Subduction Zone Magmatism Research Articles

Detrital zircon record of continental collision: Assembly of the Qilian Orogen, China

The Qilian Orogen, Northwest China, preserves a record of Early to Middle Paleozoic subduction and collision between the Central Qilian Block and North China Craton through closure of the intervening Late Neoproterozoic to Early Paleozoic North Qilian Ocean. Ordovician strata in the Hexi-Corridor Basin document the initial collisional history with a change in tectonic setting from retro-arc to foreland basin. Ages of zircons from the Upper Ordovician strata range from Archean to early Paleozoic with prominent peaks at around 2.5 Ga, 1.6 Ga, 1.0 Ga, and 0.47 Ga. The early Paleozoic ages are consistent with the time of magmatic arc activity related to northward subduction of the Qilian Ocean. Precambrian grains are restricted to the upper part of the sampled sections, with the proportion of Paleozoic grains descending up the section. This trend is interpreted to reflect the gradual termination of subduction zone magmatism and the swamping of the basin by material from the colliding Central Qilian Block and North China Craton. Archean and the early Proterozoic detritus were likely derived from the North China Craton to the north, whereas the Central Qilian Block to the south provided the late Proterozoic grains. Silurian and Devonian strata within the Hexi-Corridor Basin continue to show this mixed source provenance with depositional environment changing from a flysch to molasse setting.

Nature of crustal terranes and the Moho in northern Costa Rica from receiver function analysis

The Central American subduction zone in northern Costa Rica shows along‐strike variations in both the incoming and overriding plates. By analyzing the subducting oceanic Moho (M1) and the upper plate Moho (M2) with receiver functions, we investigate the variability in the hydration state of the subducting Cocos Plate and the nature of crustal terranes within the overriding Caribbean Plate. We calculate high‐quality P and PP wave receiver functions using broadband data of the Global Seismology Network; Geoscope Project; and the CRSEIZE, Pocosol, and Corisubmod experiments. In addition, we estimate the depth (H) and vertically averaged Vp/Vs (κ) to Moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the H and κ estimates. Our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the Nicoya Peninsula, as compared to a serpentinized oceanic mantle subducting beneath the northern part. In the Caribbean Plate, we describe the nature of the Mesquito, Nicoya, and Chorotega terranes by integrating new and published Vp/Vs estimates. Both the Nicoya and Chorotega terranes display high Vp/Vs (1.80–1.92) consistent with their oceanic character. In contrast, the oceanic Mesquito Terrane mostly displays lower Vp/Vs (1.62–1.80) more compatible with continental crust, which may indicate that subduction zone magmatism is modifying the crust to display continental character. Our estimates show that the deepest M2 (∼42 km) is observed in the volcanic arc region whereas the shallowest M2 (∼27–33 km) is observed in parts of the fore‐arc and back‐arc regions.

日本列島の誕生場―古太平洋の沈み込み開始を示す飛騨外縁帯の５２０Ｍａの熱水活動―

This paper examines the early stage of the geotectonic history of the Japanese Islands on the basis of finding hydrothermal jadeitite including zircons of ca. 520 Ma in serpentinite mélange of the Itoigawa-Omi area of the Hida-Gaien belt, Central Japan. Hydrothermal jadeitite contains euhedral jadeite in natrolite veins and patches, and consists of jadeite-albite and jadeite-natrolite without quartz. These minerals were crystallized from an aqueous fluid phase at the low-pressure and high-temperature side of the reaction boundary of albite = jadeite + quartz in the system NaAlSiO4-SiO2-H2O. The occurrence of rounded relict hornblende mantled by omphacite rimmed by fine-grained aggregates of jadeite in the matrix of jadeite and albite suggests a pervasive hydrothermal fluid flow, through which metabasite was extensively replaced by jadeitite. This rather high-temperature hydrothermal activity of ca. 520 Ma did not occur in an ordinary subduction zone but in a newly-formed mantle wedge suffering severe hydration from a subducting slab. Recently accumulated U-Pb ages of zircon of ca. 450-500 Ma from paleozoic sediments and granitic rocks of the Hida-Gaien belt were due to initiation of subduction followed by subduction zone magmatism. Protolith of serpentinite in the Hida-Gaien belt includes highly depleted harzburgite, thus requiring tectonic setting of a high-temperature-rift zone rather than a low-temperature-slow spreading ridge. Subduction was initiated at ca. 520 Ma along the boundary between low-density harzburgitic rift zone peridotite and lherzolitic spreading ridge peridotite with a slightly higher density, resulting in the common occurrence of harzburgitic serpentinite in the oldest part of the accretionary complex of Southwest Japan. An area including the Japanese Islands was born around the Yangtze block by the breaking up of the Rodinia supercontinent, because the oldest K-Ar age of biotite actinolit rock of 672 Ma (Matsumoto et al., 1981) and the subduction initiation of ca. 520 Ma are in accord with the paleogeographic history of the Yangtze block, and because ca. 300 Ma Renge schists of the Hida-Gaien belt did not suffer the ca. 280-200 Ma collision-type metamorphism of the Hida metamorphic belt that is an eastern extension of the suture between the Sino-Korea and Yangtze blocks.

Geochemistry and tectonic significance of the Stony Mountain gabbro, North Carolina: Implications for the Early Paleozoic evolution of Carolinia

Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia. The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO 2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas. Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~ 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.

Magma genesis beneath Northeast Japan arc: A new perspective on subduction zone magmatism

It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealed by latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow (< 120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. The position of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled by dynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in the mantle wedge.

CH 4 inclusions in orogenic harzburgite: Evidence for reduced slab fluids and implication for redox melting in mantle wedge

Fluids released from the subducting oceanic lithosphere are generally accepted to cause mantle wedge peridotite melting that produces arc magmas. These fluids have long been considered to be dominated by highly oxidized H 2O and CO 2 as inferred from erupted arc lavas. This inference is also consistent with the geochemistry of peridotite xenoliths in some arc basalts. However, the exact nature of these fluids in the mantle wedge melting region is unknown. Here, we report observations of abundant CH 4 + C + H 2 fluid inclusions in olivine of a fresh orogenic harzburgite in the Early Paleozoic Qilian suture zone in Northwest China. The petrotectonic association suggests that this harzburgite body represents a remnant of a Paleozoic mantle wedge exhumed subsequently in response to the tectonic collision. The mineralogy, mineral compositions and bulk-rock trace element systematics of the harzburgite corroborate further that the harzburgite represents a high-degree melting residue in a mantle wedge environment. Furthermore, existing and new C, He, Ne and Ar isotopes of these fluid inclusions are consistent with their being of shallow (i.e., crustal vs. deep mantle) origin, likely released from serpentinized peridotites and sediments of the subducting oceanic lithosphere. These observations, if common to subduction systems, provide additional perspectives on mantle wedge melting and subduction-zone magmatism. That is, mantle wedge melting may in some cases be triggered by redox reactions; the highly reduced (∼ΔFMQ–5, i.e., 5 log units below the fayalite–magnetite–quartz oxygen fugacity buffer) CH 4-rich fluids released from the subducting slab interact with the relatively oxidized (∼ΔFMQ–1) mantle wedge peridotite, producing H 2O and CO 2 that then lowers the solidus and incites partial melting for arc magmatism. The significance of slab-component contribution to the geochemistry of arc magmatism would depend on elemental selection and solubility in highly reduced fluids, for which experimental data are needed. We do not advocate the above to be the primary mechanism of arc magmatism, but we do suggest that the observed highly reduced fluids are present in mantle wedge peridotites and their potential roles in arc magmatism need attention.

Albemarle Q2 profit up

Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia.The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas.Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~ 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.

Boron and arsenic in highly evolved Archean felsic rocks: Implications for Archean subduction processes

The importance of subduction zone magmatism for Archean crustal evolution is highly debated, particularly with regard to the direct melting of subducted slabs. The debate centers largely around trace element geochemical observations. Among the fluid mobile elements, boron is widely accepted to be one of the most sensitive tracers of subduction zone processes. In modern subduction zone magmas, the B/Be ratio can be used to constrain supra-subduction zone mantle metasomatism. We present the first systematic database for concentrations of fluid mobile elements (Li, Be, B, and As) in 60 Archean granitoids spanning ∼ 3.8 to ∼ 2.5 Ga in age. The behavior of the very fluid mobile B and As relative to the similarly incompatible Be and Pr, respectively, demonstrates that two end-member types of Archean felsic magmas existed. Those with low fluid mobile element abundances have the steepest La/Yb, lowest W/Th and highest Nb/Ta and Sc concentrations, features compatible with an origin as direct slab melts that ascended through the mantle. On the other end of the spectrum are TTG with strong B and As enrichment, which in all regards other than their high La/Yb cannot be distinguished from modern arc melts. In between these extremes are TTG that share some characteristics with each end-member. These could have formed by melting of underplated basaltic eclogite at the base of the crust, in fluid-mobile element poor mantle above (hot) subduction zones where B and As were lost at shallow depth, or they could represent differentiates from very hydrous basaltic precursors in which garnet and amphibole replace olivine and pyroxene as the earliest crystallizing phases. The diversity of geochemical signatures of Archean granitoids suggests that they have formed in a wide range of geodynamic environments.

Evidence from high‐Ni olivines for a hybridized peridotite/pyroxenite source for orogenic andesites from the central Mexican Volcanic Belt

Subduction zone magmatism produces calc‐alkaline andesite melts that combine the high SiO2, Na2O, and K2O abundances of the differentiated continental crust with low FeO, FeO/MgO, and TiO2 typical of melts from depleted mantle. Ni‐rich olivines in basaltic andesites and andesites of the central Mexican Volcanic Belt suggest that this dichotomy reflects a particular mechanism of mantle processing in the subduction environment. Hydrous slab components rich in Si, Na, and fluid mobile large‐ion lithophile elements (LILE) transform mantle olivine to “reaction orthopyroxene.” Along the ascent paths, and embedded into surrounding peridotite, secondary pyroxenite lithologies are created that are composed of “reaction orthopyroxene” next to mantle clinopyroxene and orthopyroxene. Partial melts from peridotite and pyroxenite then mix to produce primary calc‐alkaline basaltic andesites and andesites that are rich in Na and LILE. The steady slab flux maintains high levels of Na and LILE in the mantle source but also induces repetitive melting that steadily depletes the subarc mantle in FeO, TiO2, and other high field strength elements. If mantle processing thus creates primary basaltic andesite and andesite melts with the fractionated major element signature of the continental crust, the high magnesium number (Mg # (=Mg/Mg + Fe2+)) ∼60–70 of these melts still requires additional differentiation to arrive at the lower Mg # ∼55 of average continental crust.

A critical evaluation of recent models for Lau–Tonga arc–backarc basin magmatic evolution

New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga–Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/ 144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas.

Hydrous Phases and Water Transport in the Subducting Slab

Arc volcanoes are typically located 90–180 km above the surface of downgoing slabs, as shown by Wadati-Benioff deep seismic foci (Gill 1981; Tatsumi 1989). The intimate relationship between the dip angles of the subducting slab and the locations of volcanic arcs indicates that subduction zone magmatism is triggered by material input from the subducting slab (Tatsumi and Eggins 1995). The slab-derived components are thought to be aqueous fluids or H2O-rich partial melts of subducted oceanic crust. Therefore, knowledge of the stability of hydrous phases and the chemical and physical properties of aqueous fluids in downgoing slabs is essential to understand the material transport in subduction zones. In this section, I will review the stability of hydrous phases in downgoing peridotite, basalt and sediment systems, and the chemical and the wetting properties of aqueous fluids. Recent experimental studies indicate that 3–4 GPa, equivalent to 90–120 km depth, is a key pressure, where (1) the chemical compositions of silicate components dissolved in aqueous fluids equilibrated with mantle minerals approach the composition of mantle peridotite itself (Stalder et al. 2001; Mibe et al. 2002; Kawamoto et al. 2004), (2) the dihedral angle between olivine and aqueous fluids starts becoming smaller than 60° (Watson et al. 1990; Mibe et al. 1998, 1999), and (3) the immisciblity gap between peridotitic melts and aqueous fluids disappears and consequently hydrous minerals liberate supercritical aqueous fluids (Mibe et al. 2004a, 2006). The similarity between these pressures and the depths of downgoing slab underneath volcanic fronts, where the maximum numbers of volcanoes are formed, 124 ± 38 km (Gill 1981) or 112 ± 19 km (Tatsumi 1986), suggests that subduction zone magmatism can be triggered by the input of supercritical fluids from the downgoing peridotite and basalt. Many hydrous crystalline phases …

Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes (R. D. Larter and P. T. Leat, eds.)

Pp. 352. Geological Society, London, Special Publications, no. 219. ISBN 1-86239-147-5. 2003. Price £85.00. This book reviews current research on subduction systems in oceanic island-arc settings. The tectonic, structural, and magmatic processes in oceanic arcs are generally simpler than in continental margin arcs, where subduction is typically a protracted process and magmas are contaminated by sialic crust. The papers in this volume present the geometry and characteristics of well-known intra-oceanic subduction systems and the implications for an improved understanding of subduction-related tectonic, magmatic, and hydrothermal processes. The book provides an up-to-date understanding of oceanic subduction systems, beyond that presented by Tatsumi and Eggins (1995) in Subduction Zone Magmatism . The latter reference is recommended for readers interested in general subduction processes. The book contains a total of 16 papers that vary in length from 14 to 30 pages. The general focus is on the geodynamics, magmatism, igneous rock geochemistry, geophysical signature, and, to a …

The subduction factory: How it operates in the evolving Earth

The subduction factory processes raw materials such as oceanic sediments and oceanic crust and manufactures magmas and continental crust as products. Aqueous fluids, which are extracted from oceanic raw materials via dehydration reactions during subduction, dissolve particular elements and overprint such elements onto the mantle wedge to generate chemically distinct arc basalt magmas. The production of calc-alkalic andesites typifies magmatism in subduction zones. One of the principal mechanisms of modern-day, calc-alkalic andesite production is thought to be mixing of two endmember magmas, a mantle-derived basaltic magma and an arc crust-derived felsic magma. This process may also have contributed greatly to continental crust formation, as the bulk continental crust possesses compositions similar to calc-alkalic andesites. If so, then the mafic melting residue after extraction of felsic melts should be removed and delaminated from the initial basaltic arc crust in order to form “andesitic” crust compositions. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic lower crust materials, are transported down to the deep mantle and recycled as mantle plumes. The subduction factory has played a central role in the evolution of the solid Earth through creating continental crust and deep mantle geochemical reservoirs.

Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data

Owing to advances in microanalytical techniques over the last 15 years, there is a growing database on the volatile contents of subduction-related magmas as recorded in melt (glass) inclusions trapped in phenocrysts in volcanic rocks. Basaltic magmas from subduction zones show a wide range of water contents, ranging from as high as 6–8 to <0.5 wt.% H 2O. Variations are related in some places to primary factors such as proximity to the arc front or extent of subduction-related mantle enrichment inferred from trace element systematics. Some low values of H 2O in melt inclusions result from shallow degassing before crystallization and entrapment of inclusions. The dissolved CO 2 contents of melt inclusions from basaltic arc lavas range from below detection (∼25 ppm) to nearly 2500 ppm. This variability is caused by degassing of low-solubility CO 2 before inclusions are trapped. The estimated primary CO 2 content of arc basaltic magma inferred from global arc volcanic CO 2 emissions and magma flux is >3000 ppm, suggesting that no melt inclusions sample undegassed arc magmas. The Cl and S contents of arc basaltic magmas are greater than midocean ridge basalts, indicating that these volatiles are also recycled from subducted sediment and altered oceanic crust back into the mantle wedge. Comparison of the fluxes of volatiles subducted back into the mantle along subduction zones and returned from the mantle to the surface reservoir (crust, ocean, and atmosphere) via magmatism suggests that there is an approximate balance for structurally bound H 2O and Cl. In contrast, ∼50% of subducted C appears to be returned to the deep mantle by subduction, but uncertainties are relatively large. For S, the amount returned to the surface reservoir by subduction zone magmatism is only ∼15–30% of the total amount being subducted. Dacitic and rhyolitic magmas in arcs contain 1–6 wt.% H 2O, a range that overlaps considerably with the values for basaltic magmas. Either basaltic parents for these differentiated magmas are relatively H 2O-poor, or intermediate to silicic arc magmas form through open-system processes involving variable amounts of crustal melting, mixing with basalt and basaltic differentiates, and fluxing of CO 2-rich vapor from mafic magma recharged into silicic magma bodies. Consideration of H 2O–CO 2 relations and gaseous SO 2 emissions for intermediate to silicic arc magmas shows that such magmas are typically vapor-saturated during crystallization in the middle to upper crust. Gas emissions thus reflect migration and accumulation of volatiles within complex open magmatic systems.

Geochemical evidence for the tectonic setting of the Harmancik ophiolites, NW Turkey

The Harmancik, Orhaneli and Tavşanlı ophiolites of NW Turkey are remnants of oceanic lithosphere formed in the Neo-Tethys during the Mesozoic. These ophiolites constitute part of the İzmir–Ankara suture zone which separates the Pontides to the north from the Anatolides and Taurides to the south. The Harmancik area includes a metamorphic basement, ophiolitic melange, peridotite (dunite, harzburgite), serpentinized peridotite, doleritic dykes and Neogene cover sediments. Metamorphic rocks in the Harmancik area are psammites, pelites and marbles, metamorphosed under blueschist facies conditions. They are overlain tectonically by peridotites cut by basic dykes. The dykes are dolerites, and based on texture and mineralogy, can be divided into three petrographic groups, coarse, medium-grained and porphyritic. These groups have similar chemical compositions with respect to major, trace and REE elements. When plotted on a basalt discrimination diagram the dykes plot in the IAT (Island Arc Tholeiite) field and are considered to have been intruded during subduction zone magmatism. Chondrite-normalized REE patterns for the dykes are Eu-depleted, similar to patterns for the mean dolerite composition of the NE Anatolian ophiolites, and are slightly LREE-enriched. Our results suggest that the Harmancik ophiolite is a supra-subduction zone ophiolite on which a volcanic arc sequence was built, and that the dolerite dykes were formed by the partial melting of the underlying mantle.

P– T history of garnet-websterites in the Sharyzhalgai complex, southwestern margin of Siberian craton: evidence for Paleoproterozoic high-pressure metamorphism

The Saramta massif in the Paleoproterozoic Sharyzhalgai complex, the southwestern margin of the Siberian craton, is mainly composed of spinel-peridotites with garnet-websterites; it is enclosed within granitic gneisses and migmatites with mafic intercalations of granulite-facies grade. The garnet-websterites occur as lenses or layers intercalated within spinel-harzburgite and spinel-lherzolite. They consist mainly of clinopyroxene (Cpx), garnet (Grt), and orthopyroxene (Opx): Grt often includes Cpx, Opx, and pargasite (Prg). Opx also occurs as kelyphite with plagioclase (Pl), spinel, olivine, Prg, and biotite. Relationships between textures and chemical compositions of these minerals suggest the following P– T stages: stage 1 (pre-peak), 0.9–1.5 GPa at 640–780 °C; stage 2 (peak), 2.3–3.0 GPa at 920–1030 °C as the minimum estimate; and stage 3 (post-peak), 750–830 °C at 0.5–0.9 GPa. Finally, the garnet-websterites are veined with lower amphibolite- to greenschist-facies minerals (stage 4). These results suggests that the Saramta massif was carried to depths of c. 100 km by subduction, and metamorphosed under eclogite-facies conditions in the Paleoproterozoic, despite the commonly held view that high geothermal gradients in those times would have prevented such deep subduction. Paleoproterozoic plate subduction at the southwestern margin of the Siberian craton might have caused subduction-zone magmatism and mantle metasomatism similar to those in the Phanerozoic.

Experimentally-determined trace element characteristics of aqueous fluid from partially dehydrated mafic oceanic crust at 3.0 GPa, 650-700C

Aqueous vapour, released from subducted dehydrated oceanic crust, has the potential to remove relatively large amounts of trace elements and to modify key trace element ratios in the overlying mantle wedge, an important source region for subduction zone magmatism. To experimentally assess this process an oceanic tholeiite (MORB) has been enriched (10–100 ppm) in 25 trace elements and crystallized at 3.0 GPa, 650 and 700°C for 2 weeks, with 30–50wt.% added water. At 650°C the fluid coexists with omphacite, lawsonite, chloritoid, coesite and accessory phengite, rutile, Ti-magnetite and talc. Changes at 700°C are that garnet is conspicuous, chloritoid absent and lawsonite, staurolite and talc are rare. LAM ICP-MS analyses establish patterns of trace element partitioning between minerals (data for omphacite, lawsonite, phengite, chloritoid and garnet) and fluid, but not absolute values. Similarly ratios of trace elements in the fluid can be well constrained. Relative to the bulk composition the fluid shows strong enrichment of LILE/HFSE and LILE/REE at both 650 and 700°C, but also shows significant decrease in Rb/Sr, Cs/Sr and Ba/Sr from 650 to 700°C, apparently controlled by residual lawsonite, and increase in La/Lu, clearly controlled by garnet. In addition, omphacite strongly fractionates both Zr/Nb and La/Lu, contributing to a decrease in Zr/Nb and an increase in La/Lu in the fluid. Residual rutile (analyzed with an electron microprobe) strongly takes up Nb and Ta, the latter more readily, causing an almost a 4-fold increase in the Nb/Ta ratio in the fluid relative to the bulk. Omphacite, garnet and chloritoid all have Nb/Ta less than and lawsonite greater than the starting composition. Omphacite and garnet strongly reject Sr, Ba, Rb and Cs; lawsonite favours all REE (HREE slightly more so), Sr, U and Th, whereas phengite favours Rb, Ba and Cu. Chloritoid most readily accommodates V, Cr, Ni and Zn. Involvement of a fluid derived by dehydration processes may increase LILE/HFSE and LILE/REE and decrease Zr/Nb in subduction-linked magmas, relative to MORB. Lowest Zr/Nb ratios point to the greatest relative involvement of clinopyroxene in the residue. Absence of major change to La/Lu restricts the role of garnet. Higher Rb/Sr, Cs/Sr and Ba/Sr may point to a low geothermal gradient in the subduction zone such that lawsonite remains stable during the dehydration process that produced the fluid. Residual rutile may explain Nb and Ta depletion and cause an increase in Nb/Ta of the coexisting fluid.

Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico

Research Article| March 01, 2003 Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico Pablo Cervantes; Pablo Cervantes 1Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77845, USA Search for other works by this author on: GSW Google Scholar Paul J. Wallace Paul J. Wallace 2Ocean Drilling Program, Texas A&M University, College Station, Texas 77845, USA Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (3): 235–238. https://doi.org/10.1130/0091-7613(2003)031<0235:ROHOIS>2.0.CO;2 Article history received: 17 Jun 2002 rev-recd: 08 Nov 2002 accepted: 14 Nov 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pablo Cervantes, Paul J. Wallace; Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico. Geology 2003;; 31 (3): 235–238. doi: https://doi.org/10.1130/0091-7613(2003)031<0235:ROHOIS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Although there is a growing body of data on H2O in arc magmas, there is still considerable uncertainty about the relationship between H2O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H2O, other volatiles (CO2, S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65–78, olivine hosts Fo85–90) trapped at upper to middle crustal pressures (1–6 kbar) contain high CO2 (250–2120 ppm). The high-pressure inclusions indicate magmatic H2O contents from 1.3 to 5.2 wt%. Enrichment of H2O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H2O and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H2O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low H2O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes <50 km apart show evidence of significant variability in the composition of H2O-rich subduction components that are added to the mantle wedge beneath central Mexico. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The subduction factory: its role in the evolution of the Earth’s crust and mantle

Abstract Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form ‘andesitic’ crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.

Detection of recycled marine sediment components in crater lake fluids using 129I

Crater lakes provide time-integrated samples of volcanic fluids, which may carry information on source components. We tested under what circumstances 129I concentrations can be used for the detection of a signal derived from the recycling of marine sediments in subduction zone magmatism. The 129I system has been successfully used to determine origin and pathways in other volcanic fluids, but the application of this system to crater lakes is complicated by the presence of anthropogenic 129I, related to recent nuclear activities. Results are reported from four crater lakes, associated with subducting crust varying in age between 23 and 98 Ma. The 129I/I ratios determined for Copahue, Argentina, (129I/I=700×10−15) and White Island, New Zealand, (129I/I=284×10−15) demonstrate the presence of iodine in the crater lakes that was derived from recycled marine sediments. A comparison to the ages of the subducted sediments in these two cases indicates that the ratios likely reflect iodine remobilization from the entire sediment column that was undergoing subduction. While the 129I signals in Poás and Rincón de la Vieja, Costa Rica also demonstrate the presence of recycled iodine, the relatively high percentage of meteoric water in these lakes prevents a reliable determination of source ages. The observed high concentrations of iodine and 129I/I ratios substantially below current surface values strongly argue for the presence of recycled marine components in the arc magmas of all four cases. Components from subducted marine sediments can be quantified and related to specific parts of the sediment column in cases where the iodine concentration in the lake waters exceeds 5 μM.