Re-equilibration Of Fluid Inclusions Research Articles

Temporal evolution of the Fogo Volcano magma storage system (Cape Verde Archipelago): a fluid inclusions perspective

The architecture of the magma storage system underneath Fogo Volcano (Cape Verde Archipelago) is characterised using novel fluid inclusion results from fifteen basanites, spanning the last 120 thousand years of volcanic activity, and encompassing a major flank collapse event at ∼73 ka. Fluid inclusions, hosted in olivine and clinopyroxene, are made of pure CO2, and based on their textural characteristics, are distinguished in early (Type I) and late (Type II) stage. Inclusions homogenize to a liquid phase in the 2.8 to 30.8 °C temperature range. Densities values, recalculated assuming an original 10% H2O content at the time of trapping, range from 543 to 952 kg·m−3, and correspond to entrapment or re-equilibration pressure ranges of 500–595 MPa, 700–740 MPa, and 245–610 MPa respectively for pre-collapse, early post-collapse, and Holocene/historical eruptions. These entrapment pressures are interpreted as reflecting the existence of two main magma accumulation zones at ∼25 km and ∼ 13–21 km depth, and a zone of fluid inclusion re-equilibration at 9–12 km depth. There is evidence of a complex temporal evolution of the magma system. Historical eruptions, and especially the three most recent ones (occurred in 1951, 1995 and 2014–25), bring fluid inclusion evidence for transient, pre-eruptive shallow (9–17 km depth) magma ponding. Early post-collapse (60 ka) volcanics, in contrast, document fast magma transport from ∼25 km, and suggest a reconfiguration of the magma system after the Monte Amarelo collapse event.

Tectono-metamorphic evolution of the Karakoram Terrane: Constrained from P–T–t–fluid history of garnet-bearing amphibolites from trans Himalaya, Ladakh, India

This study presents new data sets on the petrological, fluid inclusion and U-Pb zircon geochronological aspects of the garnet-bearing amphibolites from the eastern Karakorum (Pangong-Tso), Ladakh. to delineate the P-T-t-fluid evolution and its tectonic implications.The calculated phase equilibria in the model system NCKFMASHTO reveal that the peak metamorphism occurred at 8.5–9.5 kbar, 630–655 °C, and retrograde metamorphism crossed a stage at 5.5–5.7 kbar, 620–630 °C. Monophase primary and secondary carbonic and carbonic-aqueous fluid inclusions occur in garnet and quartz. Trapping of the primary inclusion occurred at pressures lower than the estimated peak P-T conditions (5.8–6.1 kbar, 629–638 °C) indicative of partially re-equilibrated fluid inclusions during exhumation. The primary inclusions were original as carbonic-aqueous fluid derived from prograde decarbonation of the amphibolite’s protolith or adjacent carbonate-rich rocks and H2O leached out during deformation, leaving a pure CO2 fluid.U–Pb dating of zircons yielded two age groups: (i) at 128 to 131 Ma interpreted as the age of detrital zircons of sedimentary derivation, and constrain the maximum depositional age of the sedimentary protolith at 131 Ma and (ii) at 114 ± 0.38 Ma is interpreted as the age of peak metamorphism remarkably similar to the metamorphic age of the surrounding metapelites. This metamorphic stage corresponds to the well-known Cretaceous phase of crustal thickening and heating along the active continental margin of South Asia.

Reequilibrated fluid inclusions in enargite-luzonite from the Chelopech high-sulfidation Cu-Au epithermal deposit (Bulgaria)

Reequilibrated fluid inclusions in enargite-luzonite from the Chelopech high-sulfidation Cu-Au epithermal deposit (Bulgaria)

A fluid inclusion study of blueschist-facies lithologies from the Indus suture zone, Ladakh (India): Implications for the exhumation of the subduction related Sapi-Shergol ophiolitic mélange

The best occurrence of blueschist-facies lithologies in Himalaya is that of the Shergol Ophiolitic Mélange along the Indus suture zone in Ladakh region of north-western India. These lithologies are characterized by well preserved lawsonite-glaucophane-garnet-quartz assemblages. This paper presents for the first time the results of a detailed fluid inclusion study on these lithologies, in order to understand the fluid P-T evolution and its tectonic implications.The blueschist rocks from Shergol Ophiolitic Mélange record metamorphic peak conditions at ∼19kbar, 470°C. Several types of fluid inclusions are trapped in quartz and garnet, most of them being two-phase at room temperature. Three types of fluid inclusions have been recognised, basing on microtextures and fluid composition: Type-I are primary two-phase carbonic-aqueous fluid inclusions (VCO2−LH2O); Type-II are two-phase (LH2O−VH2O) aqueous fluid inclusions, either primary (Type-IIa) or secondary (Type-IIb); Type-III are re-equilibrated fluid inclusions. In the Type-I primary carbonic-aqueous inclusions, H2O is strongly predominant with respect to CO2; the homogenization temperature of CO2 range from −7 to −2°C. The clathrate melting temperature in such inclusions varies in between +7.1 and +8.6°C. Type-II two-phase aqueous fluid inclusions show a wide range of salinity, from 7.8–14wt.% NaCleq (Type-IIa) to 1.65–6.37wt.% NaCleq (Type-IIb) with accuracy ±0.4wt.% NaCleq.Type-I and Type-IIa primary fluid inclusions are hosted in peak minerals (garnet and quartz included in garnet), therefore they were likely entrapped at, or near to, peak P-T conditions. The dominantly aqueous fluid of both Type-I and Type-IIa inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab. Despite their primary nature, the isochores of Type-I and Type-IIa inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage, suggesting that these inclusions stretched or re-equilibrated during nearly isothermal decompression from 19kbar to 3kbar or less, at T=290°C. This conclusion is further supported by their large variability in shapes and sizes which range from irregular inclusions (‘C’/arc shaped, hook shape and satellite type). This decompression stage was followed by nearly isobaric cooling, testified by the occurrence of dendritic networks of decrepitated and ‘imploded’ fluid inclusions.

Metamorphic P–T conditions and CO2 influx history of medium-grade metapelites from Karakorum, Trans-Himalaya, India

The medium grade metapelites of Pangong-Tso area in the trans-Himalayan region underwent sillimanite-grade metamorphism initiated during the Cretaceous, associated with the collision of the Kohistan arc and the Indian plate with Asia. This paper present results from a petrological and fluid inclusion study to understand the metamorphic P–T conditions and fluid history of these rocks. The calculated phase equilibria in the Na2O–CaO–K2O–FeO–MgO–MnO–Al2O3–SiO2–H2O–TiO2 (NCKFMMnASHT) system suggest P–T conditions of 8kbar and 650°C for the peak metamorphic event. Primary fluid inclusions occur in staurolite and garnet, whereas quartz carries mostly secondary fluid inclusions. The trapped fluids in primary inclusions show initial melting temperatures in the range of −56.9 to −56.6°C, suggesting nearly pure CO2 composition. The secondary fluids are of mixed carbonic-aqueous nature. The re-equilibrated inclusions show annular morphology as well as necking phenomena. The CO2 isochores for the primary inclusions indicate pressures of 6.1–6.7kbar, suggesting that the CO2-rich fluids were trapped during post-peak exhumation of the rocks, or that synmetamorphic carbonic fluids underwent density reversal during isothermal decompression. The secondary CO2–H2O fluids must have been trapped during the late exhumation stage, as their isochores define further lower pressures of 4.8kbar. The morphology of re-equilibrated fluid inclusions and the rapid decrease in pressure are consistent with a near-isothermal decompression trajectory following the peak metamorphism. The carbonic fluids were probably derived locally from decarbonation reactions of the associated carbonate rocks during metamorphism or from a deep-seated reservoir through Karakorum fault.

Hydrocarbon charge history of the Tazhong Ordovician reservoirs, Tarim Basin as revealed from an integrated fluid inclusion study

Abstract This paper presents an integrated workflow for investigating hydrocarbon charge history using fluid inclusions using the Ordovician reservoirs from the Tazhong Oilfield, Tarim Basin as an example. The work flow involves the delineation of fluid inclusion assemblage (FIA) using fluid inclusion petrography and spectroscopy for microthermometric analysis. The interpretation of microthermometric data is based on data derived from synthetic inclusion experiments and takes consideration of the P-T re-equilibration of fluid inclusions in carbonate and over pressure effect on fluid inclusion trapping. The workflow also emphasizes on the requirements of adequate sample preparation, FIA identification, quantitative spectroscopic data (CIE), coeval petroleum and aqueous fluid inclusions, adequate number of microthermometric measurements and integrated interpretation using borehole temperature. In the study reservoirs in the Tazhong area, fluid inclusion spectroscopy reveals the presence of three groups of oil inclusions of near yellow, near blue and near white fluorescence colours. The fluid inclusion microthermometry data indicate the presence of at least two predominant hydrocarbon fluid inclusion assemblages including: homogenous liquid phase yellow fluorescence inclusions and liquid phase oil and condensate fluid inclusion assemblages with homogenous near blue, near white and near yellow fluorescence colours. The aqueous inclusions from the Ordovician reservoirs in the Tazhong area reveal the presence of low, moderate, high and very high salinities in the reservoirs. The extremely high salinity is believed to be caused by regional brine intrusions relating to regional structural movement and deformation.

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

Abstract This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (T h) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147–351°C) and calcite (108–352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO2-H2O fluid. Progressive heating and hydrothermal pulses, involving a CO2-H2O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.

Quartz recrystallization regimes, c-axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone (Ribeira Shear Zone, SE Brazil)

The crystal-plastic behavior of quartz mylonites from the Ribeira Shear Zone (SE Brazil), a major strike–slip structure that was active during a prograde metamorphic phase related to the Neoproterozoic Brasiliano–Pan African Orogeny, was investigated using a multi-method approach. Geothermobarometry results indicate deformational conditions ranging from ∼ 300 to ∼ 630 °C and 500–700 MPa. A strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone. Bulging recrystallization (BLG) dominates within the chlorite zone between 300 and 410 °C, subgrain rotation recrystallization (SGR) operates within the biotite zone from 410 to 520 °C, and grain boundary migration recrystallization (GBM) dominates in the garnet zone above 520 °C. The development of quartz c-axis textures is mainly governed by temperature and dynamic recrystallization mechanisms. Textures from BLG zone mylonites are characterized by maxima around Z; SGR zone mylonites display single girdles or asymmetric type I crossed girdles; and GBM zone mylonites comprise maxima around Y and intermediate between X and Z. The scarcity or absence of water-bearing fluid inclusions in quartz mylonites from the SGR and GBM zones, which are dominated by carbonic inclusions, suggests water-deficient conditions, whereas BLG zone mylonites are dominated by water-bearing inclusions. This evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature. No effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected, and little influence on c-axis texture development was observed. Most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history, recording a decompression in the range of 300–500 MPa, while microstructural reequilibration effects related to the prograde metamorphism are largely preserved. Fluid inclusion microstructures and densities from two SGR zone samples preserved evidence for a near isothermal compression within the interior of the Ribeira Shear Zone during the prograde metamorphism.

Reequilibration of fluid inclusions: Bulk-diffusion

Bulk diffusion of fluid components through quartz, which is a nominally anhydrous mineral, is one of a series of processes that may alter fluid inclusions in crystals. Diffusion of water through quartz has been considered an important factor for re-equilibration of fluid inclusions. However, the solubility of water in quartz, the nature of the diffusing water-related molecules, and diffusion coefficients have not been adequately determined to calculate reliable rates of inclusion alteration. A new three-dimensional mathematical diffusion model is presented to characterize bulk diffusion in quartz with randomly distributed fluid inclusions. This model assumes an infinite external fluid source and treats fluid inclusions as instantaneous point sources. The computer program “ReqDif” from the software package “FLUIDS” ( http://fluids.unileoben.ac.at) can be used to calculate the effect of diffusion with self-defined parameters. Using a diffusion coefficient of 10 − 12 m 2 s − 1 (± 60%), the rate of H 2O loss or gain of most inclusions is relatively high, adapting them within several 10,000 of years to new pore fluid conditions. However, diffusion according to this model may be restricted and partly inhibited by several important factors, such as the low porosity in metamorphic rock and relative low temperatures in sedimentary and diagenetic rocks. The imprecision of estimated diffusion coefficients, and the undetermined accuracy do not allow reliable diffusion calculations. Diffusion along micro-cracks and dislocations is a more efficient process to transport fluid components through quartz.

Reequilibrated fluid inclusions as proxy for PTX-conditions of deformation and recrystallisation in vein quartz

Reequilibrated fluid inclusions as proxy for PTX-conditions of deformation and recrystallisation in vein quartz

Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P– T– d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by ( L ≫ S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO 2 inclusions by preferential loss of H 2O. Those inclusions armoured within garnet retained their primary 3-phase H 2O–CO 2 compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H 2O leakage along dislocations and planar lattice defects which results in the predominance of CO 2 inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO 2-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear. This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P– T conditions, are a welcome supplement for the reconstruction of a rather detailed P– T– d path.

Shock reequilibration of fluid inclusions in Coconino sandstone from Meteor Crater, Arizona

This study examines the effects of natural shock metamorphism on fluid inclusions trapped in porous sedimentary target rocks and compares these results to previous experimental work on single crystal quartz. Samples of shock metamorphosed Coconino sandstone were collected from Barringer Meteorite Crater (Meteor Crater, Arizona) and classified based on their shock features into the six shock stages described by Kieffer [S.W. Kieffer, 1971. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona, Journal of Geophysical Research 76, 5449-5473.]. The frequency of two-phase fluid inclusions decreases dramatically from unshocked samples of Coconino sandstone through shock stages 1a, 1b, and 2. No two-phase fluid inclusions were observed in shock stage 3 or 4 samples. However, the total number of grains containing fluid inclusions remains approximately the same for shock stages 1a–2, suggesting that two-phase fluid inclusions reequilibrated during impact to form single-phase inclusions. In shock stages 3 and 4, the total number of inclusions also decreases, indicating that at these higher shock pressures fluid inclusions are destroyed by plastic deformation and phase changes within the host mineral. Entrained quartz grains within a shock stage 5 sample contain two-phase inclusions, emphasizing the short duration of melting associated with the impact and the heterogeneous nature of impact processes. These results are similar to those observed in single-crystal experiments, although inclusions survive to slightly higher shock pressures in samples of naturally shocked Coconino sandstone. Results of this study suggest that the rarity of fluid inclusions in meteorites does not preclude the presence of fluids on meteorite parent bodies. Instead, fluid inclusions trapped during alteration events may have been destroyed due to shock processing. In addition, loss of fluids from inclusion vesicles along fractures and microcracks may lead to shock devolatilization, even in unsaturated target rocks.

Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Ramepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz–tourmaline–sulphide veins located mainly within an intensively altered and sheared zone about 20–30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H2O–CH4 (≤9 equiv. wt% NaCl), and (2) H2O–CO2 (≤12 equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310–350 and 220–250 °C respectively. Both types of fluid inclusions contain 1–3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH4) to a more oxidising (CO2) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.

Fluid inclusions in sedimentary and diagenetic systems

Some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals. Important fluids in the sedimentary realm include atmospheric gases, fresh water of meteoric origin, lake water, seawater, mixed water, evaporated water, formation waters deep in basins, oil, and natural gas. Preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global-change research, is essential for improving understanding of diagenetic systems, and provides useful information in petroleum geology. Applications of fluid inclusions to sedimentary systems are not without their complexities. Some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration, which results in stretching, or leakage and refilling, of some fluid inclusions. Similarly, overheating in the laboratory can also cause reequilibration of fluid inclusions, so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating. Natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment, fluid inclusion assemblages (FIA). Consistency in homogenization temperatures within a fluid inclusion assemblage, consisting of variably sized and shaped inclusions, is the hallmark of a data set that has not been altered through thermal reequilibration. In contrast, fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration. If a fluid inclusion assemblage has not been altered by thermal reequilibration, its fluid inclusions may be useful as geothermometers for low- and high-temperature systems, or useful as geobarometers applicable throughout the sedimentary realm. If a fluid inclusion assemblage has been altered partially by thermal reequilibration, techniques for distinguishing between altered and unaltered fluid inclusions may be applied. In studies of global change, fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments. Fluid inclusions also preserve microsamples of ancient seawater and atmosphere, the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans. In petroleum geology, fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons; they can delineate the evolution of the chemistry of hydrocarbons; and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems. In studies of diagenesis, fluid inclusions can be the most definitive record. Most diagenetic systems are closely linked to temperature and salinity of the fluid. Thus, fluid inclusions are sensitive indicators of diagenetic environments.

Synthetic fluid inclusions. XV. TEM investigation of plastic flow associated with reequilibration of fluid inclusions in natural quartz

The nature and abundance of dislocations in quartz surrounding fluid inclusions were studied to obtain a better understanding of processes associated with fluid inclusion reequilibration. Synthetic fluid inclusions containing 10 wt% NaCl aqueous solution were formed in three samples at 700 °C and 5 kbar. One of the samples was quenched along an isochore to serve as a reference sample. The other two samples were quenched along a P-T path that generated internal pressures in excess of the confining pressure. The two samples were held at the final reequilibration P-T conditions of 625 °C and 2 kbar for 30 and 180 days, respectively. Following the experiments, microstructures associated with fluid inclusions were examined with the TEM. Quartz in healed fractures in the reference sample that was quenched isochorically shows a moderate dislocation activity. Quartz adjacent to reequilibrated fluid inclusions in the other two samples, however, showed a marked increase in dislocation activity compared to the un-reequilibrated sample. Deformation of the inclusion walls occurred anisotropically by expansion of mobile dislocations in their slip systems. Dislocation expansion was controlled by glide in the rhombohedral planes {1 0 1 1} that was restricted to narrow zones (≤3 μm) in the immediate vicinity of the fluid inclusion walls outside of the healed fracture plane. These plastic zones were observed after both short term (30 days) and long term (180 days) experiments and are attributed to hydrolytic weakening of quartz around fluid inclusions owing to diffusion of water into the quartz matrix during the experiment. The close spatial association of submicroscopic water bubbles with dislocations, and the rarity of water bubbles in the reference sample, show clearly that in both the 30 and 180 day experiments reequilibration involves water loss from the fluid inclusions. Our results indicate that synthetic fluid inclusions in this study recover (chemically and volumetrically), even at relatively fast experimental loading rates, such that internal stresses never reach the point of brittle failure. The driving force for fluid inclusion deformation involves two related mechanisms: plastic deformation of hydrolytically weakened wet quartz in the healed fracture, and water leakage associated with preexisting and strain-induced dislocations.

Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia

Fluid inclusions that are interpreted to be related to vein filling of quartz veins at the Three Mile Hill and Marvel Loch gold deposits in amphibolite-facies rocks, and the Griffins Find deposits in granulite-facies rocks in the Yilgarn Craton of Western Australia are dominantly low-salinity H 2O–CO 2±CH 4 mixtures or CO 2–CH 4 fluids. At each deposit, inclusions have a wide range of compositions with respect to both the aqueous:carbonic ratio and the CO 2:CH 4 ratio of the carbonic phase, and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock. Inclusion densities suggest a range of P–T conditions of entrapment, and are in general not consistent with the conditions of vein formation indicated by vein assemblages. Diffusional addition of H 2 into inclusions, diffusional loss of H 2O, and reduction of inclusion volume are possible during cooling and uplift along the inferred P–T path. The inclusion populations could have been derived from an originally uniform population of low-salinity, aqueous dominated H 2O–CO 2 inclusions by a combination of these processes. Inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition, and, to an extent, by variations in fluid salinity. If inclusion re-equilibration is the cause of inclusion variability, it was of variable intensity within a single vein system, and within individual clusters of inclusions in some samples, and is suggested to have been a function of the local petrological and textural environment.

Statistical microthermometry of synthetic fluid inclusions in quartz during decompression reequilibration

Fluid inclusion microthermometric data are often reported as homogenization temperature frequency histograms. Interpretation of such histograms for a single fluid inclusion assemblage (FIA) of non-reequilibrated fluid inclusions is usually straightforward and provides an accurate determination of the original density (Th) of that FIA. However, interpretation of such histograms for reequilibrated inclusions is more problematic. Decompression experiments using synthetic inclusions in natural quartz and conducted at 2–5 kbar and 600–700 °C with a maximum internal overpressure of 2 kbar indicate that histogram shape reflects the sample's P-T history. Our results further indicate that the mean, mode, range, standard deviation, extreme values, etc., all have a significance with respect to the P-T history of the sample. Thus, a mound-shaped, unimodal histogram with low range is indicative of a nearly isochoric cooling P-T path. A unimodal histogram that is slightly skewed to the right, and with a low standard deviation but high range, results from inclusion deformation in the plastic regime (high temperature/low strain rates). Fluid inclusions deformed plastically show no correlation between size and density. Histogram outliers should not be ignored and may be used to determine an isochore that passes close to the conditions of entrapment (minimum Th) or close to the final reequilibration conditions (maximum Th). The histogram mean Th value corresponds to an isochore that represents the internal overpressure (about 1 kbar) that can be maintained over geologic time by a majority of reequilibrated fluid inclusions. A multimodal histogram with high range and high standard deviation indicates inclusion brittle deformation (low temperature/high strain environments). Fluid inclusions deformed in a brittle manner show strong positive correlation between size and density. Histograms produced in the laboratory show many similarities to histograms for natural samples, offering the hope that laboratory results may be used to interpret P-T histories of natural samples.

Fluid-Deposited Graphitic Inclusions in Quartz: Comparison Between KTB (German Continental Deep-Drilling) Core Samples and Artificially Reequilibrated Natural Inclusions

We used Raman microsampling spectroscopy (RMS) to determine the degree of crystallinity of minute (2–15 μm) graphite inclusions in quartz in two sets of samples: experimentally reequilibrated fluid inclusions in a natural quartz grain and biotite-bearing paragneisses from the KTB deep drillhole in SE Germany. Our sequential reequilibration experiments at 725°C on initially pure CO 2 inclusions in a quartz wafer and the J. Krautheim 1993experiments at 900–1100°C on organic compounds heated in gold or platinum capsules suggest that, at a given temperature, (1) fluid-deposited graphite will have a lower crystallinity than metamorphosed organic matter and (2) that the crystallinity of fluid-deposited graphite is affected by the composition of the fluid from which it was deposited. We determined that the precipitation of more-crystalline graphite is favored by lower f H 2 (higher f O 2), and that the crystallinity of graphite is established by the conditions (including gas fugacities) that pertain as the fluid first reaches graphite saturation. Graphite inclusions within quartz grains in the KTB rocks show a wide range in crystallinity index, reflecting three episodes of carbon entrapment under different metamorphic conditions. Isolated graphite inclusions have the spectral properties of totally ordered, completely crystalline graphite. Such crystallinity suggests that the graphite was incorporated from the surrounding metasedimentary rocks, which underwent metamorphism at upper amphibolite-facies conditions. Much of the fluid-deposited graphite in fluid inclusions, however, shows some spectral disorder. The properties of that graphite resemble those of experimental precipitates at temperatures in excess of 700°C and at elevated pressures, suggesting that the inclusions represent precipitates from C-O-H fluids trapped under conditions near those of peak metamorphism at the KTB site. In contrast, graphite that is intimately associated with chlorite and other (presumably low-temperature) silicates in inclusions is highly disordered and spectrally resembles kerogens. This graphite probably was deposited during later greenschist-facies retrograde metamorphism at about 400–500°C. The degree of crystallinity of fluid-deposited graphite is shown to be a much more complex function of temperature than is the crystallinity of metamorphic graphite. To some extent, experiments can provide temperature-calibration of the crystallinity index. However, the difference in time scales between experimental runs and geologic processes makes it difficult to infer specific temperatures for naturally precipitated graphite.

Thermochronological and Petroleum Generation Studies next to Dikes in the Upper Jurassic-Lower Cretaceous Strzelecki Group, Onshore Gippsland Basin, Australia: ABSTRACT

Fluid inclusion, apatite fission track analysis (AFTA[reg sign]) annealing, mean random vitrinite reflectance (Rv-r) data and heat flow modeling studies of nine basalt dikes ranging from 0.06 m to 40 in thick in the Strzelecki Group were used to constrain the conditions of contact metamorphism of coaly sedimentary organic matter (SOM) to investigate it as an extended-duration natural analog to hydrous pyrolysis experiments. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a paleotemperature of 100 to 120[degrees]C. Reequilibrated fluid inclusions in the host rocks next to thin dikes (< 3.4 m thick) suggest maximum temperature (T[sub peak]) systematically increases towards the dike margin to around 550[degrees]C. These T[sub peak] data are in fair agreement with those predicted by a conductive heat flow model and measured Rv-r. AFTA indicates the time of uplift and cooling as mid to Late Cretaceous and that paleotemperatures were too high for heating duration information to be recorded by annealing of tracks in apatite near the dikes. The estimated T[sub peak] next to some larger dikes (20 to 40 m thick) as indicated by an extended zone of elevated Rv-r did not fit the pattern seen next tomore » the thin dikes. Heat flow modeling along with whole rock elemental and isotopic data suggest that the extended zone of increased Rv-r is caused by convection of heated fluids. This is in contrast to the apparent advection effects found adjacent to thin dikes. Laboratory hydrous pyrolysis compares well with contact metamorphism next to thin dikes with respect to the composition of the free hydrocarbons except in samples very close to the dike. Close to the dike, fluid inclusion evidence indicates a water vapor or super-critical phase in the rock making the organic geochemistry results diverge from.« less

Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction

Experimental reequilibration of synthetic 10 wt% NaCl-H2O inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the P-T path followed by the inclusion after formation. These differences combined with other geological information may be used to determine whether the sample (rock) followed a dominantly isothermal or isobaricP-T path following entrapment. The intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of theP-T path from the original isochore for the inclusion. Thus, fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradeP-T path, but also the magnitude of departure of this path from one that is isochoric.