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Does the Oxygen Isotope Diagram Provide Clues to the Distribution of Meteoritic Material in the Early Solar System? T

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Does the Oxygen Isotope Diagram Provide Clues to the Distribution of Meteoritic Material in the Early Solar System? T 51st Lunar and Planetary Science Conference (2020) 2214.pdf DOES THE OXYGEN ISOTOPE DIAGRAM PROVIDE CLUES TO THE DISTRIBUTION OF METEORITIC MATERIAL IN THE EARLY SOLAR SYSTEM? T. H. Burbine1 and R. C. Greenwood2, 1Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, MA 01075, USA ([email protected]), 2Planetary and Space Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Introduction: Following the pioneering studies of Robert Clayton [1], oxygen three isotope analyses have been widely used as a means of constraining the genetic relationships between meteorite groups. Most meteorites in the same group, particularly achondrites, tend to display coherent relationships on oxygen three isotope diagrams (Figure 1). Here we discuss how the distribution of different meteorite groups on the oxy- gen three isotope diagram may provide clues to their original location within the protoplanetary disk. Figure 2. Plot of Δ17O versus ε54Cr for a range of me- teorites. The plot is modified after Warren [2]. The formation of Jupiter’s core would stop signifi- cant material exchange between the NC and CC re- gions and isotopic homogenization, resulting in the dichotomy that we see today. The NC meteorites would have formed in the inner Solar System and the CC meteorites would have formed in the outer Solar System. As Jupiter and Saturn migrated outwards during the later stages of the “Grand Tack” [6], CC parent bodies would have been scattered into the re- Figure 1. Oxygen three isotope diagram showing the gion now known as the asteroid belt. distribution of the principal achondrite groups. Oxygen Isotope Diagram: A number of trends are apparent on the oxygen three isotope diagram Models: Recent astrophysical models have fo- (Figures 1 and 2). For example, the distance (∆17O) cused on the dichotomy of isotopic compositions be- that the enstatite, ordinary, and R chondrites fall away tween two groups of material [2], which are labeled as from the terrestrial fractionation line (TFL) may be the carbonaceous chondrite (CC) and non- correlated with the sequence that these meteorites carbonaceous (NC) groups (Figure 2). The CC group would have condensed out in the solar nebula [7]. tend to have higher concentrations of nucleosynthetic Almost all carbonaceous chondrites fall below the isotopes (e.g., ε50Ti, ε54Cr). The NC group includes TFL [1,8] (Figure 2). The CI and “CY” chondrites non-carbonaceous chondrites and most achondrites. are exceptions and fall slightly above the TFL [1,8,9]. The isotopic differences are proposed [3-5] to be Almost all the meteorites that experienced melting due to early infalling material in the solar nebula be- also fall below the TFL [1,8] (Figures 1 and 2). ing enriched in nuclides produced in neutron-rich These meteorites include the HEDs (howardites, eu- stellar environments. CAIs record these early-Solar crites, diogenites) and ureilites. The aubrites fall rela- System isotopic enrichments. This material would tively near the TFL. Differentiated meteorites that fall have been transported outwards through viscous above the TFL are relatively rare. Heating of these spreading. The later infalling material, assumed to be bodies is attributed to 26Al. depleted in neutron-rich nuclides, would tend to ac- Also, the aubrites and enstatite chondrites, which cumulate in the inner part of the disk and dilute the are the best analogs for the chondritic precursor of the isotopic signature of the NC region. aubrites, have virtually indistinguishable ∆17O values 51st Lunar and Planetary Science Conference (2020) 2214.pdf [10] and fall very near the TFL. Both aubrites and The observation that the aubrites and enstatite enstatite chondrites are extremely reduced. chondrites have virtually indistinguishable ∆17O val- Also, there is considerable overlap in oxygen iso- ues may imply that bodies that formed in the same topic space (Figure 2) between meteorites that fall in region of the Solar System may have accumulated the CC region and those that fall in the NCC region similar abundances of refractory inclusions. These [2]. While nucleosynthetic anomalies can be used to bodies would have similar ∆17O values but the differ- easily separate CC and NC meteorites, ∆17O does not. entiated bodies would have melted due to either form- The ∆17O variations are usually assumed to be related ing earlier or accumulating more 26Al. to photochemical processes in the solar nebula [11]. Could this observation imply that NC differentiat- Finally, refractory inclusions, which would have ed meteorites that formed with negative ∆17O values condensed out at extremely high temperatures [7] and formed in the outer Solar System from carbonaceous very early in Solar System history, have extremely chondritic material, which also have negative ∆17O 16O-rich compositions and, therefore, extremely nega- values, and not in the inner Solar System? But why tive ∆17O values [12]. These inclusions include CAIs don’t NC differentiated meteorites have nucleosyn- (calcium-aluminum-rich inclusions) and AOAs thetic anomalies consistent with carbonaceous chon- (amoeboid olivine condensates). CAIs are the oldest drites? Maybe these bodies formed early in Solar Sys- dated material in the Solar System [13]. tem history and did not incorporate a significant frac- Proposed Model: We propose that the location of tion of interstellar dust with nucleosynthetic isotopic a meteoritic group on the oxygen three isotope dia- excesses. The carbonaceous chondrites would have gram could be a function of the original concentration formed later and incorporated more dust. of refractory inclusions incorporated into the body. Or did the NC differentiated meteorites that The more negative the ∆17O, the higher concentration formed with negative ∆17O values just form from in- of refractory inclusions that the parent body received. ner Solar System material that accumulated more re- However, relict CAIs are relatively rare in chon- fractory inclusions? This accumulation would have drules [14] and refractory inclusions are also relatively moved the resulting meteorites downwards on an oxy- rare in ordinary, enstatite, and R chondrites [15]. gen three isotope diagram. Could the majority of these inclusions incorporated Conclusions: We propose a scenario where the into the chondrites be “destroyed” by parent body pro- distribution of meteorites on the oxygen isotope dia- cesses? Despite the proposed “destruction” of the gram is due to the early incorporation of refractory CAIs, the bulk oxygen isotopic composition of the inclusions. Further modeling needs to be done to in- body would not be expected to change. Some refracto- vestigate this scenario in more detail. ry inclusions may have been extremely small, increas- Acknowledgments: THB would like to thank the ing the probability of their “destruction”. The refrac- RISE2 SSERVI for support. Meteorite studies at The tory inclusions we see today were either “lucky” or Open University are supported by a Consolidated were incorporated late. Grant from STFC. We assume that the NC chondrites formed in the References: [1] Clayton R. N. (1993) Ann. Rev. inner Solar System and that carbonaceous chondrites Earth. Planet. Sci., 21, 115-149. [2] Warren P. H. formed in the outer Solar System. We assume that the (2011) EPSL, 311, 93-100. [3] Kruijer T. S. et al. formation location of the NC chondrites (in order of (2017) PNAS, 114, 6712-6716. [4] Nanne J. A. M. et increasing distance from the Sun) would be enstatite, al. (2019) EPSL, 511, 44-54. [5] Burkhardt C. et al. H, L, LL, and R chondrites. This sequence also in- (2019) GCA, 261, 145-170. [6] Walsh K. J. et al. creases in ∆17O, which could be due to each meteorite (2011) Nature, 475, 206-209. [7] Grossman L. (1972) parent body receiving smaller fluxes of refractory in- GCA, 36, 597-619. [8] Greenwood R. C. et al. (2017) clusions with increasing distance from the Sun. Chemie der Erde, 77, 1-43. [9] King A. J. et al. The negative ∆17O values for most carbonaceous (2019) Geochemistry, 79, 125531. [10] Newton J. et chondrites implies that the abundances of refractory al. (2000) Meteoritics & Planet. Sci., 35, 689-698. inclusions incorporated by outer Solar System bodies [11] Lyons J. R. and Young E. D. (2005) Nature, 435, were higher than those incorporated by bodies in the 317-320. [12] Krot A. N. et al. (2010) Astrophys. J., inner Solar System. The CI chondrites, which may 713, 1159-1166. [13] Bouvier A. and Wadhwa M. have formed far from the Sun, would have incorpo- (2010) Nature Geoscience, 3, 637-641. [14] Krot A. rated the smallest flux of refractory inclusions of the N. et al. (2006) Astrophys. J., 639, 1227-1237. [15] carbonaceous chondrites, and, therefore, have a slight- Hezel D. C. et al. (2008) Meteoritics & Planet. Sci., ly positive ∆17O. 43, 1879-1894. .
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