Potassic magmatism as a key to trace-element enrichment processes in the upper mantle

Abstract

Basaltic magmas can be divided into two major associations on the basis of minor-element variations, one characterised by high K/Ti ratios and low-Ti abundances and the second by lower K/Ti ratios and high-Ti. These two associations appear to be the result of contrasting incompatible-element enrichment processes operating in the upper mantle, the most extreme expressions of which are found in potassic magmas. The latter, therefore, offer a key to understanding the processes that scavenge and mobilise these elements in the upper mantle. Two examples of potassic magmatism are discussed and compared. The Italian province is characterised by low-Ti magmas (TiO 2 < 1%) which show strong fractionation between the large ion lithophile (LIL) and high field strength (HFS) elements. Potassium enrichment is associated with the development of very high Rb/Sr ratios (0.2–0.4), low Sm/Nd and moderate Th/U (4–5) and is considered to be related to recent subduction, probably involving the incorporation of subducted sediment into the magma source. In contrast, the Virunga province in the East African Rift has high-Ti magmas which do not show LIL/ HFS fractionation. Potassium enrichment is the result of mafic melt migration into the source during the Proterozoic, producing moderate Rb/Sr ratios (∼ 0.1), low Sm/Nd and unusually high Th/U (5–7). These two processes, which are responsible for potassium enrichment, lead to contrasting fractionation trends for many trace elements and thus have implications for the evolution of isotope heterogeneities in the mantle. These are discussed with particular reference to continental flood basalts and ocean island basalts.