Melting Of Sediments Research Articles

Variable sediment flux in generation of Permian subduction-related mafic intrusions from the Yanbian region, NE China

This paper presents petrology, mineralogy, zircon U–Pb ages, and whole-rock major, trace element and Sr–Nd–Hf isotopic compositions of four Permian (273–253Ma) subduction-related mafic intrusions (including the Qinggoushan and Qianshan gabbros, and the Wangqing and Shuguang diorites) from the Yanbian region, NE China, with aims to understand the role of subducted sediment flux in generation of arc mafic cumulates. These intrusions have mineral assemblages crystallized in water-saturated parental magmas and show variable degrees of crystal accumulation as observed in mafic cumulates in subduction zones. Mass-balance consideration indicates that their parental magmas were calc-alkaline with arc-type trace element features (enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depletions in Nb–Ta). They also have Sr–Nd–Hf isotopic compositions, i.e., 87Sr/86Sr(i)=0.7029–0.7047, εNd(t)=+0.9~+6.8, εHf(t)=+5.6~+14.6, similar to modern arc basalts. The parental magmas were likely derived from a mantle wedge variably metasomatized by sediment melt and fluid from the subducting paleo-Asian Oceanic slab. Combined trace elemental and isotopic modeling results suggest that the parental magma of Qinggoushan gabbro was formed through 5–20% melting of the mantle wedge with 1% and 1.5% additions of sediment fluid and sediment melt, respectively; 5–10% melting of the mantle wedge through inputs of 1% sediment fluid and 2% sediment melt produced the Qianshan gabbro; 10–20% melting of the mantle wedge with additions of 1% sediment fluid and 3% sediment melt formed the Wangqing diorite; whereas 5–20% melting of the mantle wedge through an input of 1.5% sediment melt produced the Shuguang diorite.The Hf–Nd isotopic array of the Yanbian Permian mafic intrusions reflected the existence of an Indian Ocean-type mantle, which was isotopically distinct from the Pacific-type mantle during early Paleozoic in the Central Asian Orogenic Belt. Such a transformation from the Pacific-type to the Indian Ocean-type mantle was primarily attributed to addition of the recycled components, which contained more radiogenic Hf than Nd isotopic compositions, from the subducting paleo-Asian Oceanic slab.

Petrology and geochemistry of metabasalts from the Taoxinghu ophiolite, central Qiangtang, northern Tibet: Evidence for a continental back-arc basin system

Newly discovered ophiolitic metabasalts in the Taoxinghu area of central Qiangtang on the Qinhai-Tibet Plateau, here described for the first time, have important implications for reconstructions of the tectonic history of the Paleo-Tethys Ocean. Most of the metaba- salts belong to the tholeiitic basalt series and most have undergone greenschist-facies metamorphism. The distribution of rare earth elements and trace elements shows that the rocks are typical of sub-continental margin arc-basin lavas, similar to the environment of formation of the present-day Okinawa lava, suggesting that the Taoxinghu metabasalts represent the upper portions of a sup- ra-subduction zone (SSZ)-type ophiolite that formed in a continental back-arc basin tectonic environment. The Taoxinghu metaba- salts are mainly sourced from a depleted spinel mantle-source region, with a spinel lherzolite content equivalent to partial melting of 6 %-25 %. In addition, lava compositions were likely affected by melting of sediments during subduction, while the influence of aqueous fluids was minor. Combining with the existing knowledge on the ophiolites of Longmuco-Shuanghu-Lancang suture zone (LSLSZ), an evolutionary model is proposed. The LSLSZ Paleo-Tethys Ocean basin may have started to form during the Cambrian or earlier, and subducted in the early Carboniferous. As subduction proceeded, a continental back-arc basin was developed, the site of generation of most of the Taoxinghu metabasalts. The LSLSZ Paleo-Tethys Ocean finally closed in the Triassic.

The thermal effect of fluid circulation in the subducting crust on slab melting in the Chile subduction zone

Fluids released from subducting slabs affect geochemical recycling and melt generation in the mantle wedge. The distribution of slab dehydration and the potential for slab melting are controlled by the composition/hydration of the slab entering a subduction zone and the pressure–temperature path that the slab follows. We examine the potential for along-strike changes in temperatures, fluid release, and slab melting for the subduction zone beneath the southern portion of the Southern Volcanic Zone (SVZ) in south central Chile. Because the age of the Nazca Plate entering the subduction zone decreases from ∼14 Ma north of the Guafo Fracture Zone to ∼6 Ma to the south, a southward warming of the subduction zone has been hypothesized. However, both north and south of Guafo Fracture Zone the geochemical signatures of southern SVZ arc lavas are similar, indicating 3–5 wt.% sediment melt and little to no contribution from melt of subducted basalt or aqueous fluids from subducted crust. We model temperatures in the system, use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the pattern of dehydration-derived fluid release, and examine the potential locations for the onset of melting of the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools the hottest parts of the system (i.e. those with the youngest subducting lithosphere). Models including the thermal effects of fluid circulation in the oceanic crust predict melting of the subducting sediment but not the basalt, consistent with the geochemical observations. In contrast, models that do not account for fluid circulation predict melting of both subducting sediment and basalt below the volcanic arc south of Guafo Fracture Zone. In our simulations with the effects of fluid circulation, the onset of sediment melting occurs under the volcanic arc, but dewatering of the subducting sediment and basalt is focused farther seaward (below the landward boundary of the stagnant mantle wedge corner). Thus, the sediment melt could enter the mantle wedge, contributing to the composition of the southern SVZ magmas, yet remain separate from the fluid derived from sediment dewatering which could migrate updip within the slab or into the wedge corner. Preferential hydrothermal cooling of the hottest segments of the system can help explain how there can be fairly uniform magma composition along the arc, despite large along-arc differences in the age of the subducting plate.

Petrology of the Motaghairat mafic–ultramafic complex, Eastern Desert, Egypt: A high-Mg post-collisional extension-related layered intrusion

The geodynamic settings of the Precambrian mafic–ultramafic complexes in the Eastern Desert of Egypt have important bearing on understanding the geotectonic evolution of the Arabian Nubian Shield. We present a detailed petrological study on a layered mafic–ultramafic intrusion that is located at the contact between the Precambrian continental crust and the Miocene Red Sea oceanic crust. The Motaghairat layered intrusion consists of basal lherzolite, orthopyroxenite, troctolite, olivine gabbro and anorthosite on the top. Variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high-Mg tholeiitic parent melt through fractional crystallization processes. The parental magma was derived from a metasomatised mantle source. The primitive mantle-normalized patterns of the calculated melts exhibit enrichment in U relative to Th and Ba relative LREE which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt. Geological and petrological evidences suggest that the layered Motaghairat intrusion was emplaced during post-orogenic extension following subduction break-off and lithospheric delamination after the collision between the amalgamated island arc terranes and the Saharan Metacraton. The heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction-break-off.

Origin of selective enrichment of Cu and Au in sulfide deposits formed at immature back-arc ridges: Examples from the Lau and Manus basins

It has long been recognized that magmatic fluids exsolved from the arc-like submarine magmas of immature back-arc basins can directly contribute metals such as Cu and Au to seafloor hydrothermal systems. The extent of this magmatic contribution, however, varies from basin to basin. In order to explain this variation, we make a comparative study of the behavior of Cu during magma differentiation in two immature back-arc ridges: Eastern Ridge (ER) of the Manus Basin and Valu Fa Ridge (VFR) of the Lau Basin. We investigate some of the factors that affect Cu behavior, including oxygen fugacity (fO2), water content, and crystallization pressure, by means of a geochemical model. Cu abundances show a continuous decreasing trend with magma evolution in the VFR lavas, whereas in the ER lavas Cu increases during the early stage of magma evolution, followed by a rapid decrease. The contrasting Cu behavior for the two lava suites is controlled on the first order by the fO2 of their primary magmas. The fO2 values of the primary ER magmas were modeled to be FMQ +1.2 to FMQ +1.8, which is sufficiently high to avoid the early sulfide saturation that typically accompanies Cu removal. By comparison, the fO2 values of the primary VFR magmas range from FMQ to FMQ +1, falling within the range of mid-ocean ridge basalts. We attribute this difference in fO2 values between the primary ER and VFR magmas to variable input of sediment melt to their mantle sources. In addition, we show for the first time that Cu content does not increase significantly until the onset of plagioclase crystallization. This finding suggests that both high water contents and high pressure, which suppress plagioclase crystallization, are unfavorable for Cu enrichment in evolved oxidized magmas. We argue that back-arc ridges that develop shallow submarine magma chambers and have a large input of subducted sediment, have a strong potential to support ore-bearing magmatic–hydrothermal systems.

Redox-variability and controls in subduction zones from an iron-isotope perspective

An ongoing controversy in mantle geochemistry concerns the oxidation state of the sources of island arc lavas (IAL). Three key factors control oxidation–reduction (redox) of IAL sources: (i) metasomatism of the mantle wedge by fluids and/or melts, liberated from the underlying subducted slab; (ii) the oxidation state of the wedge prior to melting and metasomatism; and (iii) the loss of melt from IAL sources. Subsequently, magmatic differentiation by fractional crystallisation, possible crustal contamination and degassing of melts en route to and at the surface can further modify the redox states of IAL. The remote nature of sub-arc processes and the complex interplay between them render direct investigations difficult. However, a possible gauge for redox-controlled, high-temperature pre-eruptive differentiation conditions is variations in stable Fe isotope compositions (expressed here as δ57Fe) in erupting IAL because Fe isotopes can preserve a record of sub-surface mass transfer reactions involving the major element Fe. Here we report Fe isotope compositions of bulk IAL along the active Banda arc, Indonesia, which is well known for a prominent subducted sediment input. In conjunction with other arc rocks, δ57Fe in erupted Banda IAL indicates that fractional crystallisation and possibly crustal contamination primarily control their Fe isotope signatures. When corrected for fractional crystallisation and filtered for contamination, arc magmas that had variable sediment melt contributions in their sources show no resolvable co-variation of δ57Fe with radiogenic isotope tracers. This indicates that crustal recycling in the form of subducted sediment does not alter the Fe isotope character of arc lavas, in agreement with mass balance estimates. Primitive sources of IAL, however, are clearly isotopically lighter than those sourced beneath mid-ocean ridges, indicating either preferential Fe3+-depletion in the mantle wedge by prior, δ57Fe-heavy melt extraction, and/or addition of an isotopically-light slab-derived agent. Based on our findings and previous models of arc redox conditions, we propose a three-stage process to explain the Fe isotope composition of IAL: (i) prior melt depletion lowers Fe3+/ΣFe (Fe3+ over total Fe) in the residues, leaving refractory, δ57Fe-light and possibly reduced mantle wedge protoliths beneath arcs. The oxygen fugacity (fO2) of these refractory protoliths may be up to −2 log10 units reduced relative to the fayalite–magnetite–quartz synthetic oxygen buffer (ΔFMQ); (ii) oxidised, slab-derived fluids, Fe-poor but possibly rich in sulphate (i.e., S6+), trigger melting of depleted protoliths with minimal effect on δ57Fe. Melts derived from this fluid-modified wedge source have high Fe3+/ΣFe, oxidised by the reduction of S6+, but importantly retain the light δ57Fe from their mantle wedge source; (iii) after melt liberation from the mantle wedge, arc magmas initially become progressively oxidised and isotopically heavier in Fe through fractional crystallisation of ferromagnesian silicates. In summary, reduction consequent to Fe3+-rich melt extraction and subsequent oxidation, likely by S6+-rich fluids, results in a “redox yo-yo” in IAL sources. Fractional crystallisation will further oxidise and elevate δ57Fe in erupting IAL. Iron isotope signatures in IAL record a complex magmatic history with no simple relation between δ57Fe and calculated fO2 in erupted lavas. Records of higher fO2 in subduction zones compared to MORB sources are inherited from the subduction component.

Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite

New geochemical data are presented for a suite of tonalites, granodiorites, trondhjemites and granites intrusive into depleted mantle harzburgites of the Oman–UAE ophiolite. A detailed field, petrological and geochemical examination suggests that these ‘mantle granitoids’ are the product of three processes: (a) the mixing of melts derived from both mafic and metasedimentary sources, (b) interaction with the mantle harzburgite host and (c) the fractional crystallisation of plagioclase, hornblende ± accessory phases. Geochemical data are used to characterise the identity of the protolith(s) by first screening the data for those samples which have experienced fractional crystallisation during emplacement. The resultant ‘reduced’ data set has moderately fractionated REE, with small negative Eu anomalies and fractionated primitive mantle-normalised trace element patterns with high concentrations of fluid mobile elements and lower concentrations of HFS elements and with positive peaks for Rb and Pb and negative troughs for Ba, Nb, Sr and Ti. The character of the protolith was quantified using a melting model based upon a MORB-type basalt similar in composition to the Oman Geotimes lavas and a model using the MUQ (MUd from Queensland) global sediment composition (Kamber et al. Geochim Cosmochim Acta 69:1041–1058, 2005) both with an amphibolite/granulite facies mineralogy. The two compositions bracket the mantle granitoid data set with partial melts of the MORB source yielding trace element compositions lower than the granitoids, whereas melts of the MUQ source yield melts with compositions higher than the granitoids. Mixing of the calculated melt compositions indicates that the measured granitoid compositions represent between 10 and 30 % mixing of a metasedimentary melt into the melt of a mafic source. Current petrological, structural and geochronological data suggest a model for the origin of the Oman ophiolite in which it is formed by spreading above a subduction zone. The results of this study support this model in three ways: (a) trace element data show that the granitoids have interacted with the mantle, implying that they originated below the mantle section of the ophiolite and were emplaced into the mantle wedge from below; (b) a most probable setting for the melting of a sediment–basalt mix is in the upper part of a subducting slab; (c) the high temperature/shallow melting of the sediment and basaltic slab mix can only have been driven by the high temperatures of the overlying mantle wedge; this is most likely during subduction initiation. Thus, sediment melting during subduction initiation represents an important means of refertilising what is otherwise highly depleted mantle.

Geochemical characteristics of Carboniferous volcanic rocks from the Wulungu-Luliang area, Junggar basin: constraints on magma source and tectonic setting

The Carboniferous volcanic rocks in the Wulungu-Luliang area are mainly andesites of medium- to high-K calc-alkalic series. Volcanic rock samples have relatively high alkali (Na2O + K2O = 4.7 % to 6.8 %) and low TiO2 contents (0.7 % to 0.9 %), relatively high MgO (2.5 % to 3.4 %) and Mg# (49.9 % to 67.1 %), high rare earth element (REE) contents, and relatively high K2O contents (1.7 % to 3.1 %). Chondrite-normalized REE patterns show light REE enrichment ((La/Yb)N = 4.15 to 5.19) with weak Eu anomalies (δEu = 0.75 to 0.92). These samples are enriched in large-ion lithophile elements but relatively depleted in high field strength elements. The trace elements and REE patterns are similar to those of Setouchi and central Ryukyu high-Mg andesites, indicating a high-Mg andesite source. Relatively high Y contents (16.7 to 24.4 ppm), and relatively low Sr/Y ratios (17.2 to 38.8) and TiO2 contents (0.7 % to 0.9 %) exclude the possibility of slab melting. Low Sr/Nd (16.6 to 42.8), Ba/Th (66.4 to 266.8), and U/Th (0.2 to 0.3) indicate that the influence of slab-derived fluids is low. The ratios of Ce/Th (4.9–7.3), Ce/Pb (1.8–4.2), Ba/Rb (7.99–22.03), Ba/Th (66.4–266.8), and La/Sm (3.6–4.3) are similar to ratios found in subducting sediment melts. Relatively high ratios of K/Nb (1357–3258), Th/La (0.28–0.42), Zr/Nb (8.8–27.1), and especially Th/Nb (0.48–1.25) suggest that the magma was assimilated and contaminated by upper continental crust. These characteristics, along with the ratios of La/Yb, Sc/Ni, Th/Yb, Ta/Yb, Ce/P2O5, and Zr/TiO2, demonstrate that the earlier Carboniferous volcanic rocks in the Wulungu-Luliang area were generated in a continental island-arc setting.

Decarbonation of subducting slabs: Insight from petrological–thermomechanical modeling

Subduction of heterogeneous lithologies (sediments and altered basalts) carries a mixture of volatile components (H2O±CO2) into the mantle, which are later mobilized during episodes of devolatilization and flux melting. Several petrologic and thermodynamic studies investigated CO2 decarbonation to better understand carbon cycling at convergent margins. A paradox arose when investigations showed little to no decarbonation along present day subduction geotherms at subarc depths despite field based observations. Sediment diapirism is invoked as one of several methods for carbon transfer from the subducting slab. We employ high-resolution 2D petrological–thermomechanical modeling to elucidate the role subduction dynamics has with respect to slab decarbonation and the sediment diapirism hypothesis. Our thermodynamic database is modified to account for H2O–CO2 binary fluids via the following lithologies: GLOSS average sediments (H2O: 7.29wt.% & CO2: 3.01wt.%), carbonated altered basalts (H2O: 2.63wt.% & CO2: 2.90wt.%), and carbonated peridotites (H2O: 1.98wt.% & CO2: 1.50wt.%). We include a CO2 solubility P–x[H2O wt.%] parameterization for sediment melts. We parameterize our model by varying two components: slab age (20, 40, 60, 80Ma) and convergence velocity (1, 2, 3, 4, 5, 6cmyear−1). 59 numerical models were run and show excellent agreement with the original code base. Three geodynamic regimes showed significant decarbonation. 1) Sedimentary diapirism acts as an efficient physical mechanism for CO2 removal from the slab as it advects into the hotter mantle wedge. 2) If subduction rates are slow, frictional coupling between the subducting and overriding plate occurs. Mafic crust is mechanically incorporated into a section of the lower crust and undergoes decarbonation. 3) During extension and slab rollback, interaction between hot asthenosphere and sediments at shallow depths result in a small window (~12.5Ma) of high integrated CO2 fluxes (205kgm−3Ma−1).

Reduced sediment melting at 7.5–12 GPa: phase relations, geochemical signals and diamond nucleation

Melting of carbonated sediment in the presence of graphite or diamond was experimentally investigated at 7.5–12 GPa and 800–1600 °C in a multianvil apparatus. Two starting materials similar to GLOSS of Plank and Langmuir (Chem Geol 145:325–394, 1998) were prepared from oxides, carbonates, hydroxides and graphite. One mixture (Na-gloss) was identical in major element composition to GLOSS, and the other was poorer in Na and richer in K (K-gloss). Both starting mixtures contained ~6 wt% CO2 and 7 wt% H2O and were doped at a ~100 ppm level with a number of trace elements, including REE, LILE and HFSE. The near-solidus mineral assemblage contained a silica polymorph (coesite or stishovite), garnet, kyanite, clinopyroxene, carbonates (aragonite and magnesite–siderite solid solution), zircon, rutile, bearthite and hydrous phases (phengite and lawsonite at 10 GPa). Hydrous phases disappear at ~900 °C, and carbonates persist up to 1000–1100 °C. At temperatures >1200 °C, the mineral assemblage consists of coesite or stishovite, kyanite and garnet. Clinopyroxene stability depends strongly on the Na content in the starting mixture; it remains in the Na-gloss composition up to 1600 °C at 12 GPa, but was not observed in K-gloss experiments above 1200 °C. The composition of melt or fluid changes gradually with increasing temperature from hydrous carbonate-rich ( 1). Aragonite and Fe–Mg carbonate have very different REE partition coefficients (DMst-Sd/L ~ 0.01 and DArg/L ~ 1). Nb, Ta, Zr and Hf are strongly incompatible in both carbonates. The bearthite/melt partition coefficients are very high for LREE (>10) and decrease to ~1 for HREE. All HFSE are strongly incompatible in bearthite. In contrast, Ta, Nb, Zr and Hf are moderately to strongly compatible in ZrSiO4 and TiO2 phases. Based on the obtained partition coefficients, the composition of a mobile phase derived by sediment melting in deep subduction zones was calculated. This phase is strongly enriched in incompatible elements and displays a pronounced negative Ta–Nb anomaly but no Zr–Hf anomaly. Although all experiments were conducted in the diamond stability field, only graphite was observed in low-temperature experiments. Spontaneous diamond nucleation and the complete transformation of graphite to diamond were observed at temperatures above 1200–1300 °C. We speculate that the observed character of graphite–diamond transformation is controlled by relationships between the kinetics of metastable graphite dissolution and diamond nucleation in a hydrous silicocarbonate melt that is oversaturated in C.

Zircon U–Pb ages and geochemical characteristics of granitoids in Nagqu area, Tibet

This paper reports zircon LA-ICP-MS U–Pb ages and Hf–O isotopic ratios, and whole-rock major and trace element data of Early Cretaceous felsic intrusive rocks from Nagqu area, the northern Lhasa subterrane, southern Tibet. LA-ICP-MS zircon U–Pb dating of biotite granites and biotite monzogranites in the area yields magmatic crystallization ages of ca. 112Ma, which suggests that they were emplaced in the late Early Cretaceous. Both rocks show high-K calc-alkaline to shoshonitic composition and slightly–moderately peraluminous signature. They are enriched in the alkalis, Rb, Th, K, U and light rare earth elements, depleted in Nb, Ta, Ti and P, and characterized by high Al2O3 contents (12–16wt.%), high Rb/Sr ratios (1.3–33) and low Mg# values (15–39). Their magmatic zircons have negative εHf(t) values (from −25.9 to 0.5) and high positive δ18O values (from 7.9‰ to 11.5‰). All the above characteristics indicate that Nagqu biotite monzogranites and biotite granites were likely derived from hybrid melts of sediments from the continent crust with minor mantle-derived input, then experienced varied degrees of fractional crystallization. The Nagqu intrusion is a component of the late Early Cretaceous magmatic flare-up event that occurred during ~120–100Ma in the northern and partly central Lhasa subterranes. This magmatic flare-up is marked with a great compositional diversity (basalt, rhyolite, adakitic rocks, dioritic enclave, biotite monzogranite and granite) that might be caused by the slab breakoff of the southward subducting Bangong–Nujiang oceanic lithosphere, or more likely by slab window opening, which may have significantly contributed to juvenile crustal growth of the northern Lhasa subterrane.

Partial melting of fertile peridotite fluxed by hydrous rhyolitic melt at 2–3 GPa: implications for mantle wedge hybridization by sediment melt and generation of ultrapotassic magmas in convergent margins

We investigated the melting behavior of peridotite fluxed with 25 wt% of H2O-bearing rhyolitic sediment melt (1.8 wt% bulk H2O), by performing experiments from 1100 to 1300 °C at 2 GPa and 1050–1350 °C at 3 GPa. The apparent solidus of our bulk composition lies between 1100 and 1150 °C at both pressures, which is at a higher temperature than the vapor-saturated solidus and close to the pargasite dehydration solidus of peridotite. With increasing temperature, reacted melt fraction increases from 20 to 36 wt% from 1200 to 1300 °C at 2 GPa and 7 to 24 wt% from 1225 to 1350 °C at 3 GPa. Orthopyroxene is present as a residual phase in all the experiments, while olivine is present as a residual phase in all the experiments at 2 GPa only. Amphibole is absent above 1100 °C at both pressures, clinopyroxene disappears above 1200 and 1300 °C at 2 and 3 GPa, respectively, and garnet (only present at 3 GPa) melts out above 1300 °C. Upon reaction with the mantle wedge and subsequent melting of the hybrid rock, subducted sediment-derived rhyolites evolve in composition to a nepheline-normative ultrapotassic leucitite, similar in major element composition to ultrapotassic lavas from active arcs such as Sunda and inactive arcs such as in the Roman Magmatic Province. Fluxing peridotite with H2O versus H2O-bearing sediment melt at similar pressures does not appear to have an effect on isobaric melt productivity, but does have significant effect on melting reactions and resultant melt composition, with influx of sediment melt adding K2O to the system, thereby stabilizing phlogopite, which in turn buffers the reacted melt to ultrapotassic compositions. Previous experimental studies, along with this study, find that phlogopite can be stable near the hotter core of the mantle wedge and, hence, is likely to be subducted to deeper mantle, thereby influencing deeper cycling of volatiles and large ion lithophile elements. Also, because D Rb phl/melt ≫ D Sr phl/melt and D Nd phl/melt , D Sm phl/melt ≪ 1, long-term stability of phlogopite in the mantle can create ‘enriched mantle’ domains (eSr and eNd ≥ 0).

On the origin of mafic magmatic enclaves (MMEs) in syn-collisional granitoids: evidence from the Baojishan pluton in the North Qilian Orogen, China

Mafic magmatic enclaves (MMEs) are abundant in Baojishan syn-collisional granitoids located in the eastern section of the North Qilian Orogen. Zircon U-Pb ages of the host granodiorite (433.7 ± 3.4 Ma) and their MMEs (431.6 ± 2.8 Ma) are the same as the time of the Qilian ocean closing and continental collision at ∼440–420 Ma, indicating that the granitoids represent a magmatic response to the collision between the Qilian-Qaidam block and the Alashan block. The MMEs have the same mineralogy as the host granodiorite except that they are more abundant in mafic phases (e.g., amphibole and biotite) and thus have higher heavy rare earth element (HREE) abundances. Both the host granodiorite and the MMEs have light REE-enriched patterns and relatively flat HREE patterns (i.e., [Dy/Yb]N = 1–1.1). They are enriched in large ion lithophile elements (LILEs; e.g., Rb, K, Pb) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and show a varying Sr anomaly (i.e., Sr/Sr* = 0.9–2.2) for the host and a negative Sr anomaly (i.e., Sr/Sr* = 0.4–0.6) for the MMEs. Both the host granodiorite and the MMEs have overlapping and indistinguishable Sr-Nd-Hf isotopic compositions (87Sr/86Sr(i) = 0.7067–0.7082, εNd(t) = −3.9–−3.2, εHf(t) = 1.0–14.7). The extremely high εHf(t) = 14.7 of sample BJS12-06MME likely results from the calculation due to nugget effect of zircons because of the unexpectedly high Hf (3.53 ppm) and too high Zr (128 ppm). All these characteristics are fully consistent with the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing model. The radiogenic Sr and unradiogenic Nd (εNd(t) <0) indicate the contribution of mature continental crust, while variably radiogenic Hf (εHf(t) > 0) for both the MMEs and their host granodiorite manifest the significant mantle input. The apparent decoupling between Nd and Hf isotopes are likely caused by partial melting of recycled terrigenous sediments and the remaining part of the North Qilian ocean crust under the amphibolite facies conditions during the collision.

Melts of sediments in the mantle wedge of the Oman ophiolite

The rocks in the crustal section of the Oman ophiolite show an increasing input of a subduction component with time, most likely reflecting the generation of the ophiolite above a subducting slab. Field relations, new geochemical data, and Nd-Hf isotope data for felsic to mafic intrusive rocks in the mantle harzburgite from the Haylayn block in the Oman ophiolite suggest late magmatic events in a mantle wedge shortly before obduction of the ophiolite. Incompatible element contents and low eNd and eHf of the felsic rocks exclude differentiation from mafic magmas, but are consistent with an origin by partial melting of pelagic sediments similar to leucogranites in continental collision zones. These melts apparently mixed with mafic magmas resembling enriched late-stage lavas from the ophiolite. The leucogranitic intrusions into the mantle wedge confirm the transfer of melts of sediments from the subducted plate into the mantle at subduction zones. We suggest that the enrichment of Rb, K, and Pb observed in the Oman boninites is caused by addition of melts of sediments similar to those from the Haylayn block to the boninite source in the mantle wedge.

Melting of pelitic sediments at subarc depths: 1. Flux vs. fluid-absent melting and a parameterization of melt productivity

To evaluate the potential of subducted sediments to contribute melts for mantle metasomatism in the subarc region we have experimentally studied and reviewed the melting behavior of pelites at subarc depths. Our experimental data set completes previous studies such that all combinations of fluid-saturated or -absent melting with or without H2O and CO2 are now investigated. Experiments were conducted at 3–4.5GPa, 750–1200°C with varying proportions of H2O (0.7–4.4wt.%) and CO2 (0–4wt.%), the maximum amount of H2O stored away in phengite being ~0.9wt.% For all pelites, the onset of melting is controlled by an assemblage of phengite+jadeitic cpx+quartz/coesite±fluid. The wet solidus locates at 715 to 830°C, 3 to 4.5GPa, increasing by ~30°C with CO2. The fluid-absent pelite solidus is at 890 to 1040°C, 3 to 4.5GPa, decreasing little with CO2. To 5GPa, quartz-saturated pelites yield highly siliceous meta- to slightly peraluminous granites.To all available experiments we fitted (i) 3rd order polynomials to describe the fluid-saturated and fluid-absent solidi, and (ii) equations to parameterize melt productivity in sediments as a function of temperature, pressure, bulk H2O- and CO2-contents (valid for 2–5GPa). For H2O-contents leading to fluid-saturation at subsolidus conditions, melt productivity at the solidus increases with pressure and depends strongly on the amount of H2O available: experimental and calculated melt fractions (F) are 2–17 and 23–38wt.% at 2.5–5GPa, 2 and 15wt.% bulk H2O, respectively. As calcite remains stable above the fluid-saturated solidus, partial melting does not lead to complete decarbonation. Near the solidus, the silicate melt may only transport 0.05 to 0.6wt.% bulk CO2. Fluid-absent melting yields 15–30% melt at the solidus with phengite melting out immediately above the solidus.Warm thermal subduction models predict temperatures that just cross the wet solidus in hot subduction zones but not in intermediate to cold ones. The only realistic scenario for extracting sediment melts from subducting slabs is flux melting where fluid is added from metamorphic reactions in underlying slab lithologies. With the meta-basaltic layer being fully dehydrated when sediments reach melting temperatures, the only viable mechanism for H2O delivery is serpentine breakdown in the subducting oceanic peridotite. However, if sediments melt in the descending slab, the same slab will almost completely dehydrate. We thus conclude that sediment melting in descending slabs cannot be a general process as otherwise the Earth interior would dry out on geological time scales. Alternatives are cold sediment diapirs rising into the mantle wedge or trace element transfer by supercritical liquids at >5GPa, the latter propelled by partial peridotite dehydration.

Role of back-arc tectonics in the origin of subduction magmas: new Sr, Nd, and Pb isotope data from Middle Miocene lavas of Kunashir Island (Kurile Island Arc)

Sr, Nd, and Pb isotope data for basaltic rocks of different ages from Kunashir Island (southern Kurile island arc) provide clues to investigate the subduction magmatic history. Signatures of a high-temperature slab component (melt and/or supercritical liquid produced by melting of slab sediments) involved in Early Miocene–Pleistocene back-arc basaltic magmatism indicate a relatively hot (>800 °C) slab surface. Depleted isotope characteristics of Holocene basaltic lavas in both volcanic front and back arc indicate their origin with the participation of a cold aqueous fluid produced by dehydration of altered oceanic crust of the Pacific MORB type. The difference in geological, geochemical and isotope patterns in the Pleistocene and the Holocene lavas may be a response to stress change from extension to compression in the Kurile back-arc basin and the Kurile arc.

Melting of pelitic sediments at subarc depths: 2. Melt chemistry, viscosities and a parameterization of melt composition

The now numerous experimental studies on sediment melting at sub-arc pressures provide the data necessary to model melt compositions. These vary systematically as a function of melt fraction (F) to F=0.6. Compositional bands as a function of melt fraction are relatively narrow for all major oxides except for the alkalis where pressure and H2O-contents lead to variations by a factor of 3–5. Most sediment melts are meta- to slightly per-aluminous granites with 71–77wt.% SiO2 (on a volatile free basis), 12–16wt.% Al2O3, and 8–12wt.% total Na2O+K2O; MgO+FeO increase from 0.5 near the solidus to ~3.0wt.% at F=0.6, while CaO remains <2wt.% for carbonate-free compositions and is 3–6wt.% (at F=0.6) for the more Ca-rich bulk compositions saturated in calcite. Concerning alkalis, H2O and relatively low pressures (2.5–3GPa) favor sodic melts (K2O/Na2O wt-ratios down to 0.5) while high pressures, low volatile contents and CO2 result in more potassic melts (K2O/Na2O up to 5). The relative compositional uniformity of these high pressure partial sediment melts results from the fact that they are in equilibrium with phengite+jadeitic cpx+garnet+quartz/coesite. Notable exceptions are partial melts at 4–5GPa under CO2-rich and/or fluid absent conditions which are distinctly alkaline granites, and melts from a bulk composition that loses SiO2-saturation at the solidus which are alkaline-rich phonolites.Melt compositions from experiments with a H2O-saturated solidus can be fit by simple second degree polynomials that converge to the average bulk composition at F=1.0. For Al2O3 a dependency on H2O, and for Na2O, K2O and SiO2 dependencies on H2O and pressure are necessary to reproduce the experimental melt compositions. These equations predict melt compositions for an average bulk sediment from solidus temperatures to F=0.6. The averaging built into such a fit corresponds to the averaging taking place across a pile of heterogeneous subducted sediments.The hydrous granites forming at the wet solidus are highly mobile: The high H2O-contents (15–30wt.%) resulting from flux melting yield melt viscosities of 10−1.5–100.5Pa·s. Porous flow, a rather slow melt migration mechanism, would lead to extraction of as little as a percent of melt within centuries, rendering mass transfer almost immediate with fluid fluxing.

Early Jurassic subduction of the Paleo-Pacific Ocean in NE China: Petrologic and geochemical evidence from the Tumen mafic intrusive complex

Subduction of the Paleo-Pacific Oceanic Plate is widely considered to have caused extensive Mesozoic magmatism, lithospheric deformation and mineralization in East Asia. However, it is still unclear when this subduction began. Here we report an Early Jurassic (~187Ma) mafic intrusive complex (including olivine norite, gabbro, and diorite) from the Tumen area in NE China. The olivine norite contains a mineral assemblage of olivine, pyroxene, Ca-plagioclase, and hornblende that crystallized in a water-saturated parental magma. The rocks in the complex show variable degrees of plagioclase and ferromagnesian mineral accumulation as reflected by positive Sr and Eu anomalies in primitive mantle-normalized incompatible element patterns. Mass-balance calculations indicate that the parental magma was calc-alkaline with arc-type trace element features (i.e., large ion incompatible and light rare earth element enrichment and Nb–Ta depletion). It also had Sr–Nd–Hf isotopic compositions (87Sr/86Sr(i)=0.7042 to 0.7044, εNd(t)=+2.5 to +3.5 and εHf(t)=+8.4 to +10.5) similar to those of modern arc basalts. The parental magma was likely derived from 5 to 20% melting of a mantle wedge metasomatized by an addition of 3–4% hydrous sediment melt from the subducting Paleo-Pacific Oceanic slab. The Tumen mafic intrusive complex, together with other contemporaneous mafic intrusions, I-type granitoids, and felsic lavas, constitutes an Early Jurassic N–S-trending arc magmatic belt that was formed by westward subduction of the Paleo-Pacific Ocean.

Neoarchean arc–juvenile back-arc magmatism in eastern Dharwar Craton, India: Geochemical fingerprints from the basalts of Kadiri greenstone belt

The Neoarchean Kadiri greenstone belt (KGB) of eastern Dharwar Craton is dominated by metavolcanic rocks including basalts, andesites, dacites and rhyolites that have experienced lower amphibolite grade of metamorphism. The geochemical investigations of the basalts and basaltic andesites of KGB show 47–56wt.% SiO2 and Mg# ranging from 38 to 64. The rocks display sub-alkaline tholeiitic to transitional affinity. Based on REEs, the KGB basalts are classified as two types. Type I basalts show flat to slightly enriched LREE patterns while type II basalts have relatively enriched LREE patterns. Both types of basalts exhibit negative Nb, Zr, Hf and Ti anomalies along with distinct enrichment in Th and U. Type II basalts are characterized by a greater magnitude of negative Nb anomaly (Nb/Nb*=0.1–0.32) compared to type I (Nb/Nb*=0.41–0.82) basalts. Prominent island arc signatures are evident in both the types in terms of their LILE, LREE enrichments and relative HFSE depletion. Ba/Th vs. Nb/Th relationships reflect a frontal arc setting for type I basalts whereas type II basalts correspond to a rear arc setting. Th/Nb, La/Nb and Ce/Yb ratios show a gradual increase from type I to type II basalts reflecting melting of sediments from the subducted slab and their increased addition in type II basalts. The Zr/Hf (31–43), Zr/Sm (11–32) and Nb/Th (1–9) ratios suggest a depleted to enriched mantle source with variable influx of subduction-derived fluids and sediments. The flat to slightly enriched LREE patterns of type I basalts indicate a gradual progression from depleted MORB-type mantle melting in a fore-arc to slab-dehydration–mantle wedge metasomatism and fluid-fluxed melting in an arc regime within spinel peridotite compositional domain. Island arc signatures in type II basalts suggest their generation through melting of a subduction-modified MORB-type mantle within spinel to garnet peridotite stability field in a juvenile back-arc rift system that developed proximal to the intraoceanic arc. Variable input of subduction derived fluids and sediments in type I and type II basalts attests to across-arc geochemical variations. The subduction processes operating within arc–back arc system provided favourable pathways for the migration of gold-bearing fluids leading to economic mineralization in the northern (Hutti) and southern (Kolar) parts of this belt.

Variations in Fe3+/∑Fe of Mariana Arc Basalts and Mantle Wedge fO2

Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe^(3+)/∑Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana Trough. For single eruptions, Fe^(3+)/∑Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation, CO_2 ± H_2O degassing) or reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO_2 + H_2O + S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H_2O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe^(3+)/∑Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean ridge basalts (∼QFM, where QFM is quartz–fayalite–magnetite buffer), ranging from QFM + 1·0 to QFM + 1·6 for Mariana arc basalts, whereas back-arc related samples record primary oxygen fugacities that range from QFM + 0·1 to QFM + 0·5. This Mariana arc sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H_2O-rich component in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from the Mariana Trough through the Mariana arc, increasing by 1·5 orders of magnitude as Ba/La increases by a factor of 10 relative to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized.