Elsevier

Icarus

Volume 329, 1 September 2019, Pages 166-181
Icarus

A radiative heating model for chondrule and chondrite formation

https://doi.org/10.1016/j.icarus.2019.03.039Get rights and content

Highlights

  • A new model is proposed for the simultaneous formation of chondrules and chondrites

  • The model accounts for the well-constrained formation epoch and inferred thermal history of chondrules.

  • Laboratory simulations show that chondrule textures can be reproduced with heating curves derived from this model.

  • The model accounts for the phenomena of complementarity and cluster chondrites.

  • If this model is correct, chondrules are rare artifacts of a fortuitous sequence of events.

Abstract

We propose that chondrules and chondrites formed together during a brief radiative heating event caused by the close encounter of a small (m to km-scale), primitive planetesimal (SPP) with incandescent lava on the surface of a large (100 km-scale) differentiated planetesimal (LDP). In our scenario, chondrite lithification occurs by hot isostatic pressing (HIP) simultaneously with chondrule formation, in accordance with the constraints of complementarity and cluster chondrites. Thermal models of LDPs formed near t = 0 predict that there will be a very narrow window of time, coincident with the chondrule formation epoch, during which crusts are thin enough to frequently rupture by impact, volcanism and/or crustal foundering, releasing hot magma to their surfaces. The heating curves we calculate are more gradual and symmetric than the “flash heating” characteristic of nebular models, but in agreement with the constraints of experimental petrology. The SPP itself is a plausible source of the excess O, Na and Si vapor pressure (compared to a solar nebula environment) that is required by chondrule observations. Laboratory experiments demonstrate that FeO-poor porphyritic olivine chondrules, the most voluminous type of chondrule, can be made using heating and cooling curves predicted by the “flyby” model. If chondrules are a by-product of chondrite lithification, then their high volume abundance within well-lithified chondritic material is not evidence that they were once widespread within the Solar System. Relatively rare events, such as the flybys modeled here, could account for their abundance in the meteorite record.

Introduction

Chondrules are mm-scale igneous spherules, first described by Sorby (1877), which are a major component of most classes of the primitive meteorites named after them — the chondrites (Grossman, 1988). Their origin remains as much a mystery in the eyes of most cosmochemists today as it was when they were discovered (Ciesla, 2005; Connolly and Jones, 2016; Palme et al., 2014). Laboratory experiments have shown that chondrule textures can be reproduced by melting pre-chondrule material at temperatures of 1400–1600 °C for times of order 20 min and then cooling it on a time scale of hours or less (Desch et al., 2012; Hewins and Radomsky, 1990). Their spherical nature suggests this was done in space, or on an object with very small surface gravity. Their ages, measured by radioactive dating techniques, show that most chondrules were formed at t = 1–4 Myr, where t = 0 is set by the age of the most primitive solids (Ca/Al-rich inclusions; CAIs), also found in chondrites (Budde et al., 2016; Connelly et al., 2012; Connelly et al., 2017; Kita et al., 2013; Villeneuve et al., 2009).

For decades, the most popular idea of chondrule formation has been that pre-existing dust aggregates embedded in the “solar nebula”, a gaseous remnant of the accretion disk through which the Sun formed, were heated and cooled by interaction with the gas (primarily H and He) that comprised the disk. This is known as the “nebular hypothesis” (Ciesla, 2005) and exists in a variety of forms today that differ according to the mechanism postulated to heat the gas. An ad hoc heating mechanism must be invoked because models of protoplanetary disk evolution, supported by observations, predict temperatures that are far below those required to form chondrules (Chiang and Youdin, 2010). Proposed mechanisms include large-scale shocks from protosolar outburts or accreting clouds, more localized shocks from giant protoplanets on elliptical orbits and heating via lightning discharges (Desch et al., 2012).

Within the last decade the nebular model has been challenged, and lost some of its popularity, as it has become clearer that the gaseous conditions under which chondrules formed were not at all representative of what one would expect in an accretion disk. In particular, the gas is inferred to be five orders of magnitude more oxidizing and has a much higher vapor pressure of Na than is plausible for nebular gas of solar composition (Alexander et al., 2008; Fedkin and Grossman, 2016; Fedkin et al., 2012; Grossman et al., 2008). These discoveries have revived an older idea that chondrules might have formed as drops of “fiery rain”, to quote (Sorby, 1877), from splashes of hot liquid created (Johnson et al., 2015) or released (Sanders and Scott, 2012) during collisions of planetesimals. This class of models is known as “planetary” and its primary attraction is that it may account for the gaseous environment in which chondrules are inferred to have formed. A collision model has also been invoked to explain unusual features of the CB chondrites (Krot et al., 2005). A major issue with planetary models, in general, is that no one has demonstrated with detailed modeling or experiments that the distinctive, often porphyritic, textures of most chondrules could be generated in such a manner. Their variable lta17O is also inconsistent with formation from planetary interior melts (Marrocchi et al., 2018).

Additional arguments can be raised against both nebular and planetary models when the narrow time frame inferred for chondrule formation, t = 1–4 Myr, is considered. Accretion disks are expected and observed to dissipate monotonically with time and solar nebula gas may have disappeared entirely from the asteroid belt before the epoch of chondrule formation even began. Mamajek (2009) finds a half-life for protoplanetary disks in local star forming regions of ∼1.7 Myr when all stellar masses are considered, but also evidence that disks last longer in very low mass stars, which shortens the estimate for stars near 1 M to ≲1 Myr. Haisch et al. (2001) and Venuti et al. (2016) report that in the young cluster NGC 2264 more than half of the stars in their sample, most of which are of lower mass than the Sun, have completely lost their accretion disks within ∼3 Myr. In one star system, known as KH 15D, with an age of 3 Myr and a total mass of 1.4 solar masses, fortuitous geometry allows us to probe its protoplanetary disk at ∼3 AU; we find abundant solids in a vertically thin ring with no evidence for accompanying nebular gas (Aronow et al., 2017; Lawler et al., 2010). Furthermore, to explain the time gap between t = 0 and the primary epoch of chondrule formation, around t = 2–3 Myr, one must invoke, in nebular models, a delayed gas heating mechanism. Among planetary models, only the proposal by Sanders and Scott (2012), which involves collisions to release (not create) liquid material, appears to address this issue.

A criticism of all chondrule formation theories to date is that, while they may account for the thermal history required by experimental petrology, they ignore other known constraints on chondrules and chondrites (Connolly and Jones, 2016). The work described here is motivated by the constraints of complementarity (Ebel et al., 2016; Hezel and Palme, 2008; Hezel, 2010; Palme et al., 2015; Wood, 1985) and cluster chondrites (Metzler, 2012), that chondrule and chondrite formation were closely linked in space and time. We consider a model in which they were simultaneous. Complementarity is the label associated with the observation that while whole chondrites may have a chemical abundance pattern that closely follows the composition of the Sun, their two primary components, chondrules and matrix do not. Underrepresentation of an element in one component is compensated for by overrepresentation in the other component, implying formation of the chondrite in a closed system. The phenomenon of complementarity has recently become an even more powerful constraint as a result of the discovery of nucleosynthetic complementarity in tungsten isotopes between chondrules and matrix in several carbonaceous chondrites (Becker et al., 2015; Budde et al., 2016). Cluster chondrites are found in all groups of unequilibrated ordinary chondrites. They are regions where chondrules of a variety of textures are closely packed and deformed in a manner indicating that the chondrite formed while the chondrules were still hot. Again, the inference is that there was close spatial and temporal coincidence of chondrule and chondrite formation.

As is the case with chondrules, there is at present no consensus among cosmochemists on how, when or where chondrite lithification occurs. Consolmagno (1999) describes some potential mechanisms and issues. One requires a source of elevated pressure and/or temperature to reduce the porosity of the material. Terrestrial rocks offer little guidance because they form under steady pressures of a magnitude that cannot be achieved on asteroids, even Ceres. Transient pressure events due to collisional impacts are often invoked as the primary mechanism for chondrite lithification, although models differ in their details (Beitz et al., 2016; Lichtenberg et al., 2018). Recently, the hot isostatic pressing (HIP) process, involving heat and mild pressure, has been invoked, but at lower temperatures and much longer timescales than are employed commercially (Atkinson and Davies, 2000; Gail et al., 2015). None of these studies address the challenges of complementarity, cluster chondrites or other links between chondrules and their host chondrites, such as the size-group relationship (Friedrich et al., 2015). Chondrule formation and chondrite lithification are currently modeled independently of one another.

In this paper we propose a model for simultaneous chondrule and chondrite formation. It employs radiative heating and predicts temperature curves that are consistent with the constraints of experimental petrology. It is an extension of the “flyby model”, originally proposed by Herbst and Greenwood (2016) (hereinafter Paper I) for chondrule formation, but now explicitly considers the heating of a larger object characterized by certain bulk parameters including opacity. In Section 2, we provide a basic overview of the model and its components. In Section 3, we develop a more comprehensive theory of the heating based on a gray, plane parallel solution to the equation of radiative transfer. We consider the full range of possible orbital parameters for the flyby from circular to hyperbolic. In Section 4, we discuss preliminary laboratory work aimed at testing the distinctive thermal histories that the flyby model predicts and demonstrate that objects with chondrule textures will form under relevant conditions. Section 5 is a discussion of some of the implications, challenges and potential tests of this model, including what happens to the chondrite after it forms. We follow Elkins-Tanton et al. (2011) in assuming that chondritic material gradually accretes to LDPs creating an undifferentiated crust where metamorphosis can occur. Our contribution to the scenario is that chondrites, with their complement of chondrules, arrive on the LDPs in already lithified form as part of that chondritic material. Section 6 is a brief summary of the paper.

Section snippets

Overview of the flyby model

A schematic representation of our model is shown in Fig. 1. The small primitive planetesimal (SPP) has been assembled and stored over 1–4 Myr from solids in its formation zone and is small enough (1–1000 m) to have avoided melting by 26Al. It may be a “free floating” object within the protosolar disk, which encounters the larger planetesimal by chance, or a gravitationally bound, orbiting moonlet or ring particle. The large differentiated planetesimal (LDP) is sufficiently large (≥100 km) and

The thermal model

Our thermal model for the heated surface of an SPP exposed during a close flyby to hot lava on the surface of an LDP (see Fig. 1) follows the gray plane layer solution to the equation of radiative transfer described in Chapter 11 of Howell et al. (2016). The assumptions of this solution are that the geometry is plane parallel, that the opacity (κ) is independent of wavelength (gray) or location within the layers and that the condition of radiative equilibrium applies, i.e. one neglects energy

Laboratory simulation of chondrule textures

As a demonstration of the ability of the flyby model to replicate important chondrule properties, we show the results of a small set (12) of laboratory experiments to simulate Type I (FeO-poor) porphyritic olivine (PO) chondrules. The FeO-poor PO chondrules are the most voluminous type of chondrule in carbonaceous, ordinary, and enstatite chondrites (Jones, 2012). A suite of minerals, chosen based on previous experimental studies of chondrule synthesis by Connolly et al. (1998), Radomsky and

Discussion

The model presented in this paper provides for a direct link between chondrule formation and chondrite lithification. In that sense it represents a major departure from the canonical view that chondrules formed first in space, then accreted to a parent body where they were lithified by processes unrelated to chondrule formation. As discussed above, a primary motivation for considering such a non-canonical model is complementarity, which provides strong evidence of a close link between chondrule

Summary

We propose that chondrules form and chondrites lithify simultaneously when m- to km-scale primitive planetesimals are exposed to infrared radiation from incandescent lava at the surfaces of much larger (100–200 km scale) differentiated planetesimals during close flybys. A thermal model is developed that predicts heating and cooling rates consistent with the constraints of experimental petrology, based on synthetic chondrule textures. The temperature of the extruding lava needs to be ∼2000 K,

Acknowledgement

We thank the Editor, Alessandro Morbidelli, and referees Guy Libourel, Dominik Hezel and an anonymous referee for helpful comments on an earlier version of this work. We also thank Jim Zareski and Kenichi Abe of Wesleyan University for assistance with the laboratory work. Funding for this project has been provided by NASA, originally through a seed grant from the CT Space Grant Consortium, and with continuing support under award NNX17AE26G. We thank the NSF for support of the SEM at Wesleyan

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