Paleomagnetic directional groups and paleointensity from the flood basalt in the Tarim large igneous province: Implications for eruption frequency 1. Geomagnetism

金沢大学理工研究域地球社会基盤学系We present paleomagnetic secular variation and paleointensity from the Early Permian Tarim large igneous province, NW China. The studied sections comprise a total of ~400 m of basaltic flows. Paleomagnetic directions were determined for 11 flows. Four successive flows with a cumulative thickness of ~150 m showed a statistically identical paleomagnetic direction. Assuming a paleosecular variation speed similar to that of the present day, the ~150-m-thick basalt was estimated to have erupted within the past few centuries. Paleointensity experiments were performed on both whole-rock and single plagioclase samples. Although alterations during the experiment and/or weak remanence degraded the data quality, the flows with the same paleomagnetic direction revealed similar paleointensity estimates, supporting the hypothesis that the eruption of these flows was rapid. More generally, flows from the Lower Kupukuziman Formation seem to record lower paleointensity compared to flows from the overlying Kaipaizileike Formation. [Figure not available: see fulltext. Caption: Left the extent of the Tarim large igneous province. Center in-situ paleomagnetic directions obtained from the Tarim large igneous province. Subashi and Yingan represents directions reported in a previous research. Right tilt-corrected paleomagnetic directions. Tight clustering of the direction indicate fast eruption relative to the paleomagnetic secular variation (PSV) speed. © 2017 The Author(s)

Non-Chained, Non-Interacting, Stable Single-Domain Magnetite Octahedra in Deep-Sea Red Clay: A New Type of Magnetofossil?

金沢大学理工研究域地球社会基盤学系Magnetic detection and classification of magnetofossils have been proposed as potential tools for paleoenvironmental studies. Magnetosomes in bacterial species living in different environmental conditions exhibit different grain morphologies and chain configurations, which determine their magnetic properties. Recently, abundant magnetofossils have been reported from unfossiliferous pelagic red clay. However, little is known about their geometry and magnetic properties. Here we report very low coercivity biogenic magnetite in red clay from Ocean Drilling Program (ODP) Site 777 in the northern Mariana Basin. Analyzed sediment showed non-interacting, stable single-domain-like magnetic behaviors. Acquisition of isothermal remanence was decomposed into five components, and a component with mean coercivity below 10 mT accounted for around 25 % of the remanence in some samples. Based on comparisons with semi-quantitative transmission electron microscopy observations of magnetic extracts, this component appears to be carried by octahedral grains with size and shape very similar to biogenic magnetite in red clay from other sites. Micromagnetic calculations indicated that isolated maghemite octahedra may be responsible for the observed low coercivity component. Based on these results, we conclude that the low coercivity component represents non-chained biogenic magnetite that has been oxidized to maghemite. The crystal morphology, geological setting, and lithology do not suggest unusual environmental conditions for ODP Site 777, so the relative amount of chained versus non-chained grains may represent subtle environmental differences. We suggest that non-chained magnetofossils may be widespread in deep-sea sediments. Some methods could overlook the presence of non-chained magnetite, affecting the identification and quantification of magnetofossils. © 2021. American Geophysical Union. All Rights Reserved

Nonlinear thermoremanence corrections for Thellier paleointensity experiments on single plagioclase crystals with exsolved magnetites: A case study for the Cretaceous Normal Superchron

金沢大学理工研究域地球社会基盤学系Single plagioclase paleointensity experiment has been an excellent tool to reliably estimate ancient geomagnetic field intensity. Although transparent plagioclases with magmatic nanophase magnetites have shown their potential for paleointensity estimation, in most cloudy plagioclases with exsolved magnetites, the problems of strong anisotropy as rod-shape magnetites, the hyperbolic tangential saturation of thermoremanence (TRM), and slow cooling of host plutons need to be addressed. In this paper, we propose experimental schemes to address these problems with considerations of error estimation and uncertainty for paleointensity experiments on exsolved magnetite. First, in order to experimentally check the effect of the hyperbolic tangential saturation of TRM, we performed Thellier simulation experiments using laboratory total TRM as simulated natural remanence (NRM). Single cloudy plagioclases were sampled from early Cretaceous granite in the Kitakami massif, Northeast Japan. We designed the experiment where the total TRM and the partial TRMs in the Thellier experiments were acquired under different field intensities. For these experiments, correction for hyperbolic tangential TRM acquisition yielded precise field intensity estimations. Next, Thellier experiments on NRM of the crystals from the same granite were performed as a demonstration of correction schemes for both magnetic anisotropy and hyperbolic tangential TRM acquisition. Precise determination of anisotropy tensor seems to be the major challenge for single plagioclase paleointensity estimation with exsolved magnetite. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Science

Electronic Structure and Electron Correlation in LaFeAsO_{1-x}F_x and LaFePO_{1-x}F_x

Photoemission spectroscopy is used to investigate the electronic structure of the newly discovered iron-based superconductors LaFeAsO_{1-x}F_x and LaFePO_{1-x}F_x. Line shapes of the Fe 2p core-level spectra suggest an itinerant character of Fe 3d electrons. The valence-band spectra are generally consistent with band-structure calculations except for the shifts of Fe 3d-derived peaks toward the Fermi level. From spectra taken in the Fe 3p -> 3d core-absorption region, we have obtained the experimental Fe 3d partial density of states, and explained it in terms of a band-structure calculation with a phenomenological self-energy correction, yielding a mass renormalization factor of ~< 2.Comment: 4 pages, 5 figure

Magnetostratigraphic evidence for post-depositional distortion of osmium isotopic records in pelagic clay and its implications for mineral flux estimates

金沢大学理工研究域地球社会基盤学系Chemical stratigraphy is useful for dating deep-sea sediments, which sometimes lack radiometric or biostratigraphic constraints. Oxic pelagic clay contains Fe–Mn oxyhydroxides that can retain seawater 187Os/188Os values, and its age can be estimated by fitting the isotopic ratios to the seawater 187Os/188Os curve. On the other hand, the stability of Fe–Mn oxyhydroxides is sensitive to redox change, and it is not clear whether the original 187Os/188Os values are always preserved in sediments. However, due to the lack of independent age constraints, the reliability of 187Os/188Os ages of pelagic clay has never been tested. Here we report inconsistency between magnetostratigraphic and 187Os/188Os ages in pelagic clay around Minamitorishima Island. In a ~ 5-m-thick interval, previous studies correlated 187Os/188Os data to a brief ( 2.9–6.9 million years duration. Quartz and feldspars content showed that while the paleomagnetic chronology gives reasonable eolian flux estimates, the 187Os/188Os chronology leads to unrealistically high values. These results suggest that the low 187Os/188Os signal has diffused from an original thin layer to the current ~ 5-m interval, causing an underestimate of the deposition duration. The preservation of the polarity patterns indicates that a mechanical mixing such as bioturbation cannot be the main process for the diffusion, so diagenetic redistribution of Fe–Mn oxyhydroxides and associated Os may be responsible. The paleomagnetic chronology presented here also demands reconsiderations of the timing, accumulation rate, and origins of the high content of rare-earth elements and yttrium in pelagic clay around Minamitorishima Island.[Figure not available: see fulltext.] © 2021, The Author(s)

Magnetite microexsolutions in silicate and magmatic flow fabric of the Goyozan granitoid (NE Japan): Significance of partial remanence anisotropy

金沢大学理工研究域地球社会基盤学系Anisotropy of magnetic susceptibility (AMS) has been widely used to infer magmatic flow patterns of granitoids where an appropriate AMS axis is parallel to an alignment of mafic minerals or magnetite. The magmatic flow fabric in Cretaceous granitic plutons from northeastern Japan was verified using an analysis of anisotropy of partial anhysteretic remanent magnetization (ApARM) which further isolates the magnetite subfabrics according to magnetite grain size. The preferred orientation of polysynthetic twins in plagioclase laths and clinopyroxene is discordant with the bulk AMS fabric along outer marginal zones of the granitoid, as shown by image analysis of microphotographs from thin sections cut in orthogonal planes. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in granitoids has risks. Scanning electron microscopy (SEM) reveals that submicroscopic, needle-shaped magnetite inclusions exsolved in anhedral plagioclase and clinopyroxene may explain such anomalous exceptions to the validity of AMS fabrics. Our ApARM measurements show that the ApARM alignment of relatively high-coercive, submicroscopic magnetite inclusions is concordant to the linear orientation of anhedral plagioclase and clinopyroxene. The combination of SEM, AMS, and ApARM was required to confirm the magmatic and submagmatic flow pattern of granitoids in this study and is generally preferable to the use of AMS alone. Copyright 2006 by the American Geophysical Union

Rock magnetism of quartz and feldspars chemically separated from pelagic red clay: a new approach to provenance study

金沢大学理工研究域地球社会基盤学系Magnetic mineral inclusions in silicates are widespread in sediments as well as in igneous rocks. Because they are isolated from surrounding environment, they have potential to preserve original magnetic signature even in chemically altered sediments. Such inclusions may provide proxies to help differentiating the source of the host silicate. We measure magnetism of quartz and feldspars separated by chemical digestion of pelagic red clay. The samples are from the upper 15 m of sediments recovered at Integrated Ocean Drilling Program Site U1366 in the South Pacific Gyre. The quartz and feldspars account for 2.3–22.7 wt% of the samples. X-ray diffraction analyses detect both plagioclase feldspar and potassium feldspar. Plagioclase is albite-rich and abundant in the top ~ 7.4 m of the core. Potassium feldspar mainly occurs below ~ 10.4 m. The dominance of albite-rich plagioclase differs from a previous investigation of coarser fraction of sediments from the South Pacific. Saturation isothermal remanence (SIRM) intensities of the quartz and feldspars are 7.45 × 10−4 to 1.98 × 10−3 Am2/kg, accounting for less than 1.02% of the SIRM of the untreated bulk samples. The depth variations of the silicate mineralogy and the previously reported geochemical end-member contributions indicate that quartz and/or plagioclase above 8.26 m is likely to be Australian dust. In contrast, the relative abundance and the magnetic properties of quartz and feldspars vary below 10.42 m, without clear correlation with geochemical end-member contributions. We consider that these changes trace a subdivision of the volcanic component. Our results demonstrate that magnetism of inclusions can reveal additional information of mineral provenance, and chemical separation is an essential approach to reveal the environmental magnetic information carried by magnetic inclusions.[Figure not available: see fulltext.]. © 2018, The Author(s)

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First-Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

金沢大学理工研究域地球社会基盤学系Relative abundance of magnetite originated from magnetotactic bacteria (magnetofossils) in sediments may influence relative paleointensity (RPI) estimations of the geomagnetic field, as some studies reported an inverse correlation between RPI and the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), a proxy of the proportion of biogenic to terrigenous magnetic minerals as well as magnetic grain size. This study aims to evaluate the influence of magnetofossils on RPI estimations more selectively using first-order reversal curve (FORC) diagrams. We studied three cores (KR0515-PC4, MD982187, and MR1402-PC1) from the western equatorial Pacific, among which large differences exist in the average natural remanent magnetization intensity normalized by ARM and kARM/SIRM. Principal component analyses (PCAs) were applied to FORC diagrams measured on bulk specimens from the three cores and silicate-hosted magnetic inclusions extracted from Core MD982187, and three endmembers (EMs) were revealed (EM1: silicate-hosted magnetic inclusions, EM2: other terrigenous, EM3: biogenic). EM3 proportions vary widely among the three cores. The average RPI decreases with increasing EM3 proportion, which is probably caused by higher ARM acquisition efficiency of magnetofossils due to small magnetostatic interactions. EM3 proportion correlates with kARM/SIRM, which confirms that kARM/SIRM represents the proportion of biogenic to terrigenous magnetic components. Core MR1402-PC1 has the highest EM3 proportion, and its within-core variation is small. From FORC-PCA applied solely to this core, we infer that the configurations of biogenic magnetite chains such as bending and collapse may also influence kARM/SIRM and RPI estimations. © 2021. The Authors

Changing Abundance of Magnetofossil Morphologies in Pelagic Red Clay Around Minamitorishima, Western North Pacific

金沢大学理工研究域地球社会基盤学系Recent investigations have discovered an unexpected abundance of magnetofossils in oxic pelagic red clays. These have potential to serve as paleoenvironmental tracers in otherwise nonfossiliferous sediments. Here, we report on variations in the abundance and morphology of magnetofossils in red clay from the western North Pacific. Magnetic measurements revealed that magnetofossils dominate the magnetic mineral assemblage of the sediments. An endmember analysis of isothermal remanent magnetization acquisition curves, supplemented by an analysis of S ratios, indicates that the magnetic assemblage can be unmixed into three endmembers, two corresponding to magnetofossils and one to terrigenous magnetic minerals. Direct counting of magnetofossil morphologies under a transmission electron microscope shows that the two magnetofossil endmembers differentiate equant magnetofossils and bullet-shaped magnetofossils, respectively. The stratigraphic variation of the endmember contributions revealed that the equant magnetofossils are dominant for the most part, while an interval at around 7 m in core depth shows higher abundance of the bullet-shaped magnetofossils. This may reflect enhanced organic carbon flux to the sediments. The organic carbon content is low throughout the sediments, and it does not show any change corresponding to the increase of bullet-shaped magnetofossils, pointing at extensive remineralization of the organic carbon. On the basis of lithostratigraphic correlation to nearby drilling sites, we tentatively estimate the age of the bullet-shaped magnetofossil increase as sometime between ∼75 and 25 Ma. These results suggest that environmental information can be obtained from magnetofossils in pelagic red clay. © 2017. American Geophysical Union. All Rights Reserved

Non-calcareous beachrock found in Akagurisaki, Ohi Town, Fukui Prefecture

Beachrock which was formed about 0.5 meter higher than high tide is found in Akagurisaki, Ohi Town, Fukui Prefecture. The outcrops always get wet by spring water. The sediments are composed of well sorted rounded gravels and sands but calcareous shell is not found at all. Intergranular space is occupied by white amorphous cement. Magnesium and silicon rich composition of the cement is obtained by EPMA analysis. There is a conjecture that the magnesium rich cement was precipitated in spite of solution of calcareous shell under saturated state by spring water, because calcium has a higher tendency to ionize than magnesium