Quaternary lavas of the Stardalur Caldera, 20 km northeast of Reykjavik, Iceland, create a 27 300 nT magnetic anomaly visible in both ground and aeromagnetic surveys. Here, we provide a comprehensive mineralogical and rock magnetic data set to analyse NRM intensities and Koenigsberger ratios of 57 drill-core samples from the critical zone (CZ) of the anomaly high at depths between 41 and 131 m. This extends previous studies and verifies that the anomaly is due to an unusually high intensity of remanent magnetization carried by magnetite. The NRM of the CZ samples was acquired during the Olduvai subchron in a field of at most today’s strength. NRM intensities range from 20 to 128 A m–1 with a median of 55 A m–1, and an average of 61 A m–1, respectively, approximately 13–15 times higher than in typical Icelandic basalts (AIB) with an NRM intensity of 4 A m–1. Our new data set shows that the magnetite concentration throughout the CZ basalts is at most twofold higher than in AIB lavas. New data on domain state and TRM efficiency prove that these properties account for an additional factor of at most 2.3. Because magnetite is the most abundant remanence carrier in rocks on Earth, and its remanence acquisition is considered to be extremely well understood, we assert that the remaining discrepancy is a critical enigma in rock magnetism. Results from scanning electron microscopy show that a significant fraction of all CZ magnetite particles have dendritic shapes with grain sizes <1 μm, indicating rapid crystallization. Most large magnetite grains are heavily subdivided by very fine oxidation-exsolution lamellae of ilmenite, and subordinate amount of exsolved spinel as needles, blebs and blades. These common microstructures found throughout the CZ subdivide the initially homogeneous mineral into separate cubicles, here denoted as compartments. The magnetite compartments then have sizes below 1 μm. Hysteresis data, Preisach maps and FORC data consistently confirm that the coercivity distribution is dominated by values above 10 mT, such that multidomain behaviour is of little relevance in the CZ. Between 5 and 20 per cent of the IRM is carried by coercivities above 100 mT, which for magnetite indicates unusually high anisotropy effects in the individual particles. Based on the quantitative analysis of all magnetic contributions to the NRM, we can demonstrate that the average efficiency of NRM acquisition in the CZ Stardalur basalts must be at least a factor 3 higher than in typical basalts. We speculate that this is related to the observed focused compartment size distribution <1 μm, and indicates thermochemical remanence acquisition below the Curie temperature of magnetite. Yet, a detailed physical mechanism for the extreme overefficiency of NRM acquisition remains enigmatic.
Pressure remanent magnetization (PRM) is acquired when a rock is compressed in the presence of a magnetic field. This process can take place in many different environments from impact and ejection processes in space, to burial and subsequent uplifting of terrestrial rocks. In this study, we systematically study the acquisition of PRM at different pressures and temperatures, using synthetic magnetite in four different grain sizes ranging from nearly single‐domain to purely multidomain. The magnitude of the PRM acquired in a 300 μ
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Page Range / eLocation ID:
- p. 2473-2483
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We examine the behavior of natural basaltic and trachytic samples during paleointensity experiments on both the original and laboratory‐acquired thermal remanences and characterize the samples using proxies for domain state including curvature (
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