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首页> 外文期刊>Contributions to Mineralogy and Petrology >Incongruent melting of biotite to spinel in quartz-free restite at EI Joyazo (SE Spain): Textures and reaction characterization
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Incongruent melting of biotite to spinel in quartz-free restite at EI Joyazo (SE Spain): Textures and reaction characterization

机译：EI Joyazo（SE西班牙）的无石英重晶石中黑云母与尖晶石的熔融不均匀：结构和反应特性

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摘要

In the Grt-Bt-Sil restitic xenoliths of El Joyazo (Cerro de Hoyazo), hercynitic spinel is a minor phase commonly associated with biotite. The possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their Contact with patches of fibrolitic sillimanite and rhyolitic melt. In these textures, resorbed biotite crystals about 1 mm long are rimmed by a layer of glass <200 #mu# m thick containing spinel and ilmenite; the same glass also fills embayments in biotite. Spinel forms euhedral crystals <100 #mu# m in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with spinel. The homogeneous felt-like melt-sillimanite aggregate ("mix") is richest in glass close to the reaction rim around biotite. Plagioclase and garnet are located> 5 mm away from the reaction texture. Biotite is chemically zoned. Cores (Bt_1) have X_(Mg) = 0.35 +-0.02 and Ti = 0.58 +-0.01 atoms; whereas the outer rims (Bt_2) have X_(Mg) = 0.45+-0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel (Spi) has low ZnO content (<0.80 wt percent) and X_(Mg)= 0.26 +-0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. This melt (melt_1) is different from that of the layer around biotite (melt_2), which is also richer in Ca and alkalis. Garnet rims (Grt) have low Ca and Mn, and X_(Mg) = 0.14. Plagioclase is characterized by large homogeneous cores (Pl_1, An_(31+-2)) and more calcic rims (P1_2, An_(49 +-6). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt_1-Bt_2-Grt-Spl-Ilm-melt_2-mix-P1_1-Pl_2) system provides the mass balance (in mole units):0.78Bt_1 + 1 .95P1_1 ~ 2.82mix + 0.32Grt = 0.25I1m + 5.00melt_2 + 0.30Bt_2 + 2.53Sp1 + 1.03P1_2This relationship is in excellent agreement with the observed textures and hence is considered a good model for the incongruent melting of biotite in the xenoliths. The mass-balance indicates that melt production is dominated by the availability of K from biotite, and that garnet and plagioclase must be involved as reactants, so that the reaction volume is larger than the melt production site. The melting of biotite, constrained at T = 900-950 deg C and P >= 5 kbar, is not a terminal reaction, as its variance in the reduced 8-component multi-system is >=3.
机译：在El Joyazo（Cerro de Hoyazo）的Grt-Bt-Sil复方异岩中，角铁尖晶石是通常与黑云母有关的次要相。在黑云母与纤维状硅线石和流纹岩熔体接触时，围绕黑云母形成的反应织构研究了黑云母和尖晶石之间可能的反应关系。在这些纹理中，被吸收的黑云母晶体长约1 mm，被一层厚度小于200＃μm的尖晶石和钛铁矿的玻璃覆盖。同样的玻璃杯也可以填充黑云母中的东西。尖晶石形成小于100＃μm的真面状晶体，钛铁矿以较小的反面晶体或针状出现，通常与尖晶石共生。均匀的毡状熔融硅线石聚集体（“混合物”）在黑云母周围反应边缘附近的玻璃中含量最高。斜长石和石榴石的位置距离反应组织5 mm。黑云母是化学分区的。核（Bt_1）的X_（Mg）= 0.35 + -0.02和Ti = 0.58 + -0.01原子;而外缘（Bt_2）的X_（Mg）= 0.45 + -0.01，Ti最多可达0.68个原子。富含海藻土的尖晶石（Spi）的ZnO含量低（<0.80 wt％），并且X_（Mg）= 0.26 + -0.04。混合骨料的化学成分代表硅线石和富含二氧化硅的熔体之间的线性组合。这种熔体（熔体_1）与黑云母（熔体_2）周围的层不同，后者也富含钙和碱。石榴石轮缘（Grt）的Ca和Mn低，X_（Mg）= 0.14。斜长石的特征是具有较大的均质核（Pl_1，An_（31 + -2））和更多的钙化边缘（P1_2，An_（49 + -6）。9组分（Al-Ca-Fe-K-Mg -Mn-Na-Si-Ti），9相（Bt_1-Bt_2-Grt-Spl-Ilm-melt_2-mix-P1_1-Pl_2）系统可提供质量平衡（以摩尔单位计）：0.78Bt_1 + 1.95P1_1〜 2.82mix + 0.32Grt = 0.25I1m + 5.00melt_2 + 0.30Bt_2 + 2.53Sp1 + 1.03P1_2这种关系与观察到的纹理非常吻合，因此被认为是黑云母在异种石中不一致熔融的良好模型。表明黑云母的熔解是由黑云母中钾的可利用性决定的，并且石榴石和斜长石必须作为反应物参与进来，因此反应体积要大于熔体的产生部位，黑云母的熔解受T = 900-950的限制。 ℃和P> = 5 kbar，不是最终反应，因为在还原的8组分多系统中，其方差> = 3。

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