An unmagnetized early planetary body

Friday, 19 December 2014
Benjamin P Weiss1, Huapei Wang2, Brynna G Downey2, David L Shuster3, Jerome Gattacceca4, Thomas G Sharp5, Roger R Fu2, Aaron T. Kuan6, Clement R Suavet2 and Anthony J Irving7, (1)MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States, (2)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (3)University of California Berkeley, Berkeley, CA, United States, (4)CEREGE, Aix-en-Provence Cedex, France, (5)Arizona State University, Tempe, AZ, United States, (6)Harvard University, Cambridge, MA, United States, (7)Univ Washington, Seattle, WA, United States
Modern paleomagnetic studies of achondrites have found that at least several planetesimals generated dynamos with inferred surface magnetic fields ranging from tens to hundreds of µT. In fact, an achondrite without evidence for paleomagnetic fields has not been previously identified, hinting that the dynamo process may have been extremely common among early planetary bodies. To further expand our understanding of the diversity of planetesimal dynamos, we have been studying the paleomagnetism the ungrouped achondrite NWA 7325. This ancient meteorite (Pb/Pb and Al/Mg ages of ~4563 Ma) is highly depleted in siderophile elements, indicating that it formed on a differentiated body that underwent large-scale metal-silicate fractionation. We present new paleomagnetic, rock magnetic transmission and scanning electron microscopy, and Ar/Ar analyses of this rock that constrain the field intensity in which it cooled. In particular, we employ a new controlled oxygen fugacity system that enables us to conduct thermal demagnetization while greatly mitigating oxidation-reduction reactions. This system is critical because of the extremely reduced conditions (at least 3 log units below the iron-wüstite buffer) in which this meteorite formed. The extremely fine grain size (<200 nm) of many FeNi metal grains in NWA 7325 (pseudo single domain-superparamagnetic size) means that it has extremely high fidelity magnetic recording properties. We find no stable remanent magnetization and no evidence for any magnetic fields greater than ~2 µT at the time of last cooling below the Curie point. Our Ar/Ar thermochronometry suggests that the last major thermal event experienced by NWA 7325 was at 500 Ma. Because this age is well after the expected lifetime of early planetesimal dynamos, our data indicate that any crustal fields on the parent body are extremely weak. This stands in stark contrast to Vesta, for which our analyses of the eucrite ALHA 81001 suggest has substantial (~10 µT) crustal fields. This makes NWA 7325 the first example of an essentially unmagnetized igneous rock from a differentiated body in the early solar system. We discuss the implications of this for early planetary thermal histories and the hypothesis that NWA 7325 may have originated from Mercury