Helium and carbon isotope variations in Liaodong Peninsula, NE China

Abstract

Chemical and C–He isotopic compositions have been measured for N2-rich hydrothermal gases from the Liaodong (abbreviation of East Liaoning Province) Peninsula from which the oldest crustal rocks in China with ⩾3.8Ga outcrop. With the exception of one sample containing tritogenic 3He and atmospheric 4He in Liaoyang, the observed 3He/4He ratios from 0.1Ra to 0.7Ra indicate 1–8% helium from mantle, 92–98% from crust and 0.1–0.8% from atmosphere. Despite the lack of Quaternary volcanism, such 3He/4He ratios suggest, together with geophysical evidences, the existence of intrusive magmas that contain mantle helium and heat within the Liaodong middle-lower crust. The 3He/4He ratios are high along the NE-trending Jinzhou faults and gradually decrease with the increase of distance from the faults. Such a spatial distribution suggests that the mantle helium exsolves from magmatic reservoir in the middle-lower crust, becomes focused into the root zones of Jinzhou faults, and subsequently traverses the crust via permeable fault zones. When transversely migrated by groundwater circulation in near surface, mantle helium with high 3He/4He ratio may have been further diluted to the observed values by addition of radiogenic helium produced in the crust. This pattern shows strong evidence that the major faults played an important role on mantle-derived components transport from mantle upwards.