Pressure Remanent Magnetization: Can a crustal PRM overprint a rock's NRM?
Abstract
Most terrestrial rocks have been subjected to burial and uplift. With depth, both pressure and temperature increase following the local geotherm, which will influence a rock's magnetic remanence and/or magnetic properties. While the effects of temperature on magnetite-bearing rocks have been studied in great detail, the effect of crustal pressures has been investigated to a much lesser extent. Here we present a series of experiments that aim to explore the acquisition process of pressure remanent magnetizations (PRMs) and the net contribution of PRMs with respect to a rock's original magnetization.
We imparted PRMs (226, 301, and 376 MPa) to synthetic rocks containing stoichiometric magnetite of four different grain sizes (65 nm, 440 nm, 16.9 µm, and 18.3 µm) and magnetic domain states. The resulting PRMs increased in magnitude as the pressure was increased with no discernable difference across the various grain sizes. These observations differ dramatically with those of thermal remanent magnetizations (TRMs), which are strongly dependent on domain state, with multidomain grains acquiring the lowest TRM. For populations of large multidomain grains, a PRM can represent ≈ 30% of a TRM acquired in the same magnetic field. When PRMs are imparted at elevated temperatures, a rock acquires both a PRM and a partial TRM (pTRM). While we observe a grain size dependence to the pTRMs, after thermally demagnetizing the pTRM, the PRM(T) is also, within error, independent of domain-state/grain-size. We hypothesize that magnetostriction - a material constant independent of domain state - is an essential factor for the acquisition of PRMs. Using the pressure and temperature dependence of magnetite's magnetostriction constant, we show that PRMs acquired in high pressure /low-temperature regimes (e.g. blueschist & zeolite facies), can potentially overprint a rock's natural remanent magnetization, leading to erroneous interpretations of the remanence direction. Determining methods to identify the presence of PRMs acquired during a rock's burial and exhumation history is an important area for future research and may ultimately lead to the development of magnetic paleobarometric tools.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP13A..03V
- Keywords:
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- 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1594 Instruments and techniques;
- GEOMAGNETISM AND PALEOMAGNETISM