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CERI 7022/8022 Global Geophysics Spring 2016

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CERI 7022/8022 Global Geophysics Spring 2016 FerrimagnetismI I Recall three types of magnetic properties of materials I Diamagnetism I Paramagnetism I Ferromagnetism I Anti-ferromagnetism I Parasitic ferromagnetism I Ferrimagnetism I Ferrimagnetism I Spinel structure is one of the common crystal structure of rock-forming minerals. I Tetrahedral and octahedral sites form two sublattices. 2+ 3+ I Fe in 1/8 of tetrahedral sites, Fe in 1/2 of octahedral sites. FerrimagnetismII I xmujpkc.xmu.edu.cn/jghx/source/chapter9.pdf Ferrimagnetism III I www.tf.uni-kiel.de/matwis/amat/def_en/kap_2/basics/b2_1_6.html I Anti-spinel structure of the most common iron oxides 3+ 3+ 2+ I Fe in 1/8 tetrahedral sites, (Fe , Fe ) in 1/2 of octahedral sites. FerrimagnetismIV I Indirect exchange involves antiparallel and unequal magnetization of the sublattices, a net spontaneous magnetization appears. This phenomenon is called ferrimagnetism. I Ferrimagnetic materials are called ferrites. I Ferrites exhibit magnetic hysteresis and retain remanent magnetization (i.e. behaves like ferromagnets.) I Above the Curie temperature, becomes paramagnetic. I Magnetite (Fe3O4), maghemite, pyrrhotite and goethite (' rust). Magnetic properties of rocksI I Matrix minerals are mainly silicates or carbonates, which are diamagnetic. I Secondary minerals (e.g., clays) have paramagnetic properties. I So, the bulk of constituent minerals have a magnetic susceptibility but not remanent magnetic properties. I Variable concentrations of ferrimagnetic and matrix minerals result in a wide range of susceptibilities in rocks. Magnetic properties of rocksII I I The weak and variable concentration of ferrimagnetic minerals plays a key role in determining the magnetic properties of the rock. Magnetic properties of rocks III I Important factors influencing rock magnetism: I The type of ferrimagnetic mineral. I its grain size. I the manner in which it acquires a remanent magnetization. I We’ll learn more about each of these. Ferrimagnetic MineralsI I The most important ferrimagnetic minerals: Fe-Ti oxides. Ferrimagnetic MineralsII I Titanomagnetite series I Responsible for the magnetic properties of oceanic basalts. I 0.5 km-thick surface basaltic layer of the oceanic crust has very fine grained titanomagnetite or titanomaghemite I Molecular fraction of ulvöspinel is about 0.6 in oceanic basalts. Ferrimagnetic Minerals III I I Magnetite (Fe3O4) 5 I Has a strong spontaneous magnetization (Ms = 4.8 × 10 Am−1. ◦ I Curie temperature of 578 C. Ferrimagnetic MineralsIV I Susceptibility is the strongest of any naturally occurring mineral. I Maghemite (γ−Fe2O3) I Produced by low-temperature oxidation of magnetite. I Likewise, titanomagnetite becomes titanomaghemite by low-T oxidation. 5 I Has a strong spontaneous magnetization (Ms = 4.5 × 10 Am−1. I Titanohematite series I The Curie temperature and cell size shows the same trend with titanomagnetite as Ti content changes. I Hematite (α−Fe2O3) I Parasitic-ferromagnetism. 3 −1 I Has a relatively weak Ms, 2.2 × 10 Am . I Important for paleomagnetics because of abundance and stability. Grain sizeI I Magnetic relaxation, i.e., decrease of magnetization with time, occurs in ferrimagnetic materials. I The relaxation is described as t M (t) = M exp − ; (1) r r0 τ and the relaxation time τ is given as 1 ν K τ = exp u ; (2) ν0 κ T where ν is the grain volume. I So, the relaxation is slower in a bigger grain. I Read Sec. 5.3.5 of Lowry (1997 or 2004) for details. Remanent MagnetizationI I The small concentration of ferrimagnetic minerals in a rock has the ability to acquire a remanent magnetization (or just remanence). I The untreated remanence of a rock is called it natural remanent magnetization (NRM). I Remanence acuqired at known times in the rock’s history, such as rock formation and subsequent alteration, is geologically important. I Primary magnetization: A remanence acquired at or close to the time of formation of the rock. Secondary if acquired at a later time. I Thermoremanent magnetization of igneous rocks or depositional remanent magnetization of sedimentary rocks are primary. Remanent MagnetizationII I Secondary remanence may be caused by chemical change of the rock during diagenesis or weathering or by sampling and lab procedure. I Thermal remanent magnetization (TRM) I What is a blocking temperature? Read Sec. 5.3.6.1. Remanent Magnetization III I Depositional and post-depositional remanent magnetization (DRM or pDRM) Remanent MagnetizationIV I DRM I Small ferrimagnetic mineral grains oriented like a compass needle. I Declination (due to water current) and inclination (due to grain rolling) error I pDRM I Very fine grains suspended in pore space can be oriented along the external magnetic field. I Occurs within the top ∼10 cm of sediments. I Lock-in time delay of 100 to 10k yrs. Remanent MagnetizationV I Chemical remanent magnetization (CRM) Remanent MagnetizationVI I Isothermal Remanent Magnetism (IRM) Remanent Magnetization VII I Isothermal Remanent Magnetism (IRM) cont’d Remanent Magnetization VIII I Environmental magnetism I Biogenic magnetite: I Evolutionary feature: When sedimentary layers are disrupted, magnetotactic bacteria can follow the geomagnetic field lines back down to the sediments rich in nutrition they need. I Submicroscopic magnetites found in the brains of dolphins and birds. I Also nanometer-scale magnetites found in the human brain, which may be related to neurological disorders such as epilepsy, Alzheimer’s disease and Parkinson’s disease..
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