Records of an ancient Martian magnetic field in ALH84001IntroductionSamples and methodsThe magnetized minerals in ALH84001SQUID microscopyThermal constraints on the carbonateThe age of the magnetization in ALH84001Implications for the timing of the Martian dynamoSummaryAcknowledgementsReferencesRecords of an ancient Martian magnetic ¢eldin ALH84001Benjamin P. Weissa; , Hojatollah Valib, Franz J. Baudenbacherc,Joseph L. Kirschvinka, Sarah T. Stewarta, David L. ShusteraaDivision of Geological and Planetary Sciences, 170-25, California Institute of Technology, Pasadena, CA 91125, USA bElectron Microscopy Centre, Department of Anatomy and Cell Biology and Department of Earth and Planetary Sciences,McGill University, 3640 University Street, Montreal, QC Canada H3A 2B2cDepartment of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235, USAReceived 12 February 2002 ; received in revised form 16 April 2002; accepted 16 May 2002AbstractAlthough Mars does not presently appear to have a global dynamo magnetic field, strong crustal fields haverecently been detected by the Mars Global Surveyor above surfaces formed V3 or more Ga. We present magneticand textural studies of Martian meteorite ALH84001 demonstrating that 4 Ga carbonates containing magnetite andpyrrhotite carry a stable natural remanent magnetization. Because40Ar/39Ar thermochronology demonstrates thatmost ALH84001 carbonates have probably been well below the Curie point of magnetite since near the time of theirformation [Weiss et al., Earth Planet. Sci. Lett. (2002) this issue], their magnetization originated at 3.9^4.1 Ga onMars. This magnetization is at least 500 million years (Myr) older than that known in any other planetary rock, andits strong intensity suggests that Mars had generated a geodynamo and global magnetic field within 450^650 Myr ofits formation. The intensity of this field was roughly within an order of magnitude of that at the surface of thepresent-day Earth, sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001and possibly for the production of the strong crustal anomalies. Chromite in ALH84001 may retain even olderrecords of Martian magnetic fields, possibly extending back to near the time of planetary formation. @ 2002 ElsevierScience B.V. All rights reserved.Keywords: Mars; paleomagnetism; dynamos; magnetic methods; Martian meteorites; atmosphere; ALH840011. IntroductionThe age of the crustal magnetization on Marshas been interpreted to be either older [2] or sub-stantially younger [3] than several large, V3.0^4.2Ga impact basins. At least 11 Martian meteoriteshave remanent magnetizations [4^10] that origi-nated sometime after the rocks formed at 1300^180 Ma. Another Martian meteorite, ALH84001,0012-821X / 02 / $ ^ see front matter @ 2002 Elsevier Science B.V. All rights reserved.PII: S0012-821X(02)00728-8* Corresponding author. Tel.: +1-626-395-6187;Fax: +1-626-568-0935.E-mail addresses: [email protected] (B.P. Weiss),[email protected] (H. Vali),[email protected] (F.J. Baudenbacher),[email protected] (J.L. Kirschvink),[email protected] (S.T. Stewart), [email protected](D.L. Shuster).EPSL 6276 5-8-02 Cyaan Magenta Geel ZwartEarth and Planetary Science Letters 201 (2002) 449^463www.elsevier.com/locate/epslis an orthopyroxene cumulate which crystallizedat 4.5 Ga [11] and contains V1% zoned carbon-ate. These carbonates contain magnetite and iron-sul¢de [20] and have Rb/Sr and Pb/Pb ages of3.90 K 0.04 Ga and 4.04 K 0.1 Ga, respectively[12] (although the Rb/Sr data have recently beenquestioned [13]). ALH84001 also possesses astable magnetization that predates its ejectionfrom Mars 15 million years ago (Ma) [4,14,15].Because it is the oldest known Martian rock,ALH84001 may contain unique records of theearliest period of Martian magnetism and evolu-tion [16].Kirschvink et al. [14] argued that iron sul¢dewithin the orthopyroxene of ALH84001 carries astable magnetization, but were unable to demon-strate that the meteorite’s carbonates are magne-tized. Based on a paleomagnetic conglomerate teston two adjacent ALH84001 orthopyroxenitegrains, they suggested that the meteorite hasbeen cooler than 110‡C since before the formationof the fracture surface separating the grains. Theyassumed this fracture was part of the internalcrushed zone created during the ‘D1’ shock event[17]. However, subsequent studies (see Section 5)have identi¢ed £ow textures and vesicles in feld-spathic glass that intrudes and disrupts carbon-ates, suggesting that one or more high-tempera-ture shock events (with peak pressures of s 45GPa and temperatures s 1400‡C) a¡ected at leastpart of the meteorite after the D1 event [17^19].Because no carbonates were identi¢ed on the frac-ture surface separating Kirschvink et al.’s twograins, this fracture need not have formed duringD1 but instead could date to any time between 4.0Ga and 15 Ma. Thus, their thermal constraintmay not apply to before the time of carbonateformation, but instead extend back to a more re-cent but unknown time. More detailed magneticstudies by Weiss et al. [15] have shown that thetransfer of ALH84001 from Mars to their labora-tory, including the impact at 15 Ma that ejected itfrom the Martian surface and its passage throughthe Earth’s atmosphere, did not heat the interiorof the meteorite above V40‡C for even short pe-riods of time ( 6 10 min). It is unknown howmuch further back in time beyond 15 Ma this40‡C thermal constraint applies.2. Samples and methodsWe analyzed two unoriented and two orientedsubsamples of ALH84001 for this study. ALH-84001,232e (Figs. 1^3) is a 1 mm thick slicefrom the interior of the meteorite previouslystudied by Weiss et al. [15]. Slice 232e was cutfrom subsample ALH84001,232 with a diamond-impregnated wire saw and mounted on a glassslide with cyanoacrylic cement [15]. An unor-iented subsample of ALH84001,236 was used forhigh resolution transmission electron microscopy(TEM) imaging (Fig. 4). TEM data on 594 indi-vidual ALH84001 magnetite crystals used for ourblocking temperature calculations (Fig. 5) weremeasured by Thomas-Keprta et al. after extrac-tion from another unoriented subsample [20].Our anhysteretic remanent acquisition studies(Fig. 6) were conducted on a second unorientedpyroxenite grain from ALH84001,236 weighingV20 mg and containing zoned carbonate. Fi-nally, subsample 227b (Figs. 7^9) is an oriented30