Abstract

Properties of fluids in high-pressure granulites were studied in HP granulites (~8.7–11 kbar, ~800–900°С) and syngranulite fluid infiltration-driven HP metasomatites (~11–9 kbar, ~920–850°С) from the Lapland granulite belt of the Fennoscandian Shield. The study involved large-scale mapping of the rocks, microthermometry of mineral-hosted fluid inclusions, multiequilibrium mineral thermobarometry, and calculations of H₂O activity based on mineral equilibria. The mafic pyroxene granulites and syngranulite metasomatites (quartz blastomylonites with orthopyroxene, sillimanite, and garnet; veins and vein-like bodies of orthopyroxene–garnet and diopside–scapolite rocks) contain similar assemblages of syngenetic fluid inclusions (which are hosted mostly in quartz and also in garnet, orthopyroxene, and scapolite) of contrasting chemical composition: nearly pure СО₂ (distinctly predominant), brines (the dominant salts are CaCl₂ and NaCl), and N₂ ± H₂O. These three types of inclusions coexist in the same generations of early inclusions: rarer primary (p) and predominant primary–secondary (ps). The CO₂ inclusions have either high or low densities, and the N₂ inclusions are of low density. The brine inclusions show a wide range of total salt contents (up to 30–35 wt%) and variable concentration proportions of the dominant salts: p-inclusions with a salinity of 20 wt% CaCl₂ + 10 wt% NaCl; ps-inclusions with a salinity of 5 wt% CaCl₂ + 20 wt% NaCl; p- and ps-inclusions with a salinity of 5–23 wt% NaCl eq; and p-inclusions with halite (up to 35 wt% NaCl). In general, CaCl₂ is the predominant salt component in the early p- and ps-inclusions of the rocks. Considered together, currently available data (including Sr, Nd, and O isotope systems) on these rocks indicate that the external fluid flow during the origin of granulites was evidently of mantle origin. At the peak P–T parameters, the inclusions were entrapped from a heterogeneous fluid in which immiscible water–salt and CO₂-rich fluids, which initially contained N₂, coexisted. Data on the chemical composition and salt concentrations of the fluids, \({{a}_{{{{{\text{H}}}_{2}}{\text{O}}}}}\) = 0.40–0.51, are compared with the theoretically predicted phase state of the fluids and the properties of the coexisting immiscible fluid phases at the estimated P–T parameters of granulite petrogenesis on the basis of numerical models in the H₂O–CO₂–NaCl and H₂O–CO₂–CaCl₂ ternary systems. The location of the tie-lines and solvus were calculated to subsequently use for the thermodynamic prediction. The paper discusses similarity and the reasons for the difference between the theoretical compositions of the generated fluid phases and the composition of fluid inclusions, geochemical consequences of the heterogenization of granulite fluids (the formation of concentrated alkaline brines and a potentially acidic CO₂-rich fluid phase, the values of the mass and volume fractions of these phases depending on variations in the composition of the initial homogeneous fluid, etc.). It follows that an extensive region of the compositions of aqueous fluids with different concentrations of CO₂ and Na and Ca chlorides exists at the P–T parameters of HP granulites in which originally homogeneous fluid splits into compositionally contrasting fluid phases with different properties. This region of coexisting immiscible fluids significantly expands with increasing CaCl₂ concentration. Hence, the lower crust at the level of the HP granulite facies may be the region where high-temperature immiscible fluids are generated. One of these fluids is a denser phase of alkaline brines, and the other is a less dense potentially acidic phase of H₂O–CO₂ fluids rich in CO₂. Ascending along regional permeable zones, these fluid phases of deep origin can play an important role in magmatic, metamorphic, metasomatic, and ore-forming processes in the middle and upper crust.

中文翻译：

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高压花岗石中的流体

摘要

研究了高压花岗石中流体的性质，包括HP粒岩（〜8.7–11 kbar，〜800–900°С）和滑石粉浸润驱动的HP交代岩（〜11–9 kbar，〜920–850°С）。 Fennoscandian Shield的拉普兰花岗石带。这项研究涉及岩石的大规模测绘，矿物载流体包裹体的显微热测量，多平衡矿物热压法以及H _2的计算。基于矿物质平衡的O活性。镁铁质的辉石花岗石和同辉石的交代物（石英与高辉石，硅线石和石榴石的珠光蒙脱石；常辉石-石榴石和透辉石-辉绿岩的脉状和脉状体）包含相似的成因流体包裹体组合（主要存在于石英中，也存在于石英中）。在对比化学成分的石榴石，斜，和方柱石）：几乎纯СО ₂（明显占优势），盐水（占主导地位的盐是氯化钙₂和NaCl），和N ₂ ±ħ ₂ O.这三种类型共存的夹杂物中的同一代早期夹杂物：罕见的小学（p）和占优势的小学—中学（ps）。一氧化碳_2个夹杂物的密度高或低，而N ₂夹杂物的密度低。盐水夹杂物显示出很宽的总盐含量（高达30-35 wt％）和主要盐的可变浓度比例：盐度为20 wt％CaCl ₂ + 10 wt％NaCl的p-夹杂物；盐度为5 wt％CaCl ₂ + 20 wt％NaCl的ps夹杂物；盐度为5-23 wt％的NaCl当量的p和ps夹杂物；以及含有盐石（含量高达35 wt％的NaCl）的p夹杂物。通常，CaCl ₂是p-和ps-早期的主要盐分。夹杂物的岩石。综合考虑，目前在这些岩石上可获得的数据（包括Sr，Nd和O同位素系统）表明，粒料生成过程中的外部流体流动显然是地幔起源的。在峰值P–T参数下，夹杂物从非均质流体中夹杂，其中不溶混的水盐和富含CO _2的流体（最初包含N ₂）共存。流体的化学成分和盐浓度的数据\（{{a）_ {{{{\ text {H}}} _ {2}} {\ text {O}}}}}}} \） = 0.40将–0.51与理论上预测的流体相态以及在估计的P–T下共存的不混溶流体相的性质进行比较数值模型的在H的基础上，粒成因的参数₂ O-CO ₂ -NaCl和H ₂ O-CO ₂ -CaCl _2个三元体系。计算联络线和固溶线的位置，以随后用于热力学预测。本文讨论了相似性以及产生的流体相的理论组成与流体包裹体组成之间差异的原因，粒状流体异质化的地球化学后果（形成浓碱性盐水和可能呈酸性的CO _2）-富流体相，这些相的质量分数和体积分数取决于初始均质流体组成的变化等）。因此，在HP颗粒的P – T参数处，存在着不同浓度的CO ₂，Na和Ca氯化物的水性流体成分的广泛区域，在这些参数中，本来均匀的流体分成具有不同性质的组成相反的流体相。共存的不混溶流体的这一区域随着CaCl ₂浓度的增加而显着扩展。因此，HP水平下的地壳较低粒岩相可能是产生高温不溶混流体的区域。其中一种流体是碱性盐水的致密相，另一种是富含CO ₂的H ₂ O-CO ₂流体的致密性较弱的潜在酸性相。这些深部成因沿区域渗透带上升，在中上地壳的岩浆，变质，变质和成矿过程中起着重要作用。