Chapter 7 Studies of short-lived radionuclides in Santorini volcanics

Abstract

ExcerptDisequilibria between the short-lived radioactive nuclides 226Ra, 230Th and 238U in young volcanic rocks can be used both for dating the time of crystallisation of a rock, and as isotopic tracers of the sources of magmas and the chemical evolution of volcanic systems. The principles of these methods have been reviewed extensively elsewhere (Condomines et al. 1988; Gill et al. 1992; Gill & Condomines 1992; Macdougall 1995). In this chapter, we present new 238U-230Th-226Ra disequilibria data for the Thera pyroclastics and for selected lavas. These data complement the radiogenic isotopic data presented in Chapter 6 on the compositional evolution through time of the pyroclastic ejecta. In particular, we find that the (230Th/232Th) isotopic ratio corrected to the time of eruption varies in a systematic way with time through the second cycle of the Thera pyroclastics and variations in this ratio are correlated with the variations of 87Sr/86Sr, 143Nd/144Nd and Pb isotopes.Systematics and conventionsThe contents of different U- and Th-decay series nuclides as measured by alpha-counting are conventionally expressed as activities (with units of 'disintegrations per minute per gram of rock'), and activities and activity ratios are given in parentheses to distinguish them from weight ratios. 238U-230Th disequilibria in a suite of cogenetic rocks or minerals may be used to date young (< 300 ka old) volcanic deposits by isochron methods (Allègre 1968), as the 75 ka half-life 230Th decays towards equilibrium with 238U. For any system, the ratio (230Th/232Th) at time t is given by: (230Th/232Th)t = (230Th/232Th)i