Table6_Role of acidic fluids in Earth’s deep lithosphere: Insights from the Neoarchean magmatic roots of the Nilgiri Block, southern India.xlsx

Fluids play a major role in facilitating igneous/metamorphic processes in the Earth’s crust and mantle. In this study, we investigate the nature and composition of fluids in Earth’s interior by studying the lower crustal rocks. We compare accessory minerals (e.g., apatite, monazite, allanite, and titanite), their texture, mineral reactions and composition among regionally distributed metamorphosed mafic and felsic rocks representing the roots of Neoarchean arc magmatism from the Nilgiri Block of the Southern Granulite Terrane in India. Regional trends in accessory minerals show the formation of monazite, allanite, and titanite in the felsic rocks. Apatite is depleted in REEs in all the rock types, irrespective of the difference in their whole-rock chemistry. Textural features and mineral reactions show that these accessory minerals were affected by fluids present in the lower crustal conditions. By comparing our results with those from previous experimental results, we further show that acidic CO2-H2O-HCl-HF fluids stable in lower crustal conditions could have resulted in these chemical and textural features. Dielectric constant of water is high (10–35 compared to lower crustal conditions) in high-pressure and low-temperature conditions of subduction zones and the upper mantle. Such conditions would enhance dissociation of HCl (compared to lower crust) and result in acidic fluids during dehydration reactions in subduction zones and in the upper mantle. Our results have important implications in understanding the nature and composition of fluids in Earth’s interior and would be helpful to model the tectonic and deep geochemical processes in both early and modern conditions in planetary interiors.

流体在地球地壳与地幔的火成作用及变质作用中发挥着关键的促进作用。本研究通过对下地壳岩石开展研究，探究地球内部流体的性质与组成。我们选取印度南部麻粒岩地体（Southern Granulite Terrane）尼尔吉里地块（Nilgiri Block）内代表新太古代（Neoarchean）弧岩浆作用（arc magmatism）根部的区域分布变质镁铁质岩与长英质岩，对比其中的副矿物（accessory minerals，如磷灰石（apatite）、独居石（monazite）、褐帘石（allanite）及榍石（titanite））的结构、矿物反应与组成特征。
副矿物的区域分布特征显示，长英质岩中形成了独居石、褐帘石与榍石。所有岩石类型中的磷灰石均亏损稀土元素（Rare Earth Elements, REEs），且不受全岩化学组成差异的影响。结构特征与矿物反应表明，这些副矿物受到了下地壳环境中流体的改造。通过将本研究结果与前人实验成果对比，我们进一步证实：下地壳环境中稳定存在的酸性CO₂-H₂O-HCl-HF流体，可引发上述化学与结构特征。
在俯冲带与上地幔的高压低温环境中，水的介电常数较高（为10~35，相较于下地壳环境）。相较于下地壳环境，此类条件会强化HCl的解离，并在俯冲带与上地幔的脱水反应过程中形成酸性流体。
本研究结果对于理解地球内部流体的性质与组成具有重要意义，同时可为行星内部早期与现代构造及深部地球化学过程的模拟提供参考。