Primary Zoning Research Articles

Application of the Probability Theory Methods for Scheming the Area of Epizootiological Survey of the Central-Caucasian High-Mountain Natural Plague Focus

It is known that territories of nature plague foci are non-equal by the degree of epizootic situation tension. There are locations (sectors of primary regions) where plague has not been registered or has been registered rarely during all the time of surveillance. In this connection, it is necessary to reestablish the borders of plague focus territory for excluding the sectors without plague manifestations or the areas where plague occurs periodically. However, mathematically reliable criteria to substantiate such decisions are currently unknown to the authors. Objective was to propose evidence-based criteria for the exclusion of certain areas from plague focus territory using probabilistic approach. Material and methods . Surveillance data on Central-Caucasian natural plague focus for the period of 28 years (1987-2013) were used. For each sector of primary zoning in the focus a multiplicity of surveillances and multiplicity of plague epizooty reports was calculated. Some nonparametric techniques (Spearman correlation, quantile analysis) and concepts of probability theory were used for statistical processing. Results and discussion . By the example of Central-Caucasian high-mountain natural plague focus, the method for probability calculation for manifestations of epizooties in the sectors of plague focus is put forward. The evidence-based criteria for the exclusion of certain areas from plague focus territory using probabilistic approach are proposed. Algorithm for excluding the sectors of primary zoning from the surveillance scheme taking into account different levels of significance is presented. The ranking of the focus by the probability of epizooty detection in a sector and probability of surveillance can be utilized alongside the index of frequency of epizooties when planning epizootiological survey of the focal territory.

Early thrusting and folding in the Snow Lake camp, Manitoba: tectonic implications and effects on volcanogenic massive sulfide deposits

The Chisel sequence occurs within the Snow Lake arc assemblage (SLA) of the Paleoproterozoic Trans-Hudson Orogen. It is host to six economic volcanogenic massive sulfide (VMS) deposits that formed within a single time-stratigraphic ore interval. The SLA was previously interpreted to share the same deformation history as the ca. 1.86–1.84 Ga Burntwood Group turbidites of the Kisseynew Basin to the north; however, early isoclinal folds identified in the arc assemblage cannot be correlated with structures in the turbidites and therefore developed prior to Burntwood Group deposition. This indicates that the SLA underwent a significant deformation event prior to 1.86 Ga, likely due to intraoceanic accretion or pre-1.86 Ga accretion to the Amisk collage during closure of the ancestral Manikewan Ocean. Evidence for this early folding event is recorded in the Chisel sequence deposits where it has influenced the geometry of the deposits and the location of the ore interval. The deposits are isoclinally folded by these folds, and a major early thrust fault at the Lalor deposit, the Lalor-Chisel fault, displaced the hanging-wall rocks of the deposit and the ore interval. Further flattening, transposition, and elongation of the ore lenses occurred during post-1.84 Ga collision of the SLA with the partially hidden Archean Sask craton. Although the VMS deposits are polydeformed and sulfides and precious metals were mechanically and hydrothermally remobilized at the metre scale, their primary base metal zoning remains intact and provides a tool to reconstruct the geometry of the deposits.

Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Čierna Lehota, Strážovské vrchy Mountains, Slovakia: crystal chemistry and evolution

AbstractSilicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulfides and other minerals occur in Devonian black shales near Čierna Lehota in the Strážovské vrchy Mountains, Slovakia. The garnets have high concentrations of V, Cr and Mn (up to 17 wt.% V2O3, ≤11 wt.% Cr2O3and ≤ 21 wt.% MnO) and several compositional types. Vanadian-chromian grossular (Grs 1) usually preserves primary metamorphic oscillatory zoning, whereas solid solutions between goldmanite (Gld 2A,B), V- and Cr-rich grossular and spessartine (Grs 2A,B, Sps 2) form irregular domains or crystals with variable zoning. Dominant substitutions in the garnets include CaMn–1and (V,Cr)Al–1, resulting in coupled Ca(V,Cr)Mn–1Al–1. Epidote-supergroup minerals occur as abundant anhedral crystals with variable compositional zoning. Nearly all crystals have a complete zoning sequence beginning withREE-rich allanite-(La), followed by mukhinite and by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor clinozoisite. In common with garnets, the epidote-supergroup minerals are enriched in V, Cr and Mn (<7 wt.% V2O3, <5 wt.% Cr2O3and <3 wt.% MnO). Lanthanum is the dominantREE(up to 11.5 wt.% La2O3) in allanite-(La). The composition of epidote-supergroup minerals is controlled byREEFe2+(CaAl)–1,REEMg(CaAl)–1,REEMn2+(CaAl)–1andREEFe2+(CaFe3+)–1substitutions introducingREE, together with VAl–1and CrAl–1substitutions. The negative Ce and slightly positive Eu anomalies displayed in chondrite-normalized patterns and enrichment in V, Cr and Mn are ascribed to the geochemical properties of the protolith. The minerals investigated exhibit multi-stage evolution: (1) presumed low-grade greenschist-facies metamorphism; and (2) development of V- and Cr-rich zones in both garnet- and epidote-supergroup minerals which result from late-Variscan contact metamorphism due to granitic intrusion of the Suchý Massif. Decreased temperature following the metamorphic peak probably resulted in the formation ofREE-, V- and Cr-poor clinozoisite and secondary garnet.

The Skaergaard trough layering: sedimentation in a convecting magma chamber

The upper parts of the floor cumulates of the Skaergaard Intrusion, East Greenland, contain abundant features known as troughs. The troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase-rich material. The origin of the troughs and layering is contentious, attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls, or to in situ development by localised recrystallisation during gravitationally-driven compaction. They are characterised by outcrop-scale features such as mineral lineations parallel to the trough axis, evidence of erosion and layer truncation associated with migration of the trough axis, and disruption of layering by syn-magmatic slumping. A detailed microstructural study of the modal trough layers, using electron backscatter diffraction together with geochemical mapping, demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution–reprecipitation. Instead, the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning. We argue that the lineations and foliations are, therefore, a consequence of grain alignment during magmatic flow. Post-accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid, with upwards migration of the Si-rich conjugate into the plagioclase-rich upper part of the layers, whereas the Fe-rich immiscible conjugate remained in the mafic base. Both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal-rich currents descending from the nearby chamber walls.

A Modeling Approach for Farmland Protection Zoning Considering Spatial Heterogeneity: A Case Study of E-Zhou City, China

Farmland protection in China is facing multiple pressures, including urbanization, population growth and ecological degradation. Primary farmland zoning was introduced as a basic state policy to ensure national food security. Previous studies about “primary farmland zoning” have always taken the factors as global linear variables, neglecting the spatial heterogeneity of the study areas. Based on the Development Priority Zoning (DPZ) strategy, we present a zoning approach using Bayesian networks (BNs) for considering regional differences. The networks were developed using quantitative biophysical and economic variables from multi-scale and historical change data of farmland. The simulated results substantiate that this method can ensure the agricultural quality, stability and connectivity of primary farmland. Furthermore, it can further optimize the spatial allocation of primary farmland to meet the development conditions and trends of different sub-regions, promoting the balance between the farmland protection, urban development and ecological protection of certain regions.

Monazite-(Ce) and Xenotime-(Y) From An Lct, NYF Tertiary Pegmatite Field: Evidence From A Regional Study In the Central Alps (Italy and Switzerland)

Abstract Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U + HREE content. The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution–reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

Northwest Africa 5790: Revisiting nakhlite petrogenesis

Northwest Africa 5790, the latest nakhlite find, is composed of 58vol.% augite, 6% olivine and 36% vitrophyric intercumulus material. Its petrology is comparable to previously discovered nakhlites but with key differences: (1) Augite cores display an unusual zoning between Mg# 54 and 60; (2) Olivine macrocrysts have a primary Fe-rich core composition (Mg#=35); (3) The modal proportion of mesostasis is the highest ever described in a nakhlite; (4) It is the most magnetite-rich nakhlite, together with MIL 03346, and exhibits the least anisotropic fabric. Complex primary zoning in cumulus augite indicates resorption due to complex processes such as remobilization of former cumulates in a new magma batch. Textural relationships indicate unambiguously that olivine was growing around resorbed augite, and that olivine growth was continuous while pyroxene growth resumed at a final stage. Olivine core compositions (Mg#=35) are out of equilibrium with the augite core compositions (Mg# 60–63) and with the previously inferred nakhlite parental magma (Mg#=29). The presence of oscillatory zoning in olivine and augite precludes subsolidus diffusion that could have modified olivine compositions. NWA 5790 evidences at least two magma batches before eruption, with the implication that melt in equilibrium with augite cores was never in contact with olivine. Iddingsite is absent.Accordingly, the previous scenarios for nakhlite petrogenesis must be revised. The first primary parent magmas of nakhlites generated varied augite cumulates at depth (Mg# 66–60) as they differentiated to different extents. A subsequent more evolved magma batch entrained accumulated augite crystals to the surface where they were partly resorbed while olivine crystallized. Trace element variations indicate unambiguously that they represent consanguineous but different magma batches. The compositional differences among the various nakhlites suggest a number of successive lava flows. To account for all observations we propose a petrogenetic model for nakhlites based on several (at least three) thick flows. Although NWA 5790 belongs to the very top of one flow, it should come from the lowest flow sampled, based on the lack of iddingsite.

Typomorphic features of placer gold of Vagran cluster (the Northern Urals) and search indicators for primary bedrock gold deposits

The Vagran placer cluster is located on the eastern slope of Northern Urals. During >100years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits.Bedrocks of the region consist of high metamorphic Upper Proterozoic and Paleozoic terrigeneous, terrigeneous-volcanogenic and igneous rocks. Gold placer deposits are mostly alluvial genesis deposits and of Quaternary to Oligocene (?) age. The alluvial deposits consist of gravel with pebbles, boulders, and sandy clay covered by sandy silt and a soil layer. The thickness of the alluvial sequence is usually 5–10m and reaches 18m in the main watercourses of the third order. Nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock.There are four different types of gold particles: (I) rounded and well-rounded particles of high fineness and homogeneous inner structure, (II) rounded to sub-rounded high fineness particles with a pure gold rim developed over a core, (III) crystallomorphic (idiomorphic) high fineness with a homogeneous inner structure, and (IV) irregular angular and subangular particles of medium fineness with a significant content of Ag (10–40wt.%) and elevated Hg (up to 1.15wt.%).The first type is prevalent and comprises up to 65% of the total gold particles; it is uniformly distributed throughout the territory. There are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation. The second and third types have a propensity for zones of the inherited erosion–tectonic depressions. Apparently, types I, II and III are related with orogenic mesothermal gold-sulfide-quartz mineralization; the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys. They were connected with middle-depth ore bodies of an orogenic gold-sulfide-quartz formation. The fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence. It is controlled by a zone of NW-SE orientation, diagonal to the main structures of Ural Fold Belt.The plot of Au content vs coefficient of heterogeneity (ratio of the Au content in the core and in the rim of the grains) is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features.No corresponding lode occurrence of gold-sulfide-quartz mineralization has been identified to date in this region. Placer gold concentrations are related to the intermediate hosts of the Mesozoic-Cenozoic surfaces of the Ural peneplain uplift in the Oligocene and eroded in Miocene-Quaternary time. This factor determines the widespread distribution of placer gold in the territory of the Vagran cluster.The large, Carlin-type Vorontsovsk gold deposit is located 60km south-east from the Vagran area. It has a shallow erosional level, small size of native gold, and its distal location from the placer deposits makes it an unlikely primary source for the Vagran placers. However, mineralization of this type of deposit is noted within the cluster.Gold of the fourth type nearly resembles the gold of the Vorontsovsk deposit and, apparently, the source is related to the same hydrothermal mineralization event. ICP MS analyses of the quartz-sulfide lodes in the floor of gold-bearing valleys revealed a gold content of 2.0–6.9g/t in the zone of type IV distribution. Therefore, gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization. The geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the Vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type.

The petrology of the Upper Albian tuffites from the Bakhchysarai district, southwestern Crimea

We present the first data on the composition of plagioclase and quartz-hosted melt inclusions, plagioclase primary magmatic zoning and mineral compositions for the Upper Albian tuffites from the Bakhchysarai district. The estimated composition of the magmatic melts corresponds to the high potassium rhyolites. The measured water content in the melt inclusions reaches 4 wt %. The phenocrysts crystallized at the temperature within the range 820–860°C and under the pressure not less than 0.1 GPa. The Upper Albian tuffites from the Bakhchysarai district were formed in the active continental margin environment.

Exploring the Moon's surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin?

AbstractRemotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan‐1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high‐Ti mare basalt 74275 are one of these ambiguous examples. In this high‐Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X‐ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe‐rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high‐pressure mineral assemblages exhibited by the Mg‐suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg‐suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low‐Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low‐Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus‐Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re‐equilibration. During the last episode of re‐equilibration, the dunite cumulate was sampled by the 74275 high‐Ti basalt and transported over a period of 30–70 days to the lunar surface.

Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40Ar/39Ar systematics, and implications for the isotopic dating of impact events

This study presents the first optical and scanning electron microscopic characterization and U–Pb SHRIMP dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large, ∼40–90km eroded Ediacaran Acraman impact structure in South Australia. Microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism, from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer-sized spots of ZrO2 (probably baddeleyite) that pseudomorph pre-impact zircon. The granular zircons correspond to the highest observed level of shock metamorphism and impact-induced recrystallization. ZrO2-bearing granular zircons indicate shock pressures in excess of ∼65–70GPa, which are considerably higher than previous shock pressure estimates for the Acraman impactites. U–Pb systematics of untreated and chemically abraded melt rock zircons indicate that U–Pb ratios of the Acraman zircons were variably reset during impact. Weakly shocked crystalline grains yield ages on concordia at ∼1.59–1.60Ga reflecting the magmatic age of the Gawler Range Volcanics. Only the entirely granular zircon population was apparently impact-reset, but based on an Ediacaran age from stratigraphic constraints on the ejecta layer, experienced significant post-impact Pb loss. The microcrystalline nature of granular zircons could have promoted Pb diffusion and α-recoil in post-impact time, as suggested by grain size-dependent diffusion and recoil modeling. A positive correlation of U concentration and shock level suggests that granularization might have preferentially occurred in initially U-rich, probably metamict, zircons. 40Ar/39Ar dating of a melted Yardea Dacite clast from the Acraman melt rock, as well as K-feldspar separated from shocked Yardea Dacite, resulted in post-impact alteration plateau ages suggestive of hydrothermal events at ∼500Ma and ∼450Ma that selectively affected the impactites exposed in the central domain of the Acraman impact structure. Our study demonstrates that the Acraman impact is particularly difficult to date. In the absence of accurate and precise isotopic ages for Acraman, the Ediacaran ejecta-stratigraphic age of ∼635–541Ma is considered the most reliable age constraint currently available for the timing of the large Acraman impact.

Peculiarities of behavior of trace elements during fractional crystallization of sulfide magmas

The behavior of elements of copper-nickel ores during fractional crystallization of sulfide magma, which is composed of more than 20 significant major and trace elements, is considered on the basis of the theory of the direct crystallization of multicomponent melts. During this process, the trajectory of variation of composition of the t-component melt moves consecutively along the elements of the liquidus surface, which corresponds to the crystallization from 1 to t phases with formation of the primary zoning of the orebody. The composition of ores gradually varies inside the zone and changes strongly when it transits from one zone to another. The zonal distribution of major ore elements is superimposed by zoning of the accompanying and trace elements including PGEs, gold, and silver. The peculiarities of the behavior of trace elements, which are cocrystallized with major ore-forming elements or form individual microphases, are analyzed. The last portions of sulfide magma is enriched in Pt, Pd, Au, and other noble metals, which are crystallized from the melt at the final stage of formation of the orebody as an assemblage of minerals of these elements.

Local variations of carbon isotope composition in diamonds from São-Luis (Brazil): Evidence for heterogenous carbon reservoir in sublithospheric mantle

Taking advantage of the common occurrence of superdeep mineral inclusion assemblages, we examined core-to-rim primary zonation in 69 diamonds from the alluvial deposits of the São-Luis River (Juina, Brazil). Syngenetic inclusions were represented by phases of superdeep paragenesis. The dominant inclusions are majoritic garnets, ferropericlases and CaSi- and CaSiTi-perovskites. Rare inclusions of MgSi-perovskites, olivines, clinopyroxenes, TAPP, phases of SiO2, kyanites, AlSi-phases, KFsp (K-hollandite?), CF, NAL, grossular, merwinite, native iron, Fe-sulphides, magnesite and CaCO3+CaMgSi2O6 (composite inclusions) were also found.The diamonds from São-Luis display wide variations in carbon isotopic compositions (δ13C), from +2.7 to −25.3‰. The diamonds with inclusions of ferropericlase have a very narrow range of δ13C values, from −2.1 to −7.7‰, which are close to the “normal” mantle values. In many cases, diamonds with inclusions of calcic majoritic garnet and CaSi- and CaSiTi-perovskites display marked differences from this range. Low δ13C values (−10 to −25 ‰) were observed exclusively in a series of diamonds with majorite garnets, Ca-silicates, aluminous silicates and SiO2. The δ13C values from the cores to rims in certain individual crystals varied substantially, indicating multiple stages of growth. The highly negative δ13C values in the cores (−20 to −25 ‰) potentially represent organic matter in metasediments of altered oceanic crust, and the lower δ13C values may represent mixing trends towards “normal” mantle compositions. In this study, we also found a set of diamonds that display an opposite trend of change of the carbon source, from primordial mantle to subducted/crust (either biotic or abiotic carbon).

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U-Pb (uranium-lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

The behavior of noble-metal admixtures during fractional crystallization of As- and Co-containing Cu–Fe–Ni sulfide melts

To study the behavior of macrocomponents and admixtures during the fractional crystallization of sulfide melts and the influence of As on noble metals in this process, we performed a quasi-equilibrium directional crystallization of melt of composition (at.%): Fe—35.5, Ni—4.9, Cu—10.4, and S—48.3, with admixtures of Pt, Pd, Rh, Ru, Ir, Au, Ag, As, and Co (each 0.1 at.%), which imitates the average (by Cu contents) compositions of massive ores at the Noril'sk Cu-Ni deposits. The following sequence of phase formation from melt has been established: mss (zone I) / mss + iss (zone II) / iss (zone III) (mss is (FezNi1–z)S1+δ, iss is (FexCuyNi1–x–y)zS1–z); it corresponds to the distribution of main elements along the sample (primary zoning). Distribution curves for macrocomponents in zones I and II of the sample were constructed, as well as the dependencies of their partition coefficients (k) between solid solutions and sulfide melt on the fraction of crystallized melt. The secondary (mineral) zoning resulted from subsolidus phase transformations has been revealed. Five subzones have been recognized: mss + cp (Ia) / mss + cp + pn (Ib) / mss + pc + pn (IIa) / mss + pc + pn + bn (IIb) / pc + bn + pn + unidentified microphases (III). Admixture species in the sample were studied: (1) admixtures dissolved in primary solid solutions and in main minerals resulted from solid-phase transformations and (2) admixtures forming their own mineral phases. The partition coefficients of Co, Rh, and Ru (mss/L), Ru, Ir, and Rh (mss/cp), and Co, Rh, and Pd (mss/pn) were determined. Minerals of noble metals have been recognized: Pt3Fe, PtFe, Au, (Ag,Pd), (Au,Pt), Ag, Ag3Cu, Au3(Cu,Ag,Pd,Pt), etc., and the regularities of their distribution in the sample have been established. It is shown that some noble-metal admixtures are prone to interact with As. Mineral arsenides and sulfoarsenides of noble metals produced during fractional crystallization have been recognized: PtAs2, Pd3As, (RhAsS), (IrAsS), and (Ir,Rh)AsS. The discovered drop-like inclusions of noble-metal arsenides suggest the separation of the initial sulfide-arsenide melt into two immiscible liquids. By indirect features, the micromineral inclusions are divided into primary, crystallized from melt, and secondary, produced in solid-phase reactions. The results of study are compared with literature experimental data obtained by the isothermal-annealing method and with the behavior of noble metals and As during the formation of zonal massive orebodies at the Noril'sk- and Sudbury-type deposits.

Physicochemical prerequisites for the formation of primary orebody zoning at copper-nickel sulfide deposits (by the example of the systems Fe–Ni–S and Cu–Fe–S)

The zoning of massive orebodies at Cu–Ni sulfide deposits such as Noril'sk and Sudbury is commonly explained by fractional crystallization of magmatic sulfide melt. On the theoretical description of fractionation of its components, the results of mineralogical studies of orebodies are usually interpreted using the Rayleigh equation or its modification. But this equation is not applicable to describe crystallization of multicomponent melt and cocrystallization of several phases. In this work we present strict equations describing the distribution of components in a directly crystallized sample. We analyzed the influence of phase reactions on the successive formation of phases during crystallization and on the formation of primary zoning in the sample. This approach permits one to compute the component distribution curves and the crystallization paths by the quantitative phase diagram model. An experimental study of fractionation in the systems Fe–Ni–S and Cu–Fe–S was carried out. They can be regarded as systems modeling the formation of Ni- or Cu-rich sulfide ores. Such studies also yield qualitative and quantitative information about the phase diagrams of geochemical systems. We demonstrated that directed crystallization can be applied to determine the equations of phase reactions and the dependence of partition coefficients on the melt composition and to construct the paths of crystallization and evolution of the tie-line position during one-phase and cotectic crystallization. By the example of the system Fe–Ni–S, all possible types of sample zoning after fractional crystallization are shown. The main regularities of fractionation have been formulated, which are also applicable to multicomponent systems, e.g., Cu–Fe–Ni–S, which is widely used on the modeling of formation of zonal Cu–Ni sulfide ores.

The geochemistry and isotopic age of eclogites from the Belomorian Belt (Kola Peninsula): evidence for subducted Archean oceanic crust

We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–repre-cipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ∼2.86–2.87, ∼2.82–2.80, and ∼2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.

The time of eclogite formation in the ultrahigh pressure rocks of the Sulu terrane: Constraints from Lu–Hf garnet geochronology

Eclogites from the main borehole of the Chinese Continental Scientific Drilling project yield highly precise Lu–Hf garnet–clinopyroxene ages of 216.9±1.2Ma (four samples) and 220.5±2.7Ma (one sample). The spatial distribution of the rare earth elements in garnet is consistent with the preservation of primary growth zoning, unmodified by diffusion, which supports the interpretation that the Lu–Hf ages date the time of formation of garnet, the major rock forming mineral in the eclogites. The preservation of primary REE-zoning, despite peak metamorphic temperatures around 800–850°C, indicates that the Lu–Hf chronometer is perfectly suitable to date garnet-forming reactions in high grade rocks. The range of Lu–Hf ages for eclogites in the Dabie–Sulu UHP terrane point to episodic rather than continuous growth of garnets and thus punctuated metamorphism during the collision of the North China Block and the Yangtze Block. The U–Pb ages and Hf-isotope systematics of zircon grains from one eclogite sample imply a protracted geologic history of the eclogite precursors that started around 2Ga and culminated in the UHP metamorphism around 220Ma.

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.

Dating coeval mafic magmatism and ultrahigh temperature metamorphism in the Anápolis–Itauçu Complex, Central Brazil

Dating granulites has always been of great interest because they represent one of the most extreme settings of an orogen. Owing to the resilience of zircon, even in such severe environments, the link between P–T conditions and geological time is possible. However, a challenge to geochronologists is to define whether the growth of new zircon is related to pre- or post-P–T peak conditions and which processes might affect the (re)crystallization. In this context, the Anápolis–Itauçu Complex, a high-grade complex in central Brazil with ultrahigh temperature (UHT) granulites, may provide valuable information within this topic.The Anápolis–Itauçu Complex (AIC) includes ortho- and paragranulites, locally presenting UHT mineral assemblages, with igneous zircon ages varying between 760 and 650Ma and metamorphic overgrowths dated at around 650–640Ma. Also common in the Anápolis–Itauçu Complex are layered mafic–ultramafic complexes metamorphosed under high-grade conditions.This article presents the first geological and geochronological constraints of three of these layered complexes within the AIC, the Damolândia, Taquaral and Goianira–Trindade complexes. U–Pb (LA-MC-ICPMS, SHRIMP and ID-TIMS) zircon analyses reveal a spread of concordant ages spanning within an age interval of ~80Ma with an “upper” intercept age of ~670Ma. Under cathodoluminescence imaging, these crystals show partially preserved primary sector zoning, as well as internal textures typical of alteration during high-grade metamorphism, such as inward-moving boundaries. Zircon grains reveal homogeneous initial 176Hf/177Hf values in distinct crystal-scale domains in all samples. Moreover, Hf isotopic ratios show correlation neither with U–Pb ages nor with Th/U ratios, suggesting that zircon grains crystallized during a single growth event. It is suggested, therefore, that the observed spread of concordant U–Pb ages may be related to a memory effect due to coupled dissolution–reprecipitation process during high grade metamorphism.Therefore, understanding the emplacement and metamorphism of this voluminous mafic magmatism is crucial as it may represent an additional heat source for the development of the ultrahigh temperature paragenesis recorded in the paragranulites.