An end-member modeling approach (EMMA) to pseudo-Thellier paleointensity data

Absolute paleointensities are notoriously hard to obtain, because conventional thermal Thellier paleointensity experiments often have low success rates for volcanic samples. The thermal treatments necessary for these experiments potentially induce (magnetic) alteration in the samples, preventing a reliable paleointensity estimate. These heating steps can be avoided by pseudo-Thellier measurements, where samples are demagnetized and remagnetized with alternating-fields. However, pseudo-Thellier experiments intrinsically produce relative paleointensities. Over the past years attempts were made to calibrate pseudo-Thellier results into absolute paleointensities for lavas by mapping laboratory induced Anhysteretic Remanent Magnetizations (ARMs) to the thermally acquired Natural Remanent Magnetizations (NRMs). Naturally occurring volcanic rocks, however, are assemblages of minerals differing in grain size, shape, and chemistry. These different minerals all have their own characteristic mapping between ARMs and thermal NRMs Here we show that it is possible to find these characteristic mappings by unmixing the NRM demagnetization and the ARM acquisition curves into end-members, with an iterative method of non-negative matrix factorization. In turn, this end-member modeling approach (EMMA) allows for the calculation of absolute paleointensities from pseudo-Thellier measurements. We tested our end-member modeling approach using a noise-free numerical data set, yielding a perfect reconstruction of the paleointensities. When adding noise up to levels beyond what is expected in natural samples, the end-member model still produces the known paleointensities well. In addition, we made a synthetic dataset with natural volcanic samples from different volcanic edifices that were given a magnetization by heating and cooling them in a controlled magnetic field in the lab. The applied fields ranged between 10 and 70 μT⁠. The average absolute difference between the calculated paleointensity and the known lab-field is around 10 μT for the models with 2 to 4 end-members, while the paleointensity of almost all flows can be retrieved within a deviation of ± 20 μT⁠. The average difference between calculated paleointensities for the 3 end-member model is -1.7 μT⁠. The deviations between the paleointensities and the known lab-fields are almost Gaussian distributed around the expected values. To assess whether the end-members produced by our analysis have a physical meaning, we measured the Curie temperatures of our samples. These Curie measurements show that there is a relationship between the abundances of the end members of the 3 end-member model in the samples and their dominant Curie temperatures. This indicates that even whilst the spectrum of Curie temperatures and hence composition of iron-oxides in the sample set is continuous, and the magnetization is also related to mineral size and shape, the calculated end-members of the 3 end-member model are somewhat related to magnetic mineral composition present in the samples. Although the two datasets in our study show that there is potential for using this end-member modeling technique for finding absolute paleointensities from pseudo-Thellier data, these synthetic datasets cannot be directly related to natural samples. Therefore, it is necessary to compile a dataset of known paleointensities from different volcanic sites that recently cooled in a known magnetic field to find the universal end-members in future studies

Paleointensity.org: An Online, Open Source, Application for the Interpretation of Paleointensity Data

AbstractPaleointensity.org is an online, open source, application to analyze paleointensity data produced by the most common paleointensity techniques. Our application currently supports four different methods: thermal Thellier (all variations), microwave Thellier, pseudo‐Thellier, and the multispecimen protocol. Data can be imported using a variety of input file formats such as ThellierTool files, the generic PmagPy file format, and a number of lab‐specific formats. The data for the individual paleointensity methods are visualized by the relevant graphs and parameters, which are updated dynamically while interpreting the data. Beyond manual interpretation, Paleointensity.org features an autointerpreter for specimen level Thellier‐type data. Interpretations and data can be exported to csv and MagIC files. Moreover, it is possible to export the local storage containing all data, saved interpretations, and settings. This file can be shared among researchers or attached to a paper as supporting information. Because of its many features and ease of use, Paleointensity.org is a major step forward in enhancing an open paleomagnetic community in which data can be shared, checked, and reused in line with the findable, accessible, interoperable, and reusable data principles.</jats:p

End-Member Modeling Analyses (EMMA) of pseudo-Thellier style experiments to derive absolute paleointensities from lavas

Over the past years several groups have made efforts to calibrate the ‘pseudo-Thellier’ technique to obtain paleointensities from materials that acquired their natural remanent magnetizations thermally, while avoiding heating the samples during the experiments. These calibrations revolve around mapping laboratory induced Anhysteretic Remanent Magnetizations (ARMs) to thermally acquired Natural Remanent Magnetizations (NRMs). One approach has been to plot pseudo-Thellier slopes against paleointensities that are either known (for very young lavas) or result from different paleointensity techniques. Although the obtained calibration relation is linear and closely follows the data, the relation worryingly misses th

The Influence of Positioning of the Nellix Endovascular Aneurysm Sealing System on Suprarenal and Renal Flow: An In Vitro Study

Purpose: To examine the influence of device positioning and infrarenal neck diameter on flow patterns in the Nellix endovascular aneurysm sealing (EVAS) system. Methods: The transition of the aortic flow lumen into two 10-mm-diameter stents after EVAS creates a mismatched area. Flow recirculation may affect local wall shear stress (WSS) profiles and residence time associated with atherosclerosis and thrombosis. To examine these issues, 7 abdominal aortic aneurysm flow phantoms were created, including 3 unstented controls and 3 stented models with infrarenal neck diameters of 24, 28, and 32 mm. Stents were positioned within the instructions for use (IFU). Another 28-mm model was created to evaluate lower positioning of the stents outside the IFU (28-mm LP). Flow was visualized using optical particle imaging velocimetry (PIV) and quantified by time-averaged WSS (TAWSS), oscillatory shear index (OSI), and relative residence time (RRT) in the aorta at the anteroposterior (AP) midplane, lateral midplane, and renal artery AP midplane levels. Results: Flow in the aorta AP midplane was similar in all models. Vortices were observed in the stented models in the lateral midplane near the anterior and posterior walls. In the 32-mm IFU and 28-mm LP models, a steady state of vortices appeared, with varying location during a cycle. In all models, a low TAWSS

Unraveling the Magnetic Signal of Individual Grains in a Hawaiian Lava Using Micromagnetic Tomography

Micromagnetic Tomography (MMT) is a new technique that allows the determination of magnetic moments of individual grains in volcanic rocks. Current MMT studies either showed that it is possible to obtain magnetic moments of relatively small numbers of grains in ideal sample material or provided important theoretical advances in MMT inversion theory and/or its statistical framework. Here, we present a large-scale application of MMT on a sample from the 1907-flow from Hawaii's Kilauea volcano producing magnetic moments of 1,646 grains. We produced 261,305 magnetic moments in total for these 1,646 grains, an increase of three orders of magnitude compared to earlier studies to assess the robustness of the MMT results, and a major step toward the number of grains that is necessary for paleomagnetic applications of MMT. Furthermore, we show that the recently proposed signal strength ratio is a powerful tool to scrutinize and select MMT results. Despite this progress, still only relatively large iron-oxide grains with diameters >1.5–2 μm can be reliably resolved, impeding a reliable paleomagnetic interpretation. To determine the magnetic moments of smaller (<1 μm) grains that may exhibit pseudo-single domain behavior and are therefore better paleomagnetic recorders, the resolution of the microcomputed tomography and magnetic scans necessary for MMT must be improved. Therefore, it is necessary to reduce the sample size in future MMT studies. Nevertheless, our study is an important step toward making MMT a useful paleomagnetic and rock-magnetic technique

The use of high frequency microwaves in absolute palaeomagnetic intensity experiments

The Microwave Palaeointensity System at the University of Liverpool has developed, over 30 years, into the current third generation version; a combined 14 GHz microwave resonant cavity and superconducting quantum interference device magnetometer integrated microwave system. The use of microwave energy minimises the bulk temperatures required to demagnetise and remagnetise palaeomagnetic material, thereby limiting the significant problem of thermo-chemical alteration of magnetic minerals. Here we review the microwave palaeointensity approach, including its development, technical details, modern usage and results. We have carried out a comprehensive analysis of 20 palaeointensity studies, published between 2008 and 2022, where data collected using the microwave system may be compared with various other methods at the site level. An assessment of microwave results revealed no statistical bias compared to thermal, and known field data. We also present results from a new controlled experiment which tests the ability of the microwave to accurately recover weak, ancient palaeointensities when strongly overprinted. We conclude that the microwave system can be used for the primary method of determining accurate absolute palaeointensities or as part of a multi-method approach, and is well suited to a wide range of material from archaeomagnetic samples to ancient rocks