Plagioclase Inclusions Research Articles

Resolving the “megacryst paradox”: Feldspar orientation relationships record crystal mobility in granites

K-feldspar megacrysts are common in silicic plutons, but there is a long-running debate around how they form and what their presence tells us about magmatic systems. Field, textural, and geochemical evidence supports growth in a melt-rich environment, but experimental evidence and phase-equilibria modeling indicate that K-feldspar grows late in the crystallization sequence, when the magma is highly crystalline. We provide a new perspective on this problem by examining the arrangement of plagioclase inclusions within megacrysts to test whether they exhibit the systematic low-energy crystallographic relationships expected from attachment by synneusis in melt-rich environments where crystals have space to rotate. We use electron backscatter diffraction to quantify the crystal orientations and find that the megacrysts’ plagioclase inclusions do occupy these preferred orientations and therefore were incorporated in a melt-rich environment. K-feldspar is also present as an interstitial network, but plagioclase crystals hosted within this network have non-systematic orientations. This transition from systematic to non-systematic plagioclase orientations marks the point at which the crystals formed a rigid, interconnected framework that impeded rotation into low-energy orientations. Phase-equilibria modeling indicates that this transition occurred when the magma was ∼55% crystalline. The remaining ∼45% melt crystallized at the eutectic, forming the interstitial phases. Thus, we resolve the “megacryst paradox”; the megacrysts grew freely in melt, and the groundmass K-feldspar formed after crystal lock-up. Megacrysts therefore provide a detailed textural and chemical record of a critical period in the system’s evolution: the transition from a mobile and potentially eruptible magma to an immobile mush.

Formation Conditions of Ignimbrites of the Khangar Volcano (Kamchatka)

Abstract —The study of minerals, melt inclusions, as well as natural glasses showed that two different melts contributed to the formation of ignimbrites of the Khangar Volcano. The first, providing the information on melt inclusions in plagioclase and quartz phenocrysts, represents the state of magma in a deep source. The other type of melt is responsible for the formation of glasses and microcrystals of feldspars in fiamme. Experimental and analytical studies of melt inclusions showed that crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano occurred at temperatures of 840–960 °C and pressures up to 1.1 kbar, from the melt with water contents up to 3.23 wt.%, under the conditions of magma chamber. The presence of syngenetic primary melt and fluid inclusions in plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano indicates phase separation (“boiling”) of the melt with mass formation of СО2 microbubbles in magma. The other type of melt is secondary relative to magmatic systems of the Khangar Volcano and is formed by sintering and melting of tuffogenic volcanoclastic material. This melt contributed to the formation of fiamme in the examined ignimbrites. Based on the study of glasses and microcrystals of feldspars in fiamme, it was found that crystallization of oligoclase occurred at temperatures of 770–840 °C in the melt between the spherules (with water content up to 2.91 wt.%). Sanidine crystals grew over spherules at lower temperatures, 680–760 °C.

Precambrian to Pleistocene 40Ar/39Ar dating of clinopyroxene-hosted melt inclusions

Abstract Clinopyroxene is a rock-forming mineral that commonly hosts melt inclusions in mafic to intermediate composition volcanic and plutonic rocks. It is highly resistant to alteration compared to other co-existing phenocrysts such as plagioclase. Several recent studies have 40Ar/39Ar dated clinopyroxene in Neoproterozoic to Miocene basalts and dolerites. To assess the viability of the technique at the youngest end of the geologic time scale, we performed 40Ar/39Ar incremental heating experiments on clinopyroxene-hosted melt inclusions from a variety of mafic lithologies and tectonic settings. Most samples produced precise plateau ages including several Quaternary basalts to andesites as young as 0.6 Ma. All data are indistinguishable from new and/or published 40Ar/39Ar ages on groundmass or plagioclase from the same samples. The source of potassium (K) and resulting 40Ar* within clinopyroxene has been debated, but thus far has only been inferred based on 40Ar/39Ar data. Using electron probe microanalysis (EPMA) we show that there is negligible K in the clinopyroxene host, but substantial K (e.g., 1–4 wt%) in trapped melt inclusions and minor amounts in plagioclase inclusions. Thus, melt inclusions, which are common in phenocrysts in basaltic magmas, can be used to obtain accurate and precise 40Ar/39Ar ages for difficult-to-date volcanic and plutonic rocks from the Precambrian to the Pleistocene.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.

FORMATION OF TRACHYTES FROM WANGTIAN’E VOLCANIC NECK OF THE CHANGBAISHAN AREA (NORTHEAST CHINA) AS A RESULT OF THOLEIITIC MAGMA CRYSTAL FRACTIONATION

Tholeiitic basalts and trachytes of Wangtian’e bimodal association are studied. It is shown that trachytes were formed under the leading role of crystal fractionation of the initial tholeiitic magma. Based on the method of melt and fluid inclusion studies, it was determined that hedenbergite phenocrysts from Wangtian’e volcanic neck crystallized in the temperature range of 1080–1100°С, and plagioclase phenocrysts crystallized at temperatures of 1050–1060°С. After thermometric experiments with melt inclusions in plagioclase from trachytes, in some cases relics of carbonates and carbon dioxide were observed in the inclusions. Trachyte phenocrysts contain hydrous Fe-rich silicate globules and CO2 inclusions containing carbonate phases. Fe-rich silicate globules are often intergrown with titanomagnetite and covered with films of amorphous carbon. A model the appearance of hydrous Fe-rich globules and CO2 inclusions with carbonate relics is proposed. It results due to the silicate-silicate and silicate-carbonate liquid immiscibility processes, that are caused by the evolution of the initial basaltic melt. As the trachyte melt has been risen to the surface, the ferrocarbonate liquid decomposed into magnetite, carbon, and carbon dioxide.

Formation of Trachytes from Wangtian’e Volcanic Neck of the Changbaishan Area (Northeast China) as a Result of Tholeiitic Magma Crystal Fractionation

Tholeiitic basalts and trachytes of the bimodal association of Wangtian’e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide.

Pressure–temperature–time path of ultrahigh-pressure metamorphic terranes constrained by zircon petrochronology: A case study of orthogneisses from the northern Sulu belt, China

Reconstruction of pressure–temperature–time (P–T–t) path of volumetrically-dominant orthogneisses in deeply subducted continental crusts is challenging due to their simple amphibolite-facies mineral assemblages. To provide new insights into this issue, a detailed study of mineral inclusions, trace elements and U-Pb ages within robust zircon containers, together with titanite U-Pb dating and petrology were conducted on ultrahigh-pressure (UHP) orthogneisses from the northern Sulu belt. The following P–T–t conditions of three metamorphic stages were constrained: (i) cathodoluminescence (CL)-grey zircon cores (ca. 238–237 Ma) with flat heavy rare earth element (HREE) patterns include garnet, phengite and omphacite that yield estimated P–T conditions of 2.3–2.8 GPa/730–800 °C, corresponding to prograde high-pressure (HP) metamorphism before the UHP peak. (ii) CL-dark zircon mantles (ca. 224 Ma) contain multiphase solid inclusions of plagioclase + K-feldspar + quartz and show extremely low Th/U ratios (≤0.08) and negative Eu anomalies, demonstrating their crystallization from hydrous melt at 2.0–1.8 GPa/890–820 °C during HP eclogite-facies exhumation. In addition, titanite grains are characterized by high LREE contents (1939–5239 ppm), Th/U ratios (0.37–0.81) and crystallization temperatures (830–801 °C), suggesting their peritectic origin. The lower intercept age (220 ± 4 Ma) of titanite is broadly consistent with the ages of the zircon mantles, indicating that they both crystallized from the melt at the same stage. (iii) CL-bright zircon rims (ca. 212–211 Ma) preserve low-pressure mineral inclusions of plagioclase, K-feldspar and quartz, and they show low Th/U ratios (0.03–0.12) and steep HREE patterns with negative Eu anomalies, witnessing amphibolite-facies overprinting at 1.0–0.8 GPa/610–550 °C. Combined with the metamorphic peak conditions of 3.2 GPa/840 °C at ca. 232 Ma constrained by previous studies from the same area, the P–T–t path of the northern Sulu UHP belt is reconstructed. The data reflects two-stage exhumation of this terrane, including an early-stage (5.8 km/m.y.) and a later-stage slower (3.1 km/m.y.) exhumation processes. The faster exhumation during early decompression was probably enhanced by extensive melting activity recorded by the CL-dark zircon mantles from the orthogneisses.

Petrographic and geochemical evidence of calc-silicate xenolith – magma interaction in the western Bushveld Complex, South Africa

Abstract Underground workings at Rowland Shaft, Lonmin Platinum, near Marikana have exposed a large (>7 x 5 m) calc-silicate xenolith in mottled anorthosite ~20 m below the UG 1 chromitite of the upper Critical Zone. The xenolith comprises a monticellite + forsterite + spinel assemblage that is consistent with a metamorphic Tmax of ≥975°C being reached at P ~1 kbar and XCO2 ~1. Internal variations in the modal proportions of these phases are interpreted as an artefact of sedimentary layering in the xenolith. The xenolith is flanked by a 20 to 30 cm wide magmatic skarn that contains several small, cm- to dm-scale, calc-silicate fragments displaying the same peak assemblage as the main xenolith, as well as a dm-scale chromitite autolith and mm- to dm-scale lenses and stringer-like masses of fine-grained uvarovite. The skarn matrix is mineralogically zoned, with an up to 10 cm wide clinopyroxenite layer proximal to the xenolith contact and calc-silicate fragments grading outwards into a gabbroic, plagioclase-clinopyroxene assemblage in which plagioclase becomes progressively more dominant distally. Notwithstanding its pseudo-ophitic texture, the distal skarn matrix displays a range of features consistent with crystallisation of magma contaminated by Ca from the decomposing xenolith under elevated fO2 conditions, including: (a) the unusual mineral compositions (zoned aluminian-ferrian diopside and anorthite, An>99), (b) the presence of calcite as inclusions in plagioclase and as a minor interstitial phase, (c) corroded wollastonite inclusions in clinopyroxene, and (d) thin grossular reaction rims between clinopyroxene and plagioclase. This chemical evolution is further substantiated by zoned spinel grains comprising Cr-rich cores and Al-enriched rims, which are, in turn, enclosed in garnet aggregates that grade outwards from uvarovite to grossular. Local development of grossular-vesuvianite symplectite indicates limited retrograde hydrous (XCO2 <0.1) fluid influx under subsolidus conditions (T <750°C). Bedding in the xenolith displays a steep westerly dip, approximately orthogonal to the magmatic layering. This, together with evidence in the skarn of both plastic shear strain and out-of-sequence magmatic autoliths, suggests that the xenolith underwent density-driven foundering within the RLS magma from an initial location above the UG 1 chromitite layer.

Crystallization conditions of plagioclase in volcanic complexes of Sulawesi island, Pacific Ocean

Mineralogical and thermobarogeochemical studies allowed us to reveal the crystallization conditions of plagioclase in volcanic complexes of Sulawesi Island (Pacific Ocean). The composition of plagioclases indicates the presence of three types of melts, which were involved in the formation of plagioclase phenocrysts in basalts, andesites and dacites, as well as crystallites in the groundmass of andesites and dacites. The analysis of glasses of heated melt inclusions in plagioclase indicates the involvement of two contrasting melts (mafic and felsic) in crystallization of phenocrysts. At the final stages of volcanic processes, plagioclase in matrix of andesites and dacites formed from another felsic melts with maximum alkali content. The presence of primary syngenetic melt and fluid inclusions in plagioclase phenocrysts indicates the phase separation of felsic magmas, which led to the crystallization of feldspar phenocrysts of different composition during the formation of dacites. Computational modeling in a program (Putirka, 2008) based on the composition of plagioclases, glasses of heated inclusions and glasses from the groundmass allowed us to estimate the crystallization temperatures of feldspar phenocrysts in basalts (1160–1120 ?С), andesites (930–890 ?С) and dacites (1020–950 ?С), as well as the feldspar microcrystals in the groundmass of andesites (875–810 ?С) and dacites (890–850 ?С). Keywords: crystallization conditions, plagioclase, melt inclusions, Sulawesi Island, Pacific Ocean.

Crystallization order effects on inclusion assemblages in magmatic accessory minerals and implications for the detrital record

Mineral inclusion assemblages in zircon are almost certainly a complex function of crystallization sequence, magma chemistry, and physical proximity to other crystallizing phases. While the latter may be largely random, the former two factors should have more systematic controls. To better constrain the effects of crystallization sequence, we investigated a suite of granitoids from the Cretaceous arc of southern California and from the Miocene Colorado River Extensional Corridor, ranging from tonalites to leucogranites. In particular we examined inclusions in a variety of accessory minerals to elucidate crystallization order effects. Apatite is almost universally overrepresented as an inclusion in the studied minerals relative to its volumetric abundance in the rock as a whole. Where crystallization order can be inferred, earlier phases usually either have a higher proportion of apatite inclusions (11 out of 20 cases) or show no significant differences in apatite content from later phases (7 cases). An increase in the proportion of apatite with progressive crystallization is observed in four out of the 20 samples with clear crystallization orders (two rocks out of the 20 contain both an increasing and a decreasing case among different mineral pairs and are included in both groups). We interpret these observations as showing the widespread early crystallization of apatite inclusions – likely by pileup of the slow-diffusing phosphorus complex along growing mineral grains. The lower apatite contents in late-crystallizing modal phases are also reflected in the very low apatite contents within late-crystallizing zircon from some studied mafic-intermediate units, contrary to earlier suggestions that apatite decreases as a proportion of the inclusion assemblage with increasing whole rock silica. Plagioclase inclusions in zircon are on average more albitic than matrix plagioclase. In some samples, sequences of plagioclase crystallization can be identified by albite content among 1) magmatic cores of matrix grains, 2) inclusions in mafic accessory minerals, 3) inclusions in zircon, and 4) post-magmatic alteration veins and exsolution lamellae. However, the relative proportions of quartz, K-feldspar, and plagioclase inclusions do not systematically vary in accessory minerals with crystallization order, suggesting other origins of the often-recognized modal shifts toward more quartz- and K-feldspar-rich inclusion assemblage compared to the matrix. Unlike Himalayan leucogranites, when muscovite is present in the matrix in our sample suite, it is a rare inclusion phase in later-crystallizing accessories and may, in some cases, even be absent in zircon even when present in other phases. It is possible to mistake non-magmatic inclusions in detrital grains for primary magmatic inclusions, and examination of internal host grain texture and geochronology (when applicable) is especially important in a detrital setting. These results should allow an improved framework for interpreting inclusion assemblages in detrital magmatic minerals and in interpreting the phase petrology of the host igneous rock. Given the remaining ambiguities in linking a number of observations (i.e., apatite contents, position on the quartz-alkali feldspar-plagioclase ternary, and the ratio of felsic to mafic silicate phases) to specific magmatic provenance, the identification of rare but petrologically significant inclusion phases such as muscovite, halides, Mn and Nb minerals, sulfides, or baddeleyite, may be more helpful in pointing to either highly evolved or more mafic provenances.

Highly heterogeneous mantle caused by recycling of oceanic lithosphere from the mantle transition zone

Geochemical heterogeneities observed in the mantle are usually attributed to recycling of oceanic lithosphere through subduction. However, it remains hotly debated where recycled material stagnates, and how quickly it can be liberated back to surface. This knowledge gap hinders our understanding of mantle circulation and the chemical evolution of the Earth. Here we address these questions using a combination of geochronology and geochemistry from South China Sea (SCS) seamounts. The Shixingbei seamount lavas formed during active seafloor spreading at c. 19.1 Ma show limited geochemical variability, whereas the Zhenbei-Huangyan seamount chain formed during the post-spreading stage at c. 7.8 Ma and displays a wide range of compositions. However, melt inclusions in olivine and plagioclase from the Zhenbei-Huangyan basalts show considerably greater isotopic variability than seen in the whole rock compositions of both the SCS syn- and post-spreading lavas. A previously unidentified third mantle source component (FOZO) revealed by olivine-hosted melt inclusions along with both depleted (DMM) and enriched (EMII) mantle components is required in the source region to explain the observed isotopic and chemical variability. On the basis of our results, the age of the recycled ocean crust and sediments in this region are estimated to be c. 120 – 350 Ma. We infer that these enriched components in the SCS lavas come from the mantle transition zone. Variations in mantle source heterogeneity coupled with melting process control spatial–temporal (spreading vs. post-spreading stage) geochemical variations of lavas from the SCS and surrounding areas. Together with the results from published studies, we propose that marginal basins are one of the major locations on Earth where oceanic and/or continental lithosphere is transferred into the upper mantle and transition zone, representing an important source of upper mantle heterogeneity. We provide a simple conceptual model linking plate subduction and upper mantle heterogeneity and the volcanism in the SCS and surrounding areas.

Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

AbstractMicron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

Hysteresis parameters and magnetic anisotropy of silicate-hosted magnetite exsolutions

SUMMARYAnisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse ‘slice-and-view’ focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within 1.0 per cent, whereas the coercivity (Hc) of the average modelled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multidomain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of palaeomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modelled small-particle fraction only explains 6 per cent of the best fit to the measured VSM hysteresis loop, it contributes 28 per cent of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is subparallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

Formation of Chromitite Seams and Associated Anorthosites in Layered Intrusion by Reactive Volatile-rich Fluid Infiltration

Abstract Drilling related to development of the platinum-group element deposit of the J-M Reef of the Stillwater Complex returned samples of a rare chromitite seam between anorthosite and norite in a discordant anorthositic body. Plagioclase core An concentrations are marginally higher and modestly reversely zoned on the norite side (average Ancore = 83·8; average Ancore – Anrim = –1·1) as compared with the anorthosite side (average Ancore 82·5; average Ancore – Anrim = +1·0). The anorthosites are also characterized by a slightly smaller average plagioclase grain size than plagioclase in the norite (1·41 mm and 1·54 mm, respectively). The chromite can contain single and polyphase inclusions of orthopyroxene, plagioclase, amphibole, biotite and Cl-rich apatite. These and other compositional and textural features, as well as inference from other discordant anorthositic bodies in the Banded series, are all consistent with a chromatographic model of chromite precipitation at a reaction front as a norite protolith reacts with a Cl-rich aqueous fluid saturated in plagioclase alone. Chromitite seam formation is modeled using an infiltration metasomatic model, in which a fluid becomes progressively undersaturated in pyroxene as it rises into the hotter part of the crystal pile. As this pyroxene-undersaturated fluid moves through a noritic protolith, it dissolves the Cr-bearing orthopyroxene to produce an anorthosite. Chromite precipitates at the reaction front between the anorthosite and the norite owing to liberation of Mg and Cr from pyroxene. Continuous redissolution and reprecipitation of chromite occurs as the pyroxene dissolution front moves in the direction of fluid flow, collecting the Cr lost from the anorthosite. Owing to Cr dissolved mainly as a neutral divalent cation complex, CrCl(OH)0, in the solution, but incorporated as a trivalent cation in chromite, the required redox reaction can involve concurrent precipitation of sulfide with chromite. This mechanism differs from some recent models in that the anorthosites are themselves replacement bodies and are not original precipitates from a magma nor formed by loss of mafic material by partial melting. The results show the need for experimental mineral solubility data at T and P conditions appropriate to upper crustal mafic–ultramafic intrusions.

Features of Mineral Crystallization at Different Stages of the Magmatism Evolution of the Gorely Volcano (Kamchatka): Data on Melt and Fluid Inclusions

Abstract ––Studies of melt and fluid inclusions and minerals as well as computational modeling (based on the data on the composition of melt inclusions, clinopyroxenes, and amphiboles) gave an insight into the physicochemical parameters of magmatic systems during the evolution of the precaldera Pra-Gorely Volcano and during the subsequent formation of rock complexes of the Young Gorely Volcano. The estimated temperatures of crystallization of olivine, clinopyroxene, and plagioclase phenocrysts (1115–1260 °С) and amphibole (740–890 °С) are in agreement with the earlier published data on the magmatism of the Gorely Volcano. Computational modeling based on the compositions and homogenization temperatures of melt inclusions showed that the established depth interval of mineral crystallization (21.0–1.5 km) with pressures of 7.0–0.5 kbar can be divided into two ranges, 21–15 km and 9.0–1.5 km. Both the Pra-Gorely and Young Gorely volcanoes have magma chambers in these depth ranges. The Pra-Gorely Volcano is characterized by higher temperatures of mineral crystallization (1240–1190 °С) as compared with the Young Gorely Volcano (1190–1125 °С). The presence of primary fluid inclusions with low-density CO2 and of syngenetic primary melt inclusions in plagioclase of the Pra-Gorely Volcano indicates that the mineral crystallized from a heterophase melt. At the same time, the cores of plagioclase phenocrysts formed from a homogeneous melt. A drastic drop in pressure led to the phase separation of magma throughout the magma column (upper and lower chambers) and to the growth of zones saturated with CO2 fluid inclusions in the plagioclase crystals formed from a two-phase melt. The subsequent closure of the system and the disappearance of CO2 phase resulted in the growth of plagioclase from a homogeneous melt.

Garnet and zircon geochronology of the Paleoproterozoic Kuru-Vaara eclogites, northern Belomorian Province, Fennoscandian Shield

The Belomorian Province of the Fennoscandian Shield exposes numerous Precambrian eclogites, which makes it significant for the study of early tectonic processes. The age of these eclogites has been discussed for more than 15 years and regarded as either Archean or Paleoproterozoic. In the Kuru-Vaara quarry within the northern Belomorian Province, the eclogitic assemblage is preserved in concordant mafic boudins in felsic gneisses and a partially eclogitized gabbro-norite dike cutting discordantly through the gneiss fabric. Both eclogite types preserve zircon cores with a magmatic geochemical signature that yield protolith ages of ca. 2.88 Ga for a mafic boudin and ca. 2.44 Ga for the eclogitized gabbro-norite. Ca. 1.9 Ga zircon rims and grains from the eclogites show low Th/U ratio and HREE depletion, reflecting the growth of metamorphic zircon in equilibrium with garnet. In the eclogite boudin, the Archean zircon cores yield δ18O = 5.1–5.9‰ typical of mantle melts; the oxygen isotope composition of garnet (δ18O = 4.0–5.0‰) is in equilibrium with that of the 1.9 Ga zircon (δ18O = 4.5–5.4‰). Garnet Lu-Hf geochronology coupled with U-Pb zircon geochronology constrains prograde metamorphism for the Kuru-Vaara eclogites at 1.92–1.89 Ga. Mineral inclusions of garnet, zoisite, plagioclase, kyanite, amphibole, quartz, and low-Na clinopyroxene in ca. 1.9 Ga zircon from the eclogite boudin imply epidote–amphibolite/amphibolite facies conditions for the prograde metamorphism. All data point to a Paleoproterozoic age of the eclogite facies metamorphism.

Jewelry and ornamental ammonites from the deposits of the Hauterivian stage of the Ulyanovsk Region: Mineral composition, gemological characteristics

Significant volumes of products from ammonites of jewelry and ornamental quality of the Ulyanovsk region are entering the Russian and world markets. Ammonites are associated with the Lower Cretaceous Hauterivian deposits and consist mainly of aragonite, calcite and pyrite. The walls and partitions of the ammonite shells retained the original aragonite composition. Also established inclusions of apatite, algodonite, pyrolusite, hematite, goethite, quartz, ferrihydrite, magnetite, uranium oxide, vernadite, todorokite, monazite, halloysite, plagioclase, kaolinite, glauconite, dolomite, graphite and organic matter. An important role in the formation of new mineral species was played by bacteria. Mineralization of undamaged ammonite chambers was carried out by the sequential formation of calcite and pyrite layers without metasomatic substitutions. The color of calcite layers is determined by the contents of Mn and Fe. Ammonites are characterized by high decorativeness and good technological characteristics; they can be mined in the coastal cliffs and the beach area of the river Volga without mining, without violating the environment.

Partial melting due to breakdown of phengite and amphibole in retrogressed eclogite of deep Precambrian crust: An example from the Algonquin terrane, western Grenville Province, Canada

Characterizing partial melting of eclogite in Precambrian orogens is critical to understanding the petrological processes that operate in the deep orogenic crust. Retrogressed eclogite from the Algonquin terrane in the western Grenville Province, Canada, was taken as an example to study such processes. The studied samples contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure. Various exhumation- and melt-related textures formed show evidence for a complex post-peak pressure history. Rutile is only found in the core of garnet, whereas ilmenite-bearing assemblages predominate in the garnet rim and the matrix. Symplectite of diopside + plagioclase + amphibole replaced former omphacite. Symplectite of amphibole and plagioclase formed as a corona around garnet. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides in garnet crystallized from melt, which was captured during garnet growth and reacted with the host. Cusps of plagioclase into the boundaries of other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during eclogite melting. Leucocratic veins without external connection, present in the sampled mafic pod at outcrop scale, are interpreted as crystallized pockets of melt which derived internally. Pseudosection modeling of the studied samples along with compositional isopleths of chemically zoned garnet yielded an exhumation path from 1.7 ± 0.1 GPa, 680 ± 50 °C to 1.3 ± 0.1 GPa, 920 ± 50 °C. Along this path, the modal content of melt increased in response to the breakdown of phengite and amphibole along with involvement of omphacite and rutile. The melt crystallized during cooling and equilibrated at 0.6–0.8 GPa, 710–750 °C, estimated using empirical thermobarometry. This study shows that metabasite from Proterozoic collisional orogens can be partially melted on the exhumation and heating path through phengite- and amphibole-dehydration melting.

Source and behavior of pyroclastic density currents generated by Vulcanian-style explosions of Popocatépetl volcano (Mexico) on 22 January 2001

Since 1994 Popocatépetl volcano has extruded 85 lava domes, each of which has been destroyed by Vulcanian-style explosions. The paroxysmal Vulcanian explosion of 22 January 2001 has been unique by generating multiple pyroclastic density currents (PDCs). Scientists and Civil Protection authorities have used those PDCs to remap the high-risk hazard zone of the volcano, although little study has been carried out to understand the generation and evolution of those PDCs. Here, we examine 38 stratigraphic sections and analyze the granulometry, componentry, vesicularity, and petrology (glass, melt inclusions in plagioclase) of collected samples to interpret the origins of the Vulcanian explosions and the PDC behavior. The 22 January event generated two series of dense PDCs that detached from the base of the eruption column. The first set of PDCs traveled northwest across a glacier on the volcano, partially scouring and melting the ice that hours later generated a lahar. Two pulses of these PDCs were confined within a large ravine, surpassed the slope break, and traveled up to 3 km away from the vent. The deposits are matrix-supported throughout their extent. The second set of PDCs traveled to the northeast, mainly as unconfined currents that split into two lobes prior to the slope break. The farthest traveled current followed a western ravine and reached the forest 3.5 km from the summit. The eastern lobe buried a local hiking hut and reached 2.1 km. The deposits from these northeastern currents are massive and block-supported. All deposits (volume of 5.71 × 105 m3) have finger-like morphology with levees and frontal snouts. All PDCs flowed as a dense basal granular current with turbulent upper clouds at minimum speeds of 7–10 m/s.The components of the deposits are, in order of abundance, black scoria (53–62 wt% SiO2), banded scoria, pumice (63 wt% SiO2), dome lithics (60–61 wt% SiO2), accidental lithics, and free crystals. Juvenile pumice and scoria contain phenocrysts of plagioclase (An47±10), clinopyroxene (En48±3), orthopyroxene (En64±1), ilmenite, and titanomagnetite set in a rhyolitic glass (pumice clast; 74.39 wt% SiO2) or microlite-rich matrix (scoria clast). The dome lithics also contain these phenocrysts as well as rare olivine phenocrysts (Fe2+/(Fe2 + Mg) = 0.1). Dissolved H2O-CO2 contents in melt inclusions hosted by pumice-plagioclase phenocrysts suggest the felsic magma was stored at depths of 2–4 km. Thus, the 2001 event was triggered by the injection of mafic magmas into a small volume of felsic magma, probably forming dikes left from the previous eruptive episode. We propose that the Vulcanian-explosions tap a column of magma (~2 km depth) equivalent to the volume that contained the felsic dikes. This mechanism has been acting during the destruction of all summit domes of the ongoing eruption of the volcano.