Isotopic composition of helium, and CO 2 and CH 4 contents in gases produced along the New Zealand part of a convergent plate boundary
New Zealand straddles an active tectonic boundary between the Indo-Australian and Pacific plates. To the NE and SW oblique convergence of oceanic and continental crusts leads to the establishment of subduction zones; in the center continental crusts collide along a transform boundary. With regard to mantle degassing, and on the basis of chemical and He isotopic analyses of 140 samples from all over New Zealand, four major environments can be distinguished: (1) R/ RA values approaching MORB values are observed in areas of andesitic and rhyolitic volcanism and high-temperature geothermal activity over the center of the North Island. C/3He ratios there vary from <10 × 10 9, typical of volatiles released from the mantle, in the W, to >40 × 10 9, suggesting considerable addition of CO 2 from other than mantle sources, in the E.(2) In areas of recent igneous activity, residual mantle He is extracted from rocks through hydrothermal alteration by percolating groundwater. The gases are low in CO 2 due to conversion to carbonate; any CH 4 present is generated within the crust. C/3He ratios are generally well below those of any magmatic vapor. (3) In the absence of recent igneous activity, but in areas of increased seismicity, mantle He may reach the surface along fractures provided by the movement of the subducting slab in forearc regions or by dilatancy pumping or fault-valving in highly compressional, but seismically active parts of an orogenic plate boundary. (4) Mantle degassing is greatly impeded in areas of crustal thickening as indicated by negative gravity anomalies, low seismicity and rapid uplift. There 3He /4He ratios approach typical crustal values of <0.1 RA. Except in areas of present-day volcanic and geothermal activity, production, transport, storage, and release processes of 3He, 4He, CO 2, and CH 4 appear to be effectively decoupled.