Micromagnetic and microstructural analyses in chondrules of the Allende meteorite

Abstract

Results of micromagnetic and microstructural studies of individual chondrules from the Allende carbonaceous meteorite are presented. Allende is a member of the CV3 carbonaceous chondrites, and part of the oxidized meteorites with iron in silicates and oxides. Magnetic hysteresis data in terms of plots of parameter ratios give relationships with chondrule size and shape, in particular with magnetization ratio (Mr/Ms) and coercivity (Hc). Morphology, internal structure and elemental composition are investigated by scanning electron microscopy and spectrometric analyses. Chondrules show low ranges of magnetization ratios (Mr/Ms from 0 to 0.22) and coercivity (Hc from 3 to 24 mT). Low values suggest that chondrules are susceptible to alteration and re-magnetization, which affects paleointensity determinations for the early planetary magnetic fields A linear relation of Mr/Ms as a function of Hc is found up to values of 0.17 and 17 mT, respectively. This relation correlates with internal microstructure and composition, with compound chondrules showing higher hysteresis ratio and parameter values. Chondrules with hysteresis parameters falling outside the major trend show internal structures, composition and textures indicative of compound chondrules, and fragmentation and alteration processes. Microprobe analyses show distinct mineralogical assemblages with spatial compositional variation related to chondrule size, shape and microstructure.