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.
The martian dynamo’s strength and duration are essential for understanding Mars' habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite Miller Range (MIL) 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1,368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1 ± 1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older subsurface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small‐scale crustal fields on Mars.
more » « less- NSF-PAR ID:
- 10446161
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Planets
- Volume:
- 126
- Issue:
- 5
- ISSN:
- 2169-9097
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Interest in magnetic fields on the ancient Earth and other planetary bodies has motivated the paleomagnetic analysis of complex rocks such as meteorites that carry heterogeneous magnetizations at <<1 mm scales. The net magnetic moment of natural remanent magnetization (NRM) in such small samples is often below the detection threshold of common cryogenic magnetometers. The quantum diamond microscope (QDM) is an emerging magnetic imaging technology with ~1 μm resolution and can, in principle, recover magnetizations as weak as 10−17 Am2. However, the typically 1–100 μm sample‐to‐sensor distance of QDM measurements can result in complex (nondipolar) magnetic field maps, from which the net magnetic moment cannot be determined using a simple algorithm. Here we generate synthetic magnetic field maps to quantify the errors introduced by sample nondipolarity and by map processing procedures such as upward continuation. We find that inversions based on least squares dipole fits of upward continued data can recover the net moment of complex samples with <5% to 10% error for maps with signal‐to‐noise ratio (SNR) in the range typical of current generation QDMs. We validate these error estimates experimentally using comparisons between QDM maps and between QDM and SQUID microscope data, concluding that, within the limitations described here, the QDM is a robust technique for recovering the net magnetic moment of weakly magnetized samples. More sophisticated net moment fitting algorithms in the future can be combined with upward continuation methods described here to improve accuracy.
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null (Ed.)The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.more » « less
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null (Ed.)Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program (IODP) Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 18 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. 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We have conducted a paleomagnetic study of Holocene sediments from Lake Victoria in order to develop a high-resolution record of paleomagnetic secular variation (PSV). This study has recovered PSV records from two cores (V95-1P and V95-7P) in northern Lake Victoria (0.5°S). The PSV is recorded in fine-grained detrital magnetite/titanomagnetite grains, but the rock magnetic data suggest that significant magnetic mineral dissolution has occurred, which limits our paleomagnetic studies to the uppermost ~5 m of both cores. Detailed alternating field (af) demagnetization of the natural remanence (NRM) shows that a distinctive characteristic remanence (ChRM) is demagnetized from ~10 to 40 mT, which decreases simply toward the origin. The resulting directional PSV records for 1P and 7P are correlatable with 22 distinct inclination features and 19 declination features. Radiocarbon dating of the cores is based on eight radiocarbon dates from core 1P, which can be correlated into core 7P using both the PSV and rock magnetic/environmental measurements. The final PSV time series cover the last 11,000 years with an average sediment accumulation rate of ~40 cm/kyr. The Lake Victoria PSV records can be correlated with new PSV records from Lake Malawi. Comparison of the correlatable PSV feature ages between the two lakes indicates that the PSV records are not significantly different in age, although Lake Victoria PSV ages might average ~100 years younger. We think that the Lake Victoria and Lake Malawi PSV records, together, provide the most accurate, well-dated, and consistent record of Holocene PSV for Africa yet developed.
