Magnetization of carbonaceous asteroids by nebular fields and the origin of CM chondrites

The solar nebula carried a strong magnetic field that had a stable intensity and direction for periods of a thousand years or more1. The solar nebular field may have produced post-accretional magnetization in at least two groups of meteorites, CM and CV chondrites [1–3], which originated from planetesimals that may have underwent aqueous alteration before gas in the solar nebula dissipated [1,3]. Magnetic minerals produced during aqueous alteration, such as magnetite and pyrrhotite [4], could acquire a chemical remanent magnetization from that nebular field [3]. However, many questions about the size, composition, formation time, and, ultimately, identity of the parent bodies that produced magnetized CM and CV chondrites await answers—including whether a parent body might exhibit a detectable magnetic field today. Here, we use thermal evolution models to show that planetesimals that formed between a few Myr after CAIs and ~1 Myr before the nebular gas dissipated could acquire from the nebular field, and retain until today, a chemical remanent magnetization throughout nearly their entire volume. Hence, in-situ magnetometer measurements of C-type asteroids could help link magnetized asteroids to magnetized meteorites. Specifically, a future mission could search for a magnetic field as part of testing the hypothesis that 2 Pallas is the parent body of the CM chondrites [5]. Overall, large carbonaceous asteroids might record ancient magnetic fields in magnetic remanence that produces strong modern magnetic fields, even without a metallic core that once hosted a dynamo.

A re-assessesment of the shallow paleomagnetic inclinations of the Western Cyclades, Greece

Consistently shallow paleomagnetic inclinations measured in Early to Middle Miocene lacustrine and dacitic volcanic rocks of the Kymi-Aliveri basin have been cited as evidence for an anomalous geomagnetic field geometry or northward drift of the Aegean Sea region. We present new paleomagnetic data from the lacustrine beds that are instead not anomalously shallow and consistent with deposition near their present-day latitude as predicted by global apparent polar wander paths. Anomalously shallow inclinations and easterly declinations reported from the Oxylithos volcanics are an artifact of an inappropriate tilt correction. The excessively shallow paleomagnetic inclinations reported from the deformed Middle Miocene plutons on Mykonos and Naxos are consistent with reorientation of an original thermoremanent magnetization acquired during cooling below 580°C by subsequent ductile strain at temperatures of 400-500°C. Magnetization overprints observed in these rocks may reflect the acquisition of a stable chemical remanent magnetization lying parallel to the transposed high-temperature magnetization as the result of low-temperature (<350°C) maghemitization. We therefore find no convincing evidence for an anomalous Middle Miocene field geometry, northward drift of the Aegean, or back-tilting of the low-angle normal faults that constitute the North Cycladic Detachment System.

Paleomagnetism and U–Pb geochronology of the Clarence Head dykes, Arctic Canada: orthogonal emplacement of mafic dykes in a large igneous province

The north–south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U–Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U–Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ∼800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route.