HR: 1330h
AN: V52A-0419    [PDF]
TI: Light element systematics of Woodlark Basin volcanic rocks - evidence for old and new slab inputs?
AU: * Hendren, M
EM: hendren8@hotmail.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620 United States
AU: Ryan, J
EM: ryan@chuma.cas.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620 United States
AB: In the Woodlark Basin/New Georgia subduction system, late Miocene collision of the Ontong Java Plateau changed the polarity of subduction, from SW subduction of the Pacific plate beneath the Solomon Is., to very steep, NE subduction of the Woodlark Basin microplate. This recent subduction has generated Plio-Pleistocene volcanism in the New Georgia Group unusually close to the trench. Questions have been raised as to whether this melting is triggered by recent slab additions beneath the New Georgia Group, or represents mantle modified by past subduction reheated by a new, hot slab. To address these questions, we have examined Li, Be and B systematics in a suite of lavas from the New Georgia Group (NG) and the Woodlark Spreading Center (WSC), which is being subducted beneath the NG. Boron enrichments can indicate relatively recent ($<$5Ma) slab inputs at arcs, while Li abundance and isotopic variations may reflect longer-term slab enrichments, as Li can be retained in olivine and other magnesian mantle minerals. Li-B decoupling has been used to suggest the presence of older slab-modified mantle in the Panamanian arc segment (Tomascak et al. 2000). The Li, B, and Be concentrations of our samples vary from 3-38 ppm, 2-34 ppm, and 0.08-2 ppm, respectively. Li and Be vary regularly with MgO content and indices of differentiation in NG lavas, while B show greater scatter. B/Be ratios range widely in NG lavas, but the minimum values encountered at each center show a pattern of decline toward the WSC-trench intersection. Li/Y and Li/Yb ratios show no change with position, and are less variable overall. The Li and B concentrations in WSC MORB samples are slightly elevated relative to MORBs (5.5-7.0 ppm Li; 2.1-3.3 ppm B) compared to normal MORBs (3-4ppm), as are Li/Yb and B/Be ratios (2.1-2.8 and 3.1-9.7, respectively). Differences in B enrichment relative to the WSC intersection point to recent subduction inputs, while elevated Li in NG and WSC samples may indicate pre-existing mantle wedge enrichments. Li isotopic results may shed further light on this question.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting