- Email: [email protected]
A natural example of superdense CO2 inclusions: Microthermometry and Raman analysis
0016-7037/90/$3.00
Cwchimica a Cosmochimica Ada Vol. 54, pp. 895-901 Copyright 0 1990 Pergamon Press plc.Printed in U.S.A.
+ .oO
LETTER
A natural example of superdense CO2 inclusions: Microthermometry and Raman analysis A.M. VANDENKERKHOF' andS.N. OLSEN* ‘Institut voor Aardwetenschappen, Vrije Universiteit, 108 1 HV Amsterdam, Netherlands *Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 2 12 18, USA (Received November 13, 1989; accepted in revised form February 14, 1990)
Abstract-The
presence of extremely dense CO2 fluid inclusions in migmatites from the Colorado Front Range has been confirmed by Raman analyses. These inclusions show metastable homogenization of liquid and vapor (L + V --t L) at temperatures below the triple point of pure CO*. Raman analyses failed to show the presence of N2, CH4, H$, or any other components found in the past in COz-rich inclusions with such low homogenization temperatures. We conclude that the high density resulted from a decrease in inclusion-cavity volume during isobaric cooling of less dense inclusions. INTRODUCTION
mainly of quartz, microcline, and plagioclase. Paleosomes (host rocks) are biotite-quartz-feldspar gneisses.
REPORTSOFCO~INCLUSIONS which homogenize below the triple point are found in several published results of microthermometric studies of fluid inclusions by a heating-freezing stage (e.g., COOLEN, 1980; SWANENBERG,1980). Such homogenization (L + V -) L) is a metastable phase transition in the case of pure CO2 and indicates an extremely high density for the fluid: the internal pressure of such an inclusion at room temperature is on the order of 2 kb. The apparent high densities have led to the conclusion that these fluids have been trapped at extreme depths, corresponding generally to pressures greater than 10 kb. Recent Raman microprobe analyses of some of these previously reported highdensity CO2 inclusions have shown that the inclusions do not contain pure CO2 but mixtures of CO*-N2 or CO&H4 with densities lower than that of CO2 at the triple point (1.178 g/cm’). This is the case for inclusions from the Furua Granulite Complex in Tanzania (KERKHOF, 1988b) and from the Rogaland area in southwestern Norway (KERKHOF et al., 1990). The phase behaviors of these fluid mixtures are very similar to those of high-density CO2 (TOURET and KERKHOF, 1986), and microthermometry alone does not provide definitive evidence for high-density CO2 fluid. To date, all inclusions with homogenization temperature (Th) below the triple point for pure CO*, when investigated by the combined techniques of microthermometry and Raman analysis, have been found to contain fluid mixtures with lower densities than originally reported. Herein, we present the first case of truly dense CO2 inclusions with metastable homogenization temperatures lower than the triple point.
METHODS AND RESULTS Microthermometry
The most commonly used technique for determination of densities in non-aqueous inclusions is a microthermometric study of the phase behavior at temperatures between - - 180 and +35”C. Most measurements in this study were made with a USGS-type heating-freezing stage modified by Fluid Inc. A Chaixmeca stage was also used in the range -100 to -40°C because it gave a slightly better optical resolution, especially for the phase transition S + L + L. The inclusions in this study are small (5- 15 pm in size), and phase transitions are difficult to observe. The heating rate around the temperature of CO2 melting was approximately 0.1 “C/s. The stages were calibrated at the COZ triple point (at -56.6”C) by using known pure CO2 inclusions of low density. Most of CO* inclusions in the Front Range samples are monophase at room temperature (OLSEN, 1987), but a vapor phase nucleates in all inclusions during cooling to about -100°C. Some supercooling is necessary for freezing any fluid. The inclusions in this study froze between about -85 and -lOO”C, or 30 to 40°C below the triple point of CO*; but they melt within a few degrees of the triple point upon subsequent heating (see below). This interval between the freezing and melting temperatures allows us to observe homogenization below the equilibrium triple point: homogenization temperatures can be measured after cooling without freezing the inclusion. Homogenization temperatures of approximately 1300 inclusions have been measured (OLSEN, 1987). Of these, four showed homogenization below the CO2 triple point at -56.6”C. Three were found in quartz in a leucosome, one in a paleosome. A homogenization temperature between the triple point and the temperature at which the inclusion freezes is denoted as metastable homogenization temperature (Thm)
STUDY SAMPLES CO2 inclusions in this study are contained in matrix quartz in migmatites from the Colorado Front Range, USA. The migmatites are Proterozoic in age (Rb/Sr whole-rock age = - 1700 Ma) and have been metamorphosed to amphibolite grade (see OLSEN, 1987, 1988, for more details). Leucosomes (leucocratic lenses and layers) in the migmatites consist 895
4. M. van den Kerkhofand
89h
because the assemblage liquid range. Thm’s for the inclusions to -61°C. In order to observe inclusions must first be frozen. clusions, the following features
+ vapor is metastable in this in this study range from -66 stable phase transitions, the Upon heating the frozen inare observed (Fig. I ):
S. N. Olsen
with liquid), but some ofthe solid remains. This transition is denoted as partial homogenization to liquid (Ths L). The solid phase can easily go undetected at this stage because of the very small difference between the refractive indices of the liquid and solid. (4) The final phase transition takes place by dissolution of the solid into liquid. This homogenization has been denoted as “sublimation” to the liquid phase (Ts L), rather than melting, by KERKHOF (1988a: 1990) because in a general case the transition can be either S + V + V or S + L + L. The two transitions for C02-CH4-N2 inclusions cannot in general be distinguished by a microscopic observation because of the very small difference in refractive index between vapor and liquid. The only exception is the case of pure CO2 inclusions for which the distinction is possible. For the inclusions in this study,
(1) A frozen inclusion contains solid CO* and a vapor phase. The filling degree for the solid is approximately 0.8; the solid occupies about 80% of the inclusion volume. (2) A liquid phase forms at about -56.6”C resulting in three phases: solid, liquid, and vapor. This transition is denoted as initial melting by formation of liquid (Ti L). (3) Both solid and vapor phases rapidly shrink during a very narrow temperature interval after the first appearance of melt. At a few tenths of a degree above the initial melting, the vapor bubble dissolves into the liquid (homogenizing
lo-3T-7-125-l -7
r~ stable ~~~-~~
meta stable
1
4
ii
-60
I
-65
0
L
’
-6LB
r
‘t FIG. I. Stable and metastable phase transitions in “superdense” CO2 inclusions observed on warming from 5 -85 to -40°C. Inclusions show a stable phase sequence, S + V +S+L+V-+S+L+L,whenwarmedafiercooling to - -100°C (temperatures at which they freeze), but met&stable homogenization L + V -) L when warmed after cooling to - -85°C without being frozen. The metastable homogenization is at a lower temperature than that of melting (see text).
Superdense CO2inclusions in migmatites Table
1.
Microthermometric
results
Sample
Thm%
TiL°C
ThsL”C
TsL”C
1 Q-3T-7-a0 1 O-3T-7-a6-2 1 O-3T-7-a6-1 IO-3T-1 O-b7
-66.2 -60.7 -64.6 -61.8
-66.6
-66.2 -66.4 -66.7 -66.6
-62.1 7’ -62.6 -54.1
‘Ultflcult
to
observe
Thm=metarlable homogenization: L+V + L TIL ~Inltial meltlng: V+S l V+S+L ThrL=parHaI homopenlzatlon to liquid: V+L+S TsL ~subllmation(/melting): S+L e L or S+V
l l
L+S V
all final transitions are S + L --) L so that the transition is referred to as melting/sublimation, or simply as melting, below. The melting/sublimation in these inclusions was observed at -52 to - -54’C (Table 1). These temperatures are significantly higher than the triple point of COZ. Thus, the following phase transitions were observed during the stable path of a microthermometric analysis from - - 100 to -4O’C: S + V + S + L + V + S + L + L. These inclusions are Type S3 in the classification proposed by KERKHOF (1988a; 1990). “S” stands for the final phase transition, “sublimation.” “ 3” stands for the total number of phase transitions. P-T phase diagrams The observed phase transitions in these inclusions, which we believe contain extremely dense CO2 fluid, can be explained by P-T phase diagrams (e.g., SWANENBERG, 1980; ROEDDER, 1984, p. 376-377). Figure 2 diagrams CO* in the region of the triple point constructed using the data of ANGUS et al. (1976) and shows stable L + V, S + V, S + L, and metastable L + V curves together with some isochores. P-T
897
paths during cooling and subsequent heating are illustrated for an inclusion with the molar volume of 36.5 cm3/mol (density = 1.2 1 gjcm3), which is representative of the densities for the inclusions in this study. The inclusion in our example is in the liquid field at room temperature. Upon cooling, the inclusion follows a stable isochore from the liquid field into the stability field of solid (through point S) along the metastable extension of the liquidfield isochore because an inclusion must be supercooled before it freezes. A solid generally needs a greater amount of supercooling to nucleate than does a bubble. In these inclusions, solid nucleates at 30 to 40°C supercooling, but a bubble at only 10 to 15°C. A bubble therefore is always the first to nucleate even though homogenization temperatures are lower than the triple point. Upon nucleation of a bubble at Bl, the P-T conditions for the fluid in the inclusion will change to B2 on the metastable extension of the L + V curve to reflect the new two-phase equilibrium. Note that a part of the metastable extension of the isochore in the region of bubble nucleation is at negative pressures down to approximately - 100 bars. Liquid is at a negative pressure when a failure to nucleate a bubble stretches it metastably to occupy a volume larger than that at equilibrium (ROEDDER, 1984, p. 298). After nucleation of a bubble, two different paths for further phase transition are possible depending on whether the inclusion is heated or cooled. If heating is started at B2, metastable homogenization will occur at H. Upon further heating, the inclusion follows the metastable extension of the liquidfield isochore through the solid field and then the stable isochore through the liquid field. On the other hand, if cooling is continued at B2, the inclusion will follow the metastable L + V curve until the solid
P bar
600
I
I
-90
-60
I
-70
I
-60
I
-50
I
-40
I
TY
FIG. 2. Phase relationship in the CO2 system in the region around the triple point (T) showing, as thick lines, the sublimation, boiling, and melting curves (S + V, L + V, S + L) and selected isochores. Stable isochores are shown as thinner solid lines and metastable extensions as dashed curves. Numbers on isochores are molar volumes in cm3/mol. The sublimation curve and the metastable extension of the L -t V curves are shown in more detail in the inset. The phase transition illustrated with letters and dotted lines is for an inclusion with a molar volume of 36.5 cm3/mol (see text for further discussion).
898
A. M. van den Kerkhof and S. N. Olsen
CO2 nucleates at F2 (as determined experimentally), resulting in an instantaneous solidification of the liquid phase with a sudden but slight drop in internal pressure (to Fl ). Heating from this point results only in stable phases. The solid and vapor phases remain stable up to the triple point (T) where a liquid phase forms (initial melting, Ti L). The presence of three phases constrains P and T at the triple point until one of the phases completely disappears, in this case the vapor phase (partial homogenization to the liquid phase, Ths L). Upon further heating, dissolution of the solid phase proceeds along the S + L curve until it is completed at S. In our example, the molar volume of the liquid phase between T and S decreases from 37.347 to 36.5 cm3/mol, which corresponds to I. 178 and 1.2 1 g/cm3, respectively, in density. Therefore, observing the solid phase becomes increasingly difficult during “dissolution” as the density of liquid approaches that of solid (1.56 g/cm3). Melting temperature of pure CO2 in the absence of a vapor phase must be higher than the CO* triple point, as observed. Such phase behaviors for high-density COZ inclusions have been discussed by SWANENBERC; (1980), but are not substantiated because his inclusions turned out not to contain pure COZ (KERKHOF, 1988a: KERKHOF et al., 1990). Comparison wilh phase behavior in CO*-N2 inclusions The phase behavior described above for high-density COZ is similar to that of some inclusions reported from the Furua Granulite Complex, Tanzania (COOLEN, 1980: KERKHOF, 1988b). Raman analyses, however, show that the Furua inclusions (which are Type S3, as are the inclusions in this study) contain CO*-N2 fluids with 25 to 30 mol% Nz and densities of approximately 0.95 g/cm3. The density corresponds to a molar volume of 40 cm3/mol (compare Fig. 2). These Type-S3, C02-Nz inclusions have Ti L = -65”C, Ths L = -63”C, Ts L = -60 to -61 “C for stable phase transitions, and Thm L = -63°C for metastable ones. Their homogenization temperatures are close to or slightly below those of melting. Although the phase behaviors of high-density COZ and lower-density CO*-N2 inclusions are similar, they are not identical. A careful observation therefore can help distinguish the two types. The most important differences can be summarized as follows:
(COOLEN,1980; KERKHOF,1988b). These inclusions, denoted as Type Sl, can also be mistaken for high-density CO2 inclusions. They contain CO>-N2 mixtures with higher N2 contents (-30 to 40 mol%) than S3 inclusions. Final melting temperatures in the absence of vapor (Ts L) range from --6 1 to -59°C: those of metastable homogenization (Thm) range from -74 to -64”C, with the lowest temperatures corresponding to critical homogenization. The other types of C02Nz and C02-CH4 inclusions showing melting or sublimation (S2 and S4) contain three phases at low temperatures (- - 180°C) and can be easily distinguished from high-density CO2 inclusions. Raman ana&s The purity of the CO* inclusions in this study was checked by Raman analysis using a Microdil-28 Raman microspectrometer (BURKEand LUSTENHOUWR, 1988) equipped with a multichannel detector. The inclusions were checked for the presence of Nz, CH4, and H2S, but no component other than CO2 could be detected. The Raman spectrum of COZ consists of two peaks, referred to as the Fermi diad, with a higherintensity peak (2~) at 1388 and a lower-intensity one (Y,) at 1285 cm-’ at 1 atm (Fig. 3). Small peaks located at the sides of the main peaks are hot bands, which represent interaction frequencies (v, + u2 and ul + 3~~)due to the thermal energy of the molecules (HOWARD-LOCKand STOICHEW, 197 1). Internal pressures of CO1 inclusions with a density of 1.21 g/ cm3 should be on the order of 2 kb at 25°C. The Raman spectrum of a high-density CO2 inclusion is compared to those of lower density inclusions in Fig. 3. The following changes in the Raman spectrum reflect higher pressures in
the inclusions (GARRABOSet al. 1980; BERTRAN, 1983): (I )
I t
(1) Initial melting in CO*-Nz inclusions (=Ti L) occurs at
(2)
lower temperatures (_ -65°C for the Furua inclusions) than for pure CO2 (-56.6”C). Final melting of high-density COz in the absence of vapor (=Ts L) takes place at higher temperatures than the triple
point of COZ (-56.6’C). (3) Filling degrees of solid, or the volume solid occupies in frozen inclusions, are lower in C02-N2 inclusions than in COZ inclusions. (4) Type-S3 inclusions are rare in C02-N2 inclusions. More common types are H3, Sl, S2, and S4 (see KERKHOF, 1988b). Inclusions which show only one transition (S + L + L: Ts L) in the temperature range -180 to +35”C were also found in the samples from the Furua Granulite Complex
FIG. 3. Raman spectra of C@ with four different densities (from the lowermost to the uucertnost s.pecUumd = 1.2, 0.8, 0.7, and ~0. I, respectively).The-lowermost spectrum (d = - 1.2g/cm3)was obtained from one of the Type-S3 inclusions of Table 1 (lO-3T-7a5-a). The Raman spectra for other “superdense” inclusions in Table I a& very similar to the one shown. The other three spectra in Fig. 3 are for “normal” H3 inclusions. Spectra of hig&er-densityfluids are characterizedby a shift to lower wavenumbers, an increased ratio in intensity of 2v2to v2peak, and suppression of the two hot bands (see text for further discussion).
Superdense CO2inclusions in migmatites a shift in the positions of both peaks to lower frequencies (for pressures lower than 6 kbar); (2) decreases in peak widths; (3) an increase in the frequency separation of the two peaks (A = 2~ - v,); and (4) an increase in peak intensity ratio (R = 12,/I,,). In addition, we also observed decreases in intensities of the hot bands (this study). The peak positions of monophase CO2 inclusions measured at 20°C are different for the gas (d < 0.19 g/cm3) and the liquid (d > 0.77 gJcm3): the 21~2peak is located at 1388 to 1387 cm-’ for the gas and 1385 to 1384 cm-’ for the liquid; the V, peak is located at 1285 to 1284 cm-’ for the gas and 128 1 to 1279 cm-’ for the liquid. There is a 2 cm-’ difference between the peak positions for the vapor and liquid in twophase inclusions with total densities between 0.19 and 0.77 g/cm3 (BURKE and KERKHOF,unpubl. data). Peak shifts within the gas and liquid regions are small compared to the accuracy in measurement of Raman lines in the range of the Fermi diadtihe accuracy in peak positions is +0.8 cm-’ for the instrument used. The instrument has been calibrated with a neon lamp and regularly checked by measuring the Raman peak of diamond at 1332 cm-‘, located between the two CO1 peaks. The position of the V, peak is shifted more than the 2vZ peak with increasing density so that the frequency separation (A = 2v2 - v,) also increases with density. The parameter has been used as an indicator of densities in fluid inclusions (PASTERISand WANAMAKER,1988). Low-density gas inclusions have A - 103 cm-‘, while the high-density inclusions (d - 1.2) of this study have A - 106.5 cm-‘-which is in acceptable agreement with the calculated value of - 105.9 cm-’ (after BERTRAN, 1983). According to DUBESSY(1989X the sum of the relative Raman cross-sections for the two peaks remains constant. The relative peak intensities (R = 12,&J of the Fermi diad, on the other hand, gradually change from about 1.6 for low densities to 1.95 for the density of the triple point (GARRABOS et al., 1980). The values of R for the present inclusions with even higher densities than that of the triple point ( 1.178 cm3/ mol) are 1.95 and 2.0, consistent with those of GARRABOS et al. (1980) within the error. The accuracy of these values is no better than +O.l with the instrument used. The suppression of the low-intensity hot bands occurs in highdensity fluids at constant temperature due to more restricted vibrational freedom of the molecules. We conclude that our observations from natural inclusions are in accordance with the experimental data of GARRABOSet al. (1980) and BERTRAN ( 1983). DISCUSSION The extremely dense CO2 inclusions of this study belong to the group of CO2 inclusions considered to be the earliest inclusions in the Front Range migmatites. These early inclusions have negative crystal shapes and occur as either isolated inclusions, in small clusters (two to 10 inclusions), or in small intragranular healed fractures (up to about 100 inclusions), but never in a healed fracture that crosscuts a grain boundary. They occur more commonly in leucosomes (leucocratic layers and lenses) than in paleosomes (host rocks), and their representative isochores are consistent with the P-T conditions of migmatization estimated from the mineralogic data. These
899
observations led OLSEN(1987) to conclude that these early CO* inclusions had formed during the migmatization episode. The isochores for the extremely dense inclusions in this study, however, are not consistent with the P-T conditions of migmatization (Fig. 4). Nevertheless, these “superdense” inclusions do not belong to a special group of inclusions but to a general population of synmigmatization inclusions, because: (1) all of the inclusions have a similar mode of occurrence; (2) there is a continuum in homogenization temperature of the early CO* inclusions, including “superdense” inclusions (from +30 to -64’C; OLSEN, 1987, Fig. 4); (3) each of the superdense inclusions occurs in a cluster together with less dense inclusions; and (4) this study has shown that the superdense inclusions contain pure COZ, as less dense inclusions do. The origin of the high-density CO2 inclusions in the Front Range rocks has been discussed by OLSEN (1988). The following is a summary of her conclusions:
(1) The high-density inclusions are not relics of an earlier high-pressure metamorphism which have escaped decrepitation, because there is no correlation between the size and density of inclusions (OLSEN, 1988, Fig. 5) and because high-density inclusions are rare in the zones of migmatites with the least signs of deformation and melting (paleosomes and quartz inclusions within feldspar grains). (2) The superdense and other high-density inclusions (with Th < - -30”) must have been modified after their formation, because inclusions with extremely high-density CO2 tend to be associated with micro-shear zones in these rocks and because high-density inclusions are associated with plagioclase which has been heavily altered by hydrothermal activity during a late event in leucosome formation. (3) A change in composition of the fluid-for example, by a loss of water-is not likely to have occurred (see OLSEN, 1988, for further discussion). Olsen also postulated that the inclusions were modified to smaller volumes (= higher densities) during nearly isobaric cooling. If the host rock for an inclusion is cooled isobarically, the cooling path must intersect COZ isochores with increasingly higher densities (Fig. 4). If the volume of an inclusion can be equilibrated to the new P-T conditions (by volume decrease), a higher density for the inclusion would result. Such reequilibrations in density have been demonstrated experimentally for synthetic aqueous inclusions by F&HER and BOULLIER(1984) and STERNERand BODNAR(1989). The observed wide variation in Th or density variation (see OLSEN, 1988, especially Figs. 3 and 6) indicated selective increases in density. Olsen suggested hydrolytic weakening of quartz (e.g., BLACK and CHRISTIE, 1984) as the factor for the selectiveness because of the association of high-density inclusions with hydrothermally altered plagioclase. The presence of water in quartz immediately adjacent to an inclusion could have caused localized hydrolytic weakening and facilitated the process of a volume decrease during deformation. There is no apparent correlation between the presence of aqueous inclusions and high-density CO* inclusions in these rocks, but this is not inconsistent with the model because
900
A. M. van den Kerkhof and S. N. Olsen
P kb
6
0
500
FIG. 4. CO2 isochores and P-T conditions of migmatization or the formation of migmatites in the Front Range. Numbers on the &chore are molar volumes in cm’/mol and corresponding homogenization temperatures (“C) are given in parentheses. “Superdense” COr inclusions with (met&able) homogenization at temperatures lower than those of melting indicate trapping or equilibration at the P-T conditions in the upper left part of the diagram. The P-T conditions of migmatization in the Colorado Front Range are indicated by the hatched rectangle, which straddles the water-saturated granite melting curve. All melting curves are from BROWN(1970). An isobaric cooling path is indicated by the arrow. The lowest reasonable temperature for quartz crack healing (Tmin), and therefore the fluid inclusions formation, is inferred as 200°C. Either the “superdense” inclusions of this study were trapped at about 250°C and 5 kb (large dot), or the present density reflects final equilibration of the inclusions at these conditions (see text).
most aqueous inclusions seem to be younger than the early CO2 inclusions (OLSEN, 1987). This study has shown conclusively that CO2 fluids with extremely high densities do occur in high-grade metamorphic rocks, but the density, in this case, is a result of modification of the inclusion after initial formation. Such an inclusion may have trapped a peak-metamorphic fluid, but the present density of fluid does not reflect the P-T conditions of the peak metamorphism.
dynamic Tables oJ‘the Fluid State, Vol. 3: Carbon Dioxide. Per-
gamon Press, Oxford. BERTRANJ. F. (1983) Study of Fermi doublet vr-2~~ in the Raman spectra of CO2 in different phases. Spectrochim Acta 39A, 119121.
Acknowledgments-We
would like to thank Dr. Ernst Burke for assistance with Raman analysis. The encouragement of Dr. Jacques Touret initiated this study. Facilities for the laser Raman microprobe analysis were provided by the Free University in Amsterdam and by the Netherlands Organization for Scientific Research. Thoughtful reviews and helpful comments by Drs. R. J. Bodnar, P. C. Hess, J. D. Pasteris, and E. Roedder led to a greatly improved final draft. The support of NSF Grant EAR-8502403 to SNO is also gratefully acknowledged.
BLACICJ. D. and CHRISTIEJ. M. (1984) Plasticity and hydrolytic weakening of quartz single crystals. .I. Geophys. Res. 89, 42234239. BROWNG. C. (I 970) A comment on the role of water in the partial fusion of crustal rocks. Earth Planet. Sci. Lett. 9, 355-358. BURKEE. A. J. and LUSTENHOUWER W. J. (1988) The application of a multichannel laser Raman microprobe (Microdil-28) to the analysis of fluid inclusions. Chem. Geology 61, 11-l 7. CWLEN J. J. M. M. M. (1980) Chemical petrology of the Furua Granulite Complex, southern Tanzania. GUA Pap. Geol. Ser. 1, 13. DUB&SYJ. (1989) Advances in C-O-H-N-S fluid geochemistry based on micro-Raman spectrometric analysis of fluid inclusions. Enropean J. Mineral. 1, 517-534. GARRABOSY ., TUFEU R., and NEINDREB. LE ( 1980) Rayleigh and Raman scattering near the critical point of carbon dioxide. J.Chem.
Editorial handling: G. Faure
HOWARDLOCKH. E. and STOICHER:B. F. (1971) Raman intensity measurements of the Fermi diad Y, , 2~2, in 12C02 and 13COz.J.
Phys. 72, 4637-465
1.
Mol. Spect. 37, 32 l-326.
REFERENCES ANGUS
S., ARMSTRONGB., DEREUKV. V., GADETSKII0. G., CHAPELAG. A., and ROWLINSONJ. S. (1976) International Thermo-
KERKHOFA. M. VAN DEN (1988a) The system C02-CHcN2 in fluid inclusions: theoretical modelling and geological applications. Ph. D. dissertation, Free University, Amsterdam. KERKHOFA. M. VAN DEN (1988b) Phase transitions and molar volumes of C02-CH4-N2 inclusions. Bull. Mineral. 111, 257-266.
Superdense CO2 inclusions in migmatites KERKHOFA. M. VAN DEN (1990) Isochoric phase diagrams in the
systems C02-CH, and COr-N2: Application to fluid inclusions. Geochim. Cosmochim. Acta 54,621-629. KERKHOF A. M. VANDEN,TOURETJ. L. R., MAIJERC., and JANSEN J. B. H. (1990) Fluid inclusions in quartz veins from high-temperature granulites of Rogaland, SW Norway. Geochim. Cosmochim. Acta (submitted). OLSENS. N. (1987) The composition and role of the fluid in migmatites: a fluid inclusion study of the Front Range rocks. Contrib. Mineral. Petrol.%, 104- 120. OLSEN S. N. (1988) High-density CO1 inclusions in the Colorado Front Range. Contrib. Mineral. Petrol. 100, 226-235. PASTERIS J. D. and WANAMAKERB. J. (1988) Laser Raman microprobe analysis of experimentally reequilibrated fluid inclusions in olivine: some implications for mantle fluids. Amer. Mineral. 73, 1074-1088.
901
PBCHERA. and BOULLIERA. M. (I 984) Evolution a pression et temperature Clev6es d’inclusions fluides dans un quartz synthttique. Bull. Mineral. 107, 139- 143. ROEDDERE. (1984) Fluid Inclusions; Reviews in Mineralogy 12. Mineral. Sot. Amer. STERNERS. M. and BODNARR. J. ( 1989) Synthetic fluid inclusionsVII. Re-equilibration of fluid inclusions in quartz during labomtorysimulated metamorphic burial and uplift. J. Metamorph. Geol. 7, 243-260. SWANENBERG H. E. C. (I 980) Fluid inclusions in high-grade metamorphic rocks from S. W. Norway. Geologica Ultraiectina25. Univ. of Utrecht, 146~. TOURETJ. L. R. and KERKHOFA. M. VANDEN(1986) High density fluids in the lower crust and upper mantle. Physica 139, MOB; Proc. Xth AIRAPT Intl. High Press. Conf 834-840.
Cwchimica a Cosmochimica Ada Vol. 54, pp. 895-901 Copyright 0 1990 Pergamon Press plc.Printed in U.S.A.
+ .oO
LETTER
A natural example of superdense CO2 inclusions: Microthermometry and Raman analysis A.M. VANDENKERKHOF' andS.N. OLSEN* ‘Institut voor Aardwetenschappen, Vrije Universiteit, 108 1 HV Amsterdam, Netherlands *Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 2 12 18, USA (Received November 13, 1989; accepted in revised form February 14, 1990)
Abstract-The
presence of extremely dense CO2 fluid inclusions in migmatites from the Colorado Front Range has been confirmed by Raman analyses. These inclusions show metastable homogenization of liquid and vapor (L + V --t L) at temperatures below the triple point of pure CO*. Raman analyses failed to show the presence of N2, CH4, H$, or any other components found in the past in COz-rich inclusions with such low homogenization temperatures. We conclude that the high density resulted from a decrease in inclusion-cavity volume during isobaric cooling of less dense inclusions. INTRODUCTION
mainly of quartz, microcline, and plagioclase. Paleosomes (host rocks) are biotite-quartz-feldspar gneisses.
REPORTSOFCO~INCLUSIONS which homogenize below the triple point are found in several published results of microthermometric studies of fluid inclusions by a heating-freezing stage (e.g., COOLEN, 1980; SWANENBERG,1980). Such homogenization (L + V -) L) is a metastable phase transition in the case of pure CO2 and indicates an extremely high density for the fluid: the internal pressure of such an inclusion at room temperature is on the order of 2 kb. The apparent high densities have led to the conclusion that these fluids have been trapped at extreme depths, corresponding generally to pressures greater than 10 kb. Recent Raman microprobe analyses of some of these previously reported highdensity CO2 inclusions have shown that the inclusions do not contain pure CO2 but mixtures of CO*-N2 or CO&H4 with densities lower than that of CO2 at the triple point (1.178 g/cm’). This is the case for inclusions from the Furua Granulite Complex in Tanzania (KERKHOF, 1988b) and from the Rogaland area in southwestern Norway (KERKHOF et al., 1990). The phase behaviors of these fluid mixtures are very similar to those of high-density CO2 (TOURET and KERKHOF, 1986), and microthermometry alone does not provide definitive evidence for high-density CO2 fluid. To date, all inclusions with homogenization temperature (Th) below the triple point for pure CO*, when investigated by the combined techniques of microthermometry and Raman analysis, have been found to contain fluid mixtures with lower densities than originally reported. Herein, we present the first case of truly dense CO2 inclusions with metastable homogenization temperatures lower than the triple point.
METHODS AND RESULTS Microthermometry
The most commonly used technique for determination of densities in non-aqueous inclusions is a microthermometric study of the phase behavior at temperatures between - - 180 and +35”C. Most measurements in this study were made with a USGS-type heating-freezing stage modified by Fluid Inc. A Chaixmeca stage was also used in the range -100 to -40°C because it gave a slightly better optical resolution, especially for the phase transition S + L + L. The inclusions in this study are small (5- 15 pm in size), and phase transitions are difficult to observe. The heating rate around the temperature of CO2 melting was approximately 0.1 “C/s. The stages were calibrated at the COZ triple point (at -56.6”C) by using known pure CO2 inclusions of low density. Most of CO* inclusions in the Front Range samples are monophase at room temperature (OLSEN, 1987), but a vapor phase nucleates in all inclusions during cooling to about -100°C. Some supercooling is necessary for freezing any fluid. The inclusions in this study froze between about -85 and -lOO”C, or 30 to 40°C below the triple point of CO*; but they melt within a few degrees of the triple point upon subsequent heating (see below). This interval between the freezing and melting temperatures allows us to observe homogenization below the equilibrium triple point: homogenization temperatures can be measured after cooling without freezing the inclusion. Homogenization temperatures of approximately 1300 inclusions have been measured (OLSEN, 1987). Of these, four showed homogenization below the CO2 triple point at -56.6”C. Three were found in quartz in a leucosome, one in a paleosome. A homogenization temperature between the triple point and the temperature at which the inclusion freezes is denoted as metastable homogenization temperature (Thm)
STUDY SAMPLES CO2 inclusions in this study are contained in matrix quartz in migmatites from the Colorado Front Range, USA. The migmatites are Proterozoic in age (Rb/Sr whole-rock age = - 1700 Ma) and have been metamorphosed to amphibolite grade (see OLSEN, 1987, 1988, for more details). Leucosomes (leucocratic lenses and layers) in the migmatites consist 895
4. M. van den Kerkhofand
89h
because the assemblage liquid range. Thm’s for the inclusions to -61°C. In order to observe inclusions must first be frozen. clusions, the following features
+ vapor is metastable in this in this study range from -66 stable phase transitions, the Upon heating the frozen inare observed (Fig. I ):
S. N. Olsen
with liquid), but some ofthe solid remains. This transition is denoted as partial homogenization to liquid (Ths L). The solid phase can easily go undetected at this stage because of the very small difference between the refractive indices of the liquid and solid. (4) The final phase transition takes place by dissolution of the solid into liquid. This homogenization has been denoted as “sublimation” to the liquid phase (Ts L), rather than melting, by KERKHOF (1988a: 1990) because in a general case the transition can be either S + V + V or S + L + L. The two transitions for C02-CH4-N2 inclusions cannot in general be distinguished by a microscopic observation because of the very small difference in refractive index between vapor and liquid. The only exception is the case of pure CO2 inclusions for which the distinction is possible. For the inclusions in this study,
(1) A frozen inclusion contains solid CO* and a vapor phase. The filling degree for the solid is approximately 0.8; the solid occupies about 80% of the inclusion volume. (2) A liquid phase forms at about -56.6”C resulting in three phases: solid, liquid, and vapor. This transition is denoted as initial melting by formation of liquid (Ti L). (3) Both solid and vapor phases rapidly shrink during a very narrow temperature interval after the first appearance of melt. At a few tenths of a degree above the initial melting, the vapor bubble dissolves into the liquid (homogenizing
lo-3T-7-125-l -7
r~ stable ~~~-~~
meta stable
1
4
ii
-60
I
-65
0
L
’
-6LB
r
‘t FIG. I. Stable and metastable phase transitions in “superdense” CO2 inclusions observed on warming from 5 -85 to -40°C. Inclusions show a stable phase sequence, S + V +S+L+V-+S+L+L,whenwarmedafiercooling to - -100°C (temperatures at which they freeze), but met&stable homogenization L + V -) L when warmed after cooling to - -85°C without being frozen. The metastable homogenization is at a lower temperature than that of melting (see text).
Superdense CO2inclusions in migmatites Table
1.
Microthermometric
results
Sample
Thm%
TiL°C
ThsL”C
TsL”C
1 Q-3T-7-a0 1 O-3T-7-a6-2 1 O-3T-7-a6-1 IO-3T-1 O-b7
-66.2 -60.7 -64.6 -61.8
-66.6
-66.2 -66.4 -66.7 -66.6
-62.1 7’ -62.6 -54.1
‘Ultflcult
to
observe
Thm=metarlable homogenization: L+V + L TIL ~Inltial meltlng: V+S l V+S+L ThrL=parHaI homopenlzatlon to liquid: V+L+S TsL ~subllmation(/melting): S+L e L or S+V
l l
L+S V
all final transitions are S + L --) L so that the transition is referred to as melting/sublimation, or simply as melting, below. The melting/sublimation in these inclusions was observed at -52 to - -54’C (Table 1). These temperatures are significantly higher than the triple point of COZ. Thus, the following phase transitions were observed during the stable path of a microthermometric analysis from - - 100 to -4O’C: S + V + S + L + V + S + L + L. These inclusions are Type S3 in the classification proposed by KERKHOF (1988a; 1990). “S” stands for the final phase transition, “sublimation.” “ 3” stands for the total number of phase transitions. P-T phase diagrams The observed phase transitions in these inclusions, which we believe contain extremely dense CO2 fluid, can be explained by P-T phase diagrams (e.g., SWANENBERG, 1980; ROEDDER, 1984, p. 376-377). Figure 2 diagrams CO* in the region of the triple point constructed using the data of ANGUS et al. (1976) and shows stable L + V, S + V, S + L, and metastable L + V curves together with some isochores. P-T
897
paths during cooling and subsequent heating are illustrated for an inclusion with the molar volume of 36.5 cm3/mol (density = 1.2 1 gjcm3), which is representative of the densities for the inclusions in this study. The inclusion in our example is in the liquid field at room temperature. Upon cooling, the inclusion follows a stable isochore from the liquid field into the stability field of solid (through point S) along the metastable extension of the liquidfield isochore because an inclusion must be supercooled before it freezes. A solid generally needs a greater amount of supercooling to nucleate than does a bubble. In these inclusions, solid nucleates at 30 to 40°C supercooling, but a bubble at only 10 to 15°C. A bubble therefore is always the first to nucleate even though homogenization temperatures are lower than the triple point. Upon nucleation of a bubble at Bl, the P-T conditions for the fluid in the inclusion will change to B2 on the metastable extension of the L + V curve to reflect the new two-phase equilibrium. Note that a part of the metastable extension of the isochore in the region of bubble nucleation is at negative pressures down to approximately - 100 bars. Liquid is at a negative pressure when a failure to nucleate a bubble stretches it metastably to occupy a volume larger than that at equilibrium (ROEDDER, 1984, p. 298). After nucleation of a bubble, two different paths for further phase transition are possible depending on whether the inclusion is heated or cooled. If heating is started at B2, metastable homogenization will occur at H. Upon further heating, the inclusion follows the metastable extension of the liquidfield isochore through the solid field and then the stable isochore through the liquid field. On the other hand, if cooling is continued at B2, the inclusion will follow the metastable L + V curve until the solid
P bar
600
I
I
-90
-60
I
-70
I
-60
I
-50
I
-40
I
TY
FIG. 2. Phase relationship in the CO2 system in the region around the triple point (T) showing, as thick lines, the sublimation, boiling, and melting curves (S + V, L + V, S + L) and selected isochores. Stable isochores are shown as thinner solid lines and metastable extensions as dashed curves. Numbers on isochores are molar volumes in cm3/mol. The sublimation curve and the metastable extension of the L -t V curves are shown in more detail in the inset. The phase transition illustrated with letters and dotted lines is for an inclusion with a molar volume of 36.5 cm3/mol (see text for further discussion).
898
A. M. van den Kerkhof and S. N. Olsen
CO2 nucleates at F2 (as determined experimentally), resulting in an instantaneous solidification of the liquid phase with a sudden but slight drop in internal pressure (to Fl ). Heating from this point results only in stable phases. The solid and vapor phases remain stable up to the triple point (T) where a liquid phase forms (initial melting, Ti L). The presence of three phases constrains P and T at the triple point until one of the phases completely disappears, in this case the vapor phase (partial homogenization to the liquid phase, Ths L). Upon further heating, dissolution of the solid phase proceeds along the S + L curve until it is completed at S. In our example, the molar volume of the liquid phase between T and S decreases from 37.347 to 36.5 cm3/mol, which corresponds to I. 178 and 1.2 1 g/cm3, respectively, in density. Therefore, observing the solid phase becomes increasingly difficult during “dissolution” as the density of liquid approaches that of solid (1.56 g/cm3). Melting temperature of pure CO2 in the absence of a vapor phase must be higher than the CO* triple point, as observed. Such phase behaviors for high-density COZ inclusions have been discussed by SWANENBERC; (1980), but are not substantiated because his inclusions turned out not to contain pure COZ (KERKHOF, 1988a: KERKHOF et al., 1990). Comparison wilh phase behavior in CO*-N2 inclusions The phase behavior described above for high-density COZ is similar to that of some inclusions reported from the Furua Granulite Complex, Tanzania (COOLEN, 1980: KERKHOF, 1988b). Raman analyses, however, show that the Furua inclusions (which are Type S3, as are the inclusions in this study) contain CO*-N2 fluids with 25 to 30 mol% Nz and densities of approximately 0.95 g/cm3. The density corresponds to a molar volume of 40 cm3/mol (compare Fig. 2). These Type-S3, C02-Nz inclusions have Ti L = -65”C, Ths L = -63”C, Ts L = -60 to -61 “C for stable phase transitions, and Thm L = -63°C for metastable ones. Their homogenization temperatures are close to or slightly below those of melting. Although the phase behaviors of high-density COZ and lower-density CO*-N2 inclusions are similar, they are not identical. A careful observation therefore can help distinguish the two types. The most important differences can be summarized as follows:
(COOLEN,1980; KERKHOF,1988b). These inclusions, denoted as Type Sl, can also be mistaken for high-density CO2 inclusions. They contain CO>-N2 mixtures with higher N2 contents (-30 to 40 mol%) than S3 inclusions. Final melting temperatures in the absence of vapor (Ts L) range from --6 1 to -59°C: those of metastable homogenization (Thm) range from -74 to -64”C, with the lowest temperatures corresponding to critical homogenization. The other types of C02Nz and C02-CH4 inclusions showing melting or sublimation (S2 and S4) contain three phases at low temperatures (- - 180°C) and can be easily distinguished from high-density CO2 inclusions. Raman ana&s The purity of the CO* inclusions in this study was checked by Raman analysis using a Microdil-28 Raman microspectrometer (BURKEand LUSTENHOUWR, 1988) equipped with a multichannel detector. The inclusions were checked for the presence of Nz, CH4, and H2S, but no component other than CO2 could be detected. The Raman spectrum of COZ consists of two peaks, referred to as the Fermi diad, with a higherintensity peak (2~) at 1388 and a lower-intensity one (Y,) at 1285 cm-’ at 1 atm (Fig. 3). Small peaks located at the sides of the main peaks are hot bands, which represent interaction frequencies (v, + u2 and ul + 3~~)due to the thermal energy of the molecules (HOWARD-LOCKand STOICHEW, 197 1). Internal pressures of CO1 inclusions with a density of 1.21 g/ cm3 should be on the order of 2 kb at 25°C. The Raman spectrum of a high-density CO2 inclusion is compared to those of lower density inclusions in Fig. 3. The following changes in the Raman spectrum reflect higher pressures in
the inclusions (GARRABOSet al. 1980; BERTRAN, 1983): (I )
I t
(1) Initial melting in CO*-Nz inclusions (=Ti L) occurs at
(2)
lower temperatures (_ -65°C for the Furua inclusions) than for pure CO2 (-56.6”C). Final melting of high-density COz in the absence of vapor (=Ts L) takes place at higher temperatures than the triple
point of COZ (-56.6’C). (3) Filling degrees of solid, or the volume solid occupies in frozen inclusions, are lower in C02-N2 inclusions than in COZ inclusions. (4) Type-S3 inclusions are rare in C02-N2 inclusions. More common types are H3, Sl, S2, and S4 (see KERKHOF, 1988b). Inclusions which show only one transition (S + L + L: Ts L) in the temperature range -180 to +35”C were also found in the samples from the Furua Granulite Complex
FIG. 3. Raman spectra of C@ with four different densities (from the lowermost to the uucertnost s.pecUumd = 1.2, 0.8, 0.7, and ~0. I, respectively).The-lowermost spectrum (d = - 1.2g/cm3)was obtained from one of the Type-S3 inclusions of Table 1 (lO-3T-7a5-a). The Raman spectra for other “superdense” inclusions in Table I a& very similar to the one shown. The other three spectra in Fig. 3 are for “normal” H3 inclusions. Spectra of hig&er-densityfluids are characterizedby a shift to lower wavenumbers, an increased ratio in intensity of 2v2to v2peak, and suppression of the two hot bands (see text for further discussion).
Superdense CO2inclusions in migmatites a shift in the positions of both peaks to lower frequencies (for pressures lower than 6 kbar); (2) decreases in peak widths; (3) an increase in the frequency separation of the two peaks (A = 2~ - v,); and (4) an increase in peak intensity ratio (R = 12,/I,,). In addition, we also observed decreases in intensities of the hot bands (this study). The peak positions of monophase CO2 inclusions measured at 20°C are different for the gas (d < 0.19 g/cm3) and the liquid (d > 0.77 gJcm3): the 21~2peak is located at 1388 to 1387 cm-’ for the gas and 1385 to 1384 cm-’ for the liquid; the V, peak is located at 1285 to 1284 cm-’ for the gas and 128 1 to 1279 cm-’ for the liquid. There is a 2 cm-’ difference between the peak positions for the vapor and liquid in twophase inclusions with total densities between 0.19 and 0.77 g/cm3 (BURKE and KERKHOF,unpubl. data). Peak shifts within the gas and liquid regions are small compared to the accuracy in measurement of Raman lines in the range of the Fermi diadtihe accuracy in peak positions is +0.8 cm-’ for the instrument used. The instrument has been calibrated with a neon lamp and regularly checked by measuring the Raman peak of diamond at 1332 cm-‘, located between the two CO1 peaks. The position of the V, peak is shifted more than the 2vZ peak with increasing density so that the frequency separation (A = 2v2 - v,) also increases with density. The parameter has been used as an indicator of densities in fluid inclusions (PASTERISand WANAMAKER,1988). Low-density gas inclusions have A - 103 cm-‘, while the high-density inclusions (d - 1.2) of this study have A - 106.5 cm-‘-which is in acceptable agreement with the calculated value of - 105.9 cm-’ (after BERTRAN, 1983). According to DUBESSY(1989X the sum of the relative Raman cross-sections for the two peaks remains constant. The relative peak intensities (R = 12,&J of the Fermi diad, on the other hand, gradually change from about 1.6 for low densities to 1.95 for the density of the triple point (GARRABOS et al., 1980). The values of R for the present inclusions with even higher densities than that of the triple point ( 1.178 cm3/ mol) are 1.95 and 2.0, consistent with those of GARRABOS et al. (1980) within the error. The accuracy of these values is no better than +O.l with the instrument used. The suppression of the low-intensity hot bands occurs in highdensity fluids at constant temperature due to more restricted vibrational freedom of the molecules. We conclude that our observations from natural inclusions are in accordance with the experimental data of GARRABOSet al. (1980) and BERTRAN ( 1983). DISCUSSION The extremely dense CO2 inclusions of this study belong to the group of CO2 inclusions considered to be the earliest inclusions in the Front Range migmatites. These early inclusions have negative crystal shapes and occur as either isolated inclusions, in small clusters (two to 10 inclusions), or in small intragranular healed fractures (up to about 100 inclusions), but never in a healed fracture that crosscuts a grain boundary. They occur more commonly in leucosomes (leucocratic layers and lenses) than in paleosomes (host rocks), and their representative isochores are consistent with the P-T conditions of migmatization estimated from the mineralogic data. These
899
observations led OLSEN(1987) to conclude that these early CO* inclusions had formed during the migmatization episode. The isochores for the extremely dense inclusions in this study, however, are not consistent with the P-T conditions of migmatization (Fig. 4). Nevertheless, these “superdense” inclusions do not belong to a special group of inclusions but to a general population of synmigmatization inclusions, because: (1) all of the inclusions have a similar mode of occurrence; (2) there is a continuum in homogenization temperature of the early CO* inclusions, including “superdense” inclusions (from +30 to -64’C; OLSEN, 1987, Fig. 4); (3) each of the superdense inclusions occurs in a cluster together with less dense inclusions; and (4) this study has shown that the superdense inclusions contain pure COZ, as less dense inclusions do. The origin of the high-density CO2 inclusions in the Front Range rocks has been discussed by OLSEN (1988). The following is a summary of her conclusions:
(1) The high-density inclusions are not relics of an earlier high-pressure metamorphism which have escaped decrepitation, because there is no correlation between the size and density of inclusions (OLSEN, 1988, Fig. 5) and because high-density inclusions are rare in the zones of migmatites with the least signs of deformation and melting (paleosomes and quartz inclusions within feldspar grains). (2) The superdense and other high-density inclusions (with Th < - -30”) must have been modified after their formation, because inclusions with extremely high-density CO2 tend to be associated with micro-shear zones in these rocks and because high-density inclusions are associated with plagioclase which has been heavily altered by hydrothermal activity during a late event in leucosome formation. (3) A change in composition of the fluid-for example, by a loss of water-is not likely to have occurred (see OLSEN, 1988, for further discussion). Olsen also postulated that the inclusions were modified to smaller volumes (= higher densities) during nearly isobaric cooling. If the host rock for an inclusion is cooled isobarically, the cooling path must intersect COZ isochores with increasingly higher densities (Fig. 4). If the volume of an inclusion can be equilibrated to the new P-T conditions (by volume decrease), a higher density for the inclusion would result. Such reequilibrations in density have been demonstrated experimentally for synthetic aqueous inclusions by F&HER and BOULLIER(1984) and STERNERand BODNAR(1989). The observed wide variation in Th or density variation (see OLSEN, 1988, especially Figs. 3 and 6) indicated selective increases in density. Olsen suggested hydrolytic weakening of quartz (e.g., BLACK and CHRISTIE, 1984) as the factor for the selectiveness because of the association of high-density inclusions with hydrothermally altered plagioclase. The presence of water in quartz immediately adjacent to an inclusion could have caused localized hydrolytic weakening and facilitated the process of a volume decrease during deformation. There is no apparent correlation between the presence of aqueous inclusions and high-density CO* inclusions in these rocks, but this is not inconsistent with the model because
900
A. M. van den Kerkhof and S. N. Olsen
P kb
6
0
500
FIG. 4. CO2 isochores and P-T conditions of migmatization or the formation of migmatites in the Front Range. Numbers on the &chore are molar volumes in cm’/mol and corresponding homogenization temperatures (“C) are given in parentheses. “Superdense” COr inclusions with (met&able) homogenization at temperatures lower than those of melting indicate trapping or equilibration at the P-T conditions in the upper left part of the diagram. The P-T conditions of migmatization in the Colorado Front Range are indicated by the hatched rectangle, which straddles the water-saturated granite melting curve. All melting curves are from BROWN(1970). An isobaric cooling path is indicated by the arrow. The lowest reasonable temperature for quartz crack healing (Tmin), and therefore the fluid inclusions formation, is inferred as 200°C. Either the “superdense” inclusions of this study were trapped at about 250°C and 5 kb (large dot), or the present density reflects final equilibration of the inclusions at these conditions (see text).
most aqueous inclusions seem to be younger than the early CO2 inclusions (OLSEN, 1987). This study has shown conclusively that CO2 fluids with extremely high densities do occur in high-grade metamorphic rocks, but the density, in this case, is a result of modification of the inclusion after initial formation. Such an inclusion may have trapped a peak-metamorphic fluid, but the present density of fluid does not reflect the P-T conditions of the peak metamorphism.
dynamic Tables oJ‘the Fluid State, Vol. 3: Carbon Dioxide. Per-
gamon Press, Oxford. BERTRANJ. F. (1983) Study of Fermi doublet vr-2~~ in the Raman spectra of CO2 in different phases. Spectrochim Acta 39A, 119121.
Acknowledgments-We
would like to thank Dr. Ernst Burke for assistance with Raman analysis. The encouragement of Dr. Jacques Touret initiated this study. Facilities for the laser Raman microprobe analysis were provided by the Free University in Amsterdam and by the Netherlands Organization for Scientific Research. Thoughtful reviews and helpful comments by Drs. R. J. Bodnar, P. C. Hess, J. D. Pasteris, and E. Roedder led to a greatly improved final draft. The support of NSF Grant EAR-8502403 to SNO is also gratefully acknowledged.
BLACICJ. D. and CHRISTIEJ. M. (1984) Plasticity and hydrolytic weakening of quartz single crystals. .I. Geophys. Res. 89, 42234239. BROWNG. C. (I 970) A comment on the role of water in the partial fusion of crustal rocks. Earth Planet. Sci. Lett. 9, 355-358. BURKEE. A. J. and LUSTENHOUWER W. J. (1988) The application of a multichannel laser Raman microprobe (Microdil-28) to the analysis of fluid inclusions. Chem. Geology 61, 11-l 7. CWLEN J. J. M. M. M. (1980) Chemical petrology of the Furua Granulite Complex, southern Tanzania. GUA Pap. Geol. Ser. 1, 13. DUB&SYJ. (1989) Advances in C-O-H-N-S fluid geochemistry based on micro-Raman spectrometric analysis of fluid inclusions. Enropean J. Mineral. 1, 517-534. GARRABOSY ., TUFEU R., and NEINDREB. LE ( 1980) Rayleigh and Raman scattering near the critical point of carbon dioxide. J.Chem.
Editorial handling: G. Faure
HOWARDLOCKH. E. and STOICHER:B. F. (1971) Raman intensity measurements of the Fermi diad Y, , 2~2, in 12C02 and 13COz.J.
Phys. 72, 4637-465
1.
Mol. Spect. 37, 32 l-326.
REFERENCES ANGUS
S., ARMSTRONGB., DEREUKV. V., GADETSKII0. G., CHAPELAG. A., and ROWLINSONJ. S. (1976) International Thermo-
KERKHOFA. M. VAN DEN (1988a) The system C02-CHcN2 in fluid inclusions: theoretical modelling and geological applications. Ph. D. dissertation, Free University, Amsterdam. KERKHOFA. M. VAN DEN (1988b) Phase transitions and molar volumes of C02-CH4-N2 inclusions. Bull. Mineral. 111, 257-266.
Superdense CO2 inclusions in migmatites KERKHOFA. M. VAN DEN (1990) Isochoric phase diagrams in the
systems C02-CH, and COr-N2: Application to fluid inclusions. Geochim. Cosmochim. Acta 54,621-629. KERKHOF A. M. VANDEN,TOURETJ. L. R., MAIJERC., and JANSEN J. B. H. (1990) Fluid inclusions in quartz veins from high-temperature granulites of Rogaland, SW Norway. Geochim. Cosmochim. Acta (submitted). OLSENS. N. (1987) The composition and role of the fluid in migmatites: a fluid inclusion study of the Front Range rocks. Contrib. Mineral. Petrol.%, 104- 120. OLSEN S. N. (1988) High-density CO1 inclusions in the Colorado Front Range. Contrib. Mineral. Petrol. 100, 226-235. PASTERIS J. D. and WANAMAKERB. J. (1988) Laser Raman microprobe analysis of experimentally reequilibrated fluid inclusions in olivine: some implications for mantle fluids. Amer. Mineral. 73, 1074-1088.
901
PBCHERA. and BOULLIERA. M. (I 984) Evolution a pression et temperature Clev6es d’inclusions fluides dans un quartz synthttique. Bull. Mineral. 107, 139- 143. ROEDDERE. (1984) Fluid Inclusions; Reviews in Mineralogy 12. Mineral. Sot. Amer. STERNERS. M. and BODNARR. J. ( 1989) Synthetic fluid inclusionsVII. Re-equilibration of fluid inclusions in quartz during labomtorysimulated metamorphic burial and uplift. J. Metamorph. Geol. 7, 243-260. SWANENBERG H. E. C. (I 980) Fluid inclusions in high-grade metamorphic rocks from S. W. Norway. Geologica Ultraiectina25. Univ. of Utrecht, 146~. TOURETJ. L. R. and KERKHOFA. M. VANDEN(1986) High density fluids in the lower crust and upper mantle. Physica 139, MOB; Proc. Xth AIRAPT Intl. High Press. Conf 834-840.