Petrophysical Characterization of Non-Magnetic Granites; Density and Magnetic Susceptibility Relationships
Emilio L. PueyoTeresa Román‐BerdielPablo CalvínJ. L. BouchezElisabet BeamudC. AyalaF. LoiRuth SotoPilar ClarianaAina MargalefNúria BachStefanía SchamuellsFélix RubioAna GimenoElena Fernández de ArévaloCarmen Rey‐MoralArturo GarcíaJoan Martı́Antonio María Casas SáinzJosé Luis García-Lobón
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In this work we establish reliable correlations between density and magnetic susceptibility in three paramagnetic granites from the Pyrenees. In total, 128 sites (310 density measurements and >2600 susceptibility ones) were studied in the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons covering the main range of variability of magnetic susceptibility. Regressions were calculated for every granitic body and an integrated linear function was obtained for the entire dataset: ρ (kg/m3) = 2566 (kg/m3) + 0.541κ (10−6 S.I.) (R:0.97). This relationship is only valid in the paramagnetic domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows for transforming magnetic susceptibility data into density data, helping to constrain gravity modelling when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows for densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signals.Keywords:
Petrophysics
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Abstract Contacts within nested plutons are crucial for constraining the relative timing of pluton emplacement and the internal geometry of composite plutons. Exposures in orogenic belts are commonly discontinuous, however, disguising these contacts. In this paper, the Merrimac plutons in the northern Sierra Nevada of California are used as an example of how composition and foliation patterns can allow the definition of unexposed contacts and identify nested plutons. Image analysis techniques were used to determine modal compositions of a total of 52 samples from the Merrimac plutons. The integrated analysis of compositional data and foliations patterns reveals a critical contact within the plutons, and suggests that the Merrimac plutons indicate way‐up towards the north‐east at the time of emplacement 142 ± 3 Ma ago. This paper provides guidelines for recognizing nested plutons in poorly exposed areas and shows that consistent structural and compositional assymmetries within nested plutons can be used as regional top‐direction indicators.
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Vanadium in the vanadium-bearing titanomagnetite (VTM) exist by replacing iron in the magnetite crystal, and magnetite should be recovered separately from titanium components in the vanadium-bearing titanomagnetite. In the present study, a sensing system was proposed by measuring magnetic susceptibility of magnetite because the magnetic susceptibility can be easily and quickly measured. The effects of mixing ratio of magnetite and ilmenite with silica and the distance between the magnetic susceptibility meter and the sample bed on the magnetic susceptibility were investigated. The magnetic susceptibility of the mixture of magnetite and ilmenite with 0.1 g or 0.2 g of silica decreased with increasing the ratio of ilmenite in the mixture, but the effect of silica addition was not significant. When the distance between the magnetic susceptibility meter and the sample bed increased, the magnetic susceptibility of magnetite decreased, but the magnetic susceptibility decreased with increasing the thickness of magnetite sample bed of magnetite. Because the magnetic susceptibility depended strongly on the content of magnetite, the sensing system using the magnetic susceptibility measurement could be successfully applied to detect magnetite.
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The magnetic susceptibility of several olivines, xFe2SiO4⋅(1-x)Mg2SiO4 with varying x has been measured. The results showed that the olivines are paramagnetic and that their molecular magnetic susceptibility χmol can be expressed empirically by χmol=2x× 10-2emn/mol at the room temperature. Using the accepted values of the Bohr magneton number of Fe+2 ion, and assuming that the Fe+2 ions are embedded isolatedly in the crystal, the paramagnetic susceptibility of olivines at the room temperature can be calculated as χmol=2.2x∼2.6x×10-2emn/mol, provided that the other constituents such as Mg+2 and SiO4 are only diamagnetic. The comparison of the empirical and the calculated values seems to justify the above simple calculation. We expect such calculation will also valid for other rock-forming paramagnetic minerals.
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