Tectonic Degassing of Ancient Igneous Rocks as a Probable Source of Mantle‐Like Helium in Hot Springs of the Capital Region of China

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Abstract
Mantle‐like helium signatures ( R c / R a up to 2.99) in hot spring gases from tectonic intersection zones in the capital region of China have repeatedly been interpreted as evidence for ongoing mantle‐fluid input. Here, we integrate gas geochemistry, heat‐flow data, geophysical observations, radiogenic 4 He ingrowth calculations, and structural geology to reassess this anomaly. Air‐corrected R c / R a values reach 2.04–2.99 in the intersection zones, well above those of the adjacent tectonic units, whereas their CO 2 is isotopically light ( = −17.1‰ to −10.0‰), is uncorrelated with helium concentrations, and lacks characteristics of active magmatic degassing. The region has relatively low heat flow and reservoir temperatures; although a localized reduced‐velocity zone occurs at ∼50–60 km depth beneath the intersection zones, it cannot be uniquely attributed to present‐day thermal or magmatic activity and is compatible with lithospheric modification inherited from Mesozoic craton destruction. Radiogenic 4 He ingrowth calculations show that relatively U‐ and Th‐poor mafic compositions can retain mantle‐like R c / R a values comparable to observations over representative Early Cretaceous timescales. We therefore propose that mantle‐derived helium introduced during Mesozoic magmatism was subsequently remobilized by Cenozoic tectonic degassing: uplift and denudation lowered pressure and promoted transfer of helium dissolved in ancient pore water into a gas phase, whereas composite strike‐slip‐extensional deformation ruptured helium‐bearing microreservoirs, enhanced permeability, and focused upward migration into the hydrothermal system. These results indicate that mantle‐like helium anomalies in tectonically active nonvolcanic regions may reflect tectonic degassing of ancient crustal reservoirs without requiring substantial present‐day mantle recharge, although a minor low‐flux mantle contribution cannot be excluded.
Funding Information
  • National Key Research and Development Program of China (2023YFC3012005)

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