MORB-type neon in an enriched mantle beneath Etna, Sicily

Abstract

Noble gas elemental and isotopic compositions were determined for five CO 2–CH 4 samples collected around Etna, Sicily, to investigate the geochemical features of the mantle beneath the volcano. The samples contain mantle-derived noble gases. The measured helium isotopic ratios ( 3He/ 4He) vary between 5.9 and 6.4 times atmospheric ratio ( R a=1.4×10 −6), which are comparable to the ratios of olivines (6.1–8.2 R a) in the lavas of the same volcano [1]. Neon in the samples is enriched in both 20Ne and 21Ne ( 20Ne/ 22Ne 9.95–10.7, 21Ne/ 22Ne 0.030–0.037), indicating derivation from the mantle. The δ( 20Ne/ 22Ne)/ δ( 21Ne/ 22Ne) values are identical with that of mid-ocean ridge basalts (MORB), indicating a similarity in the time-integrated (U/Ne) ratio between the Etnean magma source and the depleted upper mantle (MORB source). Argon in the samples has 40Ar/ 36Ar ratios up to of 1800, which are higher than in atmosphere. These ratios are positively correlated with the 20Ne/ 22Ne ratios, indicating a mantle origin of the radiogenic argon. Compared with olivines from the Etnean lavas [1], the argon in our natural gas samples is less contaminated by atmospheric argon. Two samples from a CO 2 well show small but resolvable excess of 129Xe and 134,136Xe. The volcanic rocks of Etna, ranging from tholeiites to alkaline, are enriched in incompatible elements. Nd, Sr and Pb isotopes of the volcanic rocks indicate that the magma source is isotopically heterogeneous and contain a component with high Sr, Pb and low Nd isotopic ratios derived from a mantle region which has been enriched in incompatible elements for a few million years [2,3,4] relative to the depleted upper mantle. The helium isotopic ratios of the gas samples are lower than those of MORB and are consistent with geochemical signatures of the solid elements in the Etnean volcanic rocks. However, the observed MORB-type neon is on a MORB correlation array without accumulation of nucleogenic 21Ne. This apparent decoupling may be explained by a recent mixing of the depleted upper mantle (MORB source) with fluid enriched in both incompatible elements and radiogenic 4He. When the fluid was formed with a small degree of partial melting, fractionation of radiogenic 4He from nucleogenic 21Ne could have occurred because of smaller partition coefficient of He than Ne.