Hydrous Melts Research Articles

Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature

Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

H2O-fluxed melting of eclogite during exhumation: an example from the eclogite type-locality, Eastern Alps (Austria)

Epidote eclogites embedded in paragneisses from the Austrian Eastern Alps host rare, decimetre- to metre-sized planar pegmatitic segregations (‘pegmatoids’) that consist of a hornblende–plagioclase–epidote–titanite–quartz assemblage. The pegmatoids cut the primary eclogite foliation at a high angle, show abrupt terminations and exhibit numerous microtextures indicating the former presence of melt, whereas the surrounding eclogites and paragneisses lack evidence for dehydration melting. We interpret the pegmatoids as crystallised hydrous melts of intermediate composition derived from a nearby eclogitic source, similar to the eclogite host rock. The host eclogite preserves high-pressure assemblages of garnet–omphacite–epidote–quartz–rutile ± phengite that yield peak P–T estimates of 21 ± 2 kbar and 700 ± 20 ∘C calculated via pseudosection modelling and Zr-in-rutile thermometry. Subsequent near-isothermal decompression occurred in a closed system under fluid-absent conditions that favoured the preservation of garnet–omphacite assemblages in eclogites. Thermodynamic modelling of the metapelitic country rock indicates that the paragneisses remained fluid-absent during early exhumation, but became fluid-saturated at mid-crustal conditions of 7.5–9 kbar and 680–690 ∘C. We suggest that, as the metapelites dehydrated, minor aqueous fluid was released and infiltrated the enclosed eclogites along discordant fractures that formed in response to unloading, further facilitated by steep gradients in μH2O between the fluid-saturated pelitic country rock and fluid-absent eclogite lenses. Eclogite distal from fluid sources/pathways experienced limited retrogression, whereas localised re-equilibration and H2O-fluxed melting is inferred for eclogite affected by extensive fluid infiltration. Zr-in-titanite thermometry on crystals from pegmatoids yields a crystallisation temperature of 697 ± 10 ∘C at 8.5 ± 1.5 kbar, similar to retrograde conditions recorded by the regionally dominant metapelites. Oxygen- and hydrogen isotopic data for eclogite, pegmatoids and paragneiss are consistent with hydrous fluid transfer from paragneisses to eclogites and a small fluid/rock ratio. We suggest that, at conditions that preclude dehydration-melting, even small-scale melting of eclogite requires addition of fluid from a suitable near-field source.

Multi-stage garnet formation and destruction in Kimberley harzburgitic xenoliths, South Africa

Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major–trace element geochemistry and garnet Sr–Nd–Hf isotope data are presented. Equilibration pressures range from 3.8–6.1 GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115–185 km). High olivine Mg# (~93.4, n = 39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ≥90%, orthopyroxene ≤10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5–3.2 wt%) and SiO2 (43.1–49.1 wt%), however, indicate heterogeneous re-introduction of garnet (≤13%) and orthopyroxene (≤50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7–13% and ~ 9–30%; n = 6) are characterised by mildly sinusoidal garnet REE patterns (Tb-Dy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7–3.3 Ga; εHfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5–7.5% and ~ 25–50%; n = 19) are characterised by highly sinuous REE patterns (Ho-Yb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7–1.6 Ga; εHfe: −16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7 Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lu-Hf isochron age of 2702 ± 64 Ma (εHfi = +44, n = 31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nb-Ta anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Sr-Nd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217 ± 58 Ma, εNdi = +4; n = 34), consistent with recent G10 garnet transformation into G9 garnets (Ca + Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.

Recycling of crustal materials and implications for lithospheric thinning: Evidence from Mesozoic volcanic rocks in the Hailar–Tamtsag Basin, NE China

Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar–Tamtsag Basin, northeast China, provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning. The Late Jurassic Nb-rich basaltic andesites (154 ± 4 Ma) are enriched in large-ion lithophile and light rare earth elements, slightly depleted in high-field-strength elements, and have high TiO2, P2O5, and Nb contents, and (Nb/Th)PM and Nb/U ratios, which together with the relatively depleted Sr–Nd–Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust. The Early Cretaceous high-Mg adakitic volcanic rocks (129–117 Ma) are characterized by low Y and heavy rare earth element contents, and high Sr contents and Sr/Y ratios, similar to those of rocks derived from partial melting of an eclogitic source. They also have high Rb/Sr, K2O/Na2O, and Mg# values, and high MgO, Cr, and Ni contents. These geochemical features suggest that the adakitic lavas were derived from partial melting of delaminated lower continental crust, followed by interaction of the resulting melts with mantle material during their ascent. Our data, along with available geological, paleomagnetic, and geophysical evidence, lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China, with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.

Formation of Amphibole‐Bearing Peridotite and Amphibole‐Bearing Pyroxenite Through Hydrous Melt‐Peridotite Reaction and In Situ Crystallization: An Experimental Study

AbstractAmphibole is a common hydrous mineral in mantle rocks. To better understand processes leading to the formation of amphibole‐bearing peridotites and pyroxenites in the lithospheric mantle, we conducted experiments by juxtaposing a lherzolite against hydrous basaltic melts in Au‐Pd capsules. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C produced a melt‐bearing orthopyroxenite‐dunite sequence. Crystallization of the partially reacted melts during cooling lead to the formation of an amphibole‐bearing gabbronorite‐orthopyroxenite‐peridotite sequence. Orthopyroxene in the peridotite and orthopyroxenite has a poikilitic texture enclosing olivines and spinels. Amphibole in the peridotite occurs interstitial to olivine, orthopyroxene, clinopyroxene, and spinel. Comparisons of texture and mineral compositions in the experimental products with those from field observations allow a better understanding of hydrous melt‐rock reaction in the lithospheric mantle. Amphibole‐bearing pyroxenite veins (or dikes) can be formed in the lithospheric mantle or at the crust‐mantle boundary by interaction between hydrous melt and peridotite and subsequent crystallization. Hornblendite or amphibole gabbronorite can be formed in the veins when the flux of hydrous melt is high. Differences in reacting melt and peridotite compositions are responsible for the variation in amphibole composition in mantle xenoliths from different tectonic settings. The extent of melt‐rock reaction is a factor that control amphibole composition across the amphibole‐bearing vein and the host peridotite.

Halogen Bearing Amphiboles, Aqueous Fluids, and Melts in Subduction Zones: Insights on Halogen Cycle From Electrical Conductivity

AbstractAmphiboles are hydrous minerals that are formed in the oceanic crust via hydrothermal alteration. The partial substitution of halogens for OH− makes amphibole one of the principal hosts of Cl and F in the subducting slab. In this study, we investigated the electrical conductivity of a suite of halogen bearing amphibole minerals at 1.5 GPa up to 1,400 K. The discontinuous electrical behavior indicates dehydration of amphibole at ∼915 K. This is followed by dehydration induced hydrous melting at temperatures above 1,070 K. We find that the released aqueous fluids have an electrical conductivity of ∼0.1 S/m. This high electrical conductivity is likely to explain anomalously high electrical conductivity observed in certain subduction zone settings. This high electrical conductivity of an order of magnitude greater than the electrical conductivity of pure aqueous fluids at similar conditions is likely due to the partitioning of the F and Cl into the aqueous fluids. We also noted that subsequent to the dehydration, secondary phases form due to the breakdown of the primary halogen bearing amphibole. Chemical analyses of these secondary phases indicate that they are repositories of F and Cl. Hence, we infer that upon dehydration of the primary halogen bearing amphibole, first the F and Cl are partitioned into the aqueous fluids and then the halogens are partitioned back to the secondary mineral phases. These secondary minerals are likely to transport the halogen to the deep Earth and may in part explain the halogen concentration observed in ocean island basalt.

Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Abstract Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt–like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc.

Remarkably fresh abyssal peridotites from Sibuyan island, Romblon Island Group, Philippines: Markers of young arc-continent collision

The bulk of Sibuyan island in the Romblon Island Group, Philippines is underlain by remarkably fresh spinel lherzolites, harzburgites and dunites collectively called as the Sibuyan Ultramafics. Based on petrographic and geochemical characteristics, the Sibuyan Ultramafics can be classified into: (1) Group I peridotites composed of spinel lherzolites and harzburgites with spinel Cr# < 0.35, (2) Group II harzburgites and dunites with intermediate spinel Cr# = 0.40–0.65, and (3) Group III dunites with very high spinel Cr# (>0.75). Increase in spinel Cr# is accompanied by decreasing Al2O3 contents and increasing Mg# in the pyroxenes from the Group I to Group II and Group III peridotites. Chondrite-normalized clinopyroxene data revealed various trace-element patterns for the three groups suggestive of different processes (e.g. anhydrous and hydrous melting, mantle-melt interaction) involved in their formation. The Sibuyan Ultramafics is closely associated with the similarly fresh Calaton Hill metamorphic/plutonic complex in Tablas island which is thought to be representative of the lower crust of the Philippine island arc. At a more regional context, the Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex can be used as markers for the young arc-continent collision in western Philippines. Rapid uplift and emplacement due to the collision of the Philippine Mobile Belt and the Palawan Microcontinental Block during the Miocene or even later may have led to the remarkably fresh state of these lithologies. This occurrence is similar to the Horoman peridotite complex in the Hidaka metamorphic belt in Northern Japan. The Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex therefore provide a rare and fresh glimpse into the upper mantle and lower crust in the Philippines due to their young ages of emplacement.

Fluid-present and fluid-absent melting of muscovite in migmatites in the Himalayan orogen: Constraints from major and trace element zoning and phase equilibrium relationships

Partial melting of the continental crust may proceed through either influx of free water (hydration, fluid-present melting) or breakdown of hydrous minerals (dehydration, fluid-absent melting). The two types of physicochemical mechanism would be operated for crustal anatexis in nature, and their effects on the composition of anatectic products were closely examined by experimental approaches as well as the study of natural leucogranites from the Himalayan orogen. However, the leucogranites usually experienced several stages of evolution during magmatism, leading to difficulties in retrieving the composition of original melts. This issue is now addressed by studying Himalayan migmatitic metapelites because they generally share similar source rocks and P–T conditions with the leucogranites. In combined with field observations, an integrated study of whole-rock geochemistry, mineral chemistry, zircon geochronology, and phase equilibrium modelling was conducted for migmatitic metapelites from the Nyalam Valley in the Greater Himalayan Sequence. The results show that the migmatitic metapelites share similar clockwise P–T paths. They experienced an early prograde metamorphism through compressional heating to reach peak pressures of 7.3–9.0 kbar at ~700 °C before ca. 31 Ma. Afterwards, the migmatitic metapelites underwent decompressional heating to ~725 °C at 6.0 kbar for upper-amphibolite facies metamorphism at 26–21 Ma. Subsequently, they experienced decompressional cooling to a temperature below the solidus at ca. 18 Ma. The crustal anatexis was dominated by muscovite melting reactions during the decompressional heating from ca. 26.4 to 18 Ma. Despite the similar P–T paths, the migmatitic metapelites experienced different types of melting reactions involving muscovite (i.e., dehydration and hydration). Whereas the hydration melting resulted in a marked increase in An contents (~65 to 75 mol%) and the absence of K-feldspar, the dehydration melting led to relatively low-An (<30 mol%) plagioclase and abundant K-feldspar. The two types of muscovite melting reaction may be common in the Himalayan orogen in the Late Cenozoic. While the fluid-present melting would take place at a shallower depth, the fluid-absent melting would occur at a greater depth. This coupled dehydration-hydration relationship has important implications for the geodynamic mechanism of crustal anatexis in the continental collision zone.

Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.

The Anomalous Seismic Behavior of Aqueous Fluids Released during Dehydration of Chlorite in Subduction Zones

Dehydration and fluid circulation are integral parts of subduction tectonics that govern the dynamics of the wedge mantle. The knowledge of the elastic behavior of aqueous fluid is crucial to understand the fluid–rock interactions in the mantle through velocity profiles. In this study, we investigated the elastic wave velocities of chlorite at high pressure beyond its dehydrating temperature, simulating the progressive dehydration of hydrous minerals in subduction zones. The dehydration resulted in an 8% increase in compressional (Vp) and a 5% decrease in shear wave (Vs) velocities at 950 K. The increase in Vp can be attributed to the stiffening of the sample due to the formation of secondary mineral phases followed by the dehydration of chlorite. The fluid-bearing samples exhibited Vp/Vs of 2.45 at 950 K. These seismic parameters are notably different from the major mantle minerals or hydrous silicate melts and provide unique seismic criteria for detecting mantle fluids through seismic tomography.

Water Content of the Dehydration Melting Layer in the Topmost Lower Mantle

AbstractThe water‐rich mantle transition zone and dry lower mantle suggest that a dehydration melting layer can form at the 660‐km depth boundary. However, the water content of the melting layer (), which dominates its gravitational stability and melt fraction, remains poorly constrained. Here, the of hydrous silicate melt by mass balance calculations is investigated and found that significantly decreases with increasing temperature, but is relatively insensitive to chemical composition (FeO and SiO2 contents) and coexisting phases. Melt at 660‐km depth should contain ∼50 wt.% water at 1600 K (slab geotherm) or ∼20 wt.% water at 2000 K (topmost lower mantle geotherm). The density of the hydrous melt is &lt;3.9 g/cm3, which makes it buoyant. With a melt fraction of ≳0.5 vol.%, the melting layer is expected to significantly reduce the viscosity and seismic velocity near slabs, which may cause slab stagnation and prohibit whole mantle convection.

Geochemistry and Geochronology of the Neoproterozoic Backarc Basin Khzama Ophiolite (Anti-Atlas Mountains, Morocco): Tectonomagmatic Implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

The Role of Lithosphere Thickness in the Formation of Ocean Islands and Seamounts: Contrasts between the Louisville and Emperor–Hawaiian Hotspot Trails

AbstractThe Hawaii–Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii–Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace-element data from five of the older (74–50 Ma) Louisville seamounts drilled during Integrated Ocean Drilling Program (IODP) Expedition 330 and compare these with published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1·5–3 % partial melting of a dominantly garnet-lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10 %) and spinel- to garnet-lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500 °C under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, whereas post-shield alkali basalt requires a lower temperature (1300–1400 °C). This is consistent with the derivation of Hawaii–Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp = 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp ≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands are the exception. Alternatively, many ocean islands may not be the product of mantle plumes but may instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.

Structure of oceanic crust in back-arc basins modulated by mantle source heterogeneity

Abstract Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4–5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5–6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ∼7 km thick, and the lower crust supports seismic velocities of 7.2–7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge–type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust.

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally. It has been exploited for nearly 100 years, yet its origin remains unresolved. In the present study, we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy. Our results indicate that the Merensky Reef formed through a range of diverse processes. Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth. The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow. Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation. The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts. Olivine-hosted melt inclusions containing Cl-rich apatite, sodic plagioclase, and phlogopite suggest the presence of highly evolved, volatile-rich melts. Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase, possibly triggered by flux of heat, acidic fluids, or hydrous melt. Together, these data suggest that the reef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes.

The formation of shoshonitic magma and its relationship to porphyry-type mineralisation: the Maronia pluton in NE Greece

The Oligocene shoshonitic Maronia pluton in NE Greece is part of an age-progressive belt of calc-alkaline intrusions and associated with a porphyry Cu-Mo±Re±Au mineralisation. We present new data on mineral and whole rock compositions as well as Sr, Nd and Pb isotopes of the monzodioritic to granitic rocks from Maronia, including the mineralised microgranite, which crosscuts the main intrusion. The age progression of the plutons in NE Greece follows the southward-directed trench retreat implying that the magmas formed during subduction of continental lithosphere after the closure of the Pindos ocean. The shoshonitic magmas of Maronia were generated by hydrous melting of a phlogopite-bearing mantle that was enriched by 3-5% of slab-derived sediment melts. A lower degree of melting controls the higher K, LILE and incompatible element contents of the shoshonitic magmas in comparison to the neighbouring calc-alkaline magmas. The mafic shoshonitic rocks in NE Greece contain much higher P2O5 contents than the associated calc-alkaline magmas, which probably reflects subducted phosphate-rich sediments in their source. Constant Sr- and Nd-isotopes with increasing SiO2 contents imply minimal crustal assimilation and that felsic magmas including the mineralised porphyritic granite were generated by fractional crystallisation of mafic magmas. Amphibole and apatite analyses provide evidence for the intrusion of S-poor, Cl-rich magmas at 5 to 6 km depths, followed by rapid uplift and exsolution of magmatic fluids. Late granitic dikes at Maronia indicate that volatiles, sulphur and metals became enriched during the late stages of the magma evolution.

Gas hydrate versus seabed morphology offshore Lebu (Chilean margin)

Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown. We report a seabed morphology growth related to fluid migration offshore Lebu associated with mud cones by analysing oxygen and deuterium stable water isotopes in pore water, bathymetric, biological and sedimentological data. A relief was observed at − 127 m water depth with five peaks. Enrichment values of δ18O (0.0–1.8‰) and δD (0.0–5.6‰) evidenced past hydrate melting. The orientation of the relief could be associated with faults and fractures, which constitute pathways for fluid migration. The benthic foraminifera observed can be associated with cold seep areas. We model that the mud cones correspond to mud growing processes related to past gas-hydrate dissociation. The integration of (i) the seismic data analysis performed in the surrounding area, (ii) the orientation of our studied relief, (iii) the infaunal foraminifera observed, (iv) the grain size and (v) the total organic matter and isotope values revealed that this area was formerly characterised by the presence of gas hydrates. Hence, this part of the Chilean margin represents a suitable area for investigating fluid-migration processes.

Origin of the spinel-pyroxene symplectites in the harzburgites from the New Caledonia Peridotite

The New Caledonia ophiolite (Peridotite Nappe) hosts one of the largest and best-exposed mantle section worldwide, providing an exceptional insight into upper mantle processes. The Peridotite Nappe is mostly dominated by harzburgites, locally overlain by mafic-ultramafic cumulates, but also includes minor spinel and plagioclase lherzolites, cropping out in the northern part of the island. The New Caledonia harzburgites are low-strain tectonites, showing dominant porphyroclastic textures. The main mantle paragenesis is constituted by olivine (~ 75-85 vol%), orthopyroxene (~ 15-25 vol%) and spinel (< 1 vol%), while primary clinopyroxene is notably absent. An important textural feature of these mantle rocks is represented by the common occurrence of spinel-pyroxene symplectitic aggregates.In this work, we present a petrographical, textural and major element chemical characterization of the spinel-pyroxene symplectitic intergrowths occurring in the New Caledonia harzburgites (Kopeto, Central Massif, and Yate, Massif du Sud). Based on textures, size and relationships with the other mineral phases, these spinel-pyroxene clusters have been divided into two types, named type-A and type-B.Type-A symplectites occur in the Kopeto harzburgites and are composed of spinel-orthopyroxene (± clinopyroxene) intergrowths. In type-A symplectites, symplectitic spinel (Spl2) occurs as abundant vermicular shaped grains, ranging in size from ~ 0.5 to 2 mm. By contrast, spinel of the porphyroclastic assemblage (Spl1) shows smaller size (in the range of few m) and notably lower abundances (< 1%). Type-A symplectites develop exclusively on porphyroclastic olivine, which in turn displays evidence of chemical disequilibrium and corroded outlines. Bulk major element composition reconstructed for type-A symplectites rule out a derivation from a pre-existing garnet phase, as the model garnet compositions do not satisfy garnet stoichiometry, being characterized by Si deficiency. By contrast, major element chemical variations of the symplectitic phases, coupled with the high abundance of Spl2 and olivine resorption, suggest an origin from reactive percolation of opx-saturated hydrous melts or slab-derived fluids in a subduction zone setting.Type-B symplectites are found in Yate sample and consist of spinel-orthopyroxene (± clinopyroxene). They are characterized by smaller size (few hundreds of m, i.e. “micro-symplectites”) and different shapes compared to type-A symplectites, growing as vermicular, “myrmekite-like” intergrowths at the rims of porphyroclastic orthopyroxene. Major element chemical compositions of type-B symplectites are consistent with an origin as “cooling textures”. These structures may derive from unmixing of a high-T, Al-Cr rich, orthopyroxene due to the decreased solubility of the Cr-Al component (CrMgTs) during post-melting lithospheric cooling at T < 900°C.

Identification of Jurassic mafic arc magmatism in the eastern North China Craton: Geochemical evidence for westward subduction of the Paleo-Pacific slab

AbstractSubduction of the Paleo-Pacific slab beneath the North China Craton (NCC) has exerted a strong influence on the Mesozoic destruction of the craton. However, no Andean-type arc magmatism has been reliably identified in the eastern NCC. Here we report the occurrence of Jurassic arc-like lamprophyres in the Liaodong Peninsula, providing a snapshot of the Paleo-Pacific slab subduction beneath the NCC in the early Mesozoic. Zircon U-Pb dating of the lamprophyres yields consistent ages of 158–155 Ma for magma crystallization. These lamprophyres all exhibit typical arc-like trace element distribution patterns, but show a series differences in their radiogenic isotope compositions and the other geochemical variables. Type 1 lamprophyres exhibit weakly enriched Sr-Nd-Hf isotopes with (87Sr/86Sr)i ratios of 0.7075–0.7085, εNd(t) values of −3.9 to −1.3 and εHf(t) values of −5.4 to −0.3, whereas Type 2 lamprophyres exhibit moderately enriched radiogenic isotopes with (87Sr/86Sr)i ratios of 0.7096–0.7117, εNd(t) values of −12.2 to −7.6 and εHf(t) values of −12.8 to −4.7. There are also systematic differences in zircon Hf isotopes and whole-rock Ba/Th, Ba/La, Sr/Nd, Th/Nd, Th/Yb, and La/Sm ratios for the two types of lamprophyre. Taken together, these similarities and differences can be accounted for by metasomatic reaction of the cratonic mantle wedge with two properties of liquid phase derived from subducting Paleo-Pacific slab. One is aqueous solutions from the subducting basaltic oceanic crust, and the other is hydrous melts from the subducting terrigenous. The two properties of subduction zone fluids were incorporated in different proportions into the mantle sources of these lamprophyres. Accordingly, the lamprophyres were derived from the metasomatic mantle sources. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer at the slab-mantle interface in a paleo-oceanic subduction zone. Therefore, the Jurassic lamprophyres in the eastern NCC provide the geochemical evidence for the crust-mantle interaction during the Paleo-Pacific slab subduction beneath eastern Asia in the early Mesozoic, when the chemical metasomatism by the slab-derived fluids would have weakened the cratonic mantle for its thinning and destruction in the Early Cretaceous.