Island arc tectonics of New Zealand manifested in helium isotope ratios

Abstract

Abstract Thirty gas samples of various types—volcanic gas, geothermal gas, CH4-rich natural gas, and petroleum gas—were collected in and around the North Island, New Zealand. 3 He 4 He and 4 He 20 Ne ratios were measured using a mass spectrometer. The observed 3 He 4 He and 4 He 20 Ne ratios of the samples varied from 0.08 × 10−6to 8.75 × 10−6 and from 4.8 to 2100, respectively. Based on a 3 He 4 He 4 He 20 Ne diagram. He in the samples can be explained by mixing of a subduction-type magmatic, a radiogenic, and an air component. The geographical distribution of 3 He 4 He ratios over the island indicates a clear contrast between forearc and volcanic arc regions: high in the former and low in the latter. A He boundary can be drawn from the Taupo Volcanic Zone to Egmont Volcano in terms of the 3 He 4 He profile. This boundary agrees well with that of high Q/lowQ in the upper mantle as well as with the geographical distribution of terrestrial heat flow values. The high 3 He 4 He ratios in the volcanic arc region are attributed to subduction-type He associated with an uprising magma from the upper mantle and the lower ratios in the forearc region to radiogenic He derived from U and Th in basement and sedimentary rocks. This tendency is very similar to that found in northeast Japan and is understood to be a general signature of the He isotope ratio in an island arc system.