剖面土壤磁性的测量结果，研究地表是否存在与深部油气藏中烃类垂向运移相关磁性蚀变带的可能性及其分布形式．结果表明，ＭＩ０２线样品显示了明显的高磁性异常特征，并与酸解烃之间呈很强的正相关．磁滞回线形态与参量显示出明显的亚铁磁性特征．电子探针分析结果表明，样品中主要磁性矿物（磁铁矿）中高温杂质元素含量很低．江汉盆地花园油田十字剖面磁滞回线参量（饱和磁化强度Ｊｓ与饱和等温剩磁Ｊｒ）的高异常位于油田周边，油田顶部土壤呈典型顺磁性特征．储家楼油田地表土壤的高磁性异常主要位于油田上方，它们均与盖层中的断裂构造对应，储家楼油田的高磁性样品中发现的典型球粒状磁铁矿为次生低温磁铁矿，它与烃运移之间存在成因关系．     

Theory of the piezomagnetic effect in titanomagnetite-bearing rocks

Summary The stress sensitivitiesS _x andS _R of susceptibility and remanence for titanomagnetite-bearing rocks are calculated in terms of magnetostriction constants ₁₀₀ and ₁₁₁ and anisotropy constantsK ₁,K ₂ of the magnetic minerals.S _x andS _R are represented by quite different algebraic expressions but happen to have comparable numerical values over the whole range of titanomagnetite compositions. Both increase strongly with titanium content. This leads to more optimistic calculations of tectonomagnetic effects than with the previously assumed stress sensitivity for pure magnetite.     

Authigenic or detrital origin of pyrrhotite in sediments?: Resolving a paleomagnetic conundrum

Monoclinic pyrrhotite (Fe₇S₈) is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments. In contrast, geochemical literature indicates that pyrrhotite formation is extremely slow below 180 °C, which makes it a highly unlikely carrier of early diagenetic remanences in sediments. This view is confirmed by the occurrence of late diagenetic Fe₇S₈ in ancient sediments and the general lack of Fe₇S₈ in modern sediments. Horng et al. [C.S. Horng, M. Torii, K.S. Shea, S.J. Kao, Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan, Earth Planet. Sci. Lett. 164 (1998) 467–481.] documented the presence of Fe₇S₈ that carries a magnetic signal indistinguishable from that of detrital magnetite in Pleistocene marine sediments from Taiwan. We tested the possibility that the Fe₇S₈ could have a detrital origin by conducting a source-to-sink study and found Fe₇S₈ in metamorphic rocks of the Taiwan Central Range and in material eroded from these rocks in the transportation pathway and in the depositional sink. This confirms that the Fe₇S₈ has a detrital origin. Rapid transportation from source to sink (e.g., by typhoon-associated flood events) probably assists preservation of the Fe₇S₈, which might otherwise oxidize during transportation. The widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of Fe₇S₈ in marginal sedimentary basins. Detrital Fe₇S₈ should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance.     

Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities

In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S_-200 values (S_-200 = IRM_-200/ SIRM, where IRM_-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.     

The middle-long term prediction of the February 3, 1996 Lijiang earthquake (MS=7) by the "criterion of activity in quiescence"

IntroductionIn the book Future CataS~ologr published in 1992, we proposed a viewpoiflt on using the"criterion of activity in quiescence" to predict big eathquake (MsZ7) (GUO, et al, 1992), and predicted in the book that in futore several years or in ten years a big earthquake (Ms27) will be possible to occur in the Zhongdian and nearby in Yunnan Province. In the 1994 nation-wide earthquake tendency consultation meeting we pointed out, once more, in the Zhongdian region of Yunnan Province…     

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d…     

Discovery and its geological significance of the mantle-derived helium in the inclusions of the Ordovician oil-bearing reservoir rocks in the Huanghua depression,China

The values of the helium isotopes in the inclusions of the Ordovician reservoir rocks in the Kongxi buried hill belt in the Huanghua depression were first measured and the source of helium and its geological significance were investigated in comparison with those of the helium isotopes in the conclusions in the Ordovician rocks in the Ordos basin and the Tarim basin. The input of the mantle-derived helium into the inclusions in the carbonate reservoir rocks was found from the Konggu 3 well, the Konggu 4 well, and the Konggu 7 well in the Kongxi buried hill belt. The 3He/4He and R/Ra in the conclusions in the Ordovician oil-bearing reservoir rocks in the Konggu 7 well average 2.54×10-6(3) (sample quantity, the same below) and 1.82(3), respectively. The percent of the mantle-derived helium in the inclusions of the reservoir rocks in the Konggu 7 well reaches up to an average of 23.0%(3). The age of the contribution of the mantle-derived helium to the inclusions in the Kongxi buried hill belt is in the La     

The structure and ability of the China Seismological Gravity Monitoring System

IntroductionMore than 30 years has passed since gravity method was used to Study the eanhquake prediction in China in later 1960s (In America, Germany, Japan and the former Soviet Union etC. it haspassed nearly half a century). In this 30 years the most remarkable success achieved are as follows.First, in the cognition aspect, the questions whether the gravity will change or not before earthquake and how much the magnitUde of change has been answered. TO the fial queStion, the answer is p…     

Changes of electric conductivity in the neighbourhood of the curie temperature of basalts

Summary In investigating the electric conductivity of rocks as a function of the temperature it was found that rocks containing ferrimagnetic minerals display a change in the slope of the functionlg =f(1/T) in the neighbourhood of the Curie temperature . In order to explain these changes the curveslg =f(1/T) and the Curie temperatures , obtained from the temperature dependence of the saturated magnetization J_s=f(T), were compared. Eight samples of basalts, 2 samples of haematite and magnetite were used to demonstrate the relation between and the changes in the pattern of the electric conductivity, caused by the variations of the exchangeable energy of ferrimagnetic minerals.     

Measuring Concentrations of Dissolved Methane and Ethane and the 13C of Methane in Shale and Till

Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration‐depth profiles of dissolved CH₄ and C₂H₆ and δ¹³C‐CH₄ in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars^® in a low O₂ headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well‐defined and reproducible depth profiles after 31‐d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes^®) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ¹³C‐CH₄ measured from mud gas, IsoTubes^®, cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high‐resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.     

Remanent hysteresis study of dolerite dykes

Saturation remanent hysteresis studies were carried out on numerous dolerite dyke samples from the peninsular India. These studies result in four types of remanent hysteresis curves which indicate that the magnetic material is magnetite or titanomagnetite with variable grain-size having remanent coercive forces (H _cr ) of 8 to 30 mT, requiring saturating fields (H _s ) up to 250 mT. Two extreme types of samples with (1) low coercive forces requiring high saturating fields and (2) high coercive forces requiring low saturating fields are noticed along with the generally observed ones. The Granulometric and Lowrie-Fuller Tests on these samples indicated that the magnetic material i.e., magnetite or titanomagnetite in these rocks is in the form of Multi-Domain (MD), Cation Deficient (CD) and a mixture of these two forms (MD+CD) within.     

A preliminary magnetic study of soil samples from west-central Minnesota

Various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west-central Minnesota. There is a marked change in magnetic properties as a function of depth in the core. X-ray analysis and Curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil. The intensity of anhysteretic remanent magnetization (ARM) decreases sharply as the depth increases. In contrast, the stability of ARM was found to be higher for the subsoil. The surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization (VRM). The VRM acquisition coefficient (S_a) of the subsoil (S_{a= 3.18 × 10^?6emu g^?1, 3.18 × 10^?6A m² kg^?1}) was about ten times weaker than that of the top soil sample (S_a = 3.868 × 10^{?7emu g?1, 3.868 × 10?7A m2 kg?1}). The magnetic domain state indicator, the ratio of coercivity of remanence to coercive force, H_cr/H_c, was 1.5 and 3.85 for the top soil and subsoil, respectively. It appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size. We conclude that the magnetic grains in surface soil samples were more single-domain (SD) like whereas the magnetite grains in the subsoil samples were more likely in pseudo-single-domain (PSD) or small multidomain (MD) range. The observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs.     

Potential petroleum sources and exploration directions around the Manjar Sag in the Tarim Basin

Since the discovery of the Tahe oilfield, it has been controversial on whether the main source rock is in the Cambrian or Middle-Upper Ordovician strata. In this paper, it is assumed that the crude oil from the Wells YM 2 and TD 2 was derived from the Middle-Upper Ordovician and Cambrian source rocks, respectively. We analyzed the biomarkers of the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions from the Lunnan and Hade areas in the North Uplift of the Tarim Basin. Results show that the ratios of tricyclic terpane C₂₁/C₂₃ in the crude oil, asphalt-adsorbed hydrocarbon and saturated hydrocarbon in bitumen inclusions are less than 1.0, indicating that they might be from Upper Ordovician source rocks; the ratios of C₂₈/(C₂₇+C₂₈+C₂₉) steranes in the saturated hydrocarbon from reservoir bitumen and bitumen inclusions are higher than 25, suggesting that they might come from the Cambrian source rocks, however, the ratios of C₂₈/(C₂₇+C₂₈+C₂₉) steranes in oil from the North Uplift are less than 25, suggesting that they might be sourced from the Upper Ordovician source rocks. These findings demonstrate that the sources of crude oil in the Tarim Basin are complicated. The chemical composition and carbon isotopes of Ordovician reservoired oil in the Tarim Basin indicated that the crude oil in the North Uplift (including the Tahe oilfield) and Tazhong Depression was within mixture areas of crude oil from the Wells YM 2 and TD 2 as the end members of the Cambrian and Middle-Upper Ordovician sourced oils, respectively. This observation suggests that the crude oil in the Ordovician strata is a mixture of oils from the Cambrian and Ordovician source rocks, with increasing contribution from the Cambrian source rocks from the southern slope of the North Uplift to northern slope of the Central Uplift of the Tarim Basin. Considering the lithology and sedimentary facies data, the spatial distribution of the Cambrian, Middle-Lower Ordovician and Upper Ordovician source rocks was reconstructed on the basis of seismic reflection characteristics, and high-quality source rocks were revealed to be mainly located in the slope belt of the basin and were longitudinally developed over the maximum flooding surface during the progressive-regressive cycle. Affected by the transformation of the tectonic framework in the basin, the overlays of source rocks in different regions are different and the distribution of oil and gas was determined by the initial basin sedimentary structure and later reformation process. The northern slope of the Central Uplift-Shuntuo-Gucheng areas would be a recent important target for oil and gas exploration, since they have been near the slope area for a long time.     

The role of dust source area and pedogenesis in three loess-palaeosol sections from North Bulgaria: A mineral magnetic study

Rock magnetic studies of three loess-palaeosol sections from Bulgaria (Harletz and Orsoja in the NW and Durankulak in the NE at the Black Sea coast) have been carried out. Thermomagnetic analyses of magnetic susceptibility point to magnetite as the major ferrimagnetic carrier in the loess and palaeosol units. Maghemite gives a significant contribution to the total magnetic signal in the recent soils (S₀), while in one of the sections-Orsoja-hematite may also be present. The effective magnetic grain sizes deduced from the hysteresis measurements and the ratios M_rs/M_s and B_cr/B_c fall in the pseudo single domain (PSD) range, but the data distributions in a Day diagram for the three different sections are shifted. This is suggested to be caused by different detrital input (e.g. different dust source areas) and varying degrees of pedogenic modification. The calculated background susceptibilities χ_bg differ significantly as well. The lowest value is obtained for the Durankulak section-χ_bg = 9.95 × 10⁻⁸ m³/kg, which is in accordance with the data from other studies of loess-palaeosol profiles from the Black Sea area. The corresponding background susceptibilities for the other two sections studied-Harletz and Orsoja-are significantly higher (33.1 and 17.75 × 10⁻⁸ m³/kg, respectively). Both parent material and pedogenesis are found to be responsible for the observed differences in the magnetic characteristics.     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×10⁸ m³ by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ ¹³C₁ is less than −20‰, carbon isotope trend of alkane gas is δ ¹³C₁<δ ¹³C₂<δ ¹³C₃ or δ ¹³C₁<δ ¹³C₂>δ ¹³C₃, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ ¹³C₁>δ ¹³C₂>δ ¹³C₃). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×10⁸–23.8×10⁸ m³/km², corresponding to R _o from 2.0% to 3.5% for organic matter.

     

Stability of remanent magnetization of some dykes from Mysore State,India

Summary The magnetic properties of some dykes from Mysore State, India, have been studied in detail. The rocks were found to have aQ _n ratio varying from 1.5 to 11.6, a remanent coercive force varying from 100 to 250 Oersteds, Curie temperature varying from 250 to 480°C and were found to have lamellae of ilmenite oriented in (111) plane of magnetite. The stable natural remanent magnetization of the rock seems to be of TRM origin with titanomagnetite and low grade titanomaghemite being the main carrier of remanent magnetization.N.G.R.I. Contribution No. 70-215.