Cosmic Ray Exposure Modification of the Molybdenum Isotopic Composition of Iron Meteorites

Accurate grouping of meteorites that represent specific planetary bodies is paramount to understanding early planetary processes, such as accretion and differentiation. Most iron meteorite groups represent the cores of differentiated planetesimals, and therefore hold insights into the timing and mechanisms of core formation and crystallization. Molybdenum isotopic compositions have been shown to vary between meteorite groups due to nucleosynthetic heterogeneities, and, thus, can be used as a genetic tracer [1]. Where isotopic differences can be resolved among iron meteorites purported to be from the same group, it is possible to reject genetic linkages. However, implicit in the use of Mo as a genetic tracer is the assumption of intra-group homogeneity, which does not hold true for Os isotopes, for example [2]. High neutron fluence due to cosmic ray exposure (CRE) has been shown to modify Os isotopic compositions [2]. We report deviations of 95Mo, normalized to 98Mo/96Mo to correct for instrumental mass bias, between iron meteorites of the same group. For instance, in group IVB, Tlacotepec has a 95Mo/96Mo ratio that is ~30 ppm lower than that of most other IVB iron meteorites. Molybdenum-95 is correlated with 189Os and 190Os in each of the three iron meteorite groups examined here, indicating that Mo isotopes are also modified by CRE. Because 95Mo has the largest neutron capture cross section and resonance integral of the Mo isotopes, the likely nuclear reaction is: 95Mo(n,γ)96Mo. The correlation between 95Mo and 189Os or 190Os can be used to correct the Mo isotopes to a pre-exposure composition, as has been done for W isotopes [e.g., 3]. The pre-exposure 95Mo for the IAB complex, IVB, and IIIAB iron meteorite groups is calculated as the y-intercept of each of the slopes defined by the correlation of 95Mo with 189Os. [1] Dauphas et al. (2002) Astrophys. J. 565, 640-644. [2] Walker (2012) EPSL 351-352, 36-44. [3] Wittig et al. (2013) EPSL 361, 152-161.