2026年6月11日に第16回岩石ー流体セミナーを行いました。講演者はIndian Institute of Technology Kanpurの Subhrajyoti Behera さんで、インドのNorth Singhbhum Mobile Beltにおける岩相境界の反応帯における物質移動や変形について活発な議論が行われました。
Fluid-Rock Interactions and Its Effect in Mid-Crustal Conditions: An Example from North Singhbhum Mobile Belt, Eastern India
Subhrajyoti Behera
Experimental Rock Deformation Laboratory, Department of Earth Sciences, Indian Institute of Technology Kanpur, Kanpur, India
Abstract:
Fluids play a major role crustal evolution by reacting with rocks, altering mineral composition, and redistributing chemical elements. However, fluid-mediated mass transfer– including mobility of traditionally “immobile” REEs and HFSEs, remains poorly understood due to complex nature of fluid-rock interaction. My work investigates metasomatic reaction zones (MRZs) preserved at lithological contacts between meta-mafic dykes and pelitic garnet-mica schists in the Proterozoic North Singhbhum Mobile Belt, India.
The outcrop sequence: Dyke (epidote-amphibolite)–Zone 1 (amphibole-epidote-calcic-plagioclase-quartz-chlorite)–Zone 2 (Zone 1 assemblage minus amphibole)–pelitic schist, records systematic fluid-rock interaction. Using EQ3/6 reaction-path modeling, bulk-rock and mineral geochemistry, pseudosection modeling, and hydrothermal experiments, I show that the MRZs formed during post-peak retrogradation and thermal relaxation at ~1–3 kbar and 300–500°C. Saline fluids pre-equilibrated with the mafic dykes drove MRZ formation upon reacting with the pelitic schist, with mineral assemblage variations reflecting differences in time-integrated fluid-to-rock ratios, resulting in significant REE (50–87%) and HFSE (43–63%) depletion within the MRZs. Additionally, EBSD analysis of quartz within MRZs reveals that fluid-rock interaction intensity, not just temperature, control microstructural evolution, intragrain structures, and slip-system activation.
These results highlight the fundamental role of fluids in governing coupled physico-chemical processes during crustal evolution.
Keywords: fluid-rock interactions, element mobility, reaction-path modeling, EBSD