Bernard Henry

Tectonic applications of magnetic susceptibility and its anisotropy

Abstract Anisotropy of low field magnetic susceptibility (AMS) is a versatile petrofabric tool. For magnetite, AMS primarily defines grain-shape anisotropy; for other minerals, AMS expresses crystallographic control on magnetic properties. Thus, we may infer the orientation-distribution of a dominant mineral from the AMS of a rock. AMS principal directions can record current directions from sediment, flow-directions from magma, finite-strain directions from tectonized rocks and stress-directions from low-strain, low-temperature, neotectonic environments. AMS measurements may reveal some aspects of the strain-path, where carefully selected. For example, we may compare different parts of a heterogeneously strained domain, different minerals in a homogeneously strained site, AMS with schistosity/mineral lineation, and AMS with remanence-anisotropy. Such measurements isolate the orientation-distributions of different minerals, adding a temporal scale to the kinematic sequence. Normally, we can interpret the principal directions of AMS distributions as a physically significant direction, such as a current direction, magmatic flow or finite-strain axis. However, calibrating the AMS ellipsoid shape against the magnitude of the controlling physical process is very difficult. Primarily, this is because the shape of the AMS ellipsoid combines contributions from several minerals whose individual AMS ellipsoids are of different shape. Thus, small variations in the proportions of minerals change the shape of the rocks AMS ellipsoid, even if the alignment process were of constant intensity. In deformed rocks, AMS is more strain-sensitive than calcite twinning or the alignment of calcite or quartz c-axes. Not all AMS fabrics relate to crystallographic or grain alignment. First, displacement fabrics generate AMS where an isotropic matrix of high susceptibility displaces unevenly spaced objects of low susceptibility and suitable scales. Second, AMS location fabrics occur where sub-isometric magnetite grains are close enough, in certain directions, for their demagnetizing fields to interact. This accounts for the AMS of many magnetite-dominated signals where there is no aligned magnetite. Third, the AMS of single-domain magnetite is inverse to shape so that such grains may oppose the AMS contribution of parallel minerals. Finally, transitional sedimentary-tectonic or magmatic-tectonic fabrics yield smeared, temporal sequences of AMS principal directions that cannot be immediately attributed to a single alignment process. These transitional AMS ellipsoids mix primary and secondary AMS components, making it difficult to characterize either component. However, such fabric combinations may permit us to recognize the sense of shearing in flow processes.

From qualitative to quantitative magnetic anisotropy analysis: The prospect of finite strain calibration

Abstract A method of separating low field susceptibility anisotropies due to ferrimagnetics and the matrix, proposed hitherto in cases where the principal axis of these two components are merged, is generalized in cases where the axes are distinct. The method consists of analysing, in specimens close in the same sample, anisotropies, the differences of which are due to the variation of respective percentages of the two components, although the strain rate remains constant. One thus uses the cause which itself renders global anisotropy unusable. A first application is presented, where a control using the anisotropy of isothermal remanent magnetization allows the validity of the method to be tested. A discussion of the choice of the best ellipsoid, relative either to the matrix or to the ferrimagnetics, to give the best indication of relative finite strain rate, is opened.

Interprétation quantitative de l'anisotropie de susceptibilité magnétique

Abstract We propose a model in which the magnetic susceptibility anisotropy is considered as resulting from the distinct properties of two rock components. This model suggests that differences in the susceptibility ellipsoid can be principally related to mineralogical variations. Thus, some difficulties in the quantitative interpretation of anisotropy data may be explained. This model is consistent with a detailed analysis carried on Dalradian rocks from Scotland. A new methodology of anisotropy analysis is derived. It is based on the fact that, in specimens from a same sample, the susceptibility ellipsoid often varies, but relations nevertheless exist between values of the principal and mean susceptibilities. For each sample, measurements are made on 6–12 specimens. The study of the relations between principal and mean susceptibilities permits separation of the anisotropies of the rock components, the anisotropy determination appearing the best for the high susceptibility component.

Magnetic properties, opaque mineralogy and magnetic anisotropies of serpentinized peridotites from ODP Hole 670A near the Mid-Atlantic Ridge

Abstract Magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites (harzburgites) from ODP Hole 670A are studied. The degree of serpentinization was determined by a new method which combines saturation magnetization and whole-rock iron content. A unique Curie temperature ( T c ) of 580°C corresponding to pure magnetite was observed. Microprobe analyses, whole-rock metal content and Curie temperatures indicate the absence of awaruite (Ni 3 Fe) or native metals. Hysteresis parameters reveal the contribution of pseudo-single-domain (PSD) grains in samples with a low degree of serpentinization and large multi-domain (MD) grains in highly serpentinized samples with J rs J s ⩽ 0.08 . Natural remanent magnetization (NRM) is mainly unstable and possibly composite, particularly in the highly serpentinized samples. The intensity ( J a ) of anhysteretic remanent magnetization (ARM) in the geomagnetic field is much higher than J n (intensity of NRM), with J a J n averaging 2.4. The Koenigsberger ratio Q is low and susceptibility ( k ) depends on degree of serpentinization, ranging from 1.2 to 51.5 (×10 −6 m 3 kg −1 ). NRM inclinations are scattered, with some tendency to lie near the horizontal plane, which is close to the foliation plane. Anisotropies of susceptibility, of ARM and of saturation remanent magnetization (SRM) were studied and compared with spinel foliation determined for three less serpentinized samples. In these samples, magnetic anisotropies seem to be mimetic of high-temperature deformation before serpentinization. When the degree of serpentinization is high, later magnetic fabrics, probably related to magnetite veins, dominate previous anisotropies. The temperature dependence of susceptibility and of viscous remanent magnetization (VRM) were studied. The thermal variation of susceptibility shows a shallow Hopkinson effect at about 300–400°C. Time dependence of induced and remanent magnetization increases with temperature, reaching maximum values at about 300–400°C. Based on these observations, induced magnetization and VRM of lower crustal and upper-mantle rocks and their contribution to marine magnetic anomalies are discussed.

THE MAGNETIC ZONE AXIS : A NEW ELEMENT OF MAGNETIC FABRIC FOR THE INTERPRETATION OF THE MAGNETIC LINEATION

Abstract The magnetic zone axis is the common direction of the intersection of the magnetic foliations of different samples. It is easily determined, using tensors, from the minimum susceptibility axes. It provides useful information for the interpretation of the magnetic fabric, particularly of the maximum axis. A bootstrap method reveals small local disturbance of the magnetic foliation. It yields also the confidence zone of the magnetic zone axis.

Anisotropy of magnetic susceptibility of heated rocks

Abstract Heating produces changes, which does not always correspond to simple enhancement of the magnetic fabric. Two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating. The first diagonalizes the tensor from the difference between each tensor term before and after heating. The second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment. When the same magnetic fabric is obtained from several thermal steps, it cannot be related to randomly oriented ferrimagnetic minerals. Instead, the newly formed fabric must be related to characteristics of the pre-existing rock. By comparing this ferrimagnetic minerals fabric with the initial whole rock fabric, we can distinguish cases where heating simply enhances pre-existing fabric from those where thermal treatment induces a different fabric. Relative to the pre-heating fabric, this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction, related to petrostructural elements other than those defining the initial fabric.

Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)

SUMMARY We report a survey carried out on basalt flows from Amsterdam Island (Southeastern Indian Ocean) in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the Brunhes epoch, completing this palaeomagnetic record with palaeointensity determinations and radiometric dating. Because the palaeomagnetic sampling was done over a few hours during the resupply of the French scientific base Martin du Viviers by the Marion Dufresne vessel, we could collect only 29 samples from four lava flows. The directional results corroborate the findings by Watkins & Nougier: normal polarity is found for two units and an intermediate direction, with associated virtual geomagnetic poles (VGPs) close to the equator, for the other two units. A notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations. 50 per cent of the samples yields reliable intensity values with high-quality factors. An original element of this study is that we made use of the thermomagnetic criterion PTRM-tail test of Shcherbakova et al. to help in the interpretation of the palaeointensity measurements. Doing this, only the high-temperature intervals, beyond 400 ◦ C, were retained to obtain the most reliable estimate of the strength of the ancient magnetic field. However, not applying the PTRM-tail test does not change the flow-mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 ◦ C. The normal units yield virtual dipole moments (VDM) of 6.2 and 7.7 (10 22 Am 2 ) and the excursional units yield values of 3.7 and 3.4 (10 22 Am 2 ). These results are quite consistent with the other Thellier determinations from Brunhes excursion records, all characterized by a decrease of the VDM as the VGP latitude decreases. 40 Ar/ 39 Ar isotopic age determinations provide an estimate of 26 ± 15 and 18 ± 9 kyr for the transitional lava flows, which could correspond to the Mono Lake excursion. However, the large error bars associated with these ages do not exclude the hypothesis that this event is the Laschamp.

Comparison of strain and magnetic fabrics in Dalradian rocks from the southwest Highlands of Scotland

Abstract The magnetic fabrics of 235 samples from 31 localities in Argyllshire, Scotland were determined to study the development of the Caledonian tectonic fabric in the southwest Highlands of Scotland. The regional fabric indicates a strong NE-SW compressional foliation due to the primary deformational phases, which in parts has been overprinted by secondary deformations. A detailed comparison of the anisotropy data and the available strain data shows that the two fabric ellipsoids are co-axial, and that their axial mean ratios seem to be related by an empirical power relationship of the type: Xi Xj = li lj a (for i = 1,2,3; j = 1,2,3 and i ≠ j) where χi and χj are orthogonal principal axes and li and lj are the corresponding orthogonal principal strain axes. The exponent a for the sites from Scotland is 0.088 ± 0.017 compared with 0.142 ± 0.001 and 0.145 ± 0.005 found in the Caledonian slates of the English Lake District and the Welsh slate belt.

Geomagnetic paleosecular variation recorded in Plio-Pleistocene volcanic rocks from Possession Island (Crozet Archipelago, southern Indian Ocean)

Possession Island, in the Crozet Archipelago, consists of volcanic units erupted mainly between ~5 and 0.5 Ma. A paleomagnetic sampling was carried out along several sections distributed near the northern, eastern, and southeastern coasts. A total of 45 independent flows were sampled (320 samples). For each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields. However, particularly complex remanence behavior is often observed. The magnetostratigraphy of the lava pile is quite simple, with reversed rocks in the lower part and normal units in the upper part of two sections. A third section is of normal polarity throughout its whole thickness, including three excursional directions. We did not find any intermediate directions between the normal and reverse magnetozones. Thus we have no evidence for the recording of the Matuyama-Brunhes transition expected from a previous study (Watkins, 1972). The amplitude of paleosecular variation, estimated from between-flow dispersion from the field of an axial dipole, is 11.8 with 95% confidence limits between 9.3 deg and 14.0 deg. This value is consistent with the general anisotropic statistical model for paleosecular variation of Constable and Johnson, 1999.

Magnetic fabric and orientation tensor of minerals in rocks

Abstract The magnetic fabric of minerals with magneto-crystalline anisotropy reflects the preferential orientation of their crystallographic axes, but strain determination from these magnetic data is only possible if the distribution function of these crystallographic axes in space, and the relation between finite strain and this function, are known. However, using the value of the principal susceptibilities of a monocrystal, the importance of preferential orientation can be modeled into the orientation tensor, for monomineralic rocks and for numerous other rocks, where magnetic fabric of a single mineral has a strongly dominant effect in the matrix anisotropy.