HR: 10:30h
AN: V52A-01 INVITED     [Abstracts]
TI: Fallout Tephra as Tool for Reconstructing the Geochemical Evolution of Arcs - a Case Study From Izu-Bonin-Mariana Arc/Trench System
AU: * Straub, S M
EM: smstraub@fas.harvard.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: * Straub, S M
EM: smstraub@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: While we are approaching an understanding of the currently active volcanic arcs, it is clear that major secular tectonic and chemical changes occur over the lifetime of arc systems. Such changes may substantially modify the arc outflux, and hence the impact of subduction zone volcanism on the global geochemical cycle. Volcanic arcs are mobile and dynamic systems that are subject to perpetual and rapid change. Therefore, a reliable reconstruction of the secular chemical evolution of arcs requires a temporally highly resolved rock record. Commonly, such a record cannot be obtained from lava series. However, it can be obtained from the distal fallout tephra that is generated from the explosive arc volcanism and that becomes embedded into the sediments of the adjacent basins. The unique geological potential of such fallout tephra is now evident from studies of the Izu Bonin Mariana arc (IBM), an intra oceanic arc in the NW Pacific that has been active since ~50 million years. Extensive drilling in the course of the DSDP and ODP Programs recovered both lava and tephra series. While the lavas record is disrupted by large gaps (>10 million years) of uncertain time and length, the fallout tephra provides a coherent, temporally highly resolved rock record (~1 Ma) of 40 million years. Fallout tephra are composed of variable proportions of volcanic glass shards, rock fragments (lithics) and phenocrysts. While alteration increases with time, fresh material is still present in tephra older than 40 million years. Careful separation of ash-sized particles prior to analysis and the usage of micro-analytical methods demonstrated that the basaltic through rhyolitic fallout tephra are the compositional counterparts of the IBM lavas series and thus provide a vital complement to studies of the temporal evolution of the IBM. The tephra profiles show that several influences are represented in the chemistry of the IBM. For one, steady-state processing is an important component of the IBM evolution, as shown by the constancy in major element oxides (except K₂O) throughout most of the IBM's history. Notably, the major element chemistry appears to be largely unrelated to the expected crustal thickening with time. To the other, the temporal trends of trace elements and radiogenic isotopes demonstrate that the arc chemistry is responsive to changes of the arc input by slab and mantle wedge, and likely to transient tectonic changes as well (e.g. backarc basin formation). Furthermore, by means of the volcanic glass, we obtained data of elements and isotopes sensitive to the water fluxes through subduction zone (e.g. Cl, B, Li, B isotopes). Our results suggest that the time-dependent variations of these elements may allow for studying the role of 'subduction serpentinization', the subduction-related hydration of the upper mantle, in the waxing and waning of arcs. In summary, the IBM case study provides strong evidence that close linkages exist between the chemistry of arcs and the evolution of the Earth's surface reservoirs, and thus reinforces the role of arc volcanism in shaping the habitable environment of Earth.
DE: 1030 Geochemical cycles (0330)
DE: 3640 Igneous petrology
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly