Across‐arc geochemical trends in the Izu‐Bonin arc: Contributions from the subducting slab, revisited

Abstract

[1] New Sr, Nd, Hf, and Pb isotope and trace element data are presented for basalts erupted in the Izu back arc. We propose that across-arc differences in the geochemistry of Izu-Bonin arc basalts are controlled by the addition of aqueous slab fluids to the volcanic front and hydrous partial melt of the slab to the back arc. The volcanic front has the lowest concentrations of incompatible elements, the strongest relative enrichments of fluid-mobile elements, and the most radiogenic Sr, Nd, Hf, and Pb, suggesting the volcanic front is the result of high degrees of partial melting of a previously depleted mantle source caused by an aqueous fluid flux from the slab. Relative to the volcanic front, the back arc has higher concentrations of incompatible elements and elevated La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less depleted or even enriched mantle source. Positive linear correlations between fluid-immobile element concentrations and the estimated degree of mantle melting suggest the slab contribution added to the mantle wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf isotopes and Th/La systematics of back-arc basalts are consistent with a slab melt composed of >90% altered oceanic crust and <10% sediment; that is, altered oceanic crust, not subducted sediment, dominates the slab contribution. High field strength element systematics require supercritical melts to be in equilibrium with residual rutile and zircon.

Expedition 350 summary

“…2. Rear-arc volcanic rocks include all the rear-arc seamount volcanoes from 3 to 17 Ma (Hochstaedter et al, 2001;Ishizuka et al, 2002Ishizuka et al, , 2003aIshizuka et al, , 2003bIshizuka et al, , 2006aIshizuka et al, , 2006bMachida et al, 2003Machida et al, , 2008Tollstrup et al, 2010), which are generally higher in Zr/Y (1.4-6.7) compared to arc-front rocks at comparable SiO 2 , but there is some overlap between R2 arc-front rhyolites and rear-arc seamount rocks.…”

“…of the arc (i.e., behind the arc front) and lie on arc crust, although the westernmost end of the rear-arc seamount chains lies on Shikoku Basin oceanic crust. The bimodal rift-type magmas differ from both the arc front and the rear-arc seamount chains in trace element and radiogenic isotopic ratios; this has been variably attributed to (1) a transition from flux to decompression mantle melting as arc rifting commences, (2) a change in the character of slabderived flux, or (3) a change in the mantle source through mantle wedge convection (Hochstaedter et al, 1990a(Hochstaedter et al, , 1990b(Hochstaedter et al, , 2001Ishizuka et al, 2003aIshizuka et al, , 2006bTollstrup et al, 2010). The Izu rear-arc seamount chains are as long as ~80 km and strike N60°E ( Figure F6).…”

“…Ig1 = igneous Unit 1. Data sources: Tamura et al (2009), Gill et al (1994, Bryant et al (2003), Straub et al (2003Straub et al ( , 2010, Hochstaedter et al (2001), Ishizuka et al (2002Ishizuka et al ( , 2003aIshizuka et al ( , 2003bIshizuka et al ( , 2006aIshizuka et al ( , 2006b), Machida et al (2003Machida et al ( , 2008, Tollstrup et al (2010). Unit III Unit III (726.50-1017.88 mbsf;age model ~4.4-6. (Figure F44), with the exception of 4 samples between 805 and 827 mbsf, which have elevated K 2 O. Sr concentrations vary greatly (up to 804 ppm) and are elevated compared to Unit I and II volcaniclastics as well as lava from the arc front and typically higher than rear-arc lava.…”

Sedimentary recycling in arc magmas: geochemical and U–Pb–Hf–O constraints on the Mesoproterozoic Suldal Arc, SW Norway

“…mantle recycling (e.g. Chauvel et al, 2009;Tollstrup et al, 2010;Nebel et al, 2011). Thus, although Hf isotopes can fingerprint crustal contamination within arc magmas, they alone do not allow the pathways of recycling to be determined, and thus also do not constrain unexposed crustal architecture.…”

Untitled

“…Furthermore, silicic volcanoes of the Quaternary arc front and Miocene granitic rocks in the Izu collision zone on Honshu are inferred to have formed by melting of Eocene-Oligocene arc crust (Tamura et al, 2009(Tamura et al, , 2010. As discussed in "Scientific results," Neogene rhyolite volcanism may be more important in the Izu rear-arc seamount chain than previously thought and could have resulted from melting of Paleogene "arc basement."…”