Acidic fluids in the Earth's lower crust.

Understanding fluid composition in Earth's interior remains a challenge due to experimental and modeling limitations at extreme conditions. Geochemical cycling is central to crustal evolution, yet fluid properties are poorly constrained. Prior studies show that halogen incorporation in minerals like apatite is sensitive to pressure and temperature. Apatite Cl and F content can reveal fluid chemistry in metamorphic settings. Regional geochemical trends suggest fluid involvement in crustal processes. However, acidic fluids in the lower crust remain poorly understood. This gap motivated investigations into apatite grains from high-grade metamorphic rocks. The Nilgiri Block provides a natural laboratory for such studies.

This study aims to determine fluid composition in the lower crust by analyzing halogen content in apatite grains. The focus is on mafic and felsic rocks from the Nilgiri Block in southern India. The goal is to infer fluid speciation during high-grade metamorphism. The motivation comes from the need to understand acidic fluid behavior in deep crustal environments. The study builds on prior work on halogen partitioning in apatite. It seeks to bridge the gap between fluid modeling and field observations. The approach involves measuring Cl and F in apatite from multiple rock types. The findings may clarify fluid-driven geochemical changes in the crust.

The study uses halogen measurements in apatite grains from metamorphic rocks in the Nilgiri Block. Apatite samples were collected from mafic and felsic rock types. Electron microprobe analysis quantified Cl and F content in the apatite. The highest Cl content observed was 2.95 wt%, indicating acidic fluid presence. Regional geochemical trends were compared with experimental data on apatite halogen incorporation. The approach relies on the known difficulty of incorporating Cl in apatite at high pressure and temperature. The analysis links halogen content to fluid speciation in the lower crust. The method integrates field observations with thermodynamic modeling.

Apatite grains in the Nilgiri Block rocks show elevated Cl content up to 2.95 wt%. This suggests the presence of acidic C-O-H fluids in the lower crust. These fluids are inferred to be active during high-grade metamorphism. The Cl/F ratio in apatite indicates fluid acidity and reactivity. The fluids are capable of altering refractory minerals in the crust. They also facilitate mass transfer on a regional scale. The findings support the role of acidic fluids in deep Earth processes. The study provides evidence for fluid-driven geochemical variations in the crust.

The study supports the presence of acidic fluids in the lower crust during high-grade metamorphism. These fluids are capable of extreme chemical alterations in refractory minerals. They also contribute to regional geochemical variations. The findings align with prior experimental work on apatite halogen incorporation. The authors suggest that acidic fluids influence the chemical and isotope records of early Earth rocks. The study highlights the importance of fluid speciation in crustal evolution. It provides a framework for understanding fluid-rock interactions in deep crustal settings. The results have implications for geochemical models of Earth's interior.