Bakker, R. J.

Diamond, L. W.

English

Bakker R. J.

Diamond L. W.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 FLUID INCLUSION RE-EQUILIBRATION EXPERIMENTS IN QUARTZ: CHEMICAL POTENTIAL GRADIENTS BAKKER, R. J.1, DIAMOND. L. W . 2 1 Institute of Geosciences, Mineralogy & Petrology, University Leoben, Austria, inst i tute of Geological Sciences, University of Bern, Switzerland. E-mail: diamond@geo.unibe.ch The post-entrapment behaviour of fluid inclusions during changing P - T - f ^ i conditions in the host rock is still a matter of discussion. So far, only little evidence has been presented from natural samples, although post-entrapment changes are invoked in many studies to explain inconsistent results obtained by petrologic and other methods. Experimental studies over the last decade have not been conclusive and, in part, they are even contradictory. The objective of this study is to continue a series of experiments with synthetic H 2 0 - C 0 2 fluid inclusions in natural quartz to examine the effect of pressure gradients and chemical potential gradients on the bulk fluid-inclusion properties (Bakker & Diamond, 1998 and 1999). Experimental method Fluid inclusions were synthesised in natural, pre-fractured quartz according to the method of Bodnar & Sterner (1987). Silver-oxalate and distilled water were used to produce an H 2 0-C0 2 - r ich mixture of 30 mol % C0 2 . The syntheses were conducted at 600 °C and 500 MPa for 29 days, within the • -quar tz stability field. In order to prevent any pressure differentials, the experimental capsules were brought to the P-T conditions of synthesis and retrieved from these conditions by following the P-T path of the relevant fluid isochore. The locations of about 100 inclusions were precisely documented and then analysed for the following properties: 7"m(C02), 7m(clathrate), 7h(C02) , 7"h(total), size, distance to quartz surface, and area-ratio of phases at room temperature. The sample was then loaded into an open gold capsule and set under the same experimental P-T, again by following the pertinent isochore. Consequently, the argon pressure medium was in direct contact with the quartz surface. A chemical potential gradient in H 2 0 , C 0 2 and Ar was maintained for 27 days between the inclusions and the surrounding pressure medium, at equal internal and external pressures. After termination of the re-equilibration experiments the same inclusions were examined again and characterised according to the previously mentioned properties. Results First, the re-equilibrated inclusions modified their morphology by attaining more perfect negative-crystal shapes (Fig. 1). However, the area ratio of vapour/liquid did not change significantly. The changes in microthermometric behaviour are illustrated in Fig.2. Tm(C02) of all inclusion has shifted to slightly lower temperatures, approximately -56.8 °C. T'h(C02) increased between 1 and 4 degrees. rm(clathrate) decreased by various amounts to temperatures as low as 4.3 °C, always in the presence of water, liquid C 0 2 and C 0 2 vapour (Q2 phase assemblage). The total homogenisation temperature increased significantly (up to >380 °C). The inclusions were analysed by micro-Raman spectroscopy but no gases other than H 2 0 and C 0 2 could be detected. Fig.l . Photomicrograph of several fluid inclusion in quartz, after synthesis (a) and after re-Special cases Some of the inclusions decrepitated during the total homogenisation measurements following the initial syntheses. These inclusions became dark and apparently lost all their H 2 0 , leaving low-density C 0 2 fluid inclusions. All the other inclusions that had not decrepitated were checked for stretching by monitoring changes in r h (C0 2 ) . After the re-equilibration experiments most of the decrepitated inclusions had completely disappeared. The quartz crystal was completely restored without any trace of the previously present inclusions. A small proportion of the decrepitated inclusions were filled again with a substantial amount of H 2 0 , which in some cases even exceeded the H 2 0 / C 0 2 ratio in the original fluid composition. These examples suggest that H 2 0 did not leave the quartz crystal upon decrepitation of the inclusions, but remained present, probably in thin films in micro-cracks or channels (such as dislocations). : C02 H20 10 ym equilibration (b). 17 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 a™ 360 у 340 jS 320 о fS 300 280-260 4 0 0 о • initial ехр. О re-equilibration ехр. 0 о о о о о ° ч • Ч , О о ! , 380 H 360-340-320-300 280 о • initial ехр. О re-equilibration ехр. о о о о оо о 0 € О °о о vapour „ o f t О о 9° о оо° о0 о critical 0 ° ° • • . liquid • • • « 10 16 22 24 18 20 Tm (clathrate) / °C Th ( C 0 2 ) / °C Fig.2. Microthermometric results of initial syntheses and re-equilibration of fluid inclusions. Clathrate melting occurs at Q2 conditions (a). Inclusions from the initial synthesis homogenise to the liquid phase (LVCL) (a and b). After re-equilibration most of the inclusions homogenise to the vapour phase (LVOV). Partial homogenisation of the C02-rich phases (b) is always to the liquid (Lc0,VDLc0,)-Discussion and conclusions As no Raman-active gases were detected in the fluid inclusions other than C 0 2 and H 2 0 , we attribute the large observed changed in 7"m(clathrate) and T^iCOi) to diffusion of argon (which is not Raman active) into the inclusions from the pressure medium of the re-equilibration experiments. Argon is known to enter into a clathrate solid-solution with C 0 2 , and for low Ar concentrations, this solid-solution is thermally less stable than pure C 0 2 clathrate. The large increase in Th(total) is also consistent with admixture of volatile Ar. Diffusion of argon through quartz into fluid inclusions appears to be effective at 600 °C and 500 MPa under the high imposed fugacity gradient of the present experiments. The possibility of Ar diffusion was apparently overlooked in previous re-equilibration experiments (e.g. Sterner et al., 1995), but it may explain some of the conflicting results in the literature. Owing to the masking effects of diffusing Ar, the mobility of H 2 0 and C 0 2 cannot be evaluated conclusively from our experimental set-up. However, the change in mode of total homogenisation, from L V D L in the initial syntheses to LVDV following re-equilibration, may indicate a decrease in bulk density. Our size measurements show that the total volume of the inclusions did not change significantly upon re-equilibration. Therefore, if Ar diffuses into the inclusions, a decrease in density can only be obtained if H 2 0 and/or C 0 2 diffuse out of the inclusions (as expected from the imposed fugacity gradients). Future experiments in a pure H 2 0 environment and a pure C 0 2 environment will give information of the effectiveness of H 2 0 and/or C 0 2 diffusion through quartz as a consequence of chemical potential gradients at equal pressures. References BARKER, R. J., DIAMOND, L. W. (1998) Reequilibration of synthetic C 0 2 - H 2 0 fluid inclusions in quartz (abst.) PACROFI VII (Las Vegas), Programs and Abstracts, 10. BARKER, R. J., DIAMOND, L. W. (1999) Reequilibration of synthetic C 0 2 - H 2 0 fluid inclusions in quartz : isofugacity experiments (abst.) ECROFI XV (Potsdam), Terra Nostra, 99/6, 20-21. BODNAR, R. J., STERNER, S. M. (1987) Synthetic Fluid Inclusions. In G. C. Ulmer & H. L. Barnes (eds.): Hydrothermal experimental techniques. John Wiley & Sons, Inc., New York, 423-457. STERNER, S. M., HALL, D. L., KEPPLER, H. (1995) Compositional re-equilibration of fluid inclusions in quartz. Contribution to Mineralogy and Petrology, 19, 1-15. 18 

Fluid inclusion re-equilibration experiments in quartz : chemical potential gradients

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Fluid inclusion re-equilibration experiments in quartz: : Chemical potential gradients

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