Crystallization Of Ilmenite Research Articles

Geochemistry of zircons from basic rocks of the Korosten anorthosite-mangerite-charnockite-granite complex, north-western region of the Ukrainian Shield

The concentrations of 26 trace elements have been determined by laser ablation ICP-MS in zircons from four samples of basic rocks of the Korosten anorthosite-mangerite-charnockite-granite plutonic complex, the Ukrainian Shield. Zircons from the Fedorivka and Torchyn gabbroic intrusions and Volynsky anorthosite massif have distinctive abundances of many trace elements (REE, Sr, Y, Mn, Th). Zircons from the gabbroic massifs are unusually enriched in trace elements, while zircons from pegmatites in anorthosite are relatively depleted in trace elements. High concentrations of trace elements in zircons from gabbroic intrusions can be explained by their crystallization from residual interstitial melts enriched in incompatible elements. The zircons studied demonstrate a wide range of Ti concentrations, which reflects their temperature of crystallization: the zircons most enriched in Ti, from mafic pegmatites of the Horbuliv quarry (20–40 ppm), have the highest temperature of crystallization (845 ± 40 °C). Lower (720–770 °C) temperatures of zircon crystallization in gabbroic rocks are explained by its crystallization from the latest portions of the interstitial melt or by simultaneous crystallization of ilmenite. The Ce anomaly in zircons correlates with the degree of oxidation of the coexisting ilmenite.

Controls on trace-element partitioning among co-crystallizing minerals: Evidence from the Panzhihua layered intrusion, SW China

The factors and processes that control trace-element partitioning among co-crystallizing cumulus minerals in layered intrusions have long been controversial. Here we address this issue using new laser ablation ICP-MS trace element data for magnetite, ilmenite, and clinopyroxene from the Panzhihua layered intrusion in the Emeishan large igneous province, SW China. The cumulus minerals display strong Ni, Co, and Cr depletions, indicative of parental magmas low in concentration of these elements probably due to prior sulfide removal and the fractionation of chromite or Cr-magnetite in a staging magma chamber at depth. Both magnetite and clinopyroxene show cyclical variations in some transition elements (e.g., Cr, V, and Ni) along the stratigraphic section. The average concentrations of these transition elements in magnetite are positively correlated with those in clinopyroxene, likely resulting from co-crystallization of magnetite and clinopyroxene. The incompatible element (e.g., Zr, Hf, and Nb) concentrations of the cumulus minerals from the Lower Zone are highly variable compared to those of the Middle and Upper Zones. These large variations in trace element compositions are attributed to a “trapped liquid shift” in the Lower Zone. Ilmenite crystals from the Panzhihua intrusion may have undergone extensive modification of transition elements during subsolidus re-equilibration with magnetite, leading to the decoupled variations of transition elements in ilmenite across the Lower Zone stratigraphy. Our study indicates that systematic trace element variations of the main cumulus mineral assemblage, rather than a single mineral, need to be considered to better constrain the magmatic differentiation and elemental fractionation of layered intrusions.

Igneous carbonates in dolerites of Franz Joseph land

Carbonates showing the signs of crystallization from a melt were found in dolerites and basalts of lava covers and dikes of Alexandra Land, Heiss, and Newcomb islands. These carbonates may fill interstitials between silicate minerals, as well as constitute wormlike or amebiform separations in dolerites. In other cases, the “bulbs” are formed within acidic glass in the centers of globules consisting of pyroxene and rimmed with ilmenite crystals in cryptocrystalline basalts. Most of the carbonate separations are constituted of siderite (80–95% of siderite minal); calcites (up to 20% of siderite minal) and carbonates of calcite–dolomite isomorphic series are found less frequently. In view of the plume nature of volcanic rocks at the archipelago, the capture of carbonatite melt by silicate magma seems to be the most likely. However, the possibility of the magma capture and melting of residual siderite fragments from the underlying terrigenous formation of Mesozoic age cannot be excluded.

Geochemical and U–Pb zircon age characterization of granites of the Bathani Volcano Sedimentary sequence, Chotanagpur Granite Gneiss Complex, eastern India: vestiges of the Nuna supercontinent in the Central Indian Tectonic Zone

Abstract The Central Indian Tectonic Zone (CITZ) marks the suture zone where the North and South Indian cratonic blocks amalgamated to form the Greater Indian Landmass (GIL). It consists of three broad domains from west to east: the central CITZ occupying the central region of mainland India juxtaposed between two mobile belts, namely the Sausar Mobile Belt (SMB) in the south and the Mahakoshal Mobile Belt (MMB) in the north; the Chotanagpur Granite Gneiss Complex (CGGC) lying east of the main CITZ; and the easternmost Shillong Plateau Gneissic Complex (SPGC). The studied granites are from the Bathani Volcano Sedimentary sequence (BVSs) from the northern margin of the CGGC. These are high-K, calc-alkaline, I-type granites related to arc magmatism and are interpreted to have formed by partial melting of an igneous source at upper-crustal depths. The granitic magma underwent extensive fractional crystallization of plagioclase, biotite, K-feldspar and ilmenite during emplacement. The U–Pb (ID-TIMS) zircon emplacement age is c. 1.7–1.6 Ga for these granites. This episode of magmatism can be correlated to the global event of the Nuna supercontinent assembly also reported from the MMB of the central CITZ. We infer that the BVSs is the eastern continuation of the MMB of the central CITZ.

Selective zircon accumulation in a new benthic foraminifer, Psammophaga zirconia, sp. nov.

Benthic foraminifera are single-celled eukaryotes that make a protective organic, agglutinated or calcareous test. Some agglutinated, single-chambered taxa, including Psammophaga Arnold, 1982, retain mineral particles in their cytoplasm, but the selective mechanism of accumulation is not clear. Here, we report the ability of a foraminiferal species to select and accumulate zircons and other heavy minerals in their cytoplasm. In particular, the use of Scanning Electron Microscope coupled with an Energy Dispersive X-ray microanalysis system (SEM-EDS) enabled a representative overview of the mineral diversity and showed that the analysed Psammophaga zirconia sp. nov. individuals contained dominantly crystals of zircon (51%), titanium oxides (27%), and ilmenite (11%) along with minor magnetite and other minerals. The studied specimens occur in the shallow central Adriatic Sea where the sediment has a content of zircon below 1% and of other heavy minerals below 4%. For that reason we hypothesize that: (i) P.zirconia may be able to chemically select minerals, specifically zircon and rutile; (ii) the chemical mechanism allowing the selection is based on electrostatic interaction, and it could work also for agglutinated foraminifera (whether for ingestion, like Xenophyophores, or incorporation in the test as in many other described taxa). In particular, this aptitude for high preferential uptake and differential ingestion or retention of zircon is reported here for the first time, together with the selection of other heavy minerals already described in members of the genus Psammophaga. They are generally counted among early foraminifera, constructing a morphologically simple test with a single chamber. Our molecular phylogenetic study confirms that P.zirconia is a new species, genetically distinctive from other Psammophaga, and occurs in the Adriatic as well as in the Black Sea.

A novel fractional crystallization route to porous TiO2-Fe2O3 composites: large scale preparation and high performances as a photocatalyst and Li-ion battery anode.

Meso/macroporous TiO2-Fe2O3 composite particles are prepared using naturally abundant ilmenite via a novel heat treatment induced fractional crystallization strategy in a fluidized bed. Fluid-bed roasting in oxidizing and reducing environments is carried out in order to realize the fractional crystallization of ilmenite. Subsequently, acid leaching is employed to remove most of the ferrous phase and form porous TiO2-Fe2O3 composites. The influences of the reaction parameters on the composition, structure and properties of the products are studied. It is found that the pore structure and composition of the porous TiO2-Fe2O3 composite particles can be controlled simply by controlling some parameters, such as the roasting time, temperature, precursor particle size, and post-roasting treatment. Photocatalytic and electrochemical cycling measurements show that the synergism of porous structures and the controlled doping of α-Fe2O3 endow the as-obtained products with excellent visible light photocatalytic activity and provide enhanced performance in lithium ion batteries. The composite porous particles thus obtained may have some promising applications in the fields of photocatalysts, electrode materials, absorbers, pigments etc. This work opens a new avenue for reasonable combination of cost-effective raw materials, a large scale fabricating process and fine control over the structure and composition in the design and preparation of functional materials.

Typomorphic characteristic features of accessory ilmenite in granitoids of the polyphase Aleisk-Zmeinogorsk complex (N-W Rudny Altai area)

The paper material is devoted to the research of crystallomorphology, accessory ilmenite expansion, characteristic features of its chemical composition, paragenetic association and conditions for mineral formation in Devonian granitoids of the polyphase Aleisk-Zmeinogorsk complex. Two ilmenite generations formed at various crystallization stages of the magmatic melt have been identified by the morphological features of the ilmenite crystals, chemical composition and paragenesis. Early-magmatic Ilmenite1 singled out at the protocrystallization stage during reduction and weakly-oxidizing environment conditions. It is characterized by the crystalline-faced shapes, low content of the pyrophanite minal and V admixture. Ilmenite 2 singled out at the major crystallization stage of the melt in the conditions of higher oxygen fugacity as the forms of the flattened, xenomorphic grains with the enhanced pyrophanite concentration and Nb admixture.

Characterization of an ilmenite ore for pressurized chemical looping combustion

This work presents the characterization results of a Canadian ilmenite ore tested in pressurized chemical looping combustion cycles. The ilmenite ore was isothermally cycled in a pressurized thermogravimetric analyzer using carbon monoxide as a reducing fuel and air as an oxidizing gas. All samples were calcined prior to submission to cycles. X-ray diffraction (XRD) comparing raw and calcined ore samples indicate the disappearance of ilmenite crystals and the formation of rutile and ferric pseudobrookite. Cycled ilmenite ore surface morphology was found to be insensitive to the total pressure (up to 51bar) and CO partial pressure (3.2–8.0bar). These findings are in agreement with thermodynamic equilibrium predictions using FactSage. SEM images reveal the development of cracks in the oxidized particles after 4 redox cycles at 950°C and 16bar, which was correlated to the original lamellar structure of the raw ilmenite ore. Increasing the number of redox cycles from 3.5 to 19.5 resulted in large cracks near iron-rich lamellae. Furthermore, the average grain size was smaller after 19.5 cycles. Increasing the reaction temperature from 850°C to 1050°C resulted in similar surface morphologies, but increased grain size. Despite being below melting temperatures, the sample cycled at 1050°C did agglomerate. Local temperature excursions during sample oxidation may have led to melting. This study provides insights into the phase transformations and morphological changes accompanying the use of this type of ilmenite ore in a pressurized chemical looping combustion process, highlighting the need for proper adjustments to the design and operation of the process.

Variations of trace element concentration of magnetite and ilmenite from the Taihe layered intrusion, Emeishan large igneous province, SW China: Implications for magmatic fractionation and origin of Fe–Ti–V oxide ore deposits

In situ LA–ICP–MS trace elemental analysis has been applied to magnetite and ilmenite of the Taihe layered intrusion, Emeishan large igneous province, SW China, in order to understand better fractionation processes of magma and origin of Fe–Ti–V oxide ore deposits. The periodic reversals in Mg, Ti, Mn in magnetite and Mg, Sc in ilmenite are found in the Middle Zone of the intrusion and agree with fractionation trends as recorded by olivine (Fo), plagioclase (An) and clinopyroxene (Mg#) compositions. These suggest the Taihe intrusion formed from open magma chamber processes in a magma conduit with multiple replenishments of more primitive magmas. The V and Cr of magnetite are well correlated with V and Cr of clinopyroxene indicating that they became liquidus phases almost simultaneously at an early stage of magma evolution. Ilmenite from the Middle and Upper Zones shows variable Cr, Ni, V, Mg, Nb, Ta and Sc contents indicating that ilmenite at some stratigraphic levels crystallized slightly earlier than magnetite and clinopyroxene. The early crystallization of magnetite and ilmenite is the result of the high FeOt and TiO2 contents in the parental magma. The ilmenite crystallization before magnetite in the Middle and Upper Zones can be attributed to higher TiO2 content of the magma due to the remelting of pre-existing ilmenite in a middle-level magma chamber. Compared to the coeval high-Ti basalts, the relatively low Zr, Hf, Nb and Ta contents in both magnetite and ilmenite throughout the Taihe intrusion indicate that they crystallized from Fe–Ti–(P)-rich silicate magmas. Positive correlations of Ti with Mg, Mn, Sc and Zr of magnetite, and Zr with Sc, Hf and Nb of ilmenite also suggest that magnetite and ilmenite crystallized continuously from the homogeneous silicate magma rather than an immiscible Fe-rich melt. Therefore, frequent replenishments of Fe–Ti–(P)-rich silicate magma and gravitational sorting and settling are crucial for the formation the massive and apatite-rich disseminated ores in the Lower and Middle Zones of the Taihe intrusion.

Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)

Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the \(\left\langle {0 10} \right\rangle_{R} \left\{ { 10 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 10} \right\}_{H}\) orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact \(\left\langle {0 10} \right\rangle_{R} \left\{ { 40 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 70} \right\}_{H}\). The intergrowth is dictated by the formation of \(\left\{ { 1 70} \right\}_{H} |\left\{ { 40 1} \right\}_{R}\) equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian–ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form \(\left\{ { 30 1} \right\}_{R} |\left\{ {0 30} \right\}_{H}\) equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian–ilmenite crystals was close to Ilm67Hem33, typical for Fe–Ti-rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian–ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.

Mn- and Zn-bearing ilmenite in porphyries of the Central Urals

Ilmenite crystals incorporated into epidote phenocrysts of porphyry dikes contain up to 8.8 wt % ZnO and 20 wt % MnO. These crystals, less than 50 μm in size, are associated with rutile, pyrite, pyrrhotite inclusions and are characterized by nonuniform Zn distribution. Ilmenite microphenocrysts 100–200 μm in size that occur in the muscovite-epidote-quartz-feldspar groundmass contain up to 2.4 wt % ZnO and 10–20 wt % MnO. It is suggested that the formation of Mn- and Zn-bearing ilmenite is related to impact of hydrous oxidized melt on the sulfide-bearing crustal rocks, giving rise to sphalerite decomposition.

Quantitative textural analysis of ilmenite in Apollo 17 high-titanium mare basalts

Quantitative textural analysis is a powerful tool in the investigation of basalt crystallization. We present the first comprehensive crystal size distribution analysis of Apollo 17 high-titanium lunar basalts, with a focus on ilmenite. Crystal size distributions of ilmenite, pyroxene, plagioclase, olivine and armalcolite were determined for 18 high-Ti mare basalt samples from the Apollo 17 mission. A subset of the ilmenite size distribution (size bins of <0.6mm in length) reflects growth at post-eruption or post-emplacement cooling rates. Growth of these small ilmenite crystals is controlled by cooling rate and not bulk composition or ilmenite abundance. CSD characteristics tied to cooling rate determined by experiments yield estimates of cooling rate in natural samples. Matrix ilmenite grew at rates up to 250°C/h, while most samples contained phenocrysts that originated in environments cooling at <3°C/h. Textural characteristics of ilmenite phenocrysts are used to develop a relative stratigraphy for the samples within a lava flow based upon comparisons with terrestrial analogues.

Trace elements in magnetite as petrogenetic indicators

We have characterized the distribution of 25 trace elements in magnetite (Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning behavior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of magnetite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibility into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the exploration of ore deposits and in sedimentology.

Silicate-iron fluid media in rhyolitic magma: Data on rhyolites from the Nilginskaya depression, Central Mongolia

Relicts of silicate-iron fluid media were found in the Early Cretaceous rhyolites of the Nilginskaya depression, Central Mongolia. They are localized in matrix cavities and in the inclusions in quartz and sanidine phenocrysts. The mineral composition of rhyolites and aggregates of silicate-iron phases has been studied. Calculations showed that crystallization of ilmenite and magnetite in a matrix occurred within a temperature range of 593–700°C and oxygen fugacity \(\Delta \log f_{O_2 }\) NNO from −2.29 to 1.68. The average compositions of the rhyolites and residual glasses in melt inclusions (MI) have A/CNK index of 1.03–1.05. The compositions of MI glasses define a trend from agpaitic to plumasitic types (A/NK and A/CNK change from 0.8–0.9 to 1.1–1.2). According to calculations, the rhyolitic melt was solidified at 640–750°C.

Zircon U–Pb geochronology and elemental and Sr–Nd–Hf isotopic geochemistry of the Daocheng granitic pluton from the Yidun Arc, SW China

The Garze–Litang suture zone, located in the eastern part of the Tethyan tectonic domain, is notable for widespread Late Triassic granitic plutons, which are genetically associated with the evolution of the Paleo–Tethys Ocean. The Daocheng granitic pluton in the eastern Yidun Arc, SW China, is located in the middle of the Garze–Litang suture zone and has an outcrop area of ∼2800km2. In the present study, we report zircon U–Pb ages and elemental and Sr–Nd–Hf isotopic data for the Daocheng granitic intrusion. Secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry U–Pb analyses on zircons yield consistent ages of ca. 216Ma for three samples from the pluton. All the Daocheng granitic rocks are enriched in Si (SiO2=68.1–76.4%) and large-ion lithophile elements but depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). Mineralogical and geochemical features indicate that these are high-K and calc-alkaline and I-type granites. They are characterized by relatively variable initial 87Sr/86Sr ratios (0.7059–0.7102), negative εNd(t) values (−5.7 to −7.8), wide-ranging εHf(t) values (−9.8 to +3.4), and two-stage Hf model ages of 1.04 to 1.88Ga. These isotopic signatures indicate that the source for the Daocheng granite was probably derived from partial melting of a Late Paleoproterozoic to Early Mesoproterozoic mafic–intermediate lower crust with a variably minor addition (<20%) of depleted mantle-derived magma. The parental magma thereafter underwent extensive fractional crystallization of ferromagnesian minerals, plagioclase, apatite, ilmenite, and rutile during emplacement, under temperatures from 785°C to 839°C. In combination with previous studies on synchronous magmatism in the Yidun Arc, we propose that the Daocheng granite was generated in a syncollisional tectonic setting. The westward subduction and closure of the Garze–Litang paleo-ocean triggered the underplating of large-scale mantle-derived magma and provided heat for the anatexis of the lower crust. Hybrid melts including minor depleted mantle-derived magma and lower crustal magma were then generated; these thereafter were continuously injected into a shallow-level chamber and gave rise to the Daocheng granite.

Petrogenetic links between lepidolite-subtype aplite-pegmatite, aplite veins and associated granites at Segura (central Portugal)

In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0±0.5Ma and 312.9±2.0Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite.The occurrence of amblygonite–montebrasite, lepidolite, cassiterite, columbite-(Fe), columbite-(Mn) and microlite suggests that lepidolite-subtype granitic aplite-pegmatite veins are highly differentiated. Montebrasite shows a heterogeneous Na distribution and secondary lacroixite was identified in some montebrasite areas enriched in Na. Unusual Mn>Fe cassiterite is zoned, with the alternating darker zones being strongly pleochroic, oscillatory zoned, and containing more Nb and Ta than the lighter zones. Inclusions of muscovite, apatite, tapiolite-(Fe), ixiolite and microlite are present both in lighter and darker zones of cassiterite. It shows exsolutions of columbite-(Fe), columbite-(Mn,Fe) and columbite-(Mn), particularly in darker zones.

High-Mg carbonatitic melts in diamonds, kimberlites and the sub-continental lithosphere

The trace elements of high-Mg carbonatitic high-density fluids (HDFs) trapped in six fibrous diamonds from Siberia exhibit patterns that are highly similar to those of Group I kimberlites, but are slightly more fractionated. The patterns of both are similar to the average pattern of post-Archaean xenoliths from the sub-continental lithospheric mantle (SCLM). The Siberian high-Mg carbonatitic HDFs are highly enriched in incompatible elements and have compositions comparable to those of high-Mg HDFs from Kankan, Guinea. However, in detail the latter show depletion of K, Rb, Cs, Nb and Ta and enrichment in Ba, Th, U and LREE relative to the Siberian HDFs. These differences correspond closely to those between the patterns of Group II and Group I kimberlites, respectively. Mixing, fractionation and melting were explored as possible scenarios to explain these similarities and to constrain the possible genetic relationships between HDFs, kimberlites and the SCLM. Addition of 2.5% of Group I kimberlitic magma or 0.5% of the Udachnaya high-Mg HDFs to a depleted peridotite closely reproduces the post-Archaean SCLM pattern. The formation of high-Mg HDFs through fractionation of kimberlitic magma calls for 80% crystallization of olivine, clinopyroxene, garnet, carbonate and ilmenite. However, mismatches in K, Rb, Y and Ho abundances, and absence of the postulated fractionating minerals as inclusions suggest other petrogenetic scenarios are more likely. High-Mg HDFs and kimberlites can be produced by melting of a common source. The pattern of the calculated source for Siberian HDF and Group I kimberlites resembles that of average post-Archean, rather than Archean, SCLM. Batch melting of such a source can produce high-Mg HDFs at 0.5% partial melting and Group I kimberlites at ~ 2%. Kankan HDFs and Group II kimberlites can be produced by 0.1 and 0.8% melting of average Archaean SCLM that carries phlogopite ± Fe–Ti oxides. The close correspondence between the trace-element composition of surface kimberlites and HDFs that were trapped at depth indicates that kimberlitic melts do not change their incompatible trace element contents much on their way to the surface (except for a possible loss of alkalis). The new data on the HDFs suggest a close genetic relation between high-Mg carbonatitic HDFs and kimberlites and reveals the similarity of the trace element of both to that of the post-Archaean SCLM. This similarity may reflect the interaction of such melts with the lithospheric keel, its melting to produce HDF and/or kimberlites or melting of deeper sources that led to formation of HDFs and kimberlite and to widespread metasomatism of the SCLM.

Sequential melting and fractional crystallization: Granites from Guarda-Sabugal area, central Portugal

Seven distinct phases of Variscan two-mica granite are recognized in the Guarda-Sabugal area. They intruded the Cambrian schist-metagraywacke complex, crystallized in the middle crust, and are syn- to late-D3 (309.2 ± 1.8 Ma), late-D3 (304–300 Ma) and late- to post-D3 (299 ± 3 Ma; ID-TIMS ages on zircon and monazite). Two of the granites, G2 and G5, are close in age and have similar Sr, Nd and O isotope characteristics but contrasting whole rock and mineral features and formed by sequential increasing degree of partial melting of a common metasedimentary protolith. During sequential melting Ti, total Fe, Mg, Ca, Zr, Zn, Sr, Ba and REE contents and (La/Yb) N increase and Si and Rb contents decrease, plagioclase becomes richer in anorthite and biotite and muscovite richer in Ti and Mg. Each of these granites evolved subsequently by fractional crystallization of quartz, K-feldspar, plagioclase, biotite and ilmenite, defining separate series G2–G3–G7 and G5–G6 containing late Sn-bearing differentiates. Two other granites G1 and G4 represent distinct pulses of magma with individual fractionation trends for major and trace elements and distinct ( 87Sr/ 86Sr) 300, ɛNd 300 and δ 18O values.

Geochemistry of granitic aplite-pegmatite sills and petrogenetic links with granites, Guarda-Belmonte area, central Portugal

Granitic amblygonite-subtype and lepidolite-subtype, aplite-pegmatite sills intruded a biotite>muscovite granite (G1). Two other biotite>muscovite granites (G2 and G3) and a muscovite>biotite granite (G4) crop out in the area. Variation diagrams for major and trace elements of the Variscan rocks show fractionation trends for a) G1 and G4; b) G2, G3 and aplite-pegmatite sills. The two series are confirmed by the two trends defined by major elements of primary muscovite. The sills also contain Li-bearing muscovite, which has higher Mn, Li, F and paragonite contents and lower Al V1 content than primary muscovite from G2, G3 and sills. All sills have pure albite and P 2 O 5 content of K-feldspar and plagioclase increases in the series G2, G3 and sills. Beryl occurs in all sills, but lepidolite and a nearly pure petalite only occur in lepidolite-subtype sills, which are the most evolved sills. Primary topaz and amblygonite have a similar composition in all sills. Aplite-pegmatite sills contain cassiterite, which shows sequences of alternating darker and lighter zones. The former are richer in (Nb + Ta + Fe + Mn) than the latter. Manganocolumbite is common in all sills, but ferrocolumbite only appears in amblygonite-subtype sills and manganotantalite in lepidolite-subtype sills. The sills richest in Li contain reversely-zoned crystals with a homogeneous microlite core and a heterogeneous uranmicrolite rim. Least squares analysis of major elements shows that granite G3 and amblygonite-subtype and lepidolite-subtype aplite-pegmatite sills can be derived from granite G2 magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Modelling of trace elements shows good results for Sr, but magmatic fluids controlled the Rb and Ba contents of the aplite-pegmatite sills and probably also their Li, F, Sn and Ta contents and crystallization of lepidolite, cassiterite and Nb-Ta oxide mineral assemblage. Schorl from the lepidolite-subtype sills that cut granite G1 has higher Mg/(Mg + Fe) than schorl from metasomatised granite at sill walls and resulted from the mixing of magmatic fluids carrying B and some Fe with a meteoric fluid that has interacted with the host granite G1 and carried Fe and Mg. Schorl and dravite, respectively from metasomatised granite and micaschist at sill walls, were also formed from the mixing processes.

Filiform ilmenite crystals from Lemolo Lake, Douglas County, Oregon

Filiform ilmenite crystals from Lemolo Lake, Douglas County, Oregon