160 PROGRESSIVE GRAPHITE ALTERATION IN A SERIES OF PROTEROZOIC GNEISSES (SASKATCHEWAN, CANADA) P. LANDAIS, J.DUBESSY and A. WANG. (CREGU - BP 23 - 54501 Vandoeuvre-l~s-Nancy cedex) X-ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been applied to the analysis of the graphite disordering in an homogeneous series of gneisses associated with an unconformity-type uranium deposit from Northern Saskatchewan (Canada). Different structural defects have been evidenced: (i) a progressive disordering of the graphite lattice mainly affecting the three dimensional order. The disordering intensity increases with decreasing distances to the unconformity and could be related to a more general hydrothermal alteration. (ii) the formation of holes in the graphite from the sample located near by the unconformity. Almost amorphous carbon phases have been recognized in the center of the holes which are surrounded by poorly organized graphite. These holes probably result from the alteration of graphite by highly oxidizing brines originated in the overlying Athabasca sedimentary formations. This reactivity of graphite at medium temperatures (T<250°C) has to be pointed out and contrasts with the blocking temperature of fluid-graphite equilibrium exemplified either by C isotope or fluid inclusions data. Emphasis has also been laid on the complementarity of the different techniques used in this study. TEM and Raman reveal surface defects while XRD investigates larger volumes of graphite and gives information on the bulk sample HYDROTHERMAL DISSOLUTION AND CRYSTALLIZATION OF FELDSPARS. EXPERIMENTAL DATA BETWEEN 500 AND 800°C, ~00AND 300 MPa. INFLUENCE OF SALINITY AND BOILING. S.C. DUJON. (D6partement de Gdologie,Ecole normale sup4rieure, 24, rue Lhomond - 75005 Paris) Experimental exchanges between plagioclases (solid solution) and aqueous chloride solution can be expressed by the reaction 2 NaAISi30 + CaCI -~ CaAI Si 0 + 2 NaCI + 4 SiO . T~is .2. 22 . .2. react,on is at least ~00 t,me faster in golng from the right to the left: a stationnary state is reached after one week at gO0°C and three weeks at 500°C. The fast reaction is enhanced by increasing pressure, temperature, dilution of the aqueous solution, and condensation (transformation of liquid plus vapor to a supercritical single phase). At 700°C condensation occurs when pressure increases from 100 to 200 Mpa. The influence on the reaction is then the same as the dilution of the aqueous homogeneous solution by a factor of 5. In natural aqueous system, increase of pressure and temperature, usually linked with burial, enhances calcic metasomatism of plagioclase. Nevertheless surrection and lowering of temperature in a two fluid phase system near of boiling may also induce such a metasomatism through fluid condensation. Dilution of connate solution by opening of the aqueous system, has the same effect. Sodic metasomatism is a much slower reaction. In natural and experimental exchanges between alkali feldspars and Na-K aqueous solution, exchanges are ruled by pressure, temperature and fluids boiling. Dilution of homogeneous f]uid~ has no inf]uence, because reaction involves elements ot tile same valency, which is not the case in the previous study. IMPROVEDOXYGEN Bbl-t-eadNG TECHNIQUES FOR C-O-H FLUID SATURATED EXPERnm:'N'I'S AT HIGH PRESSURE-TEMPERATURE W.R.TAYLOR ~ (Univm~dy ~:~Tmmm~l~ Hobart. Aus~atia TOO1) We have developed new oxygen buffering techniques for experiments in nigh-pressure, high-temperanire solidmedia apparatus where satuxation with reduced C-O-H fluidsare required. The first technique, abbreviated 'WCWO', uses a buffer consisting of a mixture of WC. WO2 and gzaphite which controls oxygen fugacity at approximately one log unit above the iron-wtistite (IW) buffer. Using an organic source for the C-O-H components, access to a range of CH4 and H20 dominated fluids with CH4/(CH4*H20) ratios between 0.8 and O.t is possible. Buffered fluid compositions can be maintained in sealed noble metal capsules for run times exceeding 50 hours. The WCWO technique capsule design overcomes problems of iron loss from the sample and melting caused by alloying between buffer and capsules. A capsule-piercing/mass spectrometry technique has been developed to analyse quenched fluids from nigh pressure experiments to determine fO2 and fluid species activities directly, thus allowing experimental 'calibration' of buffers. WCWO fluids were controlled to fO 2 neea IW*I log unit with a reproducibility of .0.2 log units. In contrast, the iron- wttstite<fraphite (IWG) buffer was unable to control fO 2 in liquidus experiments, although it performed well in the ebsence of a silicate sampte. The second technique, abbreviated 'CWI', closely controls fluid compositions at high H20/CH 4. Although fO2 is not controlled by a sotid buffer, it is ctosety bracketed by fluid species abundances. Using the mass spectrometric fluid monitoring technique the exact fO2 of CWI experiments can be determined after each run. Both these techniques have been employed in melting studies: WCWO on peridotite, and CVC[ on lamproite. WCWO should have wide application in experiments above 1000°C where control of fO 2 and aH20 is required, whereas CWI has the advantage of producing H20 rich fluids at all P.T which is particularly relevant to melting studies. Large scale infdtration of fluids during regional metamorphism. H and C isotope evidence from Central Nepal. Christian France-Lanord & Simon M.F. Sheppard CRPG-CNRS Nancy, BP-20, 54501 Vandoeuvre. Thrustuing of the hot High Himalayan Crystalline Formations over unmetamorphosed Lesser Himalaya Formations (Midlands) induce a prograde and inverted metamorphism in the Midlands (Le Fort, 1975). 3D and 313C have been measured in various sandstones and schists of the Midland from chlorite zone (= 400°C) to kyanite zone (= 620°C) 20 km to the north and close to the MCT thrust. Whole rock bD values tend to increase with metamorphic grade from -104 / -66 ~,- in chlorite & biotite zones to -84 / -45 o/~ in the garnet and kyanite zones. However in the garnet & kyanite zones bD decreases with the fraction of the rock water contained in the biotite. This trend is consistent with distillation of H during dehydration and biotite in/chlorite out reactions, b13C of trace graphite increases towards the MCT from = -30~ in the chlorite & biotite zones to =-20~ in the garnet and kyanite zones. Calculated isotopic compositions of fluids in equilibrium with the rocks, taking into account metamorphic temperatures and modal compositions are rather uniform over the proflls. ~D of water ranges from -55 to -25~ and bl3c of CO 2 varies between -15 & -10~. In the high grade zones these values are similar to those of water and CO 2 from fluid inclusions of quartz pods. Combined with H20 and C mass balance, modal compositions of rocks and fluid composition, stable isotope data imply that primary values of the Midlands were = -45 to -70'~ for ~D and = -30~ for ~13C of graphite. In situ volatilization tend to decrease bD values of rock (=15~ for 50% dehydration). Migration of these fluids into the lower grade zone tend to decrease ~D of these zones through exchange. Migrating CO 2 with ~13C = -10%~,probably produced in deeper zones, buffers all the graphite poor rocks.