76 THE HYDROTHERMAL KINETICS OF CRISTOBALITE H.L. BARNES and C.H. GAMMONS (Ore Deposits Research Section, Penn State University, University Park, Pa 16802, U.S.A.) Cristobalite (xtbl) has been reported only rarely from natural hydrothermal systems, yet many have fluids with high degrees of silica supersaturation. However, measured rates of dissolution of quartz (qtz) and amorphous silica ('SIO2') and of precipitation of qtz suggest that silica-water rate4imiting reactions are elemental, and that at equivalent degrees of supersaturation qtz and xtbl should precipitate at the same rates. To test whether the rarity of xtbl is due to its rapid recrystallization to qtz or to its rate of precipitation being much less than predicted, we measured its precipitation and dissolution rates. The results of dissolution experiments conducted in a rocking autoclave at 200°C are in good agreement with published studies. Precipitation measurements were made by saturating a solution with qtz in a bomb at temperatures up to 400°C and circulating the fluid in sequence through a sampling valve, a heat exchanger to quench to an intermediate temperature, a second sampling valve, a closed system pump, and back into the bomb. Samples of solution entering and exiting the exchanger were withdrawn for silica analysis, and the exchanger was periodically opened to examine the nature and quantity of precipitate. To avoid the randomness of nucleation processes, qtz or xtbl seeds of known surface area were added. Supersaturation levels of the solutions entering the heat exchanger were 1.7 - 3.5x xtbl solubility and 0-1.4x 'SiO2' solubility at quench temperatures of 135-200°C. Only qtz precipitated on the substrates. The rates ofqtz growth in the binary H20-SiO 2 tested here exceed those of xtbl regardless of the degree of supersturation in silica or of the seed crystals present, and there is no effective epitaxial control on the overgrowth of xtbl. The absence of growth even under favorable conditions explains the rarity of hydrothermal xtbl. HYDROTHERMAL ALTERATION OF QUARTZ: NEW INSIGHT BASED ON HYDROGEN DEPTH-PROFILING WITH A RESONANT NUCLEAR REACTION G. BERGER, J. SCHOTT, J C PETIT and ~C DRAN (Laboratoire de Min~ralogie, U.P.S., 31062 Toulouse, France) The dissolution kinetics of quartz under hydrothermal conditions has been investigated in order to check whether it is controlled by surface reaction or by transport across a hydrated layer where continuous depolymerisation/repolymerisa- tion reactions occur. Indeed such an observation would have important implications for the stable isotope geothermometry of the system quartz-water. Our experiments are based on the analysis of the altered surface by means of a resonant nuclear reaction which provides the hydrogen concentration and depth-profile. Dissolution runs performed at 400°C on both natural surfaces and polished sections and for times up to 8 days do not yet show clear evidence of important surficial hydration. Such a behavior markedly departs from that of feldspars and olivines (see companion paper) and seems to imply that ion exchange is a necessary step in the formation of hydrated leached layers, thus excluding any significant role for the permeation of molecular water contrary to the case of silicate glasses. However, ~he fact that radiatlon-damaged quartz exhibits a marked hydration above a critical density of defects, suggests that any factor activating water diffusion would favour the formation of a hydrated layer on this mineral. Therefore, we are currently testing this assumption by conducting experiments at higher temperatures and for longer times. RATE CONTROLLEDADVECTIVE INFILTRATION AND THE DECOUPLINGOF ISOTOPE EXCHANGE FRONTS P. BLATTNER and K.R. LASSEY (DSIR, P.O. Box 30368, Lower Hutt, New Zealand) The infiltration of lithosphere by internal or external fluids leads to characteristic profiles of stable isotope ratios with distance in the aquifer rock. The systematic evolution of such profiles is discussed for the model case of a uniformly permeable l-dimensional straight or curved rock column, with any steady state non-uniform temperature profile and corresponding profiles of isotope fractionation factors ~. Transport equations have been solved analytically, which allows to connect confidently causes and effects in model simulations. All possible profile shapes (Figure) are captured by the non-dimen~ DamkShler number (= exchange rate constant ~ over interstitial fluid velocity q times infiltration distance L), whereas real profile length is L. Profile evolution involves 6-transients ahead of the geochemical exchange front and a gradual approach to steady state equilibrium 'behind' the front. Values of L, and therefore apparent isotope temperatures, change from equilibrium at infinite <, or zero q, to rate controlled in real situations. Strong decoup]ing of exchange fronts for isotopes of different elements is dictated oy different concentration ratios in fluid/rock. Similar decoupling is predicted to occur for one set uf isotopes but mineral phases witn differing 's. i g~/ / I o L KINETICS OF A MEIASOMATIC REACTION UNDER GRANULIT[ FACIES CONDITIONS AS DEDUCED FROM STRONTIUM ISOTOPIC DISEQUILIBRIA D. BUHL and B. GRAUERI (University of MCinster, MOnster, FRO) Objekt of the study are the metasomatlc reac- tlon zones (MRZ) between a basic layer (BL) and a quartz-fe]dspar gneiss tn a charnockite terrane of the South Indian craton (ioeatity Swarna~atl, Mysore). ]he Bl (3 cm) probab]y represents a for- mer dike. The MRZ (each about 5 cm) ~ere formed during granutite-facies metamorphism 2.5 Ga ago. From the concentration profiles it seems that elements migrated only from the B[ into the gneiss and from the inner Lo the outer parts of the MRZ. Among the migrating elements Sr is of partJcutar interest due to its isotopic signature and the peculLarJties of its concentration pat- tern. During the metamorphism the whole-cock 87Sr/86Sr ratios in the gneiss ~ece homogenized to 0.715 whereas the ratio in the BI remained at 0.702. The migration of 5r from the BL into the MRZ produced concentration peaks Jn the MRZ (300-400 ppm) which ace ~eJI above the concentra- tion levels For the BL (lO0 ppm) and fhe gneiss (200 ppm). The boundary between the two types off Sz" has been found in the middle of the peaks. ~rom these observations and on the basis of 5[" mass balance calc:u]atJons tt is conctuded that Sr mtqrated From the BE into the gneiss as a frorlt~ driving out and replacing the 5r (if the grle]ss without isotopic mi×~nq. Thus, the melasomatic r~acl icon must hav{z taken place #tthin a very short time interval, otherwise, the two types I)f Sr should have been isotopit:aJ]y mixed. After the metasomat ic leact ion, however, the high gradients in the 87Sr/B6Sr ratios at the metasomatic front ill the BI Sr have been lowered by Sr self-dir- fusH~n brcaufm the temperatures in th{? rl~cks r('rriaJr/ecJ high for at ]east ~00 Ha.