Apparent decoupling of the He and Ne isotope systematics of the Icelandic mantle: The role of He depletion, melt mixing, degassing fractionation and air interaction

Abstract

We present new He–Ne data for geothermal fluids and He–Ne–Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios ( 3 He/ 4 He) encompassing typical MORB-like ratios through values as high as 36.8 RA (where RA = air 3 He/ 4 He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic 21 Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by 20 Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, ‘solar’ mantle component. However, binary mixing between these two endmembers can account for the He–Ne isotope characteristics of the basalts only if the 3 He/ 22 Ne ratio of the primordial mantle endmember is lower than in the MORB component. Indeed, the helium to neon elemental ratios ( 4 He/ 21 Ne* and 3 He/ 22 Nes where 21 Ne* = nucleogenic 21 Ne and 22 Nes = ‘solar’-derived 22 Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth’s mantle, as observed previously for other OIB samples. Helium may be depleted relative to neon in high- 3 He/ 4 He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher 4 He/ 40 Ar* ( 40 Ar* = radiogenic Ar) values for a given 4 He/ 21 Ne* value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He–Ne–Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high- 3 He/ 4 He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He–Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive ‘solar-like’ characteristics in central and southern Iceland.