Pierre Rochette

Rock magnetism and the interpretation of anisotropy of magnetic susceptibility

The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. As discussed in this article, various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence. 93 refs., 11 figs., 1 tab.

Magnetic transition at 30–34 Kelvin in pyrrhotite: insight into a widespread occurrence of this mineral in rocks

Abstract A characteristic magnetic transition at 30–34 K is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low-temperature techniques as applied to magnetite and hematite, extended down to liquid helium temperature. A review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals. Unblocking temperature up to 350°C and extreme resistance against AF may be encountered in fine grained pyrrhotite.

Is this magnetic fabric normal? A review and case studies in volcanic formations

Abstract When studying weakly anisotropic rocks non-standard relationships between magnetic and expected petrofabric axes are sometimes observed, in particular the so-called inverse or intermediate fabrics. A review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics, often encountered in magnetite-rich volcanic rocks, are due to abnormal preferred orientations or distributions of magnetite grains. Through case studies in dikes (from Skye Island, Yemen and the Iberian Peninsula), and magma and ash flows (from the Ethiopian traps and southern Corsica) the various possible origins of such fabrics are examined. It is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed, leading to non-unique interpretations.

Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism

Abstract A combined magnetic and mineralogical study of 10 Shergotty–Nakhla–Chassigny type (SNC) meteorites, including six recent desert finds, provides strong evidence that, like in ALH84001, the natural remanence (NRM) of basaltic shergottites (apart from Los Angeles, LA) is carried by pyrrhotite. Low Curie point titanomagnetite is the major magnetic carrier only in nakhlites, Chassigny and LA. LA appears to be by far the most magnetic SNC. The dominant role of pyrrhotite has major implications on the interpretation of Martian magnetic anomalies and SNC paleomagnetism: in particular pyrrhotite undergoes magnetic and/or phase transition at room temperature in a pressure range of 1.6–3.6 GPa, much lower than the peak shock pressure experienced by shergottites. Any impact-related pressure above this value should thus totally reset remanent magnetism. It is also suggested that the very low thermal stability of remanence in some SNCs may be due to hexagonal pyrrhotite in metastable ferrimagnetic state or shock-induced substructures in monoclinic pyrrhotite.

Inverse magnetic fabric in carbonate-bearing rocks

Abstract The observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric (pole of bedding or schistosity) is referred to as an inverse magnetic fabric. The investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c -axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction. Inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones. A mineralogical model based on the property of single-domain grains to have a zero susceptibility parallel to their long axis is proposed. However, more complex cases are also encountered.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Relationship between heavy metals and magnetic properties in a large polluted catchment: The Etang de Berre (south of France)

Abstract The purpose of this study was to test possible extension of the relationship found by several authors between heavy metals enriched materials and magnetic properties to environmental system by the analysis of various materials of mixed anthropic and natural origins. The samples studied came from the Berre lake (pre-industrial sediments, recent surface sediments, saltmarsh materials, atmospheric dusts) and from other sources such as leaves of plane trees and Marseille urban sewage sludge. Clear correlations between metallic pollutants (mainly Zn, Cd & Cr with r 2 > 0.75, and weaker correlation for Cu, Ni, Pb & Fe) concentrations and simple magnetic parameters (Saturation Isothermal Remanent Magnetisation, Susceptibility), are observed. The results show that correlation factors are frequently higher between magnetic parameters and metallic pollutants than between iron and the same metallic elements. For the same chemical element, SIRM values are frequently better correlated than x values. Nature of magnetic grains involved is discussed.

Magnetic susceptibility of the Mont-Louis andorra ilmenite-type granite (Pyrenees): A new tool for the petrographic characterization and regional mapping of zoned granite plutons

We have observed remarkably consistent patterns of concentric zonation in the values of low-field magnetic susceptibility measurements over the Variscan age Mont-Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe-bearing silicates, and this is demonstrated by the fact that low-field susceptibilities of representative specimens are almost equal to (1) their high-field susceptibilities and (2) their calculated susceptibilities using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.

The Gangotri granite (Garhwal Himalaya): Laccolithic emplacement in an extending collisional belt

The Gangotri Miocene leucogranite is composed of several laccoliths (6–7 km long, 1.5–2 km thick), which can be divided into two sets of lenses: the southern lenses, intruded in the lower part of the High Himalaya sedimentary cover, and the northern lenses, intruded in an older porphyritic biotite-bearing granite. In both cases, the magmatic fabric is commonly weak with no dominant stretching direction, although a rough E-W trend is present in the northern lenses. The magnetic fabric is characterized by the fact that drastic changes in the direction, in the magnitude of the magnetic parameters, or in the fabric type (planar or linear) may take place over very short distances. Measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses, in agreement with the large dispersion observed in both the field and magnetic lineations. The comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the High Himalaya range and is not related to a southward directed thrust event associated with the Main Central Thrust. This is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses. The presence of these dikes indicates in turn that magma ascent occurred by fracture propagation. The spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the Tibetan Slab or from the intrusion of independent laccoliths. The use of the elastic bending theory of Pollard and Johnson (1973) shows that in both hypotheses, the current laccolith sizes are compatible with a laccolithic mode of magma emplacement. However, neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the Tibetan Slab and the overlying Tibetan metasedimentary series controlled the level of magma emplacement. Rather, flat-lying collapse structures, which intersected the upward propagating magma dikes, are the most likely causes of magma arrest. Such a mechanism was favored by the schist-rich lithology of the metasedimentary host rocks. In addition, field relationships indicate that the melt supply through the dike system was a continuous, rather than pulsed, process. Existing numerical treatments on the rates of magma transport through fractures show that in such a case, the laccoliths could have been built in less than 100 years. This short time of emplacement, the small size of the laccoliths, and their peripheral disposition relative to the Badrinath granite suggest that the Gangotri lenses may represent the initial stage of pluton accretion in the High Himalaya which ultimately gave rise to a much larger massif such as the Manaslu granite.

Metamorphic control of the magnetic mineralogy of black shales in the Swiss Alps: toward the use of “magnetic isogrades”

Abstract The magnetic susceptibility of the Helvetic Jurassic black shales, sampled around the Aar and Gothard massifs in the Swiss Alps, is due to iron-bearing silicates, magnetite or pyrrhotite. It exhibits a clear correlation with the Alpine metamorphism, from zeolite to amphibolite facies: it firstly decreases due to the breakdown of original magnetite, then sharply increases when pyrite is transformed into magnetic pyrrhotite. Using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite-in isograde, which appears to correspond closely to the stilpnomelane-out isograde. The matrix susceptibility, obtained by means of high-field measurements with a cryogenic magnetometer, is due to iron-bearing silicates and shows only smooth variations with metamorphism. Both monoclinic and hexagonal pyrrhotites are observed in the high-grade area.