Geochemistry of the Northern Izu-Bonin arc: implications for crustal recycling and mantle processes

The aim of this study is to better understand chemical recycling at destructive plate boundaries.<br/>Isotopic, major and trace element data of lavas sampled at varying distances from the trench in<br/>the Izu-Bonin arc provide constraints on the different types of flux that influence subduction zone<br/>magmas. Geochemical models are employed to quantify the exchange of elements between flux<br/>and mantle that lead to the creation of arc magmas. These calculations allow the changes in<br/>fluid flux from the slab and the variations of mantle composition to be correlated with the<br/>variations in across-arc geochemistry.<br/>All of the analysed lavas show arc-related geochemical signatures such as enriched LILEs,<br/>depleted HFSEs and fluid-modified strontium isotopes. However, a systematic variation in<br/>isotope and trace element ratios is recognised across the Izu-Bonin arc. Radiogenic 87Sr/86Sr<br/>and 143Nd/144Nd, low Nb/Y and high Ba/La in the volcanic front suggests melting of a depleted<br/>mantle source influenced by an aqueous fluid (AQF). Less radiogenic 87Sr/86Sr and 143Nd/144Nd<br/>and higher Th/Ce and Nb/Y in the rear and back arc suggest melting of a more fertile mantle<br/>metasomatised by supercritical fluids (SCF).<br/>Positive correlations between 87Sr/86Sr and 143Nd/144Nd are observed in lavas sampled across<br/>the width of arc. The correlation is also found within some volcanoes found behind the volcanic<br/>front (Taga and Hakone). This suggests that magmas affected by AQF and by SCF are tapped<br/>by single volcanoes and at least partially mixed prior to eruption. Analysis of chemostratigraphic<br/>variations in the Ashtaka, Taga, Hakone, Hachijojima, and Oshima volcanoes reveal that the<br/>fluxes are not in a steady state. Individual volcanoes can tap heterogeneous mantle sources and<br/>variable fluid input over tens of ka’s.<br/>High-precision double-spike Pb isotope measurements provide estimates of the amount of<br/>overprinting of the slab fluid isotope signature by the mantle wedge. The effect is greatest along<br/>the volcanic front where 70% of the Pb is derived from slab fluids compared to 34% in the back<br/>arc. Pb isotopes also highlight that the mantle source is heterogeneous along the arc with<br/>?8/4Pb and ?7/4Pb increasing toward the north.<br/>The composition of the crust beneath the arc is estimated using the Tanzawa plutonic complex,<br/>and is found to be isotopically similar to the northern Izu-Bonin arc. This suggests that crustal<br/>contamination will only have a minor effect on the isotopic composition of the arc. The isotopic<br/>composition of Tanzawa complex indicates a source resembling the Indian Ocean mantle<br/>domain was present in the mantle wedge before 7Ma.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2007

Author dc:creator

• Clifford, David James