Saturation magnetostriction and its low-temperature variation inferred for natural titanomaghemites: implications for internal stress control of coercivity in oceanic basalts

Highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity (stability), helping preserve the oceanic magnetic anomaly pattern. We study the source of this high coercivity in four oceanic basalts (from ODP sites 238, 572D, 470A and 556) containing highly oxidized titanomaghemite (titanium content parameter x ≈ 0.55 and oxidation parameter z ≈ 0.9 on average). Most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence J RS to saturation magnetization Js approaches 0.50 (JRS/JS = 0.46 on average). We show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses σi in the titanomaghemite grains. This allows us to use a novel indirect method to estimate the saturation magnetostriction λS of the titanomaghemite. A saturation remanence J RS is given along the axis of a cylindrical sample of each basalt. Then a small compression σ is applied repeatedly along this axis and the reversible change ∆JRS in JRS is measured. Combining equations from single-domain theory for this piezomagnetic effect and for the sample’s coercive force HC gives λS = 1.39HCJS 1/σ ∆JRS/JRS (using cgs units, or with HC in mT, J S in kA m and σ in Pa). This yields four λS estimates (with ca 50 per cent expected error) ranging from 3 × 10−6 to 10 × 10−6 and averaging 6 × 10−6. Theory for the piezomagnetic effect yields four σ i estimates averaging 2 × 108 Pa. This is similar to the internal stress magnitude thought to be responsible for the high coercivity of ball-milled single-domain titanomagnetite (x ≈ 0.6) and natural single-domain haematite. We also show that cooling to 120 ◦K causes HC J S for each oceanic basalt to vary in approximate proportion to (1− T TC)n with n between 1.9 and 2.0 (where T is temperature and T C is Curie point, both in ◦K). This implies that λS of titanomaghemite with x ≈ 0.55 and z ≈ 0.9 also varies in approximate proportion to (1− T TC)n with n near 1.9 or 2.0 on cooling to 120 ◦K (assuming that σ i remains constant on cooling). Our results support the hypothesis that coercivity (magnetic stability) is often magnetostrictively controlled by internal stresses in the highly oxidized titanomaghemites typical of oceanic basalts older than ca 10 Myr.We suggest that this hypothesis can be further tested by more extensive observation of whether cooling to 120 ◦K often causes HC J S of such basalts to vary in approximate proportion to (1 − T TC)n with n near 1.9 or 2.0

Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds

Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50\% over that of initial conditions by 2 GPa, and then decreases until only 33\% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth’s surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used

Evidence for coeval Late Triassic terrestrial impacts from the Rochechouart (France) meteorite crater

High temperature impact melt breccias from the Rochechouart (France) meteorite crater record magnetization component with antipodal, normal and reverse polarities. The corresponding paleomagnetic pole for this component lies between the 220 Ma and 210 Ma reference poles on the Eurasian apparent polar wander path, consistent with the 214 $\pm$ 8 Ma 40Ar/39Ar age of the crater. Late Triassic tectonic reconstructions of the Eurasian and North American plates place this pole within 95% confidence limits of the paleomagnetic pole from the Manicouagan (Canada) meteorite impact crater, which is dated at 214 $\pm$ 1 Ma. Together, these observations reinforce the hypothesis of a Late Triassic, multiple meteorite impact event on Earth

Low-temperature alteration and magnetic changes of variably altered pillow basalts

Pillow basalt fragments from the East Pacific Rise, dredged during the Phoenix expedition, often show discoloured rims due to alteration. A suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their Fe–Ti-oxide mineralogy and rock magnetic properties. These large pillow fragments show relatively unaltered grey interiors, surrounded by darker, concentric halos, which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior. The discoloured zones are characterized by precipitation of abundant secondary minerals, such as Fe 3+ -rich clays that filled vesicle spaces. Fe–Ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques. The subsamples come from traverses that are parallel to the outer glassy pillow rims, allowing us to study the low-temperature alteration effects and rock magnetic properties without having to take variable grain size into account. Not surprisingly, titanomaghemites in discoloured rims are, in a general sense, oxidized to a higher degree ( z typically >0.55) than those in the relatively unaltered grey interior ( z typically 0.6 within 200 000 yr. The difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age, so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim. Our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z . Because average Ti content can vary from sample to sample, Curie temperatures are also inaccurate proxies for z . If one wants to study possible correlations between z and rock magnetic parameters, the best technique is to determine z for each subsample by using transmission electron microscopy (TEM), electron microprobe, MÖssbauer or similar techniques. In agreement with many (but not all) previous observations on natural samples, we find that bulk coercivity ( H c ) , and high-field susceptibility (Χ hf ) increase, whereas low-field susceptibility (Χ l f ) , natural remanent magnetization intensity and saturation magnetization ( M s ) generally decrease with increasing oxidation state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73443/1/j.1365-246X.2005.02819.x.pd

Domain wall pinning and dislocations: Investigating magnetite deformed under conditions analogous to nature using transmission electron microscopy

In this study, we deformed samples cut from a single magnetite octahedron and used transmission electron microscopy (TEM) and magnetic measurements to experimentally verify earlier computational models of magnetic domain wall pinning by dislocations and to better understand the nature of dislocations in magnetite. Dislocations in magnetite have been of interest for many decades because they are often cited as a likely source of stable thermoremanent magnetizations in larger multidomain (MD) magnetite grains, so a better understanding of dislocation effects on coercivity in MD magnetite is crucial. TEM imaging shows, for the first time, domain walls sweeping through the magnetite sample and being pinned at dislocations. In agreement with theory, these findings demonstrate that domain walls are more strongly pinned at networks of dislocations than at single dislocations and that domain walls pinned at longer dislocations have higher microcoercivities than those pinned at shorter dislocations. This experimentally illustrates the ability of dislocations to increase the coercivity of larger multidomain magnetite grains. The observed values for microcoercivity and bulk coercivity are in reasonable agreement with theoretical calculations. Burgers vectors were determined for some dislocations to verify that they were in keeping with expected dislocation orientations. The dislocations were found to be primarily located on close-packed {111} planes within the magnetite. Deformation caused only a minor change in bulk coercivity, but first-order reversal curve diagrams show populations with increased coercivity not visible in hysteresis loops.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement 320750. The Institute for Rock Magnetism and LacCore are supported by the NSF EAR Instrumentation and Facilities Program and by the University of Minnesota, Earth Sciences Division, National Science Foundation. To obtain the data used for this paper, please contact A.K.L. This work was funded by EAR-0810085 to J.M.F., by EAR-0810252 to A.J.N., and by a Geological Society of America grant to A.K.L. This is IRM publication 1406.This is the final published version. It first appeared at http://onlinelibrary.wiley.com/doi/10.1002/2014JB011335/abstract?rememberMePresent=false

Late Permian palaeomagnetic data east and west of the Urals

We studied Upper Permian redbeds from two areas, one between the Urals and the Volga River in the southeastern part of Baltica and the other in north Kazakhstan within the Ural-Mongol belt, which are about 900 km apart; a limited collection of Lower-Middle Triassic volcanics from north Kazakhstan was also studied. A high-temperature component that shows rectilinear decay to the origin was isolated from most samples of all three collections. For the Late Permian of north Kazakhstan, the area-mean direction of this component is D = 224.3°, I =−56.8°, k = 161, Α 95 = 2.7°, N = 18 sites, palaeopole at 53.4°N, 161.3°E; the fold test is positive. The Triassic result ( D = 55.9°, I =+69.1°, k = 208, Α 95 = 4.2°, N = 7 sites, pole at 57.0°N, 134.1°E) is confirmed by a positive reversal test. The corresponding palaeomagnetic poles from north Kazakhstan show good agreement with the APWP for Baltica, thus indicating no substantial motion between the two areas that are separated by the Urals. Our new mean Late Permian direction for SE Baltica ( D = 42.2°, I = 39.2°, k = 94, Α 95 = 3.5°, N = 17 sites; palaeopole at 45.6°N, 170.2°E) is confirmed as near-primary by a positive tilt test and the presence of dual-polarity directions. The corresponding pole also falls on the APWP of Baltica, but is far-sided with respect to the coeval reference poles, as the observed mean inclination is shallower than expected by 13°± 4°. In principle, lower-than-expected inclinations may be attributed to one or more of the following causes: relative tectonic displacements, quadrupole and octupole terms in the geomagnetic field, higher-order harmonics (incl. secular variation) of the same field, random scatter, non-removed overprints, or inclination error during remanence acquisition and/or diagenetic compaction. Our analysis shows that most mechanisms from the above list cannot explain the observed pattern, leaving as the most likely option that it must be accounted for by inclination shallowing. Comparison with selected coeval results from eastern Baltica (all within Russia) shows that all of them are biased in the same way. This implies that they cannot be used for analysis of geomagnetic field characteristics, such as non-dipole contributions, without a more adequate knowledge of the required correction for inclination shallowing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71899/1/j.1365-246X.2008.03727.x.pd

Limestones of western Newfoundland that magnetized before Devonian folding but after Middle Ordovician lithification

A positive fold test and a negative conglomerate test help determine when and how stable remanence was acquired in the Middle Ordovician Table Head Group limestones of the Port au Port Peninsula of Newfoundland. The limestones magnetized after lithification and incorporation as clasts into a Middle Ordovician breccia. Hence, the limestones do not carry a detrital or other primary remanence despite their very low conodont colour alteration index of 1. The remanence may be thermoviscous or diagenetic and was acquired before Devonian folding. This suggests the need for caution in interpreting paleomagnetic results from other early Paleozoic limestones whose remanence resides in magnetite of blocking temperature lower than 400°C