HR: 17:00h
AN: V44C-05    [Abstracts]
TI: Constraining differentiation processes and timescales from mineral-scale isotopic data
AU: * Davidson, J
EM: j.p.davidson@durham.ac.uk
AF: University of Durham, Department of Earth Sciences, University of Durham, Durham, DH13LE, United Kingdom
AU: Charlier, B
EM: b.l.a.charlier@open.ac.uk
AF: The Open University, Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
AU: Morgan, D
EM: d.morgan@see.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Earth Science Building, University of Leeds, Leeds, LS2 9JT, United Kingdom
AB: The mechanisms by which magmas diversify en route to the surface and the timescales over which this differentiation occurs have received a great deal of attention over the past decade. Many magma systems appear to be characterised by crystal recycling and cannibalisation of progenitor products on short timescales. Single crystal isotopic data indicate that magmas contain crystal cargoes that have formed in different places and at different times, and have been aggregated shortly before, or during, emplacement/ eruption. Integration of isotopic zoning with textural features in crystals commonly reflects multiple recharge and mixing of different composition magmas. The fact that individual crystals from the same rock may have different isotopic zoning profiles further indicates that they do not share a common differentiation history. The c. 5000 cubic kilometer Fish Canyon Tuff, for example, exhibits extreme inter- and intra-crystal Sr isotopic diversity, despite being relatively homogeneous with respect to major and trace elements at the bulk rock scale. Such heterogeneity is more likely inherited from progenitor magma systems which have incubated in and interacted with the crust, rather than from mantle sources. The timescales over which the processes of contamination, recharge and mixing operate can be estimated from the compositional gradients in the crystals. The failure of small biotite grains in the Fish Canyon Tuff to isotopically equilibrate with the host magma requires entrainment and eruption within a timescale less than that required for diffusive equilibration – in this case a few thousand years. The common observation that crystal rims are sometimes not in isotopic equilibrium with the host glass is consistent with entrainment of crystals into the host melt shortly before eruption. The resolution of isotopic sampling is poor but timescales indicated are typically less than 1000 years. Trace element profiling constrains timescales better and can suggest pre- eruption mixing timescales of weeks to years.
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting