MORB-TYPE NEON IN AN ENRICHED MANTLE BENEATH ETNA, SICILY

Noble gas elemental and isotopic compositions were determined for five CO2-CH4 samples collected around Etna, Sicily, to investigate the geo chemical features of the mantle beneath the volcano. The samples conta in mantle-derived noble The measured helium isotopic ratios (He-3/He-4 ) vary between 5.9 and 6.4 times atmospheric ratio (R-a = 1.4 x 10(-6) ), which are comparable to the ratios of olivines (6.1-8.2R(a)) in the lavas of the same volcano [1]. Neon in the samples is enriched in bot h Ne-20 and Ne-21 (Ne-20/Ne-22 9.95-10.7,Ne-21/Ne-22 0.030-0.037), ind icating derivation from the mantle. The delta(Ne-20/Ne-22)/delta(Ne-21 /Ne-22) values are identical with that of mid-ocean ridge basalts (MOR E), indicating a similarity in the time-integrated (U/Ne) ratio betwee n the Etnean source and the depleted upper mantle (MORE source). Argon in the samples has Ar-40/Ar-36 ratios up to of 1800, which are higher than in atmosphere. These ratios are positively correlated with the N e-20/Ne-22 ratios, indicating a mantle origin of the radiogenic argon. Compared with olivines from the Etnean lavas [1], the argon in our na tural gas samples is less contaminated by atmospheric argon. Two sampl es from a CO, well show small but resolvable excess of Xe-129 and Xe-1 34,Xe-136. The volcanic rocks of Etna, ranging from tholeiites to alka line, are enriched in incompatible elements. Nd, Sr and Pb isotopes of the volcanic rocks indicate that the magma source is isotopically het erogeneous and contain a component with high Sr, Pb and low Nd isotopi c ratios derived from a mantle region which has been enriched in incom patible elements fora few million years [2,3,4] relative to the deplet ed upper mantle. The helium isotopic ratios of the gas samples are low er than those of MORE and are consistent with geochemical signatures o f the solid elements in the Etnean volcanic rocks. However, the observ ed MORE-type neon is on a MORE correlation array without accumulation of nucleogenic Ne-21. This apparent decoupling may be explained by a r ecent mixing of the depleted upper mantle (MORE source) with fluid enr iched in both incompatible elements and radiogenic He-4. When the flui d was formed with a small degree of partial melting, fractionation of radiogenic He-4 from nucleogenic Ne-21 could have occurred because of smaller partition coefficient of He than Ne. (C) 1997 Elsevier Science B.V.