[meteorite-list] Paleomagnetism and ALH 84001

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:54:06 2004
Message-ID: <3C6FA929.760CCA9D_at_lehrer1.rz.uni-karlsruhe.de>

Tracy weote:

> I would be interested in the results of a study on the paleomagnetic
> memory of meteorites. Who did it, and what specifically was found
> about ALH8001 ... ?

Hello Tracy and List,

Kirschvink, J.L., Maine A.T., and Vali H. (1997) Paleomagnetic evidence
of a low-temperature origin of carbonate in the martian meteorite ALH
84001 (Science, 275, 1629-1633, excerpts):

When a rock forms or cools down, it can trap some of the local magnetic
field; magnetic minerals in the rock become little bar magnets, aligned
with the planet's magnetic field. This trapped magnetic field, called
natural remnant magnetism or NRM, can stay in the rock indefinitely,
and can be used to unravel the history of the magnetic minerals and the
rock. The strength of the trapped magnetic field can tell how strong the
planet's field was. If the rock is broken or bent, the magnetic field
trapped in it will point in a different direction from the original
field. If the rock gets heated above a critical temperature, the old
trapped magnetic field is lost, and a new one is trapped when it cools
down again.

Kirschvink et al. measured the trapped magnetic fields (NRM) in two
adjacent fragments of ALH 84001 from the fracture zone where McKay
et al. found the most carbonate globules. The trapped fields in the two
fragments were strong, equally strong, but in different orientations;
the "bar-magnets" of the magnetic minerals were pointed in different
directions. This meant that the two fragments had probably trapped the
same original field, but had been rotated or jostled when the fracture
between them formed. If ALH 84001 had ever been hotter than 325°C since
the fragments were jostled, they would have lost their original magnetic
fields; when they cooled, the fragments would have trapped the new
magnetic field, with the same direction in both fragments! Because the
fragments do have magnetic fields in different directions, ALH 84001
could not have been hotter that 325°C at any time after the fractures
formed. Now, the carbonate globules are in these same fractures, and
must have formed after the fractures did, and so must not have formed at
temperatures hotter than 325°C (otherwise the rock fragments would have
their trapped magnetic fields pointing in the same direction)!


Best regs,

Bernd
Received on Sun 17 Feb 2002 07:59:21 AM PST