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SUMMARY Exsolved iron oxides in silicate minerals can be nearly ideal palaeomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in palaeodirectional and palaeointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second-order tensors usually used to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections. 

Abstract. Numerous studies have revealed genetic similarities between Tethyanophiolites and oceanic “proto-arc” sequences formed above nascent subductionzones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as ananalogue for this proto-arc crust. Though proto-arc magmatism and themechanisms of subduction initiation are of great interest, insight isdifficult to gain from drilling and limited surface outcrops in marinesettings. In contrast, the 3–5 km thick upper-crustal succession of theSemail ophiolite, which is exposed in an oblique cross section, presents anopportunity to assess the architecture and volumes of different volcanicrocks that form during the proto-arc stage. To determine the distribution ofthe volcanic rocks and to aid exploration for the volcanogenic massivesulfide (VMS) deposits that they host, we have remapped the volcanic unitsof the Semail ophiolite by integrating new field observations, geochemicalanalyses, and geophysical interpretations with pre-existing geological maps.By linking the major-element compositions of the volcanic units to rockmagnetic properties, we were able to use aeromagnetic data to infer theextension of each outcropping unit below sedimentary cover, resulting ina new map showing 2100 km2 of upper-crustal bedrock. Whereas earlier maps distinguished two main volcanostratigraphic units, wehave distinguished four, recording the progression from early spreading-axisbasalts (Geotimes), through axial to off-axial depleted basalts (Lasail), topost-axial tholeiites (Tholeiitic Alley), and finally boninites (BoniniticAlley). Geotimes (“Phase 1”) axial dykes and lavas make up ∼55 vol % of the Semail upper crust, whereas post-axial (“Phase 2”) lavasconstitute the remaining ∼45 vol % and ubiquitously coverthe underlying axial crust. Highly depleted boninitic members of the Lasailunit locally occur within and directly atop the axial sequence, marking anearlier onset of boninitic magmatism than previously known for theophiolite. The vast majority of the Semail boninites, however, belong to theBoninitic Alley unit and occur as discontinuous accumulations up to 2 kmthick at the top of the ophiolite sequence and constitute ∼15 vol % of the upper crust. The new map provides a basis for targetedexploration of the gold-bearing VMS deposits hosted by these boninites. Thethickest boninite accumulations occur in the Fizh block, where magma ascentoccurred along crustal-scale faults that are connected to shear zones in theunderlying mantle rocks, which in turn are associated with economicchromitite deposits. Locating major boninite feeder zones may thus be anindirect means to explore for chromitites in the underlying mantle. 

Magnetic fabrics are powerful tools in structural geology and tectonic studies, because they provide a fast and efficient measurement of mineral alignment, which helps interpret a rock's (de)formation history. The magnetic fabric of remanence‐carrying minerals provides useful information when these grains record different deformation stages than the bulk minerals in a rock. When rocks contain several subpopulations of remanence‐carrying minerals, each of these potentially displays a distinct fabric. This can lead to complex remanence anisotropies, being a superposition of all subpopulations' individual anisotropies. Characterization of partial remanence anisotropies has been used to investigate changes in fabric with grain size. However, most studies still report one bulk remanence anisotropy tensor per sample, and it remains to be determined how commonly different populations of remanence‐carrying grains reflect different subfabrics. Based on a large sample collection including 93 specimens from different lithologies, we have investigated the coercivity dependence of anisotropy of (partial) anhysteretic remanent magnetization A(p)ARM. We find that the principal directions, degree, and shape of A(p)ARM are generally dependent on the coercivity window used to impart the anhysteretic remanent magnetizations (ARMs). Depending on the carrier minerals and their fabrics, ARM anisotropy can either increase or decrease when the ARMs are applied over larger coercivity windows. Additionally, the coercivity fraction that dominates the ARM anisotropy is not always the coercivity fraction that acquires the strongest mean ARM. This illustrates the complexity of characterizing remanence anisotropy, and highlights the importance of carefully choosing experimental parameters in A(p)ARM determination for both magnetic fabric and anisotropy correction studies.

 

Several types or grain sizes of ferromagnetic minerals can contribute to a rock's remanence and anisotropy of remanence. Each subpopulation may have a different fabric. Measuring anisotropy of partial anhysteretic remanent magnetization (ApARM) allows one to determine the anisotropy contribution of subpopulations with different coercivity distributions. Separating these contributions to remanence anisotropy can provide information about early versus late stages of deformation in fabric studies and is the basis for improved anisotropy corrections in paleomagnetic studies. Unfortunately, collecting multiple ApARM tensors on each specimen is time‐consuming and not often done. Measuring a smaller number of carefully chosen ApARM tensors and obtaining the remaining tensors of interest by tensor calculation would be more efficient. This can only be done, however, when ApARM tensors are additive. Here we investigate the additivity of ApARM tensors in a range of lithologies, by measuring a total of seven ApARM and anisotropy of anhysteretic remanent magnetization (AARM) tensors for each specimen, and comparing the tensors calculated from a combination of ApARM tensors to the corresponding measured AARM. Differences in principal directions between measured and calculated tensors are often smaller than the confidence angles of the measurements. Mean anhysteretic remanences are additive to within ±5%. The anisotropy degree varies by ±30% (k′) or ±0.15 (P), and the shape parameterUby ±0.4. These error limits will help to determine whether or not it is necessary to measure each ApARM tensor in future fabric or paleomagnetic studies, or if these tensors can be calculated from a smaller set of measurements.