Mineralogy and Magnetic Behavior of Yellow to Red Xuanhua-Type Agate and Its Indication to the Forming Condition

Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the Xuanhua District (China) was investigated by temperature dependence of magnetic susceptibility, hysteresis loop, isothermal remanent magnetization and the analysis of remanent coercivity components from the gradient acquisition plot. Yellow goethite and red hematite can be quantitatively identified by XRD and Raman spectroscopy due to their relatively higher content. Results showed that the red, yellow and orange Xuanhua-type agate had different magnetic behavior, and magnetite existed in the yellow and orange ones. Fluid inclusions in such agate had the homogenization temperature of ~168 °C to 264 °C. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reflects the transformation from Fe2+ to Fe3+, indicating the change in the redox property of the environment. Unique patterns mainly formed by hematite and goethite make it a popular gem-material with high research value.

Traffic-related activities driven dust-bound magnetic pollution in urban road-system: Case study of Xiamen island, China

Sources of magnetic phases in urban dust include traffic-related activities, industrial products and detrital minerals. Exposure to magnetic phases in urban dust co-associated with trace metals is associated with respiratory ailments in sensitive organisms. Here, we collected samples of surface roadside dust and topsoil from Xiamen island, Fujian, China and then magnetic properties were determined using rock-magnetic methods. Magnetic particle concentrations in surface roadside dust was found to be significantly high compared with magnetic content in topsoil using magnetic susceptibility (𝜒LF), saturation isothermal remanent magnetization (SIRM) and susceptibility of anhysteretic remanent magnetization (𝜒ARM). Roadside dusts had much lower percent frequency-dependent susceptibility (1.83 ± 0.12%) than topsoil (2.96 ± 0.14%). Given the 𝜒ARM/SIRM vs 𝜒fd% and first-order reversal curves, most of the magnetic particles in roadside dust were large than in topsoil. Variable temperature dependence of magnetization, isothermal remanent magnetization decomposition spectra and first-order reversal curves suggested the enhancement of dust 𝜒LF, SIRM and 𝜒ARM were related to the exogenous pseudo-single-domain magnetite-like materials. This study provides a better understanding of magnetic ageing of dust particles in coastal cities. These results will be useful in optimizing environmental policies aimed at regulating magnetic dust particles pollution in coastal cities.

Ultrafine Magnetic Particles: A DIET-Proxy in Organic Rich Sediments?

In this work we present results of the magnetic properties characterization of sediment samples from a brownfield site that is generating methane biogas in São Paulo–Brazil. We applied interpretation procedures (frequency dependent susceptibility and time-dependent Isothermal Remanent Magnetization) appropriate to study the ultrafine magnetic fraction response of the samples. The higher content of superparamagnetic (SP) particles correlates well with the detected biogas pockets, suggesting that the methanogens activity produces these ultrafine particles, different from the magnetic particles at other depth levels. We propose the use of two simple measurement and interpretation techniques to identify such magnetic particles fingerprints. The results presented here support the use of environmental magnetism techniques to investigate biogeochemical processes of anaerobic microbial activity.

Preliminary absolute paleointensity estimation from a single volcanic-glass grain extracted from an unwelded pyroclastic flow

<p>Many pyroclastic flows are distributed around Japan. They usually involve volcanic-glass grains. These grains are considered to form at the timing of volcanic eruptions and are expected to have magnetic inclusions consisting of tiny single (titano)magnetites with recording the paleomagnetic field. We have extracted single volcanic-glass grains of pumice-type with a diameter of 0.60-0.84 mm from an unwelded part of the Ito pyroclastic flow deposits (A-Ito, 26-29 ka; Machida and Arai, 2003), Kyusyu, Japan. A series of rock- and paleomagnetic measurements have been made on the grains.</p><p>Sixty-seven out of 88 grains had detectable intensities of natural remanent magnetization. Some of such grains were further investigated. Results of low-temperature magnetometry exhibited inflection points at 105-120 K, suggesting magnetite as a main remenence carrier. Stepwise alternating field demagnetization revealed an existence of stable characteristic remanence (ChRM) which was interpreted to be a primary component. </p><p>Tsunakawa-Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003), one of the latest absolute paleointensity (API) techniques to date, was applied to selected grains having stable ChRMs. On the application we newly included measurements related to an isothermal remanent magnetization (IRM). Four successful results were obtained by an adoption of IRM corrections, giving an average API value of about 25 μT. This corresponds to a virtual axial dipole moment (VADM) of about 50 ZAm<sup>2</sup>, which is consistent with the contemporaneous VADM of the sedimentary record (PISO-1500; Channell et al., 2009). </p>

Large field impressed anisotropy of magnetic susceptibility (AMS) in metamorphic volcanoclastic rocks from the western Central Pamir with ilmeno-hematite as the main magnetic carrier.

<p>Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM). Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan). These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite. The magnetic properties are very homogeneous across the sequence. Bulk magnetic susceptibilities vary between 150-250 μ SI. The AMS magnetic fabrics correspond to triaxial tensors with a well defined foliation plane and a steeply dipping magnetic lineation. The degree of anisotropy varies between 1.03 and 1.2. This fabric was likely acquired during the deformation associated with the emplacement of Middle Miocene gneiss domes. SEM/EDS data indicate that the main iron oxide mineral is hematite with up to 15% of ilmenite in solid solutions. This is in agreement with unblocking temperatures of SIRM around 630 °C, lower than the one of pure hematite. One of the most surprising magnetic characteristics of these rocks is the effect of strong-field remanent magnetizations upon the AMS. During the acquisition of an Isothermal Remanent Magnetization (IRM), the initial AMS is progressively obliterated by a new AMS fabric. The field-impressed AMS is characterized by a decrease of the magnetic susceptibility along the direction of the IRM and an increase in magnetic susceptibility in the orthogonal plane. The field-impressed AMS is thus mainly oblate with a degree of anisotropy usually between 1.2 and 1.4. As far as we know, such a strong effect has never been reported. In sandstone with detrital hematite as the main carrier, the degree of the induced AMS fabric is less than 1.02 suggesting that the ilmenite content in the metamorphic hematite is the main cause of the large observed field induced fabric in these rocks.</p><p> </p>

Anisotropy of (partial) isothermal remanent magnetization: DC-field-dependence and additivity

SUMMARY Anisotropy of isothermal remanent magnetization (AIRM) is useful for describing the fabrics of high-coercivity grains, or alternatively, the fabrics of all remanence-carrying grains in rocks with weak remanence. Comparisons between AIRM and other measures of magnetic fabric allow for description of mineral-specific or grain-size-dependent fabrics, and their relation to one another. Additionally, when the natural remanence of a rock is carried by high-coercivity minerals, it is essential to isolate the anisotropy of this grain fraction to correct paleodirectional and paleointensity data. AIRMs have been measured using a wide range of applied fields, from a few mT to several T. It has been shown that the degree and shape of AIRM can vary with the strength of the applied field, for example, due to the contribution of separate grain subpopulations or due to field-dependent properties. To improve our understanding of these processes, we systematically investigate the variation of AIRM and the anisotropy of partial isothermal remanence (ApIRM) with applied field for a variety of rocks with different magnetic mineralogies. We also test the additivity of A(p)IRMs and provide a definition of their error limits. While A(p)IRM principal directions can be similar for a range of applied field strengths on the same specimen, the degree and shape of anisotropy often show systematic changes with the field over which the (p)IRM was applied. Also, the data uncertainty varies with field window; typically, larger windows lead to better-defined principal directions. Therefore, the choice of an appropriate field window is crucial for successful anisotropy corrections in paleomagnetic studies. Due to relatively large deviations between AIRMs calculated by tensor addition and directly measured AIRMs, we recommend that the desired A(p)IRM be measured directly for anisotropy corrections.

Studying the dynamics of cosmic dust flux on the earth’s surface from peat deposits

Peat cores sampled from different climatic zones are studied. The petromagnetic and microprobe methods are used to find peat layers enriched with cosmic dust. It is established that the behavior of saturation isothermal remanent magnetization (SIRM) of peat deposits from the zones where the aeolian transfer of terrigenous particles is negligible can be used for studying the dynamics of the fall of cosmic matter on the Earth’s surface. The cosmic dust flux can be conditionally divided into the background and burst components. Here, the background flux of cosmic dust varies cyclically. The characteristic times of these cycles are about 100 years. The cyclicity in the background flux of cosmic material most clearly manifested itself in the interval of 1200 to 500 years ago. The most significant burst in the influx of cosmic material (by an order of magnitude above the background) is revealed in the layer that was formed about 5000 years ago. The microprobe studies established that the mineralogical content of cosmic dust differs between the background and burst fluxes.