The effects of K2O on the compositions of near-solidus melts of garnet peridotite at 3 GPa and the origin of basalts from enriched mantle

Enrichment in K₂O in oceanic island basalts (OIB) is correlated with high SiO₂, low CaO/Al₂O₃, and radiogenic isotopic signatures indicative of enriched mantle sources (EM1 and EM2). These are also chemical characteristics of the petit-spot lavas, which are highly enriched in K₂O (3–4 wt%) compared to other primitive oceanic basalts. We present experimentally derived liquids with varying concentrations of K₂O in equilibrium with a garnet lherzolite residue at 3 GPa to test the hypothesis that the major element characteristics of EM-type basalts are related to their enrichment in K₂O. SiO₂ is known to increase with K₂O at pressures less than 3 GPa, but it was previously unknown if this effect was significant at the high pressures associated with partial melting at the base of the lithosphere. We find that at 3 GPa for each 1 wt% increase in the K₂O content of a garnet lherzolite saturated melt, SiO₂ increases by ~0.5 wt% and CaO decreases by ~0.5 wt%. MgO and $K_{D}^{{{\text{Fe}} - {\text{Mg}}}}$ K D Fe - Mg each decrease slightly with K₂O concentration, as do Na₂O and Cr₂O₃. The effect of K₂O alone is not strong enough to account for the SiO₂ and CaO signatures associated with high-K₂O OIB. The SiO₂, CaO, and K₂O concentrations of experimentally derived partial melts presented here resemble those of petit-spot lavas, but the Al₂O₃ concentrations from the experimental melts are greater. Partitioning of K₂O between peridotite and melt suggests that petit spots, previously considered to sample ambient asthenosphere, require a source more enriched in K₂O than the MORB source.     

Anomalous variation of the ocean’s inertial oscillations at the Hawaii shelf

The data of acoustic Doppler profilometers placed at the edge of the steep shelf of Oahu Island, Hawaii, are analyzed for the currents. The specific character of inertial oscillations is revealed in the region: strong elongation of inertial orbits, a large amount of anomalous (cyclonic) current rotation, and sharp weakening in the divergence layer of the background (low-frequency) oscillations, the strong variation of which results in significant deviations of the effective inertial frequency from its local geographical value. It is suggested that cyclonic rotation of the inertial currents is related to a strong decrease in the effective oscillation frequency of the oceanic waves on the shelf.     

Origin of K-rich diamond-forming immiscible melts and CO2 fluid via partial melting of carbonated pelites at a depth of 180–200 km

Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO₂ inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K₂O enters potassium feldspar or K₂TiSi₃O₉ wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K_0.90Na_0.10)₂CO₃·60Ca_0.55Mg_0.24Fe_0.21CO₃ + 1.9 mol% SiO₂ + 0.7 mol% TiO₂ + 1.4 mol% Al₂O₃ at the solidus established near 1000 °C. Molecular CO₂ liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO₂ = 57.0/52.4, TiO₂ = 1.8/2.3, Al₂O₃ = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na₂O = 3.2/3.0, K₂O = 10.5/15.2, CO₂ = 12.0/8.0, while carbonatite melt can be approximated as 24(K_0.81Na_0.19)₂CO₃·76Ca_0.59Mg_0.21Fe_0.20CO₃ + 3.0 mol% SiO₂ + 1.6 mol% TiO₂ + 1.4 mol% Al₂O₃. Both melts remain stable to at least 1500 °C coexisting with CO₂ fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K₂O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali‑carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.     

Evolution of melts of the Changbaishan Tianchi volcano (China–North Korea) as a model of ore–magmatic system formation: Data from melt inclusions studies

The composition and evolution of the melts of trachytes from the volcano were studied based on examining the inclusions of mineral-forming media by means of X-ray and ion microanalysis. A correlation was shown between the degree of enrichment of these melts in rare elements and the processes of magmatic differentiation. It was found that trachytes of the volcano were generated in highly differentiated alkaline melts enriched in Hf, Nb, Zr, Ta, U, Th, Rb, Y, and REEs under 1020–1060°C. The evolution of melts was determined by the processes of crystal fractionation. The main volatile components in the melts are water, fluorine, and chlorine with the concentrations of 0.1–0.5, 0.2–0.5, and 0.2–0.3 wt %, respectively. The melt crystallization was accompanied by degassing caused by the decrease in the outer pressure. The low concentrations of water and fluorine represent the melt composition by these components exclusively at the time of the melt movement towards the Earth’s surface just before the eruption.     

Basaltic melts in olivine from alkaline pumice of Primor’e: Evidence from the study of melt inclusions

Doklady Earth Sciences -     

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

The melting behaviour of three carbonated pelites containing 0–1 wt% water was studied at 8 and 13 GPa, 900–1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluid-absent and dry carbonated pelite solidi locate at 950 and 1,075°C, respectively; >100°C lower than in carbonated basalts and 150–300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H₂O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H₂O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca–Fe–Mg-carbonatites, with extreme K₂O/Na₂O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with D_\textNa^{\textcpx/\textcarbonatite} = 10-18 D_{\text{Na}}^{{{\text{cpx}}/{\text{carbonatite}}}} = 10{-}18 at the solidus at 8 GPa. The melt K₂O/Na₂O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali- and CO₂-rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated pelites at 8 GPa contrasts that at ≤5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO₂-metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of >200 km. As long as the mantle remains adiabatic, low-viscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca–Fe–Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond- and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.     

Beyond passive consumption: Dis/ordering water supply and sanitation at Hanoi’s urban edge

In Hanoi people access, expand and create water and sanitation infrastructures in multiple ways that include, but are not restricted to, external provision of networked services. Urban master planning and the construction of large technological networks aim at integrating the urban region based on circulating ‘modern ideals’ of ubiquity and standardization of infrastructures. However, centralized infrastructure provision has remained unstable and spatially uneven. We examine differently networked spaces that have emerged at the edge of Hanoi along with rapid urban change and new financing mechanisms in the past thirty years, and the ways in which urban residents engage with the various water and sanitation systems. This engagement is shaped by circulating ideals, place-specific processes of urban re-production, sector-specific dynamics, and individuals. Not only in periurban villages, but also in modern housing estates, people rebut a role as passive receptors of external services. In some instances, they create relatively stable collectives through which they provide, negotiate and complement networked infrastructure connection. Thus, people living at Hanoi’s urban edge actively re-produce water and sanitation systems beyond passive consumption of externally provided services.     

Holocene fire occurrence and alluvial responses at the leading edge of pinyon–juniper migration in the Northern Great Basin,USA

Fire and vegetation records at the City of Rocks National Reserve (CIRO), south-central Idaho, display the interaction of changing climate, fire and vegetation along the migrating front of single-leaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma). Radiocarbon dating of alluvial charcoal reconstructed local fire occurrence and geomorphic response, and fossil woodrat (Neotoma) middens revealed pinyon and juniper arrivals. Fire peaks occurred ~ 10,700–9500, 7200–6700, 2400–2000, 850–700, and 550–400 cal yr BP, whereas ~ 9500–7200, 6700–4700 and ~ 1500–1000 cal yr BP are fire-free. Wetter climates and denser vegetation fueled episodic fires and debris flows during the early and late Holocene, whereas drier climates and reduced vegetation caused frequent sheetflooding during the mid-Holocene. Increased fires during the wetter and more variable late Holocene suggest variable climate and adequate fuels augment fires at CIRO. Utah juniper and single-leaf pinyon colonized CIRO by 3800 and 2800 cal yr BP, respectively, though pinyon did not expand broadly until ~ 700 cal yr BP. Increased fire-related deposition coincided with regional droughts and pinyon infilling ~ 850–700 and 550–400 cal yr BP. Early and late Holocene vegetation change probably played a major role in accelerated fire activity, which may be sustained into the future due to pinyon–juniper densification and cheatgrass invasion.     

Equation of state of silicate melts with densified intermediate-range order at the pressure condition of the Earth’s deep upper mantle

The knowledge from the compression behavior of densified SiO₂ glass suggests that SiO₂ melt may behave as a single phase having a densified network structure (intermediate-range order) at the pressure condition of the Earth’s deep upper mantle, including the transition zone. A simple and easy-to-use equation of state of silicate melts which is applicable to a wide range of chemical composition at the pressure condition of the deep upper mantle is proposed based on the assumption that SiO₂ component is in its densified state (or phase). The equation of state proposed in this study is consistent with all the available density data of silicate melts with an SiO₂ content of about 35–55 mol% measured with large-volume presses at pressures between 8 and 22 GPa. The equations of state in previous studies differ considerably from each other. The main reason for the discrepancies seems to be that the compression behavior of multiple states (or phases) of silicate melts has been described in most cases with a single equation of state. It is necessary to consider that silicate melts are in their densified states (or phases) in the deep upper mantle.     

Detrital zircon provenance of early Palaeozoic sediments at the southwestern margin of the Siberian Craton: Insights from U–Pb geochronology

Detrital zircons from the Ordovician and Devonian sedimentary cover of the Siberian Craton were analyzed for U/Pb geochronology to understand their sediment provenances. Five main age-peaks were identified in the zircon U/Pb age-spectra: (1) Neoarchaean – early Palaeoproterozoic (2.7–2.4 Ga); (2) late Palaeoproterozoic (2.0–1.65 Ga); (3) minor early Neoproterozoic (1.0–0.75 Ga); (4) Ediacaran (0.65–0.60 Ga) and (5) Cambrian – Early Ordovician (0.54–0.47 Ga), reflecting the main magmatic events in the sediment source regions. The oldest zircons (groups 1 and 2) are derived from the Siberian Craton which amalgamated during the Neoarchean – Palaeoproterozoic. The Neoproterozoic zircons (groups 3 and 4) likely sourced from southwestern basement uplifts and Neoproterozoic belts of the Siberian margin such as the Yenisey Ridge and Baikal-Muya region. The provenance of the youngest zircons (group 5) can be traced to the Altai–Sayan fold-belt, where peri-Gondwanan microcontinents and island-arcs accreted to Siberia during late Neoproterozoic – early Palaeozoic progressive consumption of the Palaeo-Asian Ocean.     

Deformation behavior of migmatites: insights from microstructural analysis of a garnet–sillimanite–mullite–quartz–feldspar-bearing anatectic migmatite at Rampura–Agucha,Aravalli–Delhi Fold Belt,NW India

International Journal of Earth Sciences - In the present study we investigate the microstructural development in mullite, quartz and garnet in an anatectic migmatite hosted within a Grenvillian-age...     

The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume

The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. ⁴⁰Ar/³⁹Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO₂-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO₂-saturated trachytes and SiO₂-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ¹⁸O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.     

Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, K_x, for the disproportionation reaction among the structural units coexisting in the melts, Si₂O₅(2Q³)SiO₃(Q²)+SiO₂(Q⁴), was calculated (K_x is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln K_x vs l/T relationships the enthalpy (H_x) for the disproportionation reaction is in the range of-30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the H_x increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li₂O·2SiO₂ (Li/Si=1), the H_x is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M=K, Na, Li), there is a positive, near linear correlation between the H_x and the Z/r² of the metal cation. The slope of the H_x vs Z/r² regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, _i, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form _i=a lnX _i+b, where a is between 0.6 and 0.87, and X _i is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.     

Physical characteristics of a lava flow determined from thermal measurements at the lava’s surface

We consider the problem about determination of characteristics of a lava flow from the physical parameters measured on its surface. The problem is formulated as an inverse boundary problem for the model simulating the dynamics of a viscous heat-conducting incompressible inhomogeneous fluid, where, on the basis of additional data at one part of the model boundary, the missing conditions at another part of the boundary have to be determined, and then the characteristics of fluid in the entire model domain have to be reconstructed. The considered problem is ill-posed. We develop a numerical approach to the solution of the problem in the case of a steady-state flow. Assuming that the temperature and the heat flow are known at the upper surface of the lava, we determine the flow characteristics inside the lava. We compute model examples and show that the lava temperature and flow velocity can be determined with a high precision when the initial data are smooth or slightly noisy.     

Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett transform fault: implications for the nature of the mantle–crust transition at fast-spreading ridges

 Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise. Received: 13 October 1997 / Accepted: 5 February 1999     

An early–middle Eocene Antarctic summer monsoon: Evidence of ‘fossil climates’

A warmer and mostly ice-free South polar region prevailed during the early–middle Eocene, indicative of a low latitudinal temperature gradient. Climatic models mostly fail to reconstruct such a low gradient, demonstrating our poor understanding of the mechanisms involved in heat transfer. Here we describe a new phenomenon that shaped the southern high latitude climate during the early–middle Eocene: the Antarctic summer monsoon. Our palaeoclimatic reconstruction is based on 25 morphotypes of fossil dicotyledonous leaves from the early–middle Eocene fossil leaf assemblage of Fossil Hill from King George Island, the Antarctic Peninsula. We use a novel CLAMP (Climate Leaf Analysis Multivariate Program) calibration which includes new climatic parameters that allow us to characterise better the seasonality in precipitation. Our reconstruction indicates a warm humid temperate climate with strong seasonality in temperature and precipitation. Seasonality in precipitation indicates a rainfall rate of 6.4 ± 1.30 mm/day during summer (summer daily rate of precipitation; SDR) and a summer precipitation representing more than 60.3 ± 8.28% of annual rainfall (ratio of summer precipitation; RSP), which fulfils the definition of a summer monsoon in the modern world. This implies a seasonal alternation of high- and low-pressure systems over Antarctica during the early–middle Eocene. Such a climate regime would have impacted upon global atmospheric circulation and heat transfer. This climatic regime presents a challenge for climatic models and their ability to reconstruct accurately palaeoclimates at high southern latitudes and thereby understand latitudinal heat transfer in a ‘greenhouse Earth’ regime.     

Geochemistry of magmatic and hydrothermal zircon from the highly evolved Baerzhe alkaline granite: implications for Zr–REE–Nb mineralization

The Baerzhe alkaline granite pluton hosts one of the largest rare metal (Zr, rare earth elements, and Nb) deposits in Asia. It contains a geological resource of about 100 Mt at 1.84 % ZrO₂, 0.30 % Ce₂O₃, and 0.26 % Nb₂O₅. Zirconium, rare earth elements (REE), and Nb are primarily hosted by zircon, yttroceberysite, fergusonite, ferrocolumbite, and pyrochlore. Three types of zircon can be identified in the deposit: magmatic, metamict, and hydrothermal. Primary magmatic zircon grains occur in the barren hypersolvus granite and are commonly prismatic, with oscillatory zones and abundant melt and mineral inclusions. The occurrence of aegirine and fluorite in the recrystallized melt inclusions hosted in the magmatic zircon indicates that the parental magma of the Baerzhe pluton is alkali- and F-rich. Metamict zircon grains occur in the mineralized subsolvus granite and are commonly prismatic and murky with cracks, pores, and mineral inclusions. They commonly show dissolution textures, indicating a magmatic origin with later metamictization due to deuteric hydrothermal alteration. Hydrothermal zircon grains occur in mineralized subsolvus granite and are dipyramidal with quartz inclusions, with murky CL images. They have 608 to 2,502 ppm light REE and 787 to 2,521 ppm Nb, much higher than magmatic zircon. The texture and composition of the three types of zircon indicate that they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system. Large amounts of Zr, REE, and Nb were enriched and precipitated during the transitional period to form the giant low-grade Baerzhe Zr–REE–Nb deposit.