Elsevier

Earth and Planetary Science Letters

Volume 496, 15 August 2018, Pages 132-141
Earth and Planetary Science Letters

Oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules

https://doi.org/10.1016/j.epsl.2018.05.042Get rights and content

Highlights

  • High-resolution X-ray maps reveal different populations of olivine in chondrules.

  • High-current microprobe analyses allow relict olivine grains to be characterized.

  • Relict olivine crystals in chondrules are Ca–Al–Ti-poor but show varying Δ17O.

  • Chondrules result from nebular condensate melting and gas–melt interactions.

  • Host olivine grains formed via gas-assisted epitaxial growth on relict olivine.

Abstract

FeO-poor (type I) porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet-elusive mechanism. Two settings are generally considered for their formation: (i) a nebular setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca–Al–Ti-poor relative to host olivine crystals. Their variable Δ17O, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120 ° triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions.

We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Δ17O values and Ca–Al–Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca–Al–Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca–Al–Ti-rich host olivine crystals via epitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i) the dilution of CAT-melts, as attested by the positive Al–Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Δ17O values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Δ17O values, possibly due to variable solid/gas mixing ratios.

Introduction

Chondrules (millimeter-sized igneous spheroids containing silicates, metal, sulfides, and glass) are the major high-temperature components of primitive meteorites (chondrites), suggesting that most inner solar system materials were affected by their formation. However, the underlying mechanism(s) of their formation remains a mystery and diverse scenarios are debated in contemporaneous literature. A key clue to their origin would be the identification of the precursor material that was melted to form chondrules. In this effort, cosmochemists may find help in the incomplete melting of most chondrules, as evidenced by their widespread porphyritic texture (Hewins et al., 2005). Indeed, relict grains inherited from chondrule precursors are identifiable. Forsteritic grains in high-FeO (type II) porphyritic chondrules were presumably inherited from precursors formed in more reducing conditions than their current host (Nagahara, 1981, Scott and Jones, 1990). Conversely, “dusty” grains, i.e., crystals speckled with Fe-rich metal beads, in low-FeO (type I) chondrules are commonly attributed to oxidized precursors that underwent reduction during formation of their host chondrules (Lemelle et al., 2001, Leroux et al., 2003, Nagahara, 1981, Rambaldi, 1981). These examples represent only a fraction of the existing relicts. In type I chondrules—the dominant type in carbonaceous chondrites and hence the main asteroid belt—relict grains from reduced precursors are not so easily recognizable.

Oxygen isotopic systematics may help to identify such relicts (Kimura et al., 2011, Kunihiro et al., 2004, Kunihiro et al., 2005, Rudraswami et al., 2011; Schrader et al., 2013, Schrader et al., 2015; Tenner et al., 2013, Tenner et al., 2015; Ushikubo et al., 2012). Indeed significant isotopic diversity is observed among solar system materials, from 16O-rich refractory inclusions (calcium–aluminum-rich inclusions (CAIs), amoeboid olivine aggregates (AOA)) to 16O-poorer chondrules (Clayton, 2003). Type I chondrules exhibit significant variability and define a broad line in the oxygen three-isotope diagram that is not specific to chondrules of a given chondrite or to particular chondrite type and known as the primitive chondrule minerals (PCM) line (Ushikubo et al., 2012). The underlying 16O variability could result from (i) physical mixing of grains of different origins within the solid chondrule precursors (Hezel and Palme, 2007, Tenner et al., 2015) and/or (ii) high-temperature exchanges between the chondrule melt and the surrounding gas (Marrocchi and Chaussidon, 2015). Secondary ion mass spectrometer (SIMS) analyses of olivines within a single chondrule typically reveal homogeneous oxygen isotopic compositions (Chaussidon et al., 2008, Tenner et al., 2015). Yet, in some chondrules, some olivines are 16O-enriched compared to their neighbors (Kunihiro et al., 2005, Rudraswami et al., 2011): these are generally considered to be relict grains, even if they are not petrographically manifest (Ushikubo et al., 2012).

Despite these additions to the known inventory of relict grains, their origin remains unclear. Relict olivine grains could correspond to (i) early condensates from the gas of the solar protoplanetary disk (Cohen et al., 2004, Jacquet and Marrocchi, 2017, Russell et al., 2005, Yurimoto and Wasson, 2002), (ii) collisional debris from early-generation planetesimals (Libourel and Chaussidon, 2011, Libourel and Krot, 2007) and/or (iii) earlier generations of chondrules (e.g., Ruzicka et al., 2007). Progress on the identification of the origin of relict olivine grains is frustrated by our poor understanding of their abundance, distribution and specific chemical compositions (Pack et al., 2005, Pack et al., 2004; Ruzicka et al., 2007). Among isotopic studies, only a few in situ measurements of olivine are generally performed within a single chondrule (usually 2–10; Rudraswami et al., 2011), leading to an misestimate of the abundance of relict olivine grains (and relict-bearing chondrules) and a lack of knowledge of their isotopic characteristics. Furthermore, there are no clear major-element signatures and/or textural features that facilitate the recognition of relict olivine grains (Jones et al., 2004, Ushikubo et al., 2012). Consequently, the message carried by relict olivine grains remains difficult to decipher, however fundamental it would be to understand the origin of chondrules.

Here we report a new method combining high-resolution X-ray maps, electron microprobe analyses, and SIMS oxygen isotope measurements to quantitatively assess for the first time the nature of relict olivine grains in type I chondrules. This method was successfully applied to type I porphyritic chondrules in the CM-related ungrouped chondrite Northwest Africa (NWA) 5958 (Jacquet et al., 2016) and provides new information on the formation conditions of the first solids of the solar system.

Section snippets

Material and methods

NWA 5958 is a carbonaceous chondrite found in the Moroccan desert in 2009 that shares numerous similarities with the CM chondrite group. This meteorite shows limited terrestrial weathering and a low degree of aqueous alteration (Jacquet et al., 2016). Its bulk oxygen isotopic composition (Δ17O = −4.3‰) is more 16O-rich than all CM chondrites, further supporting a limited alteration episode (Marrocchi et al., 2018, Verdier-Paoletti et al., 2017). As previous O isotope studies have revealed CO

Results

All porphyritic chondrule textural types are present in NWA 5958, i.e., porphyritic olivine-rich (PO), porphyritic olivine–pyroxene (POP), and porphyritic pyroxene-rich (PP); Fig. 1, Fig. 2, Fig. 3). Among the chondrules examined, two PO chondrules (Ch-1 and Ch-7) were chosen for detailed examination. They are characterized by many variably-sized (≈30–300 μm) subhedral to euhedral olivine grains meeting in 120 ° triple junctions (Fig. 1, Fig. 2, S1–S2) first described by (Libourel and Krot, 2007

Chemical characterization of relict olivines

Our high-resolution titanium X-ray maps of porphyritic chondrules reveal different populations of olivine grains characterized by variable titanium contents (Fig. 2B, S2–S3). Most Ti-poor olivine grains have oxygen isotopic compositions markedly distinct from their hosts (olivine grains with higher Ti concentrations; Fig. 3B, 4B and S4B, Tables S1 and S2). Following Ushikubo et al. (2012), we consider olivines with Δ17O deviating by >3σ from their host Δ17O as relicts (host being the olivine

Concluding remarks

We developed a method combining high-current electron microprobe X-ray mapping (at 500 nA) and quantitative measurements (at 900 nA) to determine the chemical characteristics of relict olivine grains in chondrules. They are Ca–Al–Ti-poor compared to their host olivine crystals and are mainly located in the center of chondrules. Ti-poor relict olivines show variable Δ17O values in each chondrule, suggesting that chondrule precursors correspond to nebular condensates that formed under changing

Acknowledgments

All the data used in the present article are available by contacting Yves Marrocchi. Laurent Tissandier, Andrey Gurenko and Mathieu Roskosz are thanked for helpful scientific discussions. Nordine Bouden is thanked for his assistance with isotopic measurements. We thanked Dominik Hezel and Ryoji Tanaka for constructive comments and Associate Editor Frederic Moynier for careful editing. This is CRPG-CNRS contribution #2564.

References (75)

  • E. Jacquet et al.

    Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy

    Geochim. Cosmochim. Acta

    (2015)
  • R.H. Jones

    FeO-rich, porphyritic pyroxene chondrules in unequilibrated ordinary chondrites

    Geochim. Cosmochim. Acta

    (1996)
  • R.H. Jones et al.

    Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite

    Geochim. Cosmochim. Acta

    (2004)
  • N.T. Kita et al.

    High precision SIMS oxygen three isotope study of chondrules in LL3 chondrites: role of ambient gas during chondrule formation

    Geochim. Cosmochim. Acta

    (2010)
  • A.N. Krot et al.

    Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

    Chem. Erde, Geochem.

    (2004)
  • A.N. Krot et al.

    Origin and chronology of chondritic components: a review

    Geochim. Cosmochim. Acta

    (2009)
  • Kunihiro et al.

    Oxygen-isotopic compositions of low-FeO relicts in high-FeO host chondrules in Acfer 094, a type 3.0 carbonaceous chondrite closely related to CM

    Geochim. Cosmochim. Acta

    (2005)
  • Kunihiro et al.

    Oxygen-isotopic compositions of relict and host grains in chondrules in the Yamato 81020 CO3.0 chondrite

    Geochim. Cosmochim. Acta

    (2004)
  • G. Libourel et al.

    Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites

    Earth Planet. Sci. Lett.

    (2011)
  • G. Libourel et al.

    Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin

    Earth Planet. Sci. Lett.

    (2007)
  • G. Libourel et al.

    Role of gas–melt interaction during chondrule formation

    Earth Planet. Sci. Lett.

    (2006)
  • Y. Marrocchi et al.

    A systematic for oxygen isotopic variation in meteoritic chondrules

    Earth Planet. Sci. Lett.

    (2015)
  • Y. Marrocchi et al.

    Sulfur and sulfides in chondrules

    Geochim. Cosmochim. Acta

    (2013)
  • Y. Marrocchi et al.

    Origin and abundance of water in carbonaceous asteroids

    Earth Planet. Sci. Lett.

    (2018)
  • K. Misawa et al.

    Demonstration of REE fractionation among individual chondrules from the Allende (CV3) chondrite

    Geochim. Cosmochim. Acta

    (1988)
  • A. Pack et al.

    Origin of chondritic forsterite grains

    Geochim. Cosmochim. Acta

    (2005)
  • A. Pack et al.

    Petrographic and oxygen-isotopic study of refractory forsterites from R-chondrite Dar al Gani 013 (R3.5–6), unequilibrated ordinary and carbonaceous chondrites

    Geochim. Cosmochim. Acta

    (2004)
  • L. Piani et al.

    Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites

    Geochim. Cosmochim. Acta

    (2016)
  • N.G. Rudraswami et al.

    Oxygen isotope systematics of chondrules in the Allende CV3 chondrite: high precision ion microprobe studies

    Geochim. Cosmochim. Acta

    (2011)
  • A. Ruzicka et al.

    Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville and Vigarano (CV3) chondrites: a record of condensate evolution in the solar nebula

    Geochim. Cosmochim. Acta

    (2012)
  • A. Ruzicka et al.

    Relict olivine, chondrule recycling, and the evolution of nebular oxygen reservoirs

    Earth Planet. Sci. Lett.

    (2007)
  • P.A. Sakyi et al.

    Inherited Pb isotopic records in olivine antercryst-hosted melt inclusions from Hawaiian lavas

    Geochim. Cosmochim. Acta

    (2012)
  • D.L. Schrader et al.

    The formation and alteration of the Renazzo-like carbonaceous chondrites II: linking O-isotope composition and oxidation state of chondrule olivine

    Geochim. Cosmochim. Acta

    (2013)
  • D.L. Schrader et al.

    The retention of dust in protoplanetary disks: evidence from agglomeratic olivine chondrules from the outer Solar System

    Geochim. Cosmochim. Acta

    (2018)
  • E. Scott et al.

    Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites

    Geochim. Cosmochim. Acta

    (1990)
  • T.J. Tenner et al.

    Oxygen isotope ratios of FeO-poor chondrules in CR3 chondrites: influence of dust enrichment and H2O during chondrule formation

    Geochim. Cosmochim. Acta

    (2015)
  • T.J. Tenner et al.

    Oxygen isotope systematics of chondrule phenocrysts from the CO3.0 chondrite Yamato 81020: evidence for two distinct oxygen isotope reservoirs

    Geochim. Cosmochim. Acta

    (2013)
  • Cited by (68)

    View all citing articles on Scopus
    View full text