Arc Magma Generation Research Articles

Heavy magnesium isotopic signatures in arc lavas may be attributed to dehydration of subducting hydrated mantle

Fluids released from subducting slabs profoundly affect mantle composition, rock melting points, and arc magma generation. However, identifying fluid sources (sediments, crust, or mantle) and their ascent paths remains challenging. Magnesium isotopes are potential tracers for subduction-related fluids, though their behavior during hydrous peridotite dehydration remains unclear. Here we determined the equilibrium magnesium isotope fractionation factors between aqueous fluids and hydrous peridotitic minerals using first-principles calculations. Aqueous fluids prefer heavy magnesium isotopes relative to mantle silicate minerals, indicating that fluids released during hydrous peridotite dehydration are enriched in heavy magnesium isotopes relative to the residual minerals. Our simulations proposed that magnesium isotope variations in arc lavas from different subduction zones could be attributed to different dehydration reactions influenced by subduction zone thermal structures. This study highlights the potential of magnesium isotopes for tracing fluids originating from subducting hydrated mantle, providing insights into the thermal structure of various subduction zones.

A negligible role for forearc serpentinites and mélange diapirism in contributing halogens to Mariana arc magmas

Trace element enrichment in arc lavas has traditionally been linked to the presence of slab-derived materials introduced by a combination of aqueous slab-fluids derived from altered oceanic crust (AOC) and subducted lithosphere and sediment-melts. However, an alternative is that slab material could be mobilised in mélange diapirs formed from forearc serpentinites admixed with sediments and AOC in a subduction channel. In order to further constrain the relative importance of slab-fluids, sediment-melts and mélange diapirs in the Marianas and track how the contributions of these components evolves across the arc, we selected submarine volcanic glasses from pillow rinds recovered from the Southeast Mariana Forearc Rift, and the Mariana arc and backarc basin. The glasses are all enriched in halogens and other fluid mobile elements relative to immobile elements of similar compatibility, demonstrating a strong subduction signature. Irrespective of their position in the arc, the lavas all have Br/Cl weight ratios of 0.0016–0.0037 and I/Cl of 8.3 × 10−6–127 × 10−6, which encompasses a similar range as reported for arc and backarc glasses from the SW Pacific. The Br/Cl and I/Cl of the subduction component overlaps with altered ocean crust suggesting dehydration of altered ocean crust (± lithospheric serpentinites) as the dominant source of halogens and related slab-fluids. The Br/Cl and I/Cl of the Marianas glasses are much lower than forearc serpentinites from the Marianas, which does not favour a role for either dehydration of forearc serpentinites entrained with the subducting slab or direct melting of mélange diapirs formed from serpentinised forearc material. The halogen data are most easily reconciled with the conventional model for arc magma generation, in which halogen-bearing aqueous fluids are progressively released from a dehydrating subducting slab. The importance of sediment-melts and/or forearc serpentinites in significantly contributing to the subduction zone budgets of other elements is not precluded, but the high halogen content of forearc serpentinites means that direct melting of mélange diapirs is unlikely.

Generation of tholeiitic and calc-alkaline arc magmas and its implications for continental growth

Arc magmas, the building blocks of the Phanerozoic continents, are compositionally diverse. The most prominent compositional diversity is that some arcs show early Fe enrichment while some others show the opposite trend. What causes this Fe differentiation diversity is important for understanding continent formation but remains highly debated. Here, we use pressure-sensitive geochemical proxies, combined with experimental results, to evaluate the petrological mechanism(s) for the different Fe trends in arc magmas that traverse through crusts of various thicknesses. We show that magnetite starts to fractionate only at MgO ≤ 4 wt% for nearly all arc magmas and thus has no bearing on the early Fe depletion in calc-alkaline arc magmas. The Fe enrichment at MgO > 4 wt% for tholeiitic arc magmas is largely caused by clinopyroxene fractionation. The culprits for the Fe depletion at MgO > 4 wt% in calc-alkaline arc magmas change from pyroxene- to garnet-dominated (±amphibole) as the arc crust thickens. The similarity between the upper continental crust and the calc-alkaline arc magmas suggests that calc-alkaline arc magmas are an essential component for the continental crust. The high La/Yb and Dy/Yb estimates for the average upper continental crust imply that garnet fractionation is an intrinsic feature of the upper continental crust. Thus, crustal thickness, that is differentiation pressure, exerts a first-order control on arc magma compositions and high-pressure intracrustal differentiation in thickened crust is necessary for making the Phanerozoic continental crust.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.

Composition of the Fluid in Carbonate- and Chlorine-Bearing Pelite near the Second Critical Point: Results of Diamond Trap Experiments

Abstract—The composition of the fluid in carbonate- and chlorine-bearing pelite was experimentally studied at 3.0 GPa and 750 and 900 ºC, using the diamond trap method. The results of inductively coupled plasma atomic-emission spectrometry (ICP AES) and mass balance calculations showed that a supercritical fluid formed in the studied system at 3.0 GPa and 750 °C. The fluid is Si- and Al-rich and contains 30–50 wt.% H2O + CO2 and up to 1 wt.% Cl. The contents of other major elements decrease in the order: K > Na > Сa ≈ Fe > Mg > Mn > Ti ≈ P. Compared with supercritical fluids appeared in the systems pelite–H2O and eclogite–H2O, the fluid with high CO2 and Cl contents is richer in Fe, Ca, Mg, and Mn but poorer in Si. Silicate melt generated in this system at 900 ºС has a composition typical of pelitic melt. Our experiments reveal a set of fingerprints of element fractionation between a supercritical fluid and solids forming an eclogite-like association, namely, high mobility of P, Sr, and B and low mobility of Li and S. Thus, a supercritical fluid compositionally similar to the pelitic melts generated in subduction zones can transfer significant amounts of both volatiles (H2O, CO2, Cl, and P) and major components to the regions of arc magma generation. It is important that supercritical fluids should have trace element signatures of diluted low-temperature fluids.

Nitrogen, helium, and argon reveal the magmatic signature of fumarole gases and episodes of outgassing from upper-crustal magma reservoirs: The case of the Nisyros caldera (Aegean Arc, Greece)

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N2, He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N2/He = 31.8 ± 4.5, N2/Ar = 281.6, δ15N = +7 ± 3 ‰, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 551.6 ± 19.8. Although N2/He is significantly low with respect to typical values for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive δ15N values of Nisyros fumaroles. The low N2/He ratio indicates N2-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018–2021 data with N2, He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996–1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO2/CH4 and He/CH4 ratios, an increase of ∼50 °C in the equilibrium temperature of the hydrothermal system (up to 325 °C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N2-He-Ar has therefore important ramifications not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir.

Sources of dehydration fluids underneath the Kamchatka arc

Fluids mediate the transport of subducted slab material and play a crucial role in the generation of arc magmas. However, the source of subduction-derived fluids remains debated. The Kamchatka arc is an ideal subduction zone to identify the source of fluids because the arc magmas are comparably mafic, their source appears to be essentially free of subducted sediment-derived components, and subducted Hawaii-Emperor Seamount Chain (HESC) is thought to contribute a substantial fluid flux to the Kamchatka magmas. Here we show that Tl isotope ratios are unique tracers of HESC contribution to Kamchatka arc magma sources. In conjunction with trace element ratios and literature data, we trace the progressive dehydration and melting of subducted HESC across the Kamchatka arc. In succession, serpentine (<100 km depth), lawsonite (100–250 km depth) and phengite (>250 km depth) break down and produce fluids that contribute to arc magmatism at the Eastern Volcanic Front (EVF), Central Kamchatka Depression (CKD), and Sredinny Ridge (SR), respectively. However, given the Tl-poor nature of serpentine and lawsonite fluids, simultaneous melting of subducted HESC is required to explain the HESC-like Tl isotope signatures observed in EVF and CKD lavas. In the absence of eclogitic crust melting processes in this region of the Kamchatka arc, we propose that progressive dehydration and melting of a HESC-dominated mélange offers the most compelling interpretation of the combined isotope and trace element data.

Thallium isotope compositions of subduction-zone fluids: Insights from ultra-high pressure eclogites and veins in the Dabie terrane, eastern China

Dehydration of the subducted slab is a crucial process in the generation of arc magmas. However, the mineralogical controls of slab dehydration remain uncertain. Thallium (Tl) isotopes have emerged as tracers of subducted slab components, but the Tl isotope characteristics of slab fluids are not known. High-pressure metamorphic veins and host eclogites in the Dabie terrane (China) provide important information on the composition and evolution of subduction zone fluids. In this study, we present the first Tl concentration and Tl isotope data for two high-pressure metamorphic epidote-rich veins formed through breakdown of lawsonite, as well as host eclogites and phengite separates, from the Ganghe and Hualiangting areas. The high Tl concentrations in phengite separates and identical Tl isotope compositions between phengites and their host bulk eclogites demonstrate that phengite controls the overall inventory of Tl in the host eclogites. Generally, the vein samples from both areas have much lower Tl concentrations than their host eclogites, suggesting that the fluids from which the veins precipitated were derived from phengite-bearing eclogites that retained Tl in the residue. The Ganghe eclogites display similar Tl isotope compositions compared with Ganghe omphacite-epidote vein. This similarity indicates that the vein-forming fluid was likely derived from the host eclogites, and that Tl isotopes did not fractionate during fluid-release. Three generations of veins in the Hualiangting area record variable Tl isotope values and were likely derived from multi-stage dehydration of heterogeneous host eclogites, some of which may have contained a sediment component. We argue that fluids derived from eclogite dehydration are likely characterized by higher Cs/Tl ratios compared to their host eclogites. Arc magmas that display high Cs/Tl (i.e. >15) that cannot be caused by variations in slab source composition, coupled with relatively low Ba/Th ratios, are likely to directly record the presence of residual phengite at subarc depths.

Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks

Metamorphic devolatilization of subducted slabs generates aqueous fluids that ascend into the mantle wedge, driving the partial melting that produces arc magmas. These magmas have oxygen fugacities some 10–1,000 times higher than magmas generated at mid-ocean ridges. Whether this oxidized magmatic character is imparted by slab fluids or is acquired during ascent and interaction with the surrounding mantle or crust is debated. Here we study the petrology of metasedimentary rocks from two Tertiary Aegean subduction complexes in combination with reactive transport modelling to investigate the oxidative potential of the sedimentary rocks that cover slabs. We find that the metasedimentary rocks preserve evidence for fluid-mediated redox reactions and could be highly oxidized. Furthermore, the modelling demonstrates that layers of these oxidized rocks less than about 200 m thick have the capacity to oxidize the ascending slab dehydration flux via redox reactions that remove H2, CH4 and/or H2S from the fluids. These fluids can then oxidize the overlying mantle wedge at rates comparable to arc magma generation rates, primarily via reactions involving sulfur species. Oxidized metasedimentary rocks need not generate large amounts of fluid themselves but could instead oxidize slab dehydration fluids ascending through them. Proposed Phanerozoic increases in arc magma oxygen fugacity may reflect the recycling of oxidative weathering products following Neoproterozoic–Palaeozoic marine and atmospheric oxygenation.

Seismic Anisotropy and Intraslab Hydrated Faults Beneath the NE Japan Forearc

AbstractThe water cycle in subduction zones is closely related to the generation of large earthquakes and arc magmas. Outer‐rise hydrated faults subducting with the oceanic lithosphere are suspected to be important for the water cycle. However, geophysical evidence for the intraslab hydrated faults beneath forearc is scarce. Here we present high‐resolution P‐wave anisotropic tomography of the Tohoku forearc derived from arrival‐time data recorded at both onshore and offshore seismic stations. Trench‐parallel intraslab fast velocity planes of anisotropy are revealed that intersect the slab upper surface at high angles (∼45°–90°), reflecting aligned hydrated faults in the slab. Ruptures of the hydrated faults may cause large intraslab earthquakes. The hydrated‐fault associated water entering a large near‐trench asperity in the megathrust zone could have triggered the great 2011 Tohoku‐oki earthquake (Mw 9.0).

Arc magmatism and porphyry-type ore deposition are primarily controlled by chlorine from seawater

A better understanding of magmatism and ore metal deposition at convergent plate margins is essential to the safety and benefit of society. Despite their spatial and, to a certain extent, temporal association, a direct connection between these two geologic processes is enigmatic. A detailed evaluation of available data reveals that seawater is key to the connection between these processes as it is the source of the ubiquitous, hot aqueous fluid from the subducting slab that is central to the generation of primary arc magmas and transfer or recycling of crustal materials at convergent margins. Particularly regarding the transfer of fluid-mobile elements or the so-called subduction component from the slab to arc lavas, however, the slab-derived hydrothermal fluid is perceived as weak and, thus, other transfer agents are proposed. Most petrogenetic studies also do not connect the source and transfer of metals in the spatially associated ore deposits to the same fluid. Chlorine from seawater can provide the crucial link between the two processes. The presence of Cl makes the slab-derived fluid a potent solvent not only to transfer but also to solubilize both the subduction component and ore trace metals from the slab, mantle wedge and lower-middle arc crust to arc lavas and ore deposits, respectively. Moreover, the dissolution and mobility of many elements in the Cl-rich fluid are primarily dependent upon their respective ionic potential (i.p.) values. In general, elements with cations that have low (soft cations) and high (hard cations) i.p. are mobile whereas those with in-between (intermediate cations) values are immobile in the Cl-rich fluid. Subducted sediment provides the bulk of subduction component and ore metals. These results led to the creation of a unified conceptual model claiming that the mobility, or immobility, of almost all elements in hydrothermal fluids at or near the Earth's surface is primarily dependent upon the amount of Cl in the fluids.

Mg-Ba-Sr-Nd isotopic evidence for a mélange origin of early Paleozoic arc magmatism

In recent years, the mélange model has been increasingly considered as an important way to transfer slab components to arc sources in modern subduction zones. This model differs from the classic slab fluid/melt metasomatism model in that it invokes physical mixing of bulk sediment, altered oceanic crust (AOC), serpentinite, and mantle wedge at the slab-mantle interface. However, due to the lack of subducted sediment compositions, the mélange model has not been applied to any significant extent in paleo-subduction zones. The lack of evidence for bulk AOC and serpentinite components in mélange sources also hinders our understanding of any mélange origin for arc-type magmas. Here, we report Mg-Ba-Sr-Nd isotope compositions of the early Paleozoic Fushui mafic rocks in the Qinling orogen of central China, to trace their origin and constrain slab component transfer. The Fushui mafic rocks show typical arc-type geochemical features and enriched Sr-Nd isotope compositions, indicating the likely contribution of subducted sediments. They have low Rb/Ba ratios (<0.1), and most samples show δ138/134Ba values of −0.38 to +0.10‰, similar to those of sediments (−0.2 to +0.1‰) but lower than those of MORBs (+0.03 to +0.14‰), indicating that these low δ138/134Ba values are most likely derived from sediment components. One Fushui sample has a high δ138/134Ba of +0.31‰, similar to that of AOC (up to +0.4‰). This high δ138/134Ba is not correlated with fluid input; instead, it results from the contribution of bulk AOC. The Fushui rocks exhibit variable δ26Mg values (−0.23 to −0.11‰), slightly higher than those of MORBs. This most likely reflects 26Mg-enriched subducted serpentinite components in their source. Our results not only identify the variable slab components (sediment, AOC, and serpentinite) in the arc source, but also suggest that these slab components may be transferred to their arc source by the mélange process. This study therefore provides solid evidence for the generation of arc magmas by mélange processes in paleo-subduction zones, which confirms an important role for the mélange model in slab material transport.

Slab-derived sulfate generates oxidized basaltic magmas in the southern Cascade arc (California, USA)

Abstract Whether and how subduction increases the oxidation state of Earth's mantle are two of the most important unresolved questions in solid Earth geochemistry. Using data from the southern Cascade arc (California, USA), we show quantitatively for the first time that increases in arc magma oxidation state are fundamentally linked to mass transfer of isotopically heavy sulfate from the subducted plate into the mantle wedge. We investigate multiple hypotheses related to plate dehydration and melting and the rise and reaction of slab melts with mantle peridotite in the wedge, focusing on electron balance between redox-sensitive iron and sulfur during these processes. These results show that unless slab-derived silicic melts contain much higher dissolved sulfur than is indicated by currently available experimental data, arc magma generation by mantle wedge melting must involve multiple stages of mantle metasomatism by slab-derived oxidized and sulfur-bearing hydrous components.

Thallium behavior during high-pressure metamorphism in the Western Alps, Europe

Thallium (Tl) is a highly incompatible element which can be redistributed during various igneous and hydrothermal events such as mineral dehydration and low-temperature hydrothermal alteration. Despite the strong influence fluid-rock interactions can have on Tl redistribution and the importance of fluids in subduction-zone volatiles cycling, the geochemical behavior of Tl during high-P/T metamorphism has not yet been quantified. Here we provide new Tl isotope composition (ε205Tl) and concentration ([Tl]) data for a suite of 18 metapelite and metacarbonate rocks from the Schistes Lustrés Complex and the nearby Lago di Cignana exposure in the western Alps. This suite represents metamorphism over a range of peak metamorphic P-T conditions of 300–550 °C and 1.5–3.0 GPa (similar to the range of P-T experienced in most modern subduction zones).With increasing metamorphic grade, [Tl] shows no statistical variation while ε205Tl initially remains fairly constant (in order of increasing grade, Fraiteve (unit A) ε205Tlavg = −2.1 (± 0.3); Assietta (unit B) ε205Tlavg = −1.6 (± 0.6); Albergian (unit C) ε205Tlavg = −1.9 (± 0.5)) before shifting to more negative values during higher grade metamorphism (Finestre (unit D) ε205Tlavg = −2.4 (± 0.3); Cignana (unit E) ε205Tlavg = −3.5 (± 0.3)). Although it appears likely that Tl is largely retained in such rocks to depths approaching those of arc magma generation, the mineralogical hosts for Tl change during prograde devolatilization. This may contribute to a significant shift in ε205Tl related to fractionation induced by only minor amounts of loss due to fluid-mineral partitioning. Prograde metamorphism results in the breakdown of chlorite and other minor mineralogical constituents, releasing chlorite-borne Tl that could be in part incorporated into phengite and in part lost to fluids. This study emphasizes the need for prograde mineral reaction history to be considered in studies of the retention or loss of trace elements such as Tl.

Architectural and Tectonic Control on the Segmentation of the Central American Volcanic Arc

Central America has a rich mix of conditions that allow comparisons of different natural experiments in the generation of arc magmas within the relatively short length of the margin. The shape of the volcanic front and this margin's architecture derive from the assemblage of exotic continental and oceanic crustal slivers, and later modification by volcanism and tectonic activity. Active tectonics of the Cocos-Caribbean plate boundary are strongly influenced by oblique subduction, resulting in a narrow volcanic front segmented by right steps occurring at ∼150-km intervals. The largest volcanic centers are located where depths to the slab are ∼90–110 km. Volcanoes that develop above deeper sections of the subducting slab are less voluminous and better record source geochemical heterogeneity. Extreme variations in isotopic and trace element ratios are derived from different components of thesubducted oceanic lithosphere. However, the extent that volcanoes sample these signatures is also influenced by lithospheric structures that control the arc segmentation. ▪ The architecture of Central America derives from the assemblage of exotic continental and oceanic crustal slivers modified by arc magmatism and tectonic processes. ▪ Active tectonics in Central America are controlled by oblique subduction. ▪ The lithospheric architecture and tectonics define the segmentation of the volcanic front, and thus the depth to the slab below a volcanic center. ▪ The composition of the subducted material is the main control of the along arc geochemical variations observed in Central American volcanoes. ▪ Geochemical heterogeneity in each segment is highlighted by extreme compositions representing the smaller centers with variations up to 65% of the total observed range.

Redox processes in subduction zones: Progress and prospect

Oxygen fugacity (fO2) is an intensive variable that describes the redox state of a system. By controlling the valence state of multivalent elements, fO2 affects the stability of iron-bearing minerals, dominants the species of volatile elements (e.g., carbon and sulfur), and controls the partitioning behaviors of multivalent elements (e.g., iron, vanadium, cerium, europium). Thus, fO2 plays a key role in understanding the generation and differentiation of arc magmas, the formation of magmatic-hydrothermal deposits, and the nature of magmatic volatiles. Subduction zones are an important site for arc magmatism and fluid action, and the study of redox processes is indispensable in subduction zone geochemistry. In this paper, we first introduce the concept, expression, and estimation methods of fO2. Then we retrospect the history and progress about the oxidation state of the metasomatized mantle wedge, summarize the redox property of slab-derived fluids, and review the latest progress on redox evolution of arc magmas during magma generation and differentiation. The main conclusions include: (1) despite its wide variation range, fO2 of the mantle wedge is generally higher than that of the oceanic mantle; (2) the redox property of the subducting slab-derived fluids is still controversial and the mechanism for the oxidization of the mantle wedge remains unclear; (3) how the fO2 varies during the generation and differentiation of the arc magmas is debated. We propose that the crux in deciphering the oxidization mechanism of the mantle wedge is to determine the mobility of iron, carbon and sulfur in subducting slab-derived fluids (especially solute-rich fluid or supercritical fluid); the key in understanding the redox evolution during arc magma generation and differentiation is to determine the partition coefficients of Fe3+ and Fe2+ between ferromagnesian minerals and silicate melts.

Evolution and maturation of the nascent Mariana arc

Long-standing debate remains regarding the processes by which nascent island arcs evolve into thicker-crust, mature volcanic edifices, long before being accreted into continental crust. How mature island arcs maintain their location for millions of years is still unclear. Arc location is currently believed to be controlled by slab dehydration or by the thermal state of the sub-arc mantle. However, most studies have focused on modern island arcs that have developed mature and stable volcanic edifices. Using published geochemical datasets, here we show that the southern Mariana arc is an incipient island arc that evolves into stable, thicker-crust stratovolcanoes to the north. Because it shares similar geochemical features with the early arc that formed during subduction infancy, it can place new important constraints onto the processes that occurred during island arc infancy. Examination of the Mariana arc along a S-N transect suggests that the growth of the serpentinized fore-arc mantle has the potential to modulate the depth of arc magma generation, which allows, in return, stabilization of the volcanic edifices during arc maturation. Arc stabilization further enables the deeply-sourced, slab fluids to infiltrate the sub-arc mantle wedge, which results in magmas with a typical arc fingerprint to the north. Melt focusing along magmatic conduits further promotes magma differentiation, magma-crust assimilation, and subsequent crustal thickening. We further propose that transient magmatic activity may also occur during island arc infancy in the Izu-Bonin-Mariana convergent margin, until serpentinization of the fore-arc mantle develops and stabilizes.

Molybdenum and boron isotope evidence for fluid-fluxed melting of intraplate upper mantle beneath the eastern North China Craton

Fluid-fluxed melting of the mantle is the principal mechanism for the generation of arc magmas above subduction zones, but it has rarely been documented as important in intraplate settings. Here, we present new molybdenum (Mo) isotopic data from a suite of well-characterized Cenozoic basalts from the eastern North China Craton (NCC) to constrain the mechanisms of mantle melting in the region. These basalts represent mixtures of three components, namely, nephelinites and basanites that variably mixed with alkali basalts. The alkali basalts have relatively low δ98/95Mo ranging from −0.56‰ to −0.22‰ relative to the NIST3134 standard, suggesting the presence of dehydrated pelagic sediments in their mantle source rocks. Interestingly, the basanites range to higher δ98/95Mo (−0.04 ± 0.02‰) compared with the nephelinites (−0.31‰ to −0.22‰) with no associated changes in their radiogenic isotopes, and they show trends of decreasing Dy/Yb, Ce/Mo and δ11B with increasing Ba/Nb and B/Nb ratios. These correlations indicate that Mo behaves as a fluid-mobile trace element in the intraplate mantle beneath the NCC, and that the mantle sources of the nephelinites and basanites were infiltrated by this fluid component. Existing data show that this fluid component has lower H2O/Ce ratios (110–130) than that of mid-ocean ridge basalts (H2O/Ce ≈ 200). This result, together with the low-SiO2 contents, relative depletion of Zr–Hf–Ti, and high δ66Zn characteristics of these strongly alkaline rocks, suggests derivation from a carbonated mantle source. Mo and B isotope systematics thus reveal carbonated fluid-fluxed mantle melting occurred beneath the eastern NCC. Increasing fluid inputs led to increases in the degree of melting of the mantle source that generated melts ranging in composition from nephelinite through basanite. The origin of such a fluid flux is enigmatic in an intraplate setting, but could be related to the decarbonization of subducted slabs stalled in the deep mantle.

Nitrogen recycling at the Costa Rican subduction zone: The role of incoming plate structure

Efficient recycling of subducted sedimentary nitrogen (N) back to the atmosphere through arc volcanism has been advocated for the Central America margin while at other locations mass balance considerations and N contents of high pressure metamorphic rocks imply massive addition of subducted N to the mantle and past the zones of arc magma generation. Here, we report new results of N isotope compositions with gas chemistry and noble gas compositions of forearc and arc front springs in Costa Rica to show that the structure of the incoming plate has a profound effect on the extent of N subduction into the mantle. N isotope compositions of emitted arc gases (9–11 N°) imply less subducted pelagic sediment contribution compared to farther north. The N isotope compositions (δ15N = −4.4 to 1.6‰) of forearc springs at 9–11 N° are consistent with previously reported values in volcanic centers (δ15N = −3.0 to 1.9‰). We advocate that subduction erosion enhanced by abundant seamount subduction at 9–11 N° introduces overlying forearc crustal materials into the Costa Rican subduction zone, releasing fluids with lighter N isotope signatures. This process supports the recycling of heavier N into the deep mantle in this section of the Central America margin.

An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle

The depth of slab dehydration is thought to be controlled by the thermal state of the downgoing slab: cold slabs are thought to mostly dehydrate beneath the arc front while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc.) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Here, we explore whether the serpentinized fore-arc mantle may be an important fluid reservoir in subduction zones and whether it can contribute to arc magma generation by being dragged down with the slab. Using simple mass balance and fluid dynamics calculations, we show that the dragged-down fore-arc mantle could provide enough water (∼7–78% of the total water injected at the trenches) to account for the water outfluxes released beneath the volcanic arc. Hence, we propose that the water captured by arc magmas may not all derive directly from the slab, but a significant component may be indirectly slab-derived via dehydration of dragged-down fore-arc serpentinites. Fore-arc serpentinite dehydration, if universal, could be a process that explains the similar geochemical fingerprint (i.e., in slab fluid proxies) of arc magmas.