Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

Abstract

[1] Hf isotope compositions, coupled with Pb-Sr-Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back-arc opening (23-14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest e Hf (+9) in the early Miocene to the highest e Hf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back-arc opening. In the early stage of the back-arc opening, when injection of hot asthenospheric mantle induced high-temperature conditions in the mantle wedge, slab melting contributed to low e Hf coupled with low e Nd , relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high e Hf , decoupling of e Hf and e Nd , high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.

Variable sediment flux in generation of Permian subduction-related mafic intrusions from the Yanbian region, NE China

“…Due to much higher solubility of Ba than Nb and La in slab fluids, Ba/La and Ba/Nb are good geochemical tracers for slab fluids (Woodhead et al, 2001;Hanyu et al, 2006). At the other hand, Ba, La and Nb are highly incompatible during melting of the subducted sediments, so Ba/Nb and Ba/La ratios of the sediment melt are similar to that of the subducted sediments.…”

“…Hf-Nd isotopic modeling results of the Yanbian Permian mafic intrusions. On the mixing curve between the sediment fluid and DM, the mobility of Nd and Hf in aqueous fluid is assumed to be 0.03 and 0.005 and the dehydration volume of sediment fluid is assumed to be 0.02 (Aizawa et al, 1999;Hanyu et al, 2006). The other calculation parameters are the same as in Fig.…”

“…One way to investigate source contributions and processes acting on the subarc mantle is by examining the Lu-Hf and Sm-Nd isotopic compositions of arc rocks, since it has been shown that these isotopic systems have the capacity to discriminate between petrogenetic processes and various source components (e.g., White and Patchett, 1984;Marini et al, 2005;Hanyu et al, 2006;Geldmacher et al, 2008;Carpentier et al, 2009;Chauvel et al, 2009). Combining data from both isotopic systems potentially provides a particularly powerful insight into petrogenetic processes because decoupling of Hf and Nd isotope systems appears specific to a very limited set of processes.…”

“…Combining data from both isotopic systems potentially provides a particularly powerful insight into petrogenetic processes because decoupling of Hf and Nd isotope systems appears specific to a very limited set of processes. For example, Nd is more fluid-mobile than Hf (Kogiso et al, 1997;Müker et al, 2004;Hanyu et al, 2006), so aqueous fluids can fractionate Nd from Hf, creating negative Hf anomalies (relative to adjacent elements on normalized incompatible trace element spidergrams) and decoupling Hf-Nd isotope systems (Pearce et al, 1999;Hanyu and Tatsumi, 2002;Kessel et al, 2005). In the case of many intra-oceanic arcs (e.g., the Izu-Bonin-Mariana subduction system, Pearce et al, 1999;Chauvel et al, 2009), where mantle enrichment is largely fluiddominated, subduction-related magmas exhibit Hf depletion (i.e., negative Zr-Hf anomalies relative to Sm and Nd in primitive mantlenormalized incompatible trace element spidergrams) and contain more radiogenic Hf than Nd (i.e., they have an 'excess of radiogenic Hf' as quantified by the equation: Δε Hf = ε Hf − 1.36 × ε Nd − 2.95, Vervoort et al, 1999).…”

Middle Jurassic MORB-type gabbro, high-Mg diorite, calc-alkaline diorite and granodiorite in the Ando area, central Tibet: Evidence for a slab roll-back of the Bangong-Nujiang Ocean

“…Hydrous minerals (hornblende) exist in the high-Mg diorites, which suggests that primitive parental melts are hydrous. This is supported by their relatively high Ba/La ratios, because Ba is more soluble in aqueous fluids than La (Hanyu et al, 2006). Therefore, we infer that not only sediment-derived melts but also slab-derived aqueous fluids have been involved in the magma generation.…”