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The modern volcanoes of Indonesia, represent the cycling of material and heat along the Southeast (SE) Asian subduction zone. This volcanic activity is well exemplified in the middle section of Java Island, which is found on the Sunda arc portion of the SE Asian convergent system. Central Java's volcanoes inherit their geochemistry from silicate melts that formed due to hydrous melting at a range of depths above their subducting slab: ∼136 to ∼272 km. It has been shown that across-arc variations in magmatic trace elements like boron (B) can shed light on the quantity and type of slab-derived fluid released in a forearc to back-arc sequence. In this study, we present major, trace and radiogenic chemistry data for five volcanoes arranged in a roughly N-S lineament across Central Java. We show that the observed chemical changes across the section of the Sunda arc can be explained by variations in different kinds of source from fluid mobile material being devolatilized across the arc. Our mixing models suggest that the magmas underlying these volcanoes see a decreasing ratio of sediment to altered oceanic crust (AOC) contributions from the volcanic front (Merapi volcano) toward the rear-arc (Muria). We estimate that a maximum ∼1% contribution of sediment-derived fluid is sufficient to explain the chemical systematics of this arc segment. Shallow slab dehydration, mostly from the sediment cover, at low temperature (at <60 km of slab depth) transfers the fluid high in B/Nb, Ba/Nb, 87Sr/86Sr and 207Pb/204Pb to the mantle beneath this region's arc front, inducing flux melting. Farther from the trench, the signature of the subducting oceanic plate increases and peaks at around ∼160–170 km of slab depth, carrying substantial slab liquids with moderate to low B/Nb and Ba/Nb with lower 87Sr/86Sr and 207Pb/204Pb to the mantle beneath middle- and rear-arc region. Beyond 200 km, in the rear-arc, evidence for slab liquid is low, but not non-existent. Unexplained Ba-enrichment in magmas from across the arc may account for the role of subduction serpentinite dehydration in the subduction channel or local crustal assimilation. This study provides a conceptual model for material transfer from the subducting slab in SE Asia. Our findings have important implications for the understanding of geochemical cycling in this region, as well as the impact of these cycles on present and past volcanism. • Slab liquid with different proportion of SED/AOC is evident across Central Java, high at arc front and low at the rear-arc • Shallow slab liquid is high in FME, 87Sr/86Sr, 207Pb/204Pb; deep slab liquid has moderate/low levels of those components • Elevated Ba/Nb across the arc magmas may hint to the importance of subducting serpentinite in subduction channel [ABSTRACT FROM AUTHOR]

Zones of fluid-bearing anomalies occur in volcanic regions, on a plate interface, or in the mantle wedge of a subduction zone. The various causes include slab dehydration, serpentinization or metamorphic reactions in different thermal regimes. To reveal the impact factors that might have contributed to slab melting and arc magmatism behind the Japan Trench, we determined high-resolution three-dimensional (3-D) structures of seismic velocities (Vp, Vs), Vp/Vs ratio and temperatures. The temperatures (T) were calculated from the inverted seismic structures, and two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage, which differ from the results of the thermal simulation. The multi-parameter structures obtained for Vp, Vs, Vp/Vs and T show good consistency in the upper mantle behind the Japan Trench. The temperatures derived from the peridotite assemblage were found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. A layer approximately 10 km thick with low-Vp and Vs, high-Vp/Vs, and slightly high-T anomalies was observed on the upper boundary of the subducting Pacific slab. This distinctive anomalous layer is interpreted as partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics are dependent on the source of fluids, mineral composition, and the thermal regime. Such a process enriches the peridotite content of the basalt underneath the island arc in the mantle wedge. Localized zones of significantly low-Vp and low-Vs, high-Vp/Vs and high-T perturbations were imaged in the mantle wedge under the active volcanoes, suggesting partial melting of peridotite-rich mantle material to produce tholeiitic magma. The present study demonstrates that fluids released from slab dehydration, mineral composition and the thermal regime play crucial roles in both arc magmatism, and slab melting in the subduction zone. [Display omitted] • High resolution seismic and thermal structures are imaged in the NE Japan subduction zone. • Slab melting is controlled by the source of fluids and thermal regime behind the Japan Trench. • Fluids and mineral composition play a key role in slab melting and arc magmatism. [ABSTRACT FROM AUTHOR]

Gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub-slab mantle to flow through the gaps and into the mantle wedge. This through-slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and/or dynamic uplift in the overriding plate, while the altered mantle flow patterns affect the trench evolution. Numerous geodynamic models have investigated the processes that form slab gaps, but few studies have examined the dynamics of slab gap-altered mantle flow, its effects on trench morphology and kinematics, or the controlling parameters on these processes. Here, laboratory subduction models with a pre-cut gap in a subducting silicone plate are used to explore how slab gap size, and slab gap depth influence the surrounding mantle flow field and trench dynamics. Results suggest that both the vertical extent and the depth of the top (trailing edge) of the slab gap are crucial parameters for modulating overall subduction dynamics. They show that a slab gap, which occurs near the surface and initially comprises 30% of the subducting plate width, can extend enough vertically in the slab to produce significant vertical flow through the gap. Changes to the trench geometry and kinematics are also evident in the models, such that double- and triple-arc geometries are formed during subduction of a shallow slab gap. All of these results are consistent with observations of slab gaps and their induced surface expressions, or the lack thereof, in Eastern Anatolia, East Java, Italy, and Argentina. Unlabelled Image • Subduction dynamics can be largely affected by the opening of slab gaps. • Effects on the surface are only observable when the slab gap is located near-surface. • Vertical mantle flow through the slab gap can happen only when the gap has several 100 km vertical extent. [ABSTRACT FROM AUTHOR]