Earliest evidence of nebular shock waves recorded in a calcium-aluminum-rich Inclusion

Abstract

Calcium-aluminum-rich inclusions (CAIs) and chondrules are among the most predominant chondritic components contained within primitive meteorites. As CAIs are the first solids to form in the solar nebula, they contain a record of its earliest chemical and physical processes. Here we combine electron backscatter diffraction (EBSD) and ²⁶Al-²⁶Mg chronology techniques to determine the crystallographic properties and ages of CAI components that provide temporal as well as spatial constraints on their origins and subsequent processing in the solar protoplanetary disk. We find evidence of shock deformation within a CAI, suggesting that it was deformed as a free-floating object soon after the CAI formation at the beginning of the Solar System. Our results suggest that even though CAIs and chondrules formed in distinct environments and on different timescales, they were likely affected by similar shock processes that operated over large temporal (0 to ∼4 Ma) and spatial (0.2 to at least 2 to 3 au) extents. Our results imply that nebular shock events were active on a wider scale in the solar protoplanetary disk than previously recognized.

Introduction

Calcium-aluminum rich inclusions (CAIs) hosted in chondritic meteorites are the oldest solids to have formed in the Solar System and define its age as 4567.30 ± 0.16 Ma (Amelin et al., 2010, Connelly et al., 2012). Based on short-lived ²⁶Al-²⁶Mg chronology and absolute Pb-Pb chronology, the formation of most CAIs is restricted to within the first ∼1 Ma of Solar System history (Connelly et al., 2012, MacPherson et al., 2012, MacPherson et al., 2017). Therefore, CAIs contain records of the processes active during the earliest period (<1 Ma) of the Solar System. The mineralogy of CAIs is consistent with thermodynamic models that predict the condensation of refractory mineral phases from the solar nebula gas (e.g., Ebel, 2006). The compositional, structural, and isotopic properties of CAIs preserve records of their high-temperature origins and contain evidence of secondary processes that were active during and after their formation, such as aggregation, melting and recrystallization, evaporation, thermal and shock processing, and aqueous alteration in nebular and parent-body settings.

In addition to CAIs, chondrules are one of the important petrographic components of chondritic meteorites. Some chondrules formed contemporaneously with CAIs, but a majority of them continued forming for ∼4 Ma afterward (Villeneuve et al., 2009, Connelly et al., 2012, Bollard et al., 2017). Thermodynamic properties of CAIs suggest that they formed close to the Sun (<1 au) under reducing conditions (Beckett et al., 1988, Grossman et al., 2008) and inherited an O-isotopic composition close to the solar value (McKeegan et al., 2011). In comparison, chondrules formed under relatively oxidizing conditions (Grossman et al., 2008) and inherited an O-isotopic composition close to that of the terrestrial planets (Yurimoto et al., 2008). Several studies have shown that there exists a chemical complementarity between the compositions of chondrules and the matrix of the host meteorite (Bland et al., 2005, Hezel and Palme, 2008, Hezel and Palme, 2010, Palme et al., 2015). The chemical complementarity between chondrules and the chondritic matrix in which they occur implies that they are cogenetic and formed in the same region, i.e., the chondrite-forming region at ∼2 to 3 au (the location of the current asteroid belt), or as some recent models have suggested, perhaps even further beyond Jupiter for carbonaceous chondrites (Kruijer et al., 2017a). Therefore, even though CAIs and chondrules coexist in chondritic meteorites, they formed in distinct nebular environments and over different timescales.

The properties of CAIs and chondrules also suggest distinct origins. Equilibrium thermodynamic models predict the condensation of refractory mineral phases directly from the solar nebular gas, such as those contained within CAIs (Ebel, 2006). In contrast, chondrules, molten silicate droplets within the solar nebula, are not predicted to form in the protoplanetary disk by such models (Connolly and Jones, 2016). The mineralogical, chemical, and isotopic compositions of chondrules indicate that they were flash heated in the solar protoplanetary disk, likely in a region with an enhanced dust-to-gas ratio that was distinct from the CAI-forming region. Chondrules were heated to peak temperatures of ∼1750 to 2370 K and then cooled quickly (∼10³ – 10⁴ K/h) above the liquidus and slowly (∼250 K/h) below the liquidus (Desch et al., 2012 and references therein).

Chondrules formed by the melting of solid precursors in dust-rich regions of the protoplanetary disk during transient heating events. However, early Solar System processes that could have led to the melting of solid precursor materials and hence the formation of chondrules in the solar protoplanetary disk remain poorly understood. Various mechanisms proposed to explain chondrule formation include shock waves in early protoplanetary disk, nebular lightening, impact plumes generated during planetary collisions, radiative heating of dust clumps, and magnetic current sheets (Connolly and Jones, 2016). It is likely that more than one such mechanism was responsible for the melting of chondrules found in different primitive meteorites. One of the chondrule formation mechanisms that has received considerable attention is rapid heating by a shock wave, through either gravitational instability driven shock (Hood and Horanyi, 1991, Wood, 1996) or planetesimal bow shock (Hood, 1998, Weidenschilling et al., 1998). Further, multiple shock waves, weaker than those responsible for the melting of chondrules, were proposed as a mechanism that compacted the fine-grained rims around chondrules (Bland et al., 2011). Similarly, electron back scatter diffraction (EBSD) and transmission electron microscope (TEM) analysis of some chondrules reveal that they record deformation features as a result of either nebular shocks or impacts on the meteorite parent bodies (Forman et al., 2016, Zolensky et al., 2020). In addition to chondrules, certain types of CAIs, i.e., igneous type-B CAIs, were also reheated and melted, possibly by mechanisms related to those responsible for chondrule formation (Richter et al., 2006). However, in either case, whether chondrules or CAIs, the presence of such shocks remains to be verified by astrophysical observations of extrasolar protoplanetary disks, and if they did occur within our solar nebula, there are few quantitative constraints on their spatial and temporal extent. Here, we report microstructural analysis of a CAI that preserves evidence of shock deformation, and we constrain the timescale of this deformation event by ²⁶Al-²⁶Mg analyses of the CAI components.