OXYGEN ISOTOPE FRACTIONATION FACTORS AMONG ROCK- FORMING MINERALS AT HIGH TEMPERATURES R.N. CLAYTON, T.K. MAYEDA, J.R. GOLDSMITH, H. CHIBA, and T. CHACKO (Enrico Fermi Institute and Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, U.S.A.) A new set of experimental calibrations of oxygen isotope geothermometers has been obtained by anhy- drous isotopic exchange between silicates and cal- cite at high pressures. These calibrations provide equilibrium constants for all pairs of minerals from the set: quartz, albite, anorthite, diopside, forsterite, and calcite, at temperatures from 600°C upward. Most of the results are in good agreement with S.W. Kieffer's theoretical calculations, but there are also some significant differences. There are also some significant differences between the new data and those previously reported based on laboratory hydrothermal experiments. The cause for these differences is unclear. Theoretical calcula- tions of mineral-water fractionation factors are in disagreement with all experiments, if a wide enough temperature range is considered. This disagreement may be due to an inadequate understanding of the vibrational behavior of water at high temperatures and pressures. The proposal of a "cross-over" in the pyroxene-olivine fractionation at high tempera- ture is not supported by the new experiments. 183 THE IMPLICATION OF THE OXYGEN ISOTOPE RECORDS IN COEXISTING CHERTS AND PHOSPHATES S. EPSTEIN (Division of Geological and Planetary Sciences, California Institute of Techno]ogy) The ~180 values of coexisting mineral pairs and HTO which are in isotopic equilibrium can provide ab~oluL~ ~emVcl auul =m vr :Hut1 |b~ =blv=l =Hal ~ll: 6t80 of the fluid medium provided that the relationships between the fractionation factors among the different components and temperature have been calibrated. The recent work of Karhu and Epstein I demonstrated the potential of 6;sO analyses of coexisting marine cherts and the phosphates extracted from the cherts. These data contain in them criteria for oxygen isotope equilibrium as well as the ability to calculate the ~;aO of the oceans of the past. The v a l i d i t y and the accuracy limitations of the temperatures calculated from these data will be discussed. In addition to the oxygen isotope analysis of the phosphate chert pairs, the results for addi- tional samples from igneous rocks and terrestrial and marine fossils will be presented. l"The implication of the oxygen isotope records in coexisting cherts and phosphates". J. Karhu and S. Epstein (1986) Geochlmlca et Cosmochimica Acta 50. No. 8. 1745-1756. HYDROTHERMAL ALTERATICN AT MURUROA ATOLL (FRENCH POLYNESIA) : pETROGRAPHIC AND ISOTROPIC DATA P. O UOO~,NON 1, A. MEUNIER I ,V. CARISTAN 2, A. GACHON2, O BUIGU ES 2 1 Laboratoire de Pdlrologie des Alt6ralicns Hydrothermalas. U.A. 721 au CNRS. Universit6 de Poitlars - 40 avenue du Recteur Pineau 86022 Poitiers - FranCe 2 C E A - B3, RCP, B P 12 - 91680 BnJy~res la Ch~llal * France The Ole/O 16. H/D and C131C 12 isotropic data intern ;d)out the odgla ol fluids, W/R ratio and temperature of crystallization el secondary phases which characledze the hydrothermal alterations associated to the successive epieodes of Iormalicn of the Mururoa Aleil. The atoll was formed from ihe three following episodes : I - the formation of Ihe guyot by submarine effusive lava flows 2 - the emersion fOllowed by the aerial volcanism 3 - the tale intrusion in the "rift zones" t In the submarine basaltic flows the alleralion is controlled by the sea waler/basafl interaction. The parageneses Iollow a lemperature and chemical gradlanl Iron the center to Ihe peripheral zone of Java gows. The protoceladonite cwslallizes in inner zones at about 100*C. Fe sapondes are domthanl in inlermethale zone (80°C-70°C) and Mg saponite + K-Nil-zeolite (phlllipsite I precipitale in periphetat brecclas. The sea waler/tNisalt rlllJo calculated from chemical balances gives a minimum value of 3. 2 In the aerial effusive rocks Ihe hydrothermal allera(icns are gouverned by meteoric waler/rock laleractions (W/R = 1/7,3l The paraganesla evolveS tree chlorite/saponite mixed tayer in inner part of lava flows Ioward AI sapontie in peripheral breCCias The oxygen isolopic dale performed on AI saponite gives 80°C to 100°C temperalure range of cr/stallization without any thermaJ gradient ; this can be assigned to the thermal isolation power of Ifle air compared Io sea waler. 3 The hydrolhermal allerations associated to the intrusions in the "dft zones" developed a vertical zonation : a) - In the deep alleraflon zone (862-a86 depth) the magnetic fluids gone(ale two successive alteration events : an early pervasive event with crystallization of talc + Mg saponile + anWA)rfle + caicae (t° 2 t60*C-2OO*C). and a late vein event with cryslallizalicn o! calcite (go*C ~ I* ~ 120°C). The water/Rock ratio estimated lot pure magmatic fluid ranges from 1/4,4 to 1/52. (b) - In the inlermediate aJleration zone (712-862 m depth) the magmatic fluids are mi~ed with meteoric ones, they generate a single vein event with AI montmori,onite + calcite + opal (l'Z 80"C-90°C) The W/R ratio calculated 1or pure magmalic and/or pure meteoric fluid range from 1/12.7 to 1/6 8 (c) - In the upper allerlztion zone (61g-712 m depth) the magm&lk: Iliads mlxed with meteoac onetl produce • single vein oxydative alteration evenls with celadonite + nOnlronite (110"C-120°C) + calcite (TO°C-gO°C) TtIERMAL HISTORIES OF ROCKS FROM Rb-Sr MINERAL DATA AND Sr DIFFUSION KINETICS B.J. GILETT1 AND J.R. FARVER. (Depl. of Geological Sciences, Brown University, Providence, R.I. 02912, U.S.A. Non-colinear Rb-Sr mineral isochrons have been ascribed to various causes, but diffusional exchange between coexisting minerals is clearly one contributor. Sr diffusion kinetics have now been measured in orthoclase, albite, apatite, and, partially, biotite. These rates are used to compute Sr isotopic composi- tions in a model rock containing these minerals, and which has experienced one of the following: 1. slow cooling from high tem- perature, or, 2. a later heating episode. If the cooling rate for a granite is <50OC/m.y., several million years wilI elapse between the solidus temperature and the highest mineral closure tempera- ture for St. During that interval, plagioclase and apatile will be equilibrating with whole rock Sr so that they will then be abnor- mally radiogenic, and will fall well above the WR isochron by an amount that depends on the cooling rate. Biotile will be cor- respondingly depleted. Given the rock mode, and the mineral [Rb], [Sr], and parlicle sizes, the present-day 87Sr/86Sr can be computed for different rates. In case 2, of a later metamorphism, the temperature and dura- tion of the heating will determine the degree to which the dif- ferent minerals exchange Sr isotopes. By employing several minerals, the lemperature and the duration of heating become separable variables that can be assessed separately. The effects on the 87Sr/BSSr ratios in the differem minerals depend on the time between rock forn'tation and metamorphism, temperature, and duration of the thermal event. Generally, however, such effects may well be >0.1%, and sometimes >1% of the a7Sr/aSSr. The analytical uncerlainties are orders of magnitude less. Con- sequently, mineral scalterchrons from complex terrains may provide valuable thermal history data. Biotite appears most susceptible to exchange, so that its "closure" temperature is really that which is determined when the other minerals are closed. This is because the radiogenic Sr musl enter some other phase when it leaves the biotite.