Biotite plays an important role in the geochemical cycle of Li, Rb, Cs, and Ba in the upper continental crust, as it is a significant carrier of Li and large-ion lithophile elements in felsic igneous rocks and high-grade detrital metasedimentary rocks. During its interaction with meteoric and hydrothermal fluids, biotite can be transformed into various types of clay minerals (mostly, interlayer-deficient biotite, vermiculites and smectites). These transformations can cause fractionation of the alkaline trace-element ratios Rb/Li, Cs/Li and Rb/Cs between biotite and its replacement products. This study examines the mineral transformations that occur when biotite interacts with aqueous and saline fluids and the poorly understood geochemical behaviour of the resulting phyllosilicates. For this purpose, we performed batch hydrothermal experiments of the interaction of biotite + quartz ± graphite with ultrapure H2O, and 2 M NaCl, 2 M CaCl2 and 1 M NaF brine fluids at 170 °C and 10 bar using Teflon bombs, and at 550 °C and 800 to 1400 bar using autoclave apparatus. At lower-T conditions, biotite was replaced by 2:1 trioctahedral clay minerals (interlayer-deficient biotite, smectite, vermiculite, and other phyllosilicate species with higher interlayer charge) and Fe oxy-hydroxide minerals by coupled dissolution-precipitation mechanisms. At higher-T conditions, these mechanisms caused the transformation of biotite into the mineral assemblages (quartz ± graphite): diopside + anorthite + titanite (CaCl2 brine experiments), albite + ilmenite + clay minerals (NaCl brine experiments), and cryolite + alkali feldspar with albite rimmed by K-feldspar + Fe-oxides (NaF brine experiments). Therefore, a significant reduction of the clay mineral stability in the presence of NaF and CaCl2 brine fluids is inferred. The biotite replacements by phyllosilicates were mostly controlled by the ion exchange of K+ by H+ (or its hydrate state H3O+), hydrated Na+ and Ca2+, and NaF in the interlayer site. Conservation of the total mass and the Si, Al and Mg abundances occurred in most experimental phyllosilicates. However, in the products of the low-T NaF brine ± graphite experiments, the total mass may have a gain of 5.3–11 % assuming Mg conservation. Sc, V, Nb and Ta abundances were also conserved, but a significant fractionation of the Rb/Li, Cs/Li, and Ba/Li ratios occurred in the experimental phyllosilicates. The experiments predict the generation of highly fractionated Rb/Li and Cs/Li phyllosilicates by replacement of biotite during interaction with aqueous fluids and, mostly, NaCl and NaF brine fluids at high-T and low-T conditions, respectively. This demonstrates a key role of biotite in the fractionation of Rb/Li, Cs/Li and Rb/Cs during the hydrothermal alteration of felsic igneous rocks. Conversely, a reversal in the mobility of Li with respect to Rb and Cs occurred in the phyllosilicate products when biotite interacted with NaCl or CaCl2 brine fluids at relatively low-T conditions. These experimental results highlight the key role of biotite-fluid interaction processes in controlling the budget of alkaline trace elements in the continental crust.

中文翻译：

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黑云母在与水流体和盐水流体相互作用过程中的地球化学行为：热液批量实验的限制


黑云母在上部陆壳Li、Rb、Cs、Ba的地球化学循环中起着重要作用，是长英质火成岩和高品位碎屑变质沉积岩中Li和大离子亲岩元素的重要载体。在与陨石和热液流体相互作用的过程中，黑云母可以转化为各种类型的粘土矿物（主要是层间缺乏的黑云母、蛭石和蒙脱石）。这些转变会导致黑云母与其替代产品之间的碱性微量元素比 Rb/Li、Cs/Li 和 Rb/Cs 分馏。这项研究考察了黑云母与水和盐水流体相互作用时发生的矿物转化，以及所得层状硅酸盐的地球化学行为知之甚少。为此，我们在 170 °C 和 10 bar 下使用特氟龙弹对黑云母 + 石英±石墨与 2 M NaCl、2 M CaCl2 和 1 M NaF 盐水液的相互作用进行了批量水热实验，并在 550 °C 和 800 至 1400 bar 下使用高压釜装置。在较低 T 条件下，黑云母被 2：1 的面体粘土矿物（层间缺乏的黑云母、蒙脱石、蛭石和其他层间电荷较高的层状硅酸盐物种）和 Fe 氢氧氧化物矿物通过耦合溶解-沉淀机制所取代。在较高 T 条件下，这些机制导致黑云母转化为矿物组合（石英±石墨）：透辉石 + 钙钛矿 + 钛铁矿（CaCl2 盐水实验），钠长石 + 钛铁矿 + 粘土矿物（NaCl 盐水实验），以及冰晶石 + 碱长石，钠长石边缘是 K 长石 + Fe 氧化物（NaF 盐水实验）。 因此，推断在 NaF 和 CaCl2 盐水流体存在下粘土矿物稳定性显着降低。层状硅酸盐对黑云母的取代主要受 K+ 与 H+ （或其水合物状态 H3O+）、水合 Na+ 和 Ca2+ 以及层间位点 NaF 的离子交换控制。总质量以及 Si、Al 和 Mg 丰度的守恒发生在大多数实验层状硅酸盐中。然而，在低 T NaF 盐水±石墨实验的产物中，假设 Mg 保存，总质量可能会增加 5.3-11%。Sc、V、Nb 和 Ta 丰度也保持保守，但在实验层状硅酸盐中，Rb/Li、Cs/Li 和 Ba/Li 比率发生显著分馏。实验预测了在高 T 和低 T 条件下，在与水性流体以及主要是 NaCl 和 NaF 盐水流体相互作用期间，通过取代黑云母生成高度分馏的 Rb/Li 和 Cs/Li 层状硅酸盐。这表明黑云母在长英质火成岩热液蚀变过程中 Rb/Li、Cs/Li 和 Rb/Cs 的分馏中起关键作用。相反，当黑云母在相对较低的 T 条件下与 NaCl 或 CaCl2 盐水流体相互作用时，层状硅酸盐产物中 Li 相对于 Rb 和 Cs 的迁移率发生了逆转。这些实验结果突出了黑云母-流体相互作用过程在控制大陆地壳中碱性微量元素预算中的关键作用。