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Accretion: Building New Worlds (2017) 2053.pdf

METEORITIC CONSTRAINTS ON TIMESCALES OF PLANETESIMAL ACCRETION IN THE EARLY SOLAR SYSTEM. M. Wadhwa1, 1Center for Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287

Introduction: The details of the physical processes accurately determining the absolute chronology of So- involved in the accretion of planetesimals in the early lar System formation. Solar System are not well understood, and have been Regardless of the absolute age of CAI formation, the focus of several recent theoretical modeling inves- high-precision 26Al-26Mg systematics indicate that tigations (e.g., [1] and references therein). The precise some CAIs that formed as condensates from the nebu- timescales of these processes are key inputs for such la, as well as the precursor solids of those that under- models. Most constraints on the timescales involved in went subsequent melting, formed during a very short the transition from dust-sized particles in the pro- time interval of £20 Ka [10-13]. Furthermore, other toplanetary disk to planetesimals (diameters ~100- CAIs appear to have undergone melting and re-melting 1000 km) come from chronologic investigations of episodes in the solar nebula over a somewhat more and their components. The following pro- extended time period lasting a few hundred Ka (e.g., vides a review of the current state of knowledge in this [13]). This requires CAIs to have remained as free- area, which has implications for timescales for accre- floating objects in the solar nebula for at least this du- tion of planetesimals in the early Solar System. ration. This time interval is consistent with the Al-Mg Constraints from chondritic components: relative ages determined for Wark-Lovering (WL) rims Chondritic meteorites are composed of variable pro- from some CAIs [14,15]. However, a recent high spa- portions of four main components formed in the solar tial resolution 26Al-26Mg study of WL rims from two nebular environment, i.e., refractory inclusions, chon- relatively pristine CAIs from the NWA 8323 CV3 drules, fine-grained matrix and Fe-Ni metal [2]. suggested that these CAIs may have re- Chronologic investigations of the refractory inclusions mained as free-floating objects (without accreting into and in particular have provided important larger parent bodies) for up to ~2-3 Ma in the solar constraints on the accretion timescales of chondritic nebula [16]. parent bodies and are discussed below. Chondrules: Chondrules are tens of micrometers to Calcium-aluminum-rich inclusions: The refractory millimeter-sized igneous-textured ferromagnesian calcium-aluminum-rich inclusions (CAI) in primitive spherules that typically comprise a significant fraction chondritic meteorites range in size from tens of mi- of chondritic meteorites [2]. The Al-Mg systematics in crometers to centimeters, and represent the earliest chondrules from a variety of chondrite types have been solids to form in the solar protoplanetary disk [2,3]. As extensively studied and suggest that most chondrules such, these objects can be considered to be markers were formed ~2-3 Ma after CAIs ([17] and references that define the beginning of the Solar System (T0) and therein). Absolute Pb-Pb ages for individual chon- provide a record of the earliest epoch in the evolution drules have so far only been reported by [7]. These of the solar protoplanetary disk. Since the confirmation authors reported 207Pb-206Pb ages ranging from of 238U/235U variation in CAIs [4], only a few studies 4567.3±0.4 Ma to 4564.7±0.3 Ma for five chondrules have been conducted where Pb-Pb ages were reported from the Allende CV3 and the NWA 5697 L3 chon- for CAIs for which the U isotope compositions were drites (assuming a 238U/235U ratio of 137.786±0.013), also measured [5-7]. These ages range from suggesting that the formation process began 4567.2±0.5 Ma and 4567.3±0.2 Ma for CAIs from essentially contemporaneously with CAI formation and Allende [5] and Efremovka [7] CV3 , re- lasted for at least 2–3 Ma thereafter. spectively, to 4567.9±0.3 Ma for a CAI from North- Given the absolute and relative ages of chondritic west Africa (NWA) 6991 CV3 chondrite [6]. Previous components (i.e., CAIs and their WL rims, as well as studies have shown that both the Allende and chondrules) discussed above, it is evident that chon- Efremovka parent bodies experienced significant but dritic parent bodies accreted at least ~3-4 Ma after CAI variable alteration [8,9] and thus CAIs in these CV formation. meteorites may be affected to varying degrees. This Constraints from differentiated meteorites: raises the possibility that the systematically younger High-precision chronologic studies of meteorites that ages of CAIs in these meteorites may be the result of formed on parent bodies that underwent wholesale isotopic disturbance, and CAIs in other, potentially less melting and differentiation can provide additional con- altered, primitive chondrites should be investigated for straints on accretionary timescales of planetesimals in Accretion: Building New Worlds (2017) 2053.pdf

the early Solar System ([18] and references therein) within a million years after the formation of the first and are discussed below. solids. Soon thereafter, these early-accreted planetesi- Achondritic meteorites: like the How- mals melted and underwent metal-silicate and silicate- ardite-Eucrite- (HED) meteorites and the silicate differentiation (primarily from internal heating represent crustal materials from differentiated resulting from the decay of 26Al). Specifically, this planetesimals [19]. Internal isochron ages obtained resulted in core formation (which extended from ~0.7 from the absolute Pb-Pb and relative Al-Mg, Mn-Cr to ~3 Ma after CAI formation for these early formed and Hf-W chronometers indicate that some basaltic planetesimals) and formation (~3-5 Ma after CAI achondrites (in particular, some noncumulate eucrites, formation) on these planetesimals. Chondritic parent quenched angrites and ungrouped basaltic achondrites bodies were accreted approximately 3-4 Ma after CAI such as Asuka 881394 and Northwest Africa 7325) formation, after most of the initial abundance of 26Al crystallized as early as ~3-5 Ma after CAIs (e.g., [20- had decayed away. 27]). This implies that their parent planetesimals had References: [1] Cuzzi J. N. et al. (2017) This accreted and differentiated within this time interval. meeting. [2] Scott E. R. D. and Krot A. N. (2014) Whole-rock isochrons for the Mn-Cr and Hf-W sys- Treatise on Geochemistry 2nd Edition, Vol. 1, 65-137 tems defined by bulk samples of the eucrites and an- pp. [3] MacPherson G. J. (2014) Treatise on Geochem- grites additionally provide evidence of early planetes- istry 2nd Edition, Vol. 1, 139-179 pp. [4] Brennecka G. imal-scale (metal-silicate and silicate-silicate) differen- A. et al. (2010) Science, 327, 449-451. [5] Amelin Y. tiation at ~1-2 Ma after CAI formation [20,25,26,28]. et al. (2010) Earth & Planet. Sci. Lett., 300, 343-350. Magmatic iron meteorites: Iron meteorites that de- [6] Bouvier A. et al. (2011) Geochim. Cosmochim. fine compositional trends consistent with fractional Acta, 75, 5310-5323. [7] Connelly J. N. et al. (2012) crystallization of a metallic core of a differentiated Science, 338, 651-655. [8] Scott E. R. D. et al. (1992) planetesimal are referred to as “magmatic” (e.g., IIAB Geochim. Cosmochim. Acta, 56, 4281-4293. [9] Krot and IIIAB irons) [29]. The Hf-W chronometer has A. N. et al. (1995), & Planet. Sci., 30, 748- been applied to magmatic irons to precisely date the 775. [10] Thrane K. et al. (2006) Astrophys. J. Lett., time of metal segregation on planetesimals, and yields 646, L159. [11] Jacobsen B. et al. (2008) Earth & Pla- core formation ages that extend from ~0.7 Ma (for net. Sci. Lett., 272, 353-364. [12] MacPherson G. J. IIAB irons) to ~3 Ma (for IID and IVB irons) after (2010) Astrophys. J., 711, L117-121. [13] MacPherson CAI formation [30]. G. J. (2012) Earth & Planet. Sci. Lett., 331-332, 43-54. The temporal constraints discussed above are for [14] Simon J. I. et al. (2005) Earth & Planet. Sci. Lett., events on differentiated planetesimals (such as 238, 272-283. [15] Kawasaki N. et al. (2017) Geochim. crystallization and metal segregation) that post-dated Cosmochim. Acta, 201, 83-102. [16] Mane P. et al. their accretion in the early Solar System. These con- (2015) LPS XXXVI, Abstract #2898. [17] Davis A. M. straints can be used with thermal modeling to estimate and McKeegan K. D. (2014) Treatise on Geochemistry the accretion times for these planetesimals. For exam- 2nd Edition, Vol. 1, 361-395 pp. [18] Kleine T. and ple, given the time interval for core formation on the Wadhwa M. (2017) Planetesimals: Early Differentia- parent bodies of the magmatic irons determined by tion and Consequences for Planets, 224-245 pp. [19] [30], these authors made model calculations (assuming Mittlefehldt D. W. (2014) Treatise on Geochemistry post-accretion heating with 26Al in parent 2nd Edition, Vol. 1, 235-266 pp. [20] Lugmair G. W. with ~40 km radii) that indicated that these parent bod- and Shukolyukov A. (1998) Geochim. Cosmochim. ies were accreted as early as ~0.1 to ~0.3 Ma after CAI Acta, 62, 2863-2886. [21] Srinivasan G. et al. (1999) formation. Science, 284, 1348-1350. [22] Amelin Y. (2008) Geo- Implications for the accretion timescales of chim. Cosmochim. Acta, 72, 221-232. [23] Connelly J. planetesimals in the early Solar System: Within the N. et al. (2008) Geochim. Cosmochim. Acta, 72, 4813- last decade, high-precision chronologic investigations 4824. [24] Wadhwa M. et al. (2009) Geochim. Cosmo- of a variety of meteoritic materials have provided un- chim. Acta, 73, 5189-5201. [25] Kleine et al. (2012) precedented temporal resolution for processes occur- Geochim. Cosmochim. Acta, 84, 186-203. [26] Tou- ring in the early Solar System. In particular, it is now boul M. et al. (2015) Geochim. Cosmochim. Acta, 156, evident that the solar protoplanetary disk was a dynam- 106-121. [27] Koefoed P. et al. (2016) Geochim. Cos- ic environment where episodic heating events over the mochim. Acta, 183, 31-45. [28] Trinquier A. et al. first ~2-3 Ma resulted in the formation and re- (2008) Geochim. Cosmochim. Acta, 72, 5146-5163. processing of chondritic components. Accretion of the [29] Benedix G. K. et al. (2014) Treatise on Geoche- earliest planetesimals (ranging from ~100 km to ~1000 mistry 2nd Edition, Vol. 1, 267-285 pp. [30] Kruijer T. km in diameter) in this protoplanetary disk began well et al. (2014) Science, 344, 1150-1154.