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The Rogaland Igneous Complex (RIC) in southern Norway intruded into Sveconorwegian granulite crust beginning ∼930 Ma. Three massif anorthosite bodies, Egersund–Ogna, Helleren and Åna-Sira, were intruded some 10 Myr later by the Bjerkreim–Sokndal layered intrusion. The Garsaknatt leuconorite and the ilmenite-rich Tellnes norite, one of the youngest rock in the complex at ∼920 Ma, intrude the anorthosite or nearby country rock. Magnetic mineralogy and palaeomagnetic studies carried out on the Tellnes norite, the Garsaknatt leuconorite and the surrounding Åna-Sira anorthosite, indicate the magnetization of all three bodies are dominated by hemo-ilmenite carrying the remanence as a thermochemical remanent magnetization, although magnetite is present in some samples. The three bodies yield steep negative inclinations with northwesterly declinations (Tellnes, I = −71.9°, D  = 305.0°, α95 = 10.6°; Garsaknatt, I = −73.1°, D  = 312.7°, α95 = 4.7°; and Åna-Sira, I = −81.2°, D  = 326.3°, α95 = 6.7°). When combined with data from other bodies in the RIC, the older anorthosites have steeper inclinations, and higher palaeolatitudes, while the younger units have less steep inclinations and shallower palaeolatitudes by nearly 10°, indicating northward plate motion during cooling of the intrusions. Age of the remanence is difficult to determine precisely, however, best estimates are ∼910 Ma for the older anorthosites and ∼900 Ma for the younger intrusions. Although these differences are significant, a unified pole position (35.6° N, 215.1° E), combining all the 111 sites from the RIC, strongly supports the assumed position of southern Baltica in Rodinia at ∼900 MA. [ABSTRACT FROM AUTHOR]

The Bjerkreim–Sokndal (BKS) intrusion in southern Norway has been studied for decades due to the presence of magnetic remanence creating anomalies 12 000 nT below background as measured by airborne magnetic surveys. The strong magnetic remanence also makes the BKS intrusion a good Earth analogue for remote studies of planets that have prominent magnetic signatures, such as Martian geological environments. Although numerous geophysical surveys and samples have been collected in the area, there are limited 3-D geological interpretations of the subsurface. Here, we used existing geophysical data to conduct forward and inversion modelling of the Bjerkreim lobe to investigate the subsurface geometry of the BKS intrusion. An extensive petrophysical property compilation was used as input data for the models, in combination with airborne magnetics and digital elevation models. This petrophysical compilation was initially analysed using principal component analysis to understand which variables would have the greatest impact on the models. Forward and inversion modelling show that cross-cutting jotunite bodies, and small anorthosite blocks within the Bjerkreim lobe have a limited depth extent of 1 km. Massive and foliated anorthosites to the west of the Bjerkreim lobe extend to depths greater than 4 km indicating that the BKS intruded into these anorthosites. Complications in magnetic field fitting during the forward modelling of megacyclic units with strong magnetic remanence and the results from a new ground magnetic survey support the need to revisit mapped contacts of the cyclical units. [ABSTRACT FROM AUTHOR]

Satellite magnetic surveys have revealed features consistent with magnetization at depth in the lithosphere. Previous studies have reported magnetic minerals preserved in mantle nodules and in some eclogite facies rocks. Deep crustal rocks are another possible source for these deep lithospheric signals, but have not been extensively studied, in many cases due to the difficulty in obtaining samples unaffected by later near-surface alteration processes. Here, we used a combined approach involving petrophysical, rock magnetic and scanning magnetic microscopy (SMM) analyses on unaltered pristine ultramafic samples from the Reinfjord Ultramafic Complex in northern Norway. The focus was to identify the magnetic carriers using SMM and link the magnetic anomalies mapped in thin section to distinct rock magnetic measurements. The dominant magnetic carriers are Cr-magnetite exsolved from grains of Al-chromite, and magnetite exsolution lamellae from clinopyroxene. In addition, some samples have exsolved magnetite from Al-Cr-spinel and Fe-rich exsolution from Cr-spinel as carriers. Rock magnetic measurements suggest that these primary magnetic carriers, could retain magnetization to considerable crustal depths. [ABSTRACT FROM AUTHOR]

The orientation of spins in a natural sample of Ti-bearing hematite ([Fe.sub.2][O.sub.3]) 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[degrees], 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 (FeTi[O.sub.3]), observed using transmission electron microscopy. It is suggested that the exsolution lamellae lead to a localization of [Fe.sup.2+] 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 Neel temperature. Keywords: Hematite, magnetic properties, spin orientation, Morin transition, exchange bias DOI: 10.2138/am.2010.3402

Lorentz transmission electron microscopy has been used to study fine-scale exsolution microstructures in ilmenite-hematite, as part of a wider investigation of the lamellar magnetism hypothesis. Pronounced asymmetric contrast is visible in out-of-focus Lorentz images of ilmenite lamellae in hematite. The likelihood that lamellar magnetism may be responsible for this contrast is assessed using simulations that incorporate interfacial magnetic moments on the (001) basal planes of hematite and ilmenite. The simulations suggest qualitatively that the asymmetric contrast is magnetic in origin. However, the magnitude of the experimental contrast is higher than that in the simulations, suggesting that an alternative origin for the observed asymmetry cannot be ruled out. Electron tomography was used to show that the lamellae have lens-like shapes and that (001) planes make up a significant proportion of the interfacial surface that they share with their host. Keywords: Hematite, ilmenite, lamellar magnetism, Lorentz electron microscopy, electron tomography, transmission electron microscopy

An approximate low-temperature, metastable phase diagram is drawn for the system (1 - X) [Fe.sub.2][O.sub.3]-(X)FeTi[O.sub.3]. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 The diagram is drawn with a bicritical point, T[lambda][lambda], at X [approximately equal to] 0.87, T [approximately equal to] 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic [right arrow] ferrimagnetic [PM(PM') [right arrow] FM] transition, [T.sub.C] (X), and the PM(PM') [right arrow] AF transition, [T.sub.N](X). In addition, the RSG phase is plotted as one of four stable phases at T[lambda][lambda], a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technological applications. Keywords: Hematite-ilmenite, magnetic properties, spin glass, phase diagram

[Fe.sup.2]/[Fe.sup.3+] charge ordering in the contact layers of lamellar magnetism in the hematite-ilmenite series is not a postulate of the lamellar magnetism hypothesis. Such ordering is possible, however, and a model was suggested earlier in which contact layer [Fe.sup.3+] octahedra share faces with ilmenite layer [Ti.sup.4+] octahedra, and contact layer [Fe.sup.3+] octahedra share faces with hematite layer [Fe.sup.3+] octahedra, thus copying the shared-face configurations of ilmenite and hematite, respectively. This model and related charge-balance matters could be explored using bond-valence theory, a simplified picture of complex bonding which takes into account the relationship between bond strength and bond distance, but ignores magnetism. This has now been done, and shows that local oxygen charge satisfaction is strongly favored by a different charge-ordering scheme, in which contact layer [Fe.sup.3+] octahedra share faces with ilmenite layer [Ti.sup.4+] octahedra, and contact layer [Fe.sup.3+] octahedra share faces with hematite layer [Fe.sup.3+] octahedra. A new, more sophisticated, Monte Carlo simulation of the interface, including electrostatic and magnetic interaction parameters of cations, reported in detail elsewhere, independently shows the same charge-ordering scheme. For the cation ordered metastable ilmenite 50 composition, bond-valence theory indicates that, unlike the contact layers, the favored charge ordering scheme in Fe layers would have the same shared-face configurations as ilmenite and hematite. Keywords: Crystal chemistry, bond valence, hematite, ilmenite, magnetic properties, lamellar magnetism, order-disorder, Fe charge ordering

Grains consisting of finely exsolved members of the hematite-ilmenite solid-solution series, such as are present in some slowly cooled middle Proterozoic igneous and metamorphic rocks, impart unusually strong and stable remanent magnetization. TEM analysis shows multiple generations of ilmenite and hematite exsolution lamellae, with lamellar widths ranging from millimeters to nanometers. Rock-magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths, yet is stronger than can be explained by canted antiferromagnetic (CAF) hematite or coexisting paramagnetic (PM) Fe-Ti-ordered (R[bar.3]) ilmenite alone. In alternating field demagnetization to 100 mT, many samples lose little remanence, indicating that the NRM is stable over billions of years. This feature has implications for understanding magnetism of deep rocks on Earth, or on planets like Mars that no longer have a magnetic field. Atomic-scale simulations of an R[bar.3] ilmenite lamella in a CAF hematite host, based on empirical cation-cation and spin-spin pair interaction parameters, show that contacts of the lamella are occupied by 'contact layers' with a hybrid composition of Fe ions intermediate between [Fe.sup.2+]-rich layers in ilmenite and [Fe.sup.3+]-rich layers in hematite. Structural configurations dictate that each lamella has two contact layers magnetically in phase with each other, and out of phase with the magnetic moment of an odd non-self-canceling [Fe.sup.3+]-rich layer in the hematite host. The two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments: a lamellar 'ferrimagnetism' made possible by the exsolution. Because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella, lamellar magnetism is unique and quite distinct from conventional ferrimagnetism. Simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent AF hematite layers and are parallel to the basal plane (001). Thus, lamellar magnetism is created at the temperature of chemical exsolution, and is a chemical remanent, rather than thermal remanent, magnetization. However, in thermal demagnetization experiments, too short for lamellar resorption, demagnetization temperatures are those of the CAF hematite, considerably higher than temperatures of original lamellae formation. Internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment, but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution. A thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host, and that once in place, neither the location nor the magnetic moment will be easily disturbed. In an ilmenite host, the external magnetic field cannot control the chemical location of a hematite lamella, which is dictated by the enclosing ilmenite, but once lamellae have formed, the field can dictate their magnetic moments. These moments, however, are not locked chemically to the host, resulting in lower coercivity. The effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field. In grains with (001) oriented parallel to the field, it would be effective in producing in-phase magnetic moments and very strong remanence. In grains with (001) normal to the field, the field would be less effective in producing in-phase magnetic moments, hence producing weak remanence. The most intense lamellar magnetism per formula unit occurs with in-phase magnetization, high lamellar yields, and the largest number of lamellae per unit volume (i.e., smallest lamellar size). Compared to the magnetic moment per formula unit ([M.sub.pfu]) and magnetic moment per unit volume ([M.sub.v]) of end-member magnetite ([M.sub.pfu] = 4 [[micro].sub.B], [M.sub.v] = 480 kA/m) and hematite ([M.sub.pfu] = 0.0115 [[micro].sub.B], [M.sub.v] = 2.1 kA/m), results for some atomic models reasonably tied to natural conditions are [M.sub.pfu] = 0.46-I .36 [[micro].sub.B] and [M.sub.v] = 84-250 kA/m.

Author(s): Peter Robinson (corresponding author) [1, 2]; Richard J. Harrison [3]; Suzanne A. McEnroe [1, 2]; Robert B. Hargraves [4] Magnetic anomalies associated with slowly cooled igneous and metamorphic rocks [...]

Chemistry of the Fe-Ti oxide minerals in the Bjerkreim-Sokndal layered intrusion and in the Tellnes Ilmenite Norite suggests some parallels in magmatic evolution and resultant petrography. Early ilmenites have high ratios of Mg/(Mg + Fe) and Cr/[R.sup.3+] and intermediate to low V/[R.sup.3+], and abundant fine hematite exsolution lamellae, in which Cr, V, and Al are concentrated. Magmatic evolution results in lower Mg/(Mg + Fe), lower Cr/[R.sup.3+] and higher V/[R.sup.3+]. With magma saturation with magnetite, V/R[R.sup.3+] falls both in ilmenite of rapidly decreasing abundance, and in magnetite, but there is an increase in Mg/(Mg + Fe) of ilmenite, probably due to late ion exchange with much more abundant magnetite. Aside from implications related to magma evolution, these results give information concerning fractionation of elements between the rhombohedral and cubic oxide series, and especially between the ilmenite hosts and their contained hematite exsolution lamellae.

Aeromagnetic maps of the Egersund Mid-Proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nT. The positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the Bjerkreim-Sokndal layered intrusion. These rocks are dominated by multi-domain (MD) magnetite. The negative magnetic anomalies are over ilmenite-rich norites of the Bjerkreim-Sokndal layered intrusion, the Tellnes ilmenite norite ore deposit, and massif anorthosites. These rocks are dominated by hemoilmenite and/or by silicates containing fine-grained oxide exsolution lamellae. Electron microprobe analyzes of coexisting Fe-Ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo-ilmenite with minor end-member magnetite, followed by more reduced oxides dominated by titanomagnetite with minor near end-member ilmenite. What is not fully understood is the property of ilmenite with hematite exsolution lamellae, or, even more striking, hematite with ilmenite lamellae, to produce strong remanent magnetization of high coercivity and with a Neel temperature equal to or above the Curie temperature of magnetite. This property makes the rhombohedral oxides an important candidate to explain some high-amplitude deep-crustal anomalies on earth, or strong remanent magnetization on other planets. A remarkable feature in the Egersund province is that primitive magmas produced rocks rich in hemo-ilmenite causing negative magnetic anomalies related to magnetic remanence, and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies, all in the course of crystallization-differentiation.

Late Triassic primary magnetization in Rhode Island dikes reveals that the intrusions occurred before the disintegration of Pangaea, but after the Permian Alleghanian orogeny. The thermal remanent magnetization during the cooling of the dikes provided the Rhode Island diabases with its magnetic properties. Magnetic secondary overprints in many of the samples in the dikes were eliminated by alternating field demagnetization. The geochemical and petrological features of the Rhode Island dikes are different from that of the Jurassic Eastern North American diabases.