Mixing of Impactor Liquid Cores and Planetesimals Constrained by Single Silicate Crystal Magnetism

Thursday, 18 December 2014: 4:15 PM
Richard K. Bono1, John Anthony Tarduno1,2, Francis Nimmo3, Edward Scott4 and Ludovic Ferrière5, (1)University of Rochester, Department of Earth & Environmental Sciences, Rochester, NY, United States, (2)University of Rochester, Department of Physics & Astronomy, Rochester, NY, United States, (3)University of California-Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States, (4)University of Hawaii at Manoa, Hawaii Institute for Geophysics and Planetology, Honolulu, HI, United States, (5)Natural History Museum Vienna, Vienna, Austria
The formation of pallasites, meteorites composed mainly of intermixed FeNi metal and angular to rounded olivine crystals, has been a paradox because these components should have separated into distinct layers at their putative location of origin: the core-mantle boundary (CMB) of an asteroid. Paleomagnetic study of gem-like angular olivine bearing minute magnetic inclusions from the main group Esquel and Imilac pallasite meteorites, however, reveals the past presence of strong magnetic fields requiring a core dynamo. A CMB location is too hot for pallasites to record these magnetizations. Alternatively, the paleomagnetic field strengths, combined with cooling rate data and thermal modeling, suggest an origin for main group pallasites in the shallow mantle of a parent body, with the FeNi metal originating from the liquid core of an impactor (Tarduno et al., Science, 2012). Here, we extend this model to include the origin of main group pallasite meteorites with rounded olivine, and pallasites of the Eagle Station group. Main group pallasites with rounded olivine (e.g., Springwater) could have formed in larger intruded liquid FeNi masses relative to the thinner dikes envisioned for the formation of pallasites with angular olivine crystals. Pallasites of the Eagle Station group have an oxygen isotope composition different from that of the main group and thus require a separate parent body. Olivine from the Eagle Station pallasite also carries a paleomagnetic record; therefore pallasites from this group may have also formed by collision. Interestingly, the magnetization of the Eagle Station pallasite is multi-component and distinctly different from the main group. This signal potentially records a combination of a dynamo signal and a later shock heating event. Mesosiderites may represent a third parent body formed by impactor liquid FeNi metal injected into the crust of an asteroid. These observations suggest liquid metal intrusion from differentiated impactors may have been a common process influencing the formation of planetesimals and protoplanets.