Sector Zoning Research Articles

Combining in situ isotopic, trace element and textural analyses of quartz from four magmatic-hydrothermal ore deposits

This study couples in situ 16O, 17O and 18O isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic-hydrothermal quartz crystals. Euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic-hydrothermal ore deposits: El Indio Au–Ag–Cu deposit; Summitville Au–Ag–Cu deposit; North Parkes Cu–Au deposit and Kingsgate quartz-Mo–Bi–W deposit. The internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps. Quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry (for 40 trace elements in quartz) and sensitive high-resolution ion microprobe (for 3 isotopes in quartz). Imaging revealed fine oscillatory zoning, sector zoning, complex “macromosaic” textures and hidden xenocrystic cores. In situ oxygen isotope analyses revealed a δ18O range of up to 12.4 ± 0.3 ‰ in a single crystal—the largest isotopic range ever ascribed to oscillatory zonation in quartz. Some of these crystals contain a heavier δ18O signature than expected by existing models. While sector-zoned crystals exhibited strong trace element variations between faces, no evidence for anisotropic isotope fractionation was found. We found: (1) isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis (e.g., δD–δ18O) using bulk analytical techniques, (2) the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and (3) in three of the four samples, both textural and isotopic data indicate non-equilibrium deposition of quartz.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

Trace element systematics of tourmaline in pegmatitic and hydrothermal systems from the Variscan Schwarzwald (Germany): The importance of major element composition, sector zoning, and fluid or melt composition

An extensive data set on the compositional variation of tourmaline from granitic pegmatites, from migmatitic gneisses and from various types of hydrothermal veins from the Schwarzwald, Germany, is provided. The investigated tourmalines are members of the alkali and X-vacant groups representing mostly dravite–schorl solid solutions with some analyses belonging to the foitite–Mg–foitite series. Oxygen isotope data on quartz–tourmaline pairs indicate formation temperatures between 550 and 350°C for most of the quartz–tourmaline veins.Most of the tourmalines show strong sector zonation, fractionating certain major (e.g., Na, Mg), minor (e.g., Ti, Ca) and trace elements (e.g., Sr, Pb, REE) among the different crystallographic sectors of the crystals. We conclude that comparison of tourmaline compositions for petrogenetic interpretations must be based on the concentrations in the neutral a sector in order to eliminate the effects of intra-crystalline fractionation.Most trace element contents vary over several orders of magnitude with median concentrations generally between 0.1 and 10μg/g. The highest concentrations are found for Zn, V, Sr, Sc, Sn and Li (median concentrations of several tens to hundreds of μg/g) whereas very low median concentrations (<0.1μg/g) were found for U, Th, Bi, Tl and Cs. The concentration of several trace elements (e.g., Sr, REE, Co) correlates with the major element composition of the tourmaline, which implies potential crystal chemical effects on their incorporation. Consequently, the general assumption that tourmaline is a passive monitor for the chemical compositions of the melts or fluids from which it crystallizes has to be questioned. Nevertheless, our study demonstrates that the trace element composition of tourmaline allows to distinguish between geochemically distinct fluid sources, and the trace element composition of tourmaline may therefore be used to unravel the chemical complexity of granite-related hydrothermal systems.

SHRIMP U–Pb dating of zircons related to the partial melting in a deep subduction zone: Case study from the Sanbagawa quartz‐bearing eclogite

AbstractDehydrated fluid from a subducting cold slab is considered to cause deep focused earthquakes and island‐arc volcanism. The Sanbagawa high P–T metamorphic rocks have been considered as typical cold oceanic material subducted during the Cretaceous. However, we have discovered an eclogite outcrop exhibiting partial melting texture in the Sanbagawa metamorphic belt, central Shikoku, Japan. The discovery is significant because the melt may play an important role in deep‐focused earthquakes and the melt itself may directly contribute to the origin of island‐arc magmatism. In order to confirm the age of partial melting of Sanbagawa metamorphic rocks, we dated zircons from both the melted portion and the host eclogite using the U–Pb SHRIMP age‐dating at the Korean Basic Science Institute. Zircons from the melted portion (SHT16, 75) are rounded and have sector zoning. The core and mantle yield U–Pb age in the 130–113 Ma range (120 Ma average), and the rim ages are in the 115–104 Ma range. Zircons from the eclogite (SHT15, 76) have a homogenous core with thin mantle and rims. The U–Pb ages are concentrated at 123–112 Ma. These pieces of evidence suggest that the eclogite metamorphism occurred at about 120 Ma and subsequent partial melting happened at about 110 Ma.

Testing the validity of detrital zircon chemistry as a provenance indicator

Zircon grains from the Nile Delta beach sediments were mineralogically investigated for their provenance perspective. Electron microprobe backscattered electron images, and wavelength dispersive spectroscopy line scans showed well-developed oscillatory and sector zoning in the majority of the studied zircon grains. In addition, some grains showed inclusions represented mainly by apatite. The zoning in zircon reflects compositional variations of Zr, Si, Hf, P, Y, and rare earth elements (REE). Based on the electron probe microanalyzer data, the calculated formula of the studied detrital zircon is A(Zr3.82Hf0.05Th0.003Ca0.10Fe0.008REE0.033Tl0.03Y0.001)∑4.04 T(Si3.90P0.08Al0.011V0.003Sc0.004)∑4.00O4. Th, Y, P, Sc, and Fe are substituted into zircon either by simple or coupled substitution mechanisms. Additional zircon grains separated from Abu Swayel granites and metamorphic gneisses, south Eastern Desert of Egypt were similarly investigated for comparison. Although evidence based on morphological characteristics, zoning, apatite inclusions, and mineral chemistry supports a derivation of zircon from granitic rocks including the hydrothermally altered varieties, the possibility that the zircon grains may have more than one source cannot be ruled out due to the large area of the Nile watershed.

X-ray Rietveld and 57Fe M^|^ouml;ssbauer study of babingtonite from Kouragahana, Shimane Peninsula, Japan

Babingtonite from Kouragahana, Shimane Peninsula, Japan, was investigated using electron microprobe, X-ray Rietveld, and 57Fe Mössbauer spectral analyses to characterize its chemical compositions, crystal structure, oxidation state of Fe, and distribution of Fe between two crystallographically independent octahedral Fe1 and Fe2 sites. The Kouragahana babingtonite occurs as single parallelohedrons with {100}, {001}, {001}, {111}, {110}, and {101} and sometimes shows penetration twinning. Both normal and sector-zoned crystals occur. Babingtonite crystals with sector zoning consist of sectors relatively enriched in Fe and of sectors enriched in Mg, Mn, and Al. Babingtonite also shows compositional zoning with higher Fe2+ and Al core and higher Fe3+ and Mn rim. The average Fe content of the babingtonite without sector zoning is similar to the Fe2+-rich sector of the sector-zoned babingtonite. The chemical formula based on the average composition of all analytical data (n = 193) is [Na0.01(2)Ca2.01(2)] [Mg0.11(4)Mn0.09(3)Fe2+0.76(7)Fe3+0.93(5)Ti0.01(1)Al0.06(5)]Si5.01(4)O14(OH). X-ray Rietveld refinement was carried out using a model of space group P1. The result of the refinement is characterized by R-weighted pattern = 9.91, R-expected pattern = 6.37, and goodness-of-fit = 1.56. The unit cell parameters are a = 7.4667(3), b = 11.6253 (6), c = 6.6820(2) Å, α = 91.533(4), β = 93.886(3), γ = 104.203(4)°, and V = 560.43(4) Å3. The refined site occupancies of atoms in the Fe1 and Fe2 sites are [Fe0.91(2)Mg0.09] and [Fe0.91(2)Al0.09], respectively, if Mg is assumed to distribute in the Fe1 site and Al in the Fe2 site. By allocating Mn, 0.09 atoms per formula unit (a.p.f.u.), to the Fe1 site, the site populations in the Fe1 and Fe2 sites are determined as [Fe0.82(2)Mn0.09Mg0.09] and [Fe0.91(2)Al0.09] a.p.f.u., respectively. The 57Fe Mössbauer spectrum taken at room temperature consists of three peaks, which were resolved into two doublets assigned to Fe2+ and Fe3+ at the two octahedral sites. The Fe2+:Fe3+ ratio was determined as Fe2+:Fe3+ = 43.3(3):56.7(4) and 47.1(4):52.9(3) by applying two fitting models, and the average Fe2+:Fe3+ ratio was 45.2(4):54.8(4). The results of X-ray Rietveld analysis and Mössbauer spectroscopy indicate that Fe2+ and Fe3+ are ordered at the Fe1 and Fe2 sites, respectively, in the Kouragahana babingtonite.

Tourmaline as a prospecting guide for the porphyry-style deposits

Tourmaline-supergroup minerals are proposed as efficient indicators of porphyry-style Cu, Au and Sn deposits and therefore are considered as useful for both the prospection and exploration of these important mineral deposits. The tourmaline-supergroup minerals from the porphyry deposits exhibit some general features: (1) sector and oscillatory chemical zoning of crystals; (2) presence in several generations; (3) Fe → Al coupled substitution; (4) evolution from Fe-rich to Mg-rich varieties resulting from sulphide deposition at the late stage; (5) Li concentration ranging from a few to ~30 ppm. On the other hand, there are distinguishing features for tourmalines from different deposits: (1) total Mg content is ~2 apfu for tourmaline in the Cu deposits, 1–2 apfu in the Au deposits, and 0–1 apfu in Sn deposits; (2) maximum Fe tot ~3 apfu (Cu, Sn) and ~5 apfu (Au), (3) maximum F content up to 0.1 apfu (Cu, Au) and up to 0.5 apfu (Sn); (4) Fe 3+ /Fe tot ~0.5–0.8 (Cu, Au) and ~0.2 (Sn).

Interpreting U–Pb data from primary and secondary features in lunar zircon

In this paper, we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon U–Pb isotopic systems and the significance of these features for understanding lunar processes. Lunar zircons can be classified according to: (i) textural relationships between zircon and surrounding minerals in the host breccias, (ii) the internal microstructures of the zircon grains as identified by optical microscopy, cathodoluminescence (CL) imaging and electron backscattered diffraction (EBSD) mapping and (iii) results of in situ ion microprobe analyses of the Th–U–Pb isotopic systems. Primary zircon can occur as part of a cogenetic mineral assemblage (lithic clast) or as an individual mineral clast and is unzoned, or has sector and/or oscillatory zoning. The age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data. A secondary set of microstructures, superimposed on primary zircon, include localised recrystallised domains, localised amorphous domains, crystal–plastic deformation, planar deformation features and fractures, and are associated with impact processes. The first two secondary microstructures often yield internally consistent and close to concordant U–Pb ages that we interpret as dating impact events. Others secondary microstructures such as planar deformation features, crystal–plastic deformation and micro-fractures can provide channels for Pb diffusion and result in partial resetting of the U–Pb isotopic systems.

Granulite facies metamorphic age and tectonic implications of BIFs from the Kongling Group in the northern Huangling anticline

We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo (赵家阳坡) area from the Kongling (崆岭) Group in the northern Huangling (黄陵) anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim tex- tures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon

Zircon U/Pb dating and Hf-O isotopes of the Zhouan ultramafic intrusion in the northern margin of the Yangtze Block, SW China: Constraints on the nature of mantle source and timing of the supercontinent Rodinia breakup

The Zhouan ultramafic intrusion in the northern margin of the Yangtze Block is mainly composed of lherzolite. Zircon grains selected from lherzolite are irregular in shape with distinct oscillatory and sector zoning and have Th/U ratios ranging from 0.8 to 10.6, indicating a magmatic origin. The weighted average 206Pb/238U age is 637±4 Ma (2σ, n=15), which can be considered as the crystallization age of the Zhouan intrusion. Zircon grains have δ18O values ranging from 5.2‰ to 7.0‰, with an averaged value of 5.8±0.4‰ (1σ, n=33), similar to the mantle δ18O value of zircon. Their 176Hf/177Hf(t) ratios range from 0.282410 to 0.282594 with ɛHf(t) values ranging from 1.3 to 7.6, lower than the corresponding value of the depleted mantle (∼15), indicating an enriched mantle source. The enriched mantle source may have generated from a metasomatized lithospheric mantle with subducted slab. A number of ∼635 Ma mafic-ultramafic intrusions in the Suizao basin are associated with coeval bimodal volcanics of the Yaolinghe Formation, indicating a continental rift setting. The ∼635 Ma magmatic event in this region may represent the product of the last breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block at Neoproterozoic.

Clinopyroxene from basaltic rocks of the Erzgebirge - Krušné hory Mts. - implications for modelling the magmatic plumbing system

*Corresponding author The Erzgebirge-Krusne hory area is part of the NW flank of the Eger-Ohře Rift. Within this rift-related area, basaltic rocks with low ( 3.5 wt. %) of TiO 2 occur. The distribution of these basaltic rock types shows strong spatial dependency. Ti-rich basalts are concentrated in the western Erzgebirge-Krusne hory Mts. with a sharp change at longitude 13.3 °E. Clinopyroxene and olivine are the most abundant phenocrysts. The zoning and chemistry of the clinopyroxene act as a recorder of the different stages of the magmatic plumbing systems. Seven different types of clinopyroxene phenocrysts are defined with four distinct chemical types: Cr-rich clinopyroxene, green-core py roxene, Mg-rich clinopyroxene and finely zoned Ti-rich clinopyroxene with sector zoning. Chemical and petrographic analysis of individual zones within clinopyroxenes allows the qualitative description of distinct steps of the multistage, regionally diverse processes yielding to the formation of both Ti-rich basalts in the W as well as Ti-poor basalts in the E. Two different types of asthenospheric melts are suggested, with increased activities of H 2 O and potassium in the western Erzgebirge Mts. During ascent they incorporated xenoliths and xenocrysts of both the mantle (peridotite, olivine and Cr-rich clinopyroxene) and the crustal (granitic and gneissic rocks, quartz and feldspar) origin. Furthermore, green cores of clinopyroxene were introduced into the magmas. Fractionation processes took place within lithospheric magma chambers, having been accompanied by reaction of xenocrysts and xenoliths with the melt. The residence time in the magma chambers associated with the eastern and western parts of the Erzgebirge appears to have been totally different: short in the eastern and long in the western parts. Within the latter, no mantle-derived xenoliths, xenocrysts, or magmatic olivines are found. It is inferred that these xenoliths and crystals settled to the bottom of the magma chambers forming cumulates and therefore did not come to the surface. In contrast, the basaltic rocks of the East still contain mantle-derived xenoliths and xenocrysts as well as magmatic olivine. The size and amount of clinopyroxene phenocrysts in the West significantly exceed that of the East. Moreover, the crystallization of giant clino pyroxenes was restricted to the western region. Mixing of the magmas with Ti-rich melts, presumably derived from an eclogitic source, took place in crustal magma chambers. In any case, the proportion of Ti-rich melts in the West significantly exceeded that of the East.

Unidirectional solidification textures and garnet layering in Y-enriched garnet-bearing aplite-pegmatites in the Cadomian Brno Batholith, Czech Republic

Layered, garnet bearing aplite pegmatite dykes of the Hlina granitic suite, ~2 to 50 m thick and up to ~200 m long with general NW SE orientation and dip 40 80 to NE or SW, cut granodiorites to granites at the SW part of the Brno Batholith, Brunovistulicum, Czech Republic. Aplite pegmatite bodies are characterized by alternation of two main textural units: (i) fine grained aplite unit with garnet stripes and (ii) coarse grained pegmatite unit with oriented Qtz, Plg and Kfs comb like crystals known as Unidirectional Solidification Textures (UST). Both units developed in zones with the thickness varying from several cm to ~1 2 m for aplite unit, and ~10 cm for the UST unit. All rock types are characterized by high contents of SiO2 (74.6 75.7 wt. %), K2O (4.61 4.94 wt. %), Na2O (3.82 4.21 wt. %), moderate concentration of CaO (0.94 1.11 wt. %), and low to very low concentrations of Fe2O3 T (0.62 0.93 wt. %), MgO (0.0 0.03 wt. %), and TiO2 ( 0.03 wt. %). Low K/Rb (212 241) and high K/Ba ratios (1034 2303) with deep Eu anomaly indicate high degree of fractionation. Both textural units consist of perthitic microcline, plagioclase An15 8 and quartz. Total amount of accessory minerals is typically very low, commonly < ~1 vol. % in aplite unit, and in the UST unit they are almost absent. Accessory minerals include relatively common Y rich garnet Sps42 38 Alm32 28 And15 7 Grs21 15 Prp2 1 (1.10 wt. % Y2O3, 0.53 wt. % Yb2O3 and 0.20 wt. % Er2O3) with oscillatory and sector zoning. Other minerals closely associated with garnet are extremely rare: magnetite, chloritized biotite, muscovite, Ta rich titanite I, Al, F rich titanite II, and ilmenite. Primary zircon, xenotime (Y), monazite (Nd), fersmite, ferrocolumbite, REE,Y rich pyrochlore are strongly altered. Geochemical and mineralogical features of the Hlina aplite pegmatites are characterized by (i) subaluminous chemistry with Nb >> Ta and HREE >> LREE, A type (NYF) affinity, indicated by elevated concentrations of Y, REE (especially HREE), Zr, U, Th, Nb, and Ta, (ii) occurrence of specific accessory minerals including Y rich garnet, and (iii) a remarkable, almost complete absence of micas and other minerals with volatiles (B, F, P and also H2O). The studied suite differs from all other granitic rocks with UST described to date, which are typically characterized by volatile bearing minerals and peraluminous signatures. Magmatic layering involving g

U–Pb zircon age of an Ordovician tuff in southeast Yukon: implications for the age of the Cambrian–Ordovician boundary1Yukon Geological Survey Contribution 013.2Geological Survey of Canada Earth Science Sector (ESS) Contribution 20110362.

Three conodont fossil occurrences stratigraphically above and below a rhyolitic tuff in the Lower Ordovician Crow Formation, southeast Yukon, constrain the tuff to the early Tremadocian Rossodus manitouensis Zone and possibly the Rossodus tenuis Zone. U–Pb dates were obtained from zircon from the tuff that was imaged with cathodoluminescence and chemically abraded to reduce the likelihood of dating grains that contain older components or suffered Pb loss, respectively. Six dates from grains with oscillatory parallel zoning are equivalent, with a weighted mean 206Pb/238U date of 491.04 ± 0.13 Ma. These grains are interpreted as being primary volcanic crystals, and the date is therefore taken as the depositional age of the tuff. Four other dates from grains with sector zoning are slightly older, up to 492.0 Ma, and are interpreted as inherited or recycled from earlier volcanics. The currently defined Cambrian–Ordovician boundary is inferred to be below the tuff and separated from it by at least two conodont biostratigraphic zones that are estimated to span at least 2.25 Ma based on the top of the Rhabdinopora flabelliformis parabola graptolite zone being older than the base of the Rossodus tenuis conodont zone. We interpret the evidence in southeast Yukon to suggest that the global Cambrian–Ordovician boundary is older (&gt;493.3 Ma) than previous estimates of ∼488 and 491 Ma based upon legacy 207Pb/206Pb zircon ages.

Ion microprobe U–Th–Pb geochronology and study of micro-inclusions in zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan

We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100μm up to 500μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6±2.4Ma (MSWD=8.5), have higher U and Th contents with a Th/U ratio>1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9±1.2Ma (MSWD=4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.

TEXTURE AND COMPOSITION OF Pb-BEARING PYRITE FROM THE COKA MARIN POLYMETALLIC DEPOSIT, SERBIA, CONTROLLED BY NANOSCALE INCLUSIONS

The Coka Marin polymetallic (Cu-Au-Ag-Zn-Pb) deposit belongs to the world-class Bor ore district in Serbia and consists of three lens-shaped orebodies hosted by Late Cretaceous andesitic volcanic rocks. Pyrite is the dominant mineral in the deposit and shows a great variety of textures and zoning. It occurs as fine- to coarse-grained crystals and massive, spongy, colloform, framboidal, oolitic, and replacement aggregates. Complex relationships with other minerals and the presence of microscale mineral inclusions and micro- to nanoscale porosity are common features of the pyrite at Coka Marin. Minor amounts of Cu (up to 7.9 wt.%), Pb (up to 7 wt.%), As (up to 4 wt.%), Ag (up to 1 wt.%), Te (up to 0.07 wt.%), and rarely Sb (up to 0.11 wt.%) were detected in pyrite using an electron microprobe. Zoning and heterogeneity in pyrite is mainly caused by variable copper content. Structurally bound Cu in pyrite from this deposit reported in a previous study was confirmed by our new electron microprobe analyses. Distinct Pb-bearing zones frequently occur in pyrite following oscillatory zoning and crystal shapes. These zones also contain lesser amounts of the other elements mentioned above. Using the focused ion beam (FIB) technique to prepare electron-transparent foils and transmission electron microscopy (TEM), we found that these elements form nanoscale Pb-(±As, Ag, Cu)-bearing inclusions (mosty <50 nm) in these distinct zones. Sector zoning in pyrite is caused by the presence of Ag and Pb and is evidence for a purely crystallographic control of a nanoscale inclusion entrapment during pyrite growth. Entrapment of nanoscale Pb-bearing inclusions in distinct crystal zones of pyrite, as well as complex mineral relationships in the deposit, were most likely associated with abrupt changes in ore-forming conditions and rapid co-precipitation of metal sulfides in a sub-volcanic environment. This study reports the first evidence of Pb-bearing pyrite zones containing nanoscale inclusions and shows a good example of the use of FIB-TEM techniques in resolving the dilemma of whether chemical impurities form solid solutions or nanoscale inclusions in a mineral.

Garnet sector and oscillatory zoning linked with changes in crystal morphology during rapid growth, North Cascades, Washington

Metamorphic garnet with sector zoning in the cores and oscillatory zoning in the rims grew during rapid heating of pelitic rocks in the Chiwaukum Schist. These types of compositional zoning are exemplified by sharp, but low amplitude, boundaries between broad petal-shaped sectors and between narrow concentric zones, respectively. Zoning is most obvious in calcium, which is inversely correlated with iron and magnesium content. Garnet habit inferred from the oscillatory calcium zoning and external morphology indicates a growth transition from early crystal forms with both trapezohedral and dodecahedral forms to later crystal forms dominated by trapezohedral faces. This transition is accompanied by changes in compositional sector zoning and may reflect the roles of local growth dynamics and external forcing mechanisms affecting growth of garnet. Subsequent modification of these textures produced patchy zoning in calcium. Electron backscatter diffraction confirms the inferred crystal growth habits and that compositional zoning occurs within single garnet crystals. Exchange thermobarometry and the peak mineral assemblages indicate that metamorphic conditions reached 640-670 °C at 6.9 kbar. These temperatures are sufficient to allow significant volume diffusion; therefore, preservation of the finely banded compositional zoning in garnet requires rapid heating and cooling. Garnet Sm-Nd ages and indistinguishable zircon U-Pb ages from adjacent orthogneiss bodies confirm that garnet grew rapidly during localized and short-lived heating adjacent to sill-like intrusions of tonalite.

Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis

High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS 13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between 12C and 13C in the growth medium to the interface of graphite precipitation in different growth sectors.

Two stages of zircon crystallization in the Jingshan monzogranite, Bengbu Uplift: Implications for the syn-collisional granites of the Dabie–Sulu UHP orogenic belt and the climax of movement on the Tan-Lu fault

The detailed study of zircon can provide vital clues about the petrogenetic environment of granitoid rocks. Morphological and chemical studies of zircon grains from the Jingshan monzogranite in the Bengbu Uplift (Anhui province, eastern China) identify three phases of growth. Zircon I is brown, translucent, ovoid, and occurs as inherited cores. It shows two groups of 206Pb/ 238U ages, corresponding to the country rock of the Jingshan monzogranite and the basement of the Bengbu Uplift. Zircon II is colorless, transparent and idiomorphic-hypidiomorphic with I pr = 0.34–0.52, I py = 0.03–0.24 and I el = 0.26–0.34. It shows a very bright CL due to high contents of trace elements (e.g., Y, U and Th), and the oscillatory zoning associated with sector zoning. It contains 1.40–1.66 wt.% HfO 2 with a mean ε Hf (t) of − 17.88. These features indicate the igneous crystallization of Zircon II from a peraluminous granite of mainly crustal origin. Based on the field geology, petrography, geochemical analysis, and especially a weighted mean age of 222 Ma for Zircon II, we argue that the Jingshan monzogranite is a syn-collisional granite of the Dabie UHP orogenic belt. This provides new evidence for the northward subduction of the South China Block beneath the North China Block just before 222 Ma, and allows quantitative estimates of the rate of post-UHP exhumation in the Dabie orogenic belt. Zircon III is also colorless and transparent, but is totally idiomorphic with I pr = 0.41–1.00, I py = 0.88–1.00 and I el = 0.39–0.83. It occurs only as overgrowths on Zircon II, and shows weak CL due to its depletion in trace elements. Zircon III has widely variable contents of HfO 2 (1.12 to 3.01 wt.%) and Hf-isotope compositions very similar to those of Zircon II. These features suggest crystallization of Zircon III from small volumes of leucosome, probably in the beginning stages of migmatisation. Zircon III has a weighted mean age of 156 Ma, interpreted as representing the climax of movement on the Tan–Lu fault. This suggests that the compressive movement of the Tan–Lu fault has decreased progressively since its climax at 156 Ma. These two major conclusions from the zircon studies contrast with previous interpretations of the Jingshan monzogranite as a late Jurassic post-collisional granite in the Dabie orogenic belt.

Growth Marks of titanian-andradite crystals from Colli Albani (Italy)

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 89-104 - DOI: 10.2451/2011PM0007 Special Issue in memory of Sergio Lucchesi Growth Marks of titanian-andradite crystals from Colli Albani (Italy) Giovanna Agrosi * , Eugenio Scandale and Gioacchino Tempesta Dipartimento Geomineralogico - Universita degli Studi di Bari Aldo Moro, Italy *Corresponding author: g.agrosi@geomin.uniba.it Abstract Growth defects and chemical zonings of Ti-rich andradite (melanite) crystals from Colli Albani were studied using X-ray Diffraction Topography and Electron Probe Microanalysis in order to determine the distinctive features, marks, characterising minerals grown directly from high temperature solution in an open system (volcanic chamber). The samples, cut in (110) and (001) slices, showed a number of primary and, above all, multiphasic melt inclusions, as well as, an unusual colour zoning consisting of a darker colour in the {211} growth sectors with the respect to the {110} sectors. The analysis of X-ray topographic contrasts of growth defects as growth bands, sector boundaries and bundles of dislocations developed parallel to the growth directions has enabled the reconstruction of the morphological evolution vs. time of the samples. The results also suggest that from their onset the crystals were faceted with {211} and predominant {110} faces. Moreover, the topographic images indicated that most of the examined dislocations were nucleated from inclusions. By using the extinction criterion, it was possible to determine that these defects are characterised by a strong edge component. The faceted morphology of garnets found in this paper and the fact that edge dislocations were only characterised suggest a layer spreading growth mechanism by two-dimensional nucleation. The reconstruction of morphology was also particularly useful in distinguishing two different types of chemical zoning: concentric and sector zoning. The results of chemical analyses confirmed that the optically observed colour zoning coincided with sector zoning because of a small but meaningful enrichment of TiO 2 and a decrease of Al 2 O 3 in correspondence with {211} growth sectors. Instead, concentration variations, chiefly of TiO 2 and SiO 2 , were recorded crossing successive growth stages. This event appeared to be associated with the physio-chemical evolution of the growth environment and was defined as concentric zoning. Comparing the results found in our study with those of previous studies on garnets from hydrothermal and metasomatic environments, the meaning of the growth marks in the Ti-rich andraditic garnets from Colli Albani could be assigned to primary multiphase melt inclusions, concentric and sector zoning, sector boundaries, growth bands and edge dislocations since these defects characterise and distinguish our samples from those grown in different genetic environments. Key words : X-ray Topography; Ti-rich andradite; structural defects; concentric zoning; sector zoning.

Cooperation of upper and lower boundary-layer fractionations in a sheet-like intrusion: Composition and microstructure of the Aosawa dolerite sill in Yamagata Prefecture, northeastern Japan

Homogeneous fractionation, involving crystal nucleation, growth, and separation from a melt-dominant magma, and boundary-layer fractionation, involving the separation of a fractionated interstitial melt from a crystal-dominant boundary layer and its mixing with the main magma body, have been advocated as two major differentiation mechanisms in a crustal magma chamber. In this study, we focus on one of the dolerite sills in the Aosawa area, Yamagata Prefecture, Japan, to elucidate the roles of the two mechanisms in the differentiation of a sheet-like magma body. The intrusion is concordantly intruded into black mudstone and is ∼ 100 m thick and more than 5 km in lateral extension. The chilled margins contain olivine (5.3 vol%) and plagioclase (1.9 vol%) as phenocrysts. There is an absence of clinopyroxene in the chilled margins, and there are systematic sill-scale variations in the whole-rock major- and trace-element contents that require the addition or removal of clinopyroxene. These findings suggest sill-scale differentiation, involving the transportation of crystals that nucleated and grew in the sill, and/or the transfer of the residual melt. The downward increases in mode and size of clinopyroxene and in the size of the most dominant plagioclase, all with maxima near the bottom, and the occurrence of pigeonite rimming augite only near the bottom, suggest a slower cooling rate in the lower boundary layer compared to the upper one. Clinopyroxene crystals with Cr-rich cores that show textures suggesting rapid growth, such as remarkable sector zoning, melt inclusions, and small euhedral plagioclase, are more abundant at 5-15 m above the lower contact. These observations lead us to believe that clinopyroxene nucleated and grew near the upper boundary layer, where large supersaturation occurred, and then settled to concentrate in the lower boundary layer, where slower cooling provided suitable conditions for pigeonite crystallization. The upward increase in the incompatible elements and the decrease in compatible elements from the zone containing a high concentration of Cr-rich clinopyroxene suggest that a fractionated melt, which was formed through enhanced overgrowth on settled clinopyroxene, was transported upwards. The sill-scale magmatic differentiation in the Aosawa dolerite intrusion took place through crystal settling from the roof boundary layer, followed by the upward transportation of the fractionated melt from the bottom boundary layer. The lower-boundary-layer fractionation was probably very effective because the settling of the crystals thickened the bottom boundary layer to facilitate an effective melt-crystal separation.