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Chondrules Reveal Large-Scale Outward Transport of Inner Solar System Materials in the Protoplanetary Disk

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Chondrules Reveal Large-Scale Outward Transport of Inner Solar System Materials in the Protoplanetary Disk Chondrules reveal large-scale outward transport of inner Solar System materials in the protoplanetary disk Curtis D. Williamsa,1, Matthew E. Sanborna, Céline Defouilloyb,2, Qing-Zhu Yina,1, Noriko T. Kitab, Denton S. Ebelc, Akane Yamakawaa,3, and Katsuyuki Yamashitad aDepartment of Earth and Planetary Sciences, University of California, Davis, CA 95616; bWiscSIMS, Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706; cDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024; and dGraduate School of Natural Science and Technology, Okayama University, Kita-ku, 700-8530 Okayama, Japan Edited by H. J. Melosh, Purdue University, West Lafayette, IN, and approved August 9, 2020 (received for review March 19, 2020) Dynamic models of the protoplanetary disk indicate there should actually be composed of solids with diverse formation histories be large-scale material transport in and out of the inner Solar Sys- and, thus, distinct isotope signatures, which can only be revealed tem, but direct evidence for such transport is scarce. Here we show by studying individual components in primitive chondritic me- that the e50Ti-e54Cr-Δ17O systematics of large individual chon- teorites (e.g., chondrules). Chondrules are millimeter-sized drules, which typically formed 2 to 3 My after the formation of spherules that evolved as free-floating objects processed by the first solids in the Solar System, indicate certain meteorites (CV transient heating in the protoplanetary disk (1) and represent a and CK chondrites) that formed in the outer Solar System accreted major solid component (by volume) of the disk that is accreted an assortment of both inner and outer Solar System materials, as into most chondrites. The nucleosynthetic anomalies in 50Ti and well as material previously unidentified through the analysis of 54Cr of individual chondrules are, in most cases, similar to those bulk meteorites. Mixing with primordial refractory components of their bulk meteorites (9–12), which suggests a close relation reveals a “missing reservoir” that bridges the gap between inner between the regions where chondrules formed and where they and outer Solar System materials. We also observe chondrules accreted into their asteroidal parent bodies. However, exceptions with positive e50Ti and e54Cr plot with a constant offset below to this observation are chondrules in CV chondrites, which dis- the primitive chondrule mineral line (PCM), indicating that they play the entire range of 50Ti and 54Cr observed for all bulk are on the slope ∼1.0 in the oxygen three-isotope diagram. In meteorite groups (9, 11). Previous Cr isotope studies that have 50 54 contrast, chondrules with negative e Ti and e Cr increasingly de- observed this larger isotopic range in CV chondrules have pro- EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 18 viate above from PCM line with increasing δ O, suggesting that posed that these chondrules or their precursor materials origi- they are on a mixing trend with an ordinary chondrite-like isotope nated from a wide (not local) spatial region of the protoplanetary 17 reservoir. Furthermore, the Δ O-Mg# systematics of these chon- disk and were transported to CV chondrite accretion regions (9). drules indicate they formed in environments characterized by dis- In contrast, Ti isotope studies have suggested that the wide range tinct abundances of dust and H2O ice. We posit that large-scale of isotope anomalies observed in individual CV chondrules could outward transport of nominally inner Solar System materials most likely occurred along the midplane associated with a viscously Significance evolving disk and that CV and CK chondrules formed in local re- gions of enhanced gas pressure and dust density created by the formation of Jupiter. We present a coordinated petrologic, mineral chemistry, and multi-isotopic investigation of individual chondrules to further elucidate the origins and formation histories of planetary ma- chondrules | bulk meteorites | isotope anomalies | disk transport | disk mixing terials. We show chondrules from certain meteorites that ac- creted in the outer Solar System contain an assortment of both inner and outer Solar System material, as well as previously hemical and isotopic signatures of primitive meteorites unidentified material. The outward transport of inner Solar Cprovide a powerful means to trace the earliest history of System material places important constraints on dynamical planet formation in our Solar System (1). Nucleosynthetic 50 54 models, as outward transport in the disk was thought possible anomalies in Ti and Cr have been identified among major only if significant barriers (e.g., Jupiter) to radial transport of meteorite classes that distinguish planetary materials into two materials do not exist. We show this “barrier” is either not groups, noncarbonaceous and carbonaceous meteorites (2–5). completely impermeable to transport of millimeter-sized ma- Bulk meteorites and their components also show a significant terials or additional mechanisms are required to transport variability in the mass-independent fractionation of O isotopes, materials to the outer Solar System. which could have resulted from photochemical reactions that occurred heterogeneously across the protoplanetary disk (6). Author contributions: Q.-Z.Y. designed research; C.D.W., M.E.S., C.D., Q.-Z.Y., N.T.K., Several other isotope systems have now been found to show a D.S.E., A.Y., and K.Y. performed research; C.D.W., M.E.S., C.D., Q.-Z.Y., N.T.K., D.S.E., A.Y., and K.Y. analyzed data; and C.D.W., M.E.S., C.D., Q.-Z.Y., N.T.K., and D.S.E. wrote similar dichotomy between noncarbonaceous and carbonaceous the paper. meteorites and, collectively, may not be easily explained by The authors declare no competing interest. thermal processing of isotopically anomalous presolar carriers or This article is a PNAS Direct Submission. addition of early-formed Ca- and Al-rich inclusions (CAIs) (7). This open access article is distributed under Creative Commons Attribution-NonCommercial- Instead, most studies have proposed that the dichotomy was NoDerivatives License 4.0 (CC BY-NC-ND). caused by spatial differences in isotopic signatures during the 1To whom correspondence may be addressed. Email: [email protected] or qyin@ earliest stage of disk evolution (5, 7, 8), such that the isotopic ucdavis.edu. signatures of noncarbonaceous and carbonaceous meteorites 2Present address: CAMECA, 92622 Gennevilliers Cedex, France. represent those of inner and outer disk materials, respectively. 3Present address: Center for Environmental Measurement and Analysis, National Institute 50 54 Nucleosynthetic anomalies in Ti and Cr associated with for Environmental Studies, Tsukuba-City, 305-8506 Ibaraki, Japan. bulk meteorites reflect the average composition of solids that This article contains supporting information online at https://www.pnas.org/lookup/suppl/ were accreted onto their parent asteroids from local regions doi:10.1073/pnas.2005235117/-/DCSupplemental. within the disk. However, local regions within the disk may www.pnas.org/cgi/doi/10.1073/pnas.2005235117 PNAS Latest Articles | 1of10 Downloaded by guest on September 25, 2021 be explained by the admixture of CAI-like precursor materials anhydrous dust versus H2O ice (17). A significant number of with highest nucleosynthetic 50Ti and 54Cr anomaly, which are high-precision Ti and Cr isotope analyses of bulk meteorites abundant in CV chondrites (11). were also obtained for both carbonaceous and noncarbonaceous Resolving these two competing interpretations would signifi- meteorites to help define the bulk meteorite 50Ti-54Cr system- cantly improve our understanding of the origin of the isotopic atics of known planetary materials. dichotomy observed for bulk meteorites and provide constraints on the disk transport mechanism(s) responsible for the potential Results mixing of material with different formation histories. However, Bulk Meteorite 50Ti and 54Cr Isotopes. We obtained analyses of Ti in earlier studies, Ti and Cr isotopes were not obtained from the and Cr isotopic compositions (reported as e50Ti and e54Cr, which same chondrules, which is required to uniquely identify their are parts per 10,000 deviations from a terrestrial standard) for 30 precursor materials and associated formation histories. Also bulk meteorites, all of which have mass-independent O isotope absent in earlier studies is documentation of the properties analyses reported as Δ17O [a vertical deviation from the terres- (petrographic and geochemical) of individual chondrules that trial fractionation line in parts per 1,000 (18)] previously repor- provide a valuable aid in interpreting Ti and Cr isotope data. ted (Dataset S1 and Fig. 1). The data clearly define two distinct Thus, we designed a coordinated chemical and Ti-Cr-O isotopic isotopic groups: 1) noncarbonaceous meteorites, including investigation of individual chondrules extracted from Allende enstatite chondrites (ECs), ordinary chondrites (OCs), and most (CV) and Karoonda (CK) chondrites (3). While the 50Ti and differentiated meteorites known as achondrites (e.g., Moon, 54Cr of chondrules tracks the average composition of solids, the Mars, Vesta, angrites, ureilites, acapulcoites, lodranites, and analyses of O isotopes and mineral chemistry of major Mg sili- winonaites) and 2) carbonaceous meteorites [including carbo- cates (olivine and pyroxene) is useful to distinguish chondrules naceous chondrites and several ungrouped achondrites (19)]. that are similar to those in ordinary chondrites (13–16) as well as These two groups display orthogonal trends and are isolated precursor materials that may have formed in regions of the disk from each other by a large area in e50Ti-e54Cr-Δ17O isotope with variable redox conditions or with distinct proportions of space that is devoid of any bulk meteorite compositions (shown A C 15.00 1.00 OC CI 2-component Mars mixing lines 0.00 10.00 CAIs / AOAs EC Tagish Lake -1.00 Aca/Lod high 50Ti 50 O Ti high Ti 54 17 50 5.00 low Cr 54 missing high Cr Δ ε -2.00 reservoir CR CM Ure -3.00 0.00 low 50Ti CV high 54Cr non-carb.
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