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1. Influence of redox conditions on the intensity of Mars crustal magnetic anomalies

   https://doi.org/10.1111/maps.12505  

   Brachfeld, Stefanie; Shah, Deepa; First, Emily; Hammer, Julia; Bowles, Julie (October 2015, Meteoritics & Planetary Science)

   Abstract We evaluate the relationship between the intensity of remanent magnetization andfO₂in natural and synthetic Mars meteorites. The olivine‐phyric shergottite meteorite Yamato 980459 (Y‐980459) and a sulfur‐free synthetic analog (Y‐98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environmentfO₂on Mars crustal anomalies. The reducing conditions under which Y‐980459 is estimated to have formed (QFM‐2.5; Shearer et al. 2006) were replicated during the synthesis of Y‐98*. Y‐980459 contains pyrrhotite and chromite. Chromite is the only magnetic phase in Y‐98*. The remanence‐carrying capacity of Y‐980459 is comparable to other shergottites that formed in thefO₂range of QFM‐3 to QFM‐1. The remanence‐carrying capacity of these lowfO₂basalts is 1–2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars's southern cratered highlands. Moderately oxidizing conditions of >QFM‐1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization. 

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

   https://doi.org/10.1029/2019GC008728  

   Abdulghafur, F.; Bowles, J. A. (December 2019, Geochemistry, Geophysics, Geosystems)

   Abstract Fine‐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 < T_cand/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. 

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

   https://doi.org/10.1029/2018GC007946  

   Santos, C_N; Tauxe, L. (January 2019, Geochemistry, Geophysics, Geosystems)

   Abstract We 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. 

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4. Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: A Late Archean analogy to modern accretionary plate margins?

   https://doi.org/doi.org/10.1016/j.precamres.2021.106322  

   Jiˇrí ˇZ´ak, Filip Tomek (July 2021, Precambrian research)

   null (Ed.)

   The Canadian Superior Province has become one of the key test pieces to discuss tectonic processes and mechanisms of crustal growth in the Late Archean. The Province consists of a >2.8 Ga proto-cratonic core intruded by voluminous arc-like plutons and surrounded by a series of narrow, elongate ca. 2.8–2.7 Ga juvenile belts, also referred to as terranes or domains. The terranes seem to wrap around the proto-cratonic core and generally young outward, but the kinematics and geodynamic causes of their assembly remain debated. In this paper, we examine the Radisson pluton in northeastern Qu´ebec, which intruded the southern, outer edge of the presumed magmatic arc (Bienville domain) along its ~WNW–ESE-trending tectonic boundary with the protocratonic crust (La Grande domain). The pluton, dominated by porphyritic monzogranite to quartz monzonite, was emplaced at around 2712 Ma and exhibits complex internal structure resulting from superposed magmatic to solid-state deformations. An early margin-parallel ~WNW–ESE magmatic foliation containing a steep lineation, recognized by the anisotropy of magnetic susceptibility (AMS), is interpreted as recording vertical stretching and horizontal flattening of highly crystallized magma, either due to emplacement and/or pure shear dominated transpression. More widespread, however, is a horizontal lineation within the same foliation that is interpreted as recording post-emplacement, but still syn-magmatic, tectonic strain (~NNE–SSW shortening and boundaryparallel stretching). Upon cooling, localized dextral S–C mylonite zones accommodated further shortening within the pluton whereas undeformed late-stage felsic dikes cross-cut the solid-state fabric at an angle to the pluton margins. We suggest that this structural succession, also reproduced by numerical fabric modeling, is a local-scale signal of a two-stage assembly of the northeastern Superior Province: the frontal, NNE-directed terrane convergence and attachment to the cratonic nucleus, operating in a ‘hot’ regime with voluminous arclike plutonism, was followed by more localized dextral shearing parallel to terrane boundaries. The latter phase is recorded at the proto-craton margin but also in the outboard Abitibi greenstone belt virtually at the same time (ca. 2700–2690 Ma). In combination, the two-stage evolution and similar deformation distributed over a broad region resemble modern large hot orogens formed in a plate-tectonic regime. 

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5. Low-Intensity Magnetic Fields Induce Anomalous Disorder-to-Order Transition in Spherical Block Copolymer Micelle Solutions

   https://doi.org/10.1021/acs.macromol.4c02346  

   Kresge, Grace V; Neal, Christopher_A P; Calabrese, Michelle A (January 2025, Macromolecules)

   Magnetic field processing is promising for directing and enhancing self-assembly of diamagnetic block copolymers (BCPs) via domain alignment, but is typically limited to high field strengths and few polymer chemistries. Herein, a novel magnetic field-induced ordering mechanism distinct from domain alignment is demonstrated in aqueous, spherical BCP micelles. Here, low-intensity magnetic fields (B< 0.5 T) induce an anomalous disorder-to-order transition, accompanied by a several order-of-magnitude increase in shear modulus-- effectively transforming a low viscosity liquid into an ordered soft solid. The induced moduli are orders of magnitude larger than those resulting from thermally-induced ordering. Further magnetization induces cubic-to-cylinder order-to-order transitions. Comprehensive characterization via magnetorheology, small- and wide-angle X-ray scattering, differential scanning calorimetry, and vibrational spectroscopy reveals a significant reduction in micelle size and aggregation number relative to zero-field temperature- or concentration-induced ordering, suggesting that B-fields strongly alter polymer-solvent interactions. This extraordinary BCP ordering strategy enables discovery of structures and d-spacings inaccessible via traditional processing routes, thus providing a new platform for developing advanced materials with precisely-controlled features. 

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