Trace element fractionation and the origin of tholeiitic and alkaline magma types

Oceanic volcanic rocks can roughly be divided into two major chemical types: (1) tholeiitic or abyssal basalts and (2) alkaline or undersaturated basalts. Trace element abundances in these two types are compared. It is shown that the high abundance of large ion elements in alkaline basalts cannot be consistently explained by fractional crystallization processes. Both the absolute abundance and relative abundance of many large ion elements in alkaline magmas are entirely consistent with the hypothesis that these liquids are produced by partial melting (3-7 %) of accepted upper mantle mineral assemblages. It is postulated that the high K/Rb and low Ba/Sr and La/Yb ratios in abyssal basalts are characteristics of the upper mantle region that produced these basalts and that these characteristics are produced by previous partial melting episodes.