Zircon Content Research Articles

Stable and ultrastable heavy minerals of alluvial to nearshore marine sediments from Central Portugal: Facies related trends

The Pliocene to Pleistocene sedimentary record from the western Iberia Coastal Margin is a thin succession (maximum thickness around 70 m) derived from the metamorphic and igneous Variscan Iberian Massif and from older sedimentary deposits. It comprises a wide variety of facies representative of deposition in environments ranging from inner shelf to alluvial fan. The facies are stacked in an overall regressive sequence. This sequence may be divided into lower order sequences with thin transgressive portions (not always present) and thicker prograding portions. Given the limited burial diagenetic transformation, the differences in heavy mineral (HM) assemblage can be explained by provenance, hydraulic segregation and chemical and mechanical selection. Transparent heavy minerals susceptible to acid treatment were not considered in this heavy mineral analysis. After the acid treatment, the HM signal is relatively monotonous. Except for the mica-rich assemblages in alluvial facies fed from the Variscan Iberian massif by short streams, the HM assemblages are dominated by tourmaline, andalusite, zircon and staurolite. However, it is still possible to recognize two opposite trends in HM composition linked to facies evolution. The first trend is widespread along the coastal margin, although typical of the earlier deposits. It is characterized by a vertical increase in staurolite and andalusite proportions coincident with prograding and regressive facies evolution. Relative sediment starvation during retrogradation, when compared to progradation, favoured the recycling of previous deposits, leading to higher proportions of ultrastable minerals (tourmaline, zircon and rutile). Hydraulic segregation in the nearshore zone also contributes to increased zircon content in the very fine sand fraction. The second trend is observed in stratigraphically higher sediments, deposited when environments were displaced basinward. This trend is common in relatively uplifted western sectors of the coastal margin, where sand and gravel dominate the record. It is characterized by a vertical increase in the content of ultrastable minerals (mostly tourmaline and zircon) throughout the regressive sequence. It may be explained by alluvial storage and the recycling of previous nearshore and coastal plain facies. Recycling, favoured by relative sediment starvation or low accommodation conditions, plays a major role in these HM assemblages.

In situ formation of zirconia–alumina–spinel–mullite ceramic composites

A study was made to prepare and characterize multicomposite containing zirconia–mullite–spinel [ZrO 2–Al 6Si 2O 13– xMgAl 2O 4 (0.25 ≤ x ≤ 4)]. This multicomposite was prepared by reaction sintering of relatively pure commercial grades of zircon, alumina and magnesium carbonate. Spinel and mullite could be in situ formed, while ZrO 2 evolved mainly in monoclinic polymorph. The sintering properties in terms of bulk density (BD) and apparent porosity (AP) were measured. The formed phases were identified by X-ray diffraction pattern (XRD). Cold crushing strength (CCS) of the fired and thermally shocked bodies was evaluated. The fractured surfaces of the fired compacts were scanned by SEM. The results showed great dependence on the firing temperature as well as zircon content, type and amount of glassy phase. XRD exhibited that all samples contain m-ZrO 2 and Al 2O 3 and with decreasing the amount of MgO and increasing zircon, the spinel phase decreases while mullite begins to form at high zircon contents. Batches containing mullite, ZrO 2 and Al 2O 3 gave higher mechanical properties. Also, all samples retained more than 90% of their original strength after being subjected to 20 cycles of thermal shock (Δ t = 1000 °C).

PETROCHEMISTRY AND RADIOACTIVITY OF GABAL ABU AQARIB ALKALI FELDSPAR GRANITE, CENTRAL EASTERN DESERT, EGYPT

The present work deals with geology, geochemistry and radiometry of Gabal Abu Aqarib Alkali Granite, Central Eastern Desert, Egypt. Gabal Abu Aqarib about 136 km2, and represents a part of the late Precambrian Arabian Nubian Shield. It is of alkali feldspar granite composition and may be considered epeiorogenic uplifts granitoids. Geochemically, the granite rocks of G. Abu Aqarib were originated frommetaluminous calc-alkaline magmas that developed in volcanic arc post orogenic tectonic setting. The studied granite rocks of G. Abu Aqarib area were generated at depth of about 9-20 km equivalent to 4-6 kb and temperature ranging from 760oC to 840oC with multi-processes of both assimilation and fractional crystallization involving plagioclase, hornblende and Fe-Ti oxides from a partial melted lithospheric magma. The U and Th contents in the studied area are associated with hematitization and silicification. Also, the radioactivity are controlled by the presence of higher content of zircon as accessory minerals and iron oxides while iron oxide and hydroxides are known to absorb U from circulating fluids.

Crystallization thermometers for zircon and rutile

Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature: $$\log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.$$ In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T −1: $$\log ({\text{Zr}}_{{{\text{rutile}}}}) = (7.36 \pm 0.10) - \frac{{4470 \pm 120}}{{T\;(\hbox{K})}}.$$ The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.

Zircon geochemistry of different intrusive phases of Weiya pluton: implications for magma genesis

The characteristics of hosted magmas and their petrogenesis based on electron microprobe determination of trace element contents in zircons were discussed. Trace element geochemistry of zircons indicates that zircons in both gabbro and quartz syenite have two-generations. Zircons of the first generations are crystallized in the magma chamber, whereas those of the second generations are formed in supercooling environment. The former is richer in Zr, but poorer in U, Th, Hf and Y. Quartz diorite porphyrite contains zircons that can be distinguished into the early and late generations. Compared to the late generation, the early generation is richer in Zr but poorer in U, Th, Hf and Y. No conspicuous disruption of zircon evolution has been found in both biotite monzogranite and fine-grained granite. However, the content of zircon in fine-grained granite is higher in U, Th and Y and lower in Zr relative to biotite monzogranite without significant contrast in mass fraction ratio of ZrO2 to HfO2 ratio. Such differences in zircon geochemistry of various intrusive phases and the occurrence of the two zircon generations within a single intrusive phase suggest that these phases of magmas are generated from diverse sources during post-collisional continental extension. These magmas ascend rapidly and cool quickly in a short interval.

Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part II: Evolving zircon and thorite trace element chemistry

The Sn–W mineralized Mole Granite in Eastern Australia hosts zircon populations that crystallized at several stages during a protracted magmatic to hydrothermal evolution. Thirty-four elements have been quantified by laser-ablation inductively-coupled-plasma mass-spectrometric microanalysis with the aim of relating the chemistry of zircon to its growth environment. Trace element contents are highly variable for all textural occurrences. Zircon inclusions in earliest quartz phenocryst suggest that zircon was a liquidus phase that crystallized probably deep in the crust. Trace element contents are conspicuously high, showing only a slight positive Ce anomaly but a pronounced negative Eu-anomaly. Successive crystallization stages of magmatic zircon are characterized by progressive depletion in trace element contents, notably the rare earth elements, with an increasingly important positive Ce-anomaly. This evolution reflects saturation of REE accepting minerals such as monazite, thorite, xenotime and possibly apatite and is affected little by the exsolution of a magmatic–hydrothermal fluid. Zircon that is interpreted to have precipitated from aqueous fluids in Sn–W-bearing quartz veins shows REE patterns indistinguishable from those of late magmatic zircon. When combined with experimental evidence on the fluid–melt partitioning of REE, it indicates that the REE distribution coefficients for zircon/melt and zircon/fluid are largely comparable. The second example of hydrothermal zircon crystallized some 2 My after the host granite. These crystals reveal an intragranular zonation of increasing trace element concentrations from core to rim. Therefore, REE abundances and patterns alone are not conclusive indicators of the geological environment in which zircon crystallized. Nevertheless, variations in trace element contents of zircon that relate to the chemistry of the melt or fluid from which zircon crystallized, as measured in cogenetic melt and fluid inclusions, are promising for future petrogenetic modeling. Lead and Cs are strongly incompatible in hydrothermal zircon, with estimated zircon–fluid distribution coefficients D ≤ 0.001, while Sn and Li are moderately incompatible, D Sn ∼ 0.6 and D Li ∼ 0.1, and Ce is compatible, D Ce ∼ 14. Moreover, hydrothermal zircon has a more pronounced negative Eu-anomaly and higher Ta/Nb and U/Th ratios than the magmatic zircons of the Mole Granite.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.

Structure, emplacement and lateral expansion of the San José tonalite pluton, Peninsular Ranges batholith, Baja California, México

The 108 km 2 San José pluton forms part of the Jurassic to Cretaceous Peninsular Ranges batholith of northern Baja California, México. The pluton was formed by three nested, southward-migrating intrusive pulses, and the internal contacts between them indicate juxtaposition while the adjoining pulses were magmas. SHRIMP U–Pb zircon data indicate that the entire pluton was emplaced in less than 4.4 m.y.; ages of the individual pulses cannot be separated at the 95% confidence interval, owing primarily to low uranium content of zircon. Detailed structural data and geologic mapping are consistent with a component of asymmetrical, lateral expansion at the site of emplacement. The direction of maximum lateral expansion may have been controlled by thermal, compositional and resulting rheological gradients in the surrounding wall rocks. A carapace of predominantly solid-state deformation marks the northern two-thirds of the pluton, and the early stages of this fabric may have formed in the presence of a small amount of melt. Lack of evidence for syn- to post-emplacement regional ductile deformation around the pluton suggests that this carapace was deformed during the lateral expansion of the pluton.

Analysis of silica fume produced by zircon desilication

Novel chemical methods have been developed to allow for the determination of the components of silica fume produced by zircon desilication. Hitherto, no methods have been described for the analysis of this material. The amorphous silica is first removed by treatment with sodium hydroxide. The residue from the hydroxide treatment may then be subjected to a suite of reagents to determine the zircon, the total zirconia, the monoclinic zirconia and the tetragonal zirconia content of the fume. The zircon content of the fume is determined by treatment of the hydroxide residue with concentrated hydrofluoric acid (HF). The total zirconia content of the fume is determined by digestion of the hydroxide residue with fuming sulphuric acid (oleum), while the relative amounts of monoclinic and tetragonal zirconia may be found by treatment of the hydroxide residue with 10%w/v HF, which attacks the less stable tetragonal phase. Both X-ray diffraction and particle size analysis were used to validate the steps in the analytical procedure. An explanation of the presence of tetragonal zirconia in the fume is proposed. A greater understanding of the composition of the fume led to the installation of a separator in the company's production line to remove the zircon. Australian Fused Materials (AFM) now produces a vastly superior grade of fume marketed under the code SF-98.

VERTICAL VARIATIONS IN THE MINERALOGY OF THE YICHUN TOPAZ LEPIDOLITE GRANITE, JIANGXI PROVINCE, SOUTHERN CHINA

The Yichun topaz–lepidolite granite is the latest and most evolved unit of the Yichun granitic complex, South China, and is well known by virtue of its unusual Ta–Nb–Li mineralization. A drill hole down to 300 m has intersected the granite from top to bottom, and revealed three petrographic zones: K-feldspar-rich facies in the lower part, albite-rich facies in the middle part, and Ab- and Kfs-rich facies in the upper part. Geochemically, the Yichun granite belongs to the phosphorus-rich type of rare-element-enriched granite. The phosphorus is concentrated in the feldspars, which contain up to 0.93 wt% P2O5. It is less enriched in feldspars in the middle part, where primary amblygonite is abundant. Principal accessory minerals include columbite–tantalite, cassiterite, zircon, and wodginite in all facies, and microlite in the middle facies. The composition of the columbite-group minerals corresponds to manganocolumbite and manganotantalite. Zircon contains mostly 4 to 8 wt% HfO2, but the HfO2 contents may reach up to 22 wt% at the rim. From the lower part to the middle part of the drill hole, the progressive increases in Ta/(Nb + Ta) of columbite and Ta-rich cassiterite, and in Hf content of zircon, are strongly suggestive of magmatic differentiation. On the other hand, in the upper part of the drill hole, the decrease in Ta/(Nb + Ta) of columbite and cassiterite, in Hf content of zircon, and the increase in Fe of lepidolite, and especially marked enrichment in Fe and W in columbite (5.76% FeO, 7.28% WO3) at the uppermost part of the drill hole, indicate another environment at the final stage of crystallization of the granite, influenced by fluid probably derived from the surrounding mica schist.

Optimal Composition of Zircon–Fused Silica Ceramic Cores for Casting Superalloys

The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon–fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of α‐cristobalite +α‐quartz crystallized from infilling silica sols exceeds ∼3 wt% when the strength decreases from ∼10 MPa to ∼6 MPa in samples sintered at 899–927°C. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to β‐ →α‐phase transformation on cooling and differential densification during sintering.

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n ≈ 1.3–1.4 and Q ≈ 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.

A note on the corrosion characterisation of ZA-27/zircon particulate composites in acidic medium

This paper is a report of an investigation made to study the effect of zircon particles on the corrosion behaviour of ZA-27 alloy-based composite materials using the weight loss method. The corrodant used was fresh 1 N HCl solution. The duration of the test was 12–60 h in steps of 12 h. In each test, the corrosion rate was found to decrease with time, probably due to the presence of aluminium in the zinc alloy. As the zircon content in the material was increased, the composite became more corrosion resistant.

Compositional modifications to floor tile glazes opacified with zircon

AbstractIn the ceramic tile industry opaque glazes with considerable zircon content are widely used. However, the high cost of zircon limits its wide use in relevant glaze compositions. In the present study, a model opaque glaze was modified by changing the alumina/silica ratio, adding potassium oxide, or using a higher content of opaque frit while gradually eliminating the zircon content in the glaze batch. After glaze preparation, application, and single firing of glazed floor tiles under industrial conditions glazes were characterised by XRD, SEM, and EDX. The optimum alumina/silica ratio was found to be 0·26. Colour parameters L, a, and b of starting and final glazes showed that an increase in opaque frit content gives more positive improvement in opacity and in dry abrasion resistance than potassium oxide addition. Removal of zircon from the glazes resulted in a decrease of ∼13–18% in glaze cost without detrimental effect on opacity.

Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): implications for source-area weathering, sorting, provenance, and tectonic setting.

The major and trace element chemical composition of Precambrian and Paleozoic shales and sandstones from the Iberian Range (Spain) has been investigated to determine the provenance and tectonic setting of these rocks, as well as to appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The samples studied belong to a prograde sequence from diagenesis to anchizonal grade, and are mainly composed of quartz, illite/mica phases and/or kaolinite, with chlorite, feldspars, and carbonates as minor components. The major element distribution (Chemical Index of Alteration (CIA) indices) reflects that recycling processes have been important in homogenizing the composition of shales and sandstones. The higher CIA indices observed in the Silurian and Devonian shales (80 and 85, respectively) compared with those of Precambrian to Ordovician and the Carboniferous age (about 70) indicate that their source area underwent more intense chemical weathering processes, possibly due to climatic and/or tectonic variations. Alternatively, the source area of the Silurian and Devonian shales may have been composed of recycled sedimentary materials. Mineral fractionation is mainly observed in coarser rocks through zircon, apatite and xenotime accumulation, although the shales are not free of these phases. The zircon content of the fine-grained rocks is not high enough to affect rare earth element (REE) contents, but phosphate minerals (apatite and xenotime) at least partially control the REE distribution. The influence of apatite and xenotime are slightly higher in the Precambrian shales as the phosphate concentration produces a decrease in the La/Sm ratios. Chondrite-normalized REE patterns and negative Eu anomaly size of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating that they originally come from a differentiated silicic source. The higher REE contents in the studied shales in relation to PAAS indicate that recycling processes in the Iberian Range sources were probably more intense than that of PAAS. The slight differences among the REE patterns of the different groups of shales probably do not reflect changes in source-area composition, but instead suggest variations in mineral sorting, chemical weathering and/or sediment recycling. The Th/Sc, Co/Th, Cr/Th, Cr/V, V/Ni ratios support a primitive silicic source for these rocks, with the higher Th/Sc and lower Co/Th in Cambrian and Ordovician shales indicating a higher proportion of felsic material in their primitive source area. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Geochemistry of granulite‐facies granitic rocks from Battye Glacier, northern Prince Charles Mountains, East Antarctica

Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.

Mechanical properties and fractography of zircon-particle-reinforced ZA-27 alloy composite materials

The mechanical properties and the fracture mechanism of composites consisting of ZA-27 alloy reinforced with Zircon particles were investigated with the primary objective of understanding the influence of the particulate reinforcement on the behaviour of the ZA-27 alloy. The zircon particle content in the composites ranged from 1 to 5% in steps of 2% by weight. The composites were fabricated by the ‘compocasting’ technique in which the molten mass was squeeze-cast under pressure. The study revealed improvements in ultimate tensile strength, yield strength, hardness, and Young's modulus of the composites, but at the cost of ductility and impact strength, which decreased with increase in zircon content. The fracture behaviour of the composites was altered significantly by the presence of the zircon particles. Crack propagation through the matrix and the reinforcing particles resulted in the final fracture. Scanning electron microscopy and fractography analyses have been carried out to furnish suitable explanations for the observed phenomena.

Hf-Nd isotope correlation in the Quaternary potassic magmatism from Central Italy: new evidence of oceanic crust contribution to the magma source

Pleistocene potassic and ultrapotassic primitive lavas and scorias (basanites, trachybasalts, basalts, phonotephrites and shoshonites) from different representative volcanic centers of the Roman Magmatic Province (RMP: Vulsini, Vico, Sabatini, Ernici), the Tuscan Magmatic Province (TMP: Cimini, Radicofani) and Sardinia (Logudoro) were collected. Nd and Hf isotopes were analysed in 27 very fresh samples using TIMS in Pisa and plasma-sector mass spectrometry in Lyon respectively. The Hf-Nd isotopic composition of the samples belonging to the RMP ranges from -0.28 (Ernici) to -8.11 (Vico) eHf and from -5.48 (Ernici) to -10.53 (Vico) eNd. In the TMP, the Cimini sample shows the lowest Hf-Nd isotope ratios (176Hf/177Hf = 0.282446; 143Nd/144Nd = 0.512055) while all the other rocks fall within the range of the RMP. Sardinian samples are characterized by eHf = -2.48 to -4.58 and eNd = -3.29 to -4.28. For a given Nd isotopic composition, all the samples from central eastern Italy are characterized by Hf isotope ratios that are significantly more radiogenic that what was usually found in most mantle-derived volcanic rocks and crustally-derived granites (Vervoort et al., 1998). In a eHf- eNd isotope space (Fig. 1), the compositions of the volcanics from the RMP and TMP plot above the Hf-Nd isotopic mantle-crust array, below the newly identified field of Mn-nodules (Albarede et al., 1998) and within the field of pelagic sediments (Vervoort et al., 1998). Therefore the Hf- Nd isotopic properties of these rocks signal a significant contribution of pelagic material to their source, whereas this component cannot be clearly resolved in Sr-Nd isotope space. Actually, with regards to a given Nd isotope ratio, radiogenic Hf isotopic composition in primitive rocks can be explained with the presence of Hf-Zr poor material in their source, that is to say with a lacking zircon content. Because of the physical properties of zircon, which due its heavy weight is unlikely to be transported by wind or suspended clay across ocean basins (Vervoort and Blichert-Toft, 1998), this material should reflect pelagic component. We suggest that the volcanics from the Roman and the Tuscan Magmatic Provinces formed with the contribution of the remelting of an oceanic crust component in which both the altered basaltic layer and its pelagic sediment veneer can be identified by radiogenic and stable isotopes (Turi and Taylor, 1976), as well as trace elements. It is likely that the isotopic signature was acquired far enough away from a terrigenous source for the pelagic component not to have become entirely diluted by terrigenous sedimentation and therefore points to an open ocean environment. Whether this pelagic component is actually present in the source or has been transported into the subcontinental mantle by suduction- related metasomatism is speculative, but we consider that metasomatic transport is not necessarily required by the data. Origin of the central Italy potassic magmas by melting of mantle that had incorporated slivers of “dirty” oceanic lithosphere by unknown geodynamic processes stands as the best interpretation of the present isotopic data.

Sliding wear behaviour of zircon particles reinforced ZA-27 alloy composite materials

The present paper is a report of an investigation made to study the unlubricated sliding wear behaviour of zinc–aluminium (ZA) alloy composites reinforced with zircon particles of size 30–50 μm. The content of zircon in the alloy was varied from 1–5% in steps of 2 wt.%. The liquid metallurgy technique was used to fabricate the composites. A pin-on-disc wear testing machine was used to evaluate the wear rate, in which a hardened EN24 steel disc was used as the counterface. The results indicated that the wear rate of the composites was lesser than that of the matrix alloy and it further decreased with the increase in zircon content. However, the material loss in terms of wear rate and the wear volume increased with the increase in load and sliding distance, respectively, both in the case of composites and the alloy. Increase in the applied load increased the wear severity by changing the wear mechanism from abrasion to particle cracking induced delamination wear. It was found that with the increase in zircon content, the wear resistance increased monotonically. The observations have been explained using scanning electron micrography (SEM) analysis of the worn surfaces and the subsurface of the composites as well as the base alloy.