Origins of Al-rich chondrules: Clues from a compound Al-rich chondrule in the Dar al Gani 978 carbonaceous chondrite
Origins of Al-rich chondrules: Clues from a compound Al-rich chondrule in the Dar al Gani 978 carbonaceous chondrite
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DOI：
10.1016/j.gca.2013.12.026
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发表时间：
2014-04
影响因子：
5
通讯作者：
A. Zhang;S. Itoh;N. Sakamoto;Rucheng Wang;H. Yurimoto
中科院分区：
文献类型：
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作者：
A. Zhang;S. Itoh;N. Sakamoto;Rucheng Wang;H. Yurimoto
富铝球粒是原始球粒陨石中最有趣的组成部分之一，因为它们具有与富钙、富铝包裹体 (CAI) 和铁镁质球粒相似的特征。然而，它们的前体和形成历史仍然受到很少的限制，特别是在它们的氧同位素分布方面。在这项研究中，我们报告了未分群球粒陨石 Dar al Gani (DaG) 978 中含蓝宝石的富铝球粒 (SARC) 的岩相学、矿物学、氧同位素比和稀土元素组成。SARC 具有复杂的核-地幔-边缘结构；虽然地核和地幔主要由富铝顽火辉石和钙长石组成，并有少量介稳态，但这些区域的特点是地核中存在富铁尖晶石和蓝宝石，而地幔中不存在富铁尖晶石和蓝宝石。 SARC 的边缘主要由富铁橄榄石、顽辉石和铁镍金属组成。 SARC中的尖晶石和一些橄榄石晶粒富含16O，Δ17O值分别低至-20‰和-23‰。顽火辉石、蓝宝石和大多数橄榄石颗粒具有相似的 Δ17O 值（～ -7‰），低于钙长石和中稳辉石（包括其中的辉石）（Δ17O：～ -3‰）。地核和地幔的中稳态均具有第二族稀土元素 (REE) 模式；然而，核心中稳态的稀土元素浓度高于地幔中稳态。这些观察结果有力地表明，SARC 是由 II 族 CAI 和铁镁球粒的材料混合物熔化和结晶形成的。富含16O特征的尖晶石和橄榄石都可能是孑遗物。富铝球粒中大部分成分的贫同位素组成可以通过球粒形成区熔化过程中熔体与贫 16 O 星云气体之间的氧同位素交换（Δ17O：∼ -7‰）来解释；而钙长石和中稳态可能与母体上 16 O 相对贫乏的储层（Δ 17 O：∼ -3‰）经历了进一步的氧同位素交换，可能是在流体辅助热变质作用期间。在同一热变质事件中，尖晶石、橄榄石、某些顽火辉石和介稳态经历了不同程度的镁-铁交换。
Aluminum-rich chondrules are one of the most interesting components of primitive chondrites, because they have characteristics that are similar to both Ca, Al-rich inclusions (CAIs) and ferromagnesian chondrules. However, their precursor and formation history remain poorly constrained, especially with respect to their oxygen isotopic distributions. In this study, we report on the petrography, mineralogy, oxygen isotope ratios, and rare-earth-element compositions of a sapphirine-bearing Al-rich chondrule (SARC) in the ungrouped chondrite Dar al Gani (DaG) 978. The SARC has a complex core-mantle-rim texture; while both the core and the mantle are mainly composed of Al-rich enstatite and anorthite with minor amounts of mesostasis, these regions are distinguished by the presence of Fe-rich spinel and sapphirine in the core and their absence in the mantle. The rim of the SARC consists mainly of Fe-rich olivine, enstatite, and Fe–Ni metal. Spinel and some olivine grains in the SARC are16O-rich, with Δ17O values down to −20‰ and −23‰, respectively. Enstatite, sapphirine, and most olivine grains have similar Δ17O values (∼ −7‰), which are lower than those of anorthite and the mesostasis (including augite therein) (Δ17O: ∼ −3‰). Mesostasis from both the core and mantle have Group II rare-earth-element (REE) patterns; however, the core mesostasis has higher REE concentrations than the mantle mesostasis. These observations provide a strong indication that the SARC formed by the melting and crystallization of a mixture of materials from Group II CAIs and ferromagnesian chondrules. Both spinel and olivine with16O-rich features could be of relict origin. The16O-poor isotopic compositions of most components in Al-rich chondrules can be explained by oxygen isotopic exchange between the melt and16O-poor nebular gas (Δ17O: ∼ −7‰) during melting in chondrule-forming regions; whereas the anorthite and mesostasis could have experienced further oxygen isotopic exchange with a relatively16O-poor reservoir (Δ17O: ∼ −3‰) on the parent body, likely during fluid-assisted thermal metamorphism. During the same thermal metamorphism event, spinel, olivine, some enstatite, and the mesostasis experienced Mg–Fe exchange to various extents.