Fluid Inclusion Thermometry Research Articles

What is the source of magnesium in hydrothermal dolomites? New insights from coupling δ26Mg - ∆47 isotopes

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18Ofluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18Ofluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆47-derived temperatures, it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids.

Continuous thermal structures of the present-day and contact-metamorphic geothermal systems revealed by drill cuttings in the Kakkonda geothermal field, Japan

Assessing the thermal structure of a geothermal reservoir and its history is of critical importance when evaluating the energy potential and heat source of the reservoir. However, the present and past thermal structures of reservoirs are not easily obtained due to temperature perturbations during drilling and/or the sparse sampling of the reservoir in rock cores. We examined drill cuttings collected from Well-20 of the Kakkonda geothermal reservoir that penetrated to a depth of ∼2700 m and into the Quaternary Kakkonda Granite, which is thought to be the heat source of the Kakkonda geothermal reservoir. Here we provide the first comprehensive report on the compositions of minerals from the Kakkonda geothermal reservoir from the Kakkonda Granite into its contact metamorphic aureole. The minimum temperature for each depth obtained by chlorite thermometry systematically changes within a range of 265–369 °C at depths of 1510–2700 m, and the temperatures are consistent with fluid inclusion thermometry and the temperature of the reservoir undisturbed by drilling. Our method provides a rapid, spatially continuous, and simple way of estimating the past or present thermal profile of a reservoir. The maximum temperature for each depth obtained by biotite thermometry shows a continuous thermal profile for the contact metamorphism with temperatures reaching ∼760 °C at the granite/host-rock interface at a depth of ∼2700 m, and gradually decreasing to ∼630 °C at a depth of ∼1700 m. These results suggest that the thermal effect of the Kakkonda Granite on the surrounding lithologies was much larger than estimated previously. Such high contact metamorphic temperatures require effective heat transfer from the Kakkonda Granite, probably enhanced by magma convection in the magma chamber and fluid convection within the supercritical geothermal reservoir.

In situ trace element compositions of sulfides constraining the genesis of the worldclass Shuangjianzishan Ag-Pb-Zn deposit, NE China

The worldclass Shuangjianzishan Ag-Pb-Zn deposit, containing proven reserves of >21,665 t of silver @128.5 g/t, >3.3 Mt of lead @1.6 wt%, and >1.9 Mt of zinc @0.6 wt%, is the largest silver deposit in Asia. This giant Ag mineralization system has attracted extensive attention since its discovery. However, the incorporation mechanism of Ag in ore sulfides and the deposit genetic type are still greatly debated. We have conducted in situ LA-ICP-MS trace element analyses for sulfides in this deposit to address the above issues. Three paragenetic stages of mineralization were recognized at Shuangjianzishan, including pyrite + quartz + K-feldspar stage (I), galena + sphalerite + Ag-bearing sulfosalt + quartz + calcite + chlorite + sericite stage (II), and post-ore quartz stage (III). Most sulfides and silver minerals were deposited in stage II. In situ time-resolved depth profiles, principal component analysis (PCA), and correlation plots of trace element for sulfides indicate that trace elements are dominantly present in sulfides as solid solutions and nano-inclusions, and less as micro-inclusions. For silver, the most critical element in the studied deposit, its substitution mechanisms vary largely among sphalerite, pyrite, and galena. In sphalerite, Ag enters the crystal lattice associated with coupled substitution of Zn by Fe, Sn, Sb, and Ga. In pyrite, Ag enters the lattice through the coupled substitution of Fe by Cu, Zn, and Sn. In galena, Ag enters lattice by the coupled substitution of Pb by Sb and Zn. Based on the GGIMFis geothermometer calculations, the temperatures for ore stage II range from 290° to 333 ℃ (mean of 319 ℃), which are almost identical to the results of the sulfur isotope geothermometry and fluid inclusion thermometry from the previous studies. Furthermore, the trace element distributions in the Shuangjianzishan sulfides, e.g., the enrichment of Fe, Mn, Sn, Cu, Ag, Sb and Pb and the depletion of Ga, Ge, Co and In in sphalerite, as well as the geological and geochemical features, infer that the studied deposit is similar to the typical epithermal type deposit. We therefore propose that the worldclass Shuangjianzishan Ag-Pb-Zn deposit is an epithermal deposit, and this contribution indicates that in situ LA-ICP-MS trace element analysis of sulfides could help shed light on identifying the genetic type for the hydrothermal ore deposits.

Characteristics and origin of the tight grainstone reservoirs of the Yijianfang Formation, middle Ordovician in the Shunnan area, Tarim Basin

Tight carbonate reservoirs are widely distributed in the world and have great economic exploration potential. The Ordovician Yijianfang Formation grainstones in the Shunnan area, Tarim Basin are considered to be hydrocarbon reservoirs. However, there are few visible pores from thin sections and cores, resulting in poor understanding of the pore systems in these tight limestone reservoirs. Core samples collected from 8 wells were prepared to characterize pore networks by combining the optical microscope, scanning electron microscope, helium gas porosimeter, nuclear magnetic resonance and micro-CT analysis. Carbon and oxygen isotopes and fluid inclusion thermometry were adopted to determine the timing of pore-related dissolution. For Yijianfang grainstones, porosity mostly ranges from 0.2 to 3.9%; permeability mostly ranges from 0.01 to 1 mD. Petrographic observations revealed that both intragranular macropores and micropores within grains constitute effective reservoir space. Besides, micropores could account for 50–90% of the total porosity. The intragranular pores only appear in the upper part of the cycle, and the micropores developed in upper and middle of the cycle. There are no pores in the lower part of the cycle. During brief subaerial exposure shortly after deposition, meteoric dissolution created intragranular pores and vugs in grainstones in upper part of the sedimentary cycle. Meanwhile, influenced by the freshwater-involved mineralogical stabilization, micropores transformed from between micrites to between calcite microcrystals within grains in grainstones in the upper and middle part of the cycle. In the burial diagenetic environment, vugs and interparticle pores were destroyed by compaction and burial cements. For packstones in the lower part of the cycle, mineralogical stabilization occurred without freshwater during progressive burial, leading to tight fabric with no or minor micropores. In summary, whether tight limestones can be effective reservoirs depends on whether freshwater is involved in dissolution and mineralogical stabilization. In general, tight grainstone reservoirs of shallowing-upward cycle could provide abundant micropores with few macropores due to the freshwater-involved dissolution and stabilization during subaerial exposure.

A tale of three fluids: Fluid-inclusion and carbonate clumped-isotope paleothermometry reveals complex dolomitization and dedolomitization history of the Latemar platform

Abstract This work focuses on an exceptionally complex natural laboratory, the Triassic Latemar isolated platform in the Dolomite Mountains of northern Italy. It explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization, and the dedolomitization of marine limestones. Rock samples were studied using dual fluid-inclusion thermometry and clumped-isotope thermometry. Independent of the spatial position at Latemar, Δ47 clumped-isotope and fluid-inclusion data provide contrasting paleotemperature estimates. An apparent lack of systematic patterns in fluid-inclusion data (homogenization temperature, salinity, density) results from analyses of micrometer-sized growth zones within a single crystal. The composition of the individual fluid inclusions represents a “snapshot” of fluid mixing with variable endmember elemental ratios. The bulk crush-leach data and slopes in Caexcessversus Nadeficit diagrams indicate different water–rock interactions and fluid signatures with evaporation sequences and crystalline rocks. The presence of three fluid types (crystalline basement brine, halite-dissolution brine, seawater) in all carbonates suggests that all fluids coexisted during contact metamorphism and dolomitization of Latemar carbonates. Non-equilibrium processes overruled thermodynamic controls on the precipitation of diagenetic phases. Fluid mixing resulted in the precipitation of two complex carbonate successions. The Δ47 data represent bulk temperatures, averaging the mixing ratio of fluids with different temperatures and their respective volume. Fluid-inclusions record patterns of remarkable complexity and shed light on the complexity of a multi-fluid system. Data shown here provide answers to the controversial interpretation of dolomitizing fluid temperature in the Latemar and exemplify the strengths of multi-proxy paleotemperature studies.

Origin of the dolomite in the Buqu Formation (Mid-Jurassic) in the south depression of the Qiangtang Basin, Tibet: Evidence from petrographic and geochemical constraints

The Qiangtang Mesozoic sedimentary basin is a new field of hydrocarbon exploration, in which the Buqu Formation dolostone reservoirs have attracted increasing attention in recent years. To determine the origin of these reservoirs, petrographic study, fluid inclusion thermometry dating, and C-O and Sr isotopic dating were performed. The results revealed the genesis and evolution of different types of dolomite matrix and cement, as follows: 1) The dolomite texture in the study area is closely related to its formation environment and process and can be categorized as primary fabric well-preserved dolomite (Rd1), primary fabric poorly-preserved dolomite, and dolomite filling. The primary fabric-poorly-preserved dolomite includes fine-grained euhedral dolomite (Rd2), fine-grained planar subhedral dolomite (Rd3), and medium-to coarse-grained anhedral dolomite (Rd4). The dolomite filling includes fine-grained planar subhedral cave-filling dolomite (Cd1) and medium-to coarse-grained anhedral, saddle dolomite (Cd2). 2) Rd1 has δ13C‰PDB of 3.42‰–4.23‰, δ18O‰PDB from −4.22‰ to −3.37‰, and 87Sr/86Sr of 0.707654–0.708176 and was formed in the contemporaneous or penecontemporaneous stage at low temperatures by mimic replacement related to seawater evaporation. Abundant supersaturated dolomitization fluids favored the preservation of its primary dolomite texture. 3) Rd2 has δ13C‰PDB of 3.18‰–4.11‰, δ18O‰PDB from −4.56‰ to −4.23‰, and 87Sr/86Sr (0.707525–0.708037), while Rd3 has δ13C‰PDB of 2.72‰–4.42‰, δ18O‰PDB from −6.57‰ to −5.56‰, and 87Sr/86Sr of 0.707432–0.707990. Both were formed at low temperatures in the shallow-burial stage, when the dolomitization fluid was mainly derived from seawater. Excessive dolomitization during the late shallow-burial stage caused the destruction of the dolomite crystals from euhedral to subhedral. 4) Rd4 has δ13C‰PDB of 3.24‰–4.14‰, δ18O‰PDB from −8.22‰ to −6.37‰, and 87Sr/86Sr of 0.707234–0.707884 and resulted from dolomitization or recrystallization at high temperatures in the medium-to deep-burial stage. The crystal curvature was caused by high environmental temperatures. 5) Cd1 has δ13C‰PDB of 3.02‰, δ18O‰PDB of −5.13‰, and 87Sr/86Sr of 0.708147 and was formed during cavern filling before the shallow-burial stage. Cd2 has δ13C‰PDB of −0.09‰−3.38‰, δ18O‰PDB from −10.41‰ to −8.56‰, and 87Sr/86Sr of 0.708180–0.708876 and was related to the collisional orogeny between the Lhasa termite and the Qiangtang Basin in the late Early Cretaceous. Fluids in the overlying and underlying clastic strata of the Buqu Formation were driven by the thermal hot spot during compressional tectonic setting. These fluids caused negative shifts in the oxygen isotope compositions of the earlier dolomite.

The maturity of Silurian Longmaxi shale in Jiaoshiba, Sichuan Basin: as revealed by laser Raman spectroscopy

Accurate estimates of the thermal maturity of organic matter are important in studies of shale oil and gas, as these data are directly related to the genesis type of shale gas, adsorption capacity of shale and formation of organic pores in organic-rich shale. Longmaxi shale is a major shale gas exploration target in South China and a typical overmature shale gas play. Because Longmaxi shale is Silurian marine strata without vitrinites, it is difficult to determine its maturity accurately. In order to evaluate the maturity of the Longmaxi shale exactly, in this paper, solid bitumen reflectance, basin modeling and laser Raman spectroscopy analyses of solid bitumen were carried out on the shale. The solid bitumen reflectance of Longmaxi shale is range of 2.77–4.14% with a mean of 3.5%, and the corresponding equivalent vitrinite reflectance (EqVRo) is 2.6%; additional information about the maturation is provided by the basin modeling with the constraint of the maximum paleo-geothermal about 210–220 °C which was indicated by thermo-acoustic emission measurement, fluid inclusion thermometry and equivalent vitrinite reflectance. In addition, the Raman was applied to Longmaxi shale in Jiaoshiba firstly, and the Raman inter-peak interval between peak G (graphitic band) and peak D (disordered band) is range of 270–279 cm−1, which suggests that equivalent vitrinite reflectance is about 2.67% and is consistent with the results of solid bitumen reflectance, infrared spectrum and basin simulation. Approximate conclusions have been drawn from these different methods, and the thermal evolution of Longmaxi shale in Jiaoshhiba area, the largest shale gas field in China, was estimated at approximately 2.5–2.7%, which means that it has reached the overmature dry gas stage. Overall, the findings also imply that Raman spectra of pyrobitumen can be used effectively to evaluate the maturation of the marine shale without vitrinites.

Mg and Sr isotopic evidence for basin wide alteration of early diagenetic dolomite in the Williston Basin by ascending crustal fluids

Dolomite is a predominantly diagenetic mineral that forms by replacing marine calcite and aragonite during burial. Dolomitization requires subsurface fluid flow to deliver Mg and remove product Ca, and only forms abiotically in laboratory experiments at temperatures above ~100°C, equivalent to burial depths of 3 km or greater. Limited sources of Mg at these depths would appear to restrict most dolomite formation to shallow burial depths using seawater as a source of reactive Mg, despite the kinetic limitation imposed by lower temperatures. The Williston Basin is replete with evaporites and dolomites that appear to have formed using Mg from evaporatively concentrated seawater at near surface temperatures. However, two dolomite bodies examined in this study record basin wide gradients in 87Sr/86Sr ratios and δ26Mg values that are inconsistent with dolomitization by seawater. We evaluate the processes that may have created these gradients. We propose a model whereby seepage reflux created early diagenetic protodolomite that was later altered during deep burial by 87Sr and Mg-enriched fluids ascending from the deep structural center of the basin with the degree of alteration decreasing towards the edges. The ascending fluid flow event is tentatively linked to anomalous heating in the Late Devonian / Early Carboniferous. Vertical faults are hypothesized to have behaved as conduits for channeling 87Sr and Mg-enriched crustal fluids upwards into the bottom of the Williston Basin, pressurizing the basal aquifer system and commencing up-dip directed fluid flow through confined aquifers. Clumped isotope and fluid inclusion thermometry indicate that the ascending fluids were hydrothermal with an estimated δ26Mg value of –0.65 ±0.20‰, which is much lower than estimates of early Paleozoic seawater δ26Mg values of +0.09 to +0.23‰. Two sources of crustal Mg are consistent with the low inferred δ26Mg values of the ascending fluid: (1) chlorite, which preferentially releases isotopically light Mg, with literature values in the range of –0.78‰ to –1.8‰, and (2) magnesite, which forms during carbonation of serpentinite with literature values in the range –0.7‰ to –1.1‰. Ultramafic rocks belonging to the Thompson Nickel Belt pass beneath the center of the Williston Basin in North Dakota, indicating that dissolution of both chlorite and magnesite may have contributed reactive Mg to the Williston Basin during the ascending fluid flow event. As dolomite is a mineral that is easily altered during burial, our findings warrant caution in choosing dolomites for reconstructing the 26Mg history of seawater.

Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCl eq. in subhorizontal veins and 6–0 wt% NaCl eq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO 2 and CH 4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide. Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031

Quartz overgrowth textures and fluid inclusion thermometry evidence for basin‐scale sedimentary recycling: An example from the Mesozoic Barents Sea Basin

AbstractSedimentary recycling has the potential to obscure source‐to‐sink relationships, provenance interpretations, burial history reconstructions and robust reservoir quality predictions in siliciclastic sedimentary basins. Here, we integrate petrographic and cathodoluminescence microtextures with fluid inclusion thermometry in quartz overgrowths to identify sedimentary recycling and to constrain the potential provenance candidate for recycled grains in Lower Mesozoic sandstone of the western Barents Sea basin. Four diagenetic imprints were recognized as proof of sediment recycling: (a) microtextural surface properties of overgrowths, (b) the presence of overgrowths at sutured grain contacts, (c) reversed diagenetic sequences and (d) fluid inclusions within quartz overgrowths. The diagenetic imprints confirm delivery of recycled sediments across the western Barents Sea basin. Their widespread distribution across the basin suggest that the recycled grains were derived from a drainage basin with regional‐scale sediment dispersal potential during the latest Triassic. Furthermore, the drainage basin must have contained sedimentary rocks. Prior to surface exposure, the precursor sedimentary basin was subjected to burial temperatures exceeding 130°C, whereby syntaxial quartz overgrowths precipitated. This temperature indicates an uplift of around 3–4 km, which represents a significant tectonic event. Recycled quartz grains can provide insights on their provenance as they retain direct temperature records. The geothermal signatures and geographically widespread distribution of recycled quartz exclude spatially restricted intrabasinal highs and higher‐temperature crystalline rocks as provenance candidates for the recycled grain portion. Our data support the contemporaneous Novaya Zemlya Fold and Thrust Belt as the most likely provenance candidate in the region. The integrated approach demonstrated herein can be used to constrain sediment recycling and partly eroded provenance candidates in sedimentary basins of equivalent setting worldwide, particularly in quartz‐rich strata susceptible to sediment supply from older uplifted sedimentary basins.

Insights into the structural evolution of the pre-Variscan rocks of the Eastern Pyrenees from La Molina quartz veins; constraints on chlorite and fluid inclusion thermometry

Quartz veins hosted in the infra and overlying series to the (Sardic) Upper Ordovician Unconformity provide new insights into the structural and thermal evolution of the pre-Variscan rocks of the Eastern Pyrenees. In the La Molina area (Canigó massif), two generations of metric quartz veins (V1 and V2) are distinguished by their distribution patterns and their relationships to the deformational macro, meso, and microstructures. P-T formation conditions are obtained by combining chlorite geothermometry and fluid inclusion microthermometry data. Discrepancy on formation temperature for chlorites located at different positions within the veins are discussed, concluding that veins grew in a low fluid/rock ratio regime. V1 veins can be related to the Late Ordovician syn-sedimentary faulting episode as revealed by their distribution patterns, formation mechanisms, and fluid-rock interactions. We propose an Alpine age for the V2 veins, based on their structure and the emplacement conditions of 318 ± 12°C and 2.4 ± 0.2kbar, with an estimated geothermal gradient of 34°C∙km-1 and a burial depth of ca. 9km. Results obtained here are compared with other quartz veins spread throughout the Paleozoic basement of the Eastern Pyrenees.

Genesis of the Xiaotongjiapuzi gold deposit of the Liaodong gold province, Northeast China: Fluid inclusion thermometry and S–Pb–H–O–He isotope constraints

The Xiaotongjiapuzi gold deposit is located in the Liaodong gold province, NE China. The deposit is hosted in the metamorphic rocks of the Proterozoic Liaohe Group, consisting of marble, schist, and granulite. Three alteration/mineralisation stages were identified, namely (a) quartz–pyrite, (b) quartz–polymetallic sulfides, and (c) quartz–carbonate. Four fluid inclusion types were distinguished: liquid‐rich two‐phase, gas‐rich two‐phase, CO2‐bearing, and pure CO2. Stage 1 quartz contains liquid‐rich two‐phase inclusions with salinity of 5.9–14.3 wt.% NaCl equiv. and homogenisation temperatures of 311°C–408°C. Stage 2 quartz contains all four inclusion types, of which the liquid‐rich two‐phase, gas‐rich two‐phase, and CO2‐bearing inclusions yielded homogenisation temperatures of 268°C–376°C and salinity of 4.1–13 wt.% NaCl equiv. In stage 3 quartz, only liquid‐rich two‐phase inclusions were found, which contain relatively low salinity (1.6–7.6 wt.% NaCl equiv.) and homogenisation temperatures (201°C–254°C), indicating an H2O–NaCl–CO2 system for ore precipitation. Stage 2 δ18Ow and δD values of quartz minerals are of 0.3‰ to 2.3‰ and −100‰ to −96‰, respectively. Hydrogen and oxygen isotopic compositions indicate that the ore fluids were a mix of magmatic fluid and meteoric water. δ34S values of the metallic sulfides from the Xiaotongjiapuzi deposit range from +4.6‰ to +12.9‰, whilst Pb isotopes show small variations: 206Pb/204Pb = 17.671 to 18.361, 207Pb/204Pb = 15.569 to 15.659, and 208Pb/204Pb = 37.695 to 37.937. Sulfur and lead isotopes suggest that the ore‐forming materials were derived from the Liaohe Group metamorphic rocks and the Triassic granitoids. 3He/4He and 40Ar/36Ar ratios of the fluid inclusions derived of ore‐bearing pyrite ranges from 0.27 to 0.53 Ra and 362.2 to 1113.1, respectively, implying that both mantle‐ and crust‐derived fluids were involved in the gold mineralisation. The Xiaotongjiapuzi deposit could be a mesothermal gold deposit, and the fluid phase separation or immiscibility is the dominant mechanism for the deposition of ore‐forming materials.

Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

Abstract. We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronometry and building on the results of recent work on the tectonic evolution. Because the Oman mountains experienced only weak post-obduction overprint, they offer a unique natural laboratory for this study. Thermal maturity data from the Adam Foothills constrain burial in the basin in front of the advancing nappes to at least 4 km. Peak temperature evolution in the carbonate platform under the ophiolite depends on the burial depth and only weakly on the temperature of the overriding nappes, which have cooled during transport from the oceanic subduction zone to emplacement. Fluid-inclusion thermometry yields pressure-corrected homogenization temperatures of 225 to 266 ∘C for veins formed during progressive burial, 296–364 ∘C for veins related to peak burial, and 184 to 213 ∘C for veins associated with late-stage strike-slip faulting. In contrast, the overlying Hawasina nappes have not been heated above 130–170 ∘C, as witnessed by only partial resetting of the zircon (U-Th)/He thermochronometer. In combination with independently determined temperatures from solid bitumen reflectance, we infer that the fluid inclusions of peak-burial-related veins formed at minimum pressures of 225–285 MPa. This implies that the rocks of the future Jebel Akhdar Dome were buried under 8–10 km of ophiolite on top of 2 km of sedimentary nappes, in agreement with thermal maturity data from solid bitumen reflectance and Raman spectroscopy. Rapid burial of the passive margin under the ophiolite results in sub-lithostatic pore pressures, as indicated by veins formed in dilatant fractures in the carbonates. We infer that overpressure is induced by rapid burial under the ophiolite. Tilting of the carbonate platform in combination with overpressure in the passive margin caused fluid migration towards the south in front of the advancing nappes. Exhumation of the Jebel Akhdar, as indicated by our zircon (U-Th)/He data and in agreement with existing work on the tectonic evolution, started as early as the Late Cretaceous to early Cenozoic, linked with extension above a major listric shear zone with top-to-NNE shear sense. In a second exhumation phase the carbonate platform and obducted nappes of the Jebel Akhdar Dome cooled together below ca. 170 ∘C between 50 and 40 Ma before the final stage of anticline formation.

A renewed look at calcite cement in marine-deltaic sandstones: the Brent Reservoir, Heather Field, northern North Sea, UK

Abstract Wireline and seismic acoustic impedance imaging show that the marine part of the clastic Brent Group reservoir in the Heather Field, northern North Sea, contains much calcite cement in the flank parts of the structure. The non-marine Ness Formation and crest parts of the structure contain negligible calcite cement. This localized calcite cement has led to relatively poor reservoir performance since first oil in 1978, although a new suite of wells has boosted production with plans to keep the field active until 2030. Understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location. We have thus used a suite of techniques, including standard point counting, SEM-EDS mineralogy, BSE microscopy, fluid inclusion thermometry and stable isotope analysis, to develop new and improved models of calcite distribution. Calcite seems to have attributes of both early and late diagenetic cement. A 30–40% intergranular volume in calcite cemented beds seems to support pre-compactional growth but high-temperature fluid inclusions and the presence of primary oil inclusions suggest late growth. Much calcite may have developed early but it seems to have recrystallized, and possibly undergone redistribution, at close to maximum burial or had a late growth event. Calcite cement probably originated as marine-derived micrite, bioclasts or early marine cement but adopted the isotopic characteristics of high-temperature growth as it recrystallized. Quartz grains have corroded outlines in calcite-cemented areas with one sample, with 79% calcite cement, displaying signs of nearly total replacement of quartz grains by calcite. The flank localization of calcite cement remains to be explained, although it could be due to primary depositional factors, early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions. Controversially, the growth of calcite seems to be associated with quartz dissolution, although the geochemical and petrophysical cause of this remains obscure. Diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in Heather sandstones.

Reservoir quality, natural fractures, and gas productivity of upper Triassic Xujiahe tight gas sandstones in western Sichuan Basin, China

The main target for hydrocarbon exploration in the western Sichuan Basin in China is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Sandstones of the Xujiahe Formation are characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. In this study, various techniques, such as scanning electron microscopy, X-ray diffraction, cathodoluminescence microscopy, casting thin-section, and fluid inclusion thermometry, were employed to investigate the composition, sedimentary facies, diagenesis history, diagenetic intensity, natural fracture characteristics, and their impact on reservoir quality. The results showed that subaqueous distributary channels and mouth bars with low rock fragment content are favorable for pore preservation and secondary pore development. Except for the medium to strong dissolution intensity, chlorite rim growth is also important for the development of high quality reservoirs. Intraformational shear fractures and horizontal shear fractures associated with tectonic movement increase pore space and improve the capability of transmitting fluids. Differences in gas productivity could be attributed to differences in reservoir quality, which were controlled by the sedimentary environment, diagenetic intensity, and fracture characteristics. A significantly positive correlation was found between total porosity and gas productivity of the tight sandstones. The high density fracture distribution was found owing to the fault system and structural deformation zone, which resulted in high gas productivity. The research results were successfully applied to improve production efficiency in the Xujiahe tight gas sandstones, and could be applicable to other similar fields involving tight gas.

Coupling Δ47 and fluid inclusion thermometry on carbonate cements to precisely reconstruct the temperature, salinity and δ18O of paleo-groundwater in sedimentary basins

Diagenetic minerals may provide information about the burial history of geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of sedimentary basins. Clumped isotope, or Δ47, thermometry on carbonates opens a new avenue for interpreting carbonate formation temperature and thermal history of rocks. Yet, most of current knowledge on Δ47 systematics has been acquired via theoretical or experimental studies with only limited validation by the rock-record at geological conditions/timescales. Here, we investigate calcitic and dolomitic cements representative of three genetically different cementation phases from a well-documented mineral paragenesis of a carbonate unit (Middle Jurassic, Paris basin, France). We compare Δ47 with fluid inclusions microthermometry (FIM) data that were independently obtained from the same calcite and dolomite crystal specimens. The range of homogenization temperatures (Th) found for Cal1, Cal2 and Dol1 fluid inclusions fit remarkably well (i.e. within <5°C) with the temperatures determined from the Δ47 measurements (TΔ47), for a temperature range between 60 and 100°C and salinities between 0 and 15wt% NaCl eq. This provides a consistent rock-based validation of the experimentally determined Δ47 calibration with formation temperature for both calcite and dolomite mineralogy. Such findings also confirm the applicability of Δ47 thermometry in low temperature diagenetic environments (i.e., below circa. 100°C), which provides higher precision than FIM measurements (i.e., typical uncertainties of ±6°C with three Δ47 measurements) though significantly less time-consuming. Importantly, this study underlines how the coupling of both techniques can help to evaluate the degree of preservation of the original temperature information captured by either fluid inclusions or Δ47 compositions, and interpret each proxy as confidently/accurately as possible. Moreover, because both FIM and Δ47 measurements can provide independent constraints on the geochemistry of diagenetic paleofluids (via their salinity and δ18O composition), this study also highlights the benefits of coupling both techniques to further unravel the nature of paleofuids. Finally, we propose a practical guideline as a basis for future applications of combined FIM and Δ47 thermometry.

Oil shows geochemistry and fluid inclusion thermometry of Mid Cretaceous carbonates from the eastern Basque Cantabrian Basin (N Spain)

Abstract Surface liquid and solid hydrocarbons are common in Cretaceous carbonate exposures throughout the Basque Cantabrian Basin (N Spain). This study investigates the oil shows that appear in the upper Albian-lower Cenomanian reefal limestones cropping out in the eastern margin of the basin in order to assess potential source rocks and common characteristics with other oil occurrences in the basin. Specifically, the study focuses on (i) the petrography and fluid inclusion thermometry of the host rock and cements, and (ii) geochemistry of the hydrocarbons including biomarkers and isotopes. Geochemistry results indicate that the studied oils, in a way similar to previously reported hydrocarbons from the eastern Basque Cantabrian Basin, were generated from marine carbonate source rocks deposited in a hypersaline environment. However, their isotopic signatures are notably different, and thus discard a common source rock. Moreover, the studied oils are notably different from those reported elsewhere in the basin, both in the western and northern parts. The petrography of the calcite cements and the fluid inclusion thermometry suggests a scenario of condensates to light oils HC migration trapped in burial calcite cements at a minimum temperature of 174 °C. The generation, migration and entrapment of these oils took likely place between Turonian and Campanian time in close association with the elevated heat flow resulting from the rift extension. Following this stage, a second entrapment of heavy to asphaltene-rich oils occurred at 72 °C, likely during the Late Cretaceous to early Cenozoic uplift related to the early stages of inversion of the Basque Cantabrian Basin. The elevated temperature of the oil entrapment suggests the occurrence of abnormal geothermal gradients in the eastern Basque Cantabrian Basin during the middle Late Cretaceous, which is ultimately associated with the opening of the Bay of Biscay.

Thermobarometry in the Sarvian Fe-skarn deposit (Central Iran) based on garnet–pyroxene chemistry and fluid inclusion studies

The Sarvian Fe skarn deposit is located in the Urumieh–Dokhtar magmatic arc, western Iran. The Sarvian quartz diorite intruded the surrounding Permian to Tertiary limy formation, culminated in thermal metamorphism as well as skarnification in which the Sarvian deposit formed. Microthermometry studies in the Sarvian skarn deposit reveal two distinct inclusion groups; group A with medium–high temperature and hypersaline and group B with low–medium temperature and low salinity. Group A inclusions which are entrapped during formation of prograde are thought to be derived from the magmatic source. Fluid boiling and subsequent developing of hydraulic fracturing led to inflow and/or mixing of early magmatic fluids (group A) with circulating groundwater culminated in formation of low salinity and low temperature fluid inclusions (group B) during the formation of retrograde assemblage. Fluid inclusion thermometry reveals the formation temperature and the salinity of 300–370 °C and 31–33 wt% NaCl for the prograde stage and 180–230 °C and 1–15 wt% NaCl for the retrograde stage of skarnification at Sarvian skarn rocks. Fe-mineralization as well as hydrothermal minerals occurred during retrograde metasomatism. The estimated depth and pressure of occurrence for prograde stage are 1000–1200 m and 100–150 bars, and for retrograde stage, these are about 200 m and 50 bars, respectively. Garnet and pyroxene, as the main constituent minerals of prograde stage, are the most informative minerals offering a suitable tool to constrain the skarnification conditions. Garnets in the Sarvian deposit are mainly grossular and andradite, showing both normal and inverse zoning as the result of variation in their chemical composition. Such types of zoning represent alternation of high acidity oxidation and low acidity oxidation conditions that were prevailed on skarnification in the Sarvian prograde assemblage. Also, chemical composition of the Sarvian pyroxenes shows an alternation of high oxygen fugacity and low oxygen fugacity conditions for their formation. This is also supported by fluctuation of the ratios of andradite to grossular and diopside to hedenbergite, denoting to an obvious shifting that was prevailed between oxidizing and redox conditions during formation of prograde assemblage in the Sarvian. Garnet–pyroxene thermometry determines the formation temperature of prograde assemblage between 370 and 550 °C at Sarvian skarn rocks which is verified also by fluid inclusion thermometry. Decomposition of limestone by reaction of high-temperature hydrothermal fluids with carbonate host rock resulted in injection of CO2 into the Sarvian system that caused oxidation, changing Fe+2 to Fe+3 culminated in the magnetite deposition.

Comparison of clumped isotope signatures of dolomite cements to fluid inclusion thermometry in the temperature range of 73–176 °C

Widespread application of the novel clumped isotope paleothermometer (Δ47) using dolomite samples from shallow crustal settings has been hindered by a lack of adequate constraints on clumped isotope systematics in dolomites that formed at temperatures greater than 50°C. Consequently, many high-temperature applications involving diagenetic dolomites have required an assumption that the relationship between temperature and Δ47 in diagenetic dolomite resembles the theoretical temperature dependence for calcite. Here we present Δ47 results from dolomite cements for which precipitation temperatures were determined independently using fluid inclusion microthermometry. We compare a rock-based “calibration” for samples from the temperature range of ∼73 to 176°C to previously published laboratory-derived calibrations for synthetic calcites. This novel combination of approaches yields results that are broadly consistent with results reported from controlled laboratory experiments, providing an important confirmation of the utility of clumped isotopes in real-world systems. Our results suggest that the Δ47 of dolomite cements may provide key information in the reconstruction of burial and thermal histories and also in the recognition of potential petroleum reservoirs.

Constraining Temperatures of Formation and Composition of Dolomitizing Fluids In the Upper Devonian Nisku Formation (Alberta, Canada) With Clumped Isotopes

Abstract Three dolomite phases in the burial dolomitization system of the Nisku Formation in central Alberta (Canada) were measured for carbonate clumped isotopes. Independent temperature constraints using fluid inclusions and traditional oxygen isotope geothermometry allow us to test the application of this method using a new calibration curve extrapolated to higher temperatures. These three phases of dolomite correspond to three diagenetic stages in the burial history. The oldest phase is replacive gray matrix dolomite formed during relatively shallow burial depths at average clumped-isotope temperatures (Δ47T) of +80 ±7°C. Previous temperature estimates from standard oxygen isotope geothermometry ranged from 50 to 80°C. A calculated δ18O(H2O) of 3 ± 1.4‰ VSMOW points to precipitation from Devonian seawater modified by evaporation and/or water–rock interaction. The first phase of saddle dolomite cement formed by pressure-solution during intermediate burial depths yielded average Δ47T of 96 ±4°C (previously calculated at ~80°C) and δ18O(H2O) of 4 ± 1‰ VSMOW. A second phase of saddle dolomite formed in association with thermochemical sulphate reduction during deep burial records average Δ47T of 151 ±5°C, comparable to fluid inclusion temperatures ranging from 125 to 145°C. The average δ18O(H2O) of the fluid that formed this saddle dolomite phase is calculated as 8 ± 1‰ VSMOW, suggesting much more advanced water–rock interaction. Δ47T are consistent with previous temperature estimates from a combination of geological data conventional O-isotope and fluid-inclusion thermometry. Another important finding is that the Δ47 signatures are preserved even after reaching deeper burial and/or during uplift and cooling, which points to a closure temperature in dolomites in excess of 180°C. Thus, dolomite appears to be a better phase for clumped-isotope analysis than calcite for rocks that have been exposed to higher temperatures upon further burial. This study demonstrates the power of the clumped-isotope thermometer to reconstruct complex diagenetic histories in burial diagenetic environments where multiple generations of diagenetic cements can be analyzed individually.