Globular Textures Research Articles

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu\u2013Bonin\u2013Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of hbox {K}_2hbox {O}-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000, mu hbox {g/g}), whereas globules and the interface are Cl poor (ca. 300, mu hbox {g/g}). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (^{87}hbox {Sr}/^{86}hbox {Sr} of 0.70580 pm 10), indicating that they likely formed from the same source. However, globules show high delta ^{11}B (ca. + 7permille), whereas their host andesites are isotopically lighter (ca. – 1 permille), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low delta ^{11}hbox {B} forearc basalt parental magmas (average ^{87}hbox {Sr}/^{86}hbox {Sr} of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.

Electrical, and Magnetic Characteristics of Homo- and Hetero-Bimetallic Macromolecular Complexes with π-Conjugated Imine-oxime Backbone

Seven homo- and hetero-bimetallic macromolecular complexes (BMCs) were prepared via an in-situ multi-step complexation reaction of CuII, NiII and FeIII metal centers with imine- oxime π-conjugated ditropic spacer (1,4-phenylene-bis(2,3-butanedeimineoxime, L2−). The structures of the obtained [M-L2-M]x, [M`-L2-M`]x, [M-L2-M`]x and their μ-oxo-[M-L2-Fe]x BMCs (M = CuII, NiII; M` = FeIII) were elucidated using different spectroscopic (UV–Vis, FT-IR), powder X-ray diffraction (p-XRD) and microscopic (SEM) techniques. The SEM images indicate that the obtained BMCs exhibit nano/micro globular and rod-like textures. The electrical and magnetic characteristics of the obtained BMCs can be described as magneto-semiconducting materials. The structural, electrical, and magnetic properties inherited in the prepared regime demonstrate that the examined semiconducting π-spacer has clear ability to semi-conduct electron transport and delocalize spin leading to antiferromagnetic coupling, thus offering a favorable magnetic exchange pathway between the paramagnetic metal sites. However, ferromagnetic enhancement is observed in the μ-oxo-[M-L2-Fe]x BMCs where the μ-oxo-bridge overplays the role of the π-spacer. A magnetic responsive thin film was conveniently obtained by solvent casting of the μ-oxo-[Ni-L2-Fe]x BMC in a transparent poly(methamethylacrylate) matrix.

Internal features, mineralogy and geochemistry of ferromanganese nodules from the Gulf of Cadiz: The role of the Mediterranean Outflow Water undercurrent

A large suite of Fe–Mn nodules (561 samples) were recovered during the Anastasya 20 01 cruise (TASYO project) along the continental margin of the Gulf of Cadiz (Eastern Central Atlantic), at the confluence of the Mediterranean Sea with the Atlantic Ocean, where extensive nodule fields were discovered. Based on wide previous studies that included swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism, heat flow probes and underwater photography surveys, nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived Mg-calcite, ankerite and dolomite chimneys and crusts. Forty-six selected samples among the various morphological types were used for the laboratory analysis of physical properties (morphology, color, surface texture, sphericity, weight and size), mineralogy (XRD, optical and electronic microscopy), geochemistry (XRF, AAS, ICP-MS, ICP-AES, EPMA, and GC-MS) and stable isotopes. The nodules show a wide range of sizes, densities, weights and morphologies. They are formed by multiple millimeter-thick layers of Fe and Mn oxyhydroxides surrounding the nucleus composed of Early–Middle Miocene plastic marl and sediment, which were derived from underlying units by fluid venting. Massive, laminated, detrital and mottled to dendritic textural features were developed by the Fe and Mn oxyhydroxide layers. The main components are Goethite, lepidocrocite, Mn oxides (7 Å manganates and 10 Å manganates), quartz, and phyllosilicates. Accessory minerals are calcite, dolomite, siderite, rhodochrosite, kutnahorite, pyrite, chalcopyrite, potassium feldspar, zircon, rutile, ilmenite and chlorite. Fe–Mn carbonates from the siderite–rhodochrosite continuous series are the principal constituent of the nuclei. Framboidal, filamentous and globular textures are observed in Fe–Mn oxides and pyrite, suggesting biogenic origin. The nodules show a high mean abundance of Fe (38.6%), moderate Mn (6.0%) and low contents of trace metals and REEs compared to the average content of deep-seabed polymetallic nodules from other oceanic areas. The Mn/Fe ratio ranges from 0.07 to 0.25. The studied nodules hold hydrocarbons ( n-alkanes) derived from marine bacterial activity in their oxide layers, with the presence of aromatic hydrocarbons as phenanthrene as well, characteristic of mature petroleum. The structure, mineralogy and chemical composition in the studied nodules are more similar to those of diagenetic–hydrogenetic continental margin nodules rather than deep-sea nodules. We suggest that the formation of this type of nodule could respond to a combined diagenetic–hydrogenetic growth process, where the fluid venting from deep-seated hydrocarbon reservoirs, the bio-mineralization processes and the bottom currents erosion and chemistry could have played an important role.

Mingling of carbonate and silicate magmas under turbulent flow conditions: Evidence from rock textures and mineral chemistry in sub-volcanic carbonatite dykes, Chagatai, Uzbekistan

The Triassic Chagatai Complex, Uzbekistan, comprises explosive pipes and dykes, dominantly of silicocarbonatite composition, with cross-cutting relationships indicating multi-stage emplacement. Although the dykes have been reported as diamond-bearing, they have not previously undergone detailed investigation in terms of their mineral chemistry or rock texture. The xenolith-rich dykes contain irregularly-shaped microscopic magmatic enclaves of silicate composition within carbonatite magma and corroded microphenocrysts with crystal overgrowths that record synmagmatic geochemical disequilibrium. Quench crystals of apatite and aegirine, and anhedral baryte, which formed after corrosion of apatite and magnetite microphenocrysts but prior to formation of crystal overgrowths and mantles, indicate contemporaneous rapid undercooling. The anhedral baryte formed as a by-product of an oxidising hydrous reaction from Ba-rich biotite and pyrite to chlorite. The rock and microphenocryst textures suggest that mingling between two magmas occurred and a post-mingling mineral assemblage, including baryte, crystallised in a partially hybridised heterogeneous magma. An initial carbonatite mineral assemblage is identified as calcite + magnetite + apatite ± augite ± barium-rich biotite ± melilite ± pyrite. Changes in mineral chemistry of the carbonatite assemblage that are contemporaneous with the disequilibrium reaction textures suggest addition of a hydrous, Na–Si–Al-rich magma, and the mineral assemblage in the magmatic enclaves is similar to that of trachyte dykes in the Chagatai Complex. Using primarily rock textures and mineral chemistry, supported by mass balance calculations and isotope data, the silicate material is interpreted as a hydrous trachyte magma that had assimilated upper crustal material. The trachyte magma was entrained by carbonatite that was rapidly and turbulently ascending through the crust, shortly before emplacement as silicocarbonatite. The interpretation of magma mingling textures in the Chagatai Complex are unique amongst reported carbonatite occurrences: previously reported carbonate–silicate magma systems either formed globular textures or were interpreted as the products of assimilation of country rock only.

Hydrocarbon-derived ferromanganese nodules in carbonate-mud mounds from the Gulf of Cadiz: Mud-breccia sediments and clasts as nucleation sites

More than 500 Fe–Mn nodules were sampled during the Anastasya-01 cruise (TASYO project) along the continental margin of the Gulf of Cadiz (eastern Central Atlantic), at the confluence of the Mediterranean Sea with the Atlantic Ocean, where extensive nodule fields were discovered. Based on wide previous studies that included swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism and underwater photography surveys, nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived ankerite and dolomite chimneys and crusts. Thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties (morphology, colour, surface texture, sphericity, weight and size), mineralogy (XRD, optical and electronic microscopy) and geochemistry (XRF, AAS, ICP-MS, EPMA, GC-MS, S, C and O isotopes). Nodules show a wide range of sizes, densities, weights and morphologies. Nodules are formed of multiple millimetre-thick layers of Fe and Mn oxyhydroxides surrounding the nucleus composed of Early–Middle Miocene plastic marl. The textures developed by the Fe and Mn oxyhydroxides layers are massive, laminated, detrital and mottled to dendritic. Goethite, lepidocrocite, Mn-oxides (7 Å-manganates and 10 Å-manganates), quartz, and phyllosilicates are the main components. Accessory minerals are calcite, dolomite, siderite, rhodochrosite, kutnohorite, pyrite, chalcopyrite, potassium feldspar, zircon, rutile, ilmenite, and chlorite. Fe–Mn carbonates from the siderite–rhodochrosite continuous series are forming part of the marl nuclei. Framboidal, filamentous, and globular textures are observed in Fe–Mn oxides and pyrite suggesting biogenic origin. Nodules show a high mean abundance of Fe (38.6%) moderate Mn (6.0%), and low contents of trace metals and REEs compared to the average content of deep-seabed polymetallic nodules from other oceanic areas. The Mn/Fe ratio ranges from 0.07 to 0.25. The studied nodules hold in their oxide layers hydrocarbons (n-alkanes) derived from marine bacterial activity. Aromatic hydrocarbons in the form of phenanthrene, are also present which is characteristic of mature petroleum. The structure, mineralogy, and chemical composition in the studied nodules are similar to those of diagenetic–hydrogenetic continental margin nodules rather than deep-sea nodules. We suggest that the formation of this type of nodule occurred through a combined diagenetic–hydrogenetic growth process involving fluid venting from deep-seated hydrocarbon reservoirs, bio-mineralization processes, and bottom current erosion.

Numerical modeling of the development of small-scale magmatic emulsions by Korteweg stress driven flow

The occurrence of micron to millimeter size globular heterogeneities in igneous rocks is frequently explained by processes of liquid immiscibility. However, such textures have also been documented in miscible magmatic pairs. In this study, the ability of miscible magmas to develop transient surface tensions and mimic the behavior of immiscible liquids is tested for the whole spectrum of magmatic compositions. We implemented a numerical model that includes the effect of gradient stresses (namely Korteweg stress) in order to investigate the role of such stresses in the evolution of diffusive interfaces. The results show that an initially elongated heterogeneity surrounded by a miscible and compositionally diverse magma will tend to minimize its contact surface by relaxing to a spherical shape, advected by a Korteweg stress driven flow. If the initial aspect ratio of the heterogeneity exceeds a critical value, surface minimization may be achieved by drop breakup. In addition, it is shown that two neighboring heterogeneities may coalesce to a single spherical drop. These results imply that even for fully miscible magmas, rheological barriers may prevent efficient mechanical intermingling and induce the formation of small-scale globular textures, analogous to those commonly observed in immiscible liquids. A better understanding of the role of Korteweg stress may be of the utmost importance for deciphering the textures generated by the interaction of compositionally diverse magmas.

Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany)

A special kind of magma mixing is extraordinarily well exposed in the Bittersberg subvolcanic complex in the Tertiary volcanic field of the German Westerwald: A trachytic melt has been penetrated by a latitic dyke which has been dispersed within the host magma as small spherical enclaves (globules). Whole rock analyses of the globules show a change in composition that cannot be explained by a simple mechanical mixing between the endmembers. The most evolved globules have a phonolitic composition. Microprobe measurements in the microlithic matrix of the host rock and the guest indicate a diffusive motion of the alkalis from the host into the globules. On the other hand, an opposite trend can be observed for Ca, Mg, Fe and Ti, which are impoverished in the globules. The trace elements and the middle rare earth elements (MREE) has also been involved in the diffusive exchange. The REE-pattern of the most evolved (phonolitic) globules shows a characteristic trough in the area of the MREE which is almost identical to the REE-pattern of many phonolites. The phonolites and the alkali-rich trachytes of the Westerwald show similar globular textures as the Bittersberg volcanics. Therefore, generation of these rocks involving diffusive element exchange during mixing processes in a magma reservoir situated on a deeper crustal level may be possible.

31P NMR investigation of a ringing gel phase and adjacent phases

The temperature-surfactant concentration phase diagram was examined for the dodecyltrimethylammonium dimethylphosphate/3-methyl-3-methoxybutanol/water ternary system. The phase diagram contained a highly elastic gel phase which is known as a “ringing gel phase”. The ringing gel phase and adjacent phases in the ternary system were investigated by polarized optical microscopy, freeze-fracture transmission electron microscopy, and 31P NMR. Globular textures were observed in an optically isotropic gel phase. Since the globules were larger than those found in an isotropic solution, the texture consists of domains of aggregated units in the cubic (I 1) phase. Structure units of domains are equivalent to microemulsions which are constructed by surfactant molecules and swollen by alcohol in the isotropic (L 1) phase. Characteristic polarized microscopic textures were visualized in two phases with higher surfactant concentrations. These phases were identified as being hexagonal (H 1) and lamellar (L α) liquid crystals which was confirmed by transmission electron microscopy. The 31P NMR signal of the ringing gel showed a sharp singlet the same as that of the L 1 phase, indicating the fully averaged anisotropic interaction of the aggregates. The characteristic NMR signals of the anisotropic hexagonal and lamellar liquid crystal phases displayed chemical shielding with an asymmetric lineshape.

Optical characteristics of amorphous kerogens and the hydrocarbon-generating potential of source rocks

Four recurring types of amorphous kerogen are optically identified in petroleum source rock samples and are related to hydrocarbon-generating potential (defined here as geochemically interpreted oil-prone or gas-prone). At this time a genetic classification is not possible. The different amorphous kerogen types are distinguished on the basis of microscopic textural differences: Type A — chunky compact masses with mottled network or weak polygonal textures; Type B — very small, dense, elongate, oval, or rounded individual grains; Type C — clumpswith granular, fragmented or globular textures; Type D — thin, platy, or rectangular individual grains. Geochemistry defined oil-prone samples generally contain Types A and/or D separately or combined; geochemically defined gas-prone samples rarely contain Type A and vary in amounts of Types B, C, and/or D. The samples in this study contain dominantly one of the four types of amorphous kerogen, so the geochemical analyses of the whole rock or bulk concentrated kerogen samples are assumed to be analyses of the dominant amorphous type. The samples were chemically analyzed by several techniques to determine the oil- or gas-generating potential. The establishment of relations between specific optical and chemical properties of kerogen types can make visual kerogen analyses more meaningful in predicting the hydrocarbon generation potential of source rocks.

The role of liquid immiscibility in the genesis of carbonatites ? An experimental study

The two-liquid field between alkali-carbonate liquids and phonolite or nephelinite magmas from the Oldoinyo Lengai volcano has been determined between 0.7 and 7.6 kb and 900°–1,250° C. The miscibility gap expands with increase in $$P_{CO_2 }$$ and decrease in temperature. Concomitantly there is a rotation of tie-lines so that the carbonate liquids become richer in CaO. The element distribution between the melts indicates that a carbonate liquid equivalent in composition to Oldoinyo Lengai natrocarbonatite lava would have separated from a phonolitic rather than a nephelinitic magma. CO2-saturated nephelinites coexist with carbonate liquids much richer in CaO than the Lengai carbonatites, but even so these liquids have high alkali concentrations. If the sovites of hypabyssal and plutonic ijolite-carbonatite complexes originated by liquid immiscibility, then large quantities of alkalis have been lost, as is suggested by fenitization and related phenomena. The miscibility gap closes away from Na2O-rich compositions, so that the tendency to exsolve a carbonatite melt is greater in salic than in mafic silicate magmas. The two-liquid field does not approach kimberlitic compositions over the range of pressures studied, suggesting that the globular textures observed in many kimberlite sills and dykes may be the result of processes other than liquid immiscibility at crustal pressures.