Silica-rich Fluids Research Articles

Geophysical evidence of large-scale silica-rich fluid flow above the continental subduction interface

Geophysical evidence of large-scale silica-rich fluid flow above the continental subduction interface

Mechanism of Opal-CT deposit from the Lam Narai Volcanics in Lop Buri, Thailand by using Petrography

Common opals were discovered in the rhyolite host rock, where various structural features such as cavities, fractures, and veins are observed. These voids served as pathways for the infiltration of silica-rich fluids originating from meteoric water, establishing a hydrothermal connection. The aqueous solution derived from meteoric water actively dissolves silica from the rhyolite host rock, thereby increasing the silica saturation within the fluid. Subsequently, the silica-saturated hydrothermal fluid fills the cavities and veins within the host rock. The margin of the host rock underwent alteration as it interacted with infiltrating hydrothermal fluid in a phenomenon known as hydrothermal fluid-rock interaction. At the periphery of the cavities, quartz crystals begin to form, effectively reducing the concentration of dissolved silica in the solution. As a result, the subsequent precipitation of amorphous opals occurs, predominantly within the inner part or central region of the vein or cavity. The formation of opals associated with Lam Narai volcanic activity occurred at a relatively high temperature of about 105-170 °C through a hydrothermal process.

Silica cycling in Neoproterozoic oceanic lithosphere: A case study from Wadi Igla carbonate-serpentinite (southern Eastern Desert of Egypt)

Serpentinites play a pivotal role in carrying fluids and different elements into the Earth’s mantle. However, their role in exchanging silica (Si) between the marine environment and the mantle remains a matter of investigation. The Wadi Igla serpentinite (southern Eastern Desert of Egypt) formed at the expense of abyssal harzburgite by ∼15–22 % melting. It contains abundant Cr-spinel with sub-microscopic serpentine and chlorite-rich pores providing a base to explore Si cycling during serpentinization-carbonatization processes. The low-grade metamorphism of the harzburgite protolith started on the ocean floor, forming lizardite and chlorite (250–300 °C). Increasing the temperature (400–450 °C) caused the formation of brucite and antigorite. With the subduction in the fore-arc and the interaction with subducting sediments-related CO2-rich fluid, the Wadi Igla serpentinite underwent metasomatism, producing chlorite (300 °C), antigorite, tremolite, dolomite, and ferritchromite rims around Cr-spinel (Type 1), with brucite loss. In the upper greenschist-lower amphibolite facies (ca. 500 °C), CO2-rich hydrothermal fluids (with XCO2 of ∼0.55) penetrated a large volume of the protolith leading to full serpentinization together with abundant magnesite replacement. The resultant silica-rich fluids percolated in the Type 1 Cr-spinel from the outward to cores through microfractures and pores, producing Type 2 and Type 3 Cr-spinel with serpentine ± chlorite along cleavages, diminished Al-cores and growing outer ferritchromite zone and/or Cr-magnetite to magnetite zones. The suprachondritic NbN/LaN (up to 39.35) and NbN/BaN (up to 13.37) of whole rock implies for HFSEs metasomatism by subduction sediments input components, while slight enrichment in LREEs (LaN/YbN = 2.5–3) and FMEs (Li, Pb, Sr, and Ba) may have resulted from serpentinization-related hydrothermal alteration. The Wadi Igla serpentinite indicates silica cycled in a closed system, suggesting that the altered Neoproterozoic oceanic lithosphere may not have shared their main components with the surrounding environment whether to the ocean floor or the subduction zone.

Multiple fluid flow events and diversity of hydrothermal minerals in Neoproterozoic to lower Paleozoic carbonate reservoirs, Tarim Basin, NW China

• Two stages of hydrothermal events account for carbonate-hosted mineralization . • Quartz precipitated from silica-rich fluids from the Precambrian sandstone layers. • Saddle dolomite, fluorite and calcite formed in the Cambrian shale-derived fluids. • The same parent fluid could precipitate different minerals in differing host rocks. • Basal sandstones do not always act as lateral conduits for hydrothermal fluids . Contrasting mineral assemblages have been frequently found in Neoproterozoic-lower Paleozoic carbonate reservoirs along high-angle faults in the Tarim Basin. However, the reason for the change in mineral types over time is poorly constrained. Field and petrographic observations, fluid inclusion microthermometry, rare earth element (REE) and C, O and Sr isotope analyses are used to constrain the changes in water chemistry and fluid flow during the development of the mineral assemblages. During the Middle Devonian-early Carboniferous, the flux of hydrothermal fluids from Neoproterozoic sandstone layers, linked to the activity of NE–SW-trending strike-slip faults, resulted in extensive silicification. The silica-bearing fluids were hot (128.1–209.4 °C), saline (17.1–21 wt% NaCl eq.) and enriched in radiogenic strontium and showed no link to large-scale petroleum migration. During the Permian, a regional thermal anomaly induced the expulsion of hydrocarbon-bearing hydrothermal fluids from lower Cambrian shales, where Mg 2+ , 87 Sr and F − were derived via the transformation and leaching of clay minerals. A more significant in situ source of Mg 2+ was the dolostone interval, as the migrating fluids contributed to the precipitation of saddle dolomite with high T h values (82–176.4 °C) and salinities (15–21.4 wt% NaCl eq.), negative δ 18 O values and radiogenic Sr compositions. The succession of saddle dolomite followed by fluorite with similar T h values, salinities and 87 Sr/ 86 Sr ratios indicates a Mg 2+ deficit and increased dissolved calcium activity in the same parent fluid system, which facilitated the breakdown of MgF − and combination of Ca 2+ and F − . Subsequently, cooling and desalination of the hydrothermal fluids occurred, which was induced by the infiltration and mixing of meteoric water related to the compression and uplift of the north-western flank of the Tarim block. These processes resulted in the precipitation of milky white calcite (Cal 1) with similar oxygen and strontium isotope compositions but wider ranges of T h (82.1–154 °C) and salinity (3.4–16.9 wt% NaCl eq.) than those of the saddle dolomite and fluorite. The lateral flow of hydrothermal fluids through suitable layers, such as bed interfaces, permeable aquifers and karstic units, facilitated the formation of stratabound mineral deposits away from the feeding faults. The termination of hydrothermal mineralization was marked by the precipitation of brown calcite (Cal 2), whose meteoric origin has been proven by the combination of its reddish staining, oscillatory luminescent zonation and low δ 18 O values. This integrated study approach improves the understanding of the diversity of exotic mineral assemblages in carbonate hosts and the evolution and migration of hydrothermal fluids connected to late Paleozoic tectonism.

İdrisyayla (Kırka-Eskişehir) Batısında Gözlenen Sferulitik Riyolitlere ait İlk Petrografik Dokusal Veriler

Spherulitic structures are classically defined as concentrically zoned crystal clusters that form in igneous systems (especially in alkaline igneous rocks), and usually occur as different mineral layers or shells.
 The volcano-sedimentary series observed in the Kırka region, south of Eskişehir, within the Kırka-Afyon-Isparta Volcanic Province, Western Anatolia, consists predominantly of Lower Miocene volcanic and pyroclastic sequences with rhyodacitic-rhyolitic in composition, lacustrine limestones, borate-bearing claystones, tuff, stromatolitic limestones and basalts. The formation mechanisms of the spherulitic structures observed in the marginal zone of the sub-alkaline rhyolitic lava dome outcropping within the Neogene series in the west of İdrisyayla in Kırka (Eskişehir) region were described for the first time with this study. The rhyolites are grayish light brown in color, have hypocrystalline hyalopilitic-holohyaline texture and present spherical structures defined as spherulites. These rocks present a completely flowing structure and are composed of volcanic glass containing rarely euhedral plagioclase and quartz microcrystals, The spherical structures observed on the surface of the rhyolites are up to 10-15 cm in size. The spherulites, which are almost round in shape, appear as concentric rings within the lava flow bands. The flow band texture of the volcanic glass phase presents continuity in the inner texture of the spherulitic structures.
 This study suggests that the sferulitic structures in the İdrisyayla (Kırka-Eskişehir) region were formed by the devitrification mechanism as a result of high temperature and rapid cooling. The petro-mineralogical features of the spherulitic rhyolites show that during the early magmatic stage, the oversaturated liquid crystallized in silica-rich fluid phases of relatively different densities, and radial quartz-feldspar crystal growths from a center, caused the formation of the spherulitic structure. It is also suggested that the lithophysae-like structures formed by the crystal growths representing the solid phase in the axial cavities due to the locally folding of the flow bands and the following radial mineral growths representing the liquid phase, were also changed the laminated texture in the marginal phase of the rhyolite.

Formation of the Nkob talc deposit by contact metamorphism and fluid infiltration into siliceous dolostones (Moroccan Anti-Atlas)

The Nkob talc deposit located in the central Anti-Atlas Pan-African belt is hosted by magnesian and siliceous metacarbonates in the contact aureole of the Ediacaran Amassine granite. It consists of a stratified succession of green serpentine-rich marbles, black dolomitic marbles, talcitites and phlogopitites (variously retrogressed into chloritites) enclosed within andalusite metapelite hornfels and quartzites. Two main metamorphic stages have been defined for the formation of the marbles and talcitites: (1) high-temperature recrystallization occurred under amphibolite facies conditions (>500 °C). Temperature increase during granite emplacement led to the formation of forsterite, tremolite, phlogopite (±diopside) bearing assemblages in siliceous marbles and to transformation of dolomitic (calcite-free) marbles from pure dolostones. This high temperature stage is also characterized by the growth of andalusite porphyroblasts in the metapelites surrounding the deposit. (2) A low temperature retrograde hydrothermal stage characterized by the reactive infiltration of aqueous silica-rich fluid (presumably derived from the granitic body), in greenschist facies conditions (<520 °C). Hydrous silicate minerals, mostly serpentine and talc (+calcite), were formed during this stage within the rocks (after fluid infiltration along grain boundaries) or within veins (marking channeled fluid circulation). Talc in ores formed by two processes at Nkob: acicular talc near the granite formed after the breakdown of tremolite in a temperature range of 350 to 500 °C, while tabular talc in the external aureole formed via reaction between dolomite and siliceous-aqueous fluids in temperatures below 350 °C. The different types of marbles derive from pure to slightly siliceous dolomite but green and green-black marbles were strongly affected by interactions with siliceous-aqueous fluids forming serpentine-phlogopite veins with calcite reaction zones. The talcitite layers were also affected by similar hydrothermal H2O-Si enrichment but their dolomitic precursor probably contained a more important detrital silicate fraction.

Possible shock-induced crystallization of skeletal quartz from supercritical SiO2-H2O fluid: A case study of impact melt from Kamil impact crater, Egypt

Abstract Since its discovery, the Kamil crater (Egypt) has been considered a natural laboratory for studying small-scale impact cratering. We report on a previously unknown shock-related phenomenon observed in impact melt masses from Kamil; that is, the shock-triggered formation of skeletal quartz aggregates from silica-rich fluids. These aggregates are unshocked and characterized by crystallographically oriented lamellar voids and rounded vesicles. The distribution of the aggregates can be correlated with former H2O- and impurity-rich heterogeneities in precursor quartz; i.e., fluid inclusions. The heterogeneities acted as hot spots for local melting. Due to the presence of H2O and the high impact pressure and temperature, the formation of a localized supercritical fluid is plausible. Below the upper critical end point of the SiO2–H2O system (temperature &amp;lt;1100 °C and pressure &amp;lt;1 GPa), SiO2 melt and H2O fluid become immiscible, leading to the rapid and complete crystallization of skeletal quartz.

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.

Solubility of K2SO4 in silica-saturated solutions and applications to determine the composition of ore-forming fluids

Sulfate-bearing geofluids are widespread in many geological environments and play important roles in ore-forming processes. Recent experiments provide evidences that in silica-rich fluids, Na2SO4 solubility exhibits a complex temperature dependence, first decreasing from 25 to 313 °C and then increasing from 313 to 425 °C. The fluid-borne silica have critical implications for sulfate-bearing hydrothermal systems and formations of rare earth element deposits. However, whether other sulfate salts share the same characteristics remains unknown.In this study, fused silica capillary capsules (FSCCs) containing K2SO4-saturated solutions and K2SO4 crystals are used to quantitatively investigate the solubility of K2SO4 at different temperatures in the K2SO4-SiO2-H2O system. In the K2SO4-SiO2-H2O system, K2SO4 shows prograde solubility from 50 to 400 °C, and does not melt through the whole investigated temperature range. The relationship between temperature and the solubility of K2SO4 can be described by a simple linear function with R2 = 0.991: Csulfate = 4.1307 × 10−3 × T + 0.5532, where Csulfate and T represent the solubility of K2SO4 in m (mol/kg H2O) and temperature in °C, respectively. These results show that the solubility of K2SO4 can be enhanced by 3 to 5 times at 300–400 °C by the presence of silica, compared with that in silica-absent system, even without the formation of sulfate melt. Considering that quartz is ubiquitous in the Earth's crust, the solubility of sulfate in natural geofluids may reach a much higher level than previously expected.The temperature dependences of the solubilities of K2SO4 and Na2SO4 in silica-saturated fluids were used to constrain the compositions of the mineralizing fluids of the synmetamorphic hematite-quartz veins at Ouro Fino (SE Brazil) and the Maoniuping REE deposit (SW China), based on the previously reported microthermometric results of fluid inclusions that contain sulfate daughter minerals. For the hematite-quartz vein at Ouro Fino, the sulfate concentration was calculated to be 8.9–19.2 wt%, which is in line with previous laser ablation measurements (~15.3 wt%). For the Maoniuping REE deposit, the total sulfate concentration was determined to be 16.3–27.4 wt%, and the REE concentration of the ore-fluid was further estimated to be 0.30–0.44 wt%.

Origin of cherts in Al Jabal al Akhdar, northeast Libya: petrographical and paleontological evidences

This paper discusses the petrographical and paleontological characteristics of the exposed siliceous (chert nodules) at Al Jabal al Akhdar in northeast Libya. Cherts are found in four rock units and at four superimposed horizons: Al Athrun Formation (Campanian–Maastrichtian) is the oldest one which is overlain by Apollonia Formation (Early–Middle Eocene), then Darnah Formation (Middle–Late Eocene) and Wadi al Qattarah Formation (Late Miocene) the youngest formations. This study is based on field observation and petrographical and paleontological examinations of both chert nodules and hosting limestone rock. Petrographically, the source of the silica in the chert nodules of the Al Athrun Formation is proved to be from silica-rich fluid due to the radiolarian. However, in Apollonia Formation, the sponges seem to be the source. Meanwhile, Darnah Formation and Wadi al Qattarah Formation are suggested to be secondary in origin (i.e., diagenetically is evidenced in the silica replacement to shells). Therefore, further study using more petrographical and paleontological samples will be demanded in order to confirm the biogenic source of Darnah and Wadi al Qattarah formations.

The influence of silica on carbonate diagenesis in chalk – Ekofisk Formation, Danish Central Graben

Upper Cretaceous–Danian chalk forms a range of complex diagenetic fabrics despite its commonly white and monotonous appearance in outcrop and core. This reflects the interaction between processes linked to composition, depositional facies, texture, sedimentation rates, bioturbation, mineralogy, burial and hydrocarbon charging history. Such fabrics have a substantial impact on reservoir quality, either maintaining or reducing porosity and permeability through diagenetic processes. The potential influence of silica, a key component in the chalk, on the diagenesis of the chalk matrix is still poorly understood and forms the topic of this study. A new approach integrating qualitative methods such as micro-computed tomography (μCT) and scanning electron microscopy (SEM) with methods like X-ray diffraction (XRD) analyses supplemented with contact angle measurements, is used to characterise chalk fabrics and investigate the impact of silica on wettability. The study is focusing on a relatively narrow stratigraphic interval, the Danian Ekofisk Formation reservoir in a single well from the Danish North Sea, the Sif-1X, to avoid uncertainties associated with regional variations in burial history, depositional environment, age and hydrocarbon charging history. The Ekofisk chalk in Sif-1X is characterised by two fabrics distinguished based on variations in calcite microcrystal texture, silica content, silicification types and clay content. They are interpreted to represent two different diagenetic pathways. The Ekofisk Fabric I is silica-poor and has the highest porosity and permeability. It is interpreted to reflect a dual-step diagenetic pathway of initial mechanical compaction, during early burial, followed by later, limited pore-filling cementation associated with pressure dissolution. The Ekofisk Fabric II occurs in intervals with more than 5% silica and displays overall inferior reservoir quality compared to the Ekofisk Fabric I. It is interpreted to be the result of local calcite dissolution and cementation related to release of silica-rich fluids during the stabilisation of metastable opal phases into α-quartz, and the progressive stabilisation of silica ultimately led to pervasive silicification of the fine-grained chalk and cementation of the calcite microcrystals. We demonstrate, based on multi-scale data integration, that silica not only affects the chalk and other fine-grained carbonates through pervasive silicification but potentially also has a significant control on the carbonate system, leading to changes in the calcite microcrystal textures. This adds to the complexity of the porosity and permeability evolution in silica-rich chalk and other mixed silica-carbonate systems.

Arthropod entombment in weathering-formed opal: new horizons for recording life in rocks

Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. Here, we present the first record of an animal fossil preserved in opal formed by weathering with such high-resolution details that even individual cuticle hairs are observed. The fossil consists of the exoskeleton of a nymphal insect belonging to the order Hemiptera and either the family Tettigarctidae or the Cicadidae. This identification is based on anatomical details such as the tibial and femoral morphology of the forelegs. The exoskeleton of the insect was primarily zeolitized during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the continental weathering of the volcanic host rocks. Organic matter is preserved in the form of amorphous carbon. This finding makes opal formed by rocks weathering a new, complementary source of animal fossils, offering new prospects for the search for ancient life in the early history of Earth and possibly other terrestrial planets such as Mars, where weathering-formed opal occurs.

Metasomatic Reactions between Archean Dunite and Trondhjemite at the Seqi Olivine Mine in Greenland

A metasomatic zone formed between the contact of a 2940 ± 5 Ma intrusive trondhjemite sheet in the Archean dunite of the Seqi Ultramafic Complex, SW Greenland, consists of three distinct mineral zones dominated by (1) talc, (2) anthophyllite, and (3) phlogopite. These zones supposedly resulted from a process of dissolution of olivine by silica rich fluid residual from the trondhjemite magma, with crystallization of secondary minerals along a compositional gradient in the fluid phase. A zircon crystal inclusion in a large (4 cm) olivine porphyroblast was dated in situ via LA-ICP-MS U–Pb isotope analysis, yielding a weighted mean 207Pb/206Pb age of 2963 ± 1 Ma, which coincides with granulite facies metamorphism and potential dehydration. Considering phase relations appropriate for the dunite composition, we deduced the talc forming conditions to be at temperatures of 600–650 °C and at a pressure below 1 GPa. This is supported by oxygen isotope data for talc, anthophyllite and phlogopite in the metasomatic zone, which suggests formation in the temperature range of 600–700 °C from fluids that had a δ18O of ~8‰ and a Δ’17O0.528 of about −40 ppm, i.e., from fluids that could have been derived from the late stage trondhjemite sheet.

Revisiting hydrocarbon phase mobilization of Au in the Au–Hg McLaughlin Mine, Geysers/Clear Lake area, California

Carbonaceous material (CM)-enriched silica is co-located with gold (Au) mineralization at the Au–Hg McLaughlin deposit, Geysers/Clear Lake area, California, U.S.A. The co-location suggests that hydrocarbons (HC) may be involved in mineralization and metal concentration processes, but little is known about the role of HC in the formation of ore deposits. Previous studies noted liquid oil inclusions in samples from the McLaughlin deposit, and proposed that the HC were liquid at the time of ore deposition. Hydrocarbon materials in the McLaughlin deposit occur as solid and liquid. Textural evidence suggests that hydrocarbon-rich and aqueous, silica-rich fluids were present simultaneously, as well as separately in alternating pulses. Synchrotron X-ray fluorescence microscopy of microscopic silica-free carbonaceous material reveals that the CM contains abundant ore metals e.g., Au, Ag, Hg, and Pb. The CM could have become metal-enriched by scavenging metals from other ore fluids, or it could have transported metals when the CM was still liquid, with subsequent in-situ degradation due to hydrothermal heat. Gold concentrations of up to 18 ppm were measured via acid digestion of solid and liquid HC material and subsequent inductively coupled plasma–mass spectrometry (ICP–MS) analyses. Hydrocarbon material with liquid to medium viscous properties bearing 10.8 ppm Au provides evidence that Au in liquid HC in the McLaughlin Au-Hg deposit is still mobile and that remobilization and/or transport of metals to the deposit by HC liquids cannot be ruled out.

Water-rock interactions in the Acoculco geothermal system, eastern Mexico: Insights from paragenesis and elemental mass-balance

Geochemical mass balances, supported by mineral characterization and whole-rock elemental analyses, were used to decipher the alteration paragenesis and to understand the elemental mobility related to the interaction of hydrothermal fluids with volcanic rocks (ignimbrites and lavas ranging from rhyolite to dacite) from the Acoculco geothermal system, in the eastern portion of the Trans-Mexican Volcanic Belt. Three extensive events of hydrothermal alteration were distinguished, from earliest to latest: (1) selective silicification and argillization, (2) hydraulic brecciation by silica-rich fluid, and (3) acid-sulfate (vuggy and argillic advanced) alteration. According to the Gresens-Grant mass-balance method, the altered volcanic rocks of Acoculco are generally enriched in Ti and Si and depleted in the rest of major elements; this chemical behavior is striking in the case of vuggy and silicic altered rocks. Small changes in some element concentrations calculated by the Gresens-Grant method may be due to chemical heterogeneity between the rock chosen to be the reference (“fresh rocks”) and the protolith of the altered ones. The chondrite-normalized REE gave different patterns for distinct types of alteration; vuggy and silicified rocks present low LREE and high HREE in opposition to the advanced argillic alteration. Boiling, primarily related to the last alteration event, may have happened at variable depths (825, 325 and 102 m below present surface); it produced changes fluid composition, resulting in acidic pH surface alterations.

Protracted fluid-metasomatism of the Siberian diamondiferous subcontinental lithospheric mantle as recorded in coated, cloudy and monocrystalline diamonds

Five typical coated diamonds (from Udachnaya, Yubileynaya, and Aikhal kimberlite pipes) with untypically low microinclusion abundances and four monocrystalline diamonds (Udachnaya, Mir, Nyurbinskaya pipes) that exhibit thin intermediate microinclusion-bearing zones were examined in details for growth structures, characteristic infrared absorption and photoluminescence, and composition of microinclusions. The internal structures of diamonds of both types imply that fluid inclusions entrapment in diamonds does not necessarily relate to the terminal stage of rapid fibrous growth. Instead, nitrogen aggregation state in some diamonds showed that both fibrous coats and inclusion-bearing layers might experience an annealing during mantle residence long enough to pre-date the ultimate kimberlite eruption, whereas the diamonds with internal inclusion-bearing zones also experienced later protracted history of monocrystalline growth. The presence of chloride-carbonate-silicate fluids/melts in monocrystalline diamonds indicate their generation from media generally similar to that observed in some fibrous diamonds. However, the composition of these metasomatizing fluids is different for the mantle beneath Udachnaya (mostly carbonatitic) and other pipes (Aikhal, Yubileynaya, Mir; variable abundance of silicic high-density fluids). The abundance of silica-rich fluids record either a heterogeneous distribution of eclogites in the subcontinental lithospheric mantle, or the operation of silica-rich slab-derived fluids. The inclusion abundance as well as the type of growth (fibrous or monocrystalline) is considered to be controlled by the volume of fluid fluxes; in this case, fluid consumption leads to decreasing growth rates, diminishing inclusion entrainment and stability of layered octahedrons. The detected minor compositional variations of high-density fluids in these diamonds may be due to local scale thermal perturbation in the host source and/or limited chemical heterogeneity of the parental fluid. The high amount of chlorides in high-density fluids from monocrystalline diamonds provide a new evidence for compositions of fluids/melts acting as primary metasomatic agent in the deep mantle of Siberian craton.

Pedogenic origin of precious opals from Wegel Tena (Ethiopia): Evidence from trace elements and oxygen isotopes

The trace element and oxygen isotope composition of Wegel Tena (Ethiopia) gem opals was measured to provide evidence of the conditions of their genesis. Elemental measurements display several behaviors, especially for K, Ca, Sr and Ba suggesting that—in agreement with previous assumptions—the silica-rich fluids that precipitate into opal are fed by the weathering of ignimbrite at several degrees. The distribution of elements in the opals indicates that the sources of silica in the ignimbrite are both glass and feldspar. Rare Earth Element (REE) signatures are also consistent with a weathering process, but underline that a wide range of physical and chemical conditions prevail at the regional, local and even intra-sample scales. The Ce anomaly emphasizes the variations in redox conditions during opal precipitation, whereas Eu anomaly indicates that feldspar dissolution feeds some of the silica-rich fluids. This suggests that the fluid responsible for opal precipitation is not homogenous across the area with underground water circulation, but rather that each sample reflects formation conditions specific to its very local environment. The oxygen isotope signatures of the opals (from 26.52 to 30.98‰ vs SMOW) allow us to formulate several hypotheses concerning their temperature of formation and the isotopic composition of the fluid. The hypothesis consistent with our other measurements is the pedogenic formation of the opals at ambient temperature (18–21 °C) involving a slightly evaporated soil water fed by meteoric water with an isotopic composition lower than at present, during an Oligocene period likely warmer and wetter than today.

Silica-rich septarian concretions in biogenic silica-poor sediments: A marker of hydrothermal activity at fossil hyper-extended rifted margins (Err nappe, Switzerland)

Silica-rich septarian concretions occur within Pliensbachian-Oxfordian syn-rift to early post-rift sediments of the fossil distal Adriatic rifted margin today exposed in the Err nappe (Central Alps). The concretions, ranging in size from few centimeters to about 2 m, consist of partially to completely siliceous spheroidal bodies developed within fine-grained sedimentary rock (mud to fine sand) with no or very scarce content in biogenic silica such as radiolarian tests. These concretions are characterized by a network of septarian cracks filled with chalcedony, quartz and locally, saddle dolomite. Some concretions are in their original position, whereas others are fragmentary and preserved in coarse-grained turbiditic beds, implying that concretionary growth had occurred prior to significant burial. Fluid inclusion temperatures (~150 °C), the positive δ18O values for original fluids (up to +7‰ SMOW), and the high 87Sr/86Sr values of dolomite point to a deep circulation of seawater into fault damage zones developed in granitic basement rocks. In their ascent, these hot, silica-rich fluids crossed compact sediments giving rise to quartz-dolomite veins. Lastly, when reaching the uncompacted soft sediments close to the seafloor, silica (quartz and chalcedony) locally precipitated in sediment pores and/or fissures leading to the formation of the siliceous septarian concretions. The anomalous quartz-rich mineralogy of the concretion body and of the authigenic crack-filling minerals, the occurrence in clastic sediments from deep-water distal rifted margins as well as high formation temperatures represent a new discovery enlarging the spectrum of septarian concretion types and the environment where they may grow. They provide a possible record of the subbottom products of hydrothermal venting typifying hyper-extended rifted margins where extensional detachment faults exhume crustal rocks to the seafloor and lead to an intense fluid-rock interaction and silica enrichment in the fluids.

Petrographic Study of Silica-rich Continental Carbonates from São José de Itaboraí Basin (Brazil)

São José de Itaboraí Basin (SJIB) is located in the city of Itaboraí, Rio de Janeiro State (Brazil), at about 60 km from the Rio de Janeiro city. Despite the small size area, about 1.5 km2, this basin represents an important source of scientific knowledge, due to the occurrence of banded carbonates and exuberant fossil assemblages. In the last decade, following the discovery of massive carbonates in South Atlantic Ocean, this basin has attracted the attention due to the presence of continental carbonates, in the form of travertines which are included the Itaboraí Formation. The SJIB carbonate rocks have been recognized as possible analogues of the pre-salt reservoir rocks or at least, similar in their depositional processes. The SJIB travertines were submitted to some diagenetic influence due to the presence of silica rich fluids, similar to that found in pre-salt carbonates. The present research aims to perform a petrographic analysis regarding the silica-rich travertine facies, and to recognize textures and structures in order to provide a better understanding of the relationship between silica and carbonate. The deposits associated with silica are easily characterized by their reddish/brown coloring and their higher resistance, due to the presence of iron oxide and silica replacing the carbonate. In some extreme cases can occur the complete substitution of the carbonate fabric generating silex materials. The relationship between silica and iron oxide/carbonate allowed the development of different structures as the banding formation, probably due to mineralogical and rheological differences during recrystallization. In some regions where the diagenetic process was more intense it could be noticed a complete replacement of the primary carbonate material. While, in other cases, the diagenesis preserved structures and past textures, such as iron oxide mouldic features.

An Experimental Study of the Formation of Talc through CaMg(CO3)2–SiO2–H2O Interaction at 100–200°C and Vapor-Saturation Pressures

The metamorphic interaction between carbonate and silica-rich fluid is common in geological environments. The formation of talc from dolomite and silica-rich fluid occurs at low temperatures in the metamorphism of the CaO–MgO–SiO2–CO2–H2O system and plays important roles in the formation of economically viable talc deposits, the modification of dolomite reservoirs, and other geological processes. However, disagreement remains over the conditions of talc formation at low temperatures. In this study, in situ Raman spectroscopy, quenched scanning electron microscopy, micro-X-ray diffraction, and thermodynamic calculations were used to explore the interplay between dolomite and silica-rich fluids at relatively low temperatures in fused silica tubes. Results showed that talc formed at ≤200°C and low CO2partial pressures (PCO2). The reaction rate increased with increasing temperature and decreased with increasingPCO2. The major contributions of this study are as follows:(1)we confirmed the formation mechanism of Mg-carbonate-hosted talc deposits and proved that talc can form at ≤200°C;(2)the presence of talc in carbonate reservoirs can indicate the activity of silica-rich hydrothermal fluids; and (3) the reactivity and solubility of silica require further consideration, when a fused silica tube is used as the reactor in highP–Texperiments.