Evidence for an impact‐induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization

Abstract

Abstract-We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix's primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix's remanent magnetization was found to be consistent and low (~6 lT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization was formed during or after an impact event. Recent mesoscale impact modeling, where chondrules and matrix are resolved, has shown that low-velocity collisions can generate significant matrix temperatures, as pore-space compaction attenuates shock energy and dramatically increases the amount of heating. Nonporous chondrules are unaffected, and act as heat-sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1 km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero-porosity chondrules make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.

Constraints on the Distances and Timescales of Solid Migration in the Early Solar System from Meteorite Magnetism

“…This scenario has been proposed previously for other carbonaceous chondrites (Cournede et al 2015;Muxworthy et al 2017) and our recovered paleointensities agree with MHD model predictions of this field (Bai & Goodman 2009, see Section 4.2). This scenario would require that WIS 91600 was impact heated within the lifetime of the nebula.…”

“…A number of field sources existed during the early solar system that could have imparted magnetic remanences to chondrites. These include the solar wind (Oran et al 2018), impacts (Muxworthy et al 2017), core dynamo activity (Bryson et al 2019a) and the solar nebula (Fu et al 2014(Fu et al , 2019Cournede et al 2015). The key parameters controlling which field was recorded by a specific chondrite are its peak temperature, cooling timescale, mechanism of heating, age of remanence acquisition and aqueous alteration history.…”

Refractory inclusions in carbonaceous chondrites: Records of early solar system processes

“…These observations do not preclude accretion of chondritic materials on the earlier formed and possibly differentiated bodies-a hypothesis recently proposed to explain paleomagnetic records of CV3 chondrites (Elkins-Tanton et al 2011;Gattacceca et al 2016). I note, however, that mechanisms of generating paleomagnetism on the CV parent asteroid remains highly controversial (e.g., Muxworthy et al 2017;Tarduno et al 2017) and requires additional studies. The virtual absence of differentiated materials in chondritic breccias (e.g., Bischoff et al 2006) and asteroid families (Delbo et al 2017) appears to contradict this hypothesis.…”

“…I note, however, that mechanisms of generating paleomagnetism on the CV parent asteroid remains highly controversial (e.g., Muxworthy et al. ; Tarduno et al. ) and requires additional studies.…”

Measuring, Processing, and Analyzing Hysteresis Data

“…The use of magnetic hysteresis data is prevalent throughout paleomagnetic and Earth science studies. It has applications in fundamental rock magnetism (Krása et al, ; Williams et al, ), analyzing paleointensity data (Carvallo et al, ; Haag et al, ; Kissel et al, ; Paterson et al, , ), paleoclimate and paleoenvironmental studies (Chang et al, ; Hatfield et al, ; Liu et al, ; R. Zhang et al, ), biomagnetism (J. Li et al, ; Lin & Pan, ; Pan et al, ), tectonics (Jackson & Swanson‐Hysell, ; S. Li et al, ; Van Hinsbergen et al, ), pollution monitoring (Muxworthy et al, ; C. Zhang et al, ), and extraterrestrial magnetism (Muxworthy et al, ; Tikoo et al, ), among many others. Despite this widespread usage, the analysis of hysteresis data can be nontrivial, and detrimental effects on the quality and accuracy of hysteresis data, such as off‐center loops and drift are routinely unaccounted for (Jackson & Solheid, ).…”