HR: 08:15h
AN: V11C-02 INVITED     [Abstracts]
TI: Beryllium Isotope and Combined Be and U-series Isotope Studies of Volcanic Arcs: Implications for Fluid and Melt Transport Through the Mantle Wedge
AU: * Morris, J D
EM: jmorris@levee.wustl.edu
AF: Dept. of Earth and Planetary Sciences, Washington University, One Brookings Dr., CB 1169, St. Louis, MO 63130-4899 United States
AU: Ryan, J G
EM: jryan@nsf.gov
AF: Dept. of Geology, University of South Florida, 4202 E. Fowler Ave., SCA 528, Tampa, FL 33620-5201 United States
AB: Beryllium isotope and combined studies of $^{10}$Be/$^{9}$Be and U-series isotopes in volcanic arcs can 1) map transport of demonstrably slab derived elements through the mantle wedge; 2) certify the relationship of U series isotopes to slab derivation; 3) identify multiple stages in subduction modification of the mantle and constrain their timescales; and 4) speak to element partitioning into fluids and melts from the slab. In the Kurile, Aleutian and Bismarck arcs, $^{10}$Be/$^{9}$Be ratios for lavas from behind the volcanic front are often comparable to, and sometimes greater than, those at the volcanic front, despite the longer path to rear-arc locations, along which $^{10}$Be is decaying in transit. Be/Zr ratios show a similar pattern of across-arc increase, without increases in enrichments of Mo and Sn, species which would be mobilized with Be if F-bearing fluids were present. The simplest interpretation is that sediment melting, and its contribution to the mantle wedge, is greater behind the front than at the volcanic front. Despite evidence for an increasing sediment melt contribution behind the front, volcanoes from the Kuriles contain progressively less B, Pb, As and Sb with increasing depth to the slab, indicating that fluid processes updip of about 180 km (beginning in the shallow forearc) strip these elements nearly quantitatively from the sedimentary portion of the downgoing slab. For studies published to date (Aleutians, Central America, S. Chile, Bismarck, Mariana) $^{10}$Be/$^{9}$Be ratios are generally highest for samples plotting furthest from the $^{238}$U-$^{230}$Th equiline (i.e. highest $^{238}$U/$^{232}$Th, $^{230}$Th/$^{232}$Th, or both). In lavas from the Southern Volcanic Zone (SVZ) of S. Chile (Sigmarsson et al., EPSL 2002), U excess (U$_{xs}$), Ra excess (Ra$_{xs}$) and $^{10}$Be/$^{9}$Be are strongly correlated (r$^{2}$=0.81-0.94). This argues that U enrichment and in some cases Ra enrichment in arc lavas is related to slab processes that are capable of mobilizing $^{10}$Be out of the sediment column, rather than reflecting only dynamic melting processes or element partitioning in a hydrous mantle wedge. If U and Ra are thought to be carried in a fluid from the altered oceanic crust, that fluid must also carry sediment-derived elements, at least in the SVZ. In Nicaragua, the Aleutians and S. Chile, volcano $^{10}$Be/$^{9}$Be ratios correlate well with $^{230}$Th/$^{232}$Th, $^{143}$Nd/$^{144}$Nd and U$_{xs}$ and Ra$_{xs}$, respectively. The well constrained mixing lines require that both the subducted component and the mantle to which it is added be relatively homogenous for these slab-derived tracers. At zero $^{10}$Be (i.e. prior to sediment addition within the last 1-2 Ma), the Nicaraguan mantle is characterized by high Ba/La, $^{87}$Sr/$^{86}$Sr and $^{230}$Th/$^{232}$Th significantly elevated above MORB or OIB values, attributed to earlier subduction modification of the mantle (Reagan et al., GCA 1994). In the SVZ of Southern Chile, at zero $^{10}$Be, the $^{230}$Th/$^{232}$Th of the inferred mantle source is ~0.9, implying sediment addition prior to the more recent event that introduced $^{10}$Be. By contrast, at zero $^{10}$Be, the inferred Aleutian mantle has a $^{143}$Nd/$^{144}$Nd ratio of $\sim$ 0.5131, suggesting little or no prior sediment addition to the mantle.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting