Preliminary Paleomagnetic Analysis of Mesosiderite Northwest Africa 8368

46th Lunar and Planetary Science Conference (2015) 2605.pdf

PRELIMINARY PALEOMAGNETIC ANALYSIS OF MESOSIDERITE NORTHWEST AFRICA 8368. R. K. Bono 1 and J. A. Tarduno 1,2 , 1Department of Earth & Environmental Sciences, University of Rochester, Roch- ester, NY 14627 ([email protected]), 2Department of Physics & Astronomy, University of Rochester, Rochester, NY 14627.

Introduction: The origin of pallasite meteorites, found within these meteorites. We describe preliminary which are composed of iron-nickel metal and olivine, data aimed at testing this hypothesis. has posed a paradox since the metal and silicate should have segregated prior to solidification. Prior hypothe- ses have focused on formation near the core-mantle boundary of a differentiated asteroid [1]. Paleomagnet- ic evidence of an ancient dynamo, from minute magnetic recorders contained within olivine grains, re- quires an alternative model for the origin of Main Group pallasites [2]. Thellier and Total Thermal Rem- anent Magnetization paleointensity results, combined with thermal modeling, have motivated a model for pallasite formation where a differentiated impactor collides with a differentiated target protoplanetary body which hosts a dynamo. Iron-nickel metal from the impactor core is injected as dikes into the silicate (olivine) mantle of the target body. Minute fractures in olivine crystals (which subsequently heal) allow for the Figure 1. Mesosiderite NWA 8368 with a large olivine injection of FeNi which record the target body dynamo grain highlighted. on cooling. This process of pallasite formation – through the injection of an impactor’s core metal into a Methods: We report magnetic data from a sample target asteroid’s mantle – has also been suggested for of the mesosiderite, Northwest Africa (NWA) 8368 the Eagle Station parent body [3, 4]. (Figure 1). Paleomagnetic data is isolated from the Here we consider mesosiderites, brecciated stony meteorite by focusing on single silicate crystals [13], iron meteorites composed of iron-nickel metal and sili- which can contain small single-domain-like magnetic cate phases [5, 6]. These bear some similarities with inclusions that are capable of recording magnetizations pallasites [7], in being mixtures of silicates and iron- on billion-year time scales [14, 15]. NWA 8368 con- nickel metal, but inferred formation mechanisms differ tains mm- to cm-scale olivine grains (Figure 1). We drastically. For example, the metal and silicate compo- selected the olivine grains for study to minimize con- nents have been interpreted to represent the reassembly tributions from multi-domain magnetic carriers which of a protoplantetary body which had a molten core pri- have relatively short relaxation times and are prone to or to impact [8]. A relatively large target body (~200- carrying spurious magnetizations. Mutually oriented 400 km diameter) has been inferred on the basis of olivine samples, ~1-2 mm on a side, were prepared cooling rate data and modeling [8]. Limited paleomag- from ~1 mm-thick thin-sections for demagnetization netic data exists for mesosiderite meteorites [9-12]; the experiments. All paleomagnetic measurements were most comprehensive study previously reported has done with a 3-component high-sensitivity DC SQUID been on the Estherville mesosiderite. Results from mu- magnetometer in a magnetically shielded room at the tually oriented alternating field and thermal demagneti- University of Rochester's paleomagnetic laboratory. zation experiments of metal and silicate fractions from Samples were first demagnetized in steps in an alternat- the Estherville mesosiderite show a range of magnetic ing field up to 10 mT, and then stepwise thermally de- stability and scattered remanent directions, predomi- magnetized using a CO 2 laser [14]. nantly carried by kamacite and tetrataenite [12]. On the Findings: In contrast to prior studies of olivine basis of these results, Collinson [12] suggested that the from pallasite meteorites [2-4], olivine from NWA magnetization was acquired prior to formation of the 8368 typically contains large (many tens of microns to meteorite. We propose, alternatively, that the incoher- ~150 µm) inclusions visible under low magnification ence in previously published mesosiderite paleomag- (e.g. 10x) which we infer to be FeNi metal and/or sul- netic data may reflect variations in the recording fideli- fides. Measurements of natural magnetic remanence ty of the different phases of silicate and FeNi metal from olivine grains from the interior of mesosiderite 46th Lunar and Planetary Science Conference (2015) 2605.pdf

NWA 8368 (Figure 2) demonstrate the presence of a References: [1] Wasson, J. T. & Choi B. G. (2003) low-unblocking temperature (LBT) component of Geochim. Cosmochim. Acta , 67 , 3079. [2] Tarduno, J. magnetization. We tentatively interpret the LBT com- A. et al. (2012) Science, 338 , 939. [3] Tarduno, J. A. et ponent to be recorded by the large inclusions; it may be al. (2014) 77th Annual Met. Soc. Meeting , Abstract a viscous overprint. We also see a different component #5453. [4] Bono, R. K. et al. (2014) AGU Fall Meet- of magnetization, isolated at higher unblocking tem- ing , P44A-02. [5] Rubin, A. E. & Mittlefehldt, D. W. peratures (HBT). Our magnetic data to date suggests (1992) Geochimica et Cosmochimica Acta, 56 , 827- that a common direction may be preserved by the HBT 840. [6] Mittlefehldt, D. W. et al. (1998) Reviews in component, however it is not fully defined, presumably Mineralogy and Geochemistry, 36 , 4.1-4.195. [7] Mit- because of the presence of non-ideal magnetic inclu- tlefehdt, D. W. (1980) Earth and Planetary Science sions. Letters, 51 , 29-40. [8] Scott, E. R. D. et al. (2001 ) Me- teoritics & Planet. Sci., 36 , 869-881. [9] Gus’kova, Y. G. (1965) Geomagnetizm i Aeronomiya, 5 , 91-101. [10] Larson, E. E. et al. (1973) Journal of Geomag- netism and Geoelectricity, 25, 331-338. [11] Kukko- nen, I. T. & Pesonen, L. J. (1983) Bull. Geol. Soc. Fin- land, 55 , 157-177. [12] Collinson D.W. (1991) Mete- oritics, 26 , 1-10. [13] Tarduno, J. A. et al. (2006) Rev. Geophys., 44, RG1002. [14] Tarduno, J. A. et al. (2007) Nature, 446 , 657-660. [15] Tarduno, J. A. et al. (2010) Science, 327 , 1238-1240.

Figure 2. Orthogonal vector plots of CO 2 laser thermal demagnetizations from two mutually oriented olivine samples from the interior of NWA 8368. Red lines, inclination; blue lines, declination. Labeled points are °C. Dashed red arrow highlights a low-unblocking temperature component of magnetization (LBT), while the green arrow highlights a higher unblocking temperature component (HBT).

Discussion: Future work will focus on a more thor- ough isolation of subsamples lacking large mul- tidomain magnetic inclusions from NWA 8368, con- tinued demagnetization experiments on mutually oriented olivine crystals to define remanent components of magnetization, and electron microscopy composi- tional analysis to characterize magnetic carriers within the olivine grains. If coherency of magnetic directions is demonstrated in future studies, it would suggest that mesosiderite meteorites could have formed by FeNi metal injection from an impactor into a parent body target in a similar manner as Main Group and Eagle Station pallasites. However, mesosiderite meteorites would represent shallower penetration depths within a parent body. Conversely, lack of coherency of magnetizations would support disruption/reassembly scenarios for the mesosiderite parent body.