ALBERT

Description: To evaluate the accuracy of Fe 3+ and Fe 2+ ratios in silicate glasses determined by Mössbauer spectroscopy, we examine in detail the temperature (47–293 K) of Mössbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 °C (VF3) and the other at 3.5 GPa, 1600 °C (M544). Variable-temperature Mössbauer spectra of these two glasses are used to characterize the recoilless fraction, f , by two different methods—a relative method (RM) based on the temperature dependence of the ratios of Fe 3+ and Fe 2+ Mössbauer doublets and the second based on the temperature dependence of the center shift (CS) of the doublets. The ratio of the recoilless fractions for Fe 3+ and Fe 2+ , C T , can then be used to adjust the observed area of the Mössbauer doublets into the Fe 3+ /Fe ratio in the sample. We also evaluated the contributions of non-paramagnetic components to the Fe in the glasses by determining the influence of applied magnetic field on sample magnetization. Finally, for the VF3 glass, we determined the Fe 3+ /Fe independently by wet chemical determination of the FeO content combined with careful electron microprobe analyses of total Fe. Recoilless fractions determined with the CS method (CSM) are significantly smaller than those determined with the relative method and suggest larger corrections to room-temperature Fe 3+ /Fe ratios. However, the RM determinations are believed to be more accurate because they depend less on the assumption of the Debye harmonic model and because they produce more nearly temperature-independent estimates of Fe 3+ /Fe ratios. Non-linear responses of sample magnetizations to applied magnetic fields indicate that the glasses contain a small (0.4–1.1% for VF3) superparamagnetic component that is most likely to be nanophase precipitates of (Fe,Mg)Fe 2 O 4 oxide, but corrections for this component have negligible influence on the total Fe 3+ /Fe determined for the glass. For the VF3 glass, the Fe 3+ /Fe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2). The Fe 3+ /Fe corrected for recoilless fraction contributions is 0.634 ± 0.078(2), which is 7.5% lower than the uncorrected room-temperature ratio, but also agrees within 5% of wet chemical ratio. At least for this andesitic glass, the room-temperature determination of Fe 3+ /Fe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe 3+ /Fe are always systematically higher compared to recoilless-fraction corrected ratios.

Description: Fine particles of titanomagnetites (Fe3-xTixO4, x 〉 0.5) in the pseudo-single-domain (PSD) size (0.5–20 μm) are important carriers of natural remanent magnetization in basalts. Understanding the mechanism of magnetic recording in these grains has important implications for paleomagnetic studies. This study reports first observations of magnetic vortex states in intermediate titanomagnetite. We imaged magnetic structures of 109 synthetic titanomagnetite grains with x = 0.54 (TM54) and 1–4-μm size using magnetic force microscopy. For six grains, we explored local energy minimum states after alternating field demagnetization and saturation isothermal remanent magnetization. According to the magnetic force microscopy results, 80% of TM54 grains display in-plane magnetization with one to four domains, vortex-like or flux-closure structures, and Néel-like domain walls. Electron backscatter diffraction data on six grains showed that their surface orientations are cutting planes of octahedral crystals and those with approximately square cross sections are within 15° of a (100) crystallographic plane. Magnetic force microscopy observations of magnetic structures in ~1.5-μm grains agree well with numerical micromagnetic modeling of a pyramidal shaped grain with a (100) square base and displayed four discrete local energy minimum states: a single vortex as a ground state and three multivortex states with higher energy. Our observations show that vortex states in titanomagnetite grains (1–5 μm) occur at the lower end of the PSD size range in this mineral and corresponding to a size range known to carry stable and reliable remanence in natural titanomagnetites. ©2018. American Geophysical Union. All Rights Reserved.

Description: The Viking Gas Chromatograph Mass Spectrometer failed to detect organic matter on Mars, and both the Viking Labeled Release and Gas Exchange experiments indicated a highly oxidizing reactive soil on Mars surface.

Description: Fe(VI) in the form of ferrate salts, with FeO(sub 4)(sup 2-) anion, was studied for its spectral and oxidative properties, with the question of whether it might be a suitable analog of the Mars soil oxidant, proposed as a result of the Viking missions of the early 1970s.

Description: The past ∼200 million years of Earth's geomagnetic field behavior have been recorded within oceanic basalts, many of which are only accessible via scientific ocean drilling. Obtaining the best possible paleomagnetic measurements from such valuable samples requires an a priori understanding of their magnetic mineralogies when choosing the most appropriate protocol for stepwise demagnetization experiments (either alternating field or thermal). Here, we present a quick, and non‐destructive method that utilizes the amplitude‐dependence of magnetic susceptibility to screen submarine basalts prior to choosing a demagnetization protocol, whenever conducting a pilot study or other detailed rock‐magnetic characterization is not possible. We demonstrate this method using samples acquired during International Ocean Discovery Program Expedition 391. Our approach is rooted in the observation that amplitude‐dependent magnetic susceptibility is observed in basalt samples whose dominant magnetic carrier is multidomain titanomagnetite (∼TM 60–65 , (Ti 0.60–0.65 Fe 0.35–0.40 )Fe 2 O 4 ). Samples with low Ti contents within titanomagnetite or samples that have experienced a high degree of oxidative weathering do not display appreciable amplitude dependence. Due to their low Curie temperatures, basalts that possess amplitude‐dependence should ideally be demagnetized either using alternating fields or via finely‐spaced thermal demagnetization heating steps below 300°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. Plain Language Summary Oceanic basalts are ideal recorders of the Earth's magnetic field. To decipher magnetic histories recorded in rocks, paleomagnetists need to isolate the magnetization directions and intensities within rocks by one of two possible methods. One method typically involves progressively heating the samples to high temperatures. The other method involves exposing samples to alternating magnetic fields with increasing peak field intensities. Both of these methods are ultimately destructive to the original magnetization preserved within rocks. However, without knowledge of a given rock's magnetic mineralogy, randomly choosing thermal or alternating field demagnetization methods may result in high failure rates. We developed a pre‐screening method to help decide which cleaning method will likely be more successful for a given sample based on low‐field magnetic susceptibility measurements. These measurements do not affect the original magnetic information recorded in a rock, thereby permitting subsequent paleomagnetic studies on the same sample. Our technique can be performed as rapidly as 2 min per sample, is non‐destructive, and does not require complicated sample preparation. Key Points Paleomagnetic studies utilize either alternating field or thermal demagnetization, but it is difficult to choose the best protocol a priori Amplitude‐dependence of magnetic susceptibility measurements permits preliminary magnetic mineralogy characterization in submarine basalts Rapid amplitude‐dependence measurements may aid in deciding upon the best demagnetization protocol for submarine basalt samples