Timescale of magma chamber processes by short-lived U-series disequilibria

Uranium series isotopes belong to three radioactive chains starting from long-lived isotopes of U and Th (i.e. 238U, 235U and 232Th) and decaying to stable Pb isotopes (206Pb, 207Pb and 208Pb, respectively) through a series of short-lived radiogenic and radioactive isotopes. In the absence of processes fractionating between the different nuclides, these chains evolve in a condition termed radioactive or secular equilibrium. In this condition the activity of each nuclide (i.e. rate of atomic disintegration: number of atoms of any nuclide multiplied by its decay constant,) is the equal and the activity ratios between any parent-daughter pair is unity, e.g., (230Th/238U)=1, where parentheses indicate activity. If fractionation occurs, and disequilibrium is produced any intermediate parent-daughter pair returns to secular equilibrium, by in-growing or decaying daughter atoms, over timescales determined by the half-life (and thus the decay constant) of the daughter nuclide, in a time equal ~5 times the half-life of the daughter nuclide after the fractionating event. The varied geochemical properties of U-series isotopes cause nuclides within the chain to be fractionated in different geological environment, while their half-lives, ranging from milliseconds to few tens thousands years, allow to investigate processes occurring at timescales from days to 105 years. During the last twenty years the development of the analytical techniques has promoted a number of studies addressing the timescale of different magmatic processes. The purpose of this contribution is to review the various applications of these isotopes in the field of magma chamber processes. U-series measurements in single volcanic samples provide first order constraints to the timescale of the fractionation event, since the occurrence of any disequilibrium constrains the process responsible for it to be younger than the time necessary to restore equilibrium. 226Ra-230Th and 228Ra-232Th pairs represent a good example of this concept, reaching equilibrium in ~8000 and 30 years respectively. Most of the volcanic rocks around the world display (226Ra/230Th) ≠ 1 and (228Ra/232Th)=1demonstrating that no Ra-Th fractionation have occurred in the 30 years before eruption. Assuming partial melting as the most likely responsible for Ra-Th fractionation, then it is possible to argue that the transfer times from the source to the surface is between 30 and 8000 years. More studies investigated the variations of the activity ratios throughout the history of a volcano, or the U-series isotope compositions of different mineral phases of a single eruption, in order to constrain the storage or residence times of the melts in magma chambers and the timescales of magma crystallisation. For this purpose classic isochron diagrams are used, where the parent and daughter activities are divided by those of a long-lived isotope of the daughter element [e.g. (230Th/232Th) vs. (238U/232Th)] or by a geochemically similar element concentrations [e.g., (226Ra)/Ba vs. (230Th)/Ba]. In such diagrams, when samples lie on straight lines, their slopes can be used to calculate to the time elapsed between the fractionating event and the eruption. These ages are significant only provided two assumption: i) that the variations among whole rock samples or minerals are due to net fractionation between parent and daughter (occurring on a timescale small respect to the half live of the daughter nuclide), and ii) that the system has remained closed after the fractionating event. The conditions necessary to generate isochrones on these diagrams are more likely to be respected for mineral separates than for whole rocks samples. Indeed, most of the U-series disequilibria measured in whole rock samples are believed not to be due to simple elemental net fractionation but to represent the result of more complex processes involving syn-melting in-growth of the daughter nuclide. Mineral isochrones for different parent-daughter pairs (mostly 230Th-238U and 226Ra-230Th) have been measured for a number of less to more differentiated volcanic rocks. These produced crystallisation ages from 102-105 years older than the eruption times, with the oldest measured in the most evolved ones, suggesting different crystallisation histories for different volcanoes (e.g. Hawkesworth et al., 2004). Moreover, the widespread lack of agreement between 230Th-238U and 226Ra-230Th ages measured on the same mineral separates has suggested that many factors needs to be considered when interpreting the results and that great attention has to be taken in the selection of the samples. Parents-daughter pairs with different half-lives register differently the prolonged crystallisation history of a magmatic system, and may reflect different mineral populations or phases of the crystal growth. Recent measurements in accessory U- and Th-rich phases (i.e. zircon, apatite) have highlighted the complexity of the crystallisation history both inside the same crystals and among different ones from the same eruption (e.g. Charlier & Zellmer 2000). Moreover, the reliability and significance of 226Ra-230Th isochrones needs also to be evaluated in the light of the possible differences between Ra and Ba partition coefficients (Cooper et al., 2001). Although not suitable for classic isochron studies, whole rocks measurements from single volcanic associations can be extremely useful to put constraints on a number of processes occurring at different timescales. The variation of U-series activity ratios [i.e. (230Th/238U), (230Th/232Th), (226Ra/230Th), (226Ra)/Ba), (210Pb/226Ra)] during the history of a volcano is strongly affected by the open or closed behaviour of the magmatic system. Therefore U-series disequilibria can be successfully utilised to discriminate volcanoes evolving in condition of closed system, periodically replenished and steady state evolution, and also to highlight period of different styles of activity within a single volcano. Models describing the isotopic composition in periodically replenished magma chambers (Hughes and Hawkesworth, 1999) are useful to describe how different parameters affect the U-series disequilibria of the erupted magmas. If the activity of a given U-series nuclide remains constant in the erupted lavas, that can be explained by an open system magma chamber having reached a steady state activity, only if the repose time between eruptions is much shorter than the half-life of the nuclide. Therefore, the same volcano can be considered in steady state for one parent-daughter pair, whilst behaving as a closed system for another, due to the different timescale investigated. For closed system the behaviour of U-series isotope is easily modelled by simply restoring secular equilibrium and, if the composition of the parental magma entering the magma chamber is known, storage times and differentiation times from differently evolved products can be estimated. For steady state systems, mass balance calculations enable to calculate the residence time of the magma in the reservoir and, from that, to estimate the volume of the reservoir (e.g. Pyle et al., 1992; Condomines et al, 1995). Recently studies of 210Pb-226Ra (Gauthier et al., 1998) disequilibria in active volcanoes have shed some lights on the magma dynamics in shallow reservoirs. The 210Pb-deficit often recorded by volcanic rocks is interpreted as resulting from degassing the highly volatile 222Rn, the precursor of 210Pb in the decay series. Models assuming physical steady state of shallow magma reservoirs have been developed to constrain residence times in the shallow magma chambers. On the other hand, 210Pb excesses in Mt St. Helens lavas have been interpreted as indicators of the build up volatiles in shallow magma chamber prior to eruptive events (Berlo et al., 2004).