Root Zone of the Bernardston Nappe and the Brennan Hill Thrust Involuted by Backfolds and Gneiss Domes in the Mount Grace Area, North-Central Massachusetts

Guidebook for field trips in southwestern New Hampshire, southeastern Vermont, and north-central Massachusetts: New England Intercollegiate Geological Conference, 80th annual meeting, October 14, 15 and 16, 1988, Keene, New Hampshire: Trip C-

Magnetic field microscopy of rock samples using a giant magnetoresistance–based scanning magnetometer

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95412/1/ggge1634.pd

Experimental evidence for lamellar magnetism in hemo-ilmenite by polarized neutron scattering

Large local anomalies in the Earth's magnetic field have been observed in Norway, Sweden, and Canada. These anomalies have been attributed to the unusual magnetic properties of naturally occurring hemo-ilmenite, consisting of a paramagnetic ilmenite host (α-Fe2O3-bearing FeTiO3) with exsolution lamellae (≈3μm thick) of canted antiferromagnetic hematite (FeTiO3-bearing α-Fe2O3) and the mutual exsolutions of the same phases on the micron to nanometer scale. The origin of stable natural remanent magnetization (NRM) in this system has been proposed to be uncompensated magnetic moments in the contact layers between the exsolution lamellae. This lamellar magnetism hypothesis is tested here by using polarized neutron diffraction to measure the orientation of hematite spins as a function of an applied magnetic field in a natural single crystal of hemo-ilmenite from South Rogaland, Norway. Polarized neutron diffraction clearly shows that the ilmenite spins do not contribute to the NRM and that hematite spins account for the full magnetization at ambient temperature. Hematite sublattice spins are shown to adopt an average angle of 56∘ with respect to a saturating magnetic field, which is intermediate between the angle of 90∘ predicted for a pure canted moment and the angle of 0∘ predicted for a pure lamellar moment. The observed NRM is consistent with the vector sum of lamellar magnetism and canted antiferromagnetic contributions. The relative importance of the two contributions varies with the length scale of the microstructure, with the lamellar contribution increasing when exsolution occurs predominantly at the nanometer rather than the micrometer scale

Magnetic properties and potential field modeling of the Peculiar Knob metamorphosed iron formation, South Australia: An analog for the source of the intense Martian magnetic anomalies?

[1] Magnetic property measurements show that the strongly metamorphosed Peculiar Knob iron formation (IF), South Australia, is coarse‐grained, high‐grade hematite with variable amounts of magnetite and maghemite. This body exhibits a relatively low magnetic susceptibility (680°C) metamorphism may be possible sources for some of the prominent Martian anomalies

Anisotropy of magnetic susceptibility versus lattice- and shape-preferred orientation in the Lac Tio hemo-ilmenite ore body (Grenville province, Quebec)

International audienceThe Lac Tio hemo-ilmenite ore body crops out in the outer portion of the 1.06 Ga Lac Allard anorthosite, a member of the Havre-Saint-Pierre anorthosite suite from the Grenville province of North America. It is made up of ilmenitite (commonly with more than 95% hemo-ilmenite) associated with noritic lithologies and anorthosite. The present study compares the magnetic fabric of the ore body, as deduced from anisotropy of magnetic susceptibility (AMS) measurements, with the crystallographic and shape fabrics, obtained from lattice-preferred orientation (LPO) and shape-preferred orientation (SPO) measurements made using electron backscattered diffraction (EBSD) and 3D image analysis, respectively. Room-temperature hysteresis measurements, thermomagnetic curves and values of the bulk magnetic susceptibility reveal a magnetic mineralogy dominated by a mixed contribution of hemo-ilmenite and magnetite. The hemo-ilmenite grains display a LPO characterized by a strong preferred orientation of the basal (0001) plane of ilmenite along which hematite was exsolved. This LPO and the magnetic fabric fit well (angle between the crystallographic c-axis and the axis of minimum susceptibility ≤ ca. 15° for most samples), and the latter is thus strongly influenced by the hemo-ilmenite magneto-crystalline anisotropy. A magnetite SPO, concordant with the hemo-ilmenite LPO, may also influence and even dominate the magnetic fabric. The rock shape fabric is coaxial with the magnetic fabric that can thus be used to perform detailed structural mapping. Interpretation of the magnetic fabric and field structural data suggests that the Lac Tio ore body would be a sag point at the margin of the Lac Allard anorthosite, deformed by ballooning during the final stage of diapiric emplacement of the anorthosite body

Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions: implications for magnetic self-reversal and exchange bias—III. Magnetic interactions in samples produced by Fe–Ti ordering

Paper II of this series described the chemical and microstructural evolution of ferri-ilmenite solid solutions during high-T quench and short-term annealing. Here we explore consequences of these Fe–Ti ordering-induced microstructures and show how they provide an explanation for both self-reversed thermoremanent magnetization and room-T magnetic exchange bias. The dominant antiferromagnetic interactions between (001) cation layers cause the net magnetic moments of ferrimagnetic ordered phases to be opposed across chemical antiphase domain boundaries. Magnetic consequences of these interactions are explored in conceptual models of four stages of microstructure evolution, all having in common that A-ordered and B-anti-ordered domains achieve different sizes, with smaller domains having higher Fe-content, lesser Fe–Ti order, and slightly higher Curie T than larger domains. Stage 1 contains small Fe-rich domains and larger Ti-rich domains separated by volumes of the disordered antiferromagnetic phase. Magnetic linkages in this conceptual model pass through disordered host, but self-reversed TRM could occur. In stage 2, ordered domains begin to impinge, but some disorder remains, creating complex magnetic interactions. In stages 3 and 4, all disordered phase is eliminated, with progressive shrinkage of Fe-rich domains, and growth of Ti-rich domains. Ordered and anti-ordered phases meet at chemical antiphase and synphase boundaries. Strong coupling across abundant antiphase boundaries provides the probable configuration for self-reversed thermoremanent magnetization. Taking the self-reversed state into strong positive fields provides a probable mechanism for room-temperature magnetic exchange bias

Spin orientation in a natural Ti-bearing hematite: evidence for an out-of-plane component

The orientation of spins in a natural sample of Ti-bearing hematite (Fe₂O₃) has been measured from 2–300 K using time-of-flight neutron powder diffraction. It is shown that the antiferromagnetic alignment vector is tilted out of the basal plane by an average angle of 30°, independent of temperature, contrary to the normal expectation that all spins lie in the basal plane due to the suppression of the Morin transition by Ti. This unusual result is related to the non-uniform spatial distribution of Ti in this sample, which takes the form of ~1 nm exsolution lamellae of ilmenite (FeTiO₃), observed using transmission electron microscopy. It is suggested that the exsolution lamellae lead to a localization of Fe²⁺ species within the lamellar interfaces, which cause tilting of some spins toward the crystallographic c axis. The presence of an out-of-plane component of spin at room temperature reconciles experimental and computational attempts to explain the phenomenon of “giant exchange bias” that appears when this sample is zero-field cooled below the ilmenite Néel temperature

Crustal magnetism lamellar magnetism and rocks that remember

b Magnetic anomalies are deviations from an internal planetary magnetic field produced by crustal materials. Crustal anomalies, measured over a wide range of vertical distances, from near-surface to satellites, are caused by magnetic minerals that respond to the changing planetary field. Previously, magnetism of continental crust was described in terms of the bulk ferrimagnetism of crustal minerals, which is mostly due to induced magnetization. The recent discovery of lamellar magnetism, a new interface-based remanence type, has changed our thinking about the contribution of remanent magnetization. Lamellar magnetism may also be an important contributor to deep-seated anomalies in the crust of the Earth and in other planets with highly magnetic crusts, like Mars.</p