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

Abstract

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
?8/4Pb and ?7/4Pb 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.

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