Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction
Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction
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DOI：
10.1111/jmg.12560
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发表时间：
2020-08
影响因子：
3.4
通讯作者：
Nanfei Cheng;D. Jenkins
中科院分区：
文献类型：
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作者：
Nanfei Cheng;D. Jenkins
蓝片岩与榴辉岩之间的过渡在俯冲带中通过大洋板块的脱水和致密化作用起着重要作用。以前有许多岩石学研究描述了在过渡时期发生的潜在矿物反应。对这类反应的实验测定有助于限制转变的压力-温度条件以及脱水过程。然而，以前的实验贡献主要集中在玄武岩成分中自发形成的含水矿物的稳定性，而不是已经形成的蓝片岩相矿物之间的反应。因此，本研究开展了三组实验，探索蓝片岩相矿物在暖俯冲条件下的变质反应，在实验室实验的时间尺度上，蓝片岩相矿物在暖俯冲条件下的反应速率可能更快。第一组实验是在2.2 ~ 3.5 GPa、650 ~ 820℃范围内建立绿帘石+paragonite→翡翠+pyrope+石英+H2O反应的实验逆转。这一反应一直被认为是蓝片岩-榴辉岩转变的关键。然而，在paragonite和jade +石英的交叉稳定性场中，只观察到蓝透石+paragonite的生长，这证实了热力学计算表明该反应在Na2O-MgO-Al2O3-SiO2-H2O体系中是不稳定的。第二组实验是在低铁和富钙体系中使用蓝帘石+黝帘石±石英进行的非反向实验，温度为1.8-2.4 GPa，温度为600-780°C。这些过程产生了辉长石+paragonite/蓝晶石+H2O，同时钠角闪石的成分也发生了变化，如蓝晶石向钠钙角闪石转变，如winchite (8.8.cana)（Mg4Al)Si8O22(OH)2）和baroite (8.8.cana)(Mg3Al2)（AlSi7)O22(OH)2）。这表明发生了两步脱水，首先是蓝帘石+黝帘石分解成含paragonite的组合，然后是paragonite分解释放H2O。钠钙角闪孔可以与榴辉岩相共存，从而将角闪孔的热稳定性扩展到更大的俯冲带深度。第三组实验是在2.0 ~ 2.4 GPa和630 ~ 850℃条件下，对高铁（Fe#=Fetotal/(Fetotal+Mg)≈0.36）的天然蓝绿石、合成paragonite及其榴辉石形成反应产物进行实验研究。结果表明，在大多数压力-温度条件下，石榴石和辉辉石生长，这表明富铁蓝铜矿在形成石榴石+辉辉石的关键榴辉岩组合中的重要性，即使在温暖的俯冲带条件下也是如此。本研究的实验结果表明，在热俯冲过程中蓝片岩-榴辉岩转变过程中，蓝绿矾+黝帘石的反应对脱水过程起着重要的控制作用。
The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H2O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na2O–MgO–Al2O3–SiO2–H2O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H2O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (☐(CaNa)(Mg4Al)Si8O22(OH)2) and barroisite (☐(CaNa)(Mg3Al2)(AlSi7)O22(OH)2). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H2O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fetotal/(Fetotal+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.