Magnetism, shock and metamorphism in chondritic meteorites

Abstract Comparative magnetic analyses have been performed on 26 selected ordinary chondrites belonging to the populous H- and L-chemical groups and spanning the spectrum of shock exposure (levels A–D) and metamorphic recrystallization (petrologic subtypes 3–6). Aims were: (1) to enlarge and establish a statistical data base for characteristic magnetic remanent behavior; (2) to search for relationships between magnetic properties and shock-metamorphic history; (3) to select samples which have preserved a primeval remanent magnetization, for further estimates of ancient-field strengths recorded (Brecher and Leung, 1979, this issue, hereafter called Paper II). The main conclusions are: 1. (i) The L-chondrites examined have narrowly-defined magnetic properties (χ, NRM and IRM S ). The NRM levels decrease systematically with petrologic subtype (L6 to L4) and with shock level (high, C, to low, A), whereas the opposite trend occurs for IRM s intensity. The metamorphic grade appears less important than shock exposure for relative NRM stability, which decreases from C to A, indicating shock-hardening (removal of soft NRM). Only lightly-shocked meteorites show some directionally stable NRM, indicating the absence of an ordered magnetic field during post-shock cooling, and the survival of an unaltered primordial NRM. 2. (ii) The H-chondrites show little magnetic contrast between metamorphic subgroups across the shock spectrum, but clearer differences are seen between petrologic subtypes within each shock class. NRM intensity and stability decrease with increasing petrologic subtype (thermal demagnetization), while the opposite trend holds for IRM s . Within a metamorphic horizon, the effect of increasing shock is to harden the coercivity spectra of NRM (shock hardening) and to lead to competing magnetic effects of grain-growth and fine-metal precipitation on IRM s stability. Magnetic Type I meteorites are those showing a good correlation between natural (NRM) and saturation (IRM s ) remanence coercive spectra; these represent all petrologic-shock classes and have also preserved a stable, directionally coherent, primary NRM component (see Paper II). Magnetic Type II H-chondrites have typically complex, chaotic NRM records, in spite of stable IRM s , which indicate a good capability for carrying a stable remanence.