Entrapment Pressures Research Articles

Application of topaz-muscovite F-OH exchange as a geothermometer

Abstract Recently reported thermodynamic data for the F end member of muscovite permitted calibration of a geo- thermometer based on the F-OH exchange between muscovite and topaz. The geothermometer provides es- timates for the temperature of greisen-forming systems that are essentially independent of pressure. At pres- sures between 500 and 4,000 bars, the temperature can be calculated using the equation T(oC) : (-0.0131P-1323)x-185, where x is equal to 1/[2 log (XoH.Toz/XF.Toz) 'l' 2 log (XF.Ms/XOH.Ms)] , using an ideal mix- ing model between topaz and muscovite end members. This geothermometer was tested with samples from four different localities where there had been previous estimates of the temperature. Calculated temperatures compared well with the previous estimates provided that the total content of doubly charged cations (Fe 2', Mg •*, and Mn •*) in the muscovite was less than 0.4 cations per formula unit (based on 22 oxygens). Introduction TEMPE•TURES at which tin greisens form have been poorly documented because of a lack of reliable geothermometers. The most widely used geothermometer is based on oxygen isotope fractionation between cassiterite and quartz. How- ever, this geothermometer cannot be confidently applied be- cause of a large discrepancy between empirical (Bor- shchevskii et al., 1983; Alderton, 1989), theoretical (Zheng, 1991), and experimental calibrations (Zhang et al., 1994). Fluid inclusions can also provide information on the temper- ature of greisen formation, but this requires that the compo- sition of the fluid and the pressure of entrapment be known. The purpose of this paper is to report on the validity of a new geothermometer that can yield convenient estimates of temperature for greisen systems. The F-OH exchange be- tween biotite and apatite has been successfully used as a ge-

Gradients in H2O, CO2, and exsolved gas in a large‐volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

Infrared spectroscopic analyses of ∼140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H2O and CO2 concentrations were present during preemptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H2O (5.3±0.4 wt %) and CO2 (62±37 ppm) than inclusions from the middle of the eruption (5.7±0.2 wt % H2O; 120±60 ppm CO2). Melt inclusions from late erupted samples have much lower H2O (4.1±0.3 wt %) and higher and variable CO2 (150–1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO2 and incompatible elements. This pattern indicates that the magma was gas‐saturated during crystallization, with CO2 partitioning into a coexisting gas phase. Quantitative modeling using H2O‐CO2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from ∼1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO2. Mass balance modeling of Cl loss to exsolving H2O‐rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H2O, CO2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H2O‐CO2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time‐stratigraphic‐compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H2O, CO2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.

Fluid inclusions in mantle and lower crustal xenoliths from the Simcoe volcanic field, Washington

Mantle (spinel lherzolite, dunite, harzburgite, websterite, orthopyroxenite) and crustal (mafic granulite, pegmatitic gabbro) xenoliths occur in basaltic lavas of the Simcoe volcanic field, Cascades arc. The mantle xenoliths are invaded by diverse alkali- and silica-rich glasses (phonolithic–trachytic) that are unrelated to the host lavas. Fluid inclusions occur in websterite, orthopyroxenite, gabbro, and rarely in dunite. Microthermometry indicates that these are nearly pure CO 2, and provides constraints on the entrapment pressures and temperatures for representative lithologies. Phlogopite-bearing olivine orthopyroxenite and websterite xenoliths contain CO 2 inclusions with densities between 1.07–0.59 g/cm 3. At independently estimated equilibration temperatures (∼1000°C), entrapment pressures of the highest density inclusions must have been at least ∼9 kbar (≥30 km depth). Associated lherzolites cut by veins of websterite also must be derived from similar depths. Textural relations indicate that invasive melts were the immediate sources for these fluids. The lowest density CO 2 inclusions formed at pressures no greater than ∼2 kbar. Inclusions recording intermediate pressures may reflect progressive fluid entrapment over a range of depths during ascent; sources for the low density secondary inclusions may be either retrapped, decrepitated primary inclusions or CO 2 exsolved from the host lava. Gabbroic pegmatite and mafic granulites also contain CO 2 inclusions with densities between 1.02–0.57 and 0.96–0.59 g/cm 3, respectively; assuming entrapment temperatures of 900±100°C, these inclusions likely formed at pressures (∼5–7 kbar) equivalent to mid-to-lower crustal depths (<20–25 km). Based on our depth estimates and other petrologic information, Simcoe xenoliths were derived from a previously metasomatized domain within the Cascades subarc lithospheric mantle. Any relation between the earlier metasomatic event(s) and Cascadia subduction remains tenuous.

Fluid-inclusion characteristics of hydrothermal Cu–Ni–PGE veins in granitic and metavolcanic rocks at the contact of the Little Stobie deposit, Sudbury, Canada

Cu- and precious-metal-enriched massive ore and veins occur at contacts of two Fe–Ni–Cu sulfide orebodies of the Little Stobie Mine, Sudbury, with metavolcanic rocks of the Elsie Mountain Formation, and the Murray granite. Veins contain chalcopyrite, pyrrhotite, pentlandite, platinum-group minerals, quartz, carbonate, chlorite, amphibole and other minerals. A granitic dike, resulting from partial re-melting of the Murray Granite, cuts back into the Sudbury Igneous Complex near Little Stobie Mine, and contains barren veinlets with similar mineralogy but no sulfides. Quartz from ore veins contains fluid inclusions that were trapped during several stages. Early high-temperature fluids (at least 180–270°C in orebody 1 and 280–350°C in orebody 2) were extremely saline and occur as polyphase, isolated and fracture-controlled inclusions with halite, sylvite, Fe–Mn-, as well as Pb–Ba-chloride daughter minerals and other unknown solids. Late secondary aqueous inclusions are not chloride-saturated; they were trapped at a minimum of 80–150°C. Their microthermometric behavior may be modeled in the CaCl 2–NaCl–H 2O system with salinity of 21–27 CaCl 2 equiv. wt.%. Very late secondary inclusions have either Ca-rich saline, or very dilute (about 1 equiv. wt.% NaCl) compositions and homogenization temperatures of 200–300°C. With the exception of these very late secondary inclusions, the association of CO 2–(CH 4)-rich inclusions with aqueous ones was usually observed. The density of these carbonic fluids decreased from early to late stages. Microthermometric data from barren veins are fundamentally different from those of early inclusions from orebodies; this implies that these heavy-metal-rich fluids were responsible for ore deposition in veins. The minimum pressure of entrapment for early fluids was 1800–2200 bars. Late Ca-rich brines were trapped at lower minimum pressure (200–900 bars). High-pressure data are in agreement with the estimated minimum paleodepth of crystallization of South Range ores. Later fluids were probably trapped during the uplift of orebodies and their host rocks. Comparison of data to other Cu–PGE–Au-rich ore of the Sudbury Structure suggests that the presence of CO 2-rich fluids in South Range deposits and their absence in North Range deposits may be related to different metamorphic histories. The high-temperature hydrothermal fluids were driven by the heat of the Sudbury Igneous Complex; these very saline fluids interacted with primary magmatic ores, remobilized metals and redeposited them along convenient structures such as fracture zones and breccias in and along various units near the footwall contact. The identification of such highly saline fluid inclusions with high heavy-metal content may be useful in the exploration for Cu–PGE–Au-enriched, footwall vein-type ores in the Sudbury Structure.

A new technique for surface and shallow subsurface paleobarometry using fluid inclusions: an example from the Upper Ordovician Viola Formation, Kansas, USA

This research illustrates a new approach for paleobarometry employing heterogeneously entrapped fluid inclusions to determine timing and depth of diagenesis. Heterogeneously entrapped fluid inclusions (gas+water) in vug-filling quartz from the Upper Ordovician Viola Formation in the Midcontinent of the United States were analyzed for their internal pressure with a fluid-inclusion crushing stage. The free gas in fluid inclusions was entrapped at near-surface temperature, as indicated by the presence of all-liquid fluid inclusions and fluid inclusions with low homogenization temperatures (<40°C). Crushing the crystal and measuring the change in bubble size determines the pressure of entrapment directly. Heterogeneous trapping is indicated by widely varying L:V ratios, from all-liquid to vapor-rich. Gas bubbles in most fluid inclusions analyzed expanded upon release to atmospheric pressure, but some collapsed. A mode of 1.5 to 2.0 atm internal pressure was indicated by the crushing runs, but pressures up to 42.9 atm were recorded. Quartz precipitation and associated fluid-inclusion entrapment therefore occurred over a wide depth-range, but principally at depths of approximately 10 m. Crushing runs done in kerosene confirmed the presence of hydrocarbon gases in most of these inclusions, and bulk analyses of gases in the quartz by quadrupole mass spectrometer revealed methane, ethane, and atmospheric gases. The hydrocarbon gases may have originated in deeper thermogenically mature sedimentary strata, and then leaked to the near-surface where they were entrapped in the precipitating quartz cement. Freezing data indicate an event of quartz precipitation from fluids of marine-fresh water intermediate salinity and other events of precipitation from more saline fluids. Considering the determined pressures, the precipitating fluids probably originated at surfaces of subaerial exposure (unconformities) and surfaces of evaporite precipitation in the overlying Silurian strata. Thus, saline inclusions most likely originated from sinking of saline surface waters during Silurian time. Lower-salinity fluids record fluxes of meteoric water during development of unconformities in the Silurian. This type of paleobarometric study may have application in many other sedimentary systems, provided low-temperature and heterogeneous entrapment of an immiscible gas phase can be demonstrated for the fluid-inclusion assemblages studied.

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path.

Fluid inclusions in the Harney Peak Granite, Black Hills, South Dakota, USA: Implications for solubility and evolution of magmatic volatiles and crystallization of leucogranite magmas

Experimental and theoretical studies have shown that magmatic fluids have strong influence on phase equilibria of granitic systems. A microthermometric study of fluid inclusions in the Proterozoic Harney Peak Granite, Black Hills, South Dakota, USA, was undertaken to obtain direct evidence for the composition and evolution of magmatic fluids in a leucogranitic system and to evaluate their effect on liquid-lines-of-descent of leucogranite magmas. A previous stable isotopic study has demonstrated that the granite has not interacted with nonmagmatic fluids since its crystallization. Therefore, fluids found in the inclusions are of magmatic origin and present an opportunity to directly sample magmatic fluids trapped during crystallization of a granitic magma and during subsolidus conditions. The Harney Peak Granite was emplaced as multiple sills, dikes, and small intrusions, many of which differentiated into coupled aplite-pegmatite layers. Three types of fluid inclusions are found in the granite. Type 1 consist of variable saline H 2OC0 2 mixtures with minor CH 4 or other gases. Inclusions of this type are found in tourmaline and quartz in aplite layers, are generally isolated, and have negative crystal shapes. They are interpreted to be primary. There is a general increase in salinity and decrease in isochore temperature with decreasing CO 2/H 2O ratio of the inclusions. The trend is ascribed to progressive change in fluid composition with crystallization due to the differential solubility of CO 2 and H 2O in the high-silica magma. Isochores, coupled with independently determined crystallization temperatures, suggest entrapment pressure ∼3.5 kbar, consistent with geobarometry in the wallrocks. Type 2 inclusions are carbonic fluids dominated by CO 2, whereas type 3 inclusions are saline aqueous fluids without a detected carbonic component. Both types are mostly found along healed fractures in quartz. They are interpreted to be secondary inclusions that formed as a result of unmixing of type 1 inclusions at subsolidus conditions during uplift and cooling of the Harney Peak Granite dome. Freezing-point depressions, birefringent daughter crystals, phase-equilibria considerations, and α-track mapping of B and Li distribution in thin sections, all indicate that both type 1 and type 3 inclusions contain solutes such as K, Li, and B in addition to NaCl. Using published solubility models for H 2O and CO 2 in granitic melts and the composition of the most CO 2-rich type 1 inclusions ( X CO 2 = 0.55), the initial H 2O and CO 2 concentrations in the Harney Peak Granite magma are deduced to have been ∼3.5 wt% and 1500 ppm, respectively. The relatively low water concentration is consistent with temperatures of dehydration-melting reactions that produced the magma. It is suggested that the aplite-pegmatite segregations found in parts of the Harney Peak Granite are the result of initial rapid near liquidus crystallization of minerals with high Na/K ratios, including plagioclase and tourmaline, along a wide range of increasing αH 2O. Aplite crystallization was followed by crystallization of relatively potassic residual liquid enriched in fluxing elements under conditions of high αH 2O.

Constraints on the thermal history of Taylorsville Basin, Virginia, U.S.A., from fluid-inclusion and fission-track analyses: implications for subsurface geomicrobiology experiments

Microbial populations have been found at the depth of 2621–2804 m in a borehole near the center of Triassic Taylorsville Basin, Virginia. To constrain possible scenarios for long-term survival in or introduction of these microbial populations to the deep subsurface, we attempted to refine models of thermal and burial history of the basin by analyzing aqueous and gaseous fluid inclusions in calcite/quartz veins or cements in cuttings from the same borehole. These results are complemented by fission-track data from the adjacent boreholes. Homogenization temperatures of secondary aqueous fluid inclusions range from 120° to 210°C between 2027- and 3069-m depth, with highest temperatures in the deepest samples. The salinities of these aqueous inclusions range from 0 to ∼ 4.3 eq wt% NaCl. Four samples from the depth between 2413 and 2931 m contain both two-phase aqueous and one-phase methane-rich inclusions in healed microcracks. The relative CH 4 and CO 2 contents of these gaseous inclusions was estimated by microthermometry and laser Raman spectroscopy. If both types of inclusions in sample 2931 m were trapped simultaneously, the density of the methane-rich inclusions calculated from the Peng-Robinson equation of state implies an entrapment pressure of 360 ± 20 bar at the homogenization temperature (162.5 ± 12.5°C) of the aqueous inclusions. This pressure falls between the hydrostatic and lithostatic pressures at the present depth 2931 m of burial. If we assume that the pressure regime was hydrostatic at the time of trapping, then the inclusions were trapped at 3.6 km in a thermal gradient of ∼ 40°C/km. The high temperatures recorded by the secondary aqueous inclusions are consistent with the pervasive resetting of zircon and apatite fission-track dates. In order to fit the fission-track length distributions of the apatite data, however, a cooling rate of 1–2°C/Ma following the thermal maximum is required. To match the integrated dates, the thermal maximum would have occurred at ∼ 200 Ma. The timing of the maximum temperature is consistent with rapid burial of the Taylorsville Basin to twice its present-day depth and thermal re-equilibration with a 40°C/km geothermal gradient, followed by slow exhumation. The results may imply that the microorganisms did not survive in situ, but were transported from the cooler portions of the basin sometime after maximum burial and heating.

Fluid pressure transients on seismogenic normal faults

Fluid inclusions in hydrothermally altered footwall rocks of the Dixie Valley fault, Nevada, and the Wasatch fault, Utah, indicate that pore fluid pressure fluctuated. Minimum entrapment pressures for fluid inclusions consisting of H 2O-CO 2-NaCl ranged from 295 MPa to 60 MPa in the temperature range 350° to 170 ° C on the Wasatch fault, and from 158 to 35 MPa in the temperature range 350° to 200 ° C on the Dixie Valley fault. Scatter in the pressure estimates at constant temperature is interpreted as paleo-fluid pressure transients at depths of up to 11 km on the Wasatch fault and 3 to 5 km on the Dixie Valley fault. Observed pressure transients range from 5 MPa, within the limits of error in pressure determination, to 120 MPa on the Wasatch fault and 7 to 126 MPa on the Dixie Valley fault. The pressure transients are greatest on both faults in the temperature range 270 ° to 310 ° C. The fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures. High fluid pressures may initiate rupture, then dilatancy, pore-pressure reduction, and dilatant hardening may arrest the rupture. However, decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90% which reduces the dilatant hardening effect and may permit ruptures to propagate.

Constraining Diagenesis Using Fluid Inclusions in Cements from Petroleum Basins: Examples from North Sea and Middle East: ABSTRACT

Cements in petroleum reservoirs commonly contain inclusions filled with brine, hydrocarbon, or mixtures of both. The physicochemical conditions of cementation as well as the quality of the inclusion oil can be derived from microthermometric and fluorescence microspectrophotometric analysis of fluid inclusions. These data, when integrated with petrographic, geochemical, engineering, and burial history data, can constrain the timing and nature of both diagenetic events and oil migration. Brine and oil-bearing inclusions occur in quartz overgrowths from the Brent Sandstone, Northwest Hutton field, North Sea. Fluorescence analysis indicates that the inclusion oil is similar to the reservoired oil. Inclusion data define the reservoir conditions that existed during the initial stages of oil accumulation and concurrent quartz cementation. The abnormally high pressure of inclusion entrapment suggests that the reservoir was overpressured. Paleo-overpressuring probably helped preserve reservoir porosity by slowing compaction-related loss of porosity in coarse-grained reservoir sands. Brine, oil, and three-phase (brine, oil, and vapor) inclusions occur in burial-related calcite and dolomite cements from a Lower Cretaceous (Thamama) carbonate reservoir in the Middle East. Inclusion data indicate that these cements were precipitated from and simultaneously trapped an invading basinal brine that contained an immiscible component of hydrocarbon. The intersection of brine and oilmore » isochores in pore-throat space allows a unique determination of the time and depth of initial reservoir fill-up. The fill-up appears to coincide with the onset of peak oil generation from the source rocks believed to have supplied these reservoirs.« less

Field Evaluation of Shallow Subsurface Drains to Vent Soil Air, Improve Infiltration, and Reduce Runoff

ABSTRACT The effect of soil air entrapment on surface runoff rate and quantity was evaluated in the field using shallow subsurface drainage and tillage treatments to modify the amount of air trapped in a fragipan soil. The volume of pore space filled with air was increased by tillage and subsurface drainage. The latter also vented entrapped soil air during infiltration events. When the entrapped soil air was vented through the drain, the average steady-state subsurface drainage rate increased 20.9 and 48.9% for grass vegetated and tilled fallow conditions, respectively. Corresponding decreases in steady state surface runoff rates were 10.4 and 14.9%, respectively. Venting air pressure, as low as 40 Pa (gauge) resulted in an increase in drain flow rate and a decrease in runoff rate. Generally, lower soil air pressures during infiltration occurred in the vegetated soil because cracking along soil aggregate faces induced by transpirational drying provided for some surface venting. As a result, soil air pressure declined more during an extended infiltration event in the grass plot than in the fallow plot. The immediate result of tillage followed by precipitation was greater infiltration and soil water retention, however as reconsolidation occurred, soil air entrapment increased and a greater portion of the applied water became surface runoff. After reconsolidation caused by 627.6 mm of precipitation, air entrapment pressure of 1.36 kPa resulted in a 36.0% decrease in tile flow rate and 20.5% increase in surface runoff rate. Likewise, with reconsolidation air entrapment pressure of 2.41 kPa resulted in an 8.8% decrease in tile flow and 32.9% increase in surface runoff volumes.

Pore-fluid chemistry and chemical reactions on the Wasatch normal fault, Utah

Abstract Mineral assemblages and fluid inclusions (F.I.) in hydrothermally altered and tectonically deformed Oligocene quartz monzonite in the footwall of the active Wasatch normal fault, Utah have been used to estimate fluid pressure, temperature, chemical composition, and chemical reactions associated with progressive displacement of the fault. Vein filling and pervasive alteration mineralogy includes (earliest to latest) biotite-K-feldspar, chlorite-epidote-sericite, and laumontite-prehnite-clay. Secondary F.I. in quartz, associated with chlorite-epidote-sericite alteration, consist of CO 2 and salt solution; the homogenization temperature mode is 285°C, X CO 2 is 0.03–0.32, and salinity ranges from 4.5–17.3 wt.% NaCl equivalent; estimated minimum entrapment pressures vary from 600–2950 bars. The homogenization temperature mode of secondary F.I. associated with the laumontite-prehnite assemblage is 100°C, salinity ranges from 2.0–16.0 wt.% NaCl equivalent, and no CO 2 was detected; entrapment pressures are presumed to have been hydrostatic and below 460 bars. Fluid pressure and temperature evolved along a path from lithostatic to hydrostatic with continued displacement of the footwall relative to the hanging wall. Age constraints are provided by a 17.6 ± 0.6 m.y. K-Ar age of hydrothermal seriate from a sample with mean T h of 309°C, and 7.3 to 9.6 m.y. fission track ages of apatite (closure temperature 120 ± 15° C ). Calculated equilibrium phase diagrams illustrate the systematic variation in cation activities, CO 2 fugacity, and alteration minerals as fluid temperature and pressure decrease. The ratios of a Ca 2+ (a H + ) 2 and a Mg 2+ (a H + ) 2 increase and the ratio a K + / a H + decreases by more than two orders of magnitude each as temperature decreases and alteration mineralogy changes from K-feldspar and muscovite to muscovite and clay. The hot aqueous fluids affect the mechanical behavior of the rock during faulting. Crack sealing by alteration product minerals reduces permeability, and the effective stress is decreased by pore fluid pressures substantially greater than hydrostatic in the early alteration.

Paleo-temperatures from Fluid Inclusions--Advances in Theory and Technique: ABSTRACT

Recent studies of fluid inclusions in diagenetic cements have attempted to determine paleosubsurface temperatures. Three sets of observations are necessary to make accurate interpretations: (1) detailed petrography to establish the relative time of formation of the inclusions, (2) careful analysis of the burial and tectonic history of the host rocks to relate the diagenetic paragenesis to the geologic history of the basin, and finally, (3) analysis of individual inclusions for homogenization and final melting temperatures, and for chemical composition to define the PVT properties of the trapped fluids. Once these observations are complete, 2 major limitations on the temperature interpretation remain. First is the assumption that the inclusions have not altered in composition or volume since entrapment. Recently published work shows that inclusions can re-equilibrate, but the extent that this affects most observations in sediments is unknown. Second, we must independently determine a during inclusion formation, and we must know whether this pressure was hydrostatic or approached lithostatic. Data from both hydrocarbon and aqueous fluid inclusions in core samples from the Mission Canyon formation, Williston basin, North Dakota, illustrate a method for independently determining both paleotemperature and paleopressure from a single set of fluid inclusion measurements. Th technique requires petrographic evidence for End_Page 241------------------------------ simultaneous trapping of 2 immiscible fluids. Theoretical analysis of the PVT properties of coexisting immiscible fluids demonstrates that the isochores for the 2 different fluids must intersect at the temperature and pressure of entrapment of the inclusions. Calculation of the PVT properties of each fluid requires detailed chemical analyses of both fluids. Recent results from new analytical techniques, especially capillary column gas chromatography to analyze hydrocarbon inclusions and laser Raman spectroscopy to analyze gases in aqueous inclusions, demonstrate that this approach to paleotemperature studies can be widely applicable in sedimentary environments. End_of_Article - Last_Page 242------------

Fluid inclusions in rocks from the amphibolite‐facies gneiss to charnockite progression in southern Karnataka, India: direct evidence concerning the fluids of granulite metamorphism

Abstract Fluid inclusion studies of rocks from the late Archaean amphibolite‐facies to granulite‐facies transition zone of southern India provide support for the hypothesis that CO2,‐rich H2O‐poor fluids were a major factor in the origin of the high‐grade terrain. Charnockites, closely associated leucogranites and quartzo‐feldspathic veins contain vast numbers of large CO2‐rich inclusions in planar arrays in quartz and feldspar, whereas amphibole‐bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed‐facies quarries contain no large fluid inclusions. Inclusions in the northernmost incipient charnockites, as at Kabbal, Karnataka, occasionally contain about 25 mol. % of immiscible H2O lining cavity walls, whereas inclusions from the charnockite massif terrane farther south do not have visibile H2OMicrothermometry of CO2 inclusions shows that miscible CH4 and N2 must be small, probably less than 10mol.%combined. Densities of CO2 increase steadily from north to south across the transitional terrane. Entrapment pressures calculated from the CO2 equation of state range from 5 kbar in the north to 7.5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750°C, in quantitative agreement with mineralogic geobarometry. This agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions.Calculations on the stability of the charnockite assemblage biotite‐orthopyroxene‐K‐feldspar‐quartz show that an associated fluid phase must have less than 0.35 H2O activity at the inferred P and T conditions, which agrees with the petrographic observations. High TiO2 content of biotite stabilizes it to lower H2O activities, and the steady increase of biotite TiO2 southward in the area suggests progressive decrease of aH2O with increasing grade. Oxygen fugacities calculated from orthopyroxene‐magnetite‐quartz are considerably higher than the graphite CO2‐O2 buffer, which explains the absence of graphite in the charnockites.The present study quantifies the nature of the vapours in the southern India granulite metamorphism. It remains to be determined whether CO2‐flushing of the crust can, by itself, create large terranes of largeion lithophile‐depleted granulites, or whether removal of H2O‐bearing anatectic melts is essential.

Manganiferous cherts of the Franciscan assemblage; I, General geology, ancient and modern analogues, and implications for hydrothermal convection at oceanic spreading centers

There are several hundred ophiolitic manganiferous chert deposits, primarily of late Jurassic to early Cretaceous age, known within the Franciscan assemblage of California. The sequences typically consist of one to three massive, manganiferous chert lenses containing 30 to 50 percent Mn, and averaging 1 m in thickness by 15 m subcircular diameter; these are separated by an average 2 to 10 m of thin-bedded radiolarian cherts and overlie basalts or greenstones. Both their geology and chemistry indicate that the ore lenses are hydrothermal and may have formed on the flanks of a mid-ocean ridge or within a back-arc basin. It is proposed that the sequences developed as a result of sea-floor spreading over a series of deep hydrothermal seawater convection cells paralleling a spreading center and spaced roughly 5 to 10 km apart. Chemical profiles of Mn, Fe, Si, Al, Cu, Ni, Zn, Co, Ba, Ti, the rare earth elements, and 87 Sr/ 86 Sr have been determined through two complete sections. These profiles indicate hydrothermal input of Mn, Si, Cu, Ni, Zn, and Ba and detrital or hydrogenous input of Al and Co; they illustrate the use of Ti as a measure of relative detrital sedimentation rates. Fe is strongly fractionated from Mn within the ores (Fe/Mn < 0.1), and Fe/Mn ratios decrease upward throughout each section suggesting preferential deposition of Fe within the sediment, and of Mn at the seawater interface. Rare earth element distributions reflect the interaction of sea-water and underlying basalts. Sr isotopic ratios of the ores and basalts demonstrate both strong and moderate seawater influences, respectively. Fluid inclusion analyses on veins of undetermined age show seawater salinity, temperatures of roughly 200 degrees C, and tentative entrapment pressures corresponding to 1,700-m water depth. Early and intermediate veins were injected into unconsolidated siliceous sediment producing a characteristic bleached and pseudobrecciated texture. An analogy is drawn with the present-day field of hydrothermal mounds near the Galapagos rift and with ophiolitic complexes of the northern Apennines and other localities.

The nature and distribution of carbon in submarine basalts and peridotite nodules

Primary carbonaceous material has been identified in submarine basaltic glasses and mantle-derived peridotite nodules from alkali basalts using electron microprobe techniques. In the submarine rocks carbon occurs (1) in quench-produced microcracks in glasses and phenocrysts, (2) in vesicles, where it is preferentially concentrated on the sulfide spherules attached to vesicle walls, and (3) in microcracks and CO 2-rich bubbles in inclusions of glass completely enclosed by phenocrysts. In peridotite nodules carbon exists in intergrain cracks, along grain boundaries, and on the walls of fluid inclusions disposed in two dimensional arrays. The carbonaceous material is believed to consist of a mixture of graphite, other forms of elemental carbon, and possibly small amounts of organic matter. It is suggested that carbon precipitates by disproportionation of CO according to the reaction 2 CO→C+CO 2 and that this reaction is catalyzed by sulfide-oxide surfaces in vesicles. Once deposition has begun, the reaction continues on carbon surfaces as well. Based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them, it is proposed that carbon condensed from a magmatic vapor in which CO was a significant constituent. This implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the QFM buffer at equivalent temperatures. This is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials. Regardless of the mechanism of its formation, the presence of carbon in CO 2-rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities. Condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks.

Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex

The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C. Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO 2 (gas or liquid or both at room temperature) through CO 2 + H 2O ± CH 4 mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism. The density and fluid composition data are derived from direct observations of phase changes between − 180 and + 380°C and from the application of published experimental data in the system CH 4-CO 2-H 2O-NaCl. The most dense, pure CO 2 inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P- T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO 2 + H 2O inclusions. There is evidence of immiscible separation of CO 2-rich and H 2O-rich fluids at temperatures at least as high as 375°C.