Distribution of 26Al in the CR chondrite chondrule-forming region of the protoplanetary disk

Abstract

We report on the mineralogy, petrography, and in situ measured oxygen-and magnesiumisotope compositions of eight porphyritic chondrules (seven FeO-poor and one FeO-rich) from the Renazzo-like carbonaceous (CR) chondrites Graves Nunataks 95229, Grosvenor Mountains 03116, Pecora Escarpment 91082, and Queen Alexandra Range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. We find no evidence that these processes modified the oxygen-or Al-Mg isotope systematics of chondrules in these meteorites. Olivine, low-Ca pyroxene, and plagioclase within an individual chondrule have similar Oisotope compositions, suggesting crystallization from isotopically uniform melts. The only exceptions are relict grains in two of the chondrules; these grains are 16 O-enriched relative to phenocrysts of the host chondrules. Only the FeO-rich chondrule shows a resolvable excesses of 26 Mg, corresponding to an inferred initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of (2.5±1.6)×10 6 (±2SE). Combining these results with the previously reported Al-Mg isotope systematics of CR chondrules (Nagashima et al., 2014, Geochem. J. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26 Mg; the presence of excess 26 Mg does not correlate with the FeO content of chondrule silicates. In contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0-3.1) unequilibrated ordinary chondrites (UOCs), Ornans-like carbonaceous (CO) chondrites, and the ungrouped carbonaceous chondrite Acfer 094 show resolvable excesses of 26 Mg. The inferred ( 26 Al/ 27 Al) 0 in CR chondrules with resolvable excesses of 26 Mg range from (1.0±0.4)×10 6 to (6.3±0.9)×10 6 , which is typically lower than ( 26 Al/ 27 Al) 0 in the majority of chondrules from UOCs, COs, and Acfer 094. Based on the inferred ( 26 Al/ 27 Al) 0 , three populations of CR chondrules are recognized; the population characterized by low ( 26 Al/ 27 Al) 0 (<3×10 6 ) is dominant. There are no noticeable trends with major and minor element or Oisotope compositions between these populations. The weighted mean ( 26 Al/ 27 Al) 0 of 22 CR chondrules measured is (1.8±0.3)×10 6 . An apparent agreement between the 26 Al-26 Mg ages (using weighted mean value) and the revised (using 238 U/ 235 U ratio for bulk CR chondrites of 137.7789±0.0085) 207 Pb-206 Pb age of a set of chondrules from CR chondrites (Amelin et al., 2002, Science 297, 1678) is consistent with the initial 26 Al/ 27 Al ratio in the CR chondrite chondrule-forming region at the canonical level (~5.210 5 ), allowing the use of 26 Mg systematics as a chronometer for CR chondrules. To prove chronological significance of 26 Al for CR chondrules, measurements of Al-Mg and U-Pb isotope systematics on individual chondrules are required. The presence of several generations among CR chondrules indicates some chondrules that accreted into the CR chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relativ...

Age of Jupiter inferred from the distinct genetics and formation times of meteorites

“…Our results show that this scattering of CC bodies and, hence, Jupiter's outward migration or runaway growth cannot have started before ∼3-4 My after CAI formation. This is because CC chondrite parent bodies continued to form until at least ∼3-4 My after CAI formation (18)(19)(20). As these chondrites plot on the CC line in Mo isotope space (Fig.…”

“…1), no meteorites plot between the CC and NC lines, meaning that the NC and CC reservoirs cannot have mixed but instead must have remained isolated from each other until parent body accretion in the NC and CC reservoirs terminated. As accretion of chondrite parent bodies occurred at ∼2 My after CAI formation in the NC reservoir (ordinary chondrites) and until ∼3-4 My in the CC reservoir (CC chondrites) (18)(19)(20), this means that the NC and CC reservoirs must have remained isolated from each other from <1 My until at least ∼3-4 My after CAI formation. This prolonged spatial separation of the NC and CC reservoirs cannot simply reflect a large distance between these reservoirs within the disk, because the rapid speed of grain drift in the disk would have facilitated efficient mixing on much shorter timescales (21,22).…”

Refractory inclusions in carbonaceous chondrites: Records of early solar system processes

“…Bollard and co-authors conclude that 26 Al was heterogeneously distributed in PPD, and therefore cannot be used as a chronometer. For a different view, see Kita et al (2015), Schrader et al (2017), and Nagashima et al (2018). Verification of these results by other research groups is urgently needed.…”

“…Therefore, it is rather difficult to estimate the possible number of chondrule generations present in a chondrite group (e.g., Schrader et al. ), which could potentially provide important constraints on the nature of chondrule‐forming mechanisms and styles of accretion of the chondrite parent bodies (e.g., Johansen et al. ).…”

“…Due to small sizes of Al-rich phases (plagioclase and glass) in chondrules suitable for in situ measurements of Al-Mg isotope systematics, the internal 26 Al- 26 Mg isochrons were measured so far only in a limited number of chondrules in an individual chondrite and to a relatively low precision and references therein). Therefore, it is rather difficult to estimate the possible number of chondrule generations present in a chondrite group (e.g., Schrader et al 2017), which could potentially provide important constraints on the nature of chondrule-forming mechanisms and styles of accretion of the chondrite parent bodies (e.g., Johansen et al 2015). Usage of a more powerful and stable Hyperion radiofrequency O-ion source on the large geometry SIMS instruments (Cameca ims-1270, 1280, 1300) could potentially help to resolve these issues (e.g., Hertwig et al 2018a;Kita et al 2018).…”