Isotopic coherence of refractory inclusions from CV and CK meteorites: Evidence from multiple isotope systems

Abstract

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated materials in the Solar System and numerous previous studies have revealed nucleosynthetic anomalies relative to terrestrial rock standards in many isotopic systems. However, most of the isotopic data from CAIs has been limited to the Allende meteorite and a handful of other CV3 chondrites. To better constrain the isotopic composition of the CAI-forming region, we report the first Sr, Mo, Ba, Nd, and Sm isotopic compositions of two CAIs hosted in the CK3 desert meteorites NWA 4964 and NWA 6254 along with two CAIs from the CV3 desert meteorites NWA 6619 and NWA 6991. After consideration of neutron capture processes and the effects of hot-desert weathering, the Sr, Mo, Ba, Nd, and Sm stable isotopic compositions of the samples show clearly resolvable nucleosynthetic anomalies that are in agreement with previous results from Allende and other CV meteorites. The extent of neutron capture, as manifested by shifts in the observed ¹⁴⁹Sm-¹⁵⁰Sm isotopic composition of the CAIs is used to estimate the neutron fluence experienced by some of these samples and ranges from 8.40 × 10¹³ to 2.11 × 10¹⁵ n/cm². Overall, regardless of CAI type or host meteorite, CAIs from CV and CK chondrites have similar nucleosynthetic anomalies within analytical uncertainty. We suggest the region that CV and CK CAIs formed was largely uniform with respect to Sr, Mo, Ba, Nd, and Sm isotopes when CAIs condensed and that CAIs hosted in CV and CK meteorites are derived from the same isotopic reservoir.

Introduction

Calcium-aluminum-rich inclusions (CAIs) are the first solids to condense in the cooling protoplanetary disk and mark the beginning of Solar System history. Therefore, these refractory inclusions provide constraints on the composition of some of the earliest reservoir(s) present in the Solar System. CAIs condensed at about 4.567 Ga (Amelin et al., 2010, Bouvier et al., 2011, Connelly et al., 2012), and most CAIs have an inferred initial ²⁶Al/²⁷Al ratio of ∼5 × 10⁻⁵ which is used to define the short time interval for CAI formation, perhaps even as short as ∼50,000 years (Bizzarro et al., 2004, Jacobsen et al., 2008, MacPherson et al., 2012). As such, these early solids represent a snapshot of the isotopic composition at the very start of the Solar System and contain clues to its early evolution. For instance, isotopic characterization of CAIs has demonstrated that they have isotopic anomalies in most elements when compared to later formed solids such as bulk chondrites and the terrestrial planets (see Dauphas and Schauble, 2016 for an extensive review on isotopic anomalies in CAIs). However, how the early Solar System evolved from the isotopic compositions measured in refractory inclusions to that of later formed solids, including chondrules and larger planetary bodies, remains unclear.

Refractory inclusions formed in the early Solar System include (1) hibonite-rich inclusions and (2) FUN (Fractionation and Unknown Nuclear effect) CAIs and (3) normal CAIs. Due to the large range of measured nucleosynthetic anomalies and non-canonical ²⁶Al/²⁷Al, hibonite-rich and FUN inclusions have been postulated to represent samples that formed prior to large-scale homogenization of the CAI-forming region (Sahijpal and Goswami, 1998, Wood, 1998, Kööp et al., 2016). As such, normal CAIs may represent a direct link between the CAI-forming region and later formed solids even though they have different nucleosynthetic anomalies. Therefore, whereas hibonite-rich and FUN inclusions are important for understanding the earliest history of the CAI-forming region, the focus of this study is on the far more abundant “normal” CAIs—hereafter referred to simply as CAIs—and their relationship to early Solar System reservoirs.

Nucleosynthetic anomalies in CAIs have been reported in many elements including: Ca, Ti, Cr, Ni, Sr, Zr, Mo, Ba, Nd, Sm, Hf, and W (e.g., Papanastassiou, 1986, Birck and Lugmair, 1988, Trinquier et al., 2009, Sprung et al., 2010, Burkhardt et al., 2011, Huang et al., 2012, Mercer et al., 2015, Armstrong, 1991, Bouvier and Boyet, 2016, Hidaka et al., 2012, Paton et al., 2013, Bermingham et al., 2014, Burkhardt et al., 2016, Bouvier and Boyet, 2016). Although there are some exceptions (e.g., Sprung et al., 2010, Burkhardt et al., 2011, Akram et al., 2013, Kruijer et al., 2014, Peters et al., 2017), broadly speaking, most CAIs have uniform and distinct nucleosynthetic anomalies indicating formation in a homogenous region (Brennecka et al., 2013). However, to this point, the vast majority of CAI isotopic studies examining elements above the Fe-peak have been limited to focusing solely on inclusions from Allende and a select few CAIs from other CV3 chondrites. It remains unknown if Allende CAIs are isotopically representative of all CAIs in all groups of meteorites, or if there are isotopic differences between host meteorites or meteorite classes. Therefore, isotopic analyses of different types of CAIs from other chondrite groups are of key importance for understanding the isotopic composition of the CAI-forming region as a whole.

The elements Sr, Mo, Ba, Nd, and Sm are well-suited to examine possible heterogeneities within the CAI-forming region because of their ample abundance in CAIs and the number of stable isotopes of each element. The individual isotopes of these five elements are produced by varying amounts of the p-, s-, and r-process nucleosynthesis pathways making them suitable to compare isotopic compositions of various CAIs. However, to this point, the sum of nucleosynthetic data from non-Allende CAIs in these elements derives from a total of seven combined measurements from Sr, Nd, and Sm (Hans et al., 2013, Paton et al., 2013, Bouvier and Boyet, 2016), with no data reported from CAIs from CK meteorites. Therefore, in order to more accurately characterize the CAI-forming region, we measured Sr, Mo, Ba, Nd, and Sm isotopes of two CAIs from CV3 chondrites and for the first time two CAIs from CK3 chondrites. This CAI isotopic data is then used to evaluate the degree of isotopic heterogeneity in the CAI-forming region with respect to Sr, Mo, Ba, Nd, and Sm.