Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite
Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite
复制标题
DOI：
10.1016/j.gca.2017.05.035
复制
发表时间：
2018-01-15
影响因子：
5
通讯作者：
Yurimoto, Hisayoshi
中科院分区：
文献类型：
--
作者：
Kawasaki, Noriyuki;Simon, Steven B.;Yurimoto, Hisayoshi
TS34为Alende CV3球粒陨石中的B1型富钙铝包裹体，由尖晶石、黄长石、富钛铝单斜辉石(方沸石)和少量火成结构的斜长岩组成。用二次离子质谱仪测定了TS34所有主要矿物中已知化学成分的点的氧和镁同位素组成。利用动态结晶实验的形成序列和黄长石、方沸石的化学成分，建立了CaI熔体的氧同位素演化。组成矿物的氧同位素组成沿碳质球粒陨石无水矿物线分布。尖晶石颗粒均匀富O-16(Delta O-17=-22.7+/-1.7%，2SD)，而黄长石颗粒均匀贫O-16(Delta O-17=-2.8+/-1.8‰)，与镁铁闪石含量和结晶相对时间无关。方沸石晶体呈生长环带，叠加不发达的扇形环带，一般由富钛组分生长到贫钛组分。方沸石晶体还显示出沿推断的晶体生长方向在O-17增量上的连续变化，从贫16O(O-17增量类似于-3‰)到富O-16(O-17增量类似于-23‰)，覆盖了TS34中观察到的所有氧同位素组成。早期结晶的贫O-16方沸石和黄长石处于氧同位素平衡状态，同时形成。氧同位素组成与方沸石中钛含量的相关性表明，在方沸石结晶过程中，CaI熔体的氧同位素组成由贫O-16演化为富O-16，可能是与周围富O-16星云气体的氧同位素交换所致。尖晶石的形成早于所有其他相的结晶，所以它的富含O-16的成分是早期阶段的遗迹。斜长岩的氧同位素组成在Delta O-17之间，接近于-2‰~-9‰，在方沸石的氧同位素组成范围内，表明这两种矿物在方沸石生长的早期和中期是共结晶的。黄长石和方沸石产生Al-26-Mg-26矿物等时线，其初始值为(Al-26/Al-27)(0)=(5.003+/-0.075)×10(-5)，对应于中国地质调查局典型铝镁年龄的相对年龄0.05Myr+/-0.02Myr。这些数据表明，在太阳系诞生后，太阳星云中同时存在富O-16和贫O-16的蓄水池，至少类似于0.05Myr。(三)2017年提交人(S)。爱思唯尔有限公司出版。
TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in TS34. Using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite, the oxygen isotopic evolution of the CAI melt was established. Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly O-16-rich (Delta O-17= -22.7 +/- 1.7%, 2SD), while the melilite grains are uniformly O-16-poor (Delta O-17= -2.8 +/- 1.8 parts per thousand) irrespective of their akermanite content and thus their relative time of crystallization. The fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow from Ti-rich to Ti-poor compositions. The fassaite crystals also show continuous variations in Delta O-17 along the inferred directions of crystal growth, from 16O-poor (Delta O-17 similar to-3 parts per thousand) to O-16-rich (Delta O-17 similar to-23 parts per thousand), covering the full range of oxygen isotopic compositions observed in TS34. The early-crystallized O-16-poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously. The correlation of oxygen isotopic compositions with Ti content in the fassaite imply that the oxygen isotopic composition of the CAI melt evolved from O-16-poor to O-16-rich during fassaite crystallization, presumably due to oxygen isotope exchange with a surrounding O-16-rich nebular gas. Formation of spinel, the liquidus phase in melts of this composition, predates crystallization of all other phases, so its O-16-rich composition is a relic of an earlier stage. Anorthite exhibits oxygen isotopic compositions between Delta O-17 similar to-2 parts per thousand and -9 parts per thousand, within the range of those of fassaite, indicating co-crystallization of these two minerals during the earliest to intermediate stage of fassaite growth. The melilite and fassaite yield an Al-26-Mg-26 mineral isochron with an initial value of (Al-26/Al-27)(0) = (5.003 +/- 0.075) x 10(-5), corresponding to a relative age of 0.05 +/- 0.02 Myr from the canonical Al-Mg age of CAIs. These data demonstrate that both O-16-rich and O-16-poor reservoirs existed in the solar nebula at least similar to 0.05 Myr after the birth of the Solar System. (C) 2017 The Author(s). Published by Elsevier Ltd.