Oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules
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 (O = −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 O deviating by >3σ from their host O 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 O 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.
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