NOBLE-GASES IN MAFIC PHENOCRYSTS AND XENOLITHS FROM NEW-ZEALAND

We have determined the elemental and isotopic compositions of noble ga ses in young subduction-related phenocrystic olivine and clinopyroxene samples from the Taupo Volcanic Zone, central North Island, New Zeala nd, and in behind-arc intraplate phenocrystic and xenolithic olivine s amples from the Northland and Auckland Volcanic Provinces, northern No rth Island, New Zealand. Helium isotopic ratios range from MORB-like H e-3/He-4 values of about 11 x 10(-6) bto lower values of about 6 x 10( -6), consistent with previous measurements of helium isotopic ratios i n subduction-related samples. Ar-40/Ar-36 and Ne-21/Ne-22 ratios range from atmosphere-like compositions to maximum values of about 700 and 0.033, respectively. In contrast, most of the Ne-20/Ne-22 ratios are g enerally indistinguishable from atmospheric values. The variations in helium, neon, and argon isotopic ratios are interpreted as resulting f rom mixing of: (a) a primordial He-3-rich component derived from the u pper mantle and characterised by MORB-like He-3/He-4 ratios of about 1 2 x 10(-6); (b) a radiogenic He-4-rich component derived from crustal materials and characterised by a He-3/He-4 ratio of less than 2 x 10(- 7), and (c) a helium-poor atmosphere-derived component which dominates the heavier noble gases. By combining the helium and argon isotopic r esults, it is possible to estimate the relative contribution of each o f these three components to the total Ar-40 observed in each sample. B ased on the present understanding of the origin and evolution of arc m agmas, a simple qualitative model of the mechanisms which introduce no ble gases to the parent magmas of the samples is developed. However, i t remains uncertain whether the atmospheric and crustal-derived noble gas components are introduced to the mantle source regions of the magm as by subduction, or alternatively, are introduced by interactions bet ween the ascending parent magmas and the overlying crust.