Imprinting chemical remanent magnetization in claystones at 95 °C

Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer.In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m^− 1/day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m^− 1/day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m^− 1/day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.     

Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H _AC ) is field-dependent. However, for fields ≤ 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k ₀ = M/H _AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k _HF (= M/H _AC ) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where |k _HF | is measured in alternating fields that peak significantly above 400 A/m the M(H _AC ) relation is forced to follow a hysteresis loop in which |k _HF | > k ₀ for small |H _AC | and where |k _HF | decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional “low-field” susceptibility (k ₀ ) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k ₀ to 80,000 A/m). This reveals M(H _AC ) dependence beyond from conventional k ₀ through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M _S ). We show k _HF increases with peak H _AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM₀ = Fe₃O₄) and TM₆₀ (Fe_2.4Ti_0.6O₄). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k ₀ increases by ≤ 5% for fields up to 2 kA/m. Above 4 kA/m k/ k ₀ increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H _AC since this exceeds the coercive force of most grains. For higher peak H _AC , k/k ₀ declines sharply as increased H _AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M₀-TM₆₀ bearing rocks, susceptibility peaks for fields ~12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M _S ) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k ₀ and the peak k/k ₀ value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k ₀ peaks at lower H _AC . Stacked k/k ₀ versus H _AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k ₀ = a + bH + cH ² + dH ³. Thus, for most M-TM bearing rocks, susceptibility and anisotropy of susceptibility (AMS) measurements made on different instruments would be sufficiently precise for most geological applications, if peak alternating fields are ≤700 A/m.     

Magnetite/Ilmenite–series Classification and Magnetic Susceptibility of the Mesozoic-Cenozoic Batholiths in Peru

Abstract: Plutonic rocks of the Coastal Batholith of Peru were evaluated in terms of the granitoid-series classification using the bulk ferric/ferrous ratio from the literature and new measurements of magnetic susceptibility. The batholith is largely composed of magnetite-series plutonic rocks; the magnetite series make up 85% by number of chemical analyses (n=130) and 80% by measurement of magnetic susceptibility (n=210). The ilmenite-series rocks are mostly found in the felsic facies of the batholith. Asymmetrical distribution of magnetic susceptibility is not clear as in the Japanese Islands and Peninsular Range Batholith, but the magnetic susceptibility may decreases continentward (i. e., Peninsular Range type).
The Cordillera Blanca Batholith and stocks are also composed of mainly magnetite series plutonic rocks, but ilmenite-series rocks may be more predominant than in the Coastal Batholith, which is also indicated by the presence of Sn and W mineralizations.     

Magnetic properties related to thermal treatment of pyrite

Detailed rock magnetic experiments were conducted on high-purity natural crystalline pyrite and its products of thermal treatments in both argon and air atmospheres. In argon atmosphere (reducing environment), the pyrite is altered by heating to magnetite and pyrrhotite; the latter is stable in argon atmosphere, and has coercive force and coercivity of remanence of ～20 and ～30 mT, respectively. Whereas in air, the pyrite is ultimately oxidized to hematite. First order reversal curve (FORC) diagram of the end product shows that the remanence coercivity of hematite is up to ～1400 mT. The corresponding thermal transformation process of pyrite in air can be simply summarized as pyrite→ pyrrhotite→magnetite→hematite. These results are helpful for understanding of sedimentary magnetism, secondary chemical remanence and meteorolite magnetic properties.     

新疆阿尔泰乔夏哈拉铁铜金矿床磁铁矿的化学成分标型特征和地质意义

位于新疆阿尔泰造山带与准噶尔盆地过渡地带的乔夏哈拉铁铜金矿原是一个小型铁矿,近年来在深部发现有铜金矿体。根据乔夏哈拉东矿区矿体磁铁矿的单矿物化学分析和东、西矿区样品中磁铁矿的电子探针分析,发现磁铁矿富集轻稀土,Eu呈正异常,Ce呈负异常;微量元素中Co和Ni含量高,而Ti含量很低。Cu含量与Fe含量呈现明显的反相关。磁铁矿化学成分标型的初步研究显示,乔夏哈拉矿床磁铁矿成因与夕卡岩型及沉积变质型铁铜矿非常相似,指示存在找铜前景。     

冲绳海槽中部的火山口

冲绳海槽位于中国东海大陆架的东部,与东海陆架相连,是一个新生的孤后盆地,海底火山活动和地震频繁,断裂体系十分发育,沉积了十分丰富的火山碎屑沉积物,根据海底沉积物中的浮岩与磁铁矿含量变化,结合水深资料推测了海底火山口的位置,结果表明Z 14—5测站(27°18′N、127°12′E)附近是一个火山口。     

广东怀集东园铁矿地质特征及找矿方向

怀集地处大珠三角的西北门户,铁矿资源较为丰富.东园铁矿为 - 中小型矿床,赋存于六庵山岩枝(γ52(3))与F1逆冲断层夹持的东岗岭组大理岩内接触带上,为隐状的富磁铁矿体,成因类型属接触交代矽卡岩型磁铁矿床.通过研究,剖析了该矿床的地质特征及控矿因素,旨在寻找隐伏矿体,扩大找矿远景,这对矿山生产和远景规划,均具有重要的实际意义.     

南海东部表层沉积物中普通角闪石和磁铁矿的特征及其成因

采用电子探针测试技术和晶体化学-成因分类图解法对南海东部表层沉积物中普通角闪石和磁铁矿的化学成分进行研究,结果表明,普通角闪石属于钙质角闪石组,主要类型有镁角闪石、钙镁闪石质角闪石、镁绿钙闪石质角闪石、浅闪石质角闪石、铁闪石质角闪石,均为中酸性岩浆成因。磁铁矿化学成分中TiO2、V2O5、Al2O3、MnO、MgO等标型组分的含量与岩浆成因的磁铁矿相似,其成因图解表明既有中酸性岩浆成因的也有基性岩浆成因的磁铁矿。普通角闪石和磁铁矿的成因性质与其共存的矿物特征和研究区的区域地质背景相吻合。     

Ore petrology of the V-Ti magnetite (lodestone) layers of the Kurihundi area of Sargur schist belt,Dharwar craton

The V-Ti magnetite layers (lodestone) occur within the layered gabbro-anorthosites-ultramafic rocks emplaced into the migmatitic gneisses close to the high grade Archeaen Sargur supracrustal rocks in the Kurihundi area. The ore petrographic studies of the lodestone reveal the presence of primary Ti-magnetite, ilmenite, ulvospinel, pleonaste, hematite and pyrite, chalcopyrite, pyrrhotite and secondary Ti-maghemite, martite and goethite as well as secondary covellite. These layers contain Ti-magnetite (60%) and ilmenite (30%) with silicates (<5%) exhibiting granular mosaic texture with well-defined triple junctions and are classified as adcumulus rocks. The grain-boundary relationships in the ores indicate considerable postcumulus growth and readjustment due to combined effects of sintering and adcumulus growth. Intergrowth textures (ulvospinel, ilmenite and pleonaste in Ti-magnetite and hematite in ilmenite) reflects exsolution features crystallized from solid-solutions compositions under different conditions of oxygen fugacities. Larger bodies of pleonaste and ilmenite in Ti-magnetite become lensoid or rounded in outline and these morphological modifications took place during the regional upper amphibolite to lower granulite facies metamorphism at 2.6 Ga ago. The lodestone contains high TiO₂ (20 to 22.59 wt%), with V₂O₅ (0.85 to 1.15%) and Fe₂O₃ ^t (72.03 to 74.25%). Ti-magnetite shows alteration to Ti-maghemite, martite and goethite due to low temperature oxidation and hydration during weathering.     

磁铁矿系列花岗岩含金差异性及其评价准则—以豫西地区为例

本文以豫西地区小秦岭、外方山为例，将磁铁矿系列花岗岩进一步划分为两大类：一类与金矿具密切的空间关系，本文命名为M1型花岗岩，以文峪岩体、合峪岩体为代表；另一类成金能力较差的岩体，本文命名为MS型花岗岩，以华山岩体、太山庙岩体为例。两类岩体在化学成分、矿物含量、同位素组成等方面均有一定差异。M1和MS型花岗岩的源岩是一致的，均为陆内腐冲碰撞地壳板块冲下插作用所产生的物质，其成分及含矿性差异与源岩部分熔融程度、岩浆演化及其分熔序列有关。M1型花岗岩是在下地壳源岩分离熔融晚阶段、源岩部分熔融程度较高、环境较氧化的条件下形成的。