Anomalous Variation of Magnetic Anisotropy with Low-field in some Volcanic Dikes and its Magnetomineralogical

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1 Anomalous Variation of Magnetic Anisotropy with Low-field in some Volcanic ikes and its Magnetomineralogical Origin Martin Chadima 1, 2, František Hrouda 1, 3 & Josef Ježek 3 1 AGICO Inc., Brno, Czech Republic 2 Institute of Geology, ASCR, v.v.i., Prague, Czech Republic 3 Faculty of Science, Charles University, Prague, Czech Republic

2 The Objective

3 Geological Setting Study Area České středohoří, Eger Graben, branch of Rhine Graben. Rocks: Basanite Bostonite Camptonite Tinguaite Age: Miocene

4 Geological Setting

5 AMS Pattern Observed in ikes Normal ike Plane Inverse Magnetic Fabrics are Mostly Normal, Seldom Inverse, and Rarely Oblique. N = 154 N = 146 Most common Relatively Unfrequent Variable Low-field Variation of Susceptibility epends on Ti Content in Titanomagnetite. Purpose of this study: Variation of Anisotropic Susceptibility with Low-field.

6 AMS Fabric in Two Selected ikes CS1 Bostonite (low Ti) (high Ti) Normal Fabric Inverse Fabric

7 Respective AAMR Fabric CS1 Bostonite (low Ti) (high Ti) Normal Fabric Normal Fabric

8 Four End-Point Scenarios Flow direction Flow direction Flow direction Flow direction Multi-domain grains Normal fabric Single-domain grains k int ike margin M grain ike margin S grain k min k max k max k max k min Multi-domain grains k i nt projection of dike plane Inverse fabric pole to dike plane k min Single-domain grains k int ike margin M grain k int ike margin S grain k min k max k min k max k int k min k max k i nt (Chadima et al. 28)

9 Low-field Variation of Principal irections (P) CS CS CS Variation in k 1, k 2, and k 3 Variation in k 1 and k 3 Locality CS1 moderate variation in P, no variation in P Locality CS34 strong variation in P, strong variation in P What is the Nature of Low-field Variation of AMS??

Low-field Anisotropy of Susceptibility Standard AMS Theory: M = k H Magnetization (vector M) is linearly related to field intensity (vector H), susceptibility (second rank tensor k) is constant.

10 Low-field Anisotropy of Susceptibility Standard AMS Theory: M = k H Magnetization (vector M) is linearly related to field intensity (vector H), susceptibility (second rank tensor k) is constant. Valid for para, dia, initial susceptibility of ferro. AMS in Rayleigh Law Region: M = k fi H + κh + αhh k = k fi + κ + αh k fi dia-, paramag. susc., κ initial susc., α Rayleigh coefficient tensors are all field independent. Field-dependent is αh. AMS above Rayleigh Law Region: M = k fi H + κh + F(κ,H) F(κ,H) matrix function of M vs. H relation

11 Low-field Anisotropy of Susceptibility Possible Causes of AMS Variation 1. Susceptibility Tensor is of higher rank than rank two 2. Superposition of field-independent and fielddependent contributions Solution: 1. Study of low-field variation of large set of directional susceptibilities 2. Evaluation of quality of fit by second rank tensor 3. Calculation of field dependent and field-independent tensors

) Single operating frequency in field range (in peak values) 122 Hz (~1kHz)

12 The Instrument KLY5 Kappabridge In-phase susceptibility Out-of-phase susceptibility (Precise and Calibrated!) Single operating frequency in field range (in peak values) 122 Hz (~1kHz) 5-75 A/m Accuracy within one range: ±.1 % Accuracy of absolute calibration: ±3. % High sensitivity of phase determination.1

independent directions 2 rotations, i.e. 64 directional susceptibilities 1.

13 3 Rotator (64 irectional Susceptibilities) Equal Area (Schmidt) N Axial N = independent directions 2 rotations, i.e. 64 directional susceptibilities 1.5 min to measure AMS

14 Even a baby can handle it.

15 Quality of Tensor Fit Fitting Error E i 1 [( Kf i Km ) / Km i i 2 ] Km i measured value Kf i fit value Excellent fit, which is more or less field independent. irectional variation of susceptibility is satisfactorily represented by ellipsoid varying with field in volume and eccentricity!!! Excellent fit in very low fields. With increasing field the fitting error increases substantially. In high lowfields, the fit is more or less constant, but almost an order worse than in very low fields.

16 Camptonite CS Very high susceptibility Strong field variation. Contour pictures differ according to field. P strongly vary with field.

17 Titanomagnetites Field and Temperature Variations No Ti - magnetite, very high susceptibility, no field variation of susceptibility, Tc = 585 C Increasing Ti Content - decreasing susceptibility, increasing field variation of susceptibility, decreasing Curie temperature. (Vahle and Kontny, 25, EPSL)

18 Temperature Variation of Susceptibility CS1 Bostonite Magnetically Monomineralic Rock. Mostly no variation in P and AMS ellipsoid shape, moderate variation in susceptibility and degree of AMS. Three carriers of AMS: with (1) Tc = 155 C, (2) Tc = 43 C, and (3) Tc =57 C. (1) Strongly field-dependent (2) Moderately field-dependent (3) Weakly field-dependent : In very low fields the AMS is affected by all three phases. In moderate fields, increasing effect of (1) phase can be observed. In strong low-fields the AMS is dominantly controlled by phase (1), which is most strongly field-dependent.

19 Tensor Separation: irectional Susceptibility Method Contributions to whole-rock susceptibility k r = K d + K p + K ma + K sd + K md + A md H contribution of field-dependent susceptibility of M fraction Ψ = K d + K p + K ma + K sd + K md initial susceptibility of M fraction Contribution of field-independent susceptibility k r = Ψ + A md H field-independent susceptibiility From fitting straight lines to the susceptibility vs. field data for each direction we are able to determine the fieldindependent directional susceptibilities and field-independent susceptibility tensor Using α = ck 2 (Néel, 1942), the initial susceptibility tensor of M ferro (except for mean susceptibility) can be determined from A md (Hrouda 28)

20 Tensor Separation: irectional Susceptibility Method Kre [E-3 SI] 95 CS Camptonite Field [A/m]

21 Field Variation of AMS 5 A/m

22 Field Variation of AMS 1 A/m

23 Field Variation of AMS 15 A/m

24 Field Variation of AMS 2 A/m

25 Field Variation of AMS 25 A/m

26 Field Variation of AMS 3 A/m

27 Field Variation of AMS 4 A/m

28 Field Variation of AMS 5 A/m

29 Field Variation of AMS 6 A/m

30 Field Variation of AMS 75 A/m

31 Field Variation of AMS 1 A/m

32 Field Variation of AMS 15 A/m

33 Field Variation of AMS 2 A/m

34 Field Variation of AMS 25 A/m

35 Field Variation of AMS 3 A/m

36 Field Variation of AMS 35 A/m

37 Field Variation of AMS 4 A/m

38 Field Variation of AMS 5 A/m

39 Field Variation of AMS 6 A/m

40 Field Variation of AMS 7 A/m

41 Field-dependent Tensor Field-dependent tensor

42 Field-independent Tensor Field-independent tensor

43 Anisotropy of Anhysteretic Remanence AARM tensor

44 Tentative Conclusion Flow direction Flow direction Flow direction Flow direction Multi-domain grains Normal fabric Single-domain grains k int ike margin M grain ike margin S grain k min k max k max k max k min Multi-domain grains k i nt projection of dike plane Inverse fabric pole to dike plane k min Single-domain grains k int ike margin M grain k int ike margin S grain k min k max k min k max k int k min k max k i nt

45 Conclusions The volcanic dikes investigated show significant increase of the mean susceptibility and degree of AMS with increasing low-field. In most dikes, the orientations of the principal directions are field independent. The contours of the directional susceptibilities have the shapes similar to those of an ellipsoid and do not change with low-field; they only increase their intensities. This holds not only for the Rayleigh Law Region, but also for the fields slightly stronger up to 7 A/m. In locality CS34, the principal directions vary with the low-field significantly and the contours change their shapes and intensities accordingly. This rock shows three magnetic phases. In very low fields the AMS is affected by all three phases. In moderate fields, increasing effect of the phase with Tc = 155 C can be observed. In strong low-fields the AMS is dominantly controlled by the same phase, which shows the strongest low-field variation. For the dikes with field invariable P, the geological interpretation in terms of magnetic foliation and lineation and lava flow is straightforward. It is recommended to investigate the low-field variation on pilot specimens.

46 Gracias por su paciencia! Obrigado pela sua paciência! Thanks for your patience! ěkujeme za Vaši trpělivost! Kiitos kärsivällisyydestäsi! მადლობას გიხდით მოთმინებისთვის!

non-zero slope of the initial ferromagnetic thermomagnetic curve. INTRODUCTION INITIAL SLOPES OF THE THERMOMAGNETIC CURVES OF SOME MINERALS

non-zero slope of the initial ferromagnetic thermomagnetic curve. INTRODUCTION INITIAL SLOPES OF THE THERMOMAGNETIC CURVES OF SOME MINERALS Geophys. J. Int. (1997) 129,715-719 Refined technique for susceptibility resolution into magnetic and magnetic components based on susceptibility temperature-variation measurement FrantiSek Hrouda,'.'