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Abstract: Hough Transform analysis is used as an objective means to constrain volcano distribution in the central Sunda Arc, Indonesia. Most volcanoes in the arc define four en echelon, linear segments, each of 500–700km length. Javan volcanoes that do not lie on these segments either (i) formed at an early stage in the history of the arc and erupted products that are petrologically and geochemically distinct from typical arc magma, or (ii) lie along other mapped structures. The en echelon distribution of volcanoes in the central Sunda Arc is best explained as originating from two possible sources. First, interaction with the subducting Indo-Australian Plate may induce stress in the arc lithosphere generating pathways for magma to exploit. Second, downward flexure of the arc lithosphere, as a result of mantle flow or loading by the arc, would also establish arc-normal tension towards the base of the lithosphere, where magma is supplied to volcanic systems. To the west and east of the central Sunda Arc deviations from the distribution of long, en echelon, linear segments can be understood as responses to specific stress fields in the arc lithosphere of Sumatra and eastern Nusa Tenggara, respectively. Control of volcano distribution by arc lithosphere explains why there are large variations in the depth from volcanoes to the zone of slab seismicity in the central Sunda Arc, where there is little variation in slab geometry or the rate of plate convergence. [Copyright &y& Elsevier]

Abstract: New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr/Y and depletion of the heavy rare earth elements. 87Sr/ 86Sr and 143Nd/ 144Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth elements, the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge. The wide range of SiO2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature. (1) Adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing garnet melted to yield adakitic magma. In either case the basaltic precursor was generated from fluid-modified mantle then differentiated within the garnet stability field. In Surigao this requires differentiation within mantle. The Surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth. This has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks. [Copyright &y& Elsevier]

Abstract: New helium and oxygen isotope data and trace element concentrations are reported for volcanic rocks from central Iceland. Basalts that are depleted in the most incompatible trace elements possess a wide range in 3He/4He but most ratios are similar to or higher than those of mid-ocean ridge basalt (MORB:∼8R A[1] [D.W. Graham, Noble gas geochemistry of mid-ocean ridge and ocean island basalts: characterisation of mantle source reservoirs, in: D.P. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Noble gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, 2002, pp. 247–317]). The low concentrations of helium in these rocks suggest that significant degassing has made them susceptible to contamination by low-3He/4He crust, therefore all measured 3He/4He are considered minimum estimates for their sources. Elevated helium isotope ratios in the source of these rocks result from interaction with high-3He/4He mantle. The highest oxygen isotope ratios in the depleted rocks are similar to those in melts from typical depleted upper mantle and the range of δ 18O values is consistent with variable, limited amounts of contamination by Icelandic crust. Most of the incompatible trace element-enriched rocks possess 3He/4He ratios that are similar to or lower than those in MORB. These rocks were erupted close to the postulated centre of the Iceland plume. This observation contradicts models in which high-3He/4He characterises the focus of mantle upwelling. A source with MORB-like 3He/4He ratios may also be common to other parts of the North Atlantic Igneous Province. The highest δ 18O values in the enriched rocks are lower than those in MORB and do not appear to have been affected by interaction with low-δ 18O Icelandic crust. Recycling of hydrothermally altered oceanic crust that has been subducted into the mantle provides a plausible mechanism for generating an 18O-poor source with the trace element and isotopic characteristics of the enriched lavas. [Copyright &y& Elsevier]

Abstract: New Nd–Hf isotope and trace element data for Javanese volcanoes are combined with recently published data to place constraints on subduction inputs at the Sunda arc in Indonesia and assess the value of Hf anomalies (expressed as Hf/Hf* and Sm/Hf ratios) as tracers of such inputs. Hf anomaly does not correlate with Hf isotope ratio in Javanese lavas, however, Hf/Hf* and Sm/Hf ratios do correlate with SiO2. Contrary to previous work, we show that Hf anomaly variation may be controlled by fractionation of clinopyroxene and/or amphibole during magmatic differentiation and does not represent the magnitude or type of subduction input in some arcs. Correlation of Sm/Hf with indices of differentiation for other arcs (e.g., Vanuatu, New Britain, and Mariana) suggests that differentiation control on Sm/Hf ratios in volcanic arc rocks may be a relatively common phenomenon. This study corroborates the use of Nd–Hf isotope co-variations in arc volcanic rocks to ascertain subduction input characteristics. The trajectories of regional volcano groups (East, Central and West Java) in Nd–Hf isotope space reveal heterogeneity in the subducted sediment input along Java, which reflects present-day spatial variations in sediment compositions on the down-going plate in the Java Trench. The high Sm/Hf ratio required in the sediment end-member for some Javanese basalts suggests that partial melting of subducted sediment occurs in the presence of residual zircon, and is inconsistent with residual monazite or allanite. [Copyright &y& Elsevier]

Most of the world's explosive volcanoes are located in volcanic arcs, formed by fluid-fluxed melting of upper mantle rocks. The fluids that facilitate melting are released from subducted tectonic plates as they sink into the mantle. Yet, we have sparse knowledge of the migration pathways of melts through the upper mantle (i.e., between the surface of the subducted plate and arc volcanoes). We are also uncertain of the time required for this migration to occur. Here, we show evidence of two earthquake swarms that occur in the upper mantle beneath the Mariana and Izu-Bonin arc systems. The best-resolved swarm occurs beneath the Mariana arc, where the earthquakes define a sub-vertical pipe-like structure with a diameter of ∼50 km and occurs between depths of ∼10–250 km. To test the robustness of depth locations, we used a fully non-linear grid search algorithm, double-difference relocation, as well as an analysis of pP-P arrival times and depth sensitive phases. In addition, we calculated centroid moment tensor solutions using a 3D Earth model to understand the mechanism of failure within the swarms. These data demonstrate that the sub-vertical earthquake swarm occurs within the upper mantle between the subducted slab and the overriding volcanic arc, with seismicity concentrated within discrete day- to month-long swarms of activity over a single two-year period. The Izu-Bonin example shares a similar sub-vertical pipe-like geometry with seismic activity bracketed within a two-year period. We infer that these rare earthquake swarms record the ascent of hydrous melt and/or fluid, from dehydration of the subducting plate. This implies that hydrous minerals within subducted slabs continue to dehydrate to depths of at least 200–250 km. Also, the short duration of earthquake swarms implies that fluids/melts can be rapidly transported through the sub-arc mantle at rates in the order of km/hr. This is consistent with rapid melt ascent rates inferred from geospeedometry and experimental petrology and is reminiscent of patterns seen during episodic tremor and slip events. • Definitive evidence of earthquakes occurring within sub-arc mantle ('mantle wedge'). • The earthquakes delineate the path of fluid/melt ascent above the subducted slab. • Earthquakes occur as day- to month long swarms, but all within a two-year period. • These earthquakes may provide insight into the rate of sub-arc fluid/melt transport. [ABSTRACT FROM AUTHOR]

We present titanium stable isotope measurements of terrestrial magmatic samples and lunar mare basalts with the aims of constraining the composition of the lunar and terrestrial mantles and evaluating the potential of Ti stable isotopes for understanding magmatic processes. Relative to the OL–Ti isotope standard, the δ 49 Ti values of terrestrial samples vary from −0.05 to +0.55‰, whereas those of lunar mare basalts vary from −0.01 to +0.03‰ (the precisions of the double spike Ti isotope measurements are ca. ±0.02‰ at 95% confidence). The Ti stable isotope compositions of differentiated terrestrial magmas define a well-defined positive correlation with SiO 2 content, which appears to result from the fractional crystallisation of Ti-bearing oxides with an inferred isotope fractionation factor of Δ Ti oxide–melt 49 = − 0.23 ‰ × 10 6 / T 2 . Primitive terrestrial basalts show no resolvable Ti isotope variations and display similar values to mantle-derived samples (peridotite and serpentinites), indicating that partial melting does not fractionate Ti stable isotopes and that the Earth's mantle has a homogeneous δ 49 Ti composition of +0.005 ± 0.005 (95% c.i., n = 29 ). Eclogites also display similar Ti stable isotope compositions, suggesting that Ti is immobile during dehydration of subducted oceanic lithosphere. Lunar basalts have variable δ 49 Ti values; low-Ti mare basalts have δ 49 Ti values similar to that of the bulk silicate Earth (BSE) while high-Ti lunar basalts display small enrichment in the heavy Ti isotopes. This is best interpreted in terms of source heterogeneity resulting from Ti stable isotope fractionation associated with ilmenite–melt equilibrium during the generation of the mantle source of high-Ti lunar mare basalts. The similarity in δ 49 Ti between terrestrial samples and low-Ti lunar basalts provides strong evidence that the Earth and Moon have identical stable Ti isotope compositions. [ABSTRACT FROM AUTHOR]

The geological carbon cycle has played a key role in controlling climate throughout Earth's history. For the last ∼3 billion years plate tectonics has driven subduction. Subducted slabs have transported CO 2 from the lithosphere, hydrosphere, and atmosphere into the Earth, from where it may be released back to the surface through processes such as arc volcanism or can be stored in the deep interior over geological time. Carbonate-bearing sediments and basalts of altered oceanic crust are the primary media by which carbon is subducted. Therefore, quantifying the depth and amount of CO 2 released from different carbonate-bearing lithologies during subduction is fundamental to understanding whether CO 2 is recycled through arc volcanism or buried in the mantle. The magnitude of CO 2 released from subducting slabs at fore- and sub-arc depths is controlled by processes including ocean crust alteration (i.e., carbonation), metamorphic decarbonation, carbonate dissolution and slab-melting. However, the relative contribution of these processes to overall slab decarbonation is still debated, and will be complex given the variety of sedimentary lithologies and subduction geodynamics. Here, we present a global arc-by-arc lithology-specific analysis of the magnitude of slab CO 2 released purely by metamorphic decarbonation of carbonate-bearing sediment and basalt during subduction of altered oceanic crust, using a thermodynamically rigorous model. We find that metamorphic decarbonation is highly efficient in low carbonate sediments, such as carbonated clay, and in carbonated basalts of altered oceanic crust, causing all of their CO 2 to be removed. Sediments with medium and higher carbonate contents, such as chalk and limestone, are only partially decarbonated, but the combination of metamorphic decarbonation and carbonate dissolution promotes efficient carbon loss. Together they can explain observed magmatic CO 2 emissions in carbonate-rich arcs. Warm slabs, such as Mexico and Cascadia, produce complete metamorphic decarbonation of carbonate minerals beneath fore-arcs. Under more common cold and intermediate thermal regimes metamorphic decarbonation of carbonate minerals occurs at depths between ∼80 and 170 km (∼2.3 to 5.5 GPa) promoting CO 2 input into the mantle sources of volcanic arcs. Overall, our results demonstrate that sub-arc decarbonation is typically considered an important potential source of slab-derived CO 2 , which needs to be considered together with carbonate dissolution to explain observed volcanic CO 2 emissions. In many arcs the modelled CO 2 flux from sediment and basalts of altered oceanic crust into the wedge exceeds the observed CO 2 output suggesting that the mantle wedge and arc lithosphere may sequester some CO 2. • Decarbonation is an important source of slab-derived CO 2 for arc volcanoes. • Cold and intermediate slabs produce decarbonation beneath sub-arcs. • Warm slabs produce decarbonation beneath fore-arcs. • The mantle wedge or arc lithosphere may accumulate and store slab-derived CO 2. [ABSTRACT FROM AUTHOR]

Abstract: Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87Sr/86Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87Sr/86Sr=0.7089–0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. measurements in the SVC crystals show extreme variation beyond the mantle range with +7.5 to for plagioclase ( ), +10.6 to for quartz ( ), +9.4 to for amphibole ( ) and +9 to for pyroxene ( ) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87Sr/86Sr=0.7041–0.7062) display values closer to mantle range: +6.4 to for plagioclase ( ) and +6 to for pyroxene ( ). We argue that the 87Sr/86Sr isotope disequilibrium and extreme values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral and whole rock 87Sr/86Sr, 143Nd/144Nd and 206,207,208Pb/204Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes. [Copyright &y& Elsevier]

• Ambient mantle metasomatism and melting degree traced through Nb/Zr, Ce/B, and δ 11 B. • δ 11 B consistent with isotopically heavy slab component(s) and DMM-OIB mantle source. • Greater δ 11 B complexity in melt inclusion suites reveals melt mixing processes. • Combined melt inclusions and whole-rocks record assembly of diverse melt batches. • Contributions from shallow crustal repositories of B remain poorly constrained. The chemistry of primitive arc rocks provides a window into compositional variability in the mantle wedge, as well as slab-derived inputs to subduction-related magmatism. However, in the long-term cycling of elements between Earth's internal and external reservoirs, a key unknown is the importance of retaining mobile elements within the subduction system, through subduction-related metasomatism of the mantle. To address these questions, we have analysed olivine-hosted melt inclusions and corresponding bulk rocks from the Kamchatka arc. Suites of melt inclusions record evidence for entrapment along melt mixing arrays during assembly of diverse parental magma compositions. Systematic variations in parental magma B/Zr, Nb/Zr, Ce/B, and δ 11 B are also apparent among the different eruptive centres studied. These element ratios constrain the nature of subduction-related metasomatism and provide evidence for ambient mantle heterogeneity and variable degrees of mantle melting. High Nb/Zr and low B/Zr in back-arc rocks indicate smaller degree melts, lower slab-derived inputs, but relatively enriched mantle compositions. Similarly, small monogenetic eruptive centres located away from the main stratocones also tend to erupt magmas with relatively lower slab contribution and overall smaller melting degrees. Conversely, arc-front compositions reflect greater slab contributions and larger degree melts of a more depleted ambient mantle. Across-arc variations in δ 11 B (ranging from ca. − 6 ‰ in the rear-arc and Sredinny Ridge to + 7 ‰ in the Central Kamchatka Depression) are generally consistent with variable addition of an isotopically heavy slab-derived component to a depleted MORB mantle composition. However, individual volcanic centres (e.g. Bakening volcano) show correlations between melt inclusion δ 11 B and other geochemical indicators (e.g. Cl/K 2 O, Ce/B) that require mixing between isotopically distinct melt batches that have undergone different extents of crustal evolution and degassing processes. Our results show that while melt inclusion volatile inventories are largely overprinted during shallower melt storage and aggregation, incompatible trace element ratios and B isotope compositions more faithfully trace initial mantle compositions and subduction inputs. Furthermore, we suggest that the signals of compositional heterogeneity generated in the sub-arc mantle by protracted metasomatism during earlier phases of subduction can be preserved during later magma assembly and storage in the crust. [ABSTRACT FROM AUTHOR]