HR: 09:45h
AN: V11A-08    [Abstracts]
TI: Crystalline Debris, Not Phenocrysts: Mineralogical Evidence for Partial Assimilative Recycling of the Mafic-Ultramafic Plutonic Roots of a Continental Arc
AU: * Dungan, M A
EM: Michael.dungan@terre.unige.ch
AF: Section of Earth Sciences, University of Geneva, 13 Rue des Maraichers, Geneva, 1205 Switzerland
AU: Costa, F
EM: Fidel.Costa-Rodriguez@ruhr-uni-bochum.de
AF: Institut fur Geologie, Mineralogie, und Geophysik Ruhr-Universit„t, Bochum, Ruhr-Universitat Bochum, Bochum, 44780 Germany
AU: Goldstein, S
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, PO Box 1000, Palisades, NY 10964 United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: C.H. Langmuir, Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138 United States
AU: Leeman, W P
EM: leeman@rice.edu
AF: W.P. Leeman, Dept. of Geology and Geophysics, Rice University, Houston, TX 77005 United States
AU: Davidson, J P
EM: j.p.davidson@durham.ac.uk
AF: Dept. Geological Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AB: Phenocryst proportions in many mafic lavas of the Quaternary Tatara-San Pedro complex (TSPC; 36°S, Chilean Andes) are minor compared to modal abundances of coarse olivine, clinopyroxene, and plagioclase xenocrysts derived from disaggregated mafic-ultramafic xenoliths. The major and trace element imprint of this process is significant, but compatible and incompatible elements are poorly anti-correlated due to irreversible blending of grain-boundary melts and variable retention of xenocrysts. Associated isotopic variations are minor and decoupled from the elemental signal, apparently due to the young age of the assimilated lithologies. Healed microfractures in embayed olivine xenocrysts (Fo_85-75), which are rampant in mafic and ultramafic xenoliths at the TSPC, are proof of subsolidus histories. Evidence for the former presence of plagioclase, othopyroxene, amphibole, and phlogopite as replacement-reaction products after olivine and other phases is preserved as secondary mineral assemblages which are the crystallization products of Mg- and alkali-rich grain-boundary melts that were encapsulated in partly replaced olivines: (1) F-phlogopite with widely varying compositions and modal abundances within and among olivines in the same thin section, (2) multiple spinel populations that require multiple origins, (3) relatively sodic plagioclase (An_40-60) with very high ferric iron, and (4) shifts in oxygen fugacity of up to +1.4 log-units in contaminated magmas relative to uncontaminated parental magmas (NNO buffer). Residence times of olivine xenocrysts from diffusion-rate modeling are 0.2-24 yrs, with a mode at 3-5 yrs. Short residence times correlate with the preservation of phlogopite-rich assemblages that crystallized from trapped grain-boundary melts, whereas those in olivines with long residence times are partially equilibrated with host magmas. Stoping and digestion of such xenoliths is fast and efficient with regard to modification of basalt compositions, but the mineralogical record may be partly obscured by magma-storage times as short as 5-25 years (i.e., much shorter than typical repose periods at arc volcanoes).
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3625 Petrography, microstructures, and textures
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005