HR: 15:25h
AN: V23D-08    [Abstracts]
TI: Diffusion-Reaction Between Basaltic Andesite and Gabbro at 0.5 GPa: an Explanation for Anorthitic Plagioclase?
AU: * Lundstrom, C C
EM: lundstro@uiuc.edu
AF: Dept. of Geology, Univ. of Illinois, Urbana, IL 61801
AU: Boudreau, A E
EM: boudreau@duke.edu
AF: School of Earth Sciences, Duke Univ., Durham, NC 27708
AU: Pertermann, M
EM: mperter@uiuc.edu
AF: Dept. of Geology, Univ. of Illinois, Urbana, IL 61801
AB: Despite the remarkably smooth variation in bulk composition of erupted lavas at Arenal volcano (1968-2003), mineral compositions vary widely. Plagioclase ranges from An$_{52}$ to An$_{95}$ while Cr2O3 in CPX varies from 0.7 to 0.05 wt % (Streck et al., 2003). To address the question "how do bulk compositions remain near-steady-state while crystal compositions vary widely," we have performed 2 diffusion-reaction experiments in the piston cylinder at 0.5 GPa. These juxtaposed Arenal basaltic andesite AR-8 at $1200\deg$C with a Stillwater Complex gabbro, lying in a thermal gradient toward the piston. In one experiment, we synthesized a glass-plagioclase (An$_{67-75}$) aggregate of AR-8 in a graphite-Pt-Ti capsule at P-T, polished one end, dried tracer solutions of 45Ca, 6Li, 84Sr and 136Ba on its surface, and juxtaposed it with gabbro for 13 days. Profiles of bulk composition as a function of distance from the interface show that AR-8 gains Al2O3, MgO and CaO from the gabbro and loses Na2O, K2O, SiO2 and FeO to it. Notably, a plagioclase rich (65%) layer develops at the interface between the two materials as CPX disappears. This layer and the compositional profiles are reproduced by diffusion-reaction models using IRIDIUM (Boudreau, 2003). Plagioclase at the interface develops a texture of homogeneous anorthitic cores (An$_{90}$) that abruptly shift to 10$\mu$m rims having compositions (An$_{67}$) in Na-Ca exchange equilibrium with the co-existing melt. A beta track map shows that 45Ca is incorporated into the plagioclase cores while SIMS analyses indicate isotopic equilibration between core and melt. Thus, these anorthitic plagioclase result from diffusion-reaction with efficient chemical communication between the melt and the plagioclase core. Microchannels cutting through the rim, rather than solid-state diffusion, appear to control re-equilibration. Other observations from the experiment parallel Arenal lavas: Mg# variation in OPX is small in both experiments and lavas while profiles of Cr show that significant amounts of Cr move from gabbro to AR-8, possibly explaining Cr2O3 variations in Arenal CPX. The near-steady-state behavior at Arenal could reflect a flux balance between ascending magmas and melt from the surrounding crust reflecting diffusion-reaction.
DE: 8439 Physics and chemistry of magma bodies
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting