EMRP3.2

The Penedos-Borralha area is located inside the Galícia-Trás-os-Montes Zone where Silurian metasediments and Variscan granites outcrop. Three Variscan ductile deformation phases were recognized: D₁ (360-337Ma); D₂ (337-320Ma) and D₃ (320-310Ma). D₃implied a regional subvertical crenulation (N120ºE) and the deformation of syn-D₃ granites (321–312Ma). This contribution results from a multidisciplinary approach (viz., fieldwork, petrography, and anisotropy of magnetic susceptibility (AMS)) for the understanding of the magnetic anisotropy patterns of Penedos (PG, post-D₃) and Borralha (BG, syn-D₃). PG occurs as circumscribed outcrop in a triple point marked by the contact between BG, Borralha tonalite and metasediments. PG is a leucocratic, medium- to coarse-grained granite with garnet. BG occurs as a WNW-ESE outcrop and is composed of biotite-rich, medium- to coarse-grained porphyritic granite. At the outcrop scale, oriented patterns were not observed in PG; however, the BG exhibits K-feldspar megacrysts and phyllosilicates N120ºE oriented. The main mineralogical features observed in BG are quartz displaying strong undulose extinction and subgranulation, K-feldspar with poikilitic texture, plagioclase presenting curved twins and well-developed kinked phyllosilicates. In turn, PG presents quartz displaying slightly undulose extinction, slightly stretched plagioclase, euhedral garnet crystals and phyllosilicates occurring as clustered flakes. The petrofabric studies were obtained using AMS providing scalar (magnetic susceptibility, K_m and paramagnetic anisotropy, P_para) and directional (magnetic foliation, ⊥K₃ and lineation, K₁) parameters. The K_mindicated paramagnetic behaviour for both BG and PG (53.9 and 30.39µSI, respectively) classifying them as ilmenite-type granites.BG exhibits the highest P_para values (4.4%), result of the strong K-feldspars and biotite alignment. In contrast, PG exhibits the lowest P_para values (1.89%) compatible with the no-oriented patterns. The ⊥K₃ in the PG are very heterogeneous ranging from NW-SE to E-W; generally, the ⊥K₃ are subvertical in the E side, where the PG is intrusive in the metasediments and subhorizontal in the W side, where the PG cuts the BG. Concerning the K₁ the PG display NNE-SSW to W-E azimuths with subhorizontal to intermediate dips. In the BG, the ⊥K₃, essentially subhorizontal, tends to be parallel to the contacts with the regional rocks and the K₁are strongly subhorizontal with azimuths ranging from WNW-ESE to ENE-WSW. Considering this multidisciplinary approach is clear that the petrofabric obtained for both granites resulted from distinct phenomena. The PG petrofabric was inherited from magmatic stages, where the K₁ trajectories suggest the location of a feeder zone in the SE border and continuous magmatic flow to NW. The evidence of subvertical ⊥K₃ in the E side of PG suggests a tongue-shaped intrusion thicker on this side. In contrast, the BG petrofabric was acquired in the subsolidus and resulted from tectonic processes. The obtained petrofabric agrees with the proposed classification of BG and PG as a syn-D₃ and post-D₃ granites, respectively. These analyses applied to two contrasting intrusions allowed us to verify that the AMS depend on several parameters and must be interpreted with caution.

Acknowledgments: This work was supported by national funding awarded by FCT - Foundation for Science and Technology, I.P., projects UIDB/04683/2020 and UIDP/04683/2020.

How to cite: Gonçalves, A., Sant'Ovaia, H., and Noronha, F.: Petrofabric patterns of two contrasting plutons: example of Penedos and Borralha granites (Montalegre, Northern Portugal), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5153, https://doi.org/10.5194/egusphere-egu22-5153, 2022.

Magnetic anisotropy is a time-efficient and powerful tool to characterize the anisotropy of pore space in ferrofluid-impregnated rocks. Empirical correlations exist between magnetic pore fabrics and the shape preferred orientation of pores, as well as between magnetic pore fabrics and permeability anisotropy. Up to now, quantitative interpretation has been challenging, and one reason is the variability of ferrofluids and their properties that have been used in different studies. Namely, the susceptibility of the ferrofluid largely controls the degree of measured magnetic anisotropy. Being a colloidal solution of superparamagnetic particles, ferrofluid displays magnetic properties that are both frequency- and time-dependent. This in turn affects the quantitative interpretation of magnetic pore fabrics. This study sheds light on the magnetic properties of the ferrofluids used in pore fabric studies, with a particular focus on processes such as particle aggregation and sedimentation, and how these affect ferrofluid impregnation as well as the measured pore fabrics. Further, interactions between the ferrofluid and the rock that lead to changes in magnetic properties are studied. These results will form the basis for future quantitative interpretation of magnetic pore fabric studies in groundwater, CO2 or hydrocarbon applications.   

How to cite: Biedermann, A. R., Pugnetti, M., Zhou, Y., and Parés, J. M.: Characterizing ferrofluid properties for a more reliable and quantitative interpretation of magnetic pore fabric studies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5251, https://doi.org/10.5194/egusphere-egu22-5251, 2022.

Anisotropy of magnetic susceptibility (AMS) studies have proven valuable in identifying subtle or cryptic petrofabrics, significantly broadening the scope of quantitative petrofabric analyses. An exciting development in magnetic anisotropy techniques is the ability to resolve an AMS response into an in-phase (ipAMS) and out-of-phase (opAMS) component using an AGICO KLY-5a Kappabridge. Because the opAMS response is produced by ferromagnetic minerals (whereas ipAMS is the result of the sum of all contributing components), it has the potential to record ferromagnetic sub-fabrics¹. However, in natural rock specimens the origin of the opAMS response remains unclear  and previous research has published conflicting reports on exactly which magnetic populations contribute to the opAMS response^1,2.

Our study attempts to understand the source of the opAMS signal in magnetite-rich mafic samples from the Younger Giant Dyke Complex in southern Greenland³.  We conduct magnetic characterisation experiments alongside AMS measurements on 22 samples, including temperature-susceptibility, magnetic saturation, hysteresis, and first-order reversal curve experiments. Our samples have absolute out-of-phase susceptibility values between 2x10^-7 and 6.75x10^-4 SI units, above the detection limit of the KLY-5a Kappabridge. Three distinct relationships are observed between ipAMS and opAMS responses; 1) a parallel ip/op response, suggesting the two AMS responses have an identical source, 2) perpendicular ip/op responses, suggesting a mineralogical control on the AMS response, and 3) ip/op responses at an oblique angle to each other, suggesting two distinct magnetic subfabrics. Surprisingly, 87% of our samples return a negative out-of-phase susceptibility, which is unexpected for ferromagnetic samples.

Magnetic characterisation experiments identify three magnetically distinct sample groups; A) relatively low coercivity, which we suggest represent a multi-domain (MD) dominated system, B) relatively high coercivity samples, which we interpret to have  a significant proportion of super-paramagnetic/single-domain (SPM/SD) magnetite as well as MD grains, and C) similar to B but with a smaller proportion of SPM/SD grains. Comparing ipAMS/opAMS Groups 1–3 to the magnetic characterisation Groups A–C, we find no consistent correlation between the rock’s magnetic mineralogy and the observed opAMS response. The difference in opAMS and ipAMS principal axes orientations therefore does not appear to be controlled by the varying proportion of MD or SPM/SD magnetite.

This result is surprising, as a correlation between ferromagnetic properties and opAMS is expected since opAMS is governed by ferromagnetic grains only. We propose three hypotheses which may explain our results; 1) there is an error in the way the data is processed using the standard software, causing negative susceptibility responses and apparent axes flipping, 2) SPM/SD magnetite may only carry an AMS signal in some of the samples, convoluting the interpretation of opAMS, or 3) viscous relaxation (caused by SPM/SD magnetite) may generate a much stronger opAMS response, resulting in a disproportionate influence on the opAMS signal whilst remaining masked in magnetic characterization experiments. Each hypothesis is assessed in our study and we recommend that further development is required before opAMS is routinely applied to petrofabric studies.

¹Hrouda et al., 2017. GJI., ²Hrouda et al., 2020. PEPI., ³Koopmans et al., 2021. GEUS.

How to cite: Koopmans, L. and McCarthy, W.: Assessing the source of out-of-phase AMS in magnetite rich igneous rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10591, https://doi.org/10.5194/egusphere-egu22-10591, 2022.

Three-dimensional shapes of 68 magnetite grains in pyroxene and 234 magnetite grains in plagioclase, were obtained by “slice-and-view” focused-ion-beam nanotomography (FIB-nt) on mineral separates from the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction (EBSD) determined the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. For each particle, hysteresis loops in 20 equidistributed field directions were calculated by the finite-element micromagnetic code MERRILL. For each direction, the averages over the particle ensemble were compared to corresponding hysteresis loops measured with a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. FIB-nt combined with micromagnetic modelling allows to explore the mechanisms controlling the magnetic anisotropy for each individual particle and to analyze the combined effect for bulk magnetic properties. This combination is of interest for anyone who interprets magnetic anisotropy because it helps understanding how domain states and crystal alignment in natural samples influence the measured anisotropy. Our results demonstrate that natural particle shapes, their orientations and domain states control the anisotropy of magnetic remanence, coercivity and susceptibility in natural particles. We can show for our specific examples, how the connection between mineral texture and magnetic anisotropy depends on specific domain states. Our data explain why natural magnetite particles at the transition between single-domain and single or multiple vortex states do not always follow the relation between axis orientation and  magnetic anisotropy which is theoretically expected for simple particle shapes.

How to cite: Nikolaisen, E. S., Harrison, R. J., Fabian, K., and McEnroe, S. A.: Anisotropy of magnetic susceptibility and hysteresis parameters for natural magnetite particle assemblages: Micromagnetic analysis of focused-ion-beam nanotomography data with MERRILL, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8528, https://doi.org/10.5194/egusphere-egu22-8528, 2022.

The recently developed Micromagnetic Tomography (MMT) technique allows precise recovery of magnetic moments of individual magnetic grains in a sample. By combining high resolution scanning magnetometry and micro X-ray computed tomography (MicroCT) MMT has the potential to become an important asset in rock-magnetic and paleointensity studies. However, uncertainties in magnetic moment solutions obtained through MMT are yet enigmatic, making a geologic application of MMT results uncertain. Therefore, we have made a first attempt in addressing those mathematical uncertainties surrounding MMT, by studying the effect of five parameters that directly influence the uncertainty of magnetic moment solutions: grain concentration of the sample, thickness of the sample, size of the sample's surface, noise level in the magnetic scan, and sampling interval of the magnetic scan. The effect of MicroCT errors are not included in this study, since those errors are better solved by improving the experimental routine than by mathematical corrections. We assess how well the magnetic moments are resolved as function of the aforementioned five parameters by setting up series of numerical models in which we assign dipole magnetizations to randomly placed grains. We perturb per model the surface magnetic field with different instrumental noise levels and sample these fields with a varying interval. The MMT inversion provides the magnetic moment per grain, and additionally produces the covariance matrix and standard deviations, which are used to define a statistical uncertainty ratio and signal strength ratio for each solution. We show that the magnetic moments of a majority of grains under realistic conditions are solved with very small uncertainties. However, increasing the grain density and sample thickness carry major challenges for the MMT inversions. Fortunately, we can use the newly defined signal strength ratio to extract grains with the most accurate solutions, even from these challenging models. Thereby we have developed an quantitative routine to individually select the most reliable grains from MMT results. This will ultimately enable determining paleodirections and paleointensities from large subsets of grains in a sample using MMT.

How to cite: Out, F., Cortés-Ortuño, D., Fabian, K., van Leeuwen, T., and de Groot, L.: Quantifying mathematical uncertainties in Micromagnetic Tomography results, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-931, https://doi.org/10.5194/egusphere-egu22-931, 2022.

In this contribution, we present a rock magnetic and magnetic fabric study of the Dejvice loess/paleosol sequence with an aim to demonstrate how rock magnetic methods can be very effective tools for detecting paleoenvironmental, pedogenic, and post-depositional processes. This study covers the 15-meter-long loess/paleosol section which was recently temporarily accessible during the underground construction works in the Vienna House Diplomat Hotel in Prague. The exposed part of the sequence contained at least four different paleosol horizons and covered the time interval from ca. 130 ky to recent. For the purpose of this study, 425 orientated samples (8 ccm) were collected evenly covering the studied section.

In general, loess sequences contain variable amount of detrital magnetic particles derided from the source material. In addition, in warmer interglacials periods, pedogenesis results in formation of paleosol horizons which are magnetic enhanced by the in-situ neo-formed nanoscale ferromagnetic particles.

The applied rock-magnetic techniques included measurements of (1) magnetic susceptibility (MS), (2) frequency-dependent susceptibility (kFD), (3) out-of-phase magnetic susceptibility (opMS), and (4) viscous magnetization (Mv). While MS very sensitively reflects the relative amount of all magnetic particles, the other methods (kFD, opMS, and Mv) mirror solely the contribution of the neo-form nanoscale particles. In addition to these rock magnetic parameters, (5) anisotropy of magnetic susceptibility (AMS) was measured in order to obtain magnetic fabric reflecting the preferred orientation of magnetic minerals. Magnetic fabric can be primarily interpreted in terms of paleotransport directions but it may also provide some evidences for post-depositional reworking and/or movements.

All paleosol horizons possess significantly higher values of MS, kFD, opMS and Mv. This indicates that the increased amount of magnetic particles in paleosols is exclusively due to the magnetic enhancement caused by the neo-formation of nanoscale particles during pedogenesis. In addition, the values of kFD, opMS, and Mv mutually intercorrelate very tightly. This indicates that all these independent methods are reliable proxies for the quantification of ultra-file particles in loess/paleosols horizons.

In addition of the paleotransport direction, the magnetic fabric reflects secondary sedimentary processes. This involves the displacement of clastic particles by flowing water and the redeposition of the material along the slope. The direction of movement of these sediments corresponds to the current geomorphology of the surroundings. We can conclude that the section was not deposited solely by the aeolian processes.

How to cite: Chadima, M., Žatecká, M., Kolaříková, K., Bradák, B., and Kadlec, J.: Magnetic fabric study of the Dejvice loess/paleosol sequence (Prague, Czech Republic), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7202, https://doi.org/10.5194/egusphere-egu22-7202, 2022.

The Central and Southern Adriatic islands, situated offshore of the Split-Dubrovnik segment of the mainland, belong to the External Dinarides. Their dominant tectonic trend is W-E, significantly different from the general Dinaric NW-SE orientation. For this reason, they are often regarded as belonging to a distinct tectonic unit, and sometimes a vertical axis counterclockwise (CCW) rotation is invoked to explain the deviation from the general tectonic trend.

In this paper, new paleomagnetic results are presented from Late Jurassic through Paleocene shallow water limestones from the Central and Southern Adriatic islands. For the localities sampled for paleomagnetic investigation, the sedimentological properties and the stratigraphic ages, based on the foraminifera population, were checked by microscopy investigation. The paleomagnetic analysis was carried out on field oriented drill cores, according to standard laboratory processing. The results were evaluated statistically on locality level. Overall-mean paleomagnetic directions were computed for several age groups and the age of the acquisition of the remanence was estimated from between-locality fold tests for a number of age groups. Comparison with earlier published results from the Northern Adriatic islands and from Stable Adria lead to the conclusion that the Central and Southern Adriatic islands had not rotate with respect to the Northern ones and even more importantly, stable Adria and the offshore External Dinarides must have moved as an integrated unit, at least from the Albian on.

This work was financially supported by the National Development and Innovation Office of Hungary project K 128625.

How to cite: Márton, E., Ćosović, V., Imre, G., and Velki, M.: Central and Southern Adriatic islands: tectonic implications of new paleomagnetic results, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12396, https://doi.org/10.5194/egusphere-egu22-12396, 2022.

In the region under study, two main types of geodynamical processes are developed: (1) final collisional and (2) initial spreading. The geodynamic-paleomagnetic mapping makes it possible to reveal not only the multilevel structural heterogeneity but also display complex elements of the geodynamics of different ages inherent in the junction (transition) zones. Just for these complex regions, paleomagnetic mapping is especially important, since makes it possible to obtain significant information unattainable by any other geological-geophysical methods. The methodology of paleomagnetic mapping of the complex junction zones is based on the integration of the mapping techniques for both continental and oceanic platforms: paleomagnetic reconstructions, results of radiometric dating of magnetized rocks, biogeographical studies, satellite data examination, plate tectonic reconstructions, and utilization of results of various geophysical surveys. All these data are used for the integrated identification of mapped geological bodies and structures. For paleomagnetic mapping, two different reference areas were selected: spreading area with younger traps (Sea of Galilee, northern Israel), and collisional area with older traps (Makhtesh Ramon, southern Israel). The supplemented edition of the paleomagnetic map of the Sea of Galilee region (northern Israel) was extended to the south where the Belvoir uplift exists with the detailed reference well, and a complex of radiometric dating of the Cenozoic traps is developed. The map has been significantly detailed due to analyzing new data from structural, radiometric, and paleomagnetic researches, which has expanded the understanding of both the tectonic-structural peculiarities and development of this complex area (Eppelbaum et al., 2022). The subterrane Makhtesh Ramon (southern Israel), is collisionally joined with the Arabian-Nubian part of Gondwana. The Ramon subterrane contains various tectonic units formed during the pre-collisional, collisional, and post-collision stages of its paleogeodynamic evolution. In this paleomagnetic scheme and diagram of geodynamic reconstructions, not all subterrane, but the edge of its submerged part corresponds to erosion-tectonic depression, and includes outcrops of Lower Cretaceous, Jurassic, Triassic, and Late Cenozoic (Eppelbaum and Katz, 2015). These rocks are penetrated by a variety of Mesozoic traps, whose radiometric age ranges from 165.7 to 93.8 Ma, and contain scattered ophiolite outcrops of the Sakharonim basalts. These ophiolites are associated with absorption of the Triassic-Jurassic crust of the Neotethys Ocean in Hauterivian and relate to the Levantine phase of tectogenesis. The pre-collision formations preceding this phase are represented by basalts dykes (Omolon and Gissar superzones) and laccolites of mainly alkalic olivine gabbro associated mostly with the Gissar superzone. Tectonic-paleomagnetic mapping as a new type of combined geological-geophysical survey contributed to an essential understanding of the junction zone of the Mesozoic Terrane Belt and the Dead Sea Transform.

Eppelbaum, L.V. and Katz, Yu.I., 2015. Eastern Mediterranean: Combined geological-geophysical zonation and paleogeodynamics of the Mesozoic and Cenozoic structural-sedimentation stages. Marine and Petroleum Geology, 65, 198-216.

Eppelbaum, L.V., Katz, Y.I. and Ben-Avraham, Z., 2022. Advanced combined geophysical-geological mapping of the Sea of Galilee and its vicinity, In: (A. di Mauro, A. Scozzari, S. Soldovieri, Eds.), "Instrumentation and Measurement Technologies for Water Cycle Management", Springer, 1-23.

How to cite: Eppelbaum, L. and Katz, Y.: Geodynamic-paleomagnetic mapping of the heterogeneous geological structure of the junction zone of the Mesozoic Terrane Belt and the Dead Sea Transform, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1919, https://doi.org/10.5194/egusphere-egu22-1919, 2022.

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant K_max axes trending NW-SE, parallel to the fold axes. The K_min axes are orientated in the NE-SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasms in the tectonic deformation that according to the ages of the rocks are 300-290 Ma (Sauce Grande to Bonete Formation deposition) and 290-276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.  

How to cite: Tomezzoli, R. N., Arzadún, G., Fortunatti, N., Cesaretti, N. N., Febbo, M. B., Calvagno, J. M., and Choque, G.: Spasmodic deformation in the Southwest of the Gondwana boundary, Upper Paleozoic of Ventana ranges in Argentina, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8926, https://doi.org/10.5194/egusphere-egu22-8926, 2022.

Oriented needle shaped magnetite inclusions in plagioclase may give rise to magnetic anisotropy of individual plagioclase grains and, in case of preferred orientation of the plagioclase grains, contribute to magnetic anisotropy of the bulk-rock. Understanding how oriented magnetite inclusions generate magnetic fabrics in single grains of plagioclase is important for interpreting rock magnetic fabrics and for correcting paleomagnetic data. Plagioclase grains from oceanic gabbro dredged at the Mid Atlantic Ridge (11-17°N) were analyzed using optical microscopy, electron backscatter diffraction (EBSD), as well as alternating field demagnetization and anisotropy of magnetic remanence (AMR) measurements to investigate the influence of the shape orientation distribution of acicular magnetite inclusions on the magnetic properties of magnetite bearing plagioclase grains.

In pristine magmatic plagioclase, the needle elongation directions form a 30° wide girdle distribution parallel to the pl(010) plane. This girdle distribution is insensitive to twinning after the Albite, Pericline, Carlsbad and Manebach laws. The statistical maximum in the inclusion orientation lays in the pl(010) plane, closely parallel to the pl[001] direction. The overall shape orientation distribution of the magnetite inclusions produces a triaxial magnetic anisotropy ellipsoid with the minimum axis direction sub-perpendicular to the pl(010) plane and the maximum axis sub-parallel to the pl[001] direction.

The vector of natural remanent magnetization parallels the maximum AMR axis direction indicating that the magnetic anisotropy caused by the magnetite inclusion fabric controls the paleomagnetic signature. In hydrothermally modified plagioclase, most or all magnetite needles are oriented parallel to the pl[001] direction and a prolate rotational ellipsoid of remanent magnetization with the maximum remanent magnetization parallel to pl[001] should occur.

Plagioclase-hosted magnetite inclusions are particularly stable recorders of the paleomagnetic filed. The magnetic anisotropy arising from the anisotropic shape orientation distribution of the magnetite inclusions may, however, bias the magnetic record, an effect that needs to be accounted for in paleomagnetic reconstructions or for absolute paleointensity experiments.

This work was supported by the Austrian Science Fund (FWF): I 3998-N29, and by the Russian Foundation for Basic Research, Grant no. 18-55-14003.

How to cite: Ageeva, O., Gilder, S., Habler, G., and Abart, R.: Ferromagnetism of magnetite-bearing plagioclase from oceanic gabbro, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13488, https://doi.org/10.5194/egusphere-egu22-13488, 2022.

In this study, we have carried out a laboratory simulation of the low-temperature oxidation (below T_c) of titanomagnetite carrying a primary thermoremanent magnetization (TRM) created in B_lab=25-50 µT. The initial material was natural basalt P72/4 from the Red Sea rift zone containing unoxidized (degree of oxidation Z =0) titanomagnetite with ulvöspinel content x=0.5 (Fe_2.5Ti_0.5O₄).

Cubic basaltic samples containing titanomagnetite were annealed in air in weak field B_an=50-100μT at temperature T_an=260 Cfor maximum time t=1300 hours. One group of samples were annealed in the magnetic field perpendicular (B_an⊥TRM) and other parallel (B_an∥TRM) to the primary TRM. Thellier's-Coe double-heating method in argon atmosphere were conducted using all samples with different Z.

As for B_an∥TRM, the calculated field value (B_calc) obtained from Thellier-Coe’s procedure coincided with the laboratory field (B_lab) for all annealing times except t=1300 hours. This result indicates the applicability of the Thellier-Coe method of paleointensity determination on basalt containing titanomagnetite of low and medium degrees of oxidation (Z<0.5).

However, inadequate results were obtained from samples annealed in a magnetic field perpendicular to primary TRM(B_an⊥TRM). In this case, the calculated value of the B_calc field for t=12 hours are overestimated by ~40%, and for t=400 and 1300 hours is underestimated by ~20% relative to the TRM creation field.

We conclude that reliability of paleointensity data of oxidized titanomagnetite depends on the direction between magnetic field and TRM during the oxidation process.

How to cite: Grachev, R.: The effect of magnetic field direction during the low-temperature oxidation of titanomagnetite bearing TRM on paleointensity determinations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12194, https://doi.org/10.5194/egusphere-egu22-12194, 2022.