Hydrous Peridotite Research Articles

Heterogeneous mush-dominated plumbing system and mantle sources of alkaline lamprophyres in Tuoyun Basin, SW Central Asian Orogenic Belt

Lamprophyres are spatially and temporally widespread in continental belts, but their magmatic differentiation process and melting dynamic mechanisms remain subjects of debate. In this study, we report mineral chemistry, bulk-rock compositions, and Sr-Nd-O isotopes of the alkaline lamprophyres (including monchiquite and camptonite) from the Tuoyun Basin of the southwestern Central Asian Orogenic Belt. Based on the P-T estimates, transcrustal magmatic reservoirs from the lower crust to upper crust are revealed for monchiquite and camptonite. These magmatic reservoirs were potentially connected, and the magmatic recharge and mixing processes that occurred were recorded by zoned clinopyroxene and amphibole. The diverse crystal populations are suggested to have originated from multiple batches of magmas, which were then involved in forming the subsequent magmas. The Sr-Nd isotopic compositions are relatively depleted, with (87Sr/86Sr)i = 0.70390−0.70452 and εNd(t) = 5.46−6.49 for the monchiquite, while the values for camptonite are 0.70391−0.70463 and 4.71−5.02. Moreover, the olivine in situ O isotopes in the monchiquite show δ18OV-SMOW values of 4.81‰−5.40‰, which are comparable to those of the mantle peridotite. Compared with the experimental melts, a hybridized mantle source of hydrous peridotite + silica-depleted pyroxenite is proposed, and partial melting of this component was induced by the upwelling asthenosphere. Ultimately, our study highlights the critical role of the magmatic plumbing system and hydrous mantle source in the formation of Tuoyun lamprophyres.

Viscosity of anhydrous and hydrous peridotite melts

The melt viscosity (η) of anhydrous and hydrous peridotite was investigated using a multipronged approach combining micropenetration viscometry, conventional DSC, flash DSC and Brillouin spectroscopy. Raman spectroscopy measurements were used to verify the absence of crystallization and/or degassing during high-temperature measurements of these extremely reactive glasses and melts, ensuring that the data corresponded to the crystal-free melt viscosity. Based on the experimental data, an accurate description of the viscosity of peridotite melts over thirteen orders of magnitude (from Tg to η ≈ 10−1 Pa s) is provided in a broad range of compositions and oxidation states. Since empirical models predict viscosities that can significantly deviate from measured data to varying degrees, a new model was developed for the temperature- and H2O-dependent viscosity of peridotite melts (up to 12 mol% H2O content).

Sea-level stability over geological time owing to limited deep subduction of hydrated mantle

Liquid surface oceans are a seemingly unique feature of Earth. Long-term, global sea level depends on the balance of water fluxes between Earth’s mantle and surface: between mantle degassing through volcanism and mantle regassing via the subduction of hydrous minerals. However, the overall balance of these fluxes at geological timescales remains uncertain. Geological observations suggest the stability of the long-term sea level and thus a near-steady-state regassing–degassing balance. In contrast, according to current thermopetrological modelling, the global input of H2O inferred from geophysical observations leads to an unequivocal excess of regassing relative to degassing. Here we use recent experimental high-pressure data on natural hydrated peridotites to update the thermopetrological models and to reassess the calculations of H2O fluxes into the mantle via subduction. Our models of 56 subduction transects show that a global input of 15−20 × 108 TgH2O every million years yields a limited global mantle regassing of 2.0−3.5 × 108 TgH2O every million years. The regassing occurs exclusively via the hydrated lithospheric mantle of the coldest subducting plates. Our requantification of the H2O budget associated with subduction matches the estimations of mantle degassing and suggests that global sea levels have been relatively stable over geological timescales. Accounting for experimental data on hydrous peridotites reduces the estimated water recycled into the deep mantle during subduction and suggests sea-level stability over geological time, according to subduction zone thermopetrological modelling.

Experimental Study of the Peridotite–Basalt–Fluid System: Phase Relations at Subcritical and Supercritical Р-Т Conditions

The paper reports experimental data on the partial melting of hydrous peridotite and basalt at pressures up to 4 GPa and temperatures up to 1400°С, as well as peridotite–basalt association in the presence of alkaline water–carbonate fluid as an experimental model of mantle reservoir with subducted oceanic protoliths. At partial melting of the hydrous peridotite at 3.7–4.0 GPa and 1000–1300°С, critical relations were observed over the entire studied pressure and temperature interval. At partial melting of hydrous basalt, critical relations between silicate melt and aqueous fluid were recorded at 1000°С and 3.7 GPa. The Na-alkaline silicate melt coexists with garnetite at 1100°С and with clinopyroxenite at 1150 and 1300°С. The peridotite–basalt–alkaline-aqueous–carbonate fluid at 4 GPa and 1400°C shows signs of critical relations between a carbonated silicate melt and a fluid. Reaction relations in the residual peridotite minerals with replacements of olivine ← orythopyroxene ← clinopyroxene ← potassic amphibole-type testify to a high chemical activity of the supercritical liquid. Experimental results suggest the existence of regions of partial melting (asthenospheric lenses) in the fluid-bearing upper mantle under supercritical conditions. These lenses contain near-solidus highly reactive supercritical liquids enriched in incompatible elements. Mantle reservoirs with supercritical liquids are geochemically similar to undepleted mantle and could serve as sources of magmas enriched in incompatible elements. The modal and cryptic upper mantle metasomatism under the effect of supercritical liquids leads to the refertilization of peridotite via enrichment of residual minerals in incompatible elements.

Reconstruction of the lithosphere-asthenosphere boundary zone beneath Ichinomegata maar, Northeast Japan, by geobarometry of spinel peridotite xenoliths

Abstract Accurate estimation of the depths of spinel peridotite xenoliths for which reliable geobarometers are not available is imperative to be able to reconstruct the precise structures of the lithosphereasthenosphere boundary (LAB). The LAB can be defined based on thermal, chemical, rheological, and petrological contrasts, and knowing its depth is crucial to understanding mantle dynamics. We attack this problem by examining spinel peridotite xenoliths from Ichinomegata maar in the back-arc side of Northeast Japan Arc. Extensive mineral compositions of nine xenolith samples revealed various patterns of chemical zoning in pyroxenes, suggesting diverse thermal histories. We examined the timescales of development of each zoning pattern and identified minerals, grain portions, and components closely approached equilibrium just before xenolith extraction as orthopyroxene and clinopyroxene, the outermost rims, and Ca-Mg-Fe components, respectively. Applying the best pair of geothermobarometers to the chosen analyses, plausible derivation depths of eight samples were obtained. They range from 0.72–1.6 GPa in pressure and from 830–1080 °C in temperature, which defines a high thermal gradient of 10 K/km or 290 K/GPa. There is an intimate correlation between the zoning patterns of pyroxenes and the depth estimates: pyroxenes in the deeper samples have zoning indicating cooling followed by heating just before xenolith extraction, and those of the shallower samples have zoning indicating monotonic cooling. Depth variations of rock microstructures, grain size of olivine, chemical compositions of minerals, and phase assemblage, including the presence or absence of glass or fluid phase, show that the mantle beneath Ichinomegata consists of two distinct layers. The shallower (28–32 km) layer is granular, less oxidized, amphibole- and plagioclase-bearing, and subsolidus, whereas the deeper (41–55 km) layer is porphyroclastic, amphibole- and plagioclase-free, oxidized, and partially molten. The contrasts between the two layers suggest that the upper layer represents a lithospheric mantle and the lower layer a LAB zone. These layers are similar to those reported from the bottom of subcontinental lithospheric mantle in various aspects, but the LAB beneath Ichinomegata is much shallower (40–60 km) and cooler (~1100 °C). The coincidence of (1) the depth of a rheological transition, marked granular to porphyroclastic textures, and (2) the depth of a phase transition, from subsolidus hydrous peridotite to a hydrous mantle with melt in localized pockets, is the remarkable feature of the LAB beneath Ichinomegata. This suggests that a rheological boundary zone in arc settings is governed by melting of the hydrous mantle and that the underlying asthenosphere is partially molten. The depth-dependent thermal history shown by chemical zoning in pyroxenes and the presence of melt as pockets suggest that the LAB beneath Ichinomegata was in a transient state that was affected by thermal and material transport.

Electrical conductivity of hydrous silicate melts: Implications for the bottom-up hydration of Earth's upper mantle

The upwelling of the hydrous mantle transition zone triggers dehydration-induced partial melting atop the 410-km discontinuity. Here we investigate the electrical conductivity of hydrous silicate melts in the 200-400 km depth range and explore whether melting at the 410-km depths is responsible for the hydration of the upper mantle. Our experimental electrical conductivity data demonstrate that the mantle at 180-350 km depths is mostly melt free, confirming the H2O under-saturated conditions. However, the residual mantle from partial melting atop the 410-km discontinuity may contain various possible amounts of water according to the initial mantle transition zone and melt concentrations. This residual H2O could contribute to the hydration of the upper mantle either through diffusion or material replacement by upwelling. Our calculations suggest that the diffusion may not be responsible for the hydration of the upper mantle to present H2O concentration of 50–200 ppm wt. Melting of the upwelling mantle transition zone with less than 1500 ppm wt. H2O produces residual peridotites with ∼ 200 ppm H2O at the 410-km discontinuity. Continuous upwelling of such hydrous residues would gradually replace the dry upper mantle with depleted residual hydrous peridotites in less than 260 Ma. In this study, we propose a bottom-up hydration mechanism for the Earth's upper mantle driven by dehydration-melting at the 410-km discontinuity. The hydrous partial melting at the top of the asthenosphere appears to be a consequence of H2O saturation in the upwelling residual peridotites.

Identifying volatile mantle trend with the water–fluorine–cerium systematics of basaltic glass

The lithophile elements and isotopic compositions of oceanic basalts suggest they derive from different mantle components. We present new analytical results of un-degassed deep-marine basaltic glasses from various regions, and we find strong linear correlations of R2 ≥ 0.997 between H2O and F as well as H2O and Ce, for which we propose mantle trends. The mantle trends represent global variations of mantle components, ranging from a depleted dry peridotite of mid-ocean ridge basalt (MORB) (DMM, depleted MORB source mantle: H2O = 100 ppm; H2O/Ce = 200; H2O/F = 10) to a hydrous peridotite of Hawaiian ocean island basalt (OIB) of deep mantle origin (FOZO, focal zone: H2O = 750 ppm; H2O/Ce = 200; H2O/F = 18.5). Accordingly, we defined the correlation as a volatile DMM–FOZO trend. Based on our findings, we report novel H2O–F–Ce systematics to discriminate the degree of water depletion in the source mantle and the rehydration of the mantle with recycled surface water through oceanic plate subduction. Using this method, most OIBs are distinguished clearly from the DMM–FOZO trend, and we find that the water in their sources originates from recycled water derived from the hydrated oceanic crust and sediment after various degrees of dehydration (75–95%) in subduction zones.

Physical mechanisms of oceanic mantle earthquakes: Comparison of natural and experimental events

Because they provide information about the spatial distribution of brittle deformation, both seismologists and experimentalists use b-values to study earthquake populations. Here, we present the b-values for intermediate-depth intraslab earthquakes in the Pacific slab beneath the Tohoku and Hokkaido regions, northeastern Japan and find a difference in the lower-plane event b-values in the double seismic zone. Lower-plane events reveal significantly larger b-values beneath Tohoku (0.96) than Hokkaido (0.86), implying that the brittle deformation beneath Hokkaido is more localized and leads to higher ratio of relatively large lower-plane events than occur beneath Tohoku. We also estimated the b-values for experimental earthquakes, and found they increase with increasing antigorite content in serpentinized peridotite. These experimental earthquakes already led to the “dehydration driven stress transfer” (DDST) model, which suggests that a highly hydrated peridotite is not required when oceanic mantle events occur. A comparison of experimental and natural earthquake b-values implies that lower-plane peridotite is more hydrated beneath the Tohoku region, which could also explain the difference in oceanic-plate velocity structures near the trench identified in Ocean Bottom Seismometer studies off Tohoku and Hokkaido. These results suggest that lower-plane events occur in fresh peridotite near serpentinized faults.

The stability of anhydrous phase B, Mg14Si5O24, at mantle transition zone conditions

The stability of anhydrous phase B, Mg14Si5O24, has been determined in the pressure range of 14–21 GPa and the temperature range of 1100–1700 °C with both normal and reversal experiments using multi-anvil apparatus. Our results demonstrate that anhydrous phase B is stable at pressure–temperature conditions corresponding to the shallow depth region of the mantle’s transition zone and it decomposes into periclase and wadsleyite at greater depths. The decomposition boundary of anhydrous phase B into wadsleyite and periclase has a positive phase transition slope and can be expressed by the following equation: P(GPa) = 7.5 + 6.6 × 10−3 T (°C). This result is consistent with a recent result on the decomposition boundary of anhydrous phase B (Kojitani et al., Am Miner 102:2032–2044, 2017). However, our phase boundary deviates significantly from this previous study at temperatures 250 km, which could contribute ferropericlase (Mg, Fe)O or magnesiowustite (Fe, Mg)O into the deep mantle. Incongruent melting of hydrous peridotite may also produce MgO-rich compounds. Anh-B could form in these conditions due to reactions between Mg-rich oxides and silicates. Anh-B might provide a new interpretation for the origin of diamonds containing ferropericlase–olivine inclusions and chromitites which have been found to have ultrahigh-pressure characteristics. We propose that directly touching ferropericlase–olivine inclusions found in natural diamonds might be the retrogressive products of anhydrous phase B decomposing via the reaction (Mg,Fe)14Si5O24 (Anh-B) = (Mg,Fe)2SiO4 (olivine) + (Mg,Fe)O (periclase). This decomposition may occur during the transportation of the host diamonds from their formation depths of < 500 km in the upper part of the mantle transition zone to the surface.

Experimental evidence supporting a global melt layer at the base of the Earth\u2019s upper mantle

The low-velocity layer (LVL) atop the 410-km discontinuity has been widely attributed to dehydration melting. In this study, we experimentally reproduced the wadsleyite-to-olivine phase transformation in the upwelling mantle across the 410-km discontinuity and investigated in situ the sound wave velocity during partial melting of hydrous peridotite. Our seismic velocity model indicates that the globally observed negative Vs anomaly (−4%) can be explained by a 0.7% melt fraction in peridotite at the base of the upper mantle. The produced melt is richer in FeO (~33 wt.%) and H2O (~16.5 wt.%) and its density is determined to be 3.56–3.74 g cm−3. The water content of this gravitationally stable melt in the LVL corresponds to a total water content in the mantle transition zone of 0.22 ± 0.02 wt.%. Such values agree with estimations based on magneto-telluric observations.

Phase relations and formation of K-bearing Al-10 Å phase in the MORB+H2O system: Implications for H2O- and K-cycles in subduction zones

The potassium (K) and water (H 2 O) cycles in subduction zones are predominately controlled by the stability of K- and H 2 O-bearing minerals, such as K-mica, lawsonite, and dense hydrous magnesium silicates (DHMS). K-micas (muscovite or phlogopite) are the principal H 2 O and K hosts in subduction zones and Earth’s upper mantle and play a significant role in the deep H 2 O and K cycles. The Mg-10 A phase, normally appearing in hydrated peridotite in high-pressure experiments, has been considered as an important water-carrier in subducted hydrated peridotite. In this study, we found a K-bearing Al-10 A phase in the MORB+H 2 O system (hydrated basalt) at high pressures according to X-ray diffraction and stoichiometry. We experimentally constrained its stability field at high pressure. By considering newly and previously documented compositions of the 10 A phase and micas, we confirmed a continuous solid solution or mixed layering between the 10 A phase and K-mica at the interlayer site, suggesting that the K cycle and the H 2 O cycle in subduction zones are coupled. From the discussion of the effect of f H 2 O on stability of the Al-10 A phase, we conclude that a cold subduction zone can host and carry more bulk H 2 O and K into Earth’s deep mantle than a hot one. This work expands the stability regions of the 10 A phase from the ultramafic system (Mg-10 A phase) to the mafic system (Al-10 A phase), and emphasizes the significance of the 10 A phase for the deep H 2 O and K cycle in subduction zone.

The effect of CO2 on the water-saturated solidus of K-poor peridotite between 4 and 6 GPa

The effect of various amounts of CO2 on the solidus of H2O–CO2-bearing peridotite was examined by determining the composition of H2O–CO2-bearing fluids and melts in equilibrium with garnet peridotite at 4–6GPa and 900–1100°C. Two capsules were placed in a rocking multi-anvil apparatus in each experiment. Both capsules contained a fertile peridotite with 10wt% H2O, one with 1 (CLZ1) and the other with 5wt% CO2 (CLZ5). In both capsules a diamond trap was placed on one end of the capsule as a fluid/melt trap. The H2O and CO2 content in the fluid or melt trapped in between the diamonds were measured using the quartz-tube-system technique by releasing the volatiles through infrared gas analyzer. The total dissolved solids in these phases were determined using the cryogenic laser-ablation – inductive couple plasma – mass spectrometry technique. The residual lherzolite consists of olivine, orthopyroxene, ±clinopyroxene, garnet, and ±magnesite as carbonate phase. The solidus of CLZ1 peridotite was located between 900 and 1000°C at 4GPa and between 1000 and 1100°C at 5 and 6GPa. CLZ5 peridotite melts below 900°C at 4GPa and between 900 and 1000°C at 5 and 6GPa. The results demonstrate a decrease in melting temperature of hydrous peridotite at pressures between 4 and 6GPa with increasing amount of CO2.The H2O–CO2-bearing fluids found in this study at 900–1000°C are similar in composition to low-Mg carbonatitic to silicic high-density fluids found in fluid inclusions in diamonds. With increasing temperature, the melts approach type II kimberlites. We propose that H2O–CO2-induced partial melting of metasomatized garnet lherzolite at 4–6GPa is a possible origin for group II kimberlites.

REEBOXPRO: A forward model simulating melting of thermally and lithologically variable upwelling mantle

AbstractThe compositions and volumes of basalts erupted in divergent margin environments provide a record of the thermal, chemical, and dynamical state of their mantle source regions. To relate basalt compositions and volumes to the underlying thermochemical and dynamical state of their mantle source regions, we have developed REEBOX PRO, a compiled stand‐alone application that simulates adiabatic decompression melting of passively or actively upwelling mantle containing up to five distinct lithologies. The model calculates melt compositions using thermodynamic and experimental constraints on the melting behaviors and mineral‐melt partitioning behavior of homogeneous and lithologically heterogeneous mantle sources containing anhydrous peridotite, hydrous peridotite, harzburgite, and/or silica‐saturated/‐undersaturated pyroxenite. Key model outputs include the mean composition and crustal thickness for the bulk basaltic crust, calculated for passive and active upwelling scenarios. Here, we present the mathematical formulations underlying the model and benchmark it against existing hydrous melting models and models for mid‐ocean ridge basalt formation. We show that the hydrous and anhydrous peridotite melting models incorporated in REEBOX PRO capture the essential differences in basalt composition and volume demonstrated by previous models, and constrain the ambient mantle beneath the global spreading ridge system to be between 1319 and 1366°C, depending on the relative fertility and/or water content of the mid‐ocean ridge mantle source. We also show how model outputs may be manipulated outside of the modeling program to calculate nontraditional melt mixing scenarios. These examples highlight the flexibility of REEBOX PRO for simulating melt generation within a range of geodynamical contexts.

U-series disequilibria in subduction zone lavas: Inherited from subducted slabs or produced by mantle in-growth melting?

U-series disequilibria of subduction zone lavas have been used to provide temporal constraints on flux transfer in convergent margins. There are debates on whether the disequilibria are inherited from the source modified by slab materials (fluids or sediments) or produced by in-growth melting of the mantle. After filtering out the effect of crustal modification on U-series data, (226Ra)/(230Th) is not positively correlated with Sr/Th and Ba/Th in arc basalts with SiO2 <52wt%. This invalidates the key evidence for addition of slab-fluids to the mantle producing 226Ra excess in subduction zone lavas. Sediment melts with a zircon-free residue may have 231Pa excess, but it is not consistent with the overwhelming Zr-Hf depletions in subduction zone lavas, which instead require a zircon-saturated residue during sediment melting.With appropriate parameters applied, in-growth melting in the mantle followed by modification in the crust can provide a self-consistent explanation of the U-series disequilibria and the relationships with other geochemical signatures in most subduction zone lavas except the few Tonga-Kermadec lavas with slight 231Pa deficit. On the basis of melting experiments of hydrous peridotites, we further propose that most mantle-derived melts and thus U-series disequilibria should be mainly produced by melting in the hot zone of the mantle wedge, as controlled by thermal structure of the convergent margins. In order to produce enough short-lived nuclides, the timescales of mass transfer and magmatism in the subduction zone should at least range from a few to a few hundred millennia. Future studies on more lavas, numerical modelling, and partition coefficients of U-series nuclides will help for better application of U-series disequilibria into subduction zone magmatism.

Dislocation microstructures developed under a cooperative deformation of hydrous peridotite

Dislocation microstructures developed under a cooperative deformation of hydrous peridotite

Evolution processes of Ordovician–Devonian arc system in the South‐Kitakami Massif and its relevance to the Ordovician ophiolite pulse

AbstractThe South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.

Elasticity of superhydrous phase B, seismic anomalies in cold slabs and implications for deep water transport

Seismic anomalies in the vicinity of cold slabs, including low velocity zones and enhanced seismic shear wave splitting have been commonly attributed to the presence of hydrated slab material. The dense hydrous magnesium silicate (DHMS) superhydrous phase B (ShyB, Mg10Si3H4O18) is considered an important water carrier to transition zone depth due to its large pressure stability and abundance (up to 20vol.%) in hydrous peridotites. To interpret the observed seismic anomalies in terms of hydration and to assess the role of ShyB in deep water recycling, we have investigated the sound velocities and single-crystal elasticity of ShyB to pressures of 17.5GPa at ambient temperature by Brillouin scattering spectroscopy in diamond anvil cells. The Voigt–Reuss–Hill averages for the adiabatic bulk moduli KS, shear moduli μ and their pressure derivatives yield KS=150(2)GPa, μ=99(1)GPa, (∂KS/∂P)=4.7(2) and (∂μ/∂P)=1.44(5). The aggregate compressional and shear wave velocities of ShyB at transition zone pressures are comparable to those of hydrous iron-bearing ringwoodite but are significantly lower than anhydrous phases in peridotites. The calculated velocity contrast between dry and ShyB-bearing hydrous peridotite (containing 1.2wt.% of water) indicate that 17vol.% of ShyB decreases both compressional VP and shear VS wave velocities by at most 2.5% at transition zone depths. Our results, combined with data for the deformation mechanisms of ShyB, indicate that ShyB aggregates develop strong textures under down-dip compression regime and yield a maximum shear wave splitting of 1.1% in the plane perpendicular to the compression axis at 17.5GPa. Although ShyB in hydrous subducted peridotite would reduce its seismic velocities, the splitting geometry VSV>VSH generated by ShyB fabrics in peridotite is incompatible with the pattern observed in Tonga and Sangihe, and would require the contribution of other phases to be explained. Textured phase D is a plausible candidate to explain the shear wave splitting in the lower transition zone, while at intermediate transition zone depths, the coexistence of textured phase D and ShyB in hydrous deformed peridotite would generate the observed array polarization. However, the later scenario will require locally very high water concentrations at depth.

Source and magma mixing processes in continental subduction factory: Geochemical evidence from postcollisional mafic igneous rocks in the Dabie orogen

Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust-mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc-like trace element distribution patterns and enriched Sr-Nd-Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole-rock εNd(t) values of −17.6 to −5.2 and zircon εHf(t) values of −29.0 to −7.7, pointing to source heterogeneities. Such whole-rock geochemical features are interpreted by the source mixing through melt-peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx-1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx-1 mantle and Cpx-2 exhibit significantly high Cr, Ni and Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite-derived mafic melt (Melt 2). Whole-rock initial 87Sr/86Sr ratios of gabbro lies between those of Pl-1core (crystallized from Melt 1) and Pl-1 mantle and Pl-2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were derived from partial melting of the heterogeneously metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes in the continental subduction factory are responsible for the significant variations in the whole-rock and mineral geochemistries of postcollisional mafic igneous rocks.

Repeated magmatism at 34Ma and 23-20Ma producing high magnesian adakitic andesites and transitional basalts on southern Okushiri Island, NE Japan arc

Abstract The southern part of Okushiri Island in the present-day back-arc margin of the NE Japan arc is one of the rare convergent plate boundaries where similar magma types (high-magnesian adakitic andesite (HMAA) and high-TiO 2 basalt (HTB)) have been erupted concurrently at more than one time. Oligocene HMAA can be divided into two types: HMAA-I is characterized by high Sr/Y and low Y, and HMAA-II by relatively low Sr/Y and high Y. HMAA-I is primitive in terms of MgO (8.5 wt.%), Mg# (67), Ni (232 ppm) and Cr (613 ppm) contents, and the most Mg-rich olivine phenocrysts plot within the mantle olivine array in terms of Fo and NiO. The similar Cr versus Ni relations of types I and II HMAA indicate some interaction of slab-derived adakitic melts with mantle peridotite, whereas Ni contents are higher than those of most boninites derived by partial melting of mantle peridotite at a given Cr content. Types I and II HMAA have more enriched Sr and Nd isotopic compositions than N-MORB. The petrography and geochemistry of these rocks, combined with published results on the genesis of high-magnesian andesite (HMA) indicate that types I and II HMAA could be produced by interaction of slab (N-MORB and sediment)-derived adakitic melts with mantle peridotite. The comagmatism of HMAA and HTB is ascribed to the following model. A cool, less hydrous, adakite magma (spherical diapir) would rise from the subducting slab (Pacific Plate) and become more hydrous as a result of its interaction with overlying hydrous peridotite. This hydrated adakitic diapir further ascends and is heated on entering the overlying mantle wedge. Subsequently, the temperature and H 2 O gradients in the ascending adakitic diapir and surrounding mantle peridotite would have been established. The HTB magma segregated from the surrounding mantle peridotite region (high temperature and low H 2 O content) at a depth of 60 km or more, whereas the adakitic diapir (low temperature and high H 2 O content) continued to rise, with its chemical composition modified due to interaction with the surrounding mantle peridotite. Type I HMAA then segregated at about 50 km. The most attractive tectono-magmatic model to account for production of adakitic magma at two different periods in the same cool subduction zone region involves upwelling of depleted hot asthenosphere into the subcontinental lithosphere beneath the back-arc margin of the NE Japan arc, coincident with back-arc rifting which took place at the initiation of the Japan Sea opening. The unusually high temperature conditions established in the mantle wedge due to upwelling of depleted hot asthenosphere caused partial melting of a limited part of the cool oceanic crust subducting beneath the NE Japan arc, resulting in the generation of adakitic magma.

Trace element mass balance in hydrous adiabatic mantle melting: The Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1)

AbstractA numerical mass balance calculation model for the adiabatic melting of a dry to hydrous peridotite has been programmed in order to simulate the trace element compositions of basalts from mid‐ocean ridges, back‐arc basins, ocean islands, and large igneous provinces. The Excel spreadsheet‐based calculator, Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) uses (1) a thermodynamic model of fractional adiabatic melting of mantle peridotite, with (2) the parameterized experimental melting relationships of primitive to depleted mantle sources in terms of pressure, temperature, water content, and degree of partial melting. The trace element composition of the model basalt is calculated from the accumulated incremental melts within the adiabatic melting regime, with consideration for source depletion. The mineralogic mode in the primitive to depleted source mantle in adiabat is calculated using parameterized experimental results. Partition coefficients of the trace elements of mantle minerals are parameterized to melt temperature mostly from a lattice strain model and are tested using the latest compilations of experimental results. The parameters that control the composition of trace elements in the model are as follows: (1) mantle potential temperature, (2) water content in the source mantle, (3) depth of termination of adiabatic melting, and (4) source mantle depletion. HAMMS1 enables us to obtain the above controlling parameters using Monte Carlo fitting calculations and by comparing the calculated basalt compositions to primary basalt compositions. Additionally, HAMMS1 compares melting parameters with a major element model, which uses petrogenetic grids formulated from experimental results, thus providing better constraints on the source conditions.