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We have quantified the magnetic fabric of the highly-compacted Heebner Shale (Pennsylvanian) of Kansas in terms of the anisotropies of partial anhysteretic remanent magnetization (pARM) and low-field susceptibility. Hysteresis loops exhibit almost purely paramagnetic behavior, and the ratio of high-field linear susceptibility to low-field susceptibility is very close to one; the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix, while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains. Both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation, with a weak northeasterly lineation, but the pARM anisotropies are generally much larger than the susceptibility anisotropy. The degree of foliation measured with pARM's decreases markedly with increasing coercivities, indicating stronger foliation in the coarser grainsize fraction. In contrast, the pARM mean lineations are grainsize-independent, suggesting a post-compactional origin, related to tectonic compression. The susceptibility lineation appears to have a slightly different azimuth, and may therefore be due to paleocurrents.

Two contrasting zones of deformation have been previously recognized in the Proterozoic Thomson Formation of east-central Minnesota. The northern zone consists of open, upright E-W-trending folds with a well-developed steeply south-dipping axial-planar cleavage. The southern zone is marked by a pervasive subhorizontal cleavage (S 1 ), crenulated by a second cleavage that is vertical to steeply south dipping (S 2 ). Paleomagnetic results and anisotropy of anhysteretic remanent magnetization (AARM) in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction: neither NRM nor AARM were much affected by tectonic deformation. By contrast, the low-field magnetic susceptibility, which is dominated by paramagnetic minerals (particularly chlorite), reflects the tectonic fabric of the rock. Anisotropy of magnetic susceptibility (AMS) is characterized by minima normal to the cleavage in the northern zone and to the S 1 cleavage in the southern zone. Thus, the S 2 deformation did not significantly affect the AMS fabric. “March strains” were obtained by measuring the (002) lattice preferred orientation of chlorite using an X-ray pole-figure goniometer. The principal orientations of March strain match well those of AMS, and AMS and March strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a Flinn diagram. Based on AMS signature, it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone. This could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north. In this scenario, the second deformation, the result of regional late-stage shortening, was only marked by a cleavage (S 2 ) in the southern zone due to the favorable orientation there of S 1 .

We have quantified effects of experimental deformation on the magnetic properties of a set of synthetic “calcite sandstone” samples containing magnetite. The deformation was carried out in a microcomputer-controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10−5 s−1. Most samples were deformed under dry conditions, but a few were deformed with a pore fluid present; the samples deformed under dry conditions required substantially higher differential stresses. Macroscopically ductile shortening strains of up to 25% produced the following irreversible changes in magnetic properties: (1) increased bulk coercivity, remanence coercivity, and mean anhysteretic remanence susceptibility; (2) decreased mean low-field susceptibility; (3) decreases in the component of remanence parallel to shortening; (4) smaller decreases for most samples in the component normal to shortening, resulting in a net “rotation” of the remanence away from the shortening axis; (5) larger decreases in the normal component in a few samples, resulting in a net “rotation” of the remanence towards the shortening axis; (6) increased magnetic anisotropy; and (7) increased “deformation” of initial magnetic ellipsoids. A comparison of data for samples deformed under dry and wet conditions (higher and lower differential stresses, respectively) indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain (i.e., rotation and displacement of particles), whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress.

[1] The natural remanent magnetization (NRM) of the Martian meteorite ALH84001 is predominantly carried by fine magnetite, which is found in association with carbonate. The magnetite is in epitaxial and topotactic relation with the carbonate and formed from the carbonate in the major impact event at 4.0 Ga. The NRM will therefore record this field. The local preferential crystallographic and shape alignment of the magnetite defines local easy directions of magnetization may account for the observed inhomogeneity of the NRM on a microscopic scale. Normalizing the intensity of the NRM by the saturation isothermal remanence (IRMs) then gives an estimate for the 4.0 Ga Martian field one order smaller than the present geomagnetic field. Such a field is unlikely to be strong enough to generate the high-intensity Martian magnetic anomalies. ALH 84001 in its pristine state as an orthopyroxenite is not a plausible source rock for the Martian anomalies because its magnetite was not formed until the 4.0 Ga event.

Stable single-domain (SSD) grains were mixed separately with superparamagnetic, pseudosingle-domain, and multidomain (MD) magnetite/maghemite particles in order to test the linearity of various magnetic parameters as a function of mixing ratio. Hysteresis loops, isothermal remanent magnetization acquisition curves, DC demagnetization curves, and low-temperature thermal demagnetization curves were measured on the mixtures. The experiments demonstrate that magnetization parameters are linearly dependent on the mixing ratio, while more complex parameters, e.g., coercivities, do not behave linearly as a function of mixing ratio. Armed with linearity, we apply a mathematical technique which, given a database of type curves, uses singular value decomposition to solve for the various concentrations of the magnetic phases in the mixture and a Monte Carlo simulation to determine the error in the inversion. We then test the technique on numerical mixtures, on the physical mixtures, and on a small set of natural samples from Lake Pepin, Minnesota. Finally, the magnetic behavior of the mixture of MD and SSD grains is considered, and two more mixture strains of MD and SSD grains (numerically produced) are considered to facilitate this discussion.

We report paleomagnetic and rock magnetic data from sedimentary carbonates of the Appalachian Basin that have implications for the mechanism of widespread late Paleozoic remagnetization in this rock type. Sampling was accomplished mostly along two across-basin transects, one in Devonian carbonates of New York State and the other in Ordovician and Mississippian carbonates of Tennessee and Alabama. Paleomagnetic investigations of the New York Devonian carbonates along a transect from Albany to Buffalo indicate that these rocks were completely remagnetized in Alleghenian time. Rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near Syracuse and decreasing gradually to the east and west. This pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a Devonian bentonite horizon which took place during Alleghenian time. We conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval, late Paleozoic events that were controlled by the same diagenetic factors. Thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity. However, our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age. We propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization, which is triggered by the introduction of potassium-rich brines. Results from the Tennessee transect from Nashville to Chattanooga show a very different pattern. Paleomagnetic studies indicate that late Paleozoic remagnetization has affected Ordovician carbonates of the Nashville Dome. Paleozoic carbonates in the overthrust belt near Chattanooga also carry the late Paleozoic remagnetization. However, Mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late Paleozoic remagnetization. Samples show only a present field magnetization or, at one locality in northwestern Alabama, a dual-polarity magnetization of probable Mississippian age. Rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the Nashville Dome and the overthrust belt than in the nonremagnetized Mississippian carbonates. We therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes. The presence of a remagnetization “shadow” in the younger rocks now exposed between the fold-thrust belt and the flexural arch is attributed to westward flow of magnetite-forming aqueous fluids through deep, sub-Carboniferous aquifers during Alleghenian time. Upward migration of the fluids through the Mississippian carbonates may have been prevented by the Devono-Mississippian Chattanooga Shale, an impermeable stratum that is present throughout the region.

Magnetic anomalies observed by the Mars Global Surveyor mission are attributed to crustal remanence. SNC (Shergotty-Nakhla-Chassigny) meteorites are likely samples of the Martian crust and are amenable to mineralogical and magnetic measurements essential to the understanding of the origin of magnetic anomalies. The recently discovered chassignite NWA 2737 and lherzolitic shergottite NWA 1950 display unusual magnetic characteristics that argue for a different magnetic carrier than the oxides and sulfides previously invoked in SNC meteorites. NWA 2737, the second member of the chassignite group, is a dunite with unusually dark-brown olivines and large magnetic susceptibility while Chassigny contains green olivines and is nearly a pure paramagnet. Dark olivines are also found in NWA 1950, a lherzolitic shergottite, which has singular magnetic properties when compared with other shergottites. The dark olivine color is due to the presence of Fe and FeNi metal nanoparticles, identified both by TEM and by magnetic measurements. Their size distribution encompasses the superparamagnetic to single domain transition at 30 K (10 nm range) and explains the magnetic properties of the bulk rocks. The formation of these nanoparticles is attributed to heating during the shock events that affected NWA 2737 and NWA 1950. The production of metal particles by shock-induced reduction of olivine has been invoked on surfaces deprived of atmosphere but never observed on Earth or Mars. Therefore, metal formed by shock in the heavily cratered Noachian crust is a possible carrier for crustal magnetic remanence. Widespread surface formation of metal nanoparticles could provide the precursor for the oxidized particles (goethite, hematite) observed in the Martian soils. (c) 2007 Elsevier B.V All rights reserved.

Detailed rock magnetic studies show that susceptibility (mass-specific χ) and anhysteretic remanent magnetization (ARM) of the Chinese loess/paleosol sequences are carried by almost identical magnetic carriers. Therefore the ratio Δχ/χARM (or equivalently χARM/Δχ, where Δχ is defined as χ − χ0, and χ0 is the intercept susceptibility of the plot of χ versus ARM, and χARM is field-normalized ARM) can be used to quantify the grain size of χ and ARM carriers. By determining this ratio for three Chinese loess/paleosol profiles (Jiuzhoutai, Yuanbao, and Yichuan) characterized by different degrees of environmentally controlled pedogenesis and sedimentation rates, we show that the lower grain-size limit of aeolian magnetic particles in the less pedogenically altered loess units is about 100–300 nm, in the finer-grained pseudosingle domain (PSD) grain-size range. In contrast, the grain sizes of pedogenically produced magnetic particles for mature paleosols dominantly cover both the superparamagnetic (SP) and single-domain (SD) ranges. On the basis of plots of Δχ/χARM against Δχ, samples can be divided into four regions (I, II, III, and IV). Region I corresponds to the least pedogenically altered primary loess samples, with Δχ/χARM of 0.165–0.24. Samples in region II, a transition zone between the least altered loess and the onset of development of paleosols, have χ values identical to those in region I but have lower Δχ/χARM of 0.09–0.165. With increasing susceptibility in zone III, Δχ/χARM is positively correlated with Δχ, indicating the gradually increasing influence of SP particles. Finally, in zone IV with Δχ higher than ∼6.5 × 10−7 m3 kg−1, Δχ/χARM is independent of the variations in Δχ, suggesting that Δχ/χARM is totally controlled by the pedogenic finest-grained particles and the size distribution of these particles remains almost constant. The development of soils in the Chinese loess revealed by these three profiles from three sites can be clearly explained by a continuous process of pedogenesis, increasing from zone I to zone IV. The definition of the pedogenic zones can help to improve our understanding of the underlying mechanisms and variability of pedogenesis and thus could enable more successful and accurate separation of the authentic pedogenic signals from the background signal of the aeolian inputs at different loess sites worldwide.

A new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described. The magnetic fabric of a collection of Trenton Limestone specimens has been determined using this method, as well as by conventional anisotropy of magnetic susceptibility. The results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the Trenton in which the bulk magnetic susceptibilities are low. A model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses. The original fabric is inferred to have been isotropic; the anisotropy resides in secondary magnetite of Late Paleozoic age. It is argued that the observed magnetic fabric must therefore be Alleghenian or younger in age. Our method has the potential to determine paleostress directions in carbonates elsewhere, provided our assumptions are correct.

Of 149 oriented specimens of the lower Ordovician Oneota dolomite subjected to stepwise thermal or alternating-field demagnetization, 50 contained a characteristic shallow east-southeasterly or west-northwesterly component of magnetization identifiable on orthogonal vector projections. For 48 other specimens the demagnetization trajectories defined greats-circle paths which intersect close to the characteristic direction. The site-mean directions for the characteristic component were determined by using both the characteristic directions and great-circle data, and these yield a formation-mean direction obtained from 10 sites with a declination of 105°, an inclination of +2°, and a precision parameter k of 17.6 (α95 = 11.9°). The mean of the site virtual poles falls at 10.4° N, 166.4° E (K = 48.0, A95 = 7.0°), which is close to previously published late Cambrian paleopoles. Both polarity groups have predominantly positive inclinations, suggesting the presence of an uncleaned downward component whose unblocking-temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization. We interpret this downward component as a drilling-induced remanent magnetization, and we hope to have removed its contribution by averaging the dual-polarity results. Multicomponent magnetizations were also present in samples collected from blocks of Oneota dolomite incorporated into the overlying New Richmond sandstone (lower Ordovician), but only six out of eight samples yielded well-defined greats-circle paths. The best fit intersection of these defines a northerly and steep recent component, while the directions of the higher-stability components are different for each block. Due to the small number of samples and imperfect resolution of magnetic directions, the conglomerate test is not conclusive; nevertheless it suggests that the stable remanence in the Oneota dolomite was acquired prior to the deposition of the New Richmond sandstone.

The concentration of magnetite in the Lower and Middle Devonian Helderberg and Onondaga carbonate formations varies in a distinct pattern along an east-west profile across New York State. Magnetite concentrations are uniformly low in the western half of the profile, increase sharply eastward to a peak value near Syracuse, and decrease farther east. The pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile. Previous paleomagnetic studies have documented that the magnetite carries a well- defined magnetization of Pennsylvanian-Permian age, and magnetite authigenesis is therefore no younger than late Paleozoic. We attribute the correlation between high magnetite concentration and high illite content to temperature-dependent diagenesis triggered by orogenic fluids. The large proportion of secondary magnetite indicates that over most of the area, the mechanism of late Paleozoic remagnetization was principally chemical. Thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence.

In humans, loss of function mutations in the SAMHD1 (AGS5) gene cause a severe form of Aicardi-Goutières syndrome (AGS), an inherited inflammatory-mediated encephalopathy characterized by increased type I IFN activity and upregulation of IFN-stimulated genes (ISGs). In particular, SAMHD1-related AGS is associated with a distinctive cerebrovascular pathology that commonly leads to stroke. Although inflammatory responses are observed in immune cells cultured from Samhd1 null mouse models, these mice are physically healthy, specifically lacking a brain phenotype. We have investigated the use of zebrafish as an alternative system for generating a clinically relevant model of SAMHD1-related AGS. Using temporal gene knockdown of zebrafish samhd1, we observe hindbrain ventricular swelling and brain hemorrhage. Furthermore, loss of samhd1 or of another AGS-associated gene, adar, leads to a significant upregulation of innate immune-related genes and an increase in the number of cells expressing the zebrafish type I IFN ifnphi1. To our knowledge, this is the first example of an in vivo model of AGS that recapitulates features of both the innate immune and neurological characteristics of the disease. The phenotypes associated with loss of samhd1 and adar suggest a function of these genes in controlling innate immune processes conserved to zebrafish, thereby also contributing to our understanding of antiviral signaling in this model organism.