HR: 09:00h
AN: V11A-05    [Abstracts]
TI: Seeking the Spoor of Reactive Bulk Assimilation.
AU: * Beard, J S
EM: jbeard@vmnh.net
AF: Virginia Museum of Natural History, 1001 Douglas Ave., Martinsville, VA 24112 United States
AB: The integration of crustal solids and liquids into mantle-derived magmas as a consequence of reactive bulk assimilation is a key process in the petrogenesis of most large-volume intermediate to silicic magmas. Feldspars, especially plagioclase, are ideal monitors for solid input into magmatic systems because they commonly preserve a chemical and isotopic record of their history. However, much of the solid material input during bulk assimilation is in the form of less robust phases, especially quartz, amphiboles, micas, pyroxenes and Fe-Ti oxides. In hydrous systems (e.g. arcs and any region where assimilation of hydrated crust occurs), incongruent melting and crystallization reactions involving amphiboles and micas are the principal controls on the redistribution of elements and isotopes amongst solids and melts during bulk assimilation. These reactions convert hydrous silicate assemblages (e.g. biotite gneiss or amphibolite) to an anhydrous (pyroxene granulite) solid assemblage coexisting with a siliceous (trondjhemitic to granitic) melt during assimilation and produce amphibole and mica via equilibrium (essentially) reaction between solid pyroxenes and oxides and evolved melts during late crystallization. The spoor of these reactions is likely to be subtle and commonly either avoided or ignored during geochemical study. Reactive crystallization features are relatively familiar and include replacement of pyroxenes and oxides by amphibole and biotite. The subtleties of chemical redistribution that can accompany these reactions, however, are less well appreciated. For example, solid reactants may carry the chemical and isotopic signatures of crustal xenoliths, mantle phenocrysts, or both, while the chemistry of the reactant melt likely reflects complex AFC (as well as low-T, equilibrium reaction crystallization) processes. Remnants of xenoliths that have undergone extensive dehydration melting may be more difficult to recognize. The presence of single plagioclase crystals that retain a crustal signature suggests disaggregation of some xenoliths down to the scale of individual crystals. In such cases, remnants of xenolithic solids will be difficult or impossible to recognize petrographically. On the other hand, if disaggregation is less complete, ghosts of individual xenoliths may still be found, even if they are microscopic or represented only by trains of xenocrysts. Easily recognizable crustal xenoliths probably contribute little to the chemical modification of the magma as a whole, either because they are refractory/non-reactive or because they were incorporated when the magma was very close to its solidus.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005