Crystallographic—magnetic correlations in single-crystal haemo-ilmenite: new evidence for lamellar magnetism

17 single crystals were identified by electron backscatter diffraction (EBSD) and isolated from coarse massive haemo-ilmenite ore from South Rogaland, Norway. These were studied using the EBSD results, natural remanent magnetization (NRM), and anisotropy of magnetic susceptibility (AMS), to gain a better understanding of angular relationships between crystallographic axes and magnetic properties of haemo-ilmenite in relation to lamellar magnetism. Electron microprobe analyses gave the following average end-member compositions for ilmenite host: 21.1 per cent MgTiO3, 73.7 FeTiO3, 0.5 MnTiO3, 4.3 Fe2O3, 0.2 Cr2O3 and 0.3 V2O3; and for the coarsest (∼3 μm) haematite exsolution lamellae: 3.5 MgTiO3, 22.4 FeTiO3, 71.4 Fe2O3, 1.6 Cr2O3, 1.0 V2O3 and 0.1 Al2O3, making this sample the most Mg- and Cr-rich haemo-ilmenite studied in the province, but with similar element fractionations between the coexisting phases. TEM work on similar material suggests the presence of much thinner exsolution down to 1-2 nm. The EBSD, NRM and AMS results from 12 out of 17 crystals indicate a good agreement between the orientation of crystallographic axes, NRM direction and principal axes of the magnetic susceptibility ellipsoid, with the NRM located in the (0001) basal plane [NRM ∧ (0001) < 6.5°] and the crystallographic c axis quasi-parallel to the minimum axis of the susceptibility ellipsoid [c∧ k3 < 13.5°]. In addition, in 10 of these 12 crystals, the remanent magnetization vector is parallel or nearly parallel to the positive direction of a crystallographic a axis [NRM ∧a < 20°], hence parallel to a principal magnetic moment direction in haematite as determined by Besser, and not parallel to the spin-canted direction of end-member haematite. This is consistent with a basic property of lamellar magnetism, where the magnetic moment is parallel to the principal moments (sublattice magnetization directions) in haematite. Relationships in three additional crystals with NRM ∧a= 22°-33°, only two with good agreement, can be interpreted as consistent with having a magnetic vector quasi-parallel to the spin-canted direction of haematit

Excavations at Gournia, 2010-2012

This article presents previous research at Gournia, the overall goals of our project, a new plan of the settlement, and our 2010-2012 excavations in eight areas: the Pit House, the Northwest Area, the North Cemetery, North Trench, the Northeast Area, House Aa, several rooms in the palace, and House He. Analytical sections discuss the textual evidence; the painted plasters; and the botanical remains. Our excavations indicate that Gournia was first settled in the Final Neolithic period and grew into an industrial town by the Protopalatial period. Following a Middle Minoan II destruction, the town was reorganized in Middle Minoan IIIA to include the palace, which in Late Minoan IB employed Linear A

A closer look at remanence-dominated aeromagnetic anomalies: Rock magnetic properties and magnetic mineralogy of the Russell Belt microcline-sillimanite gneiss, northwest Adirondack Mountains, New York

A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline-sillimanite-quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence-dominated, although the carriers of remanence were not well documented. Russell Belt gneisses have a strong natural rémanent magnetization with steep remanence directions, D = 263°, I = −58°, an average intensity of 3.6 A/m, and typical susceptibilities of 10−4 SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650°–750°C during the Ottawan Orogen (1050–1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self-reversed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susceptibility values and highly stable remanence. Optical observations show titanohematite grains with multiple generations of ilmenite, pyrophanite, rutile, and spinel exsolution lamellae. Microprobe analyses confirm titanohematite compositions ranging from 72 to 97% Fe2O3 with hematite83 being most typical. In rare samples, inclusions of magnetite were identified. The ubiquitous presence of titanohematite, and the rare occurrence of magnetite, is supported by thermal and alternating field demagnetization studies, saturation magnetization measurements, hysteresis properties, temperature-hysteresis studies, and low-temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this may contribute a strong rémanent component to what have previously been considered to be solely induced anomalies

Magnetic petrology of the Devonian Achala Batholith, Argentina: Titanohaematite as an indicator of highly oxidized magma during crystallization and cooling

Devonian magmatism in the Eastern Sierras Pampeanas (Córdoba, Argentina) is represented by elliptical, porphyritic, batholithic, late- to post-orogenic monzogranites emplaced in a metamorphic-plutonic basement. Two groups of Devonian granitoids are distinguished: (1) metaaluminous to weakly peraluminous, hornblende-biotite bearing granitoids, and (2) peraluminous, biotite-(muscovite) bearing granitoids. Groups 1 and 2 are characterized by accessory phases of sphene-magnetite with high susceptibility, and ilmenite-(monazite-uraninite) with low susceptibility, respectively. The magnetic assemblage of the Group 2 Achala Batholith, predominantly paramagnetic, is the subject of this paper Forty-three sites were sampled in the Achala monzogranite. Overall, it is weakly magnetic, with a mean magnetic susceptibility of 1.5 × 10 -4 SI. Twenty-one sites showed titanohaematite as the magnetic carrier of a stable remanence. The titanohaematite is an accessory mineral and contains exsolved disc-shaped intergrowths of ilmenite. The weak magnetism of Achala monzogranite facies contrasts with the magnetic character of other Devonian porphyritic, batholithic monzogranites in the region. The virtual absence of magnetite, together with the presence of exsolved titanohaematite and Mn-ilmenite indicate crystallization and cooling under more oxidizing conditions. Oxidizing conditions continued into the subsolidus, causing late-stage reactions of ilmenite host grains to ilmenite + rutile+ haematite, and titanohaematite to ilmenite-poorer haematite + rutile. These reactions resulted in low susceptibility values and a strong reduction of natural remanent magnetization (NRM) intensities. Thermal demagnetization, up to 620-640°C, isolated steeply dipping, dual-polarity remanence directions. The coincidence of the Achala paleomagnetic pole with the 380-360 Myr segment of the Gondwana Apparent Polar Wander Path and the U-Pb zircon age implies that the remanence was acquired soon after emplacement.Fil: Geuna, Silvana Evangelina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Instituto de Geofísica "Daniel Valencio"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Mcenroe, S. A.. Geological Survey of Norway; NoruegaFil: Robinson, P.. Geological Survey of Norway; NoruegaFil: Escosteguy, Leonardo Darío. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Mapping and Modeling Sources of Natural Remanent Magnetization in the Microcline–Sillimanite Gneiss, Northwest Adirondack Mountains: Implications for Crustal Magnetism

Abstract Distinct crustal remanent magnetic anomalies are a strong indicator of rocks with stable natural remanent magnetization (NRM) carriers. The latter are able to store information on the history of a rock over long geological periods, and can therefore be used for a variety of applications in the field of paleomagnetism and rock magnetism. Typically, paleomagnetic and rock‐magnetic studies rely on rocks bulk magnetic properties. With the advent of high‐resolution magnetometric scanning techniques, it is now possible to map magnetic sources at the mineral scale, identify the different magnetic carriers, and analyze the effect of geometry, microstructures, and composition on their magnetic response. We investigate the stability of discrete remanent magnetization sources of a microcline–sillimanite gneiss sample from Russell Belt with a strong NRM, by scanning the sample before, and after alternating field demagnetization to 100 mT. We quantified changes in the magnetization intensity and direction by inverting the magnetic scan data. Here, we confirm that the exsolved titanohematite with ilmenite lamellae is the major source of magnetization, and the coexisting multidomain hematite grains contribute little to the NRM, or the magnetic anomalies. The microstructures in the titanohematite control local magnetic properties at the mineral scale. Magnetic modeling results suggest a consistent average magnetization direction before and after demagnetization at both the grain and thin section scale, with a decrease in the magnetization intensity of ≈30%. Results are consistent with previous bulk magnetic measurements and the likelihood to use high‐resolution magnetometric techniques in future magnetic studies is high