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Investigating mass exchanges between the subduction zones and mid-ocean ridges provides essential clues to understanding recycling process and mantle convection in the Earth's mantle. In this study, we found evidence of recycled subduction signals in the abyssal peridotites from mid-ocean ridges through their extremely low bulk (Dy/Yb) N and bulk Al 2 O 3 , which could not be explained by normal decompression melting at mid-ocean ridges, but required a hydrous melting history. We show here that abyssal peridotites from the fast-spreading Pacific Oceanic Ridges (East-Pacific Rise and Pacific-Antarctic Ridge), to the ultraslow-spreading Southwest Indian Ridge and to the slow-spreading Mid-Atlantic Ridge, require increasing amount of slab-derived fluid in their mantle sources. Therefore, the subduction influence on abyssal peridotites is independent of the spreading rates of the overlying ridges. Locations, such as the Fifteen-Twenty Fracture Zone, the Rainbow Massif from the MAR and the Hess Deep from EPR, have abyssal peridotites overlap well with forearc peridotites, suggesting that the prominent characteristics of arc mantle wedge could be preserved in abyssal peridotites when the traveling distance or time is short for the recycled arc mantle migrating in the mantle convection flow. The well-preserved subduction-related signature in ridge areas that are relatively proximal to modern subduction zones imply that mass exchanges between the subduction zones and mid-ocean ridges may be more efficient than previously thought. • Subduction signals seen in abyssal peridotites expand subduction-modified ridge mantle inferred from mid-ocean basalts. • Manifestation of subduction influence on abyssal peridotites is independent of ridge spreading rates. • Travel time of sub arc mantle to mid-ocean ridges should be <200 Ma to maintain key hydrous melting geochemical traits. [ABSTRACT FROM AUTHOR]

The Mbengwi plutonics consist of intermediate to felsic granitoids forming a continuous magmatic series from monzonite to granite and mafic intrusions. Their mineralogical composition consists of quartz, plagioclases, K-feldspars, biotite, muscovite, and amphibole. The accessory phase includes opaque minerals + titanite ± apatite ± zircon, while secondary minerals are pyrite, phengite, chlorite, epidote, and rarely calcite. These plutonics are assigned high–K calc–alkaline to shoshonitic series, metaluminous to weakly peraluminous and mostly belong to an I–type suite (A/CNK = 0.63–1.2). They are typically post–collisional, with a subduction signature probably being inherited from their protoliths emplaced during the subduction phase. The Sr and Nd isotopic data evidence that these plutonics result from melting of the lower continental crust with variable contribution of the oceanic crust. Their geochemical features are similar to those of western Cameroon granitoids related to the Pan–African D 1 event in Cameroon. [ABSTRACT FROM AUTHOR]

• First δ 11 B ratios for young post-collisional volcanic rocks gives δ 11 B of −5 to +2‰. • Sr-Nd-B isotopes are distinct from both arcs and ocean ridges/islands. • Boron inherited from previous subduction and stored in amphibole for >40 Myr. • The slab component is dominated by sediment melts, rather than aqueous fluids. We present the first boron abundance and δ 11 B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ 11 B of post-collisional volcanic rocks (−5 to +2‰) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ 11 B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB δ 11 B, due to previous fluid depletion. Instead, the similarity of δ 11 B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ 11 B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr. [ABSTRACT FROM AUTHOR]

Volcanic gas sampling and SO 2 flux measurements were performed on Taftan volcano (3920 m height, SE Iran, Makran volcanic arc) and Damavand volcano (5610 m height, Northern Iran, Alborz Mountains). Both volcanoes possess near-summit fumarolic fields with moderately intensive gas jets and temperatures up to 160 °C (Taftan) and 175 °C (Damavand). Gases of both volcanoes contain (mmol/mol): H 2 O (910–930), CO 2 (50–80), SO 2 (3–7), H 2 S (2–5.5), HCl (5–8.2) and HF (0.1–0.13). Both volcanoes are also similar in terms of minor gas species (mmol/mol): He (0.0005–0.0011), H 2 (0.00077–0.0046), N 2 (ex) (0.15–0.30) and Ar(ex) (0.0006–0.0012), where subscript "ex" denotes non-atmospheric fraction. δD–δ18O systematics has shown that Damavand fumarolic gases contain 60–65% of magmatic fraction whereas the Taftan emissions correspond to almost pure magmatic ("andesitic") vapor. Low amount of nitrogen in Taftan gases and its isotopic composition may be explained by low input of nitrogen-containing organic matter with the subducting slab due to specific geometry of the Makran subduction zone, namely, by the presence of an accretionary prism of a considerable size. The origin of high CH 4 content in Damavand gases and very low concentrations of C 2 H 6 and higher hydrocarbons (0.65 mmol/mol; CO 2 /CH 4 ~ 80; CH 4 /C 2 H 6 ~ 104) is unclear. Fumaroles of Taftan volcano are poor in CH 4 , similar to many other arc volcanoes (CO 2 /CH 4 ~ 105). 3He/4He isotopic composition expressed as R/Ra was measured at 7.0–7.5 for Taftan and 6.65 for Damavand, corresponding to the CO 2 /3He ratio of ~1.0E+10. The δ34S of the total sulfur is +7.6 ± 2‰ (2σ) for Taftan fumaroles and + 8.1 ± 2‰ (2σ) for Damavand fumaroles; δ13C (CO 2) is −5.9 ± 2.0‰ (Damavand) and − 4.3 ± 0.15‰ (Taftan). Concentrations of major gas species (H 2 O/CO 2 /S/HCl ratios) and isotopic data (R/Ra, CO 2 /3He, δ34S) show that the volcanic gas composition on both volcanoes have a distinct arc signature. This conclusion is especially important for the intraplate Damavand volcano, which has an uncertain tectonic affinity but according to the latest geochemical studies was considered having hotspot/rift origin. Mini-DOAS measurements of the SO 2 fluxes showed 20 ± 12 t/d SO 2 at Taftan and 43 ± 20 t/d SO 2 at Damavand, which places both volcanoes as small SO 2 emitters. • Fumarolic gases were studied on Taftan and Damavand, the Iranian volcanoes. • Major gas species and isotopic data suggest arc gas signature for both volcanoes. • SO 2 fluxes of 20 t/d at Taftan and 43 t/d at Damavand were measured. • Low N 2 content in Taftan gases is explained by low input of subducted nitrogen. [ABSTRACT FROM AUTHOR]