Mineral Textures Research Articles

Clay mineral signatures of fault-related fluid flows in a sandstone reservoir: A case study from the Teloua Formation, Tim Mersoї Basin, Niger

During the evolution of sedimentary basins, clay minerals may record changes in their mineralogy, chemistry, and texture as a response to diagenesis, fluid flows, and tectonic activities. Therefore, studying clay minerals in sedimentary rocks provides important information that may assist to decipher the basin history. The focus of the present study is the Teloua Sandstone Formation (Fm) which belongs to the Tim Mersoï Basin located in the NE of Niger republic (West Africa). The formation is cut by the major Arlit fault which is the largest tectonic structure impacting the Tim Mersoï Basin. The aim of the present work is to use the texture, mineralogy, and chemistry of clay minerals in the Teloua Fm as geochemical markers for burial diagenesis history and fluids flows (i.e., attributed mainly to the Arlit fault). Therefore, three drill holes (DH1, DH2, and DH3) were investigated as a function of their distance from the fault and samples have been collected all along the Teloua Fm (Upper Triassic-Lower Jurassic), at the unconformity surface with the underlaying Moradi Fm (Upper Permian), and at the top of the Moradi Formation.Petrographic observations demonstrate that the sediments of the Teloua Fm have undergone transformations induced by burial diagenesis, brittle deformation, and multiple stages of fluid flows. These transformations are marked by grain compaction, fragmentation, dissolution of detrital minerals, recrystallization of authigenic clay minerals, and modifications in the primary porosity. XRD and SEM results indicate no significant variation in the detrital mineralogy of the three drill holes; Teloua Fm samples are composed of quartz, feldspars (microcline and albite), micas (biotite and muscovite), minor heavy minerals such as hematite, anatase, phosphates (apatite and monazite), and zircon. The authigenic clay minerals observed in the three drill holes are rosette-like tosudite (i.e., a regular interstratified chlorite/smectite), saponite, montmorillonite, and kaolinite/smectite mixed layer. Regarding their vertical distribution, tosudite occurs throughout the Teloua Fm, whereas montmorillonite is mainly present at the top of the formation, and saponite is only observed at the unconformity between Teloua and Moradi formations. The Mg content of tosudite tends to decrease with increasing vertical distance from the basal unconformity and the lateral distance from the Arlit fault. This is interpreted as an evidence of Mg-rich fluid circulations related to the Arlit fault activity that penetrated the formation through the unconformity surface and fractures. This is also evidenced by the enrichment of dolomite at the unconformity when moving closer to the Arlit fault. In addition, the occurrence of both authigenic saponite (at the unconformity) and montmorillonite (upper part of the Teloua Fm) that post-dated tosudite is attributed to another stage of Mg-rich fluid circulation during late diagenesis. Furthermore, late downward circulation of meteoric fluids of low pH are evidenced by the partial alteration and replacement of montmorillonite by kaolinite/smectite mixed layer in the Teloua Fm (at shallow depth of the DH3). These results demonstrate the validity of clay minerals as reliable tools to provide information about fluid/rock interaction in complex geological and tectonic setting.

Pre-Alpine high-pressure metamorphism in the Gemer unit: mineral textures and their geodynamic implications for Variscan Orogeny in the Western Carpathians

We report mineral textures of retrogressed eclogites from the gneiss–amphibolite complex, which is represented by a series of isolated bodies that were thrusted over the greenschist-facies Rakovec group in the Gemer unit (Western Carpathians). Peak pressure and temperature conditions of about 2.0 GPa and 650 °C are estimated for the eclogite-facies assemblage (omphacite, garnet and rutile). The eclogites show strong amphibolite-facies re-equilibration that is characteristic for the rest of gneiss–amphibolite complex rocks. Pressure and temperature conditions for both eclogite and subsequent amphibolite-facies stages are comparable with those reported from the retrogressed eclogites in the Vepor and Tatra units. The underlying Rakovec group rocks indicate greenschist-facies conditions, but relicts of high-pressure (blueschist facies) metamorphism are also preserved. The Pre-Alpine age of the eclogite-facies metamorphism in the gneiss–amphibolite complex and blueschist-facies metamorphism in the Rakovec group is confirmed by the presence of their pebbles in the overlaying upper Carboniferous conglomerates. The origin of eclogites and blueschists in the Gemer unit and their relationship to eclogites in the Vepor and Tatra units are discussed with respect to the subduction zone along which they originated, but also in relation to the high-pressure rocks occurring along the European Variscan Belt. As the whole Gemer unit underwent Alpine metamorphism, the retrogressed eclogites and associated gneisses were also studied to clarify possible degree of this metamorphic overprint in the gneiss–amphibolite complex rocks.

The Robustness of the Gray Level Co-Occurrence Matrices and X-Ray Computed Tomography Method for the Quantification of 3D Mineral Texture

Mineral textural quantification methods have become critical in both geosciences and mineral processing as mineral texture is a critical factor contributing to ore variability. However, the lack of objective mineral texture classification has made quantification difficult. The aim of this study is therefore to investigate the robustness of applying the gray level co-occurrence matrices (GLCM) to 3-dimensional (3D) gray scale images measured by X-ray computed tomography (XCT) for the quantification of mineral texture in 3D. The data quality of the GLCM outputs like statistics, heat maps and histograms in response to changes in XCT conditions such as artefacts, resolution, and calibration was tested. The response of the GLCM outputs with respect to different mineral texture types with anisotropic features and inter-sample variability was also explored. The methodology included testing core sizes of 26, 19, 14, and 6 mm diameter. Calibration was tested using copper and tungsten wires. The study demonstrated the versatility of the method for different sample types. Inter-sample calibration and optimal scanning conditions (quality and integrity) were also demonstrated, and a basic link between the 3D GLCM statistical descriptors with the mineral texture features of rocks was established. The 3D mineral texture method can potentially bypass the XCT segmentation process for direct automation of 3D mineral texture information.

Geochemistry and petrogenesis of the post-collisional high-K calc-alkaline magmatic rocks in Tengchong, SE Tibet

Post-collisional high-K calc-alkaline (HKCA) magmatic rocks (8 Ma to present) are extensively distributed in the Tengchong volcanic field (TVF), SE Tibet. Here, an integrated study of whole-rock major-trace elements, Sr-Nd-Pb-O isotopes and zircon U-Pb-Hf-O isotopes, together with mineral texture and geochemistry, are presented for these volcanic suites. The HKCA magmatic rocks exhibit high concentrations of incompatible trace elements with negative Nb-Ta-Ti anomalies and high (87Sr/86Sr)i (0.705–0.710), low (143Nd/144Nd)i (0.5120–0.5126) with variable δ18O (5.7–9.3‰). They may be divided into four groups from early to late stages: Groups I and III are basaltic-trachybasaltic-trachyandesitic assemblages (MgO = 3.3–9.2 wt.%); Groups II and IV mainly consist of trachyandesites and dacites (MgO = 0.6–5.1 wt.%). Incompatible trace element and Sr-Nd isotope compositions of the mafic magmas (MgO = 6.0–9.2 wt.%) from Groups I and III are postulated to reflect fluid enrichment of a mantle source by silicate and carbonatitic metasomatism as a consequence of eastward underthrusting of the Indian continental slab following the India-Asia collision. The petrogenesis of differentiated magmas (Groups II and IV) is interpreted in terms of control by crustal magma chamber AFC processes. The results of P-T estimation and EC-AFC modelling suggest at least two magma storage regions: a deep one (17–28 km) in which the trachyandesitic magmas were produced by AFC of basaltic magmas and a shallow one (9–14 km) fed by trachyandesitic magmas which gave rise to dacitic magmas via AFC. Phenocryst zoning textures also record mafic replenishment into silicic magma chambers.

Origin of the Triassic Lincang granites in the southeastern Tibetan Plateau: Crystallization from crystal mush

The crustal generation of the Paleo-Tethys orogenic belt in the southeastern Tibetan Plateau is a major unresolved geological problem. The Changning-Menglian suture in the southeastern Tibetan Plateau represents a remnant of the Paleo-Tethys Ocean. The Lincang batholith and associated volcanic rocks (the Manghuai rhyolites and Xiaodingxi basalts) are located to the east of the Changning-Menglian suture, and represent important constraints for the abovementioned geological problem. New zircon UPb ages, together with previously published results, indicate that the Manghuai rhyolites were emplaced at 229–239 Ma and the Lincang batholith yielded longer age span (211–239 Ma). Broad age span of zircons (to 28 million years) from these rocks record a long-lived crustal magmatic system in the Changning-Menglian Paleo-Tethys orogenic belt. These intrusive and associated volcanic rocks have significant differences between them in terms of geochemistry. In this study, these rocks are geochemically classified into three groups: (1) Group 1 comprises biotite granite and granodiorite rocks; (2) Group 2 includes alkali feldspar granite, two-mica granite, and rhyolite, which are high-silica (generally >70 wt%) rocks that have low TiO2, Al2O3, FeOtot, MgO, CaO, P2O5, Eu, and Sr contents, compared to those of Group 1. Based on their geochemical relationships and mineral textures, Groups 1 and 2 include cumulative- and fractionated-type granitoids, respectively; (3) while Group 3 is comprised of dioritic enclaves and the Xiaodingxi basalts. Based on more robust evidence, dioritic enclaves are derived from a mantle source magma. The Lincang granites have high and variable initial 87Sr/86Sr ratios (0.703122–0.728476), significantly negative εNd (t = 220 Ma) values (−13.4 to −7.5), negative εHf (t) values (−17.7 to −6.8), and old TDM2 ages. The Manghuai rhyolites and Xiaodingxi basalts have lower initial 87Sr/86Sr ratios (0.695970–0.709398), and higher εNd (t) values (−3.4 to −0.6) and εHf (t) values (−1.2 to +3.7) than those of the Lincang granites. Geochemical data suggest that the Manghuai rhyolites could be seen as fractionated and erupted melts from crystal mush reservoirs, which later crystallized to form the Lincang granites. The Manghuai rhyolites provide an instantaneous snapshot of the state of the crystal mush at the time of eruption. The Lincang granites are interpreted to record prolonged histories of open-system magmatic evolution, involving magma mixing, wall-rock assimilation, and multiple cycles of crystallization from the crystal mush reservoirs. Magmatic evolution was likely related to mantle upwelling in a post-collisional setting, forming large-volume granites and volcanics in the Changning-Menglian Paleo-Tethys orogenic belt. Thus, this crystal mush model provides an integrated picture of the Triassic silicic magmatism, linking the evolution of the Lincang batholith and Manghuai rhyolites before storage in the Changning-Menglian Paleo-Tethys orogenic belt.

Muscovite Dehydration Melting in Silica-Undersaturated Systems: A Case Study from Corundum-Bearing Anatectic Rocks in the Dabie Orogen

Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.

Silicate-Carbonate Liquid Immiscibility: Insights from the Crevier Alkaline Intrusion (Quebec)

Abstract This contribution explores the petrogenetic relationships between silicate and carbonatitic rocks in the Crevier Alkaline Intrusion (CAI, Québec, Canada). The CAI is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated peralkaline rocks from ijolite to nepheline syenite and carbonatites. Petrogenetic relationships between different undersaturated alkaline igneous rocks, carbonate-bearing and carbonate-free nepheline syenite and carbonatites observed in the CAI suggest that (1) carbonate-bearing and carbonate-free silicate rocks are comagmatic with carbonatite, and that (2) both silicate and carbonatitic liquids are fractionated from an ijolitic parental magma that has undergone liquid immiscibility. One of the observed facies is characterized by spectacular ocelli of carbonate-bearing nepheline syenite in a matrix of carbonatite. The younger nepheline syenite facies can be divided into two groups based on the presence or absence of magmatic carbonates. Both groups are characterized by the presence of pyrochlore-group minerals that carry the Nb–Ta mineralization. We specifically use accessory minerals such as zircon, pyrochlore and apatite to constrain the temporal and physicochemical parameters of the immiscibility process. By coupling (1) mineral textures, (2) trace elements, (3) Ti-in-zircon thermometry, and (4) oxygen isotope compositions, we have traced the crystallization of zircon before, during and after the immiscibility process. The results allowed us to constrain the minimum temperature of this process at ∼815–865°C. In addition, two magmatic populations of pyrochlore are identified through their petrographic and geochemical characteristics within the younger nepheline syenite facies. Pyrochlore from the earlier ocelli facies of carbonate-bearing nepheline syenite follow a Nb–Ta differentiation trend, whereas pyrochlore from the younger carbonate-free nepheline syenite follow a more classical Nb–Ti trend. Following the complete immiscibility between the silicate and carbonatitic liquids, the fractionation between Nb and Ta stopped while a new generation of Nb-rich pyrochlore grew, displaying a more classical Nb–Ti fractionation trend and a higher Nb/Ta ratio in the nepheline syenite.

Enrichment of REE and HFSE during the magmatic-hydrothermal evolution of the Baerzhe alkaline granite, NE China: Implications for rare metal mineralization

The Baerzhe Cretaceous (123.7 ± 0.9 Ma) peralkaline pluton underwent extensive fractional crystallization and extreme enrichment in incompatible elements including rare earth elements (REE) and the high field strength elements (HFSE). It is one of the largest resources of rare metals in China, containing approximately 100 million tons of ore with an average grade of 1.84 wt% ZrO2, 1.00 wt% REE2O3 (34% heavy rare-earth oxides), and 0.26 wt% Nb2O5. Zr, REE and Nb are mainly hosted by hydrothermal minerals, such as zircon, hingganite-(Y), monazite-(Ce), polycrase, pyrochlore, fergusonite and columbite. An integrated investigation of field geology, mineral textures and compositions of minerals and host granites was carried out to examine the evolution of the Baerzhe pluton and the roles of magmatic and hydrothermal processes in concentrating REE and HFSE. Most minerals in the intensively altered subsolvus granite show secondary textures or replaced pseudomorphs, such as the replacement of arfvedsonite by aegirine, zircon dissolution and reprecipitation, and the replacement of monazite-(Ce) and polycrase-(Y) by hingganite-(Y). Compositions of the key economic minerals and changes with respect to these alteration stages reveal evidence that hydrothermal alteration played a role in the mineralization of the pluton. In-situ analyses and element mappings suggest that large volume of metals were remobilized and redistributed during hydrothermal replacement, such as the replacing of monazite-(Ce) and polycrase by hingganite-(Y). It is suggested that subsolidus re-equilibration and hydrothermal alteration, in addition to magmatic fractionation, is critical for further concentrating REE and HSFE in peralkaline granitic systems.

The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland

Hosting up to 3.3 billion barrels of oil in place, the upfaulted Precambrian crystalline rocks of the Lancaster field, offshore west of Shetland, give key insights into how fractured hydrocarbon reservoirs can form in such old rocks. The Neoarchean ( c. 2700–2740 Ma) charnockitic basement is cut by deeply penetrating oil-, mineral- and sediment-filled fissure systems seen in geophysical and production logs and thin sections of core. Mineral textures and fluid inclusion geothermometry suggest that a low-temperature (<200°C) near-surface hydrothermal system is associated with these fissures. The fills help to permanently prop open fissures in the basement, permitting the ingress of hydrocarbons into extensive well-connected oil-saturated fracture networks. U–Pb dating of calcite mineral fills constrains the onset of mineralization and contemporaneous oil charge to the mid-Cretaceous and later from Jurassic source rocks flanking the upfaulted ridge. Late Cretaceous subsidence and deposition of mudstones sealed the ridge, and was followed by buoyancy-driven migration of oil into the pre-existing propped fracture systems. These new observations provide an explanation for the preservation of intra-reservoir fractures (‘joints’) with effective apertures of 2 m or more, thereby highlighting a new mechanism for generating and preserving fracture permeability in sub-unconformity fractured basement reservoirs worldwide. Supplementary material: Analytical methods and isotopic compositions and ages are available at https://doi.org/10.6084/m9.figshare.c.4763237 Thematic Collection: This article is part of the Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Phoscorites of the Salitre I complex: Origin and petrogenetic implications

The Salitre I complex is a kidney-shaped alkaline-carbonatite-phoscorite complex part of the Late Cretaceous Alto Paranaíba Igneous Province, central Brazil. This complex is a key example where the phoscorite-carbonatite association was preserved from metasomatism by late-stage fluids and, thus, provides an opportunity to address the genesis of phoscorites. Salitre I is composed of two main coalescing intrusions: a) a northern one dominantly containing perovskite bebedourites (B1), with swarms of ring dykes of carbonatites and phoscorites and, b) a southern one containing melanite bebedourites (B2). Salitre I phoscorites can be subdivided into P1 and P2 based on modal abundance, mineral texture, chemistry and mode of emplacement. P1 phoscorites contain essential olivine, phlogopite and apatite, with accessory magnetite and traces of perovskite whereas P2 phoscorites are composed of the same essential phases but also contain carbonate-rich pockets, accessory pyrochlore and rare phlogopite. Carbonate C-O isotopes and whole-rock Sr-Nd show that Salitre I phoscorites crystallized from mantle-derived parental magmas, similar to other APIP carbonatite complexes. However, chemical and textural discrepancies between P1 and P2 suggest derivation from different magma sources and through distinct processes. For instance, P1 Al- and Ti-rich phlogopite, accessory perovskite and general trace element geochemical behaviour can be associated with the evolution of B1 perovskite bebedourites, i.e. genesis through fractional crystallization from a bebedourite magma. P2 phoscorites, on the other hand, have their evolution linked to a magmatic phase that underwent melt immiscibility, as suggested by trace element fractionation. Additionally, P2 contains pyrochlore, which is generally lacking in bebedourites. Therefore, whereas P1 phoscorites might have derived from a bebedourite magma, P2 phoscorites were generated from a carbonated magma enriched in phosphate and iron oxides.

Sulfide ore facies, fluid inclusion and sulfur isotope characteristics of the Tappehsorkh Zn-Pb (± Ag-Ba) deposit, South Esfahan, Iran

The stratiform, stratabound Tappehsorkh Zn-Pb (±Ag-Ba) deposit, located in the southeastern part of the Malayer-Esfahan Metallogenic Belt of Iran, formed during Lower Cretaceous back-arc extension. Sulfide mineralization occurs within dolostone, black siltstone, and crystal lithic tuff and andesite associated with the Gushfil-Baghabrisham synsedimentary normal fault. Three sulfide ore facies (massive, bedded, and stockwork) occur in the deposit. Sulfide minerals are sphalerite, galena, tetrahedrite and pyrite with minor chalcopyrite and bornite, and gangue minerals are barite, dolomite and quartz. Sulfide mineralization textures are massive, replacement, vein-veinlet, laminated, disseminated, and brecciaThree mineralization stages are distinguished: early, main and post-ore. An early fine-grained sulfide ore (early ore, stage I) occurs in the bedded facies which has synsedimentary textures such as laminated, disseminated and framboidal pyrite, galena and sphalerite. Coarse-grained sulfides in the brecciated, massive replacement, and vein-veinlet textures comprise main ore stage of mineralization (main ore, stage II) and formed during diagenesis via sub-seafloor replacement of protore by mixing of hydrothermal brines with cold seawater. Late, coarse-grained sulfides (post-ore, stage III) were formed during deformation and orogenic events and occur as strain fringes, foliation-like, and spindle textures. Pervasive hydrothermal alteration styles are predominantly dolomitization and silicification and minor sericitization occurs in all host rocks in the hanging-wall of the GBF.Fluid inclusions within massive sphalerite of the main ore stage have trapping temperatures ranging from 118–199 °C, and salinities ranging from 6.2 to >23.32 wt. % NaCl equivalent. The δ34S values of the sulfide minerals from main ore stage ranges from -2.4 to -34.1‰, whereas barite ranges from +10.3 to +16.9‰. The negative values of the δ34S as well as the narrow range of δ34S in galena and sphalerite suggest that sulfur was derived by bacterial reduction of seawater sulfate.The back-arc extensional tectonic setting, presence of a normal fault controlling sulfide mineralization and hydrothermal alteration, the siliciclastic-carbonate-volcanic lithology of the host rocks, magmatism in the region (pyroclastic and volcanic rocks, that also host the sulfide mineralization), the distribution of the three ore facies, and the negative sulfur isotope values all support a sub-seafloor diagenetic replacement SEDEX-type classification for the Tappehsorkh deposit, which was then affected by Laramide (?) orogenic events which imparted the post-ore stage mineral textures.

A study of igneous rocks related to Zn–Pb mineralization in the Shinyemi and Gagok deposits of the Taebaeksan Basin, South Korea

AbstractThe Shinyemi and Gagok deposits, located in the Taebaeksan Basin, South Korea, display Zn–Pb mineralization along a contact between Cretaceous granitoids and Cambrian–Ordovician carbonates of the Joseon Supergroup. The Shinyemi mine is one of the largest polymetallic skarn‐type magnetite deposits in South Korea and comprises Fe and Fe–Mo–Zn skarns, and Zn–Cu–Pb replacement deposits. Both deposits yield similar Cretaceous mineralization ages, and granitoids associated with the two deposits displaying similar mineral textures and compositions, are highly evolved, and were emplaced at a shallow depth. They are classified as calc‐alkaline, I‐type granites (magnetite series) and were formed in a volcanic arc. Compositional variation is less in the Shinyemi granites and aplites (e.g., SiO2 = 74.4–76.6 wt% and 74.4–75.1 wt%, respectively) than in the Gagok granites and aplites (e.g., SiO2 = 65.6–68.0 wt% and 74.9–76.5 wt%, respectively). Furthermore, SiO2 vs K/Rb and SiO2 vs Rb/Sr diagrams indicate that the Shinyemi granitoids are more evolved than the Gagok granitoids. Shinyemi granitoids had been already differentiated highly in deep depth and then intruded into shallow depth, so both granite and aplite show the highly evolved similar chemical compositions. Whereas, less differentiated Gagok granitoids were separated into two phases in the last stage at shallow depth, so granite and aplite show different compositions. The amounts of granites and aplite are similar in the Shinyemi deposit, whereas the aplite appears in an amount less than the granite in the Gagok deposit. For this reason, the Shinyemi granitoids caused not only Fe enrichment during formation of the dolomite‐hosted magnesian skarn but also was associated with Mo mineralization in the Shinyemi deposit. Zn mineralization of the Gagok deposit was mainly caused by granite rather than aplite. Our data suggest that the variation in mineralization displayed by the two deposits resulted from differences in the compositions of their associated igneous intrusions.

Late Cretaceous and Cenozoic paleoceanography from north-east Atlantic ferromanganese crust microstratigraphy

Oceanic hydrogenetic ferromanganese (Fe-Mn) crusts precipitate directly from ambient seawater over millions of years. Their very slow growth rates and physio-chemical properties mean that they adsorb numerous elements from seawater. As such, they provide condensed records of seawater evolution through time that can be used for paleoceanographic reconstruction. Here, we present the results of a high-resolution, stratigraphic, textural and geochemical investigation of a core sample, obtained from a Fe-Mn crust pavement, located on the summit of Tropic Seamount in the tropical north-east Atlantic Ocean. A number of observations and interpretations are proposed, within the context of a well-constrained age model, spanning the last 75 ± 2 Myr. This core has textural stratigraphic coherence with Pacific Fe-Mn crusts formed since the Late Cretaceous, highlighting that global oceanic and climatic phenomena exert first-order controls on Fe-Mn crust development. All major hiatuses observed in the Fe-Mn crusts are contemporaneous with erosion events occurring throughout the Atlantic Ocean. High-resolution geochemical data indicate that there is variability in the composition of Fe-Mn crusts at the cm to μm scale. The dominant factors controlling this include major oceanographic events, mineral textures and micro-topography.

Petrology, geochemistry, and zircon U–Pb isotopes of Xintian Complex in Yanbian area, Northeast China: Evidence for magma mixing and geodynamics processes

Mixing/mingling of magmas is an important mode of magmatic evolution, crust–mantle interaction, and compositional exchanges, and the mafic microgranular enclaves (MMEs) in the host granites are commonly generated by the injection of hot mafic magmas into felsic reservoir at the crustal level. However, the exhaustion of most mafic melts causes the absence of the critical mantle marks after homogenization. The Xintian Complex in Yanbian area is composed of the gabbro diorite and granitic rocks enclosing the abundant MMEs, which likely provides the significant, but rare mantle end‐member information. On the basis of geochronology study, both MMEs (126.3 ± 1.2 Ma) and granodiorite (126.0 ± 2.0 Ma) were formed in the Early Cretaceous, which is coincident with the earlier emplacement of gabbro diorite (128.5 ± 1.0 Ma) within the errors. Geochemically, all of these distinguished phases have arc affinities related to subduction of oceanic slabs. Nevertheless, the dominant granodiorite rocks, typical high‐K calc‐alkaline I‐type granites, have the source of partial melting of mafic lower crust modified by subducted sediment melts, with additions of mafic melts as indicated by high Mg# values (48–56) and special textures (acicular apatite and re‐equilibrium plagioclase) of minerals in the hosted MMEs. In contrast, gabbro diorites are typical of calc‐alkaline affinities with deficit of the incompatible Zr and Hf, which were formed by re‐melting with low degree (3–10%) of mantle wedges metasomatized by subduction fluids with the crystallization of dominant pyroxene minerals. However, the MMEs is characterized by higher SiO2 (55.71–56.02 wt%), Cr (546–569 ppm), Ni (83.7–92.8 ppm), MgO (6.03–6.63 wt%), and heavy rare earth element (HREE) (546–569 ppm) with weak enrichment in Zr and Hf and lower (La/Yb)N values (17.98–20.93) compared to gabbro diorite. Therefore, the MMEs and host granodiorite are the products of heterogeneous mixing of weakly evolved mafic melts and subsolid felsic melts in the shallow storages, and this crust–mantle interaction is under a back‐arc extensional setting related to rollback of subducting palaeo‐Pacific slab.

The evolution of the Dominga Fe-Cu deposit (northern Chile): Insights from mineral textures and micro-CT analysis

The Fe-Cu Dominga deposit (2082 Mt at 23% Fe, 0.07% Cu), located in the Coastal Cordillera of northern Chile, is hosted by volcanic rocks of the Punta del Cobre Formation (131.5 ± 1.5 Ma zircon U-Pb) and into subvolcanic units (Dioritic Complex, 131.6 ± 1.0 Ma zircon U-Pb). The Fe-Cu mineralization is controlled by three structural systems which developed from a transtensional to a transpressional tectonic regime and can be divided into three groups: Early iron, Late iron and Early copper ores. Early iron ores are comprised of magnetite + pyrite + biotite breccia (1A ore), veins (1B ores), layers (1C ores) and disseminated ores (1D ores). Late iron ores are characterized by two groups of magnetite-apatite-actinolite hydrothermal breccias (2A, 2C ores) and syntaxial/antitaxial veins (2B, 2D, 2E ores). Early copper ores occur as syntaxial K-feldspar and quartz + epidote + chalcopyrite veins (3A ores), and as anhydrite + chalcopyrite-rich matrix breccia and veins (3B ores). This work presents a detailed mineral texture study of veins, hydrothermal breccias and disseminated iron-rich layers of the Dominga deposit which aims to determine fluid flow mechanisms associated with both iron and copper ores. The description of vein and breccia textural and internal structures was conducted in thin/polished sections perpendicular and parallel to the wall using optical and Scanning Electron Microscope techniques. In addition, two oriented surface samples were analyzed by computerized X-ray microtomography and numerical fluid flow simulations through the Lattice-Boltzmann method to obtain (3D) permeability anisotropy associated with early iron ores. Microtextures associated with Dominga iron and copper ores suggest that the main mass transfer fluid flow mechanism corresponds to advection (channelized and pervasive fluid flow), regardless of the tectonic regime. However, early copper ores have more complex mineral textures and internal structure due to the recurrence of crack-seal episodes. We propose that the various mineral textures and structures indicate changes in fluid flow direction over time, controlled by the permeability anisotropy of each tectonic regime. Results from numerical fluid flow simulations of early iron ore (1B) veins show a higher value of structural permeability in the vertical direction (kVz), which is consistent with a transtensional tectonic regime and the formation of vertical veins. Moreover, permeability related to 1C layered ore is higher in horizontal directions (kHx, kHy) rather than vertical (kVz) because of the natural permeability anisotropy of volcaniclastic rocks parallel to bedding. However, the kVz value suggests that the development of such layered ores also exhibits a degree of structural control at several length scales consistent with a transtensional regime. These results indicate that the occurrence of early iron ores as veins and layers may be controlled by both primary permeability anisotropy related to each lithology present at the Dominga Fe-Cu deposit and to the tectonic regime at the time. Finally, the Dominga Fe-Cu deposit attests to long lived hydrothermal activity with a transition from early fluids capable of precipitating iron ores under a transtensional system to later-stage fluids which precipitated copper ores under a transpressional regime that generated multiple crack and seal episodes.

Formation and metamorphism of sedimentary rocks beneath the Dingxi Basin: Record of the evolution from subduction to collision in the North Qilian Orogen

AbstractThe North Qilian Orogen (NQO) in northwest China underwent oceanic subduction and subsequent continental collision. Metasedimentary rocks from a deep borehole in the Dingxi Basin, NQO, contain garnet, biotite, plagioclase, quartz and minor cummingtonite and chlorite in the matrix, with inclusions of kyanite and staurolite in garnet. The mineral textures and compositions define clockwise pressure–temperature evolution with peak conditions of ~10.5 kbar and ~670°C, followed by isothermal decompression down to ~6.5 kbar. Age and Hf isotope data of detrital zircon support the formation of the sedimentary protolith in an arc setting at ~460 Ma, and the age and rare earth element characteristics of metamorphic monazite reflect exhumation at ~425 Ma. These results indicate a complete cycle of deposition–burial–exhumation for the sedimentary rocks, and directly constrain the continental collision process in NQO to yield a geotherm of ~21°C/km and to culminate before 425 Ma.

Automatic Recognition of Palaeobios Images Under Microscope Based on Machine Learning

The research of paleontology is an essential part of contemporary earth science. However, the time-consuming manual identification process has always been cumbrous in the field of paleontology. Since conventional algorithms have limited efficiency in processing images of complicated paleontological fossils. In this work, a combinational machine learning method, which comprises appropriate image preprocessing, Scale-invariant feature transform (SIFT), K-means clustering (K-means), and Support Vector Machine (SVM) are applied to realize automatic recognition of paleontological images under microscope. It is demonstrated that this combined algorithm has superior performance in morphological feature extraction in the case of complex mineral textures. With this technique, the overall average accuracy of image recognition is 84.5%, which significantly improved the efficiency of sample analysis in the field of paleontology.

Pyrrhotite oxidation as a proxy for thermal structure of eruption clouds: A comparative study of Plinian and Vulcanian eruptions of Asama and Sakurajima volcanoes, Japan

Thermal expansion of a volcanic eruption cloud by entrainment of air is a primary control of its behavior. Oxidation of the pyroclasts in the eruption cloud can be an indicator of the degree of air entrainment and thermal disequilibrium during its development. Here, the oxidation texture of sulfide minerals (pyrrhotite) in four andesitic pyroclastic eruptions of Asama and Sakurajima volcanoes was investigated, in order to highlight the contrast between the air entrainment and thermal disequilibrium processes of the Plinian and Vulcanian eruption columns. Though oxidized pyrrhotite crystals were commonly found in the pumice-fall deposit of these four Plinian and Vulcanian eruptions, significant differences in the component proportion and degree of oxidation of the pyrrhotite crystals were found between Plinian and Vulcanian products. Greater proportions of oxidized pyrrhotite crystals in the Asama 1783 and Sakurajima 1914 Plinian pumice deposits than those in the Asama 2004 and Sakurajima 2010 Vulcanian eruptions indicate that heat retainment of pumice clasts and the continuous ejection of eruption clouds in the Plinian eruptions kept pumice in the eruption cloud at high temperature during the entrainment of air and expansion of the cloud. In contrast, the rapid thermal equilibrium of pumice clasts and the short-lived episodic ejection of Vulcanian eruptions reduced the temperature of pumice in the eruption cloud quickly. This suggests that pyrrhotite crystals have the potential to be indicators of the development of temperature inside an eruption cloud, reflecting the process of thermal disequilibrium of pyroclasts and of air entrainment.

Relationships Between Eclogite‐Facies Mineral Assemblages, Deformation Microstructures, and Seismic Properties in the Yuka Terrane, North Qaidam Ultrahigh‐Pressure Metamorphic Belt, NW China

AbstractTo understand the relationships between eclogite‐facies mineral assemblages, deformation microstructures, and the seismic properties of subducting oceanic crust, eclogites from the Yuka terrane, North Qaidam ultrahigh‐pressure metamorphic belt, NW China, were studied. Observations of mineral textures, deformation microstructures, and petrofabrics in the eclogites indicate that garnet, omphacite, and phengite were deformed by intracrystalline deformation (i.e., dislocation creep) during prograde metamorphism. In contrast, amphibole, which was formed by the topotactic replacement of omphacite at fluid‐present conditions, is considered to have been deformed by diffusional flow (dissolution‐precipitation creep) during amphibolite‐facies retrogression associated with exhumation. Based on the petrofabrics in the samples, the seismic properties of the eclogites were calculated depending on eclogite‐facies mineral assemblages such as garnet + omphacite, garnet + omphacite + phengite, garnet + omphacite + phengite + lawsonite, garnet + omphacite + phengite + amphibole, and garnet + omphacite + amphibole. We found that the seismic signatures of each of the eclogite‐facies mineral assemblages were different. In particular, phengite‐bearing eclogites (the garnet + omphacite + phengite/garnet + omphacite + phengite + amphibole assemblages), depending on phengite content, produced the strongest seismic anisotropy (AVp and AVs), with a strong polarization anisotropy, that was at least three times higher than bimineralic (phengite‐absent) eclogites (garnet + omphacite assemblage). Our results indicate that phengite, as a stable phase at high‐pressure and high‐temperature conditions, can play an important role in the creation of trench‐parallel seismic anisotropy in the eclogite‐facies mineral assemblages found in subduction zones.

A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H2O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and vice-versa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and, in most cases, more evolved amphiboles present in the lava flows. Mineral thermo-barometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710–790 °C; 5.6–6.5 wt% H2O) than their explosive counterparts (815–850 °C; 4.2–4.6 wt% H2O). At storage pressures of 1.5–2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at >5 wt% H2O. Therefore, it is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved volatiles in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions.