Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Abstract

Abstract-Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single-domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650-450°C interval for higher petrographic types (consistent with an onion-shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock-related transient heating events above approximately 500°C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.

Solar nebula magnetic fields recorded in the Semarkona meteorite

“…Analogous LC components are also present in some non-dusty olivine-bearing chondrules (NDOC; see Section 3.5), strongly suggesting that LC magnetizations are carried by recrystallized Ni-rich metal grains, which are present in the mesostases of all chondrules. Furthermore, the low coercivity range of these components is consistent with their presence in MD mesostasis metal grains (22). As discussed in Section 2.1, the plessitic unmixing of kamacite and Ni-rich taenite phases in such grains during metamorphism on the LL parent body in a weak field likely resulted in the loss of any pre-accretional remanence and the acquisition of randomly oriented, spontaneous magnetizations.…”

“…Two samples, C2 and C4b, have no stable magnetization above 32.5 mT while C4a loses directional stability by 140 mT. These maximum coercivities are lower than those of dusty olivine-bearing chondrules, suggesting that the main carrier of these magnetizations is mesostasis metal grains, which are much larger than dusty olivine metals and exist in the magnetically soft MD state (22). In contrast, the remaining four NDOC samples are not fully demagnetized by 290 mT, possibly due to the presence of fine FeNi grains in the mesostasis.…”

Kinetics of tetrataenite disordering

“…Although the stoichiometric composition Fe 50 Ni 50 characteristic of tetrataenite was not reported in Jeremine and Sandrea [30], the magnetic properties of Guidder meteorite suggest that tetrataenite occurs in large proportions. Indeed, as shown by Gattacceca et al [8] with hysteresis properties and first-order reversal curves (FORC), among the 91 studied ordinary chondrite falls, Guidder is the meteorite in which tetrataenite magnetic properties are best expressed. Thus, we also used the bulk sample to characterize the magnetic behavior of tetrataenite.…”

“…In particular, coercivity of remanence and remanent saturation magnetization are very sensitive for identification of tetrataenite, so that these properties can be used to monitor the disordering of tetrataenite [8,31].…”

“…), and because it disorders at high temperature, its presence has been recently shown to be a sensitive proxy for the thermal history of meteorites [8]. In particular, short thermal event like those associated to hypervelocity impacts can be detected by evidencing tetrataenite disordering into taenite.…”