NOBLE GAS ISOTOPE GEOCHEMISTRY OF MID-OCEAN RIDGE AND OCEAN ISLAND BASALTS: CHARACTERIZATION OF MANTLE SOURCE RESERVOIRS

Abstract

The study of noble gases in oceanic basalts is central to understanding chemical heterogeneity of the Earth’s mantle and origin of the atmosphere. In the terrestrial environment the abundances of noble gases are quite low because they were excluded from solid materials during planetary formation in the inner solar system. This low background inventory helps to make the noble gases excellent tracers of mantle reservoirs. In this context, mid-ocean ridge and ocean island basalts provide valuable windows into the Earth’s mantle. These oceanic basalts are not prone to the degree of contamination often observed in continental lavas that results from their passage through thick continental lithosphere and crust. Mid-ocean ridge basalts (MORBs) form by partial melting as the ascending mantle beneath spreading ridges reaches its solidus temperature, and MORBs are generally accepted to represent a broad sampling of the convecting upper mantle. Ocean island basalts (OIBs) represent melting ‘anomalies’ that are generally related to mantle upwelling. The extent to which ocean islands are derived from a thermal boundary layer in the deep mantle (e.g., as a mantle plume) or from chemical heterogeneities embedded within the mantle convective flow (e.g., as a mantle ‘blob’) has been debated for decades, and is not currently resolved. The isotope compositions of noble gases in oceanic basalts bear significantly on such debates over the chemical structure of the mantle. When oceanic basalts erupt as submarine lavas, their quenched rims of glass may contain high volatile abundances (especially when they are deeply erupted under elevated hydrostatic pressure), providing the best available opportunity for precisely characterizing the noble gas composition of the Earth’s mantle. In favorable cases, inclusions of melt or fluids trapped within magmatic phenocrysts and mantle xenoliths can also be precisely analyzed for noble gas composition. Measurable changes in the isotope compositions of noble gases …