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La ungavaite, Pd 4 Sb 3 , est une nouvelle espece minerale intermetallique decouverte dans le gisement Mesamax Northwest, dans la ceinture orogenique du Cap Smith, region d'Ungava, dans le nord du Quebec. Lui sont associes pyrrhotite monoclinique, pentlandite, chalcopyrite, galene, sphalerite, cobaltite, un membre du groupe de la chlorite, et magnetite. Les mineraux suivants porteurs de metaux precieux lui sont aussi associes: naldrettite (Pd 2 Sb), aussi une nouvelle espece minerale, sperrylite, sudburyite, michenerite, alliage Au-Ag, altaite, petzite (Ag 3 AuTe 4 ) et hessite (Ag 2 Te). L'ungavaite se presente en rares grains xenomorphes avec inclusions d'un alliage Au-Ag ou rattachee a la chalcopyrite et un mineral du groupe de la chlorite. Ces grains varient entre environ 36 to 116 μm (diametre d'un cercle equivalent), et sont en moyenne 73 μm (n = 4). Nous ne pouvons deceler ni clivage, ni fracture. Le mineral est distinctement anisotrope et non-pleochroique, fait preuve d'une faible bireflectance, et ne montre aucune reflexion interne discernable. L'ungavaite est blanc cremeux brillant en association avec la pentlandite, la pyrrhotite, la chlorite, et la chalcopyrite. Les valeurs de reflectance dans l'air (et dans l'huile) pour R 1 et R 2 sont: 50.2, 50.5 (37.6, 38.0) a 470 nm, 55.6, 55.9 (43.2,43.5) a 546 nm, 57.9, 58.3 (45.9,46.3) a 589 nm, et 60.2, 60.7 (48.1, 48.5) a 650 nm. Le resultat moyen de seize analyses d'une particule, effectuees avec une microsonde electronique, est: Pd 54.53, Fe 0.13, Te 0.09, Sb 44.59, Bi 0.42, Hg 0.19, et As 0.20, pour un total de 100.15% (poids), ce qui correspond a la formule empirique (fondee sur sept atomes) Pd 4 . 0 6 2 (Sb 2 . 8 9 3 Fe 0 . 0 1 7 Bi 0 . 0 1 7 Hg 0 . 0 0 6 Te 0 . 0 0 5 ) Σ 2 . 9 3 8 ou, de facon ideale, Pd 4 Sb 3 . Le mineral serait l'analogue a dominance de Pd de la genkinite. La ungavaite est tetragonale, et cristallise dans un des groupes d'espace suivants: P4 1 2 1 2, P4 1 22, P4 3 2 1 2. P4 2 2 1 2, ou P4 2 22. Les dimensions de la maille sont: a 7.7388 (4), c 24.145(1) A, avec V= 1446.02(1) A 3 et Z = 8. La densite calculee est 7.264(1) g/cm 3 . Les six raies les plus intenses du spectre de diffraction (methode des poudres) [d en A(I)(hkl)] sont: 3.008(90)(008), 2.268(100)(134), 2.147(30)(230), 1.9404(60)(400), 1.2043(30)(2218, 452), et 1.2002(30)(624). On trouve l'ungavaite dans une etroite zone enrichie en (Pd + Sb) entre les sulfures massifs et les sulfures dissemines. Elle resulterait probablement de la mobilisation hydrothermale du Pd (et possiblement du Sb) aux depens des sulfures massifs. D'apres les relations de phases, l'ungavaite se serait developpee au dessous de 400°C, peut-etre au cours d'une reaction de mise en ordre.

We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ∼50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.

We have measured the interfacial tension γSF between sulfide and silicate melts over the temperature range from 1250 to 1325 °C by in situ measurement of the shapes of sessile drops of sulfide immersed in silicate melt, sitting upon level alumina substrates. The silicate melt was an alkali-free synthetic basaltic liquid. The sulfide melts were synthetic mixtures of Fe–Cu–Ni–S or Fe–S–O. The interfacial tension ranges from about 500 mJ m−2 for sulfide of FeS composition to approximately 600 mJ m−2 for sulfide containing 16 wt.% Cu or 6.5 wt.% Ni, and up to 650 mJ m−2 for Fe–S–O liquid containing 4 wt.% O. Contact angles vary from 150° to 180°, showing that sulfide liquid does not wet oxide minerals in silicate magmas. The density of sulfide melt does not depend strongly on Cu or Ni content, remaining between 4.0 and 4.5 g cm−3 over the range of Cu and Ni contents expected in sulfide magmas, but increases substantially to about 5.5 g cm−3 in Fe–S–O melts containing 4 wt.% O. Our results can be combined with conventional concepts of nucleation theory to indicate that when silicate magmas exist at small degrees of supersaturation, sulfide drops may nucleate rarely and at widely spaced intervals, leading to kinetic control of the compositions of the resulting droplets. In light of our results, we use simple scaling arguments to argue that sulfide magmas are not capable of migrating through partially molten silicate rocks by capillary forces alone, but may be forced through narrow grain boundaries or grain edges only by the flow of the enclosing silicate melt. During melt extraction by compaction of partially molten mantle peridotite, it is highly unlikely that silicate flow rates could be sufficiently rapid to permit the entrainment of droplets of sulfide liquid. At temperatures below the solidus of enclosing silicate rocks, sulfide melt will be free to travel along fractures or grain boundaries. The mobility of sulfide melts through completely solidified silicate rocks may account for the widespread observation that late-stage, highly fractionated sulfide liquids have escaped from cooling bodies of massive sulfide to form veins and disseminations of Cu-rich sulfide minerals in their silicate host rocks.

Naldrettite, Pd2Sb, is a new intermetallic mineral discovered in the Mesamax Northwest deposit, Cape Smith fold belt, Ungava region, northern Québec. It is associated with monoclinic pyrrhotite, pentlandite, chalcopyrite, galena, sphalerite, cobaltite, clinochlore, magnetite, sudburyite (PdSb), electrum and altaite. Other rarer associated minerals include a second new mineral (ungavaite, Pd4Sb3), sperrylite (PtAs2), michenerite (PdBiTe), petzite (Ag3AuTe4) and hessite (Ag2Te). Naldrettite occurs as anhedral grains, which are commonly attached or moulded to sulphide minerals, and also associated with clinochlore. Grains of naldrettite vary in size (equivalent circle diameter) from ~10 to 239 μm, with an average of 74.4 mm (n = 632). Cleavage was not observed and fracture is irregular. The mineral has a mean micro-indentation hardness of 393 kg/mm2. It is distinctly anisotropic, non-pleochroic, has weak bireflectance, and does not exhibit discernible internal reflections. Some grains display evidence of strain-induced polysynthetic twinning. Naldrettite appears bright creamy white in association with pentlandite, pyrrhotite, clinochlore and chalcopyrite. Reflectance values in air (and in oil) for R1 and R2 are: 49.0, 50.9 (35.9, 37.6) at 470 nm, 53.2, 55.1 (40.3, 42.1) at 546 nm, 55.4, 57.5 (42.5, 44.3) at 589 nm and 58.5, 60.1 (45.4, 47.2) at 650 nm. The average of 69 electron-microprobe analyses on 19 particles gives: Pd 63.49, Fe 0.11, Sb 35.75, As 0.31, and S 0.02, total 99.68 wt.%, corresponding to (Pd1.995Fe0.007)2.002(Sb0.982AS0.014S0.002)0.998. The mineral is orthorhombic, space group Cmc21, a 3.3906(1), b 17.5551(5), c 6.957(2) Å , V 414.097(3) Å3, Z = 8. Dcalc is 10.694(1) g/cm3. The six strongest lines in the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 2.2454(100)(132), 2.0567(52)(043), 2.0009(40)(152), 1.2842(42)(115), 1.2122(50)(204) and 0.8584(56)(1.17.4).

La transition entre les domaines ou la breche de Sudbury est sterile et ceux ou elle renferme un minerai a sulfures de Cu-Ni-PGE a la mine Fraser, en Ontario, est accompagnee d'une augmentation de la concentration de Cl et de Br dans les roches globales, dans la fraction possible a dissoudre dans l'eau (a 20°C), et du rapport Cl:Br dans la matrice (pseudotachylite) de la breche. Nous pouvons detecter des concentrations anomales de Cl et de Br dans les roches totales (1.5-10 et 1.5-2 fois le bruit de fond, respectivement) sur des distances d'au moins 150 m des premiers indices visibles de mineralisation en sulfures dans les roches du socle. Le rapport Cl:Br dans la roche globale augmente en s'approchant des zones mineralisees, temoignant de l'augmentation dans la proportion des mineraux aptes a accepter le Cl et a exclure le Br. Jusqu'a -70% du Cl (en moyenne, 35-40%) et ∼80% du Br (en moyenne, 25-35%) dans la matrice de la breche de Sudbury sont solubles dans l'eau a temperature ambiante, et resident (i) dans des inclusions fluides primaires le long des interfaces recristallisees entre grains, qui augmentent en s'approchant du minerai, et (ii) dans des cristaux de halite dans les vacuoles et les pores de la matrice. Des solutions aqueuses preparees par lessivage des echantillons de la matrice de la breche pulverises indiquent les rapports Ca:Na et Cl:Br variables, qui seraient en partie le resultat de precipitation d'infimes quantites de halite (

The spatial relations between outliers of Huronian metasediment and the current outline of the Sudbury Igneous Complex (SIC) have been used previously to constrain the original size of the meteorite impact crater, despite uncertainty regarding their structure and origin. The outliers are tight to isoclinal, gently plunging synclines with northeast‐southwest‐trending axes, slightly overturned to the northwest. Major deformation of the Huronian Supergroup preceded the 1849‐Ma Sudbury meteorite impact event by sufficient time to permit peneplanation of the region. Impact occurred in a foredeep related to the subduction of the Laurentia plate under the Wisconson Magmatic Terranes. Faults truncate the northwest margins of most of the outliers. Where examined, at several localities, the northwest margin–bounding faults are invariably occupied by pseudotachylite. Structural considerations provide evidence for thousands of meters of slip along these surfaces, each of which was apparently accomplished in ...

An assemblage of hydrous, Cl-bearing alteration-induced minerals is present within 150 m of footwall-style Cu–Ni–PGE sulfide mineralization hosted by the Sudbury Breccia at the Fraser Copper zone, Fraser mine, Onaping–Levack area, Ontario. The pyroxene hornfels to hornblende hornfels contact-metamorphic aureole of the Sudbury Igneous Complex (SIC) contains less than 15 modal % hydroxysilicates. Near the mineralized areas, a diagnostic assemblage of secondary minerals of the albite–epidote hornfels facies is present in the matrix of the Sudbury Breccia; it contains 25 to 80% actinolite + chamosite as well as epidot e + sodic plagioclase (An2.8–42) + quartz ± titaniferous magnetite ± biotite ± K-feldspar ± titanite. The Cl content and Cl/(Cl + F) values of amphibole and biotite in this matrix are elevated within 150 m of mineralization, and increase toward known occurrences of sulfides. Parameters of K, Na, and A-site occupancy correlate with Cl content in actinolite in such matrix material. The value of Mg# is lower in biotite in the matrix near mineralized areas and is inversely correlated with Cl/(Cl + F) in the bioti te, reflecting Mg–Cl or Fe–F avoidance. Calculated temperatures of amphibole–plagioclase equilibration for the diagnostic hydrous assemblage are between 442 to 540 ± 75°C. The assemblage formed during a retrograde metamorphic event during or after footwall Cu–Ni–PGE mineralization, and after passage of peak conditions of contact metamorphism at the base of the SIC. The enrichment of Cl in this assemblage was the result of the increased amounts of free Cl relative to F in the equilibrating fluid , and favorable cation proportions for incorporation of Cl into the structure of hydroxysilicate minerals. The Cl-bearing assemblage accounts for a significant portion of the bulk halogen content of Sudbury Breccia matrix. It is unique to zones of Sudbury Brec cia that host mineralization, and hence may be used to identify prospective areas.

Nous avons etudie la repartition de F, Cl. Br, et I entre liquides silicate et sulfure coexistants a une pression allant de 0.9 a a 1.8 GPa et a une temperature entre 1150° et 1450°C, et la repartition du CI entre liquide sulfure riche en Cu et en Ni d'une part, et solution solide monosulfuree (mss) a pression atmospherique et une temperature de 1000° et 1100°C. Nous avons calcule les coefficients de partage de Nernst, exprimes en termes de la proportion ponderale d'halogene dans le sulfure divise par la proportion dans le silicate. Nous n'avons pas detecte de fluor dans le liquide sulfure, quelles que soient les conditions experimentales. Il n'y a pas de changement discernable dans les coefficients Dc s u l / s i l , DBr s u l / s i l et DI s u l / s i l avec la temperature. Les valeurs de Dc s u l / s i l , D B r s u l / s i l et D I s u l / s i l sont dispersees de part et d'autre de 0.038, 0.026, et 0. 15, respectivement. Une augmentation de la pression de 0.9 a 1.8 GPa mene a une augmentation des coefficients D c l sul/silD B r sul/sil et D 1 sul/silselon un facteur de 2, environ. Le chlor est moderement incompatible avec la phase mss: D c l m s s / l i q varie de 0.09 a 1000°C a 0.19 a 1100°C, indication que la cristallisation de cette phase solide a partir d'un liquide sulfure entrainera un enrichissement progressif du Cl dans le liquide residuel. Le point de fusion de mss dans le systeme Fe-Cu-Ni-S est abaisse par la presence du Cl. Nos resultats montrent que l'association commune des halogenures dans les minerais de sulfures massifs, y inclus la lawrencite, mineral deliquescent, concorde parfaitement avec l'hypothese d'une origine purement magmatique du minerai. Les halogenes pourraient bien avoir ete dissous dans le liquide sulfure, pour ensuite etre rejete pendant sa cristallisation finale, pour former des halogenures comme mineraux accessoires dans les amas de sulfures massifs, ou bien des fluides hypersalins qui pourront se melanger avec les fluides aqueux dans les roches encaissantes. Comme signe revelateur d'une phase fluide orthomagmatique derivee d'un magma sulfure, on s'attend a voir des valeurs extremement elevees des rapports Cl/F et Cl/Br, traduisant l'affinite preferentielle du Cl pour les sulfures en comparaison des autres halogenes. Des signes de la presence d'une telle phase fluide hypersaline ayant un rapport C1/ Br anormalement eleve pourrait fournir un outil precieux dans la prospection pour les amas sulfures enrichis par fractionnement en Cu et en elements du groupe du platine.

The interstitial melt in partially molten cumulate piles in layered intrusions must at some point reach saturation with a volatile phase such as water vapor or hydrosaline melt. A number of models have been proposed in which orthomagmatic fluids migrate through partially solidified cumulates and participate in the formation of ore deposits. Here I examine the topology of the crystal—melt—vapor system in a cumulate and relate this to the role of capillary forces in governing the size and mobility of individual vapor bubbles. Capillary forces will play a dominant role in setting the number density and sizes of bubbles. In any cumulate rock with crystals smaller than several cm in diameter, bubbles of the postcumulus aqueous phase will be unable to migrate away from their sites of nucleation and growth. Although bubble stranding does not preclude the eventual loss of volatile ­constituents from the cumulate after complete solidification, it does prevent such constituents from migrating while the system remains partially molten and therefore prohibits them from participation in magmatic-hydrothermal interactions within the magma chamber.

Massive sulfide in the contact sublayer in three different deposits of the Sudbury Igneous Complex is overlain by substantial thicknesses of norite-hosted disseminated sulfide mineralization with anomalous compositions. In the Trill embayment, the Levack trough (McCreedy East), and the Victor-Nickel Rim embayment (Victor Deep), disseminated sulfides are present at concentrations of 1 to 5 percent over a vertical thickness locally exceeding 100 m within the main mass (i.e., texturally homogeneous, inclusion- free) norites above massive contact ores of the sublayer. Where primary textures are preserved, they show that sulfide liquid remained present as an interstitial liquid during the entire crystallization history of the enclosing silicate cumulate rocks. The disseminated sulfides show remarkably constant compositions over a large stratigraphic interval. They cannot have formed due to fractional segregation of sulfide liquid from the main mass magma accompanying the deposition of the enclosing silicate cumulus phases, because this would lead to extreme depletion in all platimun-group elements within a stratigraphic interval much shorter than the hundreds of meters in which their compositions are observed to remain approximately constant. Geochemical modeling of sulfide liquation and fractional crystallization shows that the massive sulfides are cumulates rich in monosulfide solid solution (mss). The overlying disseminated sulfides represent mss left behind by an upward-migrating fractionated sulfide liquid residual to the formation of the massive cumulates at the base. These results show that Cu-rich sulfide liquids are capable of migrating distances greater than 100 m through almost completely solidified cumulate rocks at volume fractions of sulfide as low as one percent. The presence of a halo of disseminated sulfide mineralization, with a distinctly fractionated geochemical character, up to several hundred meters wide above zones of massive sulfide mineralization could be used to detect deeply buried massive sulfide orebodies at Sudbury and elsewhere.

Mass transfer between sulfide and silicate liquids is often discussed elements (PGE), has resulted from liquation (immiscible by reference to a mass ratio between hypothetical liquid reservoirs liquid phase separation) of sulfide liquid from mafic or at equilibrium, defined as R. In an open system not at equilibrium ultramafic silicate magma. A central assumption of modthe mass of silicate melt from which a growing sulfide droplet els of magmatic sulfide deposit formation is that the base appears to have scavenged chalcophile elements will depend on metals and PGE are strongly partitioned into the sulfide diffusivities Di and DFeS of the element and of the FeS component melt. Previous discussions of the process by which sulfide in the silicate melt, respectively, on the degree of supersaturation of liquids become enriched in chalcophile metals have fothe melt with FeS C o , on the radius a of the droplet and on the cused almost exclusively on models in which coexisting velocity v with which the droplet is advected through the melt, so bodies of sulfide and silicate melt are considered to be that in equilibrium at all times. The intent of this paper is to demonstrate that natural sulfide deposits preserved in igneous rocks will in many R = 1 C FeS o Di + 1/2 aDiv DFeS + 1/2 aDFeSv . instances record metal distributions that never attained equilibrium, as a result of the kinetic constraints on metal Chalcophile elements are expected to show inter-element variations partitioning between phases. The consequences are that in D spanning several orders of magnitude in some melts, leading different elements will seem to show evidence for equito large inter-element variations in effective R in a given sulfide libration at different silicate/sulfide mass ratios within body. These variations in effective R will lead to spatial separations single sulfide bodies. I describe the time-dependent evolubetween zones of maximum enrichment of chalcophile elements tion of metal distributions in silicate–sulfide melt systems, produced during fractional segregation of sulfide in layered intrusions. starting with analytical solutions for simple boundary Whereas it has proven impossible to model the generation of spatially conditions and then adapting them to more complex offset stratiform horizons in the Australian Munni Munni layered systems as numerical models. intrusion using equilibrium partition coefficients, the present kinetic model reproduces the observed patterns with great fidelity.

The Adam-Gibbs equations describing relaxation in silicate melts are applied to diffusion of trace components of multicomponent liquids. The Adam-Gibbs theory is used as a starting point to derive an explicit relation between viscosity and diffusion including non-Arrhenian temperature dependence. The general form of the equation is Diη = Aiexp{Δ(scEi)/TSc}, where D is diffusivity, η is melt viscosity, T is absolute temperature, Δ(scEi) is the difference between the products of activation energies and local configurational entropies for viscous and diffusive relaxation, Ai is a constant that depends on the characteristics of the diffusing solute particles, and Sc is configurational entropy of the melt. The general equation will be impractical for most predictive purposes due to the paucity of configurational entropy data for silicate melts. Under most magmatic conditions the proposed non-Arrhenian behaviour can be neglected, allowing the general equation to be simplified to a generalized form of the Eyring equation to describe diffusion of solutes that interact weakly with the melt structure: Diη/T = Qiexp{ΔEi/RT}, where Qi and ΔEi depend on the characteristics of the solute and the melt structure. If the diffusing solute interacts strongly with the melt structure or is a network-forming cation itself, then ΔEi = 0, and the relation between viscosity and diffusion has the functional form of the classic Eyring and Stokes-Einstein equations; Diη/T = Qi. If the diffusing solute can make diffusive jumps without requiring cooperative rearrangement of the melt structure, the diffusivity is entirely decoupled from melt viscosity and should be Arrhenian, i.e., Di = Qiexp{Bi/T}. A dataset of 594 published diffusivities in melts ranging from the system CAS through diopside, basalt, andesite, anhydrous rhyolite, hydrous rhyolite, and peralkaline rhyolite to albite, orthoclase, and jadeite is compared with the model equations. Alkali diffusion is completely decoupled from melt viscosity but is related to melt structure. Network-modifying cations with field strength Zi2/r between 1 and 10 interact weakly with the melt network and can be modelled with the extended form of the Eyring equation. Diffusivities of cations with high field strength have activation energies essentially equal to that of viscous flow and can be modelled with a simple reciprocal Eyring-type dependence on viscosity. The values of Qi, ΔEi and Bi for each cation are different and can be related to the cation charge and radius as well as the composition of the melt through the parameters Zi2/r, M/O, and Al/(Na + K + H). I present empirical fit parameters to the model equations that permit prediction of cation diffusivities given only charge and radius of the cation and temperature, composition and viscosity of the melt, for the entire range of temperatures accessible to magmas near to or above their liquidus, for magmas ranging in composition from basalt through andesite to hydrous or anhydrous rhyolite. Pressure effects are implicitly accounted for by corrections to melt viscosity. Ninety percent of diffusivities predicted by the models are within 0.6 log units of the measured values.

We have measured effective binary chemical diffusion coefficients for Be, B, Mg, Ca, Ti, Ge, Sr, Y, Zr, Nb, Cs, Ba, Nd, Tb, Lu, Hf, Ta, and W at concentrations below 1000 ppm in synthetic granitoid melts by performing infinite couple diffusion anneals. By imposing systematic variations in the viscosity of a granitic melt through changes in temperature and composition, we have created an internally consistent set of diffusivities that can be used to assess quantitatively the relationship between melt viscosity, melt composition, and tracer diffusivities. One set of experiments were performed at 1 atm in an anhydrous haplogranitic melt near to the eutectic composition under air at temperatures of 1600, 1400, and 1137°C. A second set of experiments investigated the independent effects of the addition of approximately 3.7 wt.% H2O at 1600 and 1300C at 1.0 GPa pressure, and of 20 wt.% Na2O at 1 atm at 1200, 960, and 810°C. Within experimental error the measured diffusivities are Arrhenian, and show clear dependencies on ionic charge and radius, as well as on the viscosity of the melt. Monovalent, divalent, and trivalent cations show increasing activation energies with increasing field strength, whereas cations with higher charges have activation energies subequal to that of viscous flow. The Eyring equation is moderately successful in relating diffusivities of high field strength cations to melt viscosity, but underestimates the diffusivities of other cations by up to 4 orders of magnitude. The database of diffusivities reported in this contribution can serve to calibrate empirical models for the prediction of tracer diffusivities over a large range in melt composition, temperature, and viscosity.

We have measured multicomponent chemical diffusion coefficients in a melt near to the low pressure water-saturated eutectic granite composition in the system K 2 O-Na 2 O-Al 2 O 3 -SiO 2 -H 2 O at 1.0 GPa and temperatures of 1300 and 1600 degrees C. The measured diffusion profiles can be accounted for within the analytical error by diffusion coefficients, which are not dependent on composition within the range of compositions accessed by our experiments. The multicomponent diffusion coefficient matrix [D] has a highly degenerate set of real, positive eigenvalues that show a regular relation to melt viscosity on an Arrhenius diagram. The smallest eigenvalue is that associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exchanges of alkalis for silica or for each other can proceed in pseudo-binary fashion without inducing fluxes of other components. The eigenvalue associated with H-Si exchange is smaller than the alkali-silica eigenvalues, but analytical uncertainties make it also effectively degenerate with the alkalis. Uphill diffusion, notably of water and alkalis, was observed in several experiments, and this would lead to transient partitioning of water and alkalis across diffusion interfaces showing large Al 2 O 3 concentration gradients. Such partitioning in natural systems would persist until Al concentration gradients were erased by continued, much slower Al-Si interdiffusion.

The generation of ores from igneous rocks depends on the operation of common igneous processes in uncommon ways. Deposits of incompatible lithophile elements form when magmas containing unusually high concentrations of these rare elements fail to freeze at eutectic points, permitting their enrichment by extreme degrees of fractional crystallization. Deposits of compatible elements, such as Cr, Ti, and V, form when high-degree mantle melts undergo unusually effective processes of mixing or phase separation, including crystal settling or liquid immiscibility. Chalcophile element deposits form because of the extremely efficient segregration of very small quantities of immiscible sulfide liquid from large volumes of fertile mafic magmas that themselves were formed in the complete absence of sulfide melt in their mantle sources.

Petrogenetic modeling of the Recent lava succession of Santa Barbara and Pico Alto volcanoes and associated basaltic lavas indicates that there are two discrete lava series present, one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of Santa Barbara. The felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including essential amphibole. Trace element abundances in the felsic lavas, particularly those of Sr and REE, cannot be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile. The difference between the comenditic and pantelleritic differentiation trends of Santa Barbara and Pico Alto is attributed primarily to FO2 control of the crystallizing assemblage, probably related to thermal dissociation of magmatic water in the Santa Barbara magma chamber. This effect may be augmented by minor differences in parent basaltic compositions, the Pico Alto pantellerites being derived from the rift basalts whereas the Santa Barbara comendites are derived from the off-rift basalts. A compositional gap between 54 and 64% silica content in the lavas is not present if the suite is extended to include co-magmatic hypabyssal xenoliths, leading to the inference that the gap in this and other bimodal suites results solely from a relative inability of magma of intermediate composition to erupt.

I present observations of interfacial tension between miscible pairs of silicate liquid and propose a theoretical model that quantitatively accounts for them. Viscoelastic rheology of the liquids permits the establishment of gradient stress completely analogous to thermal stress in Maxwell solids; this is expected whenever the time scale of diffusion is shorter than the Maxwell relaxation time. The existence of gradient stress may profoundly affect interface processes during mixing of miscible fluids.

Pumice produced during the subplinian phase of eruption of a peralkaline trachyte lava on the island of Terceira, Azores, has undergone extensive leaching at ambient conditions by groundwater, while remaining in a glassy state. Over 25% of F, Na, K, Si, Fe, Ti, and Mn originally present has been removed, whereas U, Al, Nb, Ca, Y, and Rb show smaller but significant losses. Thorium, Zr, Hf, Ta, and Lu have remained immobile. Water content (measured as LOI) has increased up to tenfold, whereas the Sr content has risen by up to 80%. The absence of devitrification requires that leached elements escaped by diffusing through the glass; a coefficient of chemical diffusion DNa between 1.8 × 10−9 cm2 s−1 and 6.1 × 10−19 cm2 s−1 has been calculated, in excellent agreement with previously published experimentally derived values for comparably low temperatures. Water has not replaced the leached cations on a charge-equivalent basis, a finding that requires an explanation other than simple binary interdiffusion. We suggest that a simultaneous influx of molecular water and cation exchange between Na+ in the glass and H+ in the groundwater opens the glass structure and frees cations, which can diffuse outward by hydrolysis of bridging oxygen atoms. Reported mobilities have important implications for such diverse fields as igneous petrology, nuclear waste management and, agriculture.

The concentration and distribution of Pt and Au in a fluid-melt system has been investigated by reacting the metals with S-free, single-phase aqueous brines (20, 50, 70 wt% eq. NaCl) ± peraluminous melt at a confining pressure of 1.5 kbar and temperatures of 600 to 800 °C, trapping the fluid in synthetic fluid inclusions (quartz-hosted) and vesicles (silicate melt-hosted), and quantifying the metal content of the trapped fluid and glass by laser ablation ICP-MS. HCl concentration was buffered using the assemblage albite-andalusite-quartz and f O 2 was buffered using the assemblage Ni-NiO. Over the range of experimental conditions, measured concentrations of Pt and Au in the brines ( C P t fluid , C A u f l u i d ) are on on the order of 1–103 ppm. Concentrations of Pt and Au in the melt ( C P t m e l t , C A u m e l t ) are ∼35–100 ppb and ∼400–1200 ppb, respectively. Nernst partition coefficients ( D P t f l u i d / m e l t , D A u f l u i d / m e l t ) are on the order of 102–103 and vary as a function of C m e t a l f l u i d (non-Henry’s Law behavior). Trapped fluids show a significant range of metal concentrations within populations of inclusions from single experiments (∼ 1 log unit variability for Au; ∼2–3 log unit variability for Pt). Variability in metal concentration within single inclusion groups is attributed to premature brine entrapment (prior to metal-fluid-melt equilibrium being reached); this allows us to make only minimum estimates of metal solubility using metal concentrations from primary inclusions. The data show two trends: (i) maximum and average values of C A u f l u i d and C P t f l u i d in inclusions decrease ∼2 orders of magnitude as fluid salinity ( m ∑ C l f l u i d ) increases from ∼4 to 40 molal (20 to 70 wt % eq. NaCl) at a constant temperature; (ii) maximum and average values of C A u f l u i d increase approximately 1 order of magnitude for every 100°C increase temperature at a fixed m ∑ C l f l u i d . The observed behavior may be described by the general expression: log ⁡ ( m m e t a l f l u i d , T , 1.5 k b a r ) = x ⋅ log ⁡ ( m ∑ C l f l u i d ) − y where x (slope of the solubility curve) is similar for both Au and Pt (−2), and y increases with increasing T. The strong negative correlation between salinity and metal concentration may result from (i) the “salting-out” of neutral hydrogen chloride complexes [e.g., PtCl3H] as NaCl is added to the solution and free H2O is consumed during the production of HCl0 by the buffer assemblage, or (ii) the consumption of free H2O by the hydration of non-chloride-based metal complexes such as hydroxides [e.g., Pt(OH)2 · (H2O)n]. The proposed solubility mechanisms would be enhanced if activity coefficients for HCl and/or metal complexes decrease significantly with increasing fluid salinity. Identification of the exact dissolution mechanism at such extreme conditions and confirmation of the equilibrium metal concentrations awaits future study. The results of this pilot study demonstrate that single-phase hypersaline fluids which exsolve from or interact with residual magmatic liquids, partially crystallized rocks, or small volumes of PGE (platinum-group element)-Au-bearing sulfide can potentially dissolve and transport economically-significant amounts of Pt and Au across the magmatic-hydrothermal transition at moderately oxidizing conditions.

We report methane-dominant hydrocarbon (fluid) inclusions (CH4±C2H6–C2H2, C3H8) coexisting with primary brine inclusions and secondary halide melt (solid NaCl) inclusions in Au–Pt-rich quartz-sulfide-epidote alteration veins associated with the footwall-style Cu–PGE (platinum-group element)–Au deposits at the Fraser Mine (North Range of the Sudbury Igneous Complex). Evidence for coentrapment of immiscible hydrocarbon–brine, and hydrocarbon–halide melt mixtures is demonstrated. A primary CH4–brine assemblage was trapped during quartz growth at relatively low T (min. T trapping∼145–315°C) and P (max. P trapping∼500 bar), prior to the crystallization of sulfide minerals in the veins. Secondary inclusions contain solid halite and a mixture of CH4, C2H6–C2H2 and C3H8 and were trapped at a minimum T of ∼710°C. The halite inclusions may represent halide melt that exsolved from crystallizing sulfide ores that texturally postdate (by replacement) early alteration quartz hosting the primary, lower T brine–CH4 assemblage. Laser ablation ICP-MS analyses show that the brine, hydrocarbon and halide melt inclusions contain significant concentrations of Cu (0.1–1 wt% range), Au, Bi, Ag and Pt (all 0.1–10 ppm range). Cu:Pt and Cu:Au ratios in the inclusions are significantly (up to 4 log units) lower than in the host alteration veins and adjacent massive sulfide ore veins, suggesting either (1) early Cu loss from the volatiles by chalcopyrite precipitation or (2) enhanced Au and Pt solubilities relative to Cu at the temperatures of entrapment. Concentration ratios between coexisting brine and CH4 inclusions $$[C^{{\rm elem}}_{{\rm brine}}/C^{{\rm elem}}_{{\rm CH_{4}}}]$$ are lower for Cu, Au, Bi and Ag than for other elements (Na, Ca, Fe, Mn, Zn, Pb) indicating that during interaction with the brine, the hydrocarbon phase was enriched in ore metals. The high concentrations of ore metals in hydrocarbon, brine and halide melt phases confirm that both aqueous and non-aqueous volatiles were carriers of precious metals in the Sudbury environment over a wide range of temperatures. Volatile evolution and magmatic sulfide differentiation were clearly part of a single, continuous process in the Sudbury footwall. The exsolution of H2O-poor volatiles from fractionated sulfide liquid may have been a principal mechanism controlling the final distribution of PGE and Au in the footwall ore systems. The study reports the first measurements of precious metal concentrations in fluid inclusions from a magmatic Ni–Cu–PGE environment (the Sudbury district).

Deformation and melting of the crust during the formation of large impact craters must have been important during the Earth's early evolution, but such processes remain poorly understood. The 1.8-billion-year-old Sudbury structure in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself.

In the absence of stress-concentrating flaws such as microfractures, vesicular glassy materials can withstand gas pressures within vesicles in excess of 100 MPa; however, vesicles within such materials are known to decrepitate explosively at much lower internal gas pressures, both in natural systems and in the laboratory. Here we present a model that quantitatively predicts the generation of microfractures in vesicle walls during cooling. Cooling of gas-bearing vesicles in glassy rock has little effect on water solubility in the glass, but leads to a rapid decrease in gas pressure in the vesicles. The drop in pressure causes disequilibrium between the water in the glass and in the vesicle. Dehydration of the glass in a diffusive boundary layer around the vesicle leads to elastic shrinkage. The resulting strain generates large tensile tangential stresses which can exceed the strength of the glass, causing microfracturing. Such microfractures present a possible means by which glassy rock surrounding vesicles could be weakened enough to permit explosive decrepitation at low pore vapor pressures. The results have implications wherever hydrous vesicular glasses are formed. For example rocks formed in shallow subvolcanic intrusions or vent plugs may spontaneously disintegrate with explosive emission of vapor; glassy submarine lavas spontaneously decrepitate upon dredging from the ocean floor (“popping rock”); vesicular glasses produced in laboratory experiments investigating vapor-melt phase equilibria have been observed to contain abundant fractures surrounding vesicles and to dehydrate at anomalously high rates.

[Figure][1] ![Figure][1] The Hawley medalists for 2004 are James E. Mungall and James M. Brenan. Their paper “Experimental evidence for the chalcophile behavior of the halogens” was selected as the best paper published in The Canadian Mineralogist in 2003. It appeared in Volume 41

An understanding of the rates and mechanisms of U and Th diffusion in silicate melts and glasses is necessary to fully understand such diverse phenomena as U-Th decay series disequilibria, melt-crystal partitioning, and leaching of actinides from nuclear waste glasses. Uranium diffusivities have been determined in anhydrous borosilicate nuclear waste glasses (Dunn, 1987), and numerous investigators have studied release rates of actinides during leaching of glass by hydrothermal solutions (e.g. Schreiber et al., 1985). Dissolved water is known to enhance diffusion of other high field strength elements in silicate melts (e.g. Harrison and Watson, 1983). In light of the abundance of water in felsic silicate melts, and the prevalence of water-rich compositions in ancient weathered natural glasses (Mungall and Martin 1994), we have investigated the effects of water content and temperature on U and Th diffusion rates in a synthetic haplogranite composition at pressures ranging from one atmosphere to 10 kbars.

The generation of large deposits of Au and Cu in suprasubduction-zone settings depends upon a combination of factors, including availability of the chalcophile elements to arc magmas in their mantle source regions, and the operation of suitable hydrothermal systems in the upper crust where the deposits eventually form. The removal of chalcophile elements from the mantle wedge into arc magmas can only occur if sulfide is absent from the melted source rock, requiring oxidation of the mantle wedge to values of log f O 2 > FMQ + 2, where f O 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer. The only agent capable of effecting this change is ferric iron, carried in solution by slab-derived partial melts or supercritical fluids. Arc magmas with high potential to generate Au and Cu deposits will have certain geochemical characteristics; they will have f O 2 more than two log units above FMQ and they will have either adakitic, sodic-alkaline, or potassic-ultrapotassic affinities. Favorable tectonic settings include subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction.