Reequilibration of fluid inclusions in low-temperature ...

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Reequilibration of fluid inclusions in low-temperature calcium-carbonate cement Robert H. Goldstein* D e p a r t m e n t of G e o l o g y a n d G e o p h y s i c s , University of W i s c o n s i n , 1215 West D a y t o n Street, Madison, W i s c o n s i n 5 3 7 0 6

ABSTRACT Calcium-carbonate cements precipitated in low-temperature, near-surface, vadose environments contain fluid inclusions of variable vapor-to-liquid ratios that yield variable homogenization temperatures. Cements precipitated in low-temperature, phreatic environments contain one-phase, all-liquid fluid inclusions. Neomorphism of unstable calcium-carbonate phases may cause reequilibration of fluid inclusions. Stable calciumcarbonate cements of low-temperature origin, which have been deeply buried, contain fluid inclusions of variable homogenization temperature and variable salt composition. Most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids. Therefore, low-temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial. Reequilibration is likely causc d by high internal pressures in inclusions which result in hydrofracturing. The resulting fluidinclusion population could contain a nearly complete record of burial fluids in which a particular rock has been bathed. INTRODUCTION In recent years, workers have used data on primary fluid inclusions to interpret conditions of calcium-carbonate cementation in the Jurassic Smackover and the Mississippian Burlington Formations (Moore and Druckman, 1981; Klosterman, 1981; Smith et al., 1984). Their fluid-inclusion data, together with other analytical results, suggested that cements grew from (or recrystallized within) hot, deep-burial brines. Other methods of study, such as stable isotope geochemistry and cement stratigraphy, have led to a low-temperature (near-surface), freshwater interpretation for some of these cements (Wagner and Matthews, 1982; Harris, 1982) and resulted in Moore and Druckman's (1981) idea that fluid inclusions may have reequilibrated during recrystallization. It is well known that much aragonite and calcite cement precipitates at low-temperature, near-surface conditions from fresh or marine waters (see Longman, 1980, for a summary). Why, then, do Smackover and Burlington fluid inclusions record high-temperature, briny conditions of precipitation? Do fluid inclusions in most deeply buried, low-temperature, near-surface cements record conditions of cement precipitation, or do they record other conditions? This study presents new data that could answer these questions. In this research, I compare my observations of primary, aqueous fluid inclusions1 in nonburied calcium-carbonate

•Present address: Department of Geology, University of Kansas, Lawrence, Kansas 66045. 'Primary fluid inclusions are petrographically recognized as having been trapped during crystal growth. 792

cements to primary, aqueous fluid inclusions in deeply buried calcium-carbonate cements, all of which have precipitated from low-tempe rature, near-surface waters. My results suggest that some fluid inclusions may be useful for interpreting conditions of cementation. However, for many fluid-inclusion populations, caution should be applied when using fluid-inclusion measurements as records of cement precipitation conditions, because many low-temp mature fluid inclusions may have reequilibrated during burial or neomorphism. The results also suggest promise in using such reequilibrated fluid inclusions as records of burial fluids and neomorphic fluids. All samples were prepared with use of cold techniques and were measured by sandard, calibrated, microthermometric methods, to prevent overheating or freezing reequilibration of inclusions.2 PRIMARY FLUID INCLUSIONS IN LOW-TEMPERATURE CALCIUMCARBONATE CEMENTS Nonburied Cements Calcium-carbonate cement occurs in many late Cenozoic carbonate rocks that have never been deeply buried. These low-temperature cements commonly have analogs in anci;nt, deeply buried sequences. The samples described below, though few, are thought to represent many low-temperature, near-surface, vadose and phreatic environments of calcium-carbonate cementation. 2 Geological Society of America Supplementary Material 86-23, describing experimental techniques, samples, and the hydrofracturing model, is available on request from Documents Secretary, Geological Society of America, P.O. Box 9140, Boulder, CO 80301.

Vadose-zone calcite cement samples studied from New Mexico, Nevada, and the Bahamas are of freshwater origin and contain primary fluid inclusions. Each inclusion may contain fresh water, air, or both in varying proportions. When the Nevada sample was heated, the inclusions with lower vapor-to-liquid ratios homogenized (all homogenization temperatures recorded are to liquid) (Fig. 1). The temperature of homogenization (Th) is controlled by the proportion of air to liquid originally trapped during crystal growth and has no relationship to the original trapping temperature of the inclusion (Th records minimum trapping temperatures only if the inclusions were trapped as one phase and the cavity volume and fluid composition remain unaltered). Final melting point iTm ice3) measurements from inclusions in the Nevada sample confirm the freshwater content of the inclusions. Shallow, phreatic-zone calcite cements from the Bahamas and southern Florida are of freshwater origin and rarely trap small fluid inclusions during growth. The predominant inclusion type (excluding all-gas inclusions of uncertain leaked, unsealed, or primary origin) contains one phase that is all liquid. A sample of aragonite submarine cement from Belize contains abundant fluid inclusions. Most inclusions are one-phase, all-liquid, and probably contain seawater. REEQUILIBRATION DURING NEOMORPHISM The neomorphism of aragonite and high-Mg calcite cements and subsequent reequilibration of their contained primary fluid-inclusion populations may be important. Bathurst (1975) has shown that aragonite consistently neomorphoses to low-Mg calcite through a "wet" process. Recrystallization of high-Mg calcite to low-Mg calcite may involve a similar "wet" process. This process may open the primary fluid-inclusion cavities to the fluids involved in neomorphism and thus reequilibrate fluid inclusions to record the temperature, pressure, and possibly the fluid composition of neomorphism, or some mixture of the original inclusion fluids and the surrounding pore fluids. Study of these

3 If 7m ice is 0 °C, water is fresh. Lower 7m ice indicates that salts are in solution. The magnitude of this depression is controlled by the concentration and types of species in solution.

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fluid inclusions could shed some needed light on the mechanism of neomorphism and the "openness" of the neomorphism process. It is unlikely that fluid inclusions in low-Mg calcite cements would reequilibrate through neomorphism of their mineral hosts. Bathurst (1980) summarized evidence supporting the likely stability of low-Mg calcite and the unlikelihood of it recrystallizing under diagenetic conditions. However, other mechanisms may cause reequilibration of these fluid inclusions. REEQUILIBRATION DURING DEEP BURIAL In the literature described in the Introduction, interpretations partially or totally based on fluid inclusions in deeply buried calcite cements do not necessarily agree with interpretations formed without fluid-inclusion data. My data, presented here, from primary fluid inclusions in ancient, deeply buried, low-temperature, freshwater calcite cements further elucidate this discrepancy. Fibrous-Bladed Calcite from MississippianPennsylvanian Karstic Surface In the Sacramento Mountains of New Mexico, a karsted unconformity occurs between Mississippian and Pennsylvanian strata (Pray, 1959). At one unconformity locality, shale and masses of in-place, sparry, flbrous-bladed calcite fill solution pockets in the Mississippian limestones of the Lake Valley Formation. This spar is closely related to the low-temperature, freshwater cements thoroughly studied by Meyers et al. (1982). Field, petrographic, and geochemical evidence (see footnote 2) suggests that the fibrous-bladed spar was precipitated as a low-Mg calcite, vadose and phreatic speleothem, or surface deposit during the subaerial exposure event. No recrystallization is indicated. Precipitating waters were probably fresh and low in temperature. After precipitation, the spar was buried by about 1 km of Paleozoic strata and possibly by several kilometres of Mesozoic and later rocks. Therefore, with burial, the spar could have been heated to temperatures in excess of 125 °C. Without reequilibration, the spar should contain (analogous to the

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Figure 1. Frequency histogram: Th of primary, two-phase fluid inclusions from near-surface, low-temperature calcite flowstone, Nevada (probably vadose), never deeply buried. Note extreme variability of measurements.

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nonburied vadose samples) fluid inclusions that have varying proportions of fresh water and air, yielding variable 7 h values. Most of the primary fluid inclusions of the unconformity-related spar are two-phase, liquid-gas inclusions. These yield Th values that range from 43.0 to 149.7 °C (Fig. 2; note that all-liquid fluid inclusions are also present below 43 °C). They apparently have stable 7m ice (vapor present; Roedder, 1967) between +1.4 and -21.1 °C, which indicates variable salinities. Although extreme care was taken in calibration, and heating runs were "cycled" slowly, the values above 0 °C probably represent minor overshooting of the 7m ice for fresh water. Observed first melting temperatures (eutectics [7e]; indicative of particular salt species in solution) ranged between -52 and about 0 °C and are indicative of the presence of both freshwater and briny aqueous fluids. The various 7e values attained indicate that major salt species in various inclusions are different. The variable 7Ti values appear analogous to the nonburied vadose examples. However, the measurements indicative of high salinity and variable dissolved species composition are not analogous to the nonburied samples and indicate that the fluid inclusions have reequilibrated with higher salinity fluids. Freshwater Phreatic Cements in the Mississippian Lake Valley Formation The Mississippian Lake Valley Formation of the Sacramento Mountains, New Mexico, consists of limestones cemented with sparry calcite. In a pioneering study, Meyers (see Meyers et al., 1982, for summary) used field relationships, cement stratigraphy, stable isotopes, and trace elements to show that the first three calcite cement zones (postsubmarine cement of Pray, 1965) precipitated from fresh, low-temperature, near-surface, dominantly phreatic groundwaters before Pennsylvanian sedimentation. The geochemical and petrographic data support a lowMg calcite mineralogy, and no recrystallization is indicated. After precipitation, these cements

Figure 2. Fluid-inclusion measurements from deeply buried, lowtemperature, nearsurface, freshwater, vadose-phreatic calcite, Mississippian-Pennsylvanian paleokarstic surface, Lake Valley Formation, New Mexico. A: Frequency histogram of Tm ice. Measurements between - 1 and +2 °C are essentially freshwater. Measurements between - 2 2 Frequency histogram of 7"h.

were buried to almost 2 km by Paleozoic strata and possibly by several more kilometres of Mesozoic and later strata. Therefore, with burial, the cement could have been heated to temperatures in excess of 150 °C. Without reequilibration, one might expect these cements to contain a primary population of one-phase, all-liquid fluid inclusions, similar to the nonburied, late Cenozoic, freshwater, phreatic zone samples. Most Lake Valley cements are devoid of primary fluid inclusions that are suitable for study. However, several samples are suitable and contain sparse populations of primary, predominantly two-phase fluid inclusions. One sample contains primary fluid inclusions within calcite cements and cemented crinoid fragments that have the cathodoluminescent properties of the low-temperature, shallow-burial, freshwater phreatic cements described by Meyers. According to Meyers and James (1978), the isotopic composition of a cemented crinoid fragment is the same as its surrounding syntaxial cement. Hence, fluid inclusions in the crinoid fragments should be the same as those in the enclosing cement. This is confirmed by similar Th and 7m ice data. Overall, Th values are highly variable and range between 85.4 and 177.7 °C (Fig. 3). 7e phenomena were difficult to observe in these small fluid inclusions. The few measurements taken ranged between -22.5 and -53 °C and indicate that different inclusions contain different dissolved species. 7m ice (with vapor present) ranged from -16.5 to -0.1 °C, which would indicate various concentrations of brines and one freshwater fluid inclusion. These fluid inclusions are not similar to the fluid inclusions in the nonburied analogous cements. Reequilibration of low-temperature, low-salinity fluid inclusions that have higher temperature, briny burial fluids could explain the results. DISCUSSION Low-temperature, nonburied cements of this study contain primary fluid inclusions. Analo-

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gous low-temperature cements, subjected to deep-burial, have similar primary fluid-inclusion cavities, yet many of these cavities seem to contain fluids that record anomalously high temperatures and salinities, which would indicate that the low-temperature fluid inclusions have reequilibrated during burial. I think this process could be the norm for low-temperature fluid inclusions in calcite subjected to deep burial. Such a process could help explain the problematic fluid-inclusion results obtained by other workers. Mechanism of Reequilibration Several mechanisms could account for the reequilibration of primary fluid inclusions in sedimentary rocks (summary in Roedder, 1984). The likely mechanisms of reequilibration of the fluid inclusions of this study are discussed below. Bodnar and Bethke (1984) documented stretching of fluid-inclusion cavities caused by high positive pressures within inclusions, resulting from heating of the fluid inclusions above their Th. The fluid-inclusion cavities increase in size predominantly through permanent plastic deformation of the matrix. No leakage is inferred. In my data, this mechanism might explain the high homogenization temperatures of the freshwater fluid inclusions found in the deeply buried cements. However, it cannot explain the salinity increase documented from fluid inclusions in low-temperature cements that have been subjected to burial. Another possible mechanism of reequilibration might be the intersection of planes of crystal imperfections (such as twin lamellae and fractures) with the primary fluid inclusions and, hence, the opening of the inclusions to exchange with external pore fluids. Although the apparent density of these imperfections is lo w

-22

in the samples studied, petrographioally undetectable planes of crystal imperfections could have caused the reequilibration. Hydrofracturing is the probable mechanism of reequilibration for most fluid inclusions of this study. Burruss and Hollister's (1979) geothermal test well data suggested that hydrofracturing, which was caused by higii internal pressures within fluid-inclusion cavities, opened the cavities and caused their reequilibration. During progressive deep burial of all-liquid, low-temperature fluid inclusions through a geothermal-geopressure gradient, high piessures develop within fluid inclusions, which could cause hydrofracturing of calcite cement (see footnote 2). If the fracture does not extend to some area open to the pore fluid, inclusionfluid composition would remain unchanged, inclusion-fluid density would be less, and thus the inclusion would yield a higher Th. If the fracture extends to some area open to the pore fluid, the density and composition of the primary fluid should equilibrate to some degree with that of the pore fluid through leakage and exchange between the two fluids. If this fracture were to reheal, the primary fluid inclusion might remain, but it would have characteristics of the fluid present during healing and would no longer retain the temperature, pressure, and composition record of crystal growth conditions. Fractures could heal completely and leave no visible trace of ever having been open. High-magnification cathodoluminesceno; and transmission electron microscope studies might be useful in identifying healed fractures. The temperature and pressure at which a particular fluid inclusion in calcite hydrot'ractures depend upon the following variable, (quantitative plot; see footnote 2): (1) the relative importance of plastic or brittle deformation of the matrix, (2) shape of the inclusion, (3)

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Figure 3. Primary fluid-inclusion measurements from deeply buried, low-temperature, nearsurface, freshwater, phreatic calcite cement, Mississippian Lake Valley Formation, New Mexico. A: Frequency histogram of 7m ice. Measurements span wide range from near 0 °C (fresh water) to below - 1 6 °C (concentrated brine). B: Frequency histogram of Th measurements. Note extreme range of measurements.

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strength of the matrix, (4) size of the inclusion, (5) composition of the inclusion, (6) orientation and position of the inclusion with respect to matrix anisotropics, and (7) geothermal-geopressure gradient. Because each inclusion may have a different shape, size, or position within a crystal, each inclusion could behave differently during progressive burial. Some fluid inclusions may never hydrofracture (these may stretch their walls), and others may hydrofracture and reequilibrate one or many times. Hydrofracturing may or may not open a fluid inclusion to the ambient pore fluids. Those not open could contain the original fluid (freshwater inclusions in my samples). From the highly variable inclusion data obtained in the deeply buried samples, it appears that individual fluid inclusions within a population behaved differently and leaked and resealed at different temperatures while bathed in many different fluid compositions. The various inclusions may provide samples of all or many of the ambient pore fluids present during burial. Whereas some of the inclusion data may be valuable for interpreting low-temperature cementation conditions, these variable data may be most valuable for interpreting burial pore fluid history. Identification of Reequilibrated FluidInclusion Populations How might one discern a reequilibrated, originally low-temperature, fluid-inclusion population from one that has undergone no reequilibration and has been entrapped during precipitation of pore-filling cement at high temperature? How might one distinguish reequilibrated fluid inclusions that have leaked, exchanged with pore fluids, and rehealed from those that have reequilibrated through a mechanism not involving exchange with pore fluids? The following, which is by no means an exhaustive list, may prove useful in distinguishing the above. 1. Reequilibrated populations of originally low-temperature, water-filled fluid inclusions. A: Fluid-inclusion populations, which have reequilibrated through a mechanism involving exchange with pore fluids, may be variable in composition and Th. The variation in Th cannot result from isothermal, isobaric mixing of fluids. B: If fluid compositions do not vary but 7"h values do vary, fluid inclusions could have reequilibrated through leakage followed by open exchange with pore fluids and subsequent resealing. This could have occurred while in the presence of fluids of only one composition and during changing temperature and/or pressure (probably unlikely), in a vadose zone, in the laboratory, on the outcrop, or in the unlikely presence of boiling fluids. If leakage out of the inclusion had occurred without introduction of material from the surrounding pore fluids, or if wall stretching or hydrofracturing had occurred

GEOLOGY, September 1986 793


without leakage and exchange with pore fluids, the inclusion population also would record one fluid composition with variable Th. C: A narrow range of Th and fluid compositions could result if the rock had been subjected to a thermal event that persisted for a long time or was of very high temperature in relation to the normal geothermal gradient. Similar characteristics could result from reequilibration during a shearing tectonic event (work of Ympa, 1963; reported in Roedder, 1984). 2. Fluid-inclusion populations that have not reequilibrated and that could be mistaken for reequilibrated populations. A: Cements precipitated in a deepening diagenetic environment, which contain fluid inclusions that have not reequilibrated, could have a wide distribution of Th and fluid compositions similar to most reequilibrated populations. However, each successive growth zone or crosscutting fracture fill would have a population of fluid inclusions that would persist throughout the time-equivalent cement zone or fracture-fill generation in many nearby spatially separated samples. Moreover, the same progression of fluid-inclusion populations should be consistent across the same growth progression in nearby samples. If each cement zone-related population is totally distinct in Th and fluid composition, the populations must not have reequilibrated through leakage, exchange with pore fluids, and rehealing, but wall-stretching or hydrofracturing without connection to a pore fluid conduit is still a possibility. B: If the values in successive cement growth zones progressively decrease, it is likely that the cement was precipitated in a decreasing thermal regime and the populations probably have not reequilibrated. However, the same decrease in Th, although with less variation possible, could have resulted from progressive pressure increase or progressive salinity change. C: If a cement contains fluid inclusions that have a narrow Th and composition range, the inclusions probably have not reequilibrated. However, considering the effects of ubiquitous reequilibration under one set of conditions, some reequilibrated populations may be indistinguishable from narrow-range populations that have not reequilibrated. It may be difficult to distinguish reequilibrated fluid inclusions from those that have not reequilibrated by using the fluid inclusion characteristics alone. To distinguish these successfully, one should examine primary fluid inclusions along with field, pétrographie, and geochemical data. CONCLUSIONS On the basis of data reported herein of fluid inclusions in deeply buried and nonburied, lowtemperature, calcium-carbonate cements, the following conclusions appear warranted. 1. Low-temperature, phreatic cements dominantly trap one phase of liquid (and other allGEOLOGY, September 1986

gas inclusions of uncertain origin), whereas vadose cements trap liquid, air, and liquid-air mixtures that yield highly variable Th values. The vadose Th values are controlled by the proportion of air and water trapped in the inclusion during growth and are not controlled by the temperature and pressure of crystal growth. 2. Aragonite cements trap fluid during growth, but because aragonite is unstable and neomorphoses to calcite, these primary compositions may be lost during neomorphism. Conversely, cements precipitated as low-Mg calcite are unlikely to neomorphose and would not have primary fluid inclusions altered by neomorphism. 3. Many primary fluid inclusions in lowtemperature, near-surface calcite cements, which have been deeply buried, may contain deep-burial pore fluids. 4. Although many processes could cause reequilibration of low-temperature fluid inclusions, hydrofracturing during increasing burial heating may be the dominant one. Reequilibration through hydrofracturing first involves cracking of calcite from fluid-inclusion overpressure, then fluid equilibration between the inclusion fluid and the pore fluid, and finally the healing of the crack. 5. Reequilibrated, low-temperature fluid-inclusion populations generally have Th and inclusion-fluid composition measurements that have wide ranges. These populations may be difficult to distinguish from inclusion populations that have not reequilibrated. In deeply buried rocks, fluid inclusions are best used when environment of calcite cementation is known from other geologic techniques. 6. During deep-burial heating, most lowtemperature fluid inclusions reequilibrate by leaking, exchanging with pore fluids, and rehealing. Some inclusions may do this repeatedly, and some may never reequilibrate by this mechanism. Reequilibrated fluid-inclusion populations may, therefore, contain a nearly complete record of a rock's pore fluid history. REFERENCES CITED Bathurst, R.G.C., 1975, Carbonate sediments and their diagenesis: New York, Elsevier, 658 p. 1980, Lithification of carbonate sediments: Oxford, England, Scientific Progress, v. 66, p. 451-471. Bodnar, R.J., and Bethke, P.M., 1984, Systematica of stretching of fluid inclusions I: Fluorite and sphalerite at 1 atmosphere confining pressure: Economic Geology, v. 79, p. 141-161. Burruss, R.C., and Hollister, L.S., 1979, Evidence from fluid inclusions for a paleogeothermal gradient at the geothermal test well sites, Los Alamos, New Mexico: Journal of Volcanology and Geothermal Research, v. 5, p. 163-177. Harris, D.C., 1982, Carbonate cement stratigraphy and diagenesis of the Burlington Limestone (Mississippian), S.E. Iowa, W. Illinois [M.S.

Printed in U.S.A.

thesis]: Stony Brook, State University of New York, 229 p. Klosterman, M.J., 1981, Applications of fluid inclusion techniques to burial diagenesis in carbonate rock sequences: Louisiana State University Applied Carbonate Research Program Technical Series, Contribution 7, 102 p. Longman, M.W., 1980, Carbonate diagenetic textures from near surface diagenetic environments: American Association of Petroleum Geologists Bulletin, v. 64, p. 461-487. Meyers, W.J., and James, A.T., 1978, Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mountains, New Mexico: Sedimentology, v. 25, p. 105-124. Meyers, W.J., Cowan, Patricia, and Lohman, K.C., 1982, Diagenesis of Mississippian skeletal limestones and bioherm muds, New Mexico, in Bolton, Keith, Lane, H.R., and LeMone, D.V., eds., Symposium on the paleoenvironmental setting and distribution of the Waulsortian Facies: El Paso Geological Society and University of Texas at El Paso, p. 80-95. Moore, C.H., and Druckman, Yehezkeel, 1981, Burial diagenesis and porosity evolution, Upper Jurassic Smackover, Arkansas and Louisiana: American Association of Petroleum Geologists Bulletin, v. 65, p. 597-628. Pray, L.C., 1959, Guidebook, Sacramento Mountains of Otero County, New Mexico: Society of Economic Paleontologists and Mineralogists, Permian Basin Section, and Roswell Geological Society Joint Field Conference, 306 p. 1965, Clastic limestone dikes and marine cementation, Mississippian bioherms, New Mexico [abs.]: Society of Economic Paleontologists and Mineralogists Program and Abstracts, Permian Basin Section, Annual Meeting, p. 21-22. Roedder, Edwin, 1967, Metastable superheated ice in liquid-water inclusions under high negative pressure: Science, v. 155, p. 1413-1416. 1984, Fluid inclusions: Mineralogical Society of America Reviews in Mineralogy, v. 12, 644 p. Smith, F.D., Reeder, R.J., and Meyers, W.J., 1984, Fluid inclusions in Burlington Limestone (Middle Mississippian)—Evidence for multiple dewatering events from Illinois Basin [abs.]: American Association of Petroleum Geologists Bulletin, v. 68, p. 528. Wagner, P.D., and Matthews, R.K., 1982, Porosity preservation in the Upper Smackover (Jurassic) carbonate grainstone, Walker Creek Field, Arkansas: Response of paleophreatic lenses to burial processes: Journal of Sedimentary Petrology, v. 52, p. 3-18. Ympa, P.J.M., 1963, Rejuvenation of ore deposits as exemplified by the Bellodone Metalliferous Province [Ph.D. thesis]: Leiden, Netherlands, University of Leiden, 213 p.

ACKNOWLEDGMENTS Supported by grants from Conoco Exploration Research and the Exxon Education Foundation. Lloyd Pray, Stephen Carey, Robert Ginsburg, and Cole Abel donated samples. I thank Lloyd Pray, Robert Monahan, Roger McLimans, Cynthia Keeffe, and the reviewers for Geology for helpful comments on the manuscript. Manuscript received February 27, 1985 Revised manuscript received June 2, 1986 Manuscript accepted June 23, 1986

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Geology Reequilibration of fluid inclusions in low-temperature calcium-carbonate cement Robert H. Goldstein Geology 1986;14;792-795 doi: 10.1130/0091-7613(1986)142.0.CO;2

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