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Aluminum-26 chronology of dust coagulation and early solar system evolution.

ABSTRACT: Dust condensation and coagulation in the early solar system are the first steps toward forming the terrestrial planets, but the time scales of these processes remain poorly constrained. Through isotopic analysis of small Ca-Al-rich inclusions (CAIs) (30 to 100 μm in size) found in one of the most pristine chondrites, Allan Hills A77307 (CO3.0), for the short-lived ²⁶Al-²⁶Mg [t _1/2 = 0.72 million years (Ma)] system, we have identified two main populations of samples characterized by well-defined ²⁶Al/²⁷Al = 5.40 (±0.13) × 10^-5 and 4.89 (±0.10) × 10^-5. The result of the first population suggests a 50,000-year time scale between the condensation of micrometer-sized dust and formation of inclusions tens of micrometers in size. The 100,000-year time gap calculated from the above two ²⁶Al/²⁷Al ratios could also represent the duration for the Sun being a class I source.

SUBMITTER: Liu MC

PROVIDER: S-EPMC6739091 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Aluminum-26 chronology of dust coagulation and early solar system evolution.

Liu M-C MC Han J J Brearley A J AJ Hertwig A T AT

Science advances 20190911 9

Dust condensation and coagulation in the early solar system are the first steps toward forming the terrestrial planets, but the time scales of these processes remain poorly constrained. Through isotopic analysis of small Ca-Al-rich inclusions (CAIs) (30 to 100 μm in size) found in one of the most pristine chondrites, Allan Hills A77307 (CO3.0), for the short-lived 26Al-26Mg [t 1/2 = 0.72 million years (Ma)] system, we have identified two main populations o ...[more]

PMID: 31535020

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Project description:26Al-26Mg ages were determined for 14 anorthite-bearing chondrules from five different unequilibrated ordinary chondrites (UOCs) with low petrologic subtypes (3.00-3.05). In addition, oxygen three isotopes of these chondrules were also measured. The selected chondrules are highly depleted in alkali elements, and anorthite is the only mesostasis phase, though they show a range of mafic mineral compositions (Mg# 76-97 mole%) that are representative of chondrules in UOCs. The mean ∆17O values in these chondrules range from -0.44 ± 0.23‰ to 0.49 ± 0.15‰, in good agreement with previous studies of plagioclase-bearing chondrules from UOCs. Anorthite in all chondrules exhibit resolvable excess 26Mg (> 1.0 ± 0.4‰). Their inferred (27Al/26Al)0 range from (6.3 ± 0.7)×10-6 to (8.9 ± 0.3)×10-6 corresponding to a timescale for chondrule formation of 1.8 ± 0.04 Ma to 2.16 ± 0.12/0.11 Ma after CAIs using a canonical (27Al/26Al)0 value of 5.25×10-5. The ages from six chondrules in LL chondrites are restricted to between 1.8 Ma and 1.9 Ma, whereas eight chondrules in L chondrites show ages from 1.8 Ma to 2.2 Ma, including three chondrules at ~2.0 Ma and two chondrules at ~2.15 Ma. The inferred chondrule formation ages from this study are at the peak of those previously determined for UOC chondrules, though with much shorter durations. This is potentially due to the time difference between formation of anorthite-bearing chondrules and typical UOC chondrules with alkali-rich compositions. Alternatively, younger chondrules ages in previous studies could have been the result of disturbance to the Al-Mg system in glassy mesostasis even at the low degree of thermal metamorphism in the parent bodies. Nevertheless, the high precision ages from this study (with uncertainties from 0.04 Ma to 0.15 Ma) indicate that there was potentially more than one chondrule forming event represented in the studied population. Considering data from LL chondrites only, the restricted duration (≤0.1 Ma) of chondrule formation ages suggests an origin in high density environments that subsequently lead to parent body formation. However, the unusually low alkali contents of the studied chondrules compared to common alkali-rich chondrules could also represent earlier chondrule formation events under relatively lower dust densities in the disk. Major chondrule forming events for UOCs might have postdated or concurrent with the younger anorthite-bearing chondrule formation at 2.15 Ma after CAIs, which are very close to the timing of accretion of ordinary chondrite parent bodies that are expected from thermal evolution of ordinary chondrite parent bodies.

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Aluminum-26 chronology of dust coagulation and early solar system evolution.

Publications

Aluminum-26 chronology of dust coagulation and early solar system evolution.

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