Thermal Remanent Magnetization Research Articles

Cracking the code of Earth's magnetic mysteries: ancient secrets unveiled by byzantine bricks reconfirmed by Mesopotamian ceramics

The study of the magnetization of artifacts discovered at archaeological sites, such as ceramics or hearth remnants, is known as archaeomagnetism. This technique is based on the idea that some materials get magnetized in the direction and intensity of the Earth's magnetic field at the time when heated or exposed to extreme heat. Development in the field has been made to improve magnetometer devices and sample size, as well as measuring techniques of thermal remanent magnetization. It helped to study the variation of the complex Earth’s magnetic field or geomagnetic field (GMF) for both geophysical and archaeological dating benefits. The rapid change in GMF within a few hundred or decades, as being of genuine origin or methodological and sample unsuitability has been a matter of debate. Data from about 40 years ago derived from well-dated Byzantine churches that first confirmed such geomagnetic spikes is discussed along with recent archaeomagnetic data from Mesopotamian inscribed bricks and Levantine slags.

Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany

AbstractThe Ries impact structure (Germany) contains well‐preserved ejecta deposits consisting of melt‐free lithic breccia (Bunte Breccia) overlain by suevite. To test their emplacement conditions, we investigated the magnetic properties and microstructures of 26 polymict breccia clasts and a stratigraphic profile from the clasts into the suevite at the Aumühle quarry. Remanent magnetization directions of the Bunte Breccia clasts fall into two groups: those whose directions mostly lie parallel to the reversed field during impact carried mostly by magnetite, and those whose directions vary widely among each clast carried by titanohematite. Basement clasts containing titanohematite acquired a chemical remanent magnetization (CRM) during the ejection process and then rotated during turbulent deposition. Clasts of sedimentary rocks grew magnetite after turbulent deposition, with CRM directions lying parallel to the paleofield. Suevite holds a thermal remanent magnetization carried by magnetite, except for ∼12 cm from the contact with the Bunte Breccia, where hematite concentrations increase due to hydrothermal alteration. These observations lead us to propose a three‐stage model of (a) turbulent deposition of the melt‐free breccia with clast rotation <580°C, (b) deposition of the overlying suevite, which acted as a semi‐permeable barrier that confined hot (<300°C) oxidizing fluids to the permeable breccia zone, and (c) prolonged hydrothermal activity producing further alteration which ended before the next geomagnetic reversal. Basement outcrops have significantly different magnetic properties than the Bunte Breccia basement clasts with similar lithology. Two basement blocks situated near the inner ring may have been thermally overprinted up to 550°C.

Insight into the formation mechanism of the Medusae fossae formation on Mars from magnetic field data

The Medusae Fossae Formation (MFF) is one of the largest unconformable deposits on Mars. The origin of the MFF is unknown, but several formation processes have been proposed, including volcanic, wind, and ice-related mechanisms. An insight into the nature and the origin of the MFF may be ascertained by examining the degree of magnetization carried by the MFF. Here we examine the high-resolution nighttime magnetic field data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft over the MFF to ascertain the degree of magnetization carried by the MFF. By showing small modulations in the magnetic field induced by impact craters superposed on the MFF, we demonstrate that the MFF is strongly magnetized close to Apollinaris Mons. Distal to Apollinaris Mons, the MFF displays a relatively lower degree of magnetization. We show that the MFF most likely got magnetized via thermal remanent magnetization, indicating that the MFF near Apollinaris Mons was emplaced as a high-temperature volcanic product. We propose that the decline in remanent magnetization |B| with distance from Apollinaris Mons could be related to lower emplacement temperature. We combine the magnetic field constraints with previous observations of the MFF and propose that the MFF was likely emplaced as a volcanic deposit.

Permian–Triassic adakitic igneous activity at Northern Mongolia: Implication for Permian–Triassic subduction system at the Siberian continental margin

The geochemistry of the Permian–Triassic large-scale igneous rock body of northern Mongolia is a key factor in understanding the subduction-related magmatism at the margin of “Siberian continent.” Several studies have been done in the Permian–Triassic igneous body; however, its detailed magmagenesis and tectonic significance remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen Formation) and the Late Permian–Middle Triassic plutonic rocks (Selenge plutonic rock complex) of the Permian–Triassic igneous body, and intermediate dike intruding into them, and discusses the Late Permian–Middle Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low K2O/Na2O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen Formation is calculated to be 37.1° N based on thermal remanent magnetization. Therefore, the Late Permian–Middle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic rock complex, indicating that adakitic igneous activity continued after the Early Triassic.

Tectonics, cooling rates and temperatures during emplacement of the Rajmahal traps, India

Paleomagnetism is used to decipher the emplacement setting of the Rajmahal traps. Sampling was restricted to flows with known age. Three magnetic minerals are identified, titanomaghemite, Ti-poor magnetite, and possibly Mg–Al-rich titanomagnetite. Detailed analysis of thermomagnetic curves revealed that the emplacement temperature for Rajmahal traps was lower than 600 °C. Two magnetization components are distinguished. The low coercivity component, probably chemical remanent magnetization, is erased in fields up to 3.1 mT. In contrast, the high coercivity thermal remanent magnetization is more stable, sometimes throughout the demagnetization. The paleomagnetic results show that the sites have suffered up to 11° of clockwise and 17° of counter-clockwise rotation. As there is no field evidence of faulting and pervasive deformation, we propose that this rotation is probably due to penecontemporaneous joints or faults. These structures could have acted as conduits for fluids that increased the cooling rate.

New criteria for selecting reliable Thellier-type paleointensity results from the 1960 Kilauea lava flows, Hawaii

Thellier-type paleointensity experiments associated with partial thermal remanent magnetization checks have been widely used to determine paleointensity values from volcanic and archaeological media. However, previous studies have revealed that a substantial portion of paleointensity results with positive checks for historical lava samples largely fails to predict known Earth magnetic field intensity values. To determine the fidelity of paleointensity values, conventional Thellier-type paleointensity experiments were performed on Kilauea lava flows that erupted in 1960. The positive partial thermal remanent magnetization checks for our results range from 30.28 ± 1.38 µT to 52.94 ± 1.89 µT. This strongly indicates that conventional paleointensity checks cannot guarantee the fidelity of paleointensity results, especially when the unblocking temperatures for the newly formed magnetic particles are higher than the treatment temperature. Therefore, in this study, to check for thermal alteration during heating, the temperature dependence of the hysteresis parameter measured at room temperature for the thermally treated samples was also measured. Our new results show that nearly all biased paleointensity values correspond to a ratio of the coercivity of remanence to the magnetic coercivity of > 3 and a chemical alteration index > ~ 10%, which indicates the strong effect of the domain state and thermal alteration on the fidelity of the paleointensity results. Our study provides feasible criteria to further improve the fidelity of paleointensity estimations.

Comparison of Thermal and Microwave Paleointensity Estimates in Specimens Displaying Non‐Ideal Behavior in Thellier‐Style Paleointensity Experiments

AbstractDetermining the strength of the ancient geomagnetic field is vital to our understanding of the core and geodynamo, but obtaining reliable measurements of the paleointensity is fraught with difficulties. Over a quarter of magnetic field strength estimates within the global paleointensity database from 0 to 5 Ma come from Hawaii. Two previous studies on the SOH1 drill core gave inconsistent, apparently method‐dependent paleointensity estimates, with an average difference of 30%. The paleointensity methods employed in the two studies differed both in demagnetization mechanism (thermal or microwave radiation) and Thellier‐style protocol (perpendicular and original Thellier protocols)—both variables that could cause the strong differences in the estimates obtained. Paleointensity experiments have therefore been conducted on 79 specimens using the previously untested combinations of thermal‐perpendicular and microwave‐original Thellier methods to analyze the effects of demagnetization mechanism and protocol in isolation. We find that, individually, neither demagnetization mechanism nor protocol entirely explains the differences in paleointensity estimates. Specifically, we found that non‐ideal multidomain‐like effects are enhanced using the original Thellier protocol (independent of demagnetization mechanism), often resulting in paleointensity overestimation. However, we also find evidence, supporting recent findings from the 1960 Kilauea lava flow, that microwave‐perpendicular experiments performed without partial thermal remanent magnetization checks can produce underestimates of the paleointensity due to unaccounted‐for sample alteration at higher microwave powers. Together, these findings support that the true paleointensities fall between the estimates previously published and emphasize the need for future studies (thermal or microwave) to use protocols with both partial thermal remanent magnetization checks and a means of detecting non‐ideal grain effects.

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

AbstractFine‐grained, Ti‐poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain‐size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier‐type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency‐dependent susceptibility, and thermomagnetic data show that sample grain‐size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality‐control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal‐chemical remanent magnetization if microcrystal growth continued at T < Tc and/or the section is affected by post‐emplacement vapor‐phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction.

Paleomagnetism of the native copper mineralization, Keweenaw Peninsula, Michigan

The age and genesis of Michigan’s world-class native copper deposits are poorly constrained. The copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 ± 2 Ma Keweenawan Portage Lake Formation. Progressive thermal demagnetization isolates stable hematite remanent magnetization components at 28 paleomagnetic sites. Paleomagnetic tilt tests show that magnetite in massive flow interiors is primary (1095 ± 2 Ma) and that hematite throughout the formation is syntectonic. The altered cupriferous deposits contain primary ∼1095 Ma and secondary ∼1053 Ma hematite in various proportions. The Caledonia Mine’s basaltic mineralization carries the ∼1053 Ma hematite dominantly whereas the Delaware Mine’s conglomeratic interbed mineralization carries the ∼1095 Ma hematite dominantly. The ∼1095 Ma hematite is attributed mostly to magnetite exsolution during flow extrusion and to weathering oxidation between extrusion events. An infusion of epigenetic hydrothermal fluids emplaced the native copper with additional hematite and polarity self-reversing titanohematite at 1053 ± 7 Ma. Importantly, paleomagnetic evidence supports a 1053 ± 7 Ma age also for the White Pine stratiform sedimentary copper mineralization, for the oxidation of the Oronto Group clastic rocks to red beds, and for the time limits of major tectonic uplift and deformation on the Keweenaw Peninsula.

Investigating the expected archaeomagnetic dating precision in Europe: A temporal and spatial analysis based on the SCHA.DIF.3K geomagnetic field model

Archaeomagnetic dating is a chronometric dating technique, based on the ability of baked clay archaeological structures and artifacts to acquire a thermal remanent magnetization (TRM) when heated at high temperatures and cooled in the presence of the Earth's magnetic field. The precision of archaeomagnetic dating depends on several factors, among others the availability of a detailed reference secular variation (SV) curve for a given territory. In this study, we investigate the precision that archaeomagnetic dating can reach in Europe. We used the SCHA.DIF.3K European geomagnetic field model to calculate the dating uncertainties for the last 3000 years, taking into consideration only the uncertainties depending on the change rate of the geomagnetic field in the past and the reference SV curve's uncertainty envelope. Such uncertainties were calculated both in time and space, they are usually asymmetrically distributed around the considered year and it have been represented as dynamic maps of uncertainties. The obtained results show that archaeomagnetic dating based solely on directional data (declination and inclination) can be very precise for the last four centuries, with total uncertainties being almost always lower than 100 years all around Europe. Inherent dating uncertainties are also low for the last millennium, with values ranging from 50 to 300 years. These results underline the great potential of archaeomagnetism on dating baked clays belonging to the last 1000 years. For older periods, the uncertainty maps show that the dating precision may importantly vary from place to place, but with local non-symmetric uncertainty ranges generally below 400 years. The obtained maps also show that adding the intensity to the computation of the inherent uncertainties does not always reduce the uncertainty. The temporal and spatial analysis presented here does not give the total dating imprecision of archaeomagnetic dating (that definitely depends on numerous, often uncontrollable, factors), but aims at offering a useful tool to archaeomagnetic and archaeological research to check the feasibility of archaeomagnetic dating for determining the age of an archaeological structure below a desired precision.

AF demagnetization and ARM acquisition at elevated temperatures in natural titanomagnetite bearing rocks

SUMMARY Understanding the temporal changes of the Earth’s magnetic field intensity is one of the main goals of modern palaeomagnetism. For most palaeointensity methods to yield reliable results, the magnetic minerals must obey a set of rules. One of these rules is the additivity of partial thermal (TRM) or anhysteretic remanent magnetizations (ARM). Additivity was previously shown for partial TRM in single-domain particles and more generally for ARMs. Additivity between these two low-field remanences, however, has not been investigated, yet. This paper presents a series of rock magnetic experiments on natural low Ti titanomagnetites (Curie temperature between 534 °C and 561 °C) examining the effects of high temperatures on alternating field (AF) demagnetization and acquisition of an ARM. One of our sample sets comes from a borehole drilled through the impact melt sheet of the Manicouagan crater (Canada), the other from the Rocche Rosse lava flow on the island of Lipari (Italy). Hysteresis parameters indicate the magnetic carriers in the pseudo-single-domain range showing no evidence for oxidation. Thermal demagnetization at 300 °C and 500 °C before AF demagnetization shifts the coercivity spectra towards higher fields. AF demagnetization experiments at 500 °C show a significant (by a factor between 1.4 and >7.6) reduction in median destructive field and a shift towards lower coercivities. A linear relationship was found between the peak magnetic field required to demagnetize a fraction of a full TRM of a sample at a specific temperature and the one necessary to demagnetize the same fraction at room temperature after heating to that temperature. The comparison of full ARM and partial TRM at successively higher temperatures with a hybrid hTARM reveals that combined additivity between the two kinds of remanences is fulfilled. These results open the possibility to demagnetize highly coercive minerals, such as hematite and goethite, which is often not achievable at elevated temperatures. Furthermore, the additivity of TRM and ARM remanences may be used to develop novel hybrid TRM/ARM palaeointensity methods for samples, where heating is problematic (e.g. in meteorites).

Domain State and Temperature Dependence of Pressure Remanent Magnetization in Synthetic Magnetite: Implications for Crustal Remagnetization

AbstractPressure remanent magnetization (PRM) is acquired when a rock is compressed in the presence of a magnetic field. This process can take place in many different environments from impact and ejection processes in space, to burial and subsequent uplifting of terrestrial rocks. In this study, we systematically study the acquisition of PRM at different pressures and temperatures, using synthetic magnetite in four different grain sizes ranging from nearly single‐domain to purely multidomain. The magnitude of the PRM acquired in a 300 μT field is, within error, independent of the domain state of the sample. We propose that the acquisition of a PRM is mainly driven by the magnetostriction of the magnetic material. We further show that compared to a thermal remanent magnetization, the acquisition of PRM in large multidomain grains can be quite efficient, and may represent a significant component of magnetization in low‐temperature–high‐pressure environments.

Investigating the Accuracy, Precision, and Cooling Rate Dependence of Laboratory‐Acquired Thermal Remanences During Paleointensity Experiments

AbstractWe examine the behavior of natural basaltic and trachytic samples during paleointensity experiments on both the original and laboratory‐acquired thermal remanences and characterize the samples using proxies for domain state including curvature (k) and the bulk domain stability parameters of Paterson (2011,https://doi.org/10.1029/2011JB008369) and Paterson et al. (2017,https://doi.org/10.1073/pnas.1714047114), respectively. A curvature value of 0.164 (suggested by Paterson, 2011,https://doi.org/10.1029/2011JB008369) as a critical threshold that separates single‐domain‐like remanences from multidomain‐like remanances on the original paleointensity data was used to separate samples into “straight” (single‐domain‐like) and “curved” (multidomain‐like) groups. Specimens from the two sample sets were given a “fresh” thermal remanent magnetization in a 70 μT field and subjected to an infield‐zerofield, zerofield‐infield (IZZI)‐type (Yu et al., 2004,https://doi.org/10.1029/2003GC000630) paleointensity experiment. The straight sample set recovered the laboratory field with high precision while the curved set had much more scattered results (70.5 ± 1.5 and 71.9 ± 5.2 μT, respectively). The average intensity of both sets for straight and curved was quite close to the laboratory field of 70 μT, however, suggesting that if experiments contain a sufficient number of specimens, there does not seem to be a large bias in the field estimate. We found that the dependence of the laboratory thermal remanent magnetization on cooling rate was significant in most samples and did not depend on domain states inferred from proxies based on hysteresis measurements and should be estimated for all samples whose cooling rates differ from that used in the laboratory.

Emplacement temperature estimation of the 2015 dome collapse of Volcán de Colima as key proxy for flow dynamics of confined and unconfined pyroclastic density currents

The recent 10th–11th of July 2015 Volcán de Colima eruption involved the collapse of the summit dome that breached to the south generating pyroclastic density currents (PDCs) along the Montegrande ravine on the southern flank of the volcano. Trees within the valley were buried, uprooted and variably transported by the PDCs, while the trees on the edges of the valley and on the overbanks, were mainly burned and folded. The emplacement temperature of valley confined and overbank PDC deposits were reconstructed using Partial Thermal Remanent Magnetization (pTRM) analysis of lithic clasts and Charcoal Reflectance analysis (Ro %) applied to the charred wood. A total of 13 sites were sampled for the pTRM study and 39 charcoaled wood fragments were collected for the charcoal optical analysis along the entire deposit length in order to detect temperature variation from proximal to distal zone. The result overlap from both data sets display a T max from ≃345°–385 °C in valley-confined area (from 3.5 to 8.5 km from the vent) and ≃170°–220 °C (from 8.0 to 10.5 km from the vent) in unconfined distal area. The emplacement temperature pattern along the 10.5 km long deposit appears related to the degree of topography confinement: valley confined and unconfined. In particular the valley confined setting is very conservative in terms of temperature, while the major drop occurs in a very narrow space where the PDC expanded over unconfined flat topography just at the exit of the main valley. This study represents the first attempt in determining the relationship between PDCs flow dynamics variation and topographic confining using deposit emplacement temperature as key proxy.

Reliable paleointensity determinations from Late Cretaceous volcanic rocks in Korea with constraint of thermochemical alteration

Paleointensity determinations were carried out from Late Cretaceous (∼77 Ma) volcanic rocks in Korea using a Thellier-type IZZI experimental protocol with systematic partial thermal remanent magnetization (pTRM) checks. Various data selection criteria were used to estimate reliable paleointensities. We set stringent threshold values for each parameter to ensure that there was: (1) a linear relationship between natural remanent magnetization (NRM) lost and TRM gained; (2) negligible thermal alteration of magnetic minerals; and (3) uni-vectorial decay of NRM towards the origin. From the 336 samples, ∼88% were rejected because of an insufficient extrapolated NRM fraction in the best-fit line (fvds < 0.6), highlighting that fvds is the most stringent selection criterion in this study. For the 31 accepted samples, paleointensities range from 6.4 to 30.4 µT. Among the 31 samples, eight samples yielded extremely low paleointensities. Although single-domain (titano)magnetite was identified as the stable paleointensity recorder, oxidation of superparamagnetic fractions upon repeated heating probably caused enhancement of pTRM acquisition and yielded a low paleointensity estimate. Apart from these low paleointensities (n = 8) as screened by a newly proposed data selection criterion of Δk < 0.2, the remaining 23 samples have a mean paleointensity of 23.1 ± 4.8 µT, corresponding to a virtual axial dipole moment (VADM) of 40.8 ± 8.5 ZAm2, which is ∼50% of the present-day VADM.

Thermal interactions of the AD79 Vesuvius pyroclastic density currents and their deposits at Villa dei Papiri (Herculaneum archaeological site, Italy)

Pyroclastic density currents (PDCs) can have devastating impacts on urban settlements, due to their dynamic pressure and high temperatures. Our degree of understanding of the interplay between these hot currents and the affected infrastructures is thus fundamental not only to implement our strategies for risk reduction, but also to better understand PDC dynamics. We studied the temperature of emplacement of PDC deposits that destroyed and buried the Villa dei Papiri, an aristocratic Roman edifice located just outside the Herculaneum city, during the AD79 plinian eruption of Mt Vesuvius (Italy) by using the thermal remanent magnetization of embedded lithic clasts. The PDC deposits around and inside the Villa show substantial internal thermal disequilibrium. In areas affected by convective mixing with surface water or with collapsed walls, temperatures average at around 270 °C (min 190 °C, max 300 °C). Where the deposits show no evidence of mixing with external material, the temperature is much higher, averaging at 350 °C (min 300 °C; max 440 °C). Numerical simulations and comparison with temperatures retrieved at the very same sites from the reflectance of charcoal fragments indicate that such thermal disequilibrium can be maintained inside the PDC deposit for time-scales well over 24 hours, i.e. the acquisition time of deposit temperatures for common proxies. We reconstructed in detail the history of the progressive destruction and burial of Villa dei Papiri and infer that the rather homogeneous highest deposit temperatures (average 350 °C) were carried by the ash-sized fraction in thermal equilibrium with the fluid phase of the incoming PDCs. These temperatures can be lowered on short time- (less than hours) and length-scales (meters to tens of meters) only where convective mixing with external materials or fluids occurs. By contrast, where the Villa walls remained standing the thermal exchange was only conductive and very slow, i.e. negligible at 50 cm distance from contact after 24 hours. We then argue that the state of conservation of materials buried by PDC deposits largely depends on the style of the thermal interactions. Here we also suggest that PDC deposit temperatures are excellent proxies for the temperatures of basal parts of PDCs close to their depositional boundary layer. This general conclusion stresses the importance of mapping of deposit temperatures for the understanding of thermal processes associated with PDC flow dynamics and during their interaction with the affected environment.

Magnetic Criticality Enhanced Hybrid Nanodiamond Thermometer under Ambient Conditions

Nitrogen vacancy (NV) centres in diamond are attractive as quantum sensors owing to their superb coherence under ambient conditions. However, the NV centre spin resonances are relatively insensitive to some important parameters such as temperature. Here we design and experimentally demonstrate a hybrid nano-thermometer composed of NV centres and a magnetic nanoparticle (MNP), in which the temperature sensitivity is enhanced by the critical magnetization of the MNP near the ferromagnetic-paramagnetic transition temperature. The temperature susceptibility of the NV center spin resonance reached 14 MHz/K, enhanced from the value without the MNP by two orders of magnitude. The sensitivity of a hybrid nano-thermometer composed of a Cu_{1-x}Ni_{x} MNP and a nanodiamond was measured to be 11 mK/Hz^{1/2} under ambient conditions. With such high-sensitivity, we monitored nanometer-scale temperature variation of 0.3 degree with a time resolution of 60 msec. This hybrid nano-thermometer provides a novel approach to studying a broad range of thermal processes at nanoscales such as nano-plasmonics, sub-cellular heat-stimulated processes, thermodynamics of nanostructures, and thermal remanent magnetization of nanoparticles.

Why magnetite is not the only indicator of past rainfall in the Chinese Loess Plateau?

This study investigates the magnetic mineralogy of palaeosol S5 from Xifeng (XF), Linyou (LY) and Baoji (BJ) sections with increasing annual precipitation from north to the south on the Chinese Loess Plateau. Palaeosol S5 samples from these three localities are further prepared as magnetic extracts and separation residues. Low-temperature magnetic measurements including field cooled and zero field cooled (FC/ZFC) remanence, in-phase magnetic susceptibility, thermal remanent magnetization and room temperature saturation isothermal remanence magnetization (RTSIRM), with X-ray diffraction measurements are carried out for all magnetic extracts and separation residues samples. The asymmetric rounded 'hump' in cooling curves on RTSIRM and the 'tilted' Verwey transition on ZFC/FC curves suggest that partially oxidized magnetite is the dominant magnetic contributor, not pure maghemite or magnetite. Furthermore, The Verwey transitions on cooling curves slightly decrease and the increased slope of 'tilted' Verwey transition on ZFC remanence curves show that the degree of oxidation of magnetite between localities increases in the order XF-LY-BJ. Hard isothermal remanent magnetization, X-ray diffraction data and the difference of magnetization inwarming curves of RTSIRM suggest that both hematite concentration in magnetic extracts and goethite concentration in separation residues increase from XF to BJ. Frequency-dependent susceptibility and ZFC/FC curves show that BJS5 layer formed under high palaeoprecipitation has less superparamagnetic (SP) but more single domain to pseudo-single domain particles, because SP maghemite was dissolved and transformed into goethite by temporary waterlogging. The increase in hematite concentration is interpreted as due to SP maghemite oxidation or original goethite dehydration within dry soil environment. Therefore, transformation of maghemite to goethite in waterlogged phases of the S5 palaeosol led to the loss of magnetization.

Forced transport of thermal energy in magmatic and phreatomagmatic large volume ignimbrites: Paleomagnetic evidence from the Colli Albani volcano, Italy

Few studies have detailed the thermal architecture of large-volume pyroclastic density current deposits, although such work has a clear importance for understanding the dynamics of eruptions of this magnitude. Here we examine the temperature of emplacement of large-volume caldera-forming ignimbrites related to magmatic and phreatomagmatic eruptions at the Colli Albani volcano, Italy, by using thermal remanent magnetization analysis on both lithic and juvenile clasts. Results show that all the magmatic ignimbrites were deposited at high temperature, between the maximum blocking temperature of the magnetic carrier (600–630 °C) and the glass transition temperature (about 710 °C). Temperature estimations for the phreatomagmatic ignimbrite range between 200 and 400 °C, with most of the clasts emplaced between 200 and 320 °C. Because all the investigated ignimbrites, magmatic and phreatomagmatic, share similar magma composition, volume and mobility, we attribute the temperature difference to magma–water interaction, highlighting its pronounced impact on thermal dissipation, even in large-volume eruptions. The homogeneity of the deposit temperature of each ignimbrite across its areal extent, which is maintained across topographic barriers, suggests that these systems are thermodynamically isolated from the external environment for several tens of kilometers. Based on these findings, we propose that these large-volume ignimbrites are dominated by the mass flux, which forces the lateral transport of mass, momentum, and thermal energy for distances up to tens of kilometers away from the vent. We conclude that spatial variation of the emplacement temperature can be used as a proxy for determining the degree of forced-convection flow.

Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

AbstractThe geological sources of large‐scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure‐dependent increase in the Curie temperature (initially in the 50–70°C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation.