A comparative study of melilite and fassaite in Types B1 and B2 refractory inclusions
A comparative study of melilite and fassaite in Types B1 and B2 refractory inclusions
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DOI：
10.1016/j.gca.2005.09.018
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发表时间：
2006-02
影响因子：
5
通讯作者：
S. Simon;L. Grossman
中科院分区：
文献类型：
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作者：
S. Simon;L. Grossman
大多数 B 型包裹体的岩石学数据来自 B1 型。 B2 矿物的产量相对较少，而且尚未对这两种最丰富的矿物的特性进行系统比较。在这项工作中，我们记录了 B2 型包裹体中黄长石和铁锰矿的成分和分带模式，并将它们与 B1 型包裹体中对应物的特征进行了比较和对比。我们发现 B2 型包裹体中的黄长石成分与 B1 型包裹体相似，最大 Åk 含量约为 75mol%，且 Åk 和 Na2O 含量呈正相关。不对称分区的黄长石在 B2 型中很常见，具有反向分区区域的黄长石晶粒也是如此，并且晶体的反向分区部分比 B1 中的更厚。在 B2 中，与 B1 一样，铁锰矿的 Ti、Sc 和 V 氧化物含量从晶粒核心到边缘逐渐降低。大约一半的 Ti 是三价的，但与 B1 中的不同，在 B2 中的铁锰矿晶粒内，Ti3+/(Ti3++Ti4+) 比率不会从核心到边缘降低，并且没有观察到 Ti3+ 和 V 的急剧富集（“尖峰”）。扇区划分的铁赛石在 B2 中比在 B1 中更为常见。我们观察到的差异可以通过 B1 和 B2 之间的本体成分差异来解释。 B2 型夹杂物往往具有更高的 SiO2 含量，因此比 B1 型夹杂物具有更高的 An/Ge 成分比。相平衡表明，与B1s相比，B2s中的黄长石应该在铁铁矿出现之前结晶，因此在B2s中，更高比例的黄长石与铁铁矿共结晶，导致更多的晶体反向分带；更多的黄长石在与其他晶体相邻时结晶，导致不对称的分区；由于可用液体较多，成分向生长中的铁赛岩的输送比 B1 更容易，从而促进晶体生长并产生扇区分区。缺乏 Ti3+/Titot 分带以及缺乏 Ti3+-、富含 V 的峰值表明，B2 型熔体在整个结晶过程中与星云气体保持平衡，而 B1 的内部可能与气体隔离，可能是通过其黄长石地幔。这使得 B1 和 B2 黄长石中 Na-Åk 关系的相似性难以理解，但显然黄长石地幔的包围对于 B 型耐火夹杂物结晶过程中 Na2O 的保留并不是必要的。
Most of the petrologic data available for Type B inclusions comes from Type B1s. Relatively little comes from the B2s, and there has not been a systematic comparison of the properties of their two most abundant minerals. In this work, we document the compositions and zoning patterns of melilite and fassaite in Type B2 inclusions, and compare and contrast them with the features of their counterparts in Type B1 inclusions. We find that melilite compositions in Type B2 inclusions are similar to those of Type B1s, with maximum Åk contents of ∼75mol% and a positive correlation between Åk and Na2O contents. Asymmetrically zoned melilite is common in Type B2s as are melilite grains with reversely zoned regions, and the reversely zoned portions of crystals are thicker than in B1s. In B2s, like B1s, fassaite is zoned with decreasing Ti, Sc, and V oxide contents from cores to rims of grains. Approximately half of the Ti is trivalent, but unlike that in B1s, within fassaite grains in B2s the Ti3+/(Ti3++Ti4+) ratio does not decrease from core to rim, and sharp enrichments (“spikes”) in Ti3+and V are not observed. Sector-zoned fassaite is much more common in B2s than in B1s. The differences we observed can be accounted for by the differences in bulk compositions between B1s and B2s. Type B2 inclusions tend to have higher SiO2contents, hence higher An/Ge component ratios, than Type B1s. Phase equilibria show that, compared to B1s, in B2s less melilite should crystallize prior to the appearance of fassaite, so that in B2s a higher proportion of melilite cocrystallizes with fassaite, causing more of the crystals to be reversely zoned; more melilite crystallizes while adjacent to other crystals, leading to asymmetrical zoning; and with more liquid available, transport of components to growing fassaite occurs more readily than in B1s, facilitating crystal growth and giving rise to sector zoning. The lack of zoning with respect to Ti3+/Titotand the absence of Ti3+-, V-rich spikes suggest that Type B2 melts maintained equilibrium with the nebular gas throughout crystallization, while the interiors of B1s were probably isolated from the gas, perhaps by their melilite mantles. This makes the similarity of Na-Åk relationships in B1 and B2 melilite difficult to understand, but apparently enclosure by melilite mantles was not necessary for the retention of Na2O during crystallization of Type B refractory inclusions.