Unique thermoremanent magnetization of multidomain sized hematite: Implications for magnetic anomalies

Magnetic remanence in the Murchison meteoriteAbstract-The Murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules, various inclusions, and matrix with occasional porphyroblasts of olivine and/or pyroxene. It also contains amino acids that may have served as the necessary components for the origin of life. Magnetic analyses of Murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence. The rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids. The level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition. We support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination. When using this approach, 1-2% of the natural remanence left in terrestrial rocks with TRM and/or CRM determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral (with the exception of hematite). The same method is applied to the Murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200-2000 nT during and/or after the formation of the parent body.

Section: New Methodsmentioning

confidence: 92%

Magnetic remanence in the Murchison meteorite

Kletetschka

Kohout

Wasilewski

2003

“…Because temperatures were not above the Curie temperature of ferrimagnetic pyrrhotite (310 °C), the nature of the stable remanent magnetization is suggested to be not a TRM but seems to be some sort of shock-induced remanent magnetization (SRM) or partial thermoremanent magnetization (pTRM). According to data given by Clark (1983; see also data compilation in Kletetschka et al 2000Kletetschka et al , 2004, single-domain pyrrhotite acquires a ten times higher remanence in small magnetic fields than multidomain pyrrhotite and a nearly equal high one than single-domain magnetite. The data from this study suggest that pyrrhotite acquired a stable remanent magnetization during shockinduced grain size reduction below its T C .…”

Section: Magnetization Mechanism For Pyrrhotitementioning

confidence: 90%

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Kontny

Elbra

Just

et al. 2007

Abstract-Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10 −6 SI) and natural remanent magnetization (10 −3 -10 −1 A/m) are generally weak, but locally higher values up to 10.6 × 10 −3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C).

“…Multidomain hematite has higher magnetic coercivity (Hc = 13 mT) than MD magnetite (Hc = 1.8 mT) due to a low demagnetizing energy and, thus, the large amount of pinning energy holding the domain walls in place (Kletetschka et al 2000c). The SD magnetite has larger coercivity (Hc = 19 mT) than MD magnetite due to absence of domain walls.…”

Section: Methodsmentioning

confidence: 99%

Pressure effects on martian crustal magnetization near large impact basins

Kletetschka

Connerney

Ness

et al. 2004

Abstract-Martian crust endured several large meteoroid impacts subsequent to the demise of an early global magnetic field. Shock pressures associated with these impacts demagnetized parts of the crust, to an extent determined by shock resistance of magnetic materials in the crust. Impacts that form large basins generate pressures in excess of 1 GPa within a few crater radii of their impact sites. Crustal materials near the surface experience significantly reduced impact pressure, which varies with depth and distance from the impact point. We present new demagnetization experiments on magnetite (Fe 3 O 4 ), hematite (α-Fe 2 O 3 ), and titanohematite (Fe 2−x Ti x O 3 where x <0.2). Our measurements show that pressures of ∼1 GPa are sufficient to partially demagnetize all of these minerals. The efficiency of demagnetization by impact pressure is proportional to the logarithm of the minerals' magnetic coercivity. The impact pressure magnetic response from exsolved titanohematite samples is consistent with the magnetization decay near Prometheus impact basin and may point to an oxidized igneous rock in Terra Sirenum region at the time of acquisition of magnetic remanence. The remaining magnetic anomalies near large impact basins suggest moderate crustal coercivity. These anomalies point to titanomagnetite as a magnetic carrier and more reduced condition during crustal formation.