Abstract
Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth. Quantification of such process needs accurate knowledge about the composition and properties of the fluids. Knowledge about the fluids in Earth’s interior is scarce due to limitations in both experimental methods and thermodynamic modeling in high/ultrahigh pressure–temperature conditions. In this study, we present halogen (Cl, F) measurements in apatite grains from the mafic (metagabbro), and felsic (two-pyroxene granulite, charnockite, hornblende-biotite gneiss) rocks preserved in the Nilgiri Block, southern India. Previous experiments show that it is difficult to incorporate Cl in apatite compared to F at high pressure and temperature conditions. Based on regional trends in Cl and F content in apatite (with highest Cl content 2.95 wt%), we suggest the presence of acidic C–O–H fluids in the lower crust (~20–40 km deep) during the high-grade metamorphism of these rocks. These fluids are capable of causing extreme chemical alterations of minerals, especially refractory ones. They also have significant potential for mass transfer, causing extensive geochemical variations on a regional scale and altering the chemical and isotope records of rocks formed in the early Earth. Our findings have important relevance in understanding speciation triggered by acidic fluids in the lower crust, as well as the role of fluids in deep Earth processes.
Highlights
Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth
We report fluid-assisted recrystallization processes that occurred in deep crustal rocks as old as 2.7–2.5 Ga12–14 from the Nilgiri Block, southern India (Fig. 1), a major high-grade crustal block located to the south of the Archean Dharwar Craton in Peninsula India[15,16] (Fig. 1a)
Fluid inclusions and textural studies in granulites worldwide suggest that C O2 and highly saline fluids/brines might represent the major fluid c ompositions[25,26,27,28,29]. These data, along with textural and experimental results suggest the possibility of fluid-assisted metamorphism in the presence of H2O–CO2-saline fluids/brines[30,31,32,33,34,35,36]
Summary
Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth. We provide first evidence for the presence of acidic fluids in the lower crust (~20–40 km) based on the trends in Cl and F content in naturally-occurred apatite grains (with a high Cl value of 2.95 wt%) under high-grade granulite facies metamorphic rocks (viz., metagabbro, two-pyroxene granulite and charnockite) preserved in the Nilgiri Block, southern India.
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