Nb and Ta intracrustal differentiation during granulite-facies metamorphism: Evidence from geochemical data of natural rocks and thermodynamic modeling
Nb and Ta intracrustal differentiation during granulite-facies metamorphism: Evidence from geochemical data of natural rocks and thermodynamic modeling
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麻粒岩相变质作用过程中 Nb 和 Ta 的壳内分异：来自天然岩石地球化学数据和热力学模型的证据
DOI：
10.2138/am-2022-8260
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发表时间：
2022-04
影响因子：
3.1
通讯作者：
Lei Zhao
中科院分区：
文献类型：
--
作者：
Guangyu Huang;Yi Chen;Jinghui Guo;Richard Palin;Lei Zhao
大陆地壳和贫化地幔的特征都是球粒下 Nb/Ta，与大块地球相比，导致质量不平衡。尽管人们已经提出在地核和未耗尽的地幔中存在几个潜在的高铌/钽储层，但很少有人关注地壳中的储层。在这里，我们提供了华北克拉通下地壳泥质麻粒岩样品的大块岩石和金红石地球化学数据，这些样品的 Nb 和 Ta 含量表现出系统变化。高温（HT）和超高温（UHT）麻粒岩残渣的 Nb/Ta 比率分别接近球粒状和球粒状下方，而 UHT 麻粒岩中的隐色体大多具有球粒状上方的 Nb/Ta。这些变化最好是通过变质作用过程中黑云母和金红石之间对 Nb 和 Ta 的竞争来解释，尽管初始块岩 Nb/Ta 值也有影响。由于黑云母优先吸收 Nb 而不是 Ta，因此黑云母脱水熔化的早期阶段会产生高 Nb/Ta 残渣和低 Nb/Ta 熔体；然而，地球化学模型表明，一旦黑云母耗尽，系统的 Nb/Ta 比率反而受到金红石生长的控制，这促进了较低 Nb/Ta 残留物和较高 Nb/Ta 熔体的形成，尽管此阶段产生的熔体体积可能很小。我们认为，超高温处理地体中的原位和源内无色体和无色脉可能保留高 Nb/Ta 地球化学特征。然而，残留地壳衍生的 A2 型花岗岩在结晶过程中经历了含铌或钽矿物的显着分异或受到其他低铌/钽来源的污染，无法保持这种高铌/钽比率，即使这些比率通常高于 S 型花岗岩。变钙岩的无水部分熔融可以生成富铌熔体，因此除了相关的 A2 型花岗岩之外，高温无色体可能代表着重要的铌矿床。
Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance when compared to the bulk Earth. Even though several potential high-Nb/Ta reservoirs in Earth’s core and undepleted mantle have been proposed, little attention has been given to those in the crust. Here we present bulk-rock and rutile geochemical data for samples from a lower crustal pelitic granulite, North China Craton, which exhibit systematic variation in their Nb and Ta contents. High-temperature (HT) and ultrahigh-temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively, whereas leucosomes from UHT granulite mostly have suprachondritic Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. As biotite preferentially incorporates Nb over Ta, the early stages of biotite dehydration melting produce a high-Nb/Ta residue and a low-Nb/Ta melt; however, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes the formation of a lower Nb/Ta residue and a higher Nb/Ta melt, even though the volume of melt produced at this stage may be small. We propose that in situ and in-source leucosomes and leucocractic veins in UHT terranes may retain a high-Nb/Ta geochemical signature. However, residual crustal-derived A2-type granites that experience significant fractionation of Nb- or Ta-bearing minerals during crystallization or contamination from other low-Nb/Ta sources cannot retain this high-Nb/Ta ratio, even though these ratios are generally higher than that of S-type granites. Anhydrous partial melting of metapelite can generate Nb-rich melts, such that high-temperature leucosomes, in addition to related A2-type granites, may represent significant Nb deposits.