Time scales of magmatic processes Chris Hawkesworth, Rhiannon George, Simon Turner, Georg Zellmer

Paper presentation by  

The General Idea Rocks come from magma… thus are called igneous rocks…. But we already know this… however we also know that igneous rocks are the dominant record of magmatic processes. Where do we look?

Groundmass Crystals Minerals and textures reflect the processes of crystallization and differentiation, once the magmas have left the sites of melt generation. Most data/information come from the application of the short lived U-series isotopes Time scales can now be found by using short-lived isotopes, from major and trace elements in crystals that have been modified by diffusion, crystal size distributions (CSD), the power output of volcanic systems, and from the rates of changes in volcanic stratigraphy

Measuring Time Scales When discussing the generation of a igneous rock it is NB to clear about what is been investigated due to number of stages involved. Groundmass (previously the melt) is best for determining the eruption due to quenching. The larger and hence older crystals can also be dated. They are normally zoned and thus have recorded conditions of crystallization and growth history.

What age info is available? Dating techniques are divided into two groups: 1.Those that give Absolute ages 2.Those that give Relative ages Absolute: use the U-series isotopes but can only be used in young rocks( <300 ka) Relative: use ages of crystal populations acquired from the CSD (crystal size distributions) and major, trace element and SR isotope profiles

238 U -> 230 Th -> 226 Ra

Ages of crystals at eruption – radiogenic isotopes Most crystal ages come from radiogenic isotope data from analyses of mineral seperates. Majority of ages are from U-Th, but Rb-Sr and Ar- Ar is also used and more commonly Ra 226 -Th 230. Zircon are increasingly been used for U-Th dating because they have high concentrations of these elements, they are robust, they are often zoned and so can preserve records of a number of events. Small amounts of zircon can be used analysed. Ages of crystals at eruption

As Si+Al (mol %) increase so does the viscosity. This is most obvious at Si+Al = 66

Ages of crystals at eruption – concordant & Relative ages Concordant testing is basically using two tests to get to the same value i.e. two different isotopes the overlap Relative ages indicate how long a mineral (or glass) has been at a certain temp. by using diffusive re-equilibrium across the boundaries of chemical zones the formed during crystallisation (both in crystals and melt inclusions) i.e. Fe-Mg in olivine, Ti in magnetite, Ca & Na in plagioclase

Often attempts to confirm the reliability of age deternination by showing that similar ages can be obtained using different techniques have often ended in failure. This in its own right may shed insights into the time scales. Discordant ages

Conclusion Many crystals appear to have formed within tens to hundreds of years prior to eruption, but more evolved rock type can preserve crystallisation histories of up to 10 5 years Generation of evolved magmas is thermally controlled, irrespective of whether that involves crustal melting or closed system fractional crystallisation

Summary of Time Scales Fluids from the subducted slab m/yr Solid diffusion rates cm 2 /s Rates of crystal growth cm/s Rates of crystal dissolution 5-20 mol/cm 2 /s Ages of crystals at eruption up to 1.5 Ma Rates of magma differentiation: Rb/Sr isochrons2.5 x km 3 of magma/yr U-Th-Ra fraction of magma crystallised 3.5x10 -4 /yr Incubation period for crustal melting years Magma ascent rates 26 km/day from xenolith-bearing magmas 10 km/yr from U-series isotopes Rates of eruption: Plinian m/s Sub-plinian <cm/s

Other references Bernard Bourdon, Simon P. Turner and Neil M. Ribe; Partial melting and upwelling rates beneath the Azores from a U- series isotope perspective, Earth and Planetary Science Letters Volume 239, Issues 1-2, 30 October 2005, Pages S Turner, S Black, K Berlo; 210 Pb– 226 Ra and 228 Ra– 232 Th systematics in young arc lavas: implications for magma degassing and ascent rates; Earth and Planetary Science Letters, 2004 G.F. Zellmer, C. Annen, B.L.A. Charlier,, R.M.M. George, S.P. Turner, C.J. Hawkesworth, Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies; Journal of Volcanology and Geothermal Research, 2005