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Mineralogy, petrography, and oxygen and aluminum-magnesium isotope systematics of grossite-bearing refractory inclusions
Geochemistry ( IF 3.7 ) Pub Date : 2019-08-15 , DOI: 10.1016/j.chemer.2019.08.001
Alexander N. Krot , Kazuhide Nagashima , Steven B. Simon , Chi Ma , Harold C. Connolly , Gary R. Huss , Andrew M. Davis , Martin Bizzarro

Grossite (CaAl4O7) is one of the one of the first minerals predicted to condense from a gas of solar composition, and therefore could have recorded isotopic compositions of reservoirs during the earliest stages of the Solar System evolution. Grossite-bearing Ca,Al-rich inclusions (CAIs) are a relatively rare type of refractory inclusions in most carbonaceous chondrite groups, except CHs, where they are dominant. We report new and summarize the existing data on the mineralogy, petrography, oxygen and aluminum-magnesium isotope systematics of grossite-bearing CAIs from the CR, CH, CB, CM, CO, and CV carbonaceous chondrites. Grossite-bearing CAIs from unmetamorphosed (petrologic type 2―3.0) carbonaceous chondrites preserved evidence for heterogeneous distribution of 26Al in the protoplanetary disk. The inferred initial 26Al/27Al ratio [(26Al/27Al)0] in grossite-bearing CAIs is generally bimodal, ˜0 and ˜5×10−5; the intermediate values are rare. CH and CB chondrites are the only groups where vast majority of grossite-bearing CAIs lacks resolvable excess of radiogenic 26Mg. Grossite-bearing CAIs with approximately the canonical (26Al/27Al)0 of ˜5×10−5 are dominant in other chondrite groups. Most grossite-bearing CAIs in type 2–3.0 carbonaceous chondrites have uniform solar-like O-isotope compositions (Δ17O ˜ ‒24±2‰). Grossite-bearing CAIs surrounded by Wark-Lovering rims in CH chondrites are also isotopically uniform, but show a large range of Δ17O, from ˜ ‒40‰ to ˜ ‒5‰, suggesting an early generation of gaseous reservoirs with different oxygen-isotope compositions in the protoplanetary disk. Igneous grossite-bearing CAIs surrounded by igneous rims of ±melilite, Al-diopside, and Ca-rich forsterite, found only in CB and CH chondrites, have uniform 16O-depleted compositions (Δ17O ˜ ‒14‰ to ‒5‰). These CAIs appear to have experienced complete melting and incomplete O-isotope exchange with a 16O-poor (Δ17O ˜ ‒2‰) gas in the CB impact plume generated about 5 Ma after CV CAIs. Grossite-bearing CAIs in metamorphosed (petrologic type >3.0) CO and CV chondrites have heterogeneous Δ17O resulted from mineralogically-controlled isotope exchange with a 16O-poor (Δ17O ˜ ‒2 to 0‰) aqueous fluid on the CO and CV parent asteroids 3–5 Ma after CV CAIs. This exchange affected grossite, krotite, melilite, and perovskite; corundum, hibonite, spinel, diopside, forsterite, and enstatite preserved their initial O-isotope compositions. The internal 26Al-26Mg isochrons in grossite-bearing CAIs from weakly-metamorphosed CO and CV chondrites were not disturbed during this oxygen-isotope exchange.

HCCJr is grateful to Klaus Keil for all his sound profession counsel and collegial friendship over the years. He has always been willing to talk and has the generous nature of listening and sharing his thoughts freely and constructively. Professor Klaus Keil has been a mentor to and played a key role in the careers of three of the authors of this paper (ANK, KN, and GRH). He has also influenced the careers of the other authors and most of the people who have worked on meteorites over the past 50+ years. We therefore dedicate this paper to Professor Keil and present it in this Special Issue of Geochemistry.



中文翻译:

含矿物的难熔夹杂物的矿物学,岩石学以及氧和铝镁同位素系统

钙石(CaAl 4 O 7)是预计会从太阳组成气体中凝结的首批矿物之一,因此可能在太阳系演化的最早阶段记录了储层的同位素组成。除CHs占主导地位外,在大多数含碳球粒陨石群中,含钙石的富含Ca,Al的夹杂物(CAIs)是一种相对罕见的难熔夹杂物。我们报告了新的资料,并总结了来自CR,CH,CB,CM,CO和CV碳质球粒陨石的含钙长石CAI的矿物学,岩石学,氧和铝镁同位素系统的现有数据。来自未变质的(岩石类型2-3〜)碳质球粒陨石的含钙质岩的CAI保留了异质分布的证据26原行星盘中的A1。推断的含钙长石的CAI中的初始26 Al / 27 Al比[(26 Al / 27 Al)0 ]通常是双峰的,分别为〜0和〜5×10 -5;中间值很少。CH和CB球粒陨石是仅有的绝大多数含钙橄榄石的CAI缺乏可解析过量的放射源26 Mg的群体。在其他球粒陨石群中,具有大约(5×10 -5)的规范(26 Al / 27 Al)0的含钙石的CAI占主导地位。最grossite轴承的CAIs在类型2-3.0含碳陨石具有均匀的太阳能型O形同位素组合物(Δ 17O〜‒24±2‰)。Grossite轴承的CAIs包围沃克-洛夫林轮圈在CH陨石也同位素均匀,但显示在大范围的Δ 17 O,从〜-40‰至〜-5‰,表明早期一代气态储层的不同氧原行星盘中的同位素组成。火成岩grossite轴承的CAIs包围±黄长石,铝-透辉石,和富钙镁橄榄石的火成岩轮辋,发现仅在CB和CH陨石,具有均匀的16 O型耗尽组合物(Δ 17 O〜-14‰至-5‰ )。这些CAIs在出现有经验的完全融化和不完整的O型同位素交换用16 O型差(Δ 17CV CAI后,CB冲击羽中的O〜‰2‰气体产生。Grossite轴承的CAIs在变质(岩石类型> 3.0)CO和CV陨石具有异质Δ 17 ö导致从与矿物学控制同位素交换16 O型差(Δ 17 O〜-2至0‰)的水溶液对CO流体和CV CAI后3-5 Ma的CV亲本小行星。这种交换影响了钙铁矿,钾铁矿,陨石和钙钛矿。刚玉,堇青石,尖晶石,透辉石,镁橄榄石和顽辉石保留了它们最初的O同位素组成。在这种氧-同位素交换过程中,由弱变质的CO和CV球粒陨石产生的含钙铁矿的CAI内部26 Al- 26 Mg等时线没有受到干扰。

HCCJr感谢克劳斯·基尔(Klaus Keil)多年来的所有声音专业顾问和同事间的友谊。他一直乐于交谈,并且乐于助人,慷慨地倾听和自由建设性地分享自己的思想。克劳斯·凯尔(Klaus Keil)教授一直是该论文三位作者(ANK,KN和GRH)的导师,并在其职业生涯中发挥了关键作用。他还影响了其他作者和过去50多年从事陨石工作的大多数人的职业生涯。因此,我们将这篇论文献给Keil教授,并在本期《地球化学》上发表

更新日期:2020-04-21
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