Demagnetization of terrestrial and extraterrestrial rocks under hydrostatic pressure up to 1.2 GPa
Introduction
Hypervelocity impacts are a major mechanism for the evolution of the solid matter in our solar system. Shock waves generated during impacts can modify both intrinsic magnetic properties (Gattacceca et al., 2007a, Louzada et al., 2007, Nishioka et al., 2007, Gilder and Le Goff, 2008) and remanent magnetization (Pohl et al., 1975, Kletetschka et al., 2004, Gattacceca et al., 2006, Gattacceca et al., 2008, Louzada et al., 2007) of rocks. Consequently, the magnetic record of solid bodies in the solar system, affected by impacts to different degrees, could have been erased or overprinted by shock events. Understanding the process and the physical mechanism of the impact remagnetization is therefore a key issue to the interpretation of the crustal magnetization of Mars (Hood et al., 2003, Hood et al., in press), the Moon (Cisowski et al., 1976, Halekas et al., 2002, Halekas et al., 2003), small solid solar system bodies such as asteroids (Chen et al., 1995) as well as paleomagnetic records of meteorites and extraterrestrial materials available from sample return. Concerning the Earth, shock-induced changes in rock magnetic properties and magnetic remanence should be taken into consideration while studying the remanent magnetism of terrestrial impacts (Halls, 1979, Pesonen et al., 1992, Pilkington and Grieve, 1992, Louzada et al., 2008).
Different authors have carried out experimental investigations of shock demagnetization (remagnetization) of rocks and pure minerals in the 1–30 GPa peak pressure range. Different techniques have been used for shock waves generation: air or gas gun accelerating aluminium or copper projectiles (Hornemann et al., 1975, Pohl et al., 1975, Martelli and Newton, 1977, Cisowski and Fuller, 1978, Srnka et al., 1979, Dickinson and Wasilewski, 2000, Louzada et al., 2007); high explosive and nuclear charges (Hargraves and Perkins, 1969, Pesonen et al., 1997, Gattacceca et al., 2007a); free falling mass (Kletetschka et al., 2004) and pulsed laser (Gattacceca et al., 2006, Gattacceca et al., 2008). The main caveats of such experiments are the complexity of dynamic pressure calibration, the possible mechanical damages of investigated samples, and deciphering of the effect of deviatoric versus hydrostatic stresses. Indeed, it is known that remanent magnetization is more sensitive to non-hydrostatic (deviatoric) than hydrostatic stresses (Nagata, 1966, Martin and Noel, 1988). Moreover, shock may permanently modify the intrinsic magnetic properties (e.g., coercivity, see Gattacceca et al., 2007a) thus complicating the interpretation.
As for meteorites, considering the relative rarity of extraterrestrial material on the Earth, it is excluded for most of them to perform shock experiments that may be destructive and require rather large sample volume. Numerous parameters must be considered when studying the effect of shock on the magnetic remanence: shock intensity and duration, background magnetic field during the shock event, magnetic mineralogy, pre-shock magnetization and temperature. This large number of parameters, that are sometimes difficult to control, complicates the comprehension of shock effect on rock magnetic remanence.
Static pressure experiments are well suited to tackle these problems. They allow better pressure calibration and can be non-destructive for samples. However, they were until recently limited to the low pressure range for under pressure measurements (<0.1 GPa, e.g., Pozzi, 1973). Experiments were also carried out by pressurizing the sample up to 2 GPa, and remeasuring the remanence outside the pressurizing device (Pearce and Karson, 1981). More recently Rochette et al. (2003) compressed a pyrrhotite sample up to 3 GPa in a piston-cylinder press and remeasured isothermal remanent magnetization (IRM) after pressure release. It was found that pyrrhotite undergoes a high-pressure magnetic transition under a pressure of 2.8 GPa, which results in a complete loss of its magnetic remanence. This experimental scheme has the disadvantage of needing a new sample and few days of experiments per each pressure value. Moreover, these experiments, by using a solid confining media, generate some deviatoric stress on the sample.
Gilder et al. (2006) performed IRM measurements of pure single domain (SD) and multidomain (MD) magnetite under quasihydrostatic load up to 4.2 GPa using a diamond anvil non-magnetic cell (in the Earth's magnetic field) and also observed a pressure demagnetization effect. Gilder and Le Goff (2008) carried out pressure experiments up to 6 GPa using a moissanite anvil cell on natural and synthesized MD titanomagnetite with different titanium concentration, but this work was focused upon the influence of stress on the acquisition of IRM. All these experiments are restricted to pure strongly magnetic minerals due to the minute sample size (e.g., for the diamond anvil cell the cylindrical sample chamber was 400 μm in diameter and 100 μm in height) and cannot be realized on bulk rock samples without extracting their magnetic fraction.
Pressure demagnetization experiments on bulk rock samples have significant implications in solid-state physics and geophysics, in particular in paleomagnetism and interpretation of crustal magnetic anomalies of the solid solar system bodies. As crustal rocks suffer the load created by overlying rocks and/or water column (for instance ∼0.06 GPa for 5 km of water and 350 m of sediments), laboratory studies of the effect of pressure on the remanent magnetism of rocks may be helpful for the comprehension and interpretation of the paleomagnetic signal of the deep seated rocks and crustal magnetic anomalies. However, together with pressure, crustal rocks undergo the concomitant influence of high temperatures, making the situation even more complex. At pressures up to 1.5–2 GPa, which corresponds to a crustal thickness of 50–70 km, titanomagnetites do not crystallize any more (Valeev, 1984): this is the upper limit of relevant pressures.
Despite previous works, the effect of pressure on the remanent magnetization is still poorly known for natural materials for pressures of the order of 1 GPa. The goal of this work is to present a thorough investigation of the effect of hydrostatic pressure up to 1.24 GPa on the magnetic remanence of rocks within a wide range of magnetic mineralogies. We investigated 50 samples of terrestrial and extraterrestrial rocks and minerals as well as synthetic samples with the following magnetic carriers: magnetite, titanomagnetite, hematite, pyrrhotite, native iron and nickel iron, goethite, greigite. For each magnetic mineralogy we studied different samples spanning a wide range of remanent coercivity (Bcr).
Section snippets
Experimental setup
In order to isolate the pressure demagnetization effect on rock magnetic remanence from the creation of piezo-remanent magnetization after pressure application (studied in many previous works, e.g., Nagata, 1966, Kinoshita, 1968, Pozzi, 1973) we always applied the pressure in a low magnetic field (<5 μT).
The experimental setup was designed for room temperature measurements of magnetic remanence of relatively large rock samples (up to 5.8 mm in diameter and 15 mm long cylinders) under hydrostatic
Main characteristics of the pressure demagnetization experiments
The used pressure cell is characterized by a relatively low but nonzero remanent magnetic moment (see above). In order to check the need for correction of the magnetic remanence of investigated samples by the cell magnetic remanence, the sample of rhyolite (rb7a) was chosen. This sample is the better suited as it was shown to be the least magnetic (initial SIRM inside pressure cell is 5.48 × 10−7 A m2). Fig. 2 displays the evolution of the magnetic moment of the cell versus pressure (after 5, 10
Discussion
In order to check the effect of repeated load (to the same pressure level) on the magnetic remanence, several cycles from 0 to 1.24 GPa were carried out on some of investigated sample (see Fig. 8). Throughout repeated loads the remanent magnetization under pressure (Fig. 8a) or upon pressure release (Fig. 8b) always shows a slight decrease as a function of number of cycles, consistent with previous works (e.g., Pozzi, 1975, Gilder et al., 2006, Bezaeva et al., 2007). This is not linked to a
Conclusions
This study gives an overview of the sensitivity to pressure demagnetization (by a purely hydrostatic load up to 1.24 GPa) of geological and extraterrestrial materials as well as synthetic samples with a variety of magnetic mineralogies: magnetite and titanomagnetite, pyrrhotite, greigite, hematite, goethite as well as iron and iron–nickel alloys. Magnetic remanence under pressure and upon decompression was investigated using a non-magnetic high-pressure cell of piston-cylinder type together with
Acknowledgements
This work was supported by the French Agence Nationale de la Recherche (project 05-JCJC-0133) and was partially funded by the CNRS-RFFI PICS program (grant no. 07-05-92165) while the stay of N.S. Bezaeva at CEREGE was funded by a research grant of the French Government (no. 2005814). We acknowledge K.L. Louzada (Harvard University, Cambridge, USA) for providing basalt samples from the Lonar crater (PD6-2-1, PD6-2-4). C. Francis (Harvard Museum of Natural History, Cambridge, USA) is acknowledged
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