Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex

Granite sheets emplaced into the migmatite zone of the easterncontact aureole of the Bushveld Complex resulted from fluid-enhanced,incongruent biotite melting of the underlying Silverton Formationshales during prograde metamorphism. Ba concentrations are extremein both the sheets (>1000 ppm) and the hornfels (>800ppm) into which they have been emplaced. We conclude that aBa-rich, hydrothermal fluid induced melting in the aureole,and that fluid transport of Ba²⁺, and to a lesser extent, Sr²⁺and Eu²⁺, persisted in the melt zones under subsolidus conditions.Sr-isotope systematics from high-Ba localities define an errorchronof 2161 ± 106 Ma with an initial (⁸⁷Sr/⁸⁶Sr) ratio of0·705 ± 0·001. Metasedimentary rocks unaffectedby fluid infiltration were homogenized at the same time butwith an increased initial ratio, suggesting that whereas isotopehomogenization was achieved between outcrops permeated by fluids,there is no evidence of regional homogenization. Oxygen-isotopecompositions of psammitic metasediments in the aureole are uncorrelatedwith distance from the contact, suggesting the infiltratingfluid equilibrated isotopically with the metasediments. Theirelevated     

OPTICAL AND RADIO SPECTRAL ANALYSIS OF A RECURRENT SURGE

Coordinated observations obtained at Meudon (MSDP) and at Ondejov provide surge spectra in optical range (H, H, H, Hµ, Ca H and K lines) and in radio range. The MSDP data allowed us to follow the time evolution of the surge. The spectra of Balmer lines were used to derive the electron density of the expelled cool material. The radio bursts indicate that the acceleration of electrons took place in the low and dense atmosphere, giving a good argument for a reconnection near the transition region.     

Metamorphism and Fluid Infiltration of the Calc-silicate Aureole of the Beinn an Dubhaich Granite, Skye

The Tertiary Beinn an Dubhaich granite intruded at 75OC and05 kb into siliceous dolostones and limestones of the UpperDurness Group in Strath, Skye, with the consequent developmentof talc, tremolite, diopside, olivine, and periclase in thebulk of the aureole, and abundant fluoro-borosilicate skarnsimmediately adjacent to the pluton. With increasing grade thelimestones develop the mineral sequence talc, tremolite, diopside,and olivine, whereas the dolostones develop the sequence talc,tremolite, olivine, and periclase. The abundant chert nodulesin the dolostones take either of the two reaction paths, dependenton their size. Those below 2–7 mm in dimension followthe dolostone reaction path, whereas larger nodules follow thelimestone reaction path. The presence of monticellite in thehighest-grade rocks points to the flushing of the contact byvolumes of water-rich fluid, derived presumably from the granite.Consideration of low-grade olivine-bearing veins and fracturesin the dolostones also points to the presence of extremely water-richfluid in the more distal parts of the aureole. Using simplebox models with constant porosity, it is shown that the observedreaction progress in oli vine-grade and talc-grade rocks canonly be accounted for if the rocks were infiltrated by substantialvolumes of water-bearing fluid. Minimum estimates for volumesof infiltrated fluid show that rocks nearest the pluton wereprobably infiltrated by greater amounts of fluid than thosefurther away. Low-grade rocks which suffered greatest amountsof infiltration are the brecciated dolostones nearest the Thrust.     

Phase Relations in the Fe-Ni-Cu-PGE-S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex

Experiments in the Fe–Ni–Cu–S system wereperformed to identify the role of the metal/S atomic ratio onmonosulphide–melt partition coefficients and closed-systemfractionation paths. In accord with previous work, D_Cu is     

Conditions of Formation and Crystallization Kinetics of Highly Oxidized Pseudotachylytes from the High Tatras (Slovakia)

Tectonic activity during the Miocene exhumation of the Tatragranitoid basement resulted in frictional melting of granite.The activity marks the early stages of the faulting that isresponsible for uplift of the High Tatras. As indicated by pre-existingcataclasite metamorphic mineral assemblages, the ambient pressurewas about 250–300 MPa, corresponding to depths between10 and 12 km. The pseudotachylytes are Fe rich, highly oxidizedand crystalline. The matrix composition suggests disequilibriumpartial melting of a biotite-dominated assemblage. Oxygen isotopiccompositions of a pseudotachylyte sample and its constituentminerals show equilibration with the host granodiorite and allowfor the introduction of oxidizing external water rather thanoxidation as a result of the dissociation of free water liberatedduring melting. The kinetic information extracted from hematitecrystal-size distributions (CSDs) that are preserved in feldspathicmatrices shows that crystals in most places were accumulatedwhile the system was open. The melt was highly mobile and proneto strong differentiation. The hematite crystals reach a maximumof 26 vol. % (45 wt % Fe₂O₃). In rare places where the flowceased, the system became closed and produced distinct CSDs.The longest apparent crystallization times (90 s) are recordedmostly in pools in the central parts of the pseudotachylyteswhereas the shortest times (10 s) come from rims and tips offractures. The estimated hematite growth rate was about fiveorders of magnitude higher than that of ilmenite in lava lakes.Such extreme crystallization rates result from high undercoolingsassociated with high cooling rates. Very high cooling ratesare promoted by the extremely high surface/volume ratios ofthe pseudotachylyte sheets. KEY WORDS: pseudotachylyte; hematite; kinetics; High Tatra; Slovakia     

Germanium Contents of USGS Standard Rocks by Flameless Atomic Absorption

Germanium in 13 USGS standard silicate rooks was determined by flameless atomic absoprtion using a procedure which permits the determination of 0.2 ppm of germanium in as little as 50 mg of sample. The mean germanium content found is (in ppm Ge) for the new USGS standards: BHVO-1, 1.60; SDC, 1. 51; STM-1, 1. 32; QLO-1, 1. 28; RGM-1, 1. 24; and for- the older USGS standards: W-l, 1.S3; BCR-1, 1.39; G-l, 1.19; GSP-1, 1.16; AGV-1, 1.06; G-2, 0.96; PCC-1, 0.80; DTS-1, 0.72.     

On the Use of Changes in Dihedral Angle to Decode Late-stage Textural Evolution in Cumulates

The melt-filled pore structure in the final stages of solidificationof cumulates must lie somewhere between the two end-membersof impingement (in which pore topology is controlled entirelyby the juxtaposition of growth faces of adjacent grains) andtextural equilibrium (in which pore topology is controlled bythe minimization of internal energies). The exact position betweenthese two end-members is controlled by the relative rates ofcrystal growth and textural equilibration. For samples in whichgrowth has stopped, or is very slow, textural equilibrium willprevail. A close examination of dihedral angles in natural examplesdemonstrates that these two end-member textures can be distinguished.The impingement end-member results in a population of apparentsolid–melt dihedral angles with a median of 60° anda standard deviation of 25–30°, whereas the texturallyequilibrated end-member population has a median of 28° anda standard deviation of 14°. For the specific case of cumulatesin the Rum Layered Intrusion, residual porosity in troctoliticcumulates was close to the impingement end-member, whereas thatin peridotites was close to melt-bearing textural equilibrium.Suites of glass-bearing samples from small, or frequently disturbed,magma systems show modification of initial impingement textures.These modifications may be a consequence of textural equilibrationor of diffusion-limited growth during quenching. Distinctioncan be made between these two processes by a consideration ofgrain shape. The geometry of interstitial phases in suites offully solidified cumulates from the Rum Layered Intrusion showsvariable approach to sub-solidus textural equilibrium from aninitial state inherited by pseudmorphing of the last melt. Texturalequilibration at pore corners occurs as a continuous process,with a gradual movement of the entire dihedral angle populationtowards the equilibrium final state. If the initial, pseudomorphedstate is one of disequilibrium (i.e. a melt-present impingementtexture) this change is accompanied by a reduction in the spreadof the population. If it is one of equilibrium, the change isaccompanied by an initial increase in the spread of the population,followed by a decrease. These observations demonstrate thatpreviously published models of dihedral angle change involvingthe instantaneous establishment of the equilibrium angle inthe immediate vicinity of the pore corner are incorrect. KEY WORDS: cumulate; dihedral angle; textural evolution; Rum intrusion; Kula; Santorini