Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

Abstract

The short-lived 26 Al radionuclide is thought to have been admixed into the initially 26 Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent 54 Cr and 26 Mg*, the decay product of 26 Al. This correlation is interpreted as reflecting progressive thermal processing of infalling 26 Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived 26 Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a 26 Mg*-depleted and 54 Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived 26 Al. The 26 Mg* and 54 Cr compositions of bulk metal-rich chondrites require significant amounts (25-50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants. molecular cloud | outer Solar System | metal-rich chondrites | isotopes | chondrite accretion regions L ow-mass stars like our Sun form by the gravitational collapse of the densest parts of molecular clouds comprising stellarderived dust and gas. Collapsing clouds swiftly evolve into deeply embedded protostars that rapidly accrete material from their surrounding envelopes via a protoplanetary disk (1), in which planetesimals and planetary embryos form over timescales of several million years (2). Chondritic meteorites (chondrites) are fragments of early-formed planetesimals that avoided melting and differentiation and, therefore, provide a record of the earliest evolutionary stages of the Sun and its protoplanetary disk. Most chondrites contain calcium−aluminum-rich inclusions (CAIs) and chondrules, which formed by high-temperature processes that included evaporation, condensation, and melting during short-lived heating events (3). CAIs represent the oldest dated solids and, thus, define the age of the Solar System at 4,567.3 ± 0.16 Ma (4). It is inferred that CAIs formed near the proto-Sun during a brief time interval (<0

Chondrules: Ubiquitous Chondritic Solids Tracking the Evolution of the Solar Protoplanetary Disk

“…For example, the absolute isotopic dates of individual chondrules suggest that the formation of these objects started contemporaneously with the condensation and melting of CAIs and lasted 3.5 Myr (Connelly et al 2012), which indicate the existence of multiple generations of chondrules within individual chondrites. Moreover, variability in the titanium and chromium stable isotope compositions of chondrules from individual chondrites suggests that these objects or their precursor was formed in distinct regions of the protoplanetary disk and subsequently transported to the accretion regions of their respective parent bodies (Trinquier et al 2009;Van Kooten et al 2016;Olsen et al 2016). These data are at odds with the traditional view of a short formation history for chondrule population from individual chondrites, the basic concept of chondrulematrix complementarity as well as the timescales and style of chondrite parent body accretion.…”

“…It has been suggested that the observed dichotomy in the abundance of the 54 Cr tracer between carbonaceous and non-carbonaceous chondrites essentially reflects distinct accretion regions of their parent bodies, namely that the carbonaceous material formed beyond the snow line whereas the non-carbonaceous material originated Sunward of the snow line (Larsen et al 2016;Warren 2011). Accretion of the carbonaceous chondrites beyond the snow line is in accord with the much higher water content of these meteorites (Robert and Epstein 1982;Kerridge 1985) compared to that of enstatite and ordinary chondrites (McNaughton et al 1981;Robert et al 1987;Hutson and Ruzicka 2000).…”

“…A model of continuous asteroidal accretion during the lifetime of the protoplanetary disk has important implications for the thermal evolution of asteroidal bodies given that the accretion process is completed beyond the time when 26 Al can provide enough energy to induce heating and differentiation (Larsen et al 2016). Thus, protracted asteroidal accretion predicts the existence of partially differentiated asteroidal bodies, namely onion-shell structured bodies with differentiated interiors consisting of silicate mantles and metallic cores surrounded by unmelted chondritic crusts (Weiss and Elkins-Tanton 2013).…”

“…Bulk carbonaceous chondrites are characterised by excesses in 54 Cr ranging from +57˙11 to +156˙6 ppm relative to the terrestrial composition (Trinquier et al 2007 (Paton et al 2013;Schiller et al 2015b;Regelous et al 2008;Trinquier et al 2009). This variability, which is interpreted as reflecting the selective unmixing of nucleosynthetic components during the earliest stages of Solar System formation (Trinquier et al 2009;Paton et al 2013;Schiller et al 2015b;Van Kooten et al 2016), provides a means of probing genetic relationships between early formed solids, asteroids and planetary bodies.…”

Age of Jupiter inferred from the distinct genetics and formation times of meteorites

“…For instance, Cr, Ti, and Mo isotope anomalies (6)(7)(8)12) reveal a fundamental dichotomy in the genetic heritage of meteorites, distinguishing between "noncarbonaceous" and "carbonaceous" meteorite reservoirs (11). This distinction may reflect either a temporal change in disk composition or the separation of materials accreted inside [noncarbonaceous (NC) meteorites] and outside [carbonaceous (CC) meteorites] the orbit of Jupiter (11)(12)(13)(14). If the latter is correct, then the age of Jupiter can be determined by assessing the formation time and longevity of the NC and CC meteorite reservoirs.…”