While iron meteorite parent bodies exhibit variable deficits in s-process Mo isotopes, they feature essentially identical stable Mo isotope compositions

Coles, B. J.

Goldberg, T.

Poole, Graeme M

Rehkämper, M.

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                          Poole, G. M., Rehkämper, M., Goldberg, T., & Coles, B. J. (2015).Nucleosynthetic and Mass-Dependent Molybdenum Isotope Anomalies inIron Meteorites: Constraints on Solar Nebula Heterogeneities and ParentBody Processes. Abstract from 46th Lunar and Planetary ScienceConference, Texas, United States.Publisher's PDF, also known as Version of recordLink to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlNUCLEOSYNTHETIC AND MASS-DEPENDENT MOLYBDENUM ISOTOPE ANOMALIES IN IRON METEORITES – CONSTRAINTS ON SOLAR NEBULA HETEROGENEITIES AND PARENT BODY PROCESSES.  G. M. Poole1, M. Rehkämper1, T. Goldberg1 and B. J. Coles1. 1Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK. (g.poole11@imperial.ac.uk)   Introduction:  Nucleosynthetic isotope anomalies have long been known to exist at the grain-size level in primitive meteorites. Numerous recent studies have, furthermore, identified planetary-scale isotope anoma-lies for some refractory elements (e.g., Ba, Cr, Ti), whilst other elements exhibit no such effects (e.g., Hf, Os). Such isotopic anomalies are typically interpreted as representing an initial state of heterogeneity in the solar nebula. This inheritance of solar nebula signa-tures can be investigated further by examining mass-dependent stable isotope anomalies. Molybdenum is ideally suited as a tracer of plane-tary-scale isotopic heterogeneity in the solar nebula as its seven isotopes were produced by distinct nucleo-synthetic processes. However, despite its suitability, previously obtained Mo isotopic evidence has been questioned by other investigations that presented con-flicting data [1-3]. Here we present Mo isotope data with improved precision and for a larger range of iron meteorites than in previous studies, to inform the debate about nucleo-synthetic isotope anomalies. We have, furthermore, investigated mass-dependent Mo isotope fractionations for these samples, based on additional stable isotope analyses via the double-spike technique. Analytical techniques:  Iron meteorite samples of 1 to 15 g from 11 different groups were obtained from the Natural History Museum London and private col-lectors. The samples were sequentially leached in 0.5 M HBr, 6 M HCl and modified aqua regia (prepared with 6 M HCl), and then digested in modified aqua regia. Separation of Mo was achieved by a two-stage anion exchange procedure, which produced purified Mo fractions that typically displayed Ru/Mo and Zr/Mo ratios of 2.7×10-5 or less. For measurements of stable Mo isotope compositions, the samples were mixed with a 100Mo-97Mo double spike [4] prior to the separation chemistry. The Mo isotopic compositions were measured by MC-ICP-MS using the Nu Plasma HR instrument at Imperial College. To determine nucleosynthetic Mo isotope anomalies, instrumental mass bias was correct-ed by internal normalisation to 98Mo/96Mo = 1.453174 [5] using the exponential law. To investigate possible analytical artefacts, internal normalization to other ratios, including 92Mo/98Mo = 0.607898 and 97Mo/95Mo = 0.602083, were performed and found to produce internally consistent results. Normalization to 98Mo/96Mo resulted in larger anomalies and more dis-tinctive anomaly patterns, in agreement with [3]. In the following, the nucleosynthetic Mo isotope anomalies are therefore described using this normalization. The internally normalized data are reported in εiMo notation (Fig. 1), where εiMo = [(iMo/96Mo)sample / (iMo/96Mo)standard - 1] × 104, calculated relative to the mean of several bracketing runs of NIST SRM 3134 Mo. External reproducibility (2σ) of the SRM meas-urements typically ranged from ±0.39 for ε92Mo to ±0.14 for ε97Mo. To determine stable Mo isotope com-positions, the double spike data reduction utilized pub-lished methods [6, 7]. The sample data are reported in δ98Mo notation, calculated using 98Mo/95Mo (Fig. 2) and determined relative to the mean obtained for bracketing analyses of NIST SRM 3134 Mo. Typical external reproducibility (2σ) was ±0.035‰ for δ98Mo. The measured stable and nucleosynthetic Mo iso-tope data were combined to determine purely mass-dependent Mo isotope fractionation, by ‘removing’ any mass-independent effects. For this, the raw data ac-quired for unspiked samples were internally normal-ized to 97Mo/95Mo, as both isotopes have similar s- and r-process contributions (97Mo: 58% s- and 42% r-; 95Mo: 55% and 45%). Therefore, variations in these components will not significantly affect the 97Mo/95Mo ratio. This is the same procedure as used by [8]. Results: Following internal normalization to 98Mo/96Mo, the Mo isotope data for all magmatic iron groups display the typical ‘w-shaped’ isotopic patterns associated with s-process deficits (Fig. 1).  Fig. 1: Mean nucleosynthetic Mo isotope anomalies for iron meteor-ite groups, with 2se error bars. The grey shaded field represents 2σ external reproducibility of the SRM measurements. Normalized to 98Mo/96Mo = 1.453174. 012392 94 95 96 97 98 100IABICIIABIICIIEIIIABIIICDIIIEIIIFIVAIVBiMo!iMo2080.pdf46th Lunar and Planetary Science Conference (2015)Whilst meteorites from the same group display consistent anomalies, the different groups show varia-ble magnitudes of s-process deficits. In detail, IIC irons display the largest anomaly of 3.0 ε92Mo, whilst IVAs displays the smallest effects of the magmatic irons at 0.8 ε92Mo (Fig. 1). These results are in agree-ment with some published data [2, 3] but stand in con-trast to others [1]. Like [3], we also find that the non-magmatic IAB and IIICD irons do not show anomalies (Fig. 1).  Fig. 2: δ98Mo = [(98Mo/95Mo)sample / (98Mo/95Mo)standard - 1] × 103. Data points represent mean of multiple analyses (n>5) for each me-teorite sample with 2se error bars. The grey shaded field defines a 2σ uncertainty around the mean δ98Mo -0.18‰ of the magmatic iron meteorite groups. Fig. 2 displays the stable fractionations. The δ98Mo values for the magmatic iron meteorites vary within a limited range, from –0.39‰ to +0.02‰. The non-magmatic irons show more variation (-0.17‰ to 0.46‰), whilst troilites (FeS) from Toluca IAB display the greatest range (-0.56‰ to +0.42‰) and exhibit the lightest and nearly the heaviest δ98Mo values measured here. Discussion: The magnitude of the observed nucle-osynthetic isotope anomalies appears to correlate with metallographic cooling rate, and hence inferred size, of the associated parent bodies (e.g., [9]). First suggested by [3], our much larger dataset for irons provides sup-port for this observation. Such a correlation may reflect progressive homogenization of the solar nebula over the timescale of iron meteorite parent body accretion, possibly due to late injection of s-process material into the protosolar disc. Alternatively, the correlation may imply more efficient homogenization in larger or more densely populated parent body feeding zones. In our study of stable Mo isotope compositions, the  iron meteorite groups (except IAB/IIICD) provide a mean δ98Mo of –0.18 ± 0.06‰ (2σ). As Mo is moder-ately siderophile, the δ98Mo value of the bulk parent body is (essentially) identical to the isotope composi-tion measured for the core. Given the consistency of the results, our mean δ98Mo value for iron meteorite groups may closely approximate the solar system aver-age. Notably, our mean is identical to a previously reported average of –0.16 ± 0.02‰ for irons and chon-drites (excluding IAB, CK, CM), which was interpret-ed to reflect the Mo isotope composition of bulk plane-tary bodies in the inner solar system [8]. The large range of δ98Mo values observed for the Toluca troilites are most likely due to Mo isotope frac-tionation during metal–sulfide partitioning, whereby the observed variability may reflect both kinetic and equilibrium isotope effects. Similarly, the IAB/IIICD group shows highly variable δ98Mo with both isotopi-cally heavy and light Mo. This stands in contrast to the results of [8], who found only heavy Mo isotope signa-tures for this group, which were interpreted to reflect evaporative loss of isotopically light Mo during ener-getic impacts. Based on our new data, we conclude that the large range of δ98Mo values observed for IAB/IIICD irons is due to the segregation and crystal-lization of S-rich melts on the parent body [10] and associated metal–sulfide partitioning. We have also found small but resolvable and sys-tematic variations of δ98Mo within magmatic iron groups. Here, the greatest variability is displayed by meteorites towards the Ni-rich end of the associated parent body fractional crystallization trend (e.g., [11]). Sulfur is incompatible in solid metal and thus becomes enriched in the liquid metal during fractional crystalli-zation of cores. We therefore posit that the variations within magmatic iron meteorite groups represent pro-cesses associated with the increased liquid metal S content, most likely metal–sulfide partitioning of Mo. References: [1] Becker, H. and Walker, R. J. (2003) Nature, 425, 152-155. [2] Dauphas, N., et al. (2002) ApJ, 565, 640-644. [3] Burkhardt, C., et al. (2011) EPSL, 312, 390-400. [4] Goldberg, T., et al. (2013) JAAS, 28, 724-735. [5] Lu, Q. and Masuda, A. (1994) Int. J. Mass Spectrom. Ion. Proc., 130, 65-72. [6] Siebert, C., et al. (2001) GGG, 2, art. no.-2000GC000124. [7] Rudge, J. F., et al. (2009) Chem. Geol., 265, 420-431. [8] Burkhardt, C., et al. (2014) EPSL, 391, 201-211. [9] Rasmussen, K. L., et al. (1995) Geochimica Et Cosmochimica Acta, 59, 3049-3059. [10] Benedix, G. K., et al. (2000) Meteoritics & Planet. Sci., 35, 1127-1141. [11] Wasson, J. T. (1970) Icarus, 12, 407-423.  -0.6 -0.4 -0.2 0 0.2 0.4 0.6!98Mo (‰)Toluca TroilitesIABICIIABIICIIEIIIABIIICDIIIEIIIFIVAIVB2080.pdf46th Lunar and Planetary Science Conference (2015)

Nucleosynthetic and Mass-Dependent Molybdenum Isotope Anomalies in Iron Meteorites:Constraints on Solar Nebula Heterogeneities and Parent Body Processes

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Nucleosynthetic and Mass-Dependent Molybdenum Isotope Anomalies in Iron Meteorites:Constraints on Solar Nebula Heterogeneities and Parent Body Processes

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