Advancement of full-vector variable-temperature magnetometry for rock-magnetic and paleointensity applications

Data on the variation of the direction and strength of Earth’s ancient magnetic field (absolute paleointensity) provide crucial information into the mechanisms of the geodynamo and the Earth’s thermal history. However, the use of conventional methods and instrumentation for absolute paleointensity determination has been hampered by physicochemical alteration of the samples caused by multiple high-temperature cycles and long experiment durations. The reliability and efficiency of the measurement process can be improved by the measurement of the full remanent magnetization vector simultaneously with the temperature cycling of a sample. Such as approach can also substantially expand the scope of materials available for rock magnetic, paleomagnetic, and paleointensity analyses. To date, no commercial instruments and only a few custom-made devices provide this capability. In addition, the existing instruments are not easily accessible, inconvenient, poorly documented, and often characterized by mediocre sensitivity and reliability. In the course of this Ph.D. study, a new full-vector variable-temperature vibrating sample magnetometer (MAG×NEAT) has been developed. The instrument allows fast and fully-automated measurements of magnetic moment as a function of temperature in a range between 20 and 800 °C temperature range. The measurements can be conducted in a vacuum or a controlled atmosphere, which, together with a short duration of thermal treatments, substantially reduces the potential for magneto-mineralogical alteration. The instrument’s capabilities have been utilized to develop a new implementation of the Lowrie method to characterize magnetic mineralogy using a combination of magnetic hysteresis and thermomagnetic analyses. The new approach (the Lowrie-Express method) has been successfully tested on a suite of synthetic and archeological samples. In particular, the method is more effective than conventional rock magnetic techniques in identifying a low Curie temperature and high-coercivity magnetic phase (interpreted as epsilon iron oxide). The new magnetometer has also been utilized to obtain absolute paleointensity with the Thellier-Coe method from six archeological objects from central and northwestern Russia, representing the 13th to 18th century period. The new instrument allows reducing the duration of a paleointensity experiment from several days to several hours, resulting in more reliable, higher quality determinations than the conventional Thellier-Coe method. The new archeointensity and inclination data suggest a period of steeper geomagnetic inclinations and a stronger geomagnetic field in central Russia in the 16th century, followed by a gradual decay in the field strength to its present-day values. These observations are consistent with the existing database for the Eastern European Plain. The results of this dissertation work open new opportunities for using full-vector variable-temperature magnetometry for rock-magnetic and paleointensity investigations

A Versatile Software for Statistical Data Analysis and Spatial Correlation

Development of new tools for interpreting of and maximizing the amount of information extracted from georeferenced experimental and observational data remains an important task of computational geoscience. One of powerful tools for data analysis is the examination of correlation dependence between different, sometimes seemingly unrelated parameters. However, the correlation analysis of geospatial data can often be hampered by inadequate sampling or lack of coincidence between coordinates in the compared datasets. The number and capabilities of affordable software that can be used for these purposes are limited. Here we present a versatile software package for correlation analysis of spatial geo-referenced data with an additional set of tools. The software is implemented as a web service and contains a toolset for performing a geographic coordinate conversion, data interpolation, mapping, spatial correlation, data visualization and other auxiliary functions for spatial data analysis. The performance of the software is checked and confirmed on the real data. We call this service “UNCORR”—the UNiversal CORRelation tool

Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

Titanomagnetite containing up to 0.6–0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3−xTixO4 (x ≈ 0.46) at temperatures of 355, 500 and 550 ◦C for up to 375 hr, by examining their magnetic properties over a wide range of temperatures. The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30 per cent for the samples annealed for 375 hr at 355 ◦C, and for 110 hr at 500 and 550 ◦C. The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205 ◦C) by up to 350 ◦C. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, that is, not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties

Nonheating methods for absolute paleointensity determination: Comparison and calibration using synthetic and natural magnetite-bearing samples

Nonheating paleointensity methods utilize an anhysteretic remanent magnetization (ARM) or a saturation isothermal remanent magnetization to model the natural thermal remanent magnetization (TRM) to avoid heating-induced alteration. We report the results of paleointensity experiments using the ARM, pseudo-Thellier, and ratio of equivalent magnetization (REM) methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors. The experiments on synthetic magnetite-bearing samples representing single-domain (SD) and pseudo-single-domain (PSD) magnetic states indicated that the correction factors for the ARM-based methods depend on the magnetic grain size/domain state changing from ~6.3 (for SD grains) to ~4.1 (for ~1.5 µm PSD grains). The pseudo-Thellier method yielded correct absolute paleointensity values when normalization by the TRM/ARM demagnetization slope was used. When applied to samples of lava flows and dikes from the ~32 kyr Lemptégy volcano (France), both the ARM and pseudo-Thellier methods produced similar paleointensity estimates (28.0 ± 5.1 μT and 26.9 ± 4.7 μT, respectively) consistent with the available Thellier data for the 31–33 kyr time interval. The correction factors estimated from our synthetic samples for the REM (~3000) and for REMc (~1500) and REM′ (~1500) variants are consistent with the previously published estimates. However, all REM variants yielded unrealistically high estimates (\u3e 110 μT) of the paleofield strength from our natural samples. Our experimental results support ARM as a better proxy of TRM and suggest that the ARM-based methods currently represent the best alternative to heating-based absolute paleointensity method

Influence of the Preparation Technique on the Magnetic Characteristics of ε-Fe₂O₃-Based Composites

ε-Fe2O3 is an iron(III) oxide polymorph attracting an increasing interest due to its unique magnetic properties combining extremely high coercivity and relatively large saturation magnetization. We review existing methods for the ε-Fe2O3 synthesis focusing on synthesis speed, repeatability, manufacturability and purity of the final product. Samples of ε-Fe2O3 have been synthesized using the two methods that appear the most promising: silica gel impregnation and microemulsion. In both cases, ε-Fe2O3 and α-Fe2O3 are present in the final product as attested by X-ray diffraction patterns and magnetic properties (maximum coercive force at 300 K~1 Tesla). Two different precursors, iron(III) nitrate and iron(II) sulfate, have been used in the silica gel impregnation method. Somewhat surprisingly, iron sulfate proved superior yielding ε-Fe2O3 content of 69% in the total iron oxide product, compared to 25% for iron nitrate under the same synthesis conditions. These results pave the way for modifying the existing ε-Fe2O3 synthesis methods aiming to increase the content of the epsilon phase in the final product and, consequently, improve its physicochemical properties

High-coercivity magnetic minerals in archaeological baked clay and bricks

The thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic or archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20 and \u3e600 °C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 and 300 K have been carried out. All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Ti, Mn and/or Al. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540 and 620 °C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140 and 240 °C, and its magnetic properties correspond to an enigmatic high coercivity, stable, low-unblocking temperature (HCSLT) phase reported earlier. In a few samples, high-and low unblocking temperature, magnetically hard phases appear to coexist; in the others, the HCSLT phase is the only magnetically hard mineral present