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Title: Geochemistry of the Northern Izu-Bonin arc : implications for crustal recycling and mantle processes
Author: Clifford, David James
ISNI:       0000 0001 3559 0555
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2007
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The aim of this study is to better understand chemical recycling at destructive plate boundaries. Isotopic, major and trace element data of lavas sampled at varying distances from the trench in the Izu-Bonin arc provide constraints on the different types of flux that influence subduction zone magmas. Geochemical models are employed to quantify the exchange of elements between flux and mantle that lead to the creation of arc magmas. These calculations allow the changes in fluid flux from the slab and the variations of mantle composition to be correlated with the variations in across-arc geochemistry. All of the analysed lavas show arc-related geochemical signatures such as enriched LILEs, depleted HFSEs and fluid-modified strontium isotopes. However, a systematic variation in isotope and trace element ratios is recognised across the Izu-Bonin arc. Radiogenic 87Sr/86Sr and 143Nd/144Nd, low Nb/Y and high Ba/La in the volcanic front suggests melting of a depleted mantle source influenced by an aqueous fluid (AQF). Less radiogenic 87Sr/86Sr and 143Nd/144Nd and higher Th/Ce and Nb/Y in the rear and back arc suggest melting of a more fertile mantle metasomatised by supercritical fluids (SCF). Positive correlations between 87Sr/86Sr and 143Nd/144Nd are observed in lavas sampled across the width of arc. The correlation is also found within some volcanoes found behind the volcanic front (Taga and Hakone). This suggests that magmas affected by AQF and by SCF are tapped by single volcanoes and at least partially mixed prior to eruption. Analysis of chemostratigraphic variations in the Ashtaka, Taga, Hakone, Hachijojima, and Oshima volcanoes reveal that the fluxes are not in a steady state. Individual volcanoes can tap heterogeneous mantle sources and variable fluid input over tens of ka’s. High-precision double-spike Pb isotope measurements provide estimates of the amount of overprinting of the slab fluid isotope signature by the mantle wedge. The effect is greatest along the volcanic front where 70% of the Pb is derived from slab fluids compared to 34% in the back arc. Pb isotopes also highlight that the mantle source is heterogeneous along the arc with Δ8/4Pb and Δ7/4Pb increasing toward the north. The composition of the crust beneath the arc is estimated using the Tanzawa plutonic complex, and is found to be isotopically similar to the northern Izu-Bonin arc. This suggests that crustal contamination will only have a minor effect on the isotopic composition of the arc. The isotopic composition of Tanzawa complex indicates a source resembling the Indian Ocean mantle domain was present in the mantle wedge before 7Ma.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology ; GC Oceanography