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

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 ⁴He 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 CO₂ is isotopically light (${\delta }^{13}{C}_{{\text{CO}}_2}$ = −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 ⁴He 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.