The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs

New helium isotope data measured in Cenozoic intraplate basalts and their m antle xenoliths are compared with present-day mantle helium emission on a r egional scale from thermal and nonthermal gas discharges on the South Islan d of New Zealand and the offshore Chatham Islands. Cenozoic intraplate basa ltic volcanism in southern New Zealand has ocean island basalt affinities b ut is restricted to continental areas and absent from adjacent Pacific ocea nic crust. Its distribution is diffuse and widespread, it is of intermitten t timing and characterised by low magma volumes. Most of the He-3/He-4 rati os measured in fluid inclusions in mantle xenocrysts and basalt phenocrysts such as olivine, garnet, and amphibole fall within the narrow range of 8.5 +/- 1.5 Ra (Ra is the atmospheric He-3/He-4 ratio) with a maximum value of 11.5 Ra. This range is characteristic of the relatively homogeneous and de gassed upper MORE-mantle helium reservoir. No helium isotope ratios typical of the lower less degassed mantle (>12 Ra), such as exemplified by the mod em hot-spot region of Hawaii (with up to 32 Ra) were measured. Helium isoto pe ratios of less than 8 Ra are interpreted in terms of dilution of upper m antle helium with a radiogenic component, due to either age of crystallisat ion or small-scale mantle heterogeneities caused by mixing of crustal mater ial into the upper mantle. The crude correlation between age of samples and helium isotopes with generally lower R/Ra values in mantle xenoliths compa red with host rock phenocrysts and the in general depleted Nd and Sr isotop e ratios and the light rare earth element enrichment of the basalts support s derivation of melts as small melt fractions from a depleted upper mantle, with posteruptive ingrowth of radiogenic helium as a function of lithosphe ric age. In comparison, the regional helium isotope survey of thermal and nonthermal gas discharges of the South Island of New Zealand shows that mantle He-3 a nomalies in general do not show an obvious relationship with either age or proximity to the Cenozoic intraplate volcanic centres or with major faults. In general, areas characterised by mantle He-3 emission are interpreted to define those regions beneath which mantle melting and basalt magma additio n to the crust are recent. The strongest mantle He-3 anomaly (equivalent to >80% mantle helium component) is centred over southern Dunedin, measured i n magmatic CO2-rich mineral water springs issuing from crystalline basement rocks which outcrop at the southern extent of Miocene intraplate basaltic volcanism which ceased 9 Ma ago. This mantle helium anomaly overlaps with a n area characterised by elevated surface high heat flow, compatible with a long-lived mantle melt/heat input into the crust. In comparison Banks Penin sula, another Miocene intraplate basaltic centre, is characterised by relat ively low surface heat flow and a small mantle helium contribution measured in a nitrogen-rich spring. Here the thermal transient induced by the magma tic event has either dissipated or has not reached the surface. In the form er case one might be dealing with storage and mixing of magmatic and crusta l gases at shallow crustal levels and in the latter with active to recent m antle-melt degassing at depth. Along the most actively deforming part of th e plate boundary zone, the transpressional Alpine Fault and Marlborough fau lt systems, mantle helium is present in gas-rich springs in all those areas underlain by actively subducting oceanic crust (the Australian plate in th e south and Pacific plate in the north), whereas the central part of the Al pine transpressional fault is characterised by pure crustal radiogenic heli um. Areas where the mantle helium component is negligible are restricted to the centre part of the South Island, extending along its length from South land to northern Canterbury and Murchison. These areas are interpreted to d elineate the extent of thicker and colder lithosphere compared to all other areas where mantle helium release from partial mantle melts at depth is re cent to active being added to the lower lithosphere and/or lower crust. Are as characterised by mantle helium anomalies are equated with areas of therm al mantle anomalies, i.e., localised mantle heterogeneities such as upwelli ng less dense silicate melts in the upper asthenospheric mantle. Copyright (C) 2000 Elsevier Science Ltd.