Paleomagnetic record of Martian meteorite ALH84001

Abstract

[1] The natural remanent magnetization (NRM) of the Martian meteorite ALH84001 is predominantly carried by fine magnetite, which is found in association with carbonate. The magnetite is in epitaxial and topotactic relation with the carbonate and formed from the carbonate in the major impact event at 4.0 Ga. The NRM will therefore record this field. The local preferential crystallographic and shape alignment of the magnetite defines local easy directions of magnetization may account for the observed inhomogeneity of the NRM on a microscopic scale. Normalizing the intensity of the NRM by the saturation isothermal remanence (IRMs) then gives an estimate for the 4.0 Ga Martian field one order smaller than the present geomagnetic field. Such a field is unlikely to be strong enough to generate the high-intensity Martian magnetic anomalies. ALH 84001 in its pristine state as an orthopyroxenite is not a plausible source rock for the Martian anomalies because its magnetite was not formed until the 4.0 Ga event.

Evidence for shock heating and constraints on Martian surface temperatures revealed by 40Ar/39Ar thermochronometry of Martian meteorites

“…Paleomagnetic studies of ALH 84001 (Kirschvink et al, 1997;Collinson, 1997;Weiss et al, 2000Weiss et al, , 2002aWeiss et al, , 2002bWeiss et al, , 2008Antretter et al, 2003;Gattacceca and Rochette, 2004) have found that the meteorite contains an unusual, heterogeneously oriented remanent magnetization that varies in direction at sub-centimeter scales. 40 Ar/ 39 Ar data in this and previous studies suggest that much of the characteristic NRM in ALH 84001 was likely acquired as a thermoremanence on Mars during a shock event that strongly heated the meteorite at $4 Ga. Weiss et al (2000Weiss et al ( , 2002aWeiss et al ( , 2002bWeiss et al ( , 2008 speculated that the heterogeneous pattern of magnetization originated as a result of one or more post-4 Ga shock events that heterogeneously heated the rock [see Auxiliary Text Section 3.0 of Weiss et al (2008) for details].…”

Meteorites: Rocks From the Outer Space

“…Generation and maintenance of magnetic field of internal origin can effectively protect the solar winds, thus pro-vide a chance to fertilize life forms once they are created. In fact, the records of four-billion-year old magnetic field were well preserved in lunar rocks (Lawrence et al 2008;Garrick-Bethell et al 2009) and Martian meteorites (Kirschvink et al 1997;Weiss et al 2000Weiss et al , 2002Antretter et al 2003). As in terrestrial igneous rocks, records of younger volcanism are also preserved in Martian meteorites (Nyquist et al 2001;Yu & Gee 2005).…”

“…For instance, the Martian meteorite SAU 008 represents the eighth object among the fallen objects collected from Sayh Al Uhaymir desert, Oman on 26 November, 1999 (Gnos et al 2002). Another example would be the famous ALH 84001 (Weiss et al 2000;Antretter et al 2003), that represents the first find object collected from Allan Hills region in Antarctica during the 1984 expedition. The ALH series requires additional temporal information of the sample collecting years because multi-year scientific expeditions were carried out in Allan Hills regions.…”

Magnetism, Iron Minerals, and Life on Mars

“…Weiss et al (2002) presented evidence for a "high intensity" paleomagnetic field (with values in the 5-50 T range), while Antretter et al (2003) and Gattacceca and reported 5 and 20 T, respectively. As the recorded field may therefore not be larger than that of other SNCs, it does not provide clear evidence of an active dynamo at the time of acquisition.…”