^1,2Stepan M. Chernonozhkin,^1,2Steven Goderis,¹Marta Costas-Rodríguez,²Philippe Claeys,¹Frank Vanhaecke
¹Ghent University, Department of Analytical Chemistry, Krijgslaan, 281 – S12, 9000 Ghent, Belgium
²Vrije Universiteit Brussel, Analytical-, Environmental-, and Geo-Chemistry, Pleinlaan 2, 1050 Brussels, Belgium

Various iron and stony-iron meteorites have been characterized for their Ni and Fe isotopic compositions using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) after sample digestion and chromatographic separation of the target elements in an attempt to further constrain the planetary differentiation processes that shifted these isotope ratios and to shed light on the formational history and evolution of selected achondrite parent body asteroids. Emphasis was placed on spatially resolved isotopic analysis of iron meteorites, known to be inhomogeneous at the μm to mm scale, and on the isotopic characterization of adjacent metal and silicate phases in main group pallasites (PMG), mesosiderites, and the IIE and IAB complex silicate-bearing iron meteorites. In a 3-isotope plot of 60/58Ni versus 62/58Ni, the slope of the best-fitting straight line through the laterally resolved Ni isotope ratio data for iron meteorites reveals kinetically controlled isotope fractionation (βexper = 1.981 ± 0.039, 1 SD), predominantly resulting from sub-solidus diffusion (with the fractionation exponent β connecting the isotope fractionation factors, as View the MathML sourceα62/58=α60/58β). The observed relation between δ56/54Fe and Ir concentration in the metal fractions of PMGs and in IIIAB iron meteorites indicates a dependence of the bulk Fe isotopic composition on the fractional crystallization of an asteroidal metal core. No such fractional crystallization trends were found for the corresponding Ni isotope ratios or for other iron meteorite groups, such as the IIABs. In the case of the IIE and IAB silicate-bearing iron meteorites, the Fe and Ni isotopic signatures potentially reflect the influence of impact processes, as the degree of diffusion-controlled Ni isotope fractionation is closer to that of Fe compared to what is observed for magmatic iron meteorite types. Between the metal and olivine counterparts of pallasites, the Fe and Ni isotopic compositions show clearly resolvable differences, similar in magnitude but opposite in sign (Δ56/54Femet-oliv of +0.178 ± 0.092 ‰ and Δ60/58Nimet-oliv of -0.212 ± 0.082 ‰, 2SD). As such, the heavier Fe isotope ratios for the metal (δ56/54Fe = +0.023 to +0.247 ‰) and lighter values for the corresponding olivines (δ56/54Fe = -0.155 to -0.075 ‰) are interpreted to reflect later-stage Fe isotopic re-equilibration between these phases, rather than a pristine record of mantle-core differentiation. In the case of mesosiderites, the similarly lighter Ni and Fe isotopic signatures found for the silicate phase (-0.149 to +0.023 ‰ for δ60/58Ni, -0.214 to -0.149 ‰ for δ56/54Fe) compared to the metal phase (+0.168 to +0.191 ‰ for δ60/58Ni, +0.018 to +0.120 ‰ for δ56/54Fe) likely result from Fe and Ni diffusion. Overall, the Fe and Ni isotopic compositions of iron-rich meteorites reflect multiple, often superimposed, processes of equilibrium or kinetic nature, illustrating convoluted parent body histories and late-stage interaction between early-formed planetesimal reservoirs.

Reference
Chernonozhkin SM,Goderis S,Costas-Rodríguez M,Claeys P,Vanhaecke F (2016) Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites.
Geochimica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.050]
Copyright Elsevier

^1,2Desireé Cotto-Figueroa, ¹Erik Asphaug, ^1,3Laurence A.J. Garvie, ⁴Ashwin Rai, ⁴Joel Johnston, ⁴Luke Borkowski, ³Siddhant Datta, ⁴Aditi Chattopadhyay, ^1,5Melissa A. Morris
¹School of Earth and Space Exploration, Arizona State University, PO Box 876004, Tempe, Arizona, 85287-6004, USA
²Department of Physics and Electronics, University of Puerto Rico at Humacao, Call Box 860, Humacao, Puerto Rico, 00792
³Center for Meteorite Studies, Arizona State University, PO Box 876004, Tempe, Arizona, 85287-6004, USA
⁴School for Engineering of Matter, Transport and Energy, Arizona State University, PO Box 876106, Tempe, Arizona, 85287, USA
⁵Physics Department, State University of New York, PO Box 2000, Cortland, New York, 13045, USA

Measuring the strengths of asteroidal materials is important for developing mitigation strategies for potential Earth impactors and for understanding properties of in situ materials on asteroids during human and robotic exploration. Studies of asteroid disruption and fragmentation have typically used the strengths determined from terrestrial analog materials, although questions have been raised regarding the suitability of these materials. The few published measurements of meteorite strength are typically significantly greater than those estimated from the stratospheric breakup of meter-sized meteoroids. Given the paucity of relevant strength data, the scale-varying strength properties of meteoritic and asteroidal materials are poorly constrained. Based on our uniaxial failure studies of centimeter-sized cubes of a carbonaceous and ordinary chondrite, we develop the first Weibull failure distribution analysis of meteorites. This Weibull distribution projected to meter scales, overlaps the strengths determined from asteroidal airbursts and can be used to predict properties of to the 100 m scale. In addition, our analysis shows that meter-scale boulders on asteroids are significantly weaker than small pieces of meteorites, while large meteorites surviving on Earth are selected by attrition. Further, the common use of terrestrial analog materials to predict scale-dependent strength properties significantly overestimates the strength of meter-sized asteroidal materials and therefore is unlikely well suited for the modeling of asteroid disruption and fragmentation. Given the strength scale-dependence determined for carbonaceous and ordinary chondrite meteorites, our results suggest that boulders of similar composition on asteroids will have compressive strengths significantly less than typical terrestrial rocks.

Reference
Cotto-Figueroa D, Asphaug E, Garvie LAJ, Rai A, Johnston J, Borkowski L, Datta S, Chattopadhyay A, Morris MA (2016) Scale-Dependent Measurements of Meteorite Strength: Implications for Asteroid Fragmentation. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.003]
Copyright Elsevier