HR: 09:15h
AN: V11A-06    [Abstracts]
TI: Evidence for Magma-Mixing and Disequilibrium in 'Primitive' Basaltic Andesites From Mount Shasta, Northern California
AU: * Leeman, W P
EM: wleeman@nsf.gov
AF: National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230 United States
AU: Streck, M J
EM: streckm@pdx.edu
AF: Portland State University, PO Box 751, Portland, OR 97207 United States
AU: Chesley, J T
EM: jchesley@geo.arizona.edu
AF: University of Arizona, PO Box 210011, Tucson, AZ 85721 United States
AU: Tonarini, S
EM: s.tonarini@igg.cnr.it
AF: IGGI/CNR, Via Maffi 36, Pisa, 56125 Italy
AB: High-Mg basaltic andesites near Mt. Shasta volcano have been considered fundamental to establishing the existence of exceptionally water-rich primary magmas in this system, implying significant slab-derived fluid fluxes into the underlying mantle wedge (Grove et al., 2002). This notion was reinvestigated via new mineralogical and geochemical studies of fresh scoria blocks from the Whaleback volcano (loc. S17; Anderson,1979). These high-Mg andesites (58% SiO₂, 8.5% MgO, Mg# = 76, 120 ppm Ni, 550 ppm Cr) carry small dunitic xenoliths and xeno/phenocrysts (ol+opx+cpx). Plagioclase is not a liquidus phase. Electron microprobe traverses and back-scattered images show that mafic silicates, particularly pyroxenes, have complex histories. Olivine compositions of larger crystals and interiors are often above Fo₉₀ up to Fo₉₄ whereas microphenocrysts and rims of larger crystals are ~Fo₈₇. Complexities among pyroxenes include: (a) Cores of opx and cpx with low Mg# (~67) containing melt inclusions; this evidence indicates these pyroxenes crystallized from magma of roughly dacitic composition; (b) Virtually all low Mg# grains are resorbed and have overgrowths (~20 microns) of high Mg# (87-92) that may be internally zoned arriving at a Mg# near 80 at the outermost euhedral rim; (c) Another variant is orthopyroxene with 'wormy' texture and either a thin (~15 microns) euhedral overgrowth or anhedral outline; compositions of resorbed interiors and overgrowth are similar ( Mg# range: 80 to 90), but distribution of lower and higher Mg# in resorbed areas is patchy whereas any compositional zoning of overgrowth follows crystal shape and arrives again at a Mg# of ~80 at the outermost rim. These data record mixing of diverse magmas (dacite and one or more basaltic liquids) combined with entrainment of ultramafic crystal debris during wall rock contamination, and eventual cooling and equilibration. Low Al₂O₃ contents in the pyroxenes imply that these minerals grew at relatively low pressure. Given these relations, the erupted hybrid magma is a product of open system processes and it is unlikely to retain a high fidelity record of its deeper origins. The inferred high water contents are associated with the highest Mg# ol and opx (Anderson, 1979), but the origin of these signatures remains enigmatic. Low B contents and δ¹¹B (ca. -5 permil) in this and other Shasta lavas are inconsistent with significant involvement of slab-derived fluids.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005