The isotope ratios of Sr, Nd, and Pb of volcanic rocks in the oceans shed light on the sources of the magmas from which they formed and on their subsequent chemical evolution. The early work by Gast (1960), Faure (1961), and Faure and Hurley (1963) indicated that volcanic rocks in the oceans have low 87Sr/86Sr ratios compared to old granitic rocks of the continental crust and therefore originated from magma sources in the mantle having low Rb/Sr ratios. Subsequent studies have disclosed significant differences in the isotope compositions of Sr, Nd, and Pb between volcanic rocks of the mid-ocean ridges, oceanic islands, and island arcs. Consequently, volcanic rocks in the ocean basins must have originated from magma sources in the mantle in which these elements have different isotope compositions. Such differences are caused by variations of their respective parent/daughter elemental ratios existing for varying lengths of time in the past. In addition, magmas that formed by partial melting of different kinds of source rocks may mix prior to eruption to form “hybrid” magmas. Alternatively, different kinds of source rocks may themselves become mixed as a consequence of tectonic processes in the mantle. In the final analysis, volcanism in the ocean basins, as well as on the continents, is a manifestation of the tectonic activity in the mantle of the Earth.
KeywordsVolcanic Rock Partial Melting Oceanic Crust Lithospheric Mantle Hawaiian Island
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