Collisional Disruption of a Primary Planetary Body

On one scale of solar system history, the nascence of planetesimal formation spanned less than 100 m.y. The most active period was determined to be the first 10–20 m.y., just after Jupiter and Saturn had formed and the protoplanetary disk was void of its gas shroud (Davison et al., 2013). During this time a dynamical, stochastic, collisional evolution played out—collisional growth proceeded in the face of ongoing disruptive impact events on contemporaneous accreting bodies. A number of these growing planetesimals accumulated heat through energetic impacts and the decay of radiogenic elements such as 26Al, beginning a stage of gravitational differentiation into a crust, silicate mantle, and metallic core. Some of the larger planestesimals developed a rotating core dynamo producing a weak magnetic field, as evidenced by the paleomagnetic signature detectable today in their associated meteorites.

Davison et al. (2013) calculated that during the first 100 m.y. only a very few planetesimals (<~50) were able to grow to very large sizes, in the range of 200–600 km in diameter, without experiencing a disruptive collision. At the same time, there is meteoritical evidence which suggests that a few planetesimals grew to protoplanetary (or even planetary) sizes before experiencing a disruptive collision; i.e., an impact by an object typically >~60 km in diameter traveling ~18–25 km/s. As an example, it was proposed by Irving et al. (2009) that the diverse meteorite lithologies with similar O-isotopic compositions to the HED clan of meteorites, generally considered to be derived from the asteroid 4 Vesta, were once part of an even larger former differentiated planetary body which they named "Opis" (the mother of Vesta in Greek mythology).

Another such hypothesized collisionally-disaggregated planetary body (here named "Antaeus") was conceived by Irving et al. (2004) to have comprised many diverse lithologies, here expanded upon to include the following: a metallic core region composed of IIF-type iron like Del Rio (Kracher et al., 1980); impact-melted zones in the upper mantle region composed of a metal+silicate assemblage that correspond to the Eagle Station pallasite group and the NWA 176 (related to Bocaiuva; Liu, 2001) silicated iron; a dunitic mantle zone possibly represented by NWA 7822; an intensely thermally-metamorphosed stratigraphy resembling the NWA 3133 metachondrite (Irving et al., 2004); and a thick insulating crust (~20 km; Davison et al., 2013), possibly involving a late accetionary stage, comprising a primitive chondrule–CAI-rich regolith consisting of several distinct lithological zones comprising reduced Vigarano-like, oxidized Allende-like, and highly aqueously-altered Bali-like material. The detailed petrogenetic sequences by which each of these meteorites acquired their present form, and the question as to whether these events occurred prior to, during, or subsequent to a catastrophic disruption of the primary planetary body (or were associated with post-disruption daughter objects), are subjects which are still under investigation.

Importantly, the O- and Cr-isotopic signatures of Eagle Station have been utilized to establish an early formation age of 4.557 b.y., or 11 m.y. years after CAI formation. According to Dauphas et al. (2005), application of the Hf–W isotopic chronometer to Eagle Station also gives a relatively late metal–silicate segregation for Eagle Station of ~10 m.y. after differentiation of the HED parent body 4 Vesta (which occurred as early as 1.3 m.y. after CAI formation; Schiller et al., 2010). Since it has been calculated that melting and core–mantle differentiation due to radiogenic heating should cease after ~7–8 m.y. (Sahijpal et al., 2007), it may be inferred that heating of the Eagle Station asteroid continued until after all radiogenic 26Al and 60Fe was extinct, and that such late heating would have been generated through large impact events. In support of that reasoning, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D'Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. Therefore, impacts were a major source of heating in early solar system history.

Likewise, the formation scenario envisioned for the silicated irons NWA 176 and Bocaiuva is consistent with impact-heating events on a small-sized asteroid. The final mixing event was accompanied by an initial rapid-cooling stage beginning at the metal–silicate equilibrium temperature of ~1100°C, and was sustained down to ~600°C. This was followed by a slow cooling stage in which a Thomson (Widmanstätten) structure was formed (Desnoyers et al., 1985). Another fast cooling stage was initiated between approximately 600°C and 300°C as indicated by the absence of tetrataenite and other petrographic features (Araujo et al., 1983). There are major structural similarities between the NWA 176 and Bocaiuva silicated irons and those silicated iron members of the IIE and IAB complex iron groups. This suggests that similar impact processes, such as a catastrophic breakup event, occurred on each of these relatively small, nonmagmatic parent bodies; however, only the IIF irons and the Eagle Station pallasites share any significant geochemical similarities with NWA 176 and Bocaiuva (Bunch et al., 1970; Curvello et al., 1983). Notably, NWA 176, Bocaiuva, and the Eagle Station pallasites, as well as other distinct meteorite lithologies, have similar O- and/or Cr-isotopic compositions to the CV chondrites (Clayton and Mayeda., 1996; Liu et al, 2001; Shukolyukov and Lugmair, 2001). Taking the many similarities into account, it seems likely that these otherwise disparate meteorites originated on a common chondritic precursor parent body (Malvin et al., 1985).

Notably, a formation scenario for pallasites was proposed by Asphaug et al. (2006) and Danielson et al. (2009) in which the wide variation in metal–silicate textures and bulk compositions that is observed among MG pallasite members is the result of a grazing collision between partially molten planetary embryos. They assert that such a collision resulted in the formation of a chain of smaller objects having diverse compositions. It may be more than coincidental that the O-isotopic composition of the ungrouped pallasite Milton plots approximate to the trend line of the Eagle Station group pallasites (now termed the Allende Mixing line: slope = 0.94 ±0.01). Both of these rapidly-cooled pallasites contain high concentrations of the refractory siderophile element Ir relative to main-group (MG) pallasites (Jones et al., 2003), and they both have overlapping Fe and Ni abundances (wt%) in their metal component; however, significant variations observed in their minor and trace element concentrations indicate that they each experienced different crystallization processes (Hillebrand, 2004). Still, there is a possibility that they share a common precursor parent body with the CV clan of meteorites, at least prior to any collisional disruption event.

Diagram adapted from Korochantsev et al., 2013

Subsequent to the catastrophic disruption of the primary planetary body that is envisioned here, and the sorting and re-accretion of material into a number of daughter objects, multiple impacts onto these small asteroids could have led to the formation of sub-surface melt pools tens of meters in size. Differentiation of these melt pools would have resulted in cumulus olivine sequestered above a metal layer, and an olivine residuum that had drained below this metal layer—a complex assemblage from which associated pallasitic and silicated-iron lithologies could be derived thereafter during less-energetic, rapidly-cooled impact events (Malvin et al., 1985). The anomalously-high Ir contents measured in some of the associated metal–silicate mixtures (e.g., Eagle Station grouplet, Milton) and segregated metal regions (IIF irons, South Byron trio) would be consistent with metal that crystallized at the lowest levels of the melt chamber. Such late-stage, rapidly-cooled, impact-heating events could have allowed for the retention of the original O- and Cr-isotopic composition of the primary planetary body (Humayun and Weiss, 2011 and references therein). The differences that exist in δ54Cr between chromite and olivine in the Eagle Station pallasite, but which are not observed in CV chondrites, could be the result of a distinct Cr source associated with impact projectile(s) which eventually led to the formation of the Eagle Station-type pallasites and other related lithologies (Papanastassiou and Chen, 2011).

On an oxygen three-isotope diagram (see example below), the CO chondrites plot along the Allende Mixing trend line (former CCAM line), overlapping near the middle of the CV chondrite field. There is a possibility that the CO chondrite group, of which Isna is a highly metamorphosed example (type 3.75), is also implicated in the sequence of events that led to the formation of the diverse CV clan of meteorites as outlined above—perhaps as another of the daughter objects that accreted after a catastrophic disruption of the primary planetary body. It is also significant that the most unequilibrated CO3 chondrites have isotopic compositions that are similar to anhydrous silicates in meteorites of the CM group, a group with which it also shares many chemical and petrographic similarities. In fact, the CO and CM groups may represent different degrees of low-temperature aqueous alteration of common precursor material which was initially similar to the primitive CO3.00 DOM 08006 and CO3.03 ALHA77307 chondrites (Clayton and Mayeda, 1999). Although it is still unresolved whether or not these two groups share a common parental source object, they both represent material from the same nebular region located beyond 3 AU (Wasson, 1988; Rubin, 2010).

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Diagram credit: Irving et al., 79th MetSoc, #6461 (2016)

Beyond that, new O-isotopic analyses conducted by Greenwood et al. (2014) on a large sampling of CM chondrites led them to suggest that a possible group relationship (same parent body) may exist between the CM and CO chondrites, previously considered to constitute a clan (groups formed at a similar heliocentric distance) based on early research on refractory lithophile abundances, chondrule size and composition, and O-isotopic composition of high-temperature phases (Kallemeyn and Wasson, 1979, 1981). Moreover, it was found that the matrix component in meteorites of both groups have nearly identical minor element compositions (Greenwood et al., 2014, reference therein). Despite the hiatus that occurs between the CM and CO groups on an oxygen three-isotope diagram, their additional data clearly shows that the CM O-isotopic trend line intersects the CO field, and they have posited a new theory based on the premise that both groups formed on a common parent body. They suggest that the CO group could represent an inner anhydrous zone of a parent body larger than ~120 km in diameter, in which the initial accreted hydrous component was rapidly liberated through endogenous heating (radiogenic) and vented to the surface and into space (Fu and Elkins-Tanton, 2013). Conversely, the outer zone represented by the CM group experienced a high degree of aqueous alteration over an extended duration. A compatable scenario was presented by Fu and Elkins-Tanton (2013) in which early accretion (within ~2 m.y. of CAI formation) of a planetesimal of significant size (>120 km in diameter), composed of low-density material akin to the CM chondrites, could experience internal differentiation without eruption of magma to the surface, thereby retaining a primitive hydrated crustal region.

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Diagram courtesy of Greenwood et al., 45th LPSC #2610 (2014)
A81:ALHA81002; A83:ALH 83100; CB:Cold Bokkeveld; E:Essebi; Ma:Maribo; MET:MET 01070; MI:Mighei; Mo:Moapa; M:Murchison; Mu:Murray; N:Nogoya; P:Paris (mean); PA:Paris-altered; PL:Paris-less altered; S:SCO06043; Q93:QUE93005; Q97:QUE97990; Y:Y791198; W:WIS91600; CO3 falls:Moss

In their comprehensive oxygen isotope study of carbonaceous chondrite groups, Clayton and Mayeda (1999) showed that many ungrouped members plot along the same mixing line and fill the hiatus between the CO and CM fields (see diagram below). They suggest that both CO and CM groups consist of a common anhydrous silicate precursor, while the CM group represents the interaction of this anhydrous precursor with an aqueous reservoir. The ungrouped members are transitional, with variable water:rock ratios as indicated by the tick marks along the mixing line. Further evidence for a possible common CO–CM parent body was presented by Schrader and Davidson (2016; #1288). They analyzed the Cr content in olivine grain cores of type-II (FeO-rich) chondrules for a number of CM chondrites spanning the full range of petrologic types (e.g., Sutter's Mill [2.0/2.1]... QUE 97990 [2.6]). Utilizing a coupled diagram comparing the mean Cr2O3 content to the standard deviation (σ) of Cr2O3 content, they demonstrated that both the CO and CM thermal metamorphism curves overlap. Their study also shows that thermal metamorphism and aqueous alteration are not coupled. Another coupled diagram presented by Schrader and Davidson (2016) comparing the Fe and Mn contents of the type-II chondrules among the CM samples is also consistent with a common CO–CM parent body.

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Image courtesy of Clayton and Mayeda, GCA, vol. 63, p. 2094 (1999)
See also the oxygen three-isotope diagram presented by Jacquet et al., MAPS, vol. 51, p. 862 (2016)

A compatable scenario was presented by Fu and Elkins-Tanton (2013) in which early accretion (within ~2 m.y. of CAI formation) of a planetesimal of significant size (>120 km in diameter), composed of low-density material akin to the CM chondrites, could experience internal differentiation without eruption of magma to the surface, thereby retaining a primitive hydrated crustal region. It could be deduced that following the catastrophic disruption and re-accretion into numerous daughter objects, these would subsequently experience impact-ejection of material into storage orbits within the outer asteroid belt. Further fragmentation events (collisional cascade processes), along with the Yarkovsky effect, would have delivered samples into mean motion resonances with some fragments eventually achieving Earth-crossing orbits.

The hypothesis of multiple daughter objects being formed following the catastrophic disruption of a large, partially differentiated, primary planetary body, could allow for the potential inclusion of several less closely-related meteorites. These may include the high-Ni irons of the South Byron trio (South Byron, ILD 83500, and Babb's Mill), which have metallographic compositions (especially siderophile element patterns) and structures similar to the metal in Milton, including kamacite spindles and associated schreibersite, consistent with their formation on the same parent body (Reynolds et al., 2006). These three irons and the metal component in Milton experienced a similar oxidation history during formation; they each have similar depletions of easily oxidized elements as well as similar abundances of siderophiles (McCoy et al., 2008). In addition to the irons mentioned above, several other ungrouped ataxites may be genetically related to this high-Ni iron group, including El Qoseir, Illinois Gulch, Morradal, Nordheim, and Tucson (Kissin, 2010). However, significant differences that exist between their refractory element contents compared to those of the South Byron trio requires further work to establish a specific relationship.

The metal in each of these high-Ni iron meteorites and in Milton is consistent with early crystallization from a metallic-melt phase that experienced a low degree of fractionation. Similarly, the FeNi-metal component of one member of the Eagle Station grouplet, Itzawisis, was derived from a metallic-melt source consistent with that of a differentiated, oxidized-CV source before 20% fractionation had occurred. In a like manner, the metal in Eagle Station derives from a 20% fractionated source (Humayun and Weiss, 2011), while another member of the grouplet, Cold Bay, was shown to derive from a melt source that crystallized after 40% fractionation. The most recent member of the Eagle Station grouplet to be analyzed, Karavannoe, crystallized from an even more evolved metallic-melt that had undergone >60% fractionation—though still not as evolved as the source melt from which MG pallasite metal crystallized. Karavannoe FeNi-metal has a lower Ni content than the other members of the grouplet (Korochantsev et al., 2013), and measurements show that its Ir content is intermediate between that of Eagle Station and Milton.

Further evidence of a large differentiated planetary body having CV-trends lies in the fact that Allende acquired a strong unidirectional natural remanent magnetization at least 8 m.y. after CAI formation, reflecting the existence of an internal core dynamo (Weiss et al., 2010). As research continues, further evidence for the catastrophic disruption of this former primary body could advance this scenario.

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