Feldspar Crystals Research Articles

Evacuation of multiple magma bodies and the onset of caldera collapse in a supereruption, captured in glass and mineral compositions

Complexities in the nature of large-scale silicic eruptions and their magmatic systems can be discerned through micro-analytical geochemical studies. We present high-resolution, stratigraphically constrained compositional data on glassy matrix material and feldspar crystals from the initial fall deposits and earliest ignimbrite (base of member A) of the 2.08 Ma, ~ 2500 km3 Huckleberry Ridge Tuff (HRT), Yellowstone. We use these data to document the nature of the magmatic system and compositional changes related to the transition from fall to widespread ignimbrite deposition, inferred to mark the onset of caldera collapse. Although major element glass compositions are relatively uniform, trace elements span a large range (e.g. Ba 10–900 ppm, Sr/Rb = 0.005–0.09), with highly evolved glasses dominating in the fall deposits. Several trace elements (e.g. Ba and light rare earth elements) in the glass samples serve to define statistically significant compositional clustering in the fall deposits and basal ignimbrite. These clusters are inferred to reflect melt compositions controlled by fractional crystallisation processes and are interpreted to represent multiple, discrete melt-dominant domains that were tapped by multiple vents. The onset of widespread ignimbrite deposition is marked by an increase in the number of erupted melt compositional clusters from four in the fall deposits to eight, representing nine melt-dominant domains. There is an absence of geographical variation of glass compositions within the basal ignimbrite, with samples from proximal to distal localities north, west and south of the HRT caldera exhibiting similar variability. Pairing of glass analyses with sanidine major and minor element compositional data suggests that the nine melt compositional domains converged at depth into two compositionally distinct upper-crustal magmatic lineages that were both active during these early stages of the eruption. Our data collectively indicate the evacuation of an exceptionally complex and heterogeneous magma system. The onset of widespread ignimbrite deposition, inferred to relate to caldera collapse, occurred after ~ 50 km3 of magma had been discharged. Although external controls were important as an eruption trigger, depressurisation of the system led to caldera collapse with the eruption of numerous discrete melt-dominant domains.

Petrogenesis of the late Mesozoic highly fractionated I-type granites in the Luanchuan district: implications for the tectono-magmatic evolution of eastern Qinling

Late Mesozoic granites are extensively distributed in the Luanchuan district of eastern Qinling and can be divided into two types: Late Jurassic to Early Cretaceous granites (ore-related plutons) and Late Cretaceous granites (Laojunshan batholith). This study presents new geochemical and zircon U-Pb-Hf isotopic data from the Shibaogou and Yuku plutons to provide robust constraints on the petrogenesis and tectonic significance of the late Mesozoic granites in the Luanchuan district. Zircon U-Pb dating results yielded weighted mean 206Pb/238U ages of 149.1 ± 0.8 Ma and 150.5 ± 0.8 Ma, which were interpreted as the crystallization ages of the Shibaogou and Yuku plutons, respectively. We propose that the late Mesozoic granites contain high concentrations of SiO2 and alkali elements (Na2O + K2O) and feature metaluminous to weakly peraluminous characteristics. Enrichment in light rare earth elements and large ion lithophile elements and depletion in high field strength elements are observed. Mineralogical and geochemical evidence reveal that the late Mesozoic granites are highly fractionated I-type granites with fractional crystallization of feldspar, plagioclase and accessory minerals (e.g., apatite and titanite or magnetite). Based on the Hf composition, we suggest that the parental magmas of the ore-related plutons were derived from remelting of the Taihua and Xiong’er groups with minor contributions of mantle-derived materials and that the Laojunshan batholith was generated by the hybridization of ancient crust- (Kuanping group) and mantle-derived components. Collectively, the above arguments indicates a tectonic transition from compression to post-collisional extension during the late Mesozoic, that was likely triggered by the continental collision of the North China Block and the Yangtze Block, which generated numerous contemporaneous granites and Mo-W-Pb-Zn-Ag-Au poly-metallic deposits.

Magmatic evolution and controls on rare metal-enrichment of the Strange Lake A-type peralkaline granitic pluton, Québec-Labrador

Although it is well known that A-type granites are enriched in the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic processes that concentrate these elements are still poorly understood. The 1.24 Ga Strange Lake pluton in northern Québec-Labrador provides an extraordinary example of hyper-enrichment in the REE, Zr, and Nb in a peralkaline A-type granite. The pluton consists of two hypersolvus granite units (southern and northern) and a transsolvus granite, all of which contain perthitic alkali feldspar as the earliest major mineral; the transsolvus granite also contains separate albite and microcline crystals. Arfvedsonite, a sodic amphibole, occurs exclusively as phenocrysts in the transsolvus granite, whereas in the hypersolvus granite it is present as a late, interstitial phase. The primary HFSE minerals are zircon, monazite-(Ce), gagarinite-(Ce) and the pyrochlore group minerals.Magma evolution was monitored by the alumina content in the bulk rock, which decreases from the southern to the northern hypersolvus granite and is lowest in the transsolvus granite. Alkalinity indices and bulk Si, Fe, Rb, REE, Zr, Nb concentrations show the opposite trend. Alkali feldspar compositions mirror the trend shown by the bulk rock, i.e., decreasing Al contents are accompanied by increasing Si, Fe3+, REE, Zr and Nb contents.The major driving forces for the evolution of the hypersolvus magma prior to emplacement were the early separation of a fluoride melt from the silicate melt and the crystallization of alkali feldspar and HFSE-rich phases (zircon, monazite-(Ce), pyrochlore group). An alkali feldspar-rich crystal-mush containing LREE-fluoride melt droplets was emplaced as the least evolved southern hypersolvus granite. Massive fractionation of alkali feldspar led to a sharp increase in ƒH2O and F− activity in the magma chamber that triggered the crystallization of arfvedsonite and was followed by emplacement of the northern hypersolvus granite, which contained a higher proportion of LREE-fluoride melt droplets. Further evolution in the magma chamber led to a transition from a miaskitic to an agpaitic composition. The transsolvus granite was intruded in the form of a low viscosity crystal mush of alkali feldspar, quartz, arfvedsonite (after appreciable crystallization of arfvedsonite) and LREE-fluoride melt droplets. Upon emplacement, arfvedsonite (and gagarinite-(Ce)) crystals segregated as cumulates in response to a combination of flow differentiation and gravity settling. The immiscible fluoride melt accumulated in a volatile-rich residual silicate magma, which migrated to the top of the pluton where it formed the F-REE-rich cores of highly mineralized pegmatites.

Phase Equilibria of Pantelleria Trachytes (Italy): Constraints on Pre-eruptive Conditions and on the Metaluminous to Peralkaline Transition in Silicic Magmas

The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites), which occur in many continental rift systems, oceanic islands and continental intraplate settings, is unclear. To fill this gap, we have performed phase equilibrium experiments on two representative metaluminous trachytes from Pantelleria to determine both their pre-eruptive equilibration conditions (pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments were performed in the temperature range 750-950 °C, pressure 0·5-1·5 kbar and fluid saturation conditions with XH2O [=H2O/(H2O+CO2)] ranging between zero and unity. Redox conditions were fixed below the nickel-nickel oxide buffer (NNO). The results show that at 950 °C and melt water contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures. Clinopyroxene is the liquidus phase, being followed by iron-rich olivine and alkali feldspar. Comparison of experimental and natural phases (abundances and compositions) yields the following pre-eruptive conditions: P=160·5 kbar, T=925±25 °C, H2Omelt = 261 wt %, and fO2 between NNO- 0·5 and NNO- 2. A decrease in temperature from 950 °C to 750 °C, as well as of H2Omelt, promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron-bearing minerals show gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk-rock compositions, residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions, owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading to a consequent increase in the peralkalinity index [PI = molar (Na2O+K2 O)/Al2 O3 ]. This is the first experimental demonstration that peralkaline felsic derivatives can be produced by low-pressure fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic magmas of basalt-trachyte-rhyolite igneous suites require at least 95 wt % of parental basalt crystallization, consistent with trace element evidence. Redox conditions, through their effect on Fe-Ti oxide stabilities, control the final iron content of the evolving melt.

Изменение строения поверхности гетерогенного тела (диорита) при трении

AbstractRaman, infrared, and photoluminescent spectroscopy has been used to study changes in the surface structure of quartz diorite caused by friction. Before friction, the diorite surface layer has contained mainly quartz and feldspar crystals. After friction, some quartz and feldspar crystals have been destructed and replaced by a newly formed hydromica mineral with a low friction coefficient such as illite.

Changes in the surface structure of the heterogeneous body (diorite) under friction

Raman, infrared, and photoluminescent spectroscopy has been used to study changes in the surface structure of quartz diorite caused by friction. Before friction, the diorite surface layer has contained mainly quartz and feldspar crystals. After friction, some quartz and feldspar crystals have been destructed and replaced by a newly formed hydromica mineral with a low friction coefficient such as illite.

Биогеохимические особенности донных отложений прибрежной зоны Черного моря

The article considers biogeochemical features of accumulation, redistribution and migration of heavy metals in bottom sediments in the conditions of oil pollution of coastal water areas. The Black Sea, like other parts of the world's oceans, is characterized by an increased content of pollutants in the coastal zone. The purpose of the study was to study the influence of the anthropogenic factor on the biogeochemical features of the bottom sediments of the coastal zone of the Black Sea. The research project is the bottom sediments of the Sevastopol Bay, as zones with increased anthropogenic load. For comparison, samples selected in the clean coastal zone of the Karadag Nature Reserve were used. Selection of the surface layer of bottom sediments was carried out according to a standard procedure. Samples were selected in 2013. The mineral composition of the selected samples was evaluated from x-ray phase analysis data. Concentration of chemical elements in samples was determined on mass spectrometer with inductively coupled plasma (ICP-MS) in the M.P. Semenenko Institute of geochemistry, mineralogy and ore formation of the National Academy of sciences of Ukraine. The investigated sediments are represented by sands with an insignificant content of clay fraction. The sand fraction is mainly represented by crystals of calcite, quartz and feldspars with an admixture of amorphous silica particles. The mineral composition of the silt fraction of bottom sediments is represented by clay minerals – montmorillonite, kaolinite, illite, and also finely dispersed calcite and quartz. The amount of organic substances did not exceed 1–1.5%. A significantly larger amount of organic matter was accumulated in the clay fraction of the bottom sediments of the Sevastopol Bay and amounted to about 3%. A distinctive feature of the silt of the Sevastopol Bay is the high degree of contamination with organic substances. The presence of clay minerals has a significant effect on the absorption of oil products by silt deposits. The influence of these processes on the microbiological composition of bottom sediments was investigated. It is shown that the natural biological processes of destruction of organic substances, leading to the formation of humic compounds, cause the fixation of heavy metals on these products and lead to a change in the microbiological composition of bottom sediments, which can disrupt the ecological balance of coastal water areas. The features of the content and distribution of microorganisms (aerobes, anaerobes and heterotrophs) are largely related to the physico-chemical properties of bottom sediments, the features of adsorption, sedimentation and accumulation of various metals in mud.

Nature of the magma storage system beneath the Damavand volcano (N. Iran): An integrated study

Damavand intraplate stratovolcano constructed upon a moderately thick crust (58–67km) over the last 2Ma. The erupted products are dominantly trachyandesite-trachyte (TT) lavas and pyroclasts, with minor mafic magmas including tephrite-basanite-trachybasalt and alkali olivine basalts emplaced as cinder cones at the base of the stratovolcano. The TT products are characterized by a mineral assemblage of clinopyroxene (diopside-augite), orthopyroxene (clinoenstatite), feldspar (An2–58, Ab6–69, Or2–56), high Ti phlogopite, F-apatite, FeTi oxides, and minor amounts of olivine (Fo73–80), amphibole and zircon, whereas olivine (Fo78–88), high Mg# (80–89) diopside, feldspar, apatite and FeTi oxide occur in the mafic magmas. The presence of hydrous and anhydrous minerals, normal zonings, mafic cumulates, and the composition of magmatic inclusions in the TT products suggest evolutionary processes in polybaric conditions. In the same way, disequilibrium textures - including orthopyroxene mantled with clinopyroxene, reaction rim of phlogopite and amphibole, the coexistence of olivine and orthopyroxene, reverse, oscillatory and complex zonings of pyroxene and feldspar crystals - suggest magmatic evolutions in open systems with a varying temperature, oxygen fugacity, water as well as pressure and, to a lesser extent, melt chemistry. Mineral assemblages are used to model the physicochemical conditions and assess default parameters for the thermodynamic simulation of crystallization using MELTS software to track the P-T-H2O-ƒO2 evolution of the magma plumbing system. Thermobarometry and MELTS models estimated the initial nucleation depth at 16–17kb (56–60km) for olivine (Fo89) and high Al diopside crystals occurring in the mafic primary magma; it then stopped and underwent fractionation between 8 and 10kb (28–35km), corresponding with Moho depth, and continued to differentiate in the lower crust, in agreement with the geophysical models. The mafic rocks were formed by crystal fractionation from the reconstructed primary magma (13wt% MgO) with the minor role of recharge and crustal assimilation. Phenocrysts in TT lavas recorded a wide range of temperature and pressure of crystallization; at least three main levels of magma storage can be recognized according to the statistical analysis of the models, at 6–8kb (22–28km), 4–6kb (15–22km), and 0.6–3kb (2–11km), respectively. The temperature of crystallization ranged from 1430 to 1180°C for primary mafic magma to alkali olivine basalts and 1180–800°C for TT suite. According to the current geophysical models, the present structure of the magma storage system in the crust beneath the Damavand volcano consists of three major accumulation zones located at 20km, 6–8km and 3–4.5km depth. Our data enlarge this scenario, suggesting a more complex magma storage system strongly controlled by the transpressional tectonic regime. Multi-depth magma reservoirs may account for the local thickening of crust below the volcano. The polybaric fractionation model, using the MELTS algorithm, reproduces mineralogy and chemical variations of minerals and whole rock of the Damavand TT lavas. However, some discrepancies between major elements of models and trends of data can be ascribed to the recharge of more mafic magma, minor crustal assimilation, disaggregation of crystal-rich mushes and uptake of magmatic inclusions, as well as crystals from different crustal levels. The polybaric differentiation as the fractionation and/or accumulation of crystals was the probable mechanism for explaining the scarcity of mafic volcanic rocks at the Damavand volcano.

REE-Th-U and Nd isotope systematics of monazites in magnetite- and ilmenite-series granitic rocks of the Japan arc: Implications for its use as a tracer of magma evolution and detrital provenance

Monazite [(Ce, La, Th) PO4], a common accessory mineral in low-Ca felsic rocks and a wide range of metamorphic rocks, is suitable for in situ U–Pb dating, rare earth element (REE) geochemistry, and Nd isotope tracer studies. To link monazite composition with igneous petrogenesis and differentiation, we carried out comprehensive LA-ICP-MS measurements of REE-Th-U abundances and Sm-Nd isotope ratios in monazites from magnetite-series (oxidized type) and ilmenite-series (reduced type) granitic rocks across the Japan arc. The data revealed systematic differences in monazite composition between magnetite-series and ilmenite-series samples, and between pegmatites and granites. In ilmenite-series granitic rocks, monazites from pegmatites showed larger negative Eu anomalies, lower light-REE/middle-REE ratios, and higher middle-REE/heavy-REE ratios than those from granites. These geochemical variations were attributed to significant fractional crystallization of feldspars, monazite, xenotime, and garnet during differentiation in relatively reduced peraluminous granitic magmas. In contrast, there was no remarkable difference in the REE fractionation pattern between magnetite-series pegmatites and granites. Furthermore, the magnitudes of the negative Eu anomalies in the magnetite-series monazites were smaller than those observed in the ilmenite-series samples. These features were interpreted to reflect the suppression of monazite, xenotime, and garnet fractionation and limited Eu incorporation into fractionating feldspars in relatively oxidized and non-peraluminous magmas. In addition, our data demonstrated intra-grain variations in REE-Th-U compositions in a monazite from the Masaki ilmenite-series pegmatite, despite its homogenous Nd isotope composition. These variations suggest that the core and rim of the grain crystallized from distinct magmas that were generated from similar crustal rocks, but were differentiated under different conditions. A comparison of the present data with previously reported data indicates that igneous monazites from granitic rocks are distinct from a wide range of metamorphic monazites in REE-Th-U composition, especially due to their larger negative Eu anomalies. This finding highlights the potential utility of monazite REE-Th-U systematics as a provenance indicator for detrital monazites.

Using thermoluminescence signals from feldspars for low-temperature thermochronology

Abstract Natural thermoluminescence (TL) signals from feldspar crystals extracted from thermally stable drill cores ( T = − 4.1 − 60.2 ∘ C) exhibit a strong dependence on geologic and laboratory thermal conditions. As burial temperature increases, the position of the TL glow curve at half-maximum intensity (i.e., the T 1 / 2 parameter) shifts to higher measurement temperatures. This shift is also observed following isothermal treatments in the laboratory. This relationship can be explained using a kinetic model originally developed for the optical luminescence dating of feldspar grains. The thermal history of a sample is preserved in the degree of electron trap saturation as a function of thermal detrapping probability, which varies with recombination distance. A natural feldspar sample contains a range of thermal stabilities: the least stable traps will remain empty, the most stable will be full, and those traps which are partially filled will, in the case of thermal equilibrium, be diagnostic of the storage temperature. The T 1 / 2 parameter of a TL glow curve reflects which sites remain occupied. This interpretation is further borne out by additive dose measurements which illustrate that samples buried at lower temperatures are fully saturated at lower TL measurement temperatures ( ∼ 200 − 300 ∘ C) relative to warmer samples. This signal is estimated to be useful in rapidly-cooling bedrock and should grow measurably for ∼ 10 2 − 10 6 years.

Variation in the structure of the surface layer of a heterogeneous solid (gneiss) on a shear

The structure of surface layers with a thickness of ~1 μm formed at destruction of gneiss is studied by means of photoluminescent and infrared spectroscopy. It is found that, in this layer, feldspar (plagioclase and microcline) crystals are completely destroyed and replaced by montmorillonite.

Mafic replenishment of multiple felsic reservoirs at the Mono domes and Mono Lake islands, California

The Mono Basin has been the site of frequent volcanic activity over the past 60,000 years, including the emplacement of the Mono domes and Mono Lake islands. The Mono Basin lavas are the youngest and most poorly understood products of the Long Valley Volcanic Field. We have undertaken a study of Mono Basin volcanism encompassing whole-rock major and trace element, Sr, Nd, Pb, and O isotopic, and electron microprobe glass, plagioclase, and amphibole analyses. Variations in major and trace elements suggest that fractional crystallization of feldspar (Sr, K2O), apatite (P2O5), titanomagnetite (V), zircon (Zr), and allanite (La, Ce) has influenced the evolution of the Mono Basin lavas. Field observations, petrography, and chemistry together demonstrate that injection of more mafic magma is a common process throughout the Mono Basin. Mafic enclaves of the Mono domes are stretched and rounded, with chilled margins between enclave and host rhyolite. Thin sections reveal millimeter-scale inclusions of rhyolite in the enclaves and vice versa along the host-enclave border. Paoha Island dacite has glass with 67–72 wt% SiO2 and contains microscopic clots of more mafic glasses, with SiO2 contents as low as 64 wt%. Isotopically, the June Lake and Black Point basalts and the Mono dome enclaves represent the least evolved material in the Long Valley Volcanic Field, with 87Sr/86Sri 0.5126. The silicic Mono Lake lavas and Mono dome rhyolites display a significant crustal component, with 87Sr/86Sri >0.7058 and 143Nd/144Nd 19 and δ18O >+6.5‰. The Mono Lake lavas generally are younger and less evolved than the Mono domes, with enrichment in trace elements including Ba and Sr accompanied by lower 143Nd/144Nd and higher 206Pb/204Pb. This implies that the Mono domes and the Mono Lake lavas are derived from different magma batches, if not from separate magma chambers. There is no systematic relationship between the degree of chemical evolution and the lava ages, indicating that several magma batches have been involved in the development of the Mono domes complex. Pronounced differences in trace element composition (Nb, Y) and isotopic values between the Negit Island and Paoha Island lavas indicate that they, too, are produced by the evolution of at least two different batches of intermediate-composition magma.

TheREE-and HFSE-bearing phases in the Itatiaia alkaline complex (Brazil) and geochemical evolution of feldspar-rich felsic melts

AbstractThe Late Cretaceous Itatiaia complex is made up of nepheline syenite grading to peralkaline varieties, quartz syenite and granite, emplaced in the metamorphic rocks of the Serra do Mar, SE Brazil. The nepheline syenites are characterized by assemblages with alkali feldspar, nepheline, Fe-Ti oxides, clinopyroxene, amphibole, apatite and titanite, while the peralkaline nepheline syenites have F-disilicates (rinkite, wöhlerite, hiortdahlite, låvenite), britholite and pyrophanite as the accessory phases. The silica-oversaturated rocks have alkali feldspar, plagioclase, quartz, amphibole, clinopyroxene and Fe-Ti oxides; the chevkinite-group minerals are the featured accessory phases and are found with allanite, fluorapatite, fluorite, zircon, thorite, yttrialite, zirconolite, pyrochlore and yttrocolumbite. The major- and trace-element composition of the Itatiaia rocks have variations linked to the amount of accessory phases, have smooth, enriched chondritenormalized rare-earth element (REE) distribution patterns in the least-evolved nepheline syenites and convex patterns in the most-evolved nepheline syenites. TheREEdistribution patterns of the quartz syenites and granites show a typical pattern caused by fractional crystallization of feldspar and amphibole, in an environment characterized by relatively high oxygen fugacity (>NiNiO buffer) and high concentrations of H2O and F, supporting the crystallization of hydrous phases, fluorite and F-disilicates. The removal of small amounts of titanite in the transition from the least-evolved to the most-evolved nepheline syenites stems from petrogenetic models involvingREE, and is shown to be a common feature of the magmatic evolution of many other syenitic/ trachytic/ phonolitic complexes of the Serra do Mar and elsewhere.

Prograde and near-peak zircon growth in a migmatitic pelitic schist of the southeastern Canadian Cordillera

U–Pb dates and trace elements acquired simultaneously on zircon from a migmatitic pelitic schist from the southeastern Canadian Cordillera are linked with results of phase equilibria modeling. The first of three phases of zircon growth occurred between 77 and 72Ma along a prograde path in the kyanite+melt field between the wet-melting and muscovite-dehydration melting reactions (675–775°C). This protracted phase of prograde growth is characterized by a shallow positive slope in heavy rare earth elements (HREE). The second phase occurred at c. 69Ma, after significant garnet growth by biotite dehydration-melting at 775–875°C, which produced zircon depleted in HREE. The last phase occurred at c. 63Ma on the retrograde path as garnet and rutile were consumed at >780°C, as indicated by enrichments in HREE and Nb, but prior to feldspar crystallization in the melt, as suggested by the moderate Eu anomaly. This path is consistent with rutile and monazite petrogenesis, documented herein and in a previous study, respectively. In contrast with the high temperatures at which zircon grew (675–875°C), Ti-in-zircon thermometry yields unrealistically low estimates (470–739°C) with 80% below the wet solidus; there are no uncertainties in the buffering assemblage, as rutile and quartz were present throughout, and no effect of pressure because a pressure-dependent calibration was used. This study demonstrates that zircon growth along a prograde path is possible and warrants further investigation on factors controlling the Ti content of zircon thermometry in metamorphic rocks.

Mineralogy, geochemistry and genesis of clays interlayered coal seams succession in the Neogene lacustrine Seyitömer coal deposit, Kütahya, western Turkey

The Seyitömer coal deposit is an important source of energy in Turkey. This study aims to determine mineralogical, geochemical characteristics, and genesis of clays associated with coal seams in order to identify their precipitation environment. The Late Miocene-Pliocene units comprise organic-rich shale, organic-rich marl, marl, coal seam, siltstone and sandstone deposited in a fluvial and lacustrine environment, with associated tuff and tuffite in the Seyitömer coal deposit. Serpentinite, sandstone, organic-rich shale and tuffaceous host rocks of the coal deposit are partly to completely argillized and partially carbonate cemented having also stockwork type silica in-fill fissures. Smectite, illite, and kaolinite are associated with quartz, feldspar, olivine, gypsum and pyrite. Oriented flaky illite with irregular outlined kaolinite, and smectite flakes edging and coating relict feldspar and volcanic glass grains developed by a dissolution-precipitation mechanism in an alkaline micro-environment during deposition and diagenetic process. The occurrence of subrounded microorganisms on resorbed feldspar and the coexistence of smectite/illite flakes with organic matter, pyrite, and siderite indicates that degradation of feldspar and devitrification of volcanic glass is biologically mediated. The slight enrichment of heavy rare earth elements (HREE) compared to light rare earth elements (LREE) with a positive Eu and Ce anomalies reflects fractional crystallization of feldspar and amphibole. Alteration of feldspar, biotite, serpentine and volcanic glass resulted in the concentration of Al, Fe, Mg, and K in a stagnant environment and precipitation of beidellitic smectite and illite in an alkaline environment. Kaolinite precipitated in an acidic open hydrological system. An increase in Ni/Co and V/(V+Ni) ratios in altered units also suggest anoxic and anoxic to dysoxic conditions. Occurrence of Pityosporites microalatus (Pinus), Laevigatosporites haardti, Pityosporites spp., Polyvestibulopollenites verus (Alnus), Compositae and Graminae in the Seyitömer coal deposit suggest subtropical to warm-temperature humid climate during Late Oligocene-Middle Miocene period. Association of argillization and silicification with coal, microorganisms, pyrite, and gypsum indicates a swampy environment and partial hydrothermal processes. This interpretation is corroborated by the calculated formation temperatures from stable isotope analysis of 30–463°C for smectite and 140–156°C for illite. The high δ34S isotope values (16.8–33.3‰) indicate a mixed diagenetic and hydrothermal origin for the sulphide and sulfate minerals.

Composite dikes in four successive granitoid suites from Transbaikalia, Russia: The effect of silicic and mafic magma interaction on the chemical features of granitoids

In the study area of 60 by 15km, located south of Lake Baikal, coeval mafic rocks form small intrusive bodies, synplutonic dikes and composite mafic-felsic dikes in four successive Late Paleozoic and Early Mesozoic syenite and granite suites. Mafic rocks in all suites are monzogabbro with similar mineral and chemical composition. This attests that mantle-derived mafic magmas produced at post-collisional and within-plate tectonic settings were not noticeably distinguished in chemical composition within a time interval of about 60–65m.y. Detailed study of the interaction zones between synplutonic mafic magma and host syenite at two key localities allowed recognition of syenite remelting stage at about 900–950°C. Remelting was accompanied by Sr and Ba enrichment by diffusion from the adjacent mafic magma. Diffusive transport of Ba also caused enhanced BaO content, 0.37–0.54wt.%, in alkali feldspars of the newly-formed syenite. Remelting of the host syenite was followed by mingling, chemical interaction and mixing of mafic and silicic magmas within the contact zone. The results of petrographic and geochemical study of the composite and synplutonic dikes are used to interpret the chemical features of the large, about 1000 km2, essentially syenitic Ust-Khilok pluton. Two varieties of syenite are recognized in the pluton, one with Ba<2000ppm and another with Ba=2000–4000ppm. The data suggest the combined effects of two processes: (1) Ba enrichment of silicic melt by diffusion from the comingling mafic magma enclaves and synplutonic dikes and (2) accumulation of alkali feldspar crystals in the course of fractional crystallization evidenced by positive Eu anomalies in cumulate-enriched syenites. It is assumed that an excess of accumulated feldspar crystals caused a decrease in the proportion of melt per unit volume; this resulted in a reduced amount of zircon and, consequently, Zr from 400–700ppm in cumulate-free to 80–150ppm in cumulate-rich syenite.

Permian–Triassic highly-fractionated I-type granites from the southwestern Qaidam Basin (NW China): Implications for the evolution of the paleo-tethys in the eastern Kunlun orogenic belt

Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and crustal growth process in the Qaidam Basin. This paper reports Permian–Triassic granites from the Kunbei area, southwestern Qaidam Basin. Detailed zircon LA-ICP MS U-Pb dating reveals that the granites from the four drilling cores (q404, q406, q1612-8, q1613-8) have identical ages of 251±3, 256±4, 247±2, and 251±6 Ma, respectively, these ages are identical with the Permian–Triassic granites from the eastern Qaidam Basin. Detailed geochemical analyses indicate that these granites display typical affinities of highly-fractionated I-type granites: (1) they have high SiO2 (up to 76.5 wt.%), Na2O+K2O (7.91 wt.% to 9.48 wt.%) contents and high FeOT/MgO values of 4.7 to 9.3, suggesting significant fractional crystallization; (2) their low A/CNK values of 0.54 to 1.03, no normative Al-rich minerals, inconsistent with the per-aluminous S-type granites; (3) their low Ga (14.5 ppm to 20.7 ppm) and 10 000×Ga/Al (2.23 to 3.03, most of them <2.6) values are inconsistent with the A-type granites; (4) the high Rb (191 ppm to 406 ppm) contents and Rb/Sr (2.1 to 13.4) ratios, as well as the significant negative Eu anomalies (0.10 to 0.42) also indicate significant fractional crystallization of feldspars; (5) their low P2O5 contents (0.02 wt.% to 0.10 wt.%) suggest the limited solubility of phosphorus in primitive metaluminous melts. In combination with the geological background, we propose that the Permian–Triassic highly-fractionated I-type granites resulted from partial melting of intra-crustal mafic rocks, and the primitive I-type granitic melts underwent significant fractional crystallization of feldspars. The occurrence of highly-fractionated I-type granites in the southwestern Qaidam Basin suggests a Permian–Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.

Geochemistry and growth morphology of alkali feldspar crystals from an IAB iron meteorite – insight into possible hypotheses of their crystallization

Alkali feldspar crystals have been recognized in the troilite-graphite nodules of the Morasko IAB iron meteorite. Their chemical, microtextural and structural properties were studied using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), transmission electron microscopy (TEM) and Raman spectroscopy. The feldspars occur as perthitic or antiperthitic intergrowths, whereas the albite lamellae are perfectly twinned. The structural properties reveal intergrown phases with fairly disordered patterns. The electron microprobe analyses demonstrate that the intergrown phases are mainly rich in sodium or potassium, resulting in compositions that are close to those of albite or orthoclase. The compositions, calculated on the basis of a segmented perthite-antiperthite image, showed that the Or-to-Ab proportions in the homogenized crystals were almost 0.3:0.7, thus indicating that the anorthoclase crystallized under high-temperature conditions. Two hypotheses of crystal formation could account for these characteristics: crystallization from a melt or from a metasomatic solution. Relics with evidence of metasomatic replacement of former minerals were not found. Accordingly, this work focuses on arguments that support the other hypothesis. Large ion lithophile elements (LILEs, e.g., Ba, Sr, Rb, LREE, Pb, and Ga) were used to track the origin of the crystals. Their concentrations indicate crystallization from a parent melt strongly depleted in LILEs. Alkali feldspar is commonly a product of a highly differentiated melt. However, highly differentiated melts are typically enriched in LILEs, which here is not the case. The melt that crystallized the feldspar cannot be related to impact-induced partial melting of the chondritic material alone. The derived melt probably was contaminated by silica-rich target material during interaction between the IAB projectile and the target material and was accompanied by metal and sulphide melts that were both immiscible with a silicate melt.

Multi-scale study of the polishing behaviour of quartz and feldspar on road surfacing aggregate

ABSTRACTIn this paper, the exact mechanisms as to how petrographic properties of aggregate influence the polishing behaviour quantitatively were identified. Since the aggregate is composed of the different rock-forming minerals, the analyses conducted in this research project focused not only on the aggregate but also on crystals/minerals. The hardest and most abundant rock-forming minerals found on earth – quartz and feldspar – were investigated with polishing tests with the Aachen Polishing Machine on granite aggregates from four sources which exhibit different mineralogical properties. Changes of the aggregate texture and minerals were studied based on texture measurements and skid resistance measurements obtained by means of the Pendulum Test and the Wehner/Schulze device. The influences of mineral composition and crystal size on the changes of the micro-texture and skid resistance of the aggregates were determined by comparing the four granites. The polishing and wearing behaviour of quartz and various feldspars (albite, microcline and plagioclase) were characterised quantitatively by means of a spectral analysis and abrasion analysis. The results show that the micro-texture of exposed minerals is greatly smoothened on wavelengths from 100 to 500 μm. Fine-grained quartz or feldspar crystals tend to be rougher than medium of large-sized crystals. Quartz exhibits more favourable properties than feldspar between 100 and 500 μm; this is especially true for fine quartz crystals. The correlation that more content of coarse-grained quartz leads to high friction value cannot be proved in this research.

Mineral composition control on inter-mineral iron isotopic fractionation in granitoids

This study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four Dabie I-type granitoids to evaluate the factors that control inter-mineral Fe isotopic fractionation in granitoids. The order of heavy iron isotope enrichment is feldspar>pyrite>magnetite>biotite≈hornblende. Feldspar has heavier iron isotopic compositions than its co-existing magnetite (Δ56Feplagioclase–magnetite=+0.376‰ to +1.084‰, Δ56Fealkali-feldspar–magnetite=+0.516‰ to +0.846‰), which can be attributed to its high Fe3+/Fetot ratio and low coordination number (tetrahedrally-coordinated) of Fe3+. Δ56Femagnetite–biotite of coexisting magnetite and biotite ranges from 0.090‰ to 0.246‰. Based on homogeneous major and iron isotopic compositions of mineral replicates, the inter-mineral fractionation in this study should reflect equilibrium fractionation. The large variations of inter-mineral fractionation among feldspar, magnetite and biotite cannot be simply explained by temperature variation, but strongly depend on mineral compositions. The Δ56Feplagioclase–magnetite and Δ56Fealkali-feldspar–magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali-feldspar, respectively. This could be explained by different Fe–O bond strength in feldspar due to different Fe3+/∑Fe or different crystal parameters. The Δ56Femagnetite–biotite increases with decreasing Fe3+/∑Febiotite and increasing mole (Na+K)/Mgbiotite, indicating a decrease of β factor in low Fe3+/∑Fe and high (Na+K)/Mg biotite.High-silica leucosomes from Dabie migmatites with a feldspar accumulation petrogenesis have higher δ56Fe values (δ56Fe=0.42–0.567‰) than leucosome that represents pristine partial melt (δ56Fe=0.117±0.016‰), indicating that accumulation of feldspar could account for high δ56Fe values of these rocks. High δ56Fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals, e.g., anorthosites. Feldspar accumulation, however, cannot explain high δ56Fe values of most high-silica granitoids reported in the literature, based on their low Sr, Ba contents and negative Eu anomalies.