New constraints from 26Al-26Mg chronology of anorthite bearing chondrules in unequilibrated ordinary chondrites
New constraints from 26Al-26Mg chronology of anorthite bearing chondrules in unequilibrated ordinary chondrites
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不平衡普通球粒陨石中含钙长石球粒的 26Al-26Mg 年代学的新约束
DOI：
10.1016/j.gca.2020.10.025
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发表时间：
2021
影响因子：
5
通讯作者：
Kita, Noriko T.
中科院分区：
文献类型：
--
作者：
Siron, Guillaume;Fukuda, Kohei;Kimura, Makoto;Kita, Noriko T.
本文测定了5种低岩石学亚型（3.00-3.05）的不平衡普通钙长石（UOC）中14个含钙长石球粒的~（26）Al-~（26）Mg年龄。此外，还测量了这些球粒的氧三种同位素。所选的球粒是高度亏损的碱性元素，钙长石是唯一的中稳态相，虽然它们显示出一系列的镁铁质矿物成分（镁# 76-97摩尔%），是代表球粒在UOC。这些球粒的平均Δ 17 O值介于−0.44 ± 0.23‰至0.49 ± 0.15‰之间，与先前对UOC中含斜长石球粒的研究结果一致。所有球粒中钙长石都具有可分解的过剩26 Mg（>1.0 ± 0.4‰）。他们推断的（27 Al/26 Al）0值范围为（6.3 ± 0.7）× 10− 6至（8.9 ± 0.3）× 10− 6，对应的球粒形成时间尺度为1.8 ± 0.04 Ma至2.16 ±0.12/0.11 Ma，使用标准（27 Al/26 Al）0值5.25 × 10−5进行CAIs。LL陨石中6个球粒的年龄限制在1.8 ~ 1.9 Ma之间，而L陨石中8个球粒的年龄在1.8 ~ 2.2 Ma之间，其中3个球粒的年龄为0.22 Ma，2个球粒的年龄为0.25 Ma。这可能是由于含钙长石球粒和典型的富含碱成分的UOC球粒形成之间的时间差。或者，年轻的球粒年龄在以前的研究可能是干扰的结果，铝镁系统在玻璃质中稳态，即使在低程度的热变质的母体。然而，高精度的年龄从这项研究（不确定性从0.04 Ma至0.15 Ma）表明，有可能是一个以上的球粒形成事件中所代表的研究人口。仅考虑来自LL陨石的数据，球粒形成年龄的有限持续时间（≤0.1 Ma）表明起源于高密度环境，随后导致母体形成。然而，与常见的富含碱的球粒相比，所研究的球粒的碱含量异常低，这也可能代表了盘中相对较低的尘埃密度下的早期球粒形成事件。UOC的主要球粒形成事件可能有滞后或同时与年轻的钙长石轴承球粒形成后2.15马CAIs，这是非常接近的普通球粒陨石母体，预计从普通球粒陨石母体的热演化的吸积时间。
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 Δ17O 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 excess26Mg (>1.0 ± 0.4‰). Their inferred (27Al/26Al)0range from (6.3 ± 0.7) × 10−6to (8.9 ± 0.3) × 10−6corresponding to a timescale for chondrule formation of 1.8 ± 0.04 Ma to 2.16 ±0.12/0.11Ma after CAIs using a canonical (27Al/26Al)0value 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.