HR: 1400h
AN: V53A-03 [Abstracts]
TI: A trace element perspective on the veined sub-arc mantle: Melts, fluids, or both?
AU: * Halama, R
EM: rhalama@geol.umd.edu
AF: Dept. of Geology, University of Maryland, College Park, MD 20742, United States
AU: Savov, I P
EM: savov@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC
20015, United States
AU: McDonough, W F
EM: mcdonoug@geol.umd.edu
AF: Dept. of Geology, University of Maryland, College Park, MD 20742, United States
AU: Rudnick, R L
EM: rudnick@geol.umd.edu
AF: Dept. of Geology, University of Maryland, College Park, MD 20742, United States
AB:
Convergent margin magmas carry slab signatures, but it is not clear whether this signature is transferred from
slab to mantle wedge via an aqueous fluid, a silicate melt, or a supercritical liquid, as the chemical
characteristics of arc volcanics can be explained by either melt-mantle interaction or slab fluid additions. In the
Kamchatka Arc, mantle xenoliths carried in lavas from Avachinsky volcano comprise refractory harzburgites
dissected by amphibole-bearing wherlitic veins. The very low modal clinopyroxene content, the refractory mineral
chemistry, and the low bulk Al2O3 and CaO contents in the harzburgites provide evidence for melt
extraction. The veins contain orthopyroxene (En88-94), clinopyroxene (En53-48Wo44-49), and
minor amounts of magnesiohornblende, but lack olivine. Primitive-mantle normalized trace element patterns of
cpx and amphibole show enrichment of U over Th (UN/ThN > 2) and relative enrichments of Pb and
Sr compared to elements with similar compatibilities (SrN/NdN > 2.7, PbN/CeN > 2.4).
Furthermore, clinopyroxene is characterized by low HREE contents, relative depletions in HFSE, and a positive Li
peak. In contrast, amphibole is enriched in HFSE, but has no Li anomaly. Trace element concentrations of bulk
veins, calculated from modal mineralogy and mineral compositions, are characterized by relative enrichments in
Sr, Pb and Li. These features are also characteristics of both basaltic and andesitic primitive island arc volcanic
rocks and suggest a mantle wedge source affected by metasomatic processes that are also evident in the host
lavas. The mantle-like Sr isotopic composition of the bulk vein material (0.7040-0.7048), along with the wehrlite
vein - host harzburgite Li isotope equilibrium (δ7Li ~ 4 ± 1.5), support the idea of melt-
dominated metasomatism. On the other hand, enrichment of the fluid-mobile elements, in particular Pb, and the
elevated U/Th ratios are more consistent with fluid-dominated metasomatism.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly