Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions
Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions
复制标题
变形虫橄榄石聚集体中低钙辉石的起源：来自氧同位素组成的证据
DOI：
10.1016/j.gca.2004.06.046
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发表时间：
2005
影响因子：
5
通讯作者：
H. Yurimoto
中科院分区：
文献类型：
--
作者：
A. Krot;T. Fagan;K. Nagashima;M. I. Petaev;H. Yurimoto
原始碳质辉长岩中的异形橄榄石集合体（AOA）由镁橄榄石（Fa<2）、铁、镍、尖晶石、铝透辉石、钙长石和罕见的钙黄长石（δ k <15）组成。·10%的AOA含有低钙辉石（Fs 1 - 3 Wo 1 -5）与镁橄榄石有蚀变关系，主要有三种结构形态：（1）AOA外围镁橄榄石颗粒周围或沿着镁橄榄石晶界的薄（<15 μm）不连续层;（ii）AOA外围Fe、Ni金属结核周围5-10 μ m厚的晕和半自形颗粒;（iii）厚度不等的壳（最大70 μm），通常在AOA周围含有大量细小（3-5 μm）的Fe、Ni金属颗粒夹杂物。具有低钙辉石壳层的AOA结构致密，由钙长石包围的自形铝透辉石晶粒组成，表明熔融程度较小（10%-20%）。具有其他低钙辉石结构的AOA相当多孔。含低钙辉石的AOA中的镁橄榄石颗粒通常具有富含16 O的同位素组成（Δ 17 O < −20‰）。结构产状（i）和（ii）的低钙辉石富16 O（Δ 17 O < −20‰），而（iii）的低钙辉石贫16 O（Δ 17 O = −6‰ ~ −4‰）。其中一个大范围熔化（>50%）的天体在结构和矿物学上介于AOA和富铝球粒之间。它由自形镁橄榄石颗粒，pigeonite，普通辉石，钙长石中稳态，丰富的自形尖晶石颗粒，和少量的铁，镍金属;它被一个粗粒的火成岩边缘包围，主要由低钙辉石和丰富的铁，镍金属硫化物结核。矿物学观察表明，只有尖晶石颗粒在这个火成岩物体没有熔化。尖晶石富含16 O（Δ 17 O <$−22‰），而邻近的斜长石中稳态则亏损16 O（Δ 17 O <$−11‰）。我们认为AOA是太阳星云凝聚物（镁橄榄石、铁、镍金属和由铝透辉石、钙长石、尖晶石和黄长石组成的CAI）的集合体，形成于富含16 O的气体储层中，可能是CAI形成区。某些AOA中的固态或初熔镁橄榄石与气态SiO在同一星云区反应生成低钙辉石。其他一些AOA似乎已经吸积了16 O-贫辉石标准尘埃，并经历了不同程度的熔融，最有可能是在球粒形成区。最广泛的熔融AOA经历了氧同位素交换与16 O-贫星云气体，并可能已转化为球粒。这种球粒的前体物质的原始富含16 O的特征只在不完全熔化的颗粒中保留下来。
Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk<15). ∼10% of AOAs contain low-Ca pyroxene (Fs1–3Wo1–5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5–10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3–5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%–20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally16O-rich isotopic compositions (Δ17O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are16O-enriched (Δ17O < −20‰), whereas those of (iii) are16O-depleted (Δ17O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is16O-rich (Δ17O ∼ −22‰), whereas the neighboring plagioclase mesostasis is16O-depleted (Δ17O ∼ −11‰). We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an16O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted16O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with16O-poor nebular gas and may have been transformed into chondrules. The original16O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.
DOI：
--
发表时间：
2005
期刊：
Astrophysical Journal 622
影响因子：
--
作者：
Krot;A.N.
通讯作者：
A.N.
DOI：
--
发表时间：
2004
期刊：
Science 305
影响因子：
--
作者：
Yurimoto H.;Kuramoto K.
通讯作者：
Kuramoto K.
从 26Al 推断的相对年龄与最小平衡球粒陨石中铁镁质球粒的整体成分之间的相关性。
DOI：
--
发表时间：
2003
期刊：
Meteorit.Planet.Sci. 38
影响因子：
--
作者：
Tachibana;S.;Nagahara;H.;Mostefaoui;S.;Kita;N.
通讯作者：
N.