U series disequilibria: Insights into mantle melting and the timescales of magma differentiation

Abstract

Several U series nuclides have half-lives (230Th, 76 kyr; 231Pa, 33 kyr; and 226Ra, 1.6 kyr) comparable to timescales of magmatic processes. We review the basic principles of extracting time information from U series nuclides and summarize variations in (230Th/238U), (226Ra/230Th), and (231Pa/235U) observed in magmas from mid-ocean ridges, within-plate settings, and subduction zones to contrast melt generation processes in different tectonic settings. U series disequilibria on melt and crystal phases of igneous rocks can provide temporal information on different stages in the magmatic history (melting duration, melt transport rates, magmatic crustal residence times, and timing of crystal growth) and potentially provide clues about the nature and mineralogy of mantle sources, mantle upwelling rates and porosity, fluid influences, and mechanisms of melt generation and transport. The subject is beginning to take a genuinely integrated approach to developing physically realistic quantitative models that offer increasingly exciting opportunities in the study of magmatic processes. Copyright 2005 by the American Geophysical Union.