Noble gases in mafic phenocrysts and xenoliths from New Zealand

Abstract

We have determined the elemental and isotopic compositions of noble gases in young subduction-related phenocrystic olivine and clinopyroxene samples from the Taupo Volcanic Zone, central North Island, New Zealand, and in behind-arc intraplate phenocrystic and xenolithic olivine samples from the Northland and Auckland Volcanic Provinces, northern North Island, New Zealand. Helium isotopic ratios range from MORB-like 3He 4He values of about 11 × 10 −6 to lower values of about 6 × 10 −6, consistent with previous measurements of helium isotopic ratios in subduction-related samples. 40Ar 36Ar and 21Ne 22Ne ratios range from atmosphere-like compositions to maximum values of about 700 and 0.033, respectively. In contrast, most of the 21Ne 22Ne ratios are generally indistinguishable from atmospheric values. The variations in helium, neon, and argon isotopic ratios are interpreted as resulting from mixing of: 1. (a) a primordial 3He-rich component derived from the upper mantle and characterised by MORB-like 3He 4He ratios of about 12 × 10 −6; 2. (b) a radiogenic 4He-rich component derived from crustal materials and characterised by a 3He 4He ratio of less than 2 × 10 −7, and 3. (c) a helium-poor atmosphere-derived component which dominates the heavier noble gases. By combining the helium and argon isotopic results, it is possible to estimate the relative contribution of each of these three components to the total 40Ar observed in each sample. Based on the present understanding of the origin and evolution of arc magmas, a simple qualitative model of the mechanisms which introduce noble gases to the parent magmas of the samples is developed. However, it remains uncertain whether the atmospheric and crustal-derived noble gas components are introduced to the mantle source regions of the magmas by subduction, or alternatively, are introduced by interactions between the ascending parent magmas and the overlying crust.