Uranium Series Disequilibrium Dating in volcanic areas

Disequilibrium existing within the uranium series radioactive family (the so called ‘U-Th’ or ‘230Th’ method) can be used for chronological purposes in volcanic areas. It can be applied to volcanic rocks as well as to travertines connected to volcanic events, provided that the age of these materials (volcanics or travertines) is younger than ∼300 ka. Many problems, capable of seriously affecting the reliability of the method, arise. This paper is intended for people who deal with chronology in volcanic areas but lack the specific knowledge necessary to assess critically the dating procedures performed by scientists with different backgrounds. Direct dating of volcanics can be affected by (a) the similar and low partition coefficients for uranium and thorium shown by many major constituents of volcanic rocks, (b) the occlusion, within the major minerals, of minor ones particularly enriched in uranium and thorium, (c) uranium loss caused by alteration, (d) the various phases used to draw the isochrone being of different ages, (e) problems connected to isotopic homogeneity of magma. The main difficulties that arise when dating travertine derive from the presence of a non-carbonate phase containing 230Th that has not been generated within the travertine post-depositionally. These troublesome occurrences are discussed and methods of assessing them are suggested.