Cathodoluminescence Imagery Research Articles

Metamorphic P–T paths of Archean granulite facies metasedimentary lithologies from the eastern Beartooth Mountains of the northern Wyoming Province, Montana, USA: constraints from quartz-in-garnet (QuiG) Raman elastic barometry, geothermobarometry, and thermodynamic modeling

Abstract. Metamorphic pressure and temperature (P–T) paths in late-Archean high-grade rocks of the eastern Beartooth Mountains of Montana (USA), a portion of the Wyoming Province, are established by a combination of imaging, analytical, and modeling approaches. Garnet inclusion mechanical and chemical thermobarometry, applied to several granulite-facies migmatites and an iron formation, is particularly useful in constraining the prograde P–T conditions. Quartz-in-garnet (QuiG) elastic Raman barometry was used on quartz inclusions in garnet for all samples studied. For a smaller subset of four representative samples, QuiG constraints were used in conjunction with Ti-in-quartz (TitaniQ) and Ti-in-biotite (TiB) thermometry to establish unique prograde inclusion entrapment P–T conditions. Ti measurements of garnet hosts and cathodoluminescence (CL) imagery of inclusion and matrix quartz grains were employed to check for Ti loss/diffusion. Lastly, inclusion studies were supplemented with thermodynamic modeling and matrix chemical thermobarometry to examine garnet nucleation temperatures and peak metamorphic conditions. Disagreement between the volume strain and elastic tensor methods used to calculate quartz inclusion pressures implies that quartz inclusions studied are under strong differential strain. Prograde entrapment results from the two inclusion thermobarometry pairs used are distinct: 0.55–0.70 GPa and 475–580 ∘C (QuiG–TitaniQ) versus 0.85–1.10 GPa and 665–780 ∘C (QuiG–TiB). Garnet modal isopleth modeling indicates that the majority of garnet growth occurred at ∼ 450–600 ∘C, implying that P–T conditions of garnet growth are interpreted to be most reliably represented by QuiG–TitaniQ inclusion thermobarometry. Normal distributions of calculated QuiG inclusion pressures and the concentration of mineral inclusions in garnet cores suggest that the majority of garnet inclusions were entrapped during a single stage of porphyroblast growth. A general lack of evidence from CL imagery for post-entrapment mechanical or chemical modifications to quartz inclusions suggests that quartz inclusions used to calculate entrapment P–T largely preserve their initial entrapment conditions. Biotite inclusions preserve higher temperatures than quartz inclusions in the same garnets, likely due to Fe–Mg exchange with garnet hosts that allowed Ti content of biotite to change after entrapment. Pseudosection modeling and matrix chemical thermobarometry of multiple, independent lithologies examined during inclusion studies suggest a range of peak granulite facies conditions of ∼ 0.50–0.70 GPa and 730–800 ∘C. Peak metamorphic P–T modeling work from this study, together with evidence of regional amphibolite facies overprinting in immediately adjacent samples, indicates an overall clockwise metamorphic P–T path with nearly isobaric prograde heating to peak temperatures. Interpreted P–T path reconstructions are consistent with metamorphism developed in a more modern-style continental arc subduction zone and are observed in portions of the northern Wyoming Province as exemplified by metasupracrustal lithologies of the eastern Beartooth Mountains.

Phenakite and bertrandite: products of post-magmatic alteration of beryl in granitic pegmatites (Tatric Superunit, Western Carpathians, Slovakia)

AbstractThe beryllium silicate minerals phenakite and bertrandite have been identified in granitic pegmatite dykes of the beryl-columbite subtype of Variscan age (~340−355 Ma), associated with S- to I-type granitic rocks of the Tatric Superunit, Western Carpathians (Slovakia). The two beryllium silicates and associated minerals were characterised by electron microprobe analysis, back-scattered electron petrography and cathodoluminescence imagery, X-ray diffraction and micro-Raman techniques. Phenakite and bertrandite form euhedral-to-anhedral crystals and aggregates in irregular domains and veinlets replacing primary magmatic beryl. A detailed textural study revealed a close genetic association of phenakite and bertrandite with secondary fine-grained quartz, K-feldspar and muscovite. Locally, clay phyllosilicate minerals, (with compositions similar to those of Fe-dominant hydrobiotite, beidellite, nontronite and saponite) occur as the youngest minerals. During the post-magmatic (hydrothermal) stage of the pegmatites, infiltration of aqueous K-bearing fluids at T ≈ 200–400°C resulted in the breakdown of magmatic beryl to secondary assemblages containing phenakite and bertrandite.

The importance of high precision in the evaluation of U-Pb zircon age spectra

We present high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon data from the Carboniferous Punteglias granodiorite from the Central Alpine basement in the Aar massif in Switzerland. These analyses yield complex age spectra consisting of both concordant and normally discordant dates, which form a linear array indicative of mixing of two age components within individual zircon crystals. While the upper intercept age reflects zircon growth during Variscan magmatism, the lower intercept of 24 ± 11 Ma is coincident with the Alpine orogeny. There is poor evidence from cathodo-luminescence imagery that magmatic zircon may have younger overgrowths, therefore, we pretreated an aliquot of zircon with physical abrasion followed by chemical abrasion to remove any potential later zircon overgrowths, and to directly date the initial magmatic domains of the zircon. This pretreatment significantly reduced age dispersion and yielded an age of 335.479 ± 0.041 Ma for the crystallization of zircon in the Punteglias granodiorite. Due to the effectiveness of this pretreatment, we interpret that the magmatic zircon was overgrown with younger domains during the Alpine orogeny. Detangling the observed complications in the Punteglias granodiorite zircons was largely possible due to the particularly high analytical precision of the ID-TIMS U-Pb dates that made it possible to detect discordance between the 206Pb/238U and 207Pb/235U systems. Lower-precision U-Pb ID-TIMS dates would not allow identification of such a two-component mixture and lead to analytically concordant points spreading over 105 to 106 years, and potentially leading to erroneous scientific interpretation. As an example, we model two hypothetical situations of simple mixing of two age components in the U-Pb concordia space to demonstrate the effect of precision on the ability to accurately interpret geological models from complicated age spectra. Combining these new high-precision data and modeling results highlights the importance of choosing the optimal, physical and/or chemical sample pre-treatment and the challenge of accurately interpreting complicated U-Pb data in terms of crystallization age.

Digital image analysis on cathodoluminescence microscopy images for ancient ceramic classification: methods, applications, and perspectives

This paper aims to investigate the application of cathodoluminescence (CL) imagery (with optical microscopy) and CL spectroscopy (with Scanning Electron Microscope) in ancient ceramics studies, for a proper classification of petro-fabrics and mineral inclusions based on their CL response. Digital image analysis (DIA) routines are proposed for both qualitative and quantitative analysis, and outcomes are reviewed in the light of classical optical microscopy (OM) and scanning electron microscopy (SEM–EDS) approaches. Obtained results demonstrated the suitability of CL in discriminating ceramic groups, offering also complementary information connected with aspects as provenance or manufacturing processes. CL imagery and spectroscopy data are also used to discuss perspectives of the method, debating bias, limits, or suspicious inconsistencies, exploring how compositional features or sample’s nature imprint CL response, and finally providing the basis for future developments. Addendum at the back of the paper includes and extensively discusses some technical aspects regarding the applied DIA routines, developed in open-sources software environments and available as supplementary materials.

Characterization of Archaeological Artefacts Using Methods Specific to Materials Science: The Case Study of Dacian Ceramics from 2nd c. BC to 1st c. AD.

Combined analysis methods such as optical microscopy (OM), cathodoluminescence (CL) microscopy, X-ray diffraction (XRD), and scanning electron microscopy–energy dispersive X-ray spectrometry (SEM–EDX) have made it possible to obtain the first physico-chemical data of Dacian potsherds, exhumed at the archeological site of Ocnița-Buridava, Romania; the samples were provided by the “Aurelian Sacerdoțeanu” County Museum Vâlcea, dating from the 2nd century BC to the 1st century AD. The mineralogical and petrographic analyses revealed two types of ceramic pastes, taking into account the granulometry of the inclusions and highlighting the choice of the potter for fabricating the ceramic either by wheel or by hand. All samples showed an abundance in quartz, mica (muscovite and biotite), and feldspars. These observations were confirmed by cathodoluminescence imagery, revealing heterogeneous pastes with varied granulometric distributions. The XRD patterns indicated the presence of the mineral phases, indicating a firing temperature below 900 °C. The wheel-made ceramics have a fine, compact matrix with very fine inclusions (<40 µm). On the other hand, the hand-made ceramics present a coarse matrix, with inclusions whose granulometry reaches approximately 2 mm. The difference between these two types of ceramics is also confirmed by the mineralogical and chemical analysis. The wheel-made potsherds are more abundant in MgO, Al2O3, and CaO contents.

Mineralogy and genesis of pyrochlore apatitite from The Good Hope Carbonatite, Ontario: A potential niobium deposit

AbstractThe Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesio-arfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1–1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (~100–500 μm × 10–25 μm) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under back-scattered electron spectroscopy, although ultraviolet and cathodoluminescence imagery shows distinct cores with thin (&lt;50 μm) overgrowths. Apatite lacks fluid or solid inclusions of other minerals. The apatite is rich in Sr (7030–13,000 ppm) and rare earth elements and exhibits depletions in La, Ce, Pr and Nd (La/NdCN ratios (0.73–1.14) relative to apatite in cumulate apatitites (La/NdCN &gt; 1.5) in the adjacent Prairie Lake complex. The pyrochlore are primarily Na–Ca pyrochlore of relatively uniform composition and minor Sr contents (&lt;2 wt.% SrO). Irregular resorbed cores of some pyrochlores are A-site deficient (&gt;50%) and enriched in Sr (6–10 wt.% SrO), BaO (0.5–3.5 wt.%), Ta2O5 (1–2 wt.%) and UO2 (0.5–2 wt.%). Many of the pyrochlores exhibit oscillatory zoning. Experimental data on the phase relationships of haplocarbonatite melts predicts the formation of apatite and pyrochlore as the initial liquidus phases in such systems. However, the texture of the clasts indicates that pyrochlore and apatite did not crystallise together and it is concluded that pyrochlores formed in one magma have been mechanically mixed with a different apatite-rich magma. Segregation of the apatite–pyrochlore assemblage followed by lithification resulted in the apatitites, which were disrupted and fragmented by subsequent batches of diverse carbonatites. The genesis of the pyrochlore apatitites is considered to be a process of magma mixing and not simple in situ crystallisation.

Supereruption quartz crystals and the hollow reentrants

AbstractHollow reentrants in quartz phenocrysts from Yellowstone (western United States) caldera’s Lava Creek Tuff are preserved vestiges of bubbles in the supereruption’s pre-eruptive magma reservoir. We characterized the reentrants using a combination of petrographic techniques, synchrotron X-ray microtomography, and cathodoluminescence imagery. One or more reentrants occur in ∼20% of quartz, and up to ∼90% of those reentrants are hollow. The earliest-erupted parts of the Lava Creek Tuff have the most empty reentrants. The hollow reentrants provide direct, physical evidence for volatile saturation, exsolution, and retention in a magma reservoir. Quartz-melt surface tension permits bubbles to attach to quartz only when bubbles have been able to nucleate and grow in the melt. Prior to eruption, the Lava Creek Tuff existed as a bubbly, volatile-saturated magma reservoir. The exsolved volatiles increased magma compressibility, helping to prevent the ever-accumulating magma from reaching a critical, eruptive overpressure until it reached a tremendous volume.

Application of titanium-in-quartz geothermometry to magmatic quartz in evolved rocks from the Bushveld Complex, South Africa

Abstract We report concentration data for the trace element titanium (Ti) in magmatic quartz in eight samples of leucogabbronorite, anorthosite, olivine ferrodiorite and quartz anorthosite from the Main and Upper Zones of the Bushveld Complex. The results, obtained by electron microprobe analysis, were used with Ti-in-quartz geothermometers (TitaniQ) to constrain the solidus temperatures of these rocks. Magmatic quartz is present sporadically in the mafic cumulate rocks of the 2950 m Bellevue borehole (Northern Limb), and commonly occurs as intercumulus, triangular patches, typically between cumulus feldspars and pyroxenes, suggesting that it crystallized late, from trapped interstitial melts. Quartz grains in rutile-bearing samples (implying TiO2 activity = 1) yield Ti concentrations of 48 to 106 ppm that constrain the solidus temperatures of the Bellevue cumulate rocks at T= 677 to 767ºC, using a calibration of the pressure-sensitive geothermometer at 3 kbar, thought to represent the emplacement pressure of the mafic cumulate rocks. These results imply a mean solidus temperature for Bushveld cumulate rocks of approximately 732 ± 31ºC (n = 7). Cathodoluminescence imagery (CL) of quartz from the studied samples reveals a variety of textures that correlate with Ti content, and hence temperature. These include rim-to-core (RC) zoning, homogeneous CL intensity and some samples show irregular patches of variable CL intensity. The observed zoning can be interpreted as intercumulus quartz grains that crystallized from their rims inward towards their cores.

A Practical Approach for Collecting Large‐n Detrital Zircon U‐Pb Data sets by Quadrupole LA‐ICP‐MS

A measurement procedure for the rapid acquisition of U‐Pb dates for detrital zircons by quadrupole LA‐ICP‐MS was developed. The procedure achieves a threefold increase in measurement efficiency compared with the most commonly used methods. Utilising reduced background counting times and a shortened ablation period, a throughput of ~ 130 measurements/h can be achieved. The measurement procedure was characterised and validated using data from thirty‐nine sessions acquired over a twelve‐month period. Systematic measurement error in 206Pb/238U dates for reference materials used for quality control with ages between 28.2 and 2672 Ma was &lt; 1.5%. Average measurement uncertainty, including both random and systematic components, was 1–4% (2s). Interrogation of time‐resolved calculated dates and signal intensities for each measurement allows for the detection and elimination of portions of measurements exhibiting age heterogeneities, zoning, lead loss and contamination by common lead. The measurement procedure diminishes the need to acquire cathodoluminescence imagery for routine detrital zircon applications further increasing throughput and reducing cost. The utility of the measurement procedure is demonstrated by the measurement of samples previously characterised by LA‐MC‐ICP‐MS.

Geochemistry, geochronology, and cathodoluminescence imagery of the Salihli and Turgutlu granites (central Menderes Massif, western Turkey): Implications for Aegean tectonics

The Menderes Massif (western Turkey) is an important metamorphic core complex located in the Aegean region; geochemical and geochronological data from this extensional domain facilitates our understanding of large-scale extension of the Earth's lithosphere. S-type, peraluminous granites (Salihli and Turgutlu) that intrude the Alasehir detachment which bounds the northern edge of the central Menderes Massif may have been generated due to subduction of the Eastern Mediterranean floor along the Hellenic trench. In situ Th–Pb ion microprobe monazite ages from the granites range from 21.7 ± 4.5 Ma to 9.6 ± 1.6 Ma (± 1 σ ). Higher uncertainty in the ages is attributed to monazite common Pb, but the range is consistent with cathodoluminescence (CL) imagery that document complex textures within the samples. Salihli and Turgutlu granites share many similar characteristics, including multiple generations of plagioclase (some with shocked cores), plagioclase replacing K-feldspar and the development of myrmekite, evidence for fluid interaction, and multiple generations of microcracks and microfaults. The granites may have evolved from compositionally distinct magma sources, as Salihli samples in general contain allanite as the major accessory mineral, whereas Turgutlu granites contain monazite. However, the CL imagery document similar alteration textures. The youngest Turgutlu monazite grain (11.5 ± 0.8 Ma) is located in the rim of a K-feldspar grain close to myrmekite, and the youngest Salihli monazite grain (9.6 ± 1.6 Ma) is located in the outer edge of an altered plagioclase crystal. These ages likely time episodes of deformation as the granites were subjected to exhumation along the Alasehir detachment. Ages reported here are similar to dates constraining extension reported elsewhere in the Aegean, but indicate a level of complexity when linking movement within the Menderes Massif to the large-scale geodynamic processes that created other metamorphic core complexes in the region. Difficulties exist in linking the ages obtained from the granites to specific tectonic events due to the presence of secondary alteration textures, generations of mineral growth, and multiple episodes of deformation.

Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: Evidence from zircon U-Pb geochronology

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge – hints at burial and exhumation velocities

AbstractIn order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone – 21 analyses: Th/U ≤ 40.023 and 337.0±2.7 Ma (2σ, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone – 23 analyses: Th/U ≥ 50.037 and 336.8±2.8 Ma; and (3) rim zone – 12 analyses: Th/U = 0.015–0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥ 50.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2σ) Ma was acquired from 113 analyses.By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (~1.8 GPa, 650ºC) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (&gt;1050ºC) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.

Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: New constraints on its tectonothermal history

The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic–gabbroic gneisses and multi-phase tonalitic–granitic gneisses. The western area is comprised of layered quartz dioritic–dioritic and tonalitic–granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies. To constrain the timing of the tectonothermal events, we have carried out U–Pb dating combined with cathodoluminescence imagery on zircon extracted from the gneisses and foliated granites. Our data reveal at least four tonalite–granite emplacement events in the eastern area, at ca. 3.94, 3.74–3.73, 3.66 and 3.59 Ga, and tonalite emplacement at ca. 3.97 Ga and granite intrusion at 3.58 Ga in the western area. The field relationships between the felsic and quartz dioritic gneisses in the eastern area demonstrate that the two quartz dioritic gneiss protoliths were emplaced prior to 3.59 and 3.66 Ga, respectively. These results confirm findings of previous zircon geochronology that the protolith ages of Acasta gneisses are 4.03–3.94, 3.74–3.72 and ca. 3.6 Ga. In addition, our comprehensive field observations, zircon internal structures and zircon U–Pb dating clearly demonstrate that the early Archean Acasta rocks suffered anatexis/recrystallization, coincident with the emplacement of younger felsic intrusions, and that the 3.59 Ga granitic gneiss protolith in the eastern area contains zircon xenocrysts with ages up to 3.9 Ga. In this context, we also discuss the tectonothermal evolution of the Acasta Gneiss Complex on the basis of these results and those from previous studies, and its implications for radiogenic isotopic studies.

The origin of jadeitite-forming subduction-zone fluids: CL-guided SIMS oxygen-isotope and trace-element evidence

Jadeitite, a rare high P/T rock, is associated spatially with blueschist and/or eclogite terranes. Scanning electron microscope (SEM) and cathodoluminescence (CL) petrography of jadeitite samples from several major occurrences [in Burma (Myanmar), Guatemala, Japan, Kazakhstan, and the U.S.A.] show that grains were deposited from s uids. Jadeite grain compositions indicate these s uid compositions changed with time. CL imagery guided the acquisition of oxygen-isotope and trace-element analyses with the ion microprobe. Jadeite grains in each rock grew in cycles that began with red- and/or blue-luminescent and ended with green-luminescent zones. The CL images were used to order the data into crystallization sequences. These data and electron-microprobe, major-element analyses document the association of green CL with increases in Ca, Mg, and Cr: (1) toward grain exteriors; (2) in Þ ne-grained matrix around porphyroblasts; (3) in shear zones that cut grains; (4) in former open spaces now Þ lled with jadeite; or (5) in veins. Abundances of many trace elements are greater in green-CL jadeitite compared with the red- or blue-CL zones. Some of these elements—in particular Li, Rb, Sr, Ti, Hf, Zr, Y, and REE—are unlikely to have been derived from serpentinite. Although crystal-chemical effects may explain some of the trace-element systematics (e.g., preferential incorporation of REE into Ca-richer jadeite), some kinetic control is suggested by sector-zoned, rhythmically zoned grains. The oxygen-isotope data suggest that jadeitite-depositing s uids either had multiple sources or evolved in composition along their s ow paths (or both).

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ18O and δ13C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ18O and 0.5‰ δ13C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ18O and −18‰ δ13C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ18O and −7‰ δ13C, with shifts towards higher δ18O values and higher and lower δ13C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ13C signatures of CO2-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ18O and −7‰ δ13C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ18O and δ13C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ18O (<10‰ calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation.

Rapid digital quantification of microfracture populations

Populations of microfractures are a structural fabric in many rocks deformed at upper crustal conditions. In some cases these fractures are visible in transmitted-light microscopy as fluid-inclusion planes or cement filled microfractures, but because SEM-based cathodoluminescence (CL) reveals more fractures and delineates their shapes, sizes, and crosscutting relations, it is a more effective structural tool. Yet at magnifications of 150–300×, at which many microfractures are visible, SEM-CL detectors image only small sample areas (0.5–0.1 mm 2) relative to fracture population patterns. The substantial effort required to image and measure centimeter-size areas at high-magnification has impeded quantitative study of microfractures. We present a method for efficient collection of mosaics of high-resolution CL imagery, a preparation method that allows samples to be any size while retaining continuous imagery of rock (no gaps), and software that facilitates fracture mapping and data reduction. Although the method introduced here was developed for CL imagery, it can be used with any other kind of images, including mosaics from petrographic microscopes. Compared with manual measurements, the new method increases several fold the number of microfractures imaged without a proportional increase in level of effort, increases the accuracy and repeatability of fracture measurements, and speeds quantification and display of fracture population attributes. We illustrate the method on microfracture arrays in dolostone from NE Mexico and sandstone from NW Scotland. We show that key aspects of microfracture population attributes are only fully manifest at scales larger than a single thin section.

SHRIMP U–Pb zircon geochronology of Neoproterozoic crustal granitoids (Southern Brazil): A case for discrimination of emplacement and inherited ages

SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc.

REE systematics in hydrothermal fluorite

REE patterns of fluorite from 63 post-Variscan hydrothermal veins of the Schwarzwald area, Germany, can be divided into two major groups. Type A fluorite shows roof-shaped PAAS normalized patterns with enriched MREE, the turning point of the curvature is at Ho or Er position. All patterns of type A show a distinct positive Eu anomaly. Type B fluorite shows roof-shaped patterns with enriched MREE and strongly depleted HREE and a characteristic, steep slope between Nd and Sm. The turning point of the curvature is at Gd to Dy, all patterns of type B show negative or no Eu anomalies. Laser ablation-ICP-MS traverses through single grains combined with cathodoluminescence imagery revealed either (i) identical REE patterns and total REE concentrations from core to rim, (ii) identical REE patterns, but concentration gradients or (iii) variable REE patterns and concentration gradients from core to rim. These features are explained by different degrees of supersaturation or different fluid/fluorite ratios during crystallization and by complexly controlled precipitation processes under closed system conditions. Petrographically late fluorites from several deposits show the characteristics of new fluid batches rather than the expected remobilization features. LREE enrichment close to the crystal faces in some grains may result from the copreciptiation of a submicrometer-sized LREE-enriched phase or from LREE clustering in late, cool fluids. By comparison with literature data on natural waters from gneissic and granitic aquifers and of leachates of granites and gneisses, we show that the hydrothermal vein fluorite preserves the characteristic, source-related REE feature of the hydrothermal fluid that migrated through either gneissic (type A) or granitic (type B) host rocks. The immediate host rocks of the veins, however, are not the source rocks of the REE, which indicates large-scale fluid convection in the basement. Gneiss-related type A fluorite shows Tb/Ca–Tb/La trends of primary hydrothermal crystallization whereas granite-related type B fluorite indicates remobilization of possibly pegmatitic or Variscan fluorite.

Characterization of an early metamorphic stage through inclusions in zircon of a diamondiferous quartzofeldspathic rock from the Erzgebirge, Germany

About 1000 zircon grains from a diamondiferous quartzofeldspathic rock of the Saxonian Erzgebirge were investigated for inclusions using optical microscopy and confocal laser-Raman spectroscopy. Cathodoluminescence imagery was applied to characterize the growth zone of zircon where the inclusions occurred. The most abundant inclusion minerals are microdiamonds. Coesite was not detected. However, garnet and jadeite occur as rare inclusions in zircon cores where diamonds are lacking. Jadeite was detected for the first time in quartzofeldspathic rocks from the crystalline complex of the Erzgebirge. The compositions of the pristine garnets in the zircons are similar to those of core areas of millimeter-sized garnets but the original garnet composition of the early metamorphic stage is only preserved in zircon. Intracrystalline diffusion at temperatures as high as 1000 °C resulted, for instance, in higher Ti concentrations in garnet cores compared with garnet enclosed in zircon. Rutile, quartz, and the compositions of jadeite and garnet inclusions in zircon and of phengite inclusions in cores of large garnets were applied for geothermobarometry. The results, related to an early metamorphic stage, are 650 °C and 18 kbar, which represent conditions at the base of a thickened continental crust before deeper subduction.

Reavaliação da evolução geológica em terrenos pré-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte II: Orógeno Araçuaí, Cinturão Mineiro e Cráton São Francisco Meridional

This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W transects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca . 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca . 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca . 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca . 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the Aracuai Orogen, the Caparao charnockitic granulite, revealed magmatic crystallization age of ca . 2195 Ma. Zircons from an amphibolite of the Pocrane Complex, also a basement inlier within the Aracuai Orogen, yielded an age of ca . 1500 Ma. Both inliers furnished well-constrained neoproterozoic metamorphic overprint at ca . 590 Ma. The subalkaline gneissic granite from the Salto da Divisa Suite yielded a magmatic crystallization age of ca . 870 Ma, close to U-Pb ages of the continental rift magmatism found in the African counterpart of the Aracuai Orogen. Four gneissic granites from the Aracuai Orogen (three of them previously interpreted as part of the Archean/Paleoproterozoic basement) were dated in the range of ca . 560-570 Ma, and were correlated to the g2 2 syn-collisional magmatism. Borders of some zircon grains from the older ( ca. 570 Ma) dated unit (Nanuque Granite) yielded ages around 500 Ma, similar to U-Pb ages obtained from titanite, monazite and zircon in other localities from the Aracuai Orogen. This youngest ages could be related to heating effects caused by melting of country rocks and ascent of magmas formed during the collapse of the orogen.