Mineralogy, petrology, and oxygen isotopic compositions of aluminum-rich chondrules from unequilibrated ordinary and the Dar al Gani 083 (CO3.1) chondrite
Introduction
Understanding a potential genetic link between chondrules and refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)] is an important goal in cosmochemistry. Relict CAIs and AOAs found within chondrules from ordinary (OCs) and carbonaceous chondrites (CCs) indicate that CAIs were present in the chondrule-forming regions, but only some of them experienced melting during chondrule formation (e.g., Krot et al., 2002, Krot et al., 2004, Krot et al., 2006a, Krot et al., 2017a, Nagashima et al., 2020, Zhang et al., 2020). These observations are interpreted as evidence for a localized nature of chondrule-forming events (Krot, 2019).
Aluminum-rich (>10 wt% bulk Al2O3) chondrules, occurring in all chondrite groups, are of special interest as they have been suggested to have formed from precursors containing refractory CAI- and/or AOA-like objects (Nagahara and Kushiro, 1982, Bischoff and Keil, 1983b, Bischoff and Keil, 1983a, Bischoff and Keil, 1984, Bischoff et al., 1985, Bischoff et al., 1989, Krot and Rubin, 1994, Krot and Keil, 2002, Russell et al., 2000, MacPherson and Huss, 2005, Guan et al., 2006, Rout and Bischoff, 2008, Zhang and Hsu, 2009, Ebert and Bischoff, 2016, Ebert et al., 2018, Ebert et al., 2019, Zhang et al., 2020). However, because Al-rich chondrules are relatively common in non-carbonaceous (NCs) chondrites, which generally contain very rare refractory inclusions, it is still not clear to what extent CAIs and AOAs make up the Al-rich chondrule precursors in these meteorites (MacPherson and Huss, 2005). Based on bulk chemical compositions, Bischoff and Keil, 1983b, Bischoff and Keil, 1983a, Bischoff and Keil, 1984 defined several subtypes of Al-rich chondrules in ordinary chondrites – Na,Al-rich (Na2O ≥ 5.0 wt%), Ca,Al-rich (Na2O ≤ 5.0 wt%), Na-Cr,Al-rich (Cr2O3 ≥ 2.4 wt%) and intermediate.
A detailed study of the Na,Al-rich chondrules from ordinary, Rumuruti, and CO3.1 chondrites has shown that these chondrules typically consist of euhedral-to-subhedral mafic minerals embedded within a brownish nepheline-normative, glassy mesostasis and often have volatility-controlled rare earth element (REE) patterns – Group II, III, and ultra-refractory (Ebert and Bischoff, 2016) – similar to those commonly observed in CAIs (Mason and Martin, 1977, MacPherson et al., 1988, Davis et al., 2018) and AOAs (Grossman et al., 1979). The combination of elevated refractory element abundances (Al, Ti, and REE), as well as CAI-like group II and group III REE patterns provide strong evidence that Al-rich chondrules contained refractory precursors chemically and mineralogically similar to CAIs and AOAs, providing a link between the two major chondritic components, chondrules and refractory inclusions (Ebert and Bischoff, 2016). It has to be mentioned that this connection between Al-rich chondrules and CAIs and AOAs based on the REE patterns has been questioned by Jacquet et al., 2018, Jacquet et al., 2019.
Similar connections between Al-rich chondrules and refractory material can be drawn by considering the Ti-isotope compositions. Ca,Al-rich inclusions in CCs and OCs typically have 50Ti excesses between ∼2–16 ɛ units (e.g., Trinquier et al., 2009, Williams et al., 2016, Ebert et al., 2018, Shollenberger et al., 2018, Render et al., 2019), also linking CAIs from different chondrites to a formation in a common isotopic reservoir. As titanium isotopes are not expected to exchange with the nebular gas during chondrule formation (Niemeyer, 1988a, Gerber et al., 2017), 50Ti is a perfect tracer to constrain the role of CAI constituents as possible precursor components of Al-rich chondrules. If 50Ti-enriched refractory material was involved as a precursor, it should be still detectable in the resulting chondrules. Indeed, Na,Al-rich chondrules in CO chondrites show positive excesses of 50Ti up to 14.5 ± 0.5ε, which is best explained by the presence of CAI-like material in their precursors (Ebert et al., 2018). However, Na,Al-rich chondrules in ordinary chondrites show no evidence for 50Ti excesses (ε50Ti range from ∼−2 to 0); these Ti-isotope compositions are similar to those of ferromagnesian chondrules and bulk ordinary chondrites (Gerber et al., 2017, Ebert et al., 2018). Based on these observations, Ebert et al. (2018) concluded that an “unknown” refractory component, mineralogically and chemically similar to known CAIs but with normal Ti-isotope compositions, was among Na,Al-rich chondrule precursors in the ordinary chondrites’ chondrule-forming region. Possible explanations for such a component could be either a heterogeneous distribution of 50Ti in the CAI-forming region or a different non-CAI origin of the refractory precursors.
Based on the solar-like oxygen isotopic compositions of refractory inclusions (McKeegan et al., 2011) and the presence of decay products of short-lived radionuclides 7Be and 10Be formed by solar energetic particle irradiation, CAIs are thought to have formed from a gas of approximately solar composition, possibly close to the young Sun, and were subsequently distributed into the different regions of the protoplanetary disk where chondrules formed and chondrites accreted (Shu et al., 1996, McKeegan et al., 2000, MacPherson et al., 2003, Brownlee et al., 2006, Chaussidon et al., 2006, Ciesla, 2007, Krot et al., 2009, Wielandt et al., 2012, MacPherson, 2014 and references therein). If CAIs or AOAs were present among Al-rich chondrule precursors, these chondrules may have preserved an 16O-rich signature of these precursors. In addition, O-isotope compositions of Al-rich chondrules in ordinary chondrites can be potentially used to constrain their formation region to the inner vs outer Solar System. Note that on a three-isotope oxygen diagram (δ17O vs δ18O), vast majority of chondrules in CCs plot below the terrestrial fractionation (TF) line, whereas those from NCs plot along or above it (e.g., Tenner et al., 2018 and references therein).
In this context, it is important to consider the effects of exchange and equilibration of O-isotopes during chondrule formation or during metasomatic/aqueous alteration in the solar nebula and/or on the chondrites’ parent bodies. Melted chondrules behaved as open systems (e.g., Kita et al., 2010) and appear to have experienced O-isotope exchange with the surrounding solar nebula gas (e.g., Clayton et al., 1991; Tenner et al., 2018). Oxygen isotopic compositions of most CAIs, AOAs, and chondrules (except relict grains) from most unmetamorphosed (petrologic type 2–3.0) chondrites are uniform (e.g., Makide et al., 2009, Ushikubo et al., 2017, Kööp et al., 2016, Krot et al., 2017b, Krot et al., 2019a). This O-isotope homogeneity can be subsequently modified by post-crystallization O-isotope exchange with either nebular gas or aqueous fluid on the chondrites’ parent asteroids; the latter process is consistent with the common presence of isotopically heterogeneous chondrules and refractory inclusions in metamorphosed (petrologic type > 3.0) CO and CV carbonaceous chondrites (e.g., Wasson et al., 2001, Itoh et al., 2004, Itoh et al., 2007, Rudraswami et al., 2011, Krot et al., 2019b, Krot et al., 2019a, Ebert et al., 2020). To minimize the effects of parent body alteration, in this study, we used only Al-rich chondrules from the weakly metamorphosed ordinary chondrites of petrologic type ≤ 3.2 [NWA 3358 (H3.1), Adrar 003 (L/LL3.1), Vicencia (L3.15), Krymka (LL3.2), and Vicencia (LL3.2)] and the CO3.1 chondrite Dar al Gani (DaG) 083.
Section snippets
Methods and samples
All polished thin sections and meteorite chips discussed in this work were provided by the Institut für Planetologie of the Westfälische Wilhelms-Universität Münster and by the Hawai‘i Institute of Geophysics and Planetology (HIGP), University of Hawai‘i (UH) at Mānoa.
For identification of Al-rich chondrules, a Zeiss Axiophot optical microscope was used for thin sections microscopy. Thick sections and meteorite chips were studied with a JEOL 6610-LV electron microscope (SEM) at the
NWA 3358 (H3.1)
NaC-1 is a porphyritic pyroxene (PP) chondrule, ∼150 µm in apparent diameter (Fig. 1a). It consists of low-Ca pyroxene (En94Wo5) phenocrysts rimmed by Ca pyroxene (En56Wo43) and embedded in glassy mesostasis (12.2 wt% Na2O). Oxygen-isotope compositions were measured for two low-Ca pyroxene grains in the core and rim of the chondrule, and for the mesostasis in the core. The pyroxenes and mesostasis are in O-isotope disequilibrium. Two low-Ca pyroxene grains have similar Δ17O, −4.3 ± 2.8‰ and
Al-rich chondrules from a CO-chondrite
Four Al-rich chondrules from the CO3.1 chondrite Dar al Gani 083 were measured for O-isotope compositions (Fig. 7, Fig. 8, Fig. 10; Table 3b). The chondrule CO-Ca-Al-2 has an internally uniform O-isotope composition, with spinel, olivine, and Ca pyroxene having Δ17O values of −2, −1, and 0‰, respectively. In chondrule CO-NaC-2, only Ca pyroxene grains were measured (Δ17O ∼ −3‰). Two other chondrules, CO-Ca-Al-1 and CO-NaC-1, are isotopically heterogeneous, with Δ17O ranging from ∼−25 to ∼−4‰
Conclusions
We investigated the mineralogy, petrology, and O-isotope compositions of 17 Al-rich chondrules and/or their constituents from unequilibrated ordinary chondrites (petrologic type < 3.2) and four Al-rich chondrules from the CO3.1 chondrite Dar al Gani 083.
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Precise Na measurements within olivine grains of selected Al-rich chondrules reveal no changes in the chondrule melts’ Na content during the olivine crystallization.
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Al-rich chondrules from the CO3.1 chondrite have a heterogeneous oxygen isotopic
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Ulla Heitmann (Münster) for sample preparation, Thorsten Grund for the technical assistance on the SEM, and Celeste Brennecka for writing support. We thank Gary Huss for the fruitful discussions. Furthermore, we thank Sarah Russell, Kohei Fukuda and one anonymous reviewer who provided very helpful reviews of the manuscript as well as Yves Marrocchi for his careful editorial handling. This work was partly supported by the German Research Foundation (DFG) within the Collaborative
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