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Chronological constraints on the thermal evolution of ordinary chondrite parent bodies from the 53Mn-53Cr system
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-05-01 , DOI: 10.1016/j.gca.2021.04.029
Aryavart Anand , Jonas Pape , Martin Wille , Klaus Mezger

The 53Mn-53Cr isotope systematics in ordinary chondrites constrains the accretion and thermal history of their parent bodies. Mineralogical observations and olivine-spinel geothermometry suggest that chromite in ordinary chondrites formed during prograde thermal metamorphism with the amount of chromite increasing with petrologic grades in type 3 to type 6 ordinary chondrites. Assuming a chondritic evolution of the respective parent bodies, 53Cr/52Cr model ages for chromite range from 3.99-0.79+0.93 to 11.1-2.8+6.0 Ma after the formation of calcium-aluminium-rich inclusions (CAIs). Chromite and silicate-metal-sulphide isochrons define an age range from 3.49-0.50+0.55 to 16.1-1.6+2.4 Ma. Both chromite model ages and isochron ages correlate with the petrological grade of the samples, which is consistent with an onion-shell structure of the chondrite parent bodies. The study shows that unlike the isochron ages, which are prone to impact-related disturbances or partial re-equilibration during cooling from high temperatures, the chromite model ages are not easily affected by thermal metamorphism or later events and yield robust mineral growth ages. The results are consistent with a homogenous distribution of 53Mn and an initial canonical 53Mn/55Mn = 6.28 x 10-6. The estimated closure temperatures for the Mn-Cr system in chromites range from ∼760 °C for type 6 to ∼540-620 °C for type 3 ordinary chondrites. The high closure temperatures estimated for type 3 and type 6 ordinary chondrites imply that the chromite ages correspond to the peak metamorphic temperature reached during the thermal history of the chondrite parent bodies. The oldest chromite model age obtained for type 3 samples along with the established Al-Mg chondrule formation ages constrain the accretion of the parent bodies to > 2.1 Ma after CAI formation, implying that planetesimal accretion immediately followed chondrule formation.



中文翻译:

53 Mn- 53 Cr系统对普通球粒陨石母体热演化的时间限制

普通球粒陨石中的53 Mn- 53 Cr同位素系统限制了其母体的积聚和热历史。矿物学观察和橄榄石-脊柱地热仪表明,普通球粒岩中的铬铁矿是在渐进热变质过程中形成的,其铬铁矿的数量随岩石等级从3型到6型普通球粒岩的增加而增加。假设各个母体的软骨演化,铬铁矿的53 Cr / 52 Cr模型年龄为3.99--0.79+0.9311.1--2.8+6.0富含钙铝夹杂物(CAIs)形成后。亚铬酸盐和硅酸盐-金属-硫化物等时线定义的年龄范围为3.49--0.50+0.5516.1--1.6+2.4嘛。亚铬酸盐模型年龄和等时年龄都与样品的岩石学等级相关,这与球粒陨石母体的洋葱壳结构一致。研究表明,与等时年龄不同,等时年龄在高温冷却过程中容易受到冲击相关的干扰或部分重新平衡,亚铬铁矿模型年龄不易受热变质作用或后来的​​事件影响,并产生稳健的矿物生长年龄。结果与均匀分布的53 Mn和初始标准的53 Mn / 55 Mn = 6.28 x 10 -6一致。铬铁矿中Mn-Cr系统的估计关闭温度范围从6型的〜760°C到3型普通球粒石的〜540-620°C。估计3型和6型普通球粒陨石的高关闭温度意味着铬铁矿的年龄对应于球粒陨石母体在热过程中达到的峰值变质温度。从3型样品获得的最古老的亚铬铁矿模型年龄以及既定的Al-Mg软骨形成年龄,将CAI形成后母体的积聚限制在> 2.1 Ma,这意味着行星状沉积物随软骨形成而立即增加。

更新日期:2021-05-02
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