Noble gases in mantle-derived xenocrysts in an alkali basalt from Japan Trench oceanward slope

Abstract

We measured noble gas isotopic ratios of olivine xenocrysts in an alkali-basalt sampled from a northwest Pacific seamount. The xenocrysts are reported to be derived from upper-mantle from major element compositions. By applying both vacuum crushing and stepwise heating extraction of the noble gases, uncontaminated 3He/4He ratios range between 7.2 and 7.7 RA and 40Ar/36Ar ratios are up to 5000 in the xenocrysts. These observations demonstrate that the upper mantle beneath the oceanward slope of the Japan Trench has a similar isotopic composition to the MORB source, suggesting a uniform MORB source is largely distributed in the upper mantle.