Continuous production of nanosized magnetite through low grade burial

Abstract

[1] Geological processes, such as burial, can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume, called blocking volume. In this study, we designed a heating experiment for claystones obtained from the Paris Basin (France), in the 50-130 C temperature range, in order to simulate <4 km burial remagnetization. At a given temperature, remanence increased rapidly within a couple of days and stabilized afterward. There was a positive relation between the experimental temperature and the obtained remanence. Remanence was determined to be carried equally by stable chemical remanent magnetization and unstable thermo-viscous remanent magnetization. By assuming that magnetite formed during the experiment, we interpreted the increase of chemical remanent magnetization and the increase of thermo-viscous remanent magnetization as the continuous growth of the >20 nm and $20 nm minerals respectively. This result led us to propose a conceptual model of nucleation-and-growth process of magnetite during low grade burial from $2 to $4 km depth. Ultrafine magnetite (≤20 nm) was predominant over single domain magnetite (>20 nm) for <4 km depth. Transposed to natural conditions, our heating steps experiment suggested that claystone-type rocks are remagnetized during burial. For temperatures higher than 200 C, the extrapolation of our results indicated that burial remagnetization, due to the chemical remanent magnetization, might be larger than the natural remanent magnetization.

Identification of nanocrystalline goethite in reduced clay formations: Application to the Callovian-Oxfordian formation of Bure (France)

“…This mineral is a classic product of early diagenesis (Roberts et al 2011). It is worth noting that the magnetic assemblage of both magnetite and greigite is expected to be representative of immature claystones according to Aubourg et al (2012).…”

“…In the Callovian-Oxfordian Bure claystones, χ is dominated by the paramagnetic contribution (χ para ) and a positive correlation exists between the magnetic susceptibility and the amount of clay minerals (Esteban 2006 and references therein). The magnetic susceptibility values and the saturation remanence at room temperature are generally low (<500 μSI and <100 μA m 2 kg -1 , respectively) and indicate that a few ferromagnetic grains are present (e.g., Moreau et al 2005;Esteban 2006; Aubourg and Pozzi 2010; Kars et al 2012). To characterize the ferromagnetic contribution, remanence should be investigated.…”

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…This setting has a relatively high geothermal gradient, determined to be 83°C/km within the upper 60 mbsf. This projects to a temperature of 50°C at about 570 mbsf, the temperature threshold for neoformation of superparamagnetic (SP) magnetite in claystone observed in experimental studies by Kars et al (2012). Further projection of the shallow geotherm to the bottom of Hole U1437E predicts a temperature of~150°C, where the beginning of neoformation of pyrrhotite in claystone has been invoked (Aubourg & Pozzi, 2010;Kars et al, 2014).…”

Burial Diagenesis of Magnetic Minerals: New Insights from the Grès d’Annot Transect (SE France)

“…Even though it could be concordant with the isotherms established by Labaume et al [33], alteration might also be evoked to explain the occurrence of goethite. Based on heating experiments, magnetite is expected to be present in rocks experiencing a wide temperature range from ~60 °C to >200 °C, including oil and gas windows [8,9,54,[65][66][67]. In the study area, magnetite is found throughout the entire transect in rocks that experienced a burial temperature <250 °C.…”

“…Finally, the remanence acquired at a low temperature (LT-SIRM) shows also a typical evolution. On the LT-SIRM curve, a fall of the remanence can be observed from 10 to 35 K. A parameter, called PM, was defined by Aubourg and Pozzi [9] to characterize this drop and aims at assessing the SP/SD ratio [54]. Other LT-SIRM curves display an inflection point at ~200-250 K, suggesting the occurrence of very small particles or minerals with a high Curie/Néel temperature (e.g., high-Ti titanomagnetite, hematite).…”