Uranium-series isotope and thermal constraints on the rate and depth of silicic magma genesis

Uranium-series isotopes provide important constraints on the timescale of magma differentiation and this can be used to identify where in the crust and silicic magmas acquire their geochemical characteristics. Timescales of differentiation can be inferred from the observed co-variations of U-series disequilibria with differentiation indexes. When crustal assimilation of secular equilibrium material is involved, inferred timescales will generally decrease. In turn, they will increase if periodical recharge (.20 wt% relative volume) of the magma body occurs. If crustal assimilation and magma recharge occur concurrently, inferred timescales for differentiation can be similar to that of closed system differentiation. We illustrate the approach with data from Mount St Helens which suggest that dacitic compositions are produced in c. 2000 years. Combining this with recent evidence for an important role for amphibole fractionation suggests that differentiation of a c. 10 km magma body at this volcanic centre occurs at 8–10 km depth in the crust. The genesis of silicic magmas is important for understanding the growth of the continental crust and the origin of explosive eruptions because the upper crust is dominated by silicic igneous rocks, and evolved lavas are responsible for many of the most dangerous volcanic eruptions. Over the last two decades, numerous efforts have been undertaken to constrain how and where silicic magmas are generated (e.g. Davidson 1985; Huppert & Sparks 1988; Bergantz 1989; Bergantz & Dawes 1992; Laube & Springer 1998; Petford & Gallagher 2001; Orozco-Esquivel et al. 2002; Annen & Sparks 2002; Annen et al. 2006). In particular, recent thermal modelling has shown that repetitive intrusions of basalt can melt pre-existing intrusions in the lower crust and that the combination of residual magmas from basalt crystallization and these crust-derived melts in a deep crustal hot zone could produce silicic magmas and account for observed compositional variations in silicic volcanic systems (Annen & Sparks 2002; Annen et al. 2006). However, the timescales of magma differentiation can also be used to constrain the depth where silicic magmas acquire their geochemical characteristics and thus provide an important test of these models. An important aspect of the model of Annen et al. (2006) is that, whilst silicic magmas can be produced rapidly as the residue of basalt crystallization, the timescales for large volumes of melt to accumulate by melting of the crust are very long (c. 100 ka). One constraint upon the timescales and relative proportions of liquid derived from recently emplaced basalt v. crustal melt in these hot zone models is that most silicic arc magmas contain significant Th/U and Ra/Th disequilibria (Cooper & Reid 2003; Turner et al. 2003a, b for a recent compilation). These disequilibria are believed to be derived from slab fluids (Turner et al. 2001; Bourdon et al. 2003). Thus, in order to preserve radioactive disequilibria, significantly less than 8 ka (five half-lives of Ra) can elapse during the evolution from mantle-derived basalts to andesitic and dacitic compositions and their eruption. Moreover, many suites of rocks from individual volcanoes show a correlated decrease in (Ra/Th) with increasing differentiation (as measured by SiO2 or Th content). This has been taken to indicate that differentiation occurred over a timescale proportional to the half-life of Ra and detailed studies have used the change in (Ra/Th) to infer the timescale in detail (e.g. Turner et al. 2003a, b; George et al. 2004). In this case, the silicic rocks must derive their geochemical characteristics almost entirely from crystallization of a single sill of basalt. Consequently, there is a need for a model that accounts for both the rapid generation of silicic magmas (in order to preserve Ra–Th disequilibrium) and their geochemical and isotopic diversity and evidence for crustal assimilation in many instances (e.g. Smith & Leeman 1987; Grove et al. 1988; Tepper et al. 1993; Bourdon From: ANNEN, C. & ZELLMER, G. F. (eds) Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society, London, Special Publications, 304, 169–181. DOI: 10.1144/SP304.9 0305-8719/08/$15.00 # The Geological Society of London 2008. et al. 2000). Moreover, if andesites are directly derived from crystallization of a basaltic sill, this requires no contribution from residual melts of previously crystallized intrusion. A second issue is that hydrous basalt cooling in the midto lower crust will crystallize amphibole and partial melts of earlier intruded basalt will probably form in the presence of residual amphibole. Thus, it now appears that amphibole fractionation may play a more critical role in the compositional evolution of arc magmas than the gabbroic assemblages with which they typically erupt (Davidson et al. 2007). Radium can be moderately compatible in amphibole (Blundy & Wood 2003) and so there is also a need to appraise the effects of amphibole fractionation on Ra disequilibria and to reconsider how this may affect our inferences about the timescales of differentiation. Accordingly, we have explored two endmember models. In the first, the radioactive disequilibrium in andesites and dacites is largely derived from zero-aged basalt mixed with crustal and residual melts just before eruption, since mixing has the potential to be an important process for the production of some intermediate silicic rocks (Zellmer et al. 2005). In the second model, crustal and residual melts are mixed with a primary basaltic magma during crystallization (assimilationfractional crystallization). The role of amphibole fractionation during this assimilation-fractional crystallization has also been assessed. Model I: mixing of zero-aged basalt with crustal and residual melts just