A Raman spectroscopic study of shock-wave densification of vitreous silica

The densification processes in SiO₂ glass induced by shock-wave compression up to 43.4 GPa are investigated by Raman spectroscopy. At first, densification increases with increasing shock pressure. A maximum densification of 11% is obtained for a shock pressure of 26.3 GPa. This densification is attributed to the reduction of the average Si−O−Si angle, which occurs first by the collapse of the largest ring cavities, then by further reduction of the average ring size. For higher shock pressures, a different structural modification is observed, resulting in decreasing densification with increasing shock pressure. Indeed, the recovered densification becomes very small, with values of 1.8 and 0.5% at 32 and 43.4 GPa, respectively. This is attributed to partial annealing of the samples due to high after shock residual temperatures. The study of the annealing process of the most densified glass by in situ high temperature Raman spectroscopy confirms that relaxation of the Si−O−Si angle starts at a lower temperature (about 800 K) than that of the siloxane rings (about 1000 K), thus explaining the high intensity of the siloxane defect bands in the samples schocked at compressions of 32 and 43.4 GPa. The large intensity of the siloxane bands in the nearly undensified samples shocked by compressions above 30 GPa may be explained by the relaxation during decompression of five- and six-fold coordinated silicon species formed at high pressure and high temperature during the shock event. Received: March 30, 1998 / Revised, accepted: August 21, 1998     

The solubility of water in NaAlSi3O8 melts: a re-examination of Ab−H2O phase relationships and critical behaviour at high pressures

The solubility of water in melts in the NaAlSi₃O₈–H₂O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at 0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to 15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to 12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above 0.45 GPa; it probably changes with T from retrograde below to prograde above 900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at 1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid.     

The elasticity of the upper mantle orthosilicates olivine and garnet to 3 GPa

The elastic moduli of single crystals of pyrope-rich garnet and San Carlos olivine have been measured over a 3 GPa pressure range at room temperature. The combination of improved ultrasonic techniques and this large pressure range provide for more reliable characterization of the pressure dependence of acoustic wave velocities than has previously been possible. First and second pressure derivatives of the velocities have been determined within 1 percent and 10 percent respectively. The Hashin-Shtrikman bounds for the pressure dependences of the bulk and shear moduli of the garnet used in this study are; K = 173.6 GPa, K = 4.93, K = –0.28 GPa^–1, G= 94.9 GPa, G = 1.56, G = –0.08 GPa^–1 and the Hashin-Shtrikman least-upper bounds and greatestlower bounds for the pressure dependences of the bulk and shear moduli of the San Carlos olivine are K=129.8 GPa, K = 4.66, K= –0.15 GPa^–1, G = 77.8 GPa, G = 1.93, G = –0.11 GPa^–1 and K = 129.2 GPa, K = 4.63, K= –0.15 GPa^–1 G = 77.3 GPa, G=1.96, G = –0.11 GPa^–1 respectively. The determination of the room-pressure elastic moduli of this pyrope-almandine garnet removes the previously observed anomaly in the predictions of systematic treatments of variations of the elastic moduli of garnets with composition. The determination of the second pressure derivatives of the moduli of garnet and olivine illustrates the importance of these terms in extrapolations to higher pressures — with K/P for these crystals being reduced by 17 percent and 9 percent respectively over the 3 GPa pressure range.     

A Raman spectroscopic study of shock-wave densification of anorthite (CaAl2Si2O8) glass

Structural modifications induced by shock-wave compression up to 40 GPa in anorthite glass are investigated by Raman spectroscopy. In the first investigation, densification increases with increasing shock pressure. A maximum densification of 2.2% is obtained for a shock pressure of 24 GPa. This densification is attributed to a decrease of the average ring size, favoring three-membered rings. The densification is much lower than in silica glass subject to shock at similar pressures (11%), because the T-O-T bond angle decrease is impeded in anorthite glass. For higher shock pressures, the decrease of the recovered densification is attributed to partial annealing of the samples due to high after-shock residual temperatures. The study of the annealing process of the most densified glass by in-situ high temperature Raman spectroscopy confirms that relaxation of the three-membered rings occurs above about 900 K. Received: 21 July 1998 / Revised and accepted: 27 January 1999     

Equation of state of iron–silicon alloys to megabar pressure

The stability and pressure–volume equation of state of iron–silicon alloys, Fe-8.7 wt% Si and Fe-17.8 wt% Si, have been investigated using diamond-anvil cell techniques up to 196 and 124 GPa, respectively. Angular–dispersive X-ray diffractions of iron–silicon alloys were measured at room temperature using monochromatic synchrotron radiation and an imaging plate (IP). A bcc–Fe-8.7 wt% Si transformed to hcp structure at around 1636 GPa. The high-pressure phase of Fe-8.7 wt% Si with hexagonal close-packed (hcp) structure was found to be stable up to 196 GPa and no phase transition of bcc–Fe-17.8 wt% Si was observed up to 124 GPa. The pressure–volume data were fitted to a third-order Birch–Murnaghan equation of state (BM EOS) with zero–pressure parameters: V₀=22.2(8) ų, K₀=198(9) GPa, and K₀=4.7(3) for hcp–Fe-8.7 wt% Si and V₀=179.41(45) ų, K₀=207(15) GPa and K₀=5.1(6) for Fe-17.8 wt% Si. The density and bulk sound velocity of hcp–Fe-8.7 wt% Si indicate that the inner core could contain 3–5 wt% Si.     

An X-ray diffraction study of shock-wave-densified SiO<Subscript>2</Subscript> glasses

 The densification and structural changes in SiO₂ glass compressed up to 43.4 GPa by shock experiments are investigated quantitatively by the X-ray diffraction technique. Direct structural data (average Si–O and Si–Si distances and Si–O–Si angles, coordination number of the Si atom) of these shock-densified SiO₂ glasses have been obtained by analyzing the radial distribution function curves, RDF(r), calculated with X-ray diffraction data. The coordination number of all densified glasses is about 4 and shows almost no pressure variation. The SiO₂ glass has shown density increase of 11% at a shock compression of 26.3 GPa. This density evolution could not be explained by the coordination change. The reduction of the average Si–O–Si angle (144° at 0 GPa to 136° at 26.3 GPa) obtained from RDF(r) data may account for this density increase. This Si–O–Si angle change may be caused by shrinkage of the network structure and the increase of small rings of SiO₄ tetrahedra. For higher shock pressure, a decrease in the Si–O–Si angle to 140° was observed. This is consistent with the decrease in density at 32.0 and 43.2 GPa. This decrease in the Si–O–Si angle and density could be attributed to an annealing effect due to high after-shock residual temperature. This pressure dependence of average Si–O–Si angles in shock-densified SiO₂ glass agrees with the results of our previous Raman spectroscopic study. On the other hand, the pressure variation for the first sharp diffraction peak (FSDP) was analyzed to estimate the evolution of intermediate range structures. It is suggested that the mean d value (d _m ) obtained from the position of FSDP strongly depends on the shock and residual temperature, as well as shock pressure. Received: 29 June 2001 / Accepted: 14 November 2001     

Shock-induced formation of kyanite (Al<Subscript>2</Subscript>SiO<Subscript>5</Subscript>) from sillimanite within a dense metamorphic rock from the Ries crater (Germany)

A dense (~3.34 g cm^–3) garnet–sillimanite-rich metamorphic rock from the suevite breccia of the Ries impact crater was studied by scanning-electron microscopy and Raman microprobe spectroscopy. In the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves. Kyanite aggregates were found as thin (~0.3–2.0 m) seams along grain boundaries between, and fractures within, sillimanite grains. Within these seams kyanite c-axes are oriented perpendicular to original grain boundaries and fractures. In addition, larger (up to 10 m) isolated kyanite grains were rarely found within host sillimanite. Filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease (~10%). Locally, broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub-micrometer-sized euhedral crystals. The silica-rich glass suggests incongruent melting of sillimanite at local post-shock temperatures significantly higher than 1,300°C. The edges of adjacent sillimanite grains are thermally and chemically altered. The local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts.     

Phase transformation studies on a synthetic hedenbergite up to 26 GPa at 1200° C

Phase transformations in a synthetic hedenbergite (CaFeSi₂O₆) have been observed with X-ray diffraction up to 26 GPa at 1200° C in a diamond anvil cell with a YAG laser heating system. Hedenbergite first decomposes into spinel, stishovite, and cubic perovskite phases at 16 GPa, and spinel further decomposes into wüstite and stishovite at 19 GPa. Between 19 GPa and 26 GPa, the phase assemblage is wüstite + stishovite+ perovskite. On decompression to 0.1 MPa, all the highpressure phases are retained except the cubic perovskite, which reverts to a retrogressive phase of CaSiO₃. A comparison of the results of this study with those from a previous study on a natural hedenbergite suggests that the garnet phase formed from natural hedenbergite is stabilized by manganese.     

An analcime mugearite-megacryst association from north-eastern New South Wales: implications for high-pressure amphibole-dominated fractionation of alkaline magmas

An analcime-rich mugearite near Spring Mount in north-eastern New South Wales contains dominant kaersutite, lesser anorthoclase and rare Ti-rich mica megacrysts, and also rare Group I peridotite xenoliths and granitoid inclusions. Mineralogical, experimental and Sr and Nd isotopic data are interpreted to indicate that the megacrysts are cognate with their host and that megacryst precipitation occurred at pressures in the vicinity of 1 GPa. Isotopic and other data indicate minimal crystal contamination of the melt. The generation of this evolved lava (100 Mg/(Mg+Fe²⁺), M55; differentiation index DI52–56) by high-pressure amphiboledominated fractionation of a mantle-derived hydrous alkali basaltic parent (M70) is inconsistent with observed megacryst abundances (<3%) and absence of Group II cumulates, and with the magnitude of the crystal extracts (45% crystals) indicated by crystal fractionation models. These data suggest the mugearite is not a high-pressure differentiate of a hypothetical primary parental magma (M70). Alternatively, its composition might approximate that of a primary magma derived from a relatively Fe-rich amphibole lherzolite mantle source.     

Geochemistry and argon thermochronology of the Variscan Sila Batholith,southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral ⁴⁰Ar/³⁹Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO₂ content in the granitic rocks ranges from 57 to 77 wt%; cumulate gabbro enclaves have SiO₂ as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to 300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (50–2100), as well as low values of Nb/U (2–28) and La/Th (1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock ¹⁸O ranges from +8.2 to +11.7; the mafic cumulate enclaves have the lowest ¹⁸O values and the two-mica granites have the highest values. ¹⁸O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7). The initial Pb isotopic range of the granitic rocks (²⁰⁶Pb/²⁰⁴Pb 18.17–18.45, ²⁰⁷Pb/²⁰⁴Pb 15.58–15.77, ²⁰⁸Pb/²⁰⁴Pb 38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.     

Distribution of the glass phase in hot-pressed,olivine-basalt aggregates: An electron microscopy study

Samples of olivine mixed with small amounts of tholeiitic basalt which were hot-pressed above the solidus temperature were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. Two sets of samples were compared. One set was hot-pressed for approximately 1 h near 1,300° C at 0.2 GPa, and the other set was held for approximately 200 h near 1,250° C at 1.0 GPa. SEM observations reveal that, in samples which were hot-pressed for 1 h, the glass phase occurs in irregular pockets surrounded by four or five olivine grains as well as in triple junctions. The crystal-glass interfaces show both positive and negative curvature. These observations and the presence of voids suggest that the microstructure is far from textural equilibrium. In contrast, in samples which were hot-pressed for 200 h, glass is largely confined to triple junctions of uniform size and the crystal-glass interfaces have uniform curvature indicating a much greater degree of textural equilibrium. TEM images reveal layers of glass 10–50 nm thick along most of the grain boundaries in the samples annealed for short times. However, within the limit of resolution, 2 nm, virtually all of the grain boundaries in the samples hot-pressed for long times appear to contain no glass. These observations indicate that segregation of melt from grain boundaries to triple junctions is an integral part of the process of textural equilibration.     

Petrology of ultramafic nodules from West Kettle River,near Kelowna,Southern British Columbia

A basanitoid flow of Miocene age, exposed near the West Kettle River, 25 km southeast of Kelowna, British Columbia, contains abundant ultramafic and mafic nodules. The subangular nodules are 1–20 cm across and typically show granular textures. A study of 250 nodules indicates that spinel lherzolite (60%) is the dominant type with subordinate olivine websterite (10%), websterite (7%), clinopyroxenite (4%), wehrlite (4%), pyroxene gabbro (4%), dunite (2%), harzburgite (1%) and granitic rocks (8%). Ultramafic nodules are of two types. Most of the wehrlites and clinopyroxenites belong to the black pyroxene (aluminous clinopyroxene) series, whereas the other clinopyroxene-bearing nodules belong to the green pyroxene (chromian diopside) series. Some spinel lherzolite nodules have distinctive pyroxene- and olivine-rich bands. Microprobe analyses of the constituent minerals of more than thirty nodules from the green pyroxene series indicate that grain to grain variations within individual nodules are small even when banding is present. Olivine, orthopyroxene, clinopyroxene and spinel in spinel lherzolite have average compositions of Fo₉₀, En₉₀, Wo₄₇Fs₅En₄₈, Cr/(Cr+ Al+Fe³)=0.1 and Mg/(Mg+Fe²⁺)=0.8. Equilibration temperatures, which were calculated using the two pyroxene geothermometer of Wells (1977), range between 920–980° C. Based on published phase stability experiments, pressures of equilibration are between 10–18 kbar. In summary, the upper mantle beneath southern British Columbia is dominated by spinel lherzolite but contains some banding on a scale of cm to meters. The temperature in the upper mantle is 950° C at a depth of 30–60 km.On leave from the Geological Institute, University of Tokyo, Japan     

Synchronous Transformations of Mineral and Organic Constituents of Sedimentary Rocks in Geological Structure with an Initial Extension and Subsequent Compression Tectonic Regime

Sedimentary rocks of the section in the Red River fold zone of northern Viet Nam are considered. It is shown that secondary mineral parageneses formed in two stages. The first stage (35–17 Ma ago) corresponded to the period of structure extension and sediment subsidence to a depth of about 6 km. This period and subsequent 10 Ma were marked by the formation of a usual dia- and catagenetic zoning of metasedimentary rocks. The second stage (5–7 Ma ago) corresponded to processes of compression that were responsible for the deformation of rocks into gentle folds and 1.5 to 2.2 times contraction of the section thickness in different places. The sequential–mineralogical zoning was disturbed at this stage. Smectites and mixed-layer minerals were replaced by chlorites and hydromicas. Organic material also responded to compression simultaneously with inorganic components. The bituminous component was released from humic matter and rocks became enriched in hydrocarbons.     

Anisotropic compression of edingtonite and thomsonite to 6 GPa at room temperature

Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, USA) were studied up to 6 GPa using monochromatic synchrotron X-ray powder diffraction and a diamond-anvil cell with a methanol:ethanol:water mixture as a penetrating pressure-transmitting fluid. Unlike natrolite, previously studied under the same conditions, edingtonite and thomsonite do not show any apparent pressure-induced hydration (PIH) or phase transitions. All these fibrous zeolites are characterized by their anisotropic compressibilities, with the linear compressibilities of the fibrous chains (c-axis) being as small as one third of those perpendicular to the chains (a-, b-axes); for edingtonite, ₀ ^a =0.0050(3) GPa^–1, ₀ ^b =0.0054(2) GPa^–1, ₀ ^c =0.0034(1) GPa^–1; for thomsonite, ₀ ^a = 0.0080(2) GPa^–1, ₀ ^b =0.0084(2) GPa^–1, ₀ ^c =0.0032(1) GPa^–1. The pressure–volume data were fitted to a second-order Birch–Murnaghan equation of state using a fixed pressure derivative of 4. As a result of the 0000-type connectivity of the chains, the bulk modulus of edingtonite is found to be about 40% larger than that of thomsonite; K^EDI ₀=73(3) GPa, K^THO ₀=52(1) GPa. Distance least-squares refinements were used to model the expected framework, following the observed linear compression behaviors. The chain-bridging T–O–T angle is proposed to be correlated with the different compressibilities across the chains in each framework type.     

Petrographie,Stoßwellenmetamorphose und Entstehung polymikter kristalliner Breccien im Nördlinger Ries

Polymict cristalline breccias are typical impact products of the Ries crater. They occur within the Ries crater (Appetshofen, Lierheim, Leopold Meyers Keller), on its rim (Maihingen-Klostermühle) and within the immediate vicinity of the crater (Itzing). Apart from very rare admixtures of sedimentary rock fragments the polymict cristalline breccias consist almost exclusively of fragments of various cristalline rocks, namely granites, gneisses and amphibolites. The petrographical and statistical investigations have shown that breccias from different localities have different composition. This reflects a possible difference in local compositions of the cristalline basement. The rocks in the breccias have been affected to various degress by shock metamorphism. The amphibolites could thus be shown to belong predominantly to stage I (diaplectic quartz and feldspar, 100–300 kb) and stage II (diaplectic quartz and feldspar glasses, 350–500 kb) whereas the granites and geisses can be attributed mostly to stage 0 (fractured quartz and feldspar, <100 kb) and stage I. This is in part the result of the bulk shock wave impedance of the rocks in question.Deformation structures resulting from shock metamorphism have been observed for the first time in sphene as well as in various planes of apatite.A large part of breccia rocks, which contain diaplectic quartz and feldspar glasses were altered into montmorillonite at a later date. The authigenic minerals were examined by x-rays and chemically by microprobe analysis.

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Herrn Prof. Dr. W. von Engelhardt danke ich für Diskussion und Beratung. Dem Bundesministerium für Bildung und Wissenschaft sei für die finanzielle Unterstützung dieser Arbeit gedankt.     

Volatile production and transport in regional metamorphism

Calculations show that H₂O and CO₂ produced during devolatilization of an average pelite will occupy 12 vol. % of the rock at 500°C and 5 kb. Because the tensional strength of well foliated rock at metamorphic conditions is vanishingly small, such a volume of fluid having any vertical extent will fracture the rock and escape upward owing to its lower density.In a simplified model of a sudden increase of heat flow from 0.8 to 2.5 H.F.U., the average pelitic rock will have a rate of fluid production averaging 9.4×10^–10 g cm² s^–1 between 400°C and 600°C. The escape of this fluid can be accomodated by a single fracture 1 cm long and 0.2 wide per cm² of rock. If the fracture is reduced to 0.02 then 1,000 cm of fracture per cm² would be required. This width is the minimum original width as calculated from the volume of fluid observed in fluid inclusions trapped along annealed fractures within quartz in metamorphic terrains. Fluid flow will be laminar if the fracture is <0.025 cm wide. Additional calculations show that grain boundary diffusion is not an effective means of fluid transport in regional metamorphism.The commonly observed quartz segregations in pelitic terrains appear to mark the site of major channelways for fluid escape. In this case the bulk of escaping fluid is not able to react pervasively with rocks higher in the metamorphisc pile. Regionally metamorphosed rocks will have a discrete fluid phase only when devolatization reactions are actually taking place. At other times only an absorbed surface monolayer of volatiles on the minerals will be present.Died April 2, 1980     

Shoshonitic and alkaline lamprophyres with elevated Au and PGE concentrations from the Kreuzeck Mountains,Eastern Alps,Austria

Summary Amphibole and mica lamprophyres and related dykes of Tertiary age from the Kreuzeck Mountains, Central Alps, Austria, have been investigated petrographically and geochemically. They intrude a sequence of early Palaeozoic metapelites, greenstones and amphibolites to the north of the Cretaceous Periadriatic Lineament, a major suture zone of 700 km E-W extent. The dykes are spatially associated with Sb, W, Hg, and Cu-Ag-Au deposits.Most lamprophyres are characterized by primitive chemistry (mg-numbers > 60 and Cr > 200 ppm) and have high contents of LIL elements (K, Rb, Sr and Ba). Geochemically, five different subgroups of calcalkaline/shoshonitic to alkaline affinity can be distinguished. These are: Group 1, amphibole-bearing shoshonitic lamprophyres (0.5–1.0 wt% Ti0₂, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10); Group 2, mica-bearing shoshonitic lamprophyres (1–1.5 wt% TiO₂, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Group 3, alkaline lamprophyres (1.5–2.1 wt% TiO₂, Zr > 250 ppm, Nb > 30 ppm, Ba/Rb 10–25); Group 4, low-MgO alkaline lamprophyres ( 2.5 wt% TiO₂, mg-number < 57, Nb 20 ppm, Ba/Rb 20); Group 5, calc-alkaline basaltic dykes ( 2.2 wt% TiO₂, mg-number <55, Nb < 10 ppm, Ba/Rb < 10). Group 2,3 and 4 dykes have NE-SW orientations and are of Oligocene age (K-Ar age 27–32 Ma); Group 1 and 5 dykes are of Lower Oligocene age (K-Ar age 36 Ma) but have mostly E-W orientations.The Kreuzeck lamprophyres were generated in post-collisional magmatic events, which were probably linked to extensional tectonics following oblique continent-continent collision between the African and Eurasian Plates during the Eocene. Group 1, 2 and 5 dyke rocks have typical calc-alkaline geochemical signatures indicating that they represent partial melting products of subduction-modified lithosphere. Group 3 and 4 alkaline lamprophyres have geochemical features transitional between calc-alkaline and within-plate alkaline igneous rocks (e.g. Ba/Nb 30–70) indicating that their mantle source-region includes both subduction-modified lithospheric and OIB-type asthenospheric components.There is no apparent relationship between mineralization in the Kreuzeck region, thought to be of Ordovician-Devonian age, and much later lamprophyre intrusion. Alteration of the dykes by late-magmatic fluids has resulted in the formation of secondary minerals, and has occasionally led to increased Au and PGE values in the 10–35 ppb range particularly in close proximity to Cu-Ag-Au deposits.

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Shoshonitische und alkalische Lamprophyre mit erhöhten Au- und PGE-Gehalten aus der Kreuzeckgruppe, Ostalpen, Österreich

Zusammenfassung Die vorliegende Arbeit untersucht die Petrographie und Geochemie tertiärer Lamprophyre und genetisch verwandter Ganggesteine aus der zentralalpinen Kreuzeckgruppe, nördlich des Periadriatischen Lineamentes, in Kärnten, Österreich. Die Ganggesteine durchschlagen die altpaläozoischen Metapelite, Grünsteine und Amphibolite des Altkristallins diskordant und stehen in räumlichem Zusammenhang mit Sb, W, Hg und Cu-Ag-Au Lagerstätten, die bereits seit dem Mittelalter abgebaut wurden.Die meisten Lamprophyre zeigen primitiven Charakter (Mg-Zahlen >60 und Cr > 200 ppm) und besitzen hohe Gehalte an LILE (K, Rb, Sr und Ba). Geochemisch lassen sich fünf verschiedeneGruppen mit kalkalkalisch/shoshonitischem bis alkalischem Charakter unterscheiden: Gruppe 1, Amphibol-führende shoshonitische Lamprophyre (0.5–1.0 Gew% TiO₂, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10);Gruppe 2, Glimmer-führende shoshonitische Lamprophyre (1–1.5 Gew% TiO₂, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Gruppe 3, alkalische Lamprophyre (1.5–2.1 Gew% TiO₂, Zr > 250 ppm, Nb > 30ppm, Ba/Rb 10–25); Gruppe 4, alkalische Lamprophyre mit geringen MgO-Anteil ( 2.5 Gew% TiO₂, Mg-Zahl < 57, Nb 20 ppm, Ba/Rb 20); Gruppe 5, kalkalkalisch basaltische Ganggesteine ( 2.2 Gew% TiO₂, Mg-Zahl < 55, Nb < 10 ppm, Ba/Rb < 10). Die Lamprophyre der Gruppen 2, 3 und 4 zeigen nordöstliches Streichen und oligozänes Intrusionsalter (K-Ar Alter 27–32 Ma), während die Ganggesteine der Gruppen 1 und 5 überwiegend östliches Streichen und UnterOligozänes Intrusionsalter (K-Ar Alter 36 Ma) aufweisen.Die Intrusionen erfolgten während einer tektonischen Dilatationsphaseim Oligozän nach der Kontinent-Kontinent Kollision zwischen derAfrikanischen und der Eurasischen Platte im unteren Eozän. Ganggesteine der Gruppen 1, 2 und 5 besitzen typisch kalkalkalischen Charakter und stellen vermutlich Produktevon aufgeschmolzener, subduzierter Lithosphäre dar. Die Geochemie der alkalischen Lamprophyre derGruppen 3 und 4 (e.g. Ba/Nb 30–70) deutet auf ihre genetische Zwischenstellung zwischen subduction-related und within-plate regime.Zwischen den tertiären Gangintrusionen und den vermutlich paläozoischen Vererzungen der Kreuzeckgruppe besteht kein genetischer Zusammenhang. Die Alteration der Ganggesteine durch postmagmatische Lösungen hat jedoch zur Bildung von sekundären Mineralen und teilweise zu überdurchschnittlich erhöhten Au und PGE Gehalten von bis zu 35 ppb geführt.

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With 6 Figures     

Formation of Ni–Cu–Platinum Group Element sulfide mineralization in the Sudbury Impact Melt Sheet

Summary The Ni–Cu–Platinum Group Element (PGE) sulfide deposits of the Sudbury Structure have provided a major portion of the worlds total nickel production and their host rocks have been the subject of numerous research studies, yet a number of perplexing problems remain to be solved. On the one hand, studies seeking to explain the formation of the Sudbury Structure have now converged on a genetic model which proposes that the Main Mass and Offset Dykes of the Sudbury Igneous Complex (SIC) were produced by crystallization of an impact-generated melt sheet. On the other hand, these models have yet to be fully reconciled with the production of the very large volume of magmatic Ni, Cu, Co, and PGE-rich sulfide mineralization and the associated mafic rock types. This paper explores this problem using new precious metal data from the Main Mass and Offset Dykes. These data are used to understand the relationships between these rocks, and to provide constraints on how the Ni–Cu–PGE sulfide ore deposits fit into the geological evolution of the Sudbury Structure.In the two drill cores selected for study in this project, the Mafic Norite has 1–5 modal percent pyrrhotite plus chalcopyrite, and elevated Ni (40–1000ppm), Cu (40–1140ppm), and PGE (1.9–7.8ppb Pd, 1.8–7.3ppb Pt); this is overlain by Felsic Norite that contains pyrrhotite, and has a wide range in concentration of Ni (13–257ppm), Cu (7–328ppm), and PGE (<0.01–6.4ppb Pd, <0.01–5ppb Pt). For a similar range of MgO, the upper portion of the Felsic Norite unit has 5–10 times lower Ni and Cu abundances than within-plate basalts and local crustal rocks, and PGE abundance levels are mostly below analytical determination limits. Stratigraphic studies of other compositional profiles around the SIC demonstrate that this depletion signature of Ni, Cu, and PGE is widespread and developed not only above mineralized embayments and offsets, but also above barren sections of the lower contact of the SIC.The depletion of the upper part of the Felsic Norite in Ni, Cu and PGE is presumably due to equilibration of the magma with magmatic sulfide, and accumulation of this dense sulfide liquid. Results of modeling indicate that the parental magma giving rise to the Mafic and Felsic Norites had initial Ni and Cu contents of 210 and 110ppm, respectively. In addition, Ni, Cu and PGE tenors calculated in 100% sulfide from the Copper Cliff Offset average 13% Cu, 6% Ni, 18ppm Pd, and 19ppm Pt indicating that these sulfides had formed by fractionation from magmas that contained 310ppm Ni, 310ppm Cu, 18ppb Pd and 19ppb Pt. These values are factors of 3 to 5 higher than the Ni, Cu, Pd, and Pt contents of the Onaping Formation with average values of 55ppm Ni, 48ppm Cu, and 4.9ppb Pd as well as the marginal sulfide-poor phase of the Worthington Offset quartz diorite, which has average values of 61ppm Ni, 59ppm Cu, 2.8ppb Pd and 4.0ppb Pt. Both the Onaping Formation and the marginal quartz diorite are believed to represent the initial composition of a large component of the melt sheet. There is therefore a fundamental problem in reconciling the initial metal contents of the SIC magma as indicated by the marginal phases of the Offset dykes and that of the Onaping Formation with the composition of the SIC magma at the times of formation of the sulfides as indicated by their Ni, Cu and PGE tenors.It is proposed that because the SIC melt sheet was initially superheated with a temperature of 1700°C, it was able to dissolve 5 times as much S as it could at its liquidus temperature of 1200°C. It was also initially composed of an emulsion of mafic and felsic melts (Marsh and Zieg, 1999), which may have formed discrete magma cells. As the temperature of the melt sheet decreased, some of these magma cells became S-saturated and the resultant Ni–Cu–PGE sulfides settled downwards and on reaching magma cells lower in the melt sheet were re-dissolved thereby raising the Ni, Cu and PGE contents of the lower magma cells. It was from these enriched magma cells that precipitation of the ore-forming Ni–Cu–PGE sulfide melts eventually took place.The mineral potential of Offset and embayment structures appears to be empirically linked to the thickness of the overlying noritic rocks; for example, the most heavily mineralized embayments and Offset Dykes are located in areas where the Felsic Norite is thickest. It appears unlikely that the entire 1–3km-thick melt sheet was convectively mixing throughout its lateral extent, and so the heterogeneity in sulfide distribution was retained after crystallization and cooling.     

Sm−Nd dating of multiple garnet growth events in an arc-continent collision zone,northwestern U.S. Cordillera

Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A⁴⁰Ar/³⁹Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.     

Late Hercynian U-vein mineralization in the Alps: fluid inclusion and C,O, H isotopic evidence for mixing between two externally derived fluids

Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with D –34 to –49. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with D –50 to –65. All fluids contain trace N₂, CO₂ and probably CH₄. The carbonates have ¹³C –8 to –14. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich ( –35) low fO₂, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.