Epidote Dissolution Research Articles

Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study

Abstract. Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming under greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote–quartz vein. The main part of the vein is deformed, producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain size reduction in the primary vein quartz. Recrystallization occurs contemporaneously with dissolution and (re)precipitation of epidote and quartz grain boundary sliding, leading to a combined process described as viscous granular flow. The combination of grain boundary sliding and dissolution locally and repeatedly produces creep cavities. These represent not only loci for nucleation of new epidote grains at the expense of dissolved ones, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability in the latter, indicate a process of chemical homogenization. The nature of the fluid that mediates deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.

Magmatic epidote in Archean granitoids of the Carajás Province, Amazonian craton, and its stability during magma rise and emplacement

In the Rio Maria and Sapucaia Domains of the Carajás Province, during the Mesoarchean, tonalite-trondhjemite-granodiorite (TTG) series (2.98–2.92 Ga; Colorado Trondhjemite) and sanukitoid rocks (∼2.87 Ga; Rio Maria Suite of Ourilândia do Norte) were formed. They were followed, in the Neoarchean (2.75–2.73 Ga), by numerous stocks of granitoids similar to A-type granites (Vila Jussara Suite). Despite the compositional differences between these granitoids, epidote is a mineral phase common to all of them, with pistacite contents of 26–29 mol% in TTGs, 22–33 mol% in sanukitoids and 25–30 mol% in Neoarchean granitoids. The study of the dissolution kinetics of Archean epidotes of the Carajás Province reveals that the partial dissolution time of their crystals was ∼4–10 years, with corresponding magma ascent rates of 4–8 km/year. Magma viscosities at liquidus temperature were estimated at 105.3 Pa s for TTG magma and 102.5 Pa s for sanukitoid magma, whereas monzogranitic magmas of the Vila Jussara Suite exhibited a viscosity of 104.4 Pa s. In contrast, the viscosity of tonalitic magma of the Vila Jussara Suite was 103.5 Pa s. The preservation of magmatic epidote in Archean granitoids requires that the plutons grew in an incremental way, similar to their Phanerozoic counterparts, with the stacking of small sill-dykes of about 100 m thickness each intruded in a rather cold crust, allowing fast cooling rates so as to prevent complete dissolution of epidote at the final emplacement level.

Experimental study of epidote dissolution rates from pH 2 to 11 and temperatures from 25 to 200 °C

The dissolution rates of Green Monster Mine epidote at temperatures of 25, 100 and 200 °C, and over the pH range 2–11, were determined from far-from equilibrium experiments performed in both batch and mixed-flow reactors. Epidote dissolution rates based on silicon release decrease with increasing pH to pH ∼ 8, then increase in response to further pH increases. The apparent relative metal release rates from epidote at chemical steady-state were found to vary with pH and temperature, in part due to secondary phase formation. At 100 and 200 °C, Al is preferentially released relative to Si at alkaline pH, but preferentially retained by the solid at 2 < pH < 8. This behaviour is interpreted to stem from the precipitation and re-dissolution of Al oxy-hydroxides such as boehmite, diaspore, and gibbsite, as observed in solids recovered after the experiments. Calcium tends to be preferentially released compared to Si and Al at acidic and near to neutral pH. Iron was found to be retained in the solid phase due to the prompt formation of secondary Fe minerals. Nevertheless, neither the dissolution of 20 wt% of the initial epidote mass nor the formation of secondary phases on the epidote surface substantially changed epidote dissolution rates. As epidote (i) is almost three times more Ca-rich than a typical basaltic glass, (ii) preferentially releases Ca at pH typical of CO2-charged water injections, and (iii) exhibits increasing dissolution rates as pH increases at alkaline conditions, it seems likely that the presence of epidote in altered basalts may be beneficial for the formation of calcite and other Ca-carbonates during mineral carbonation efforts. In addition, the relatively rapid rates of epidote dissolution at acidic conditions suggest that epidote has a significant role in the weathering of altered mafic rocks.

Rapid magma ascent and formation of the Águas Belas-Canindé granitic batholith, NE Brazil: evidence of epidote dissolution and thermobarometry

Abstract Mineral chemistry and intensive parameter estimates for the Major Isidoro (626 Ma) and Monteiropolis (627 Ma) magmatic epidote-bearing granitic plutons, emplaced along the Jacare dos Homens transpressional shear zone (JHSZ), Borborema Province, are focused in this study. These plutons consist of medium-to-coarse grained equigranular to porphyritic tonalite to granite that show abundant dioritic enclaves. These granites contain biotite (Fe# 0.44 to 0.55), Fe-edenite (Major Isidoro), hastingsite (Monteiropolis), titanite, and epidote that often show allanite core as key mafic mineral phases. Pistacite molecular content in epidote is in the interval of 27 to 31 mol%, presenting TiO2 < 0.30%, typical for magmatic epidote. Estimated intensive parameters reveal crystallization at 6.5 ± 1 (Major Isidoro) and 4.7 ± 0.6 kbar (Monteiropolis), temperatures from ~940°C (near-liquidus) to 675 ± 35°C (near-solidus) and oxidizing conditions. Partial corrosion of epidote took place during 15.6 to 32 (Major Isidoro) and 27-49 years (Monteiropolis), corresponding to rather high magma ascension rates of 365 to 750 and 395 to 635 m.years-1, respectively. The JHSZ likely favored upward magma transport at the Sergipano and Pernambuco-Alagoas domains boundary, during the onset of the Brasiliano orogeny (650-620 Ma).

Constrains from magmatic and hydrothermal epidotes on crystallization of granitic magma and sulfide mineralization in Paleoproterozoic Malanjkhand Granitoid, Central India

The Paleoproterozoic Malanjkhand Granitoid (MG) pluton in the Central India was studied to constrain the depth of emplacement, upward magma transport rate, and quantification of physicochemical condition of sulfide deposition. In this study, the magmatic and hydrothermal epidotes are of two varieties, reported from a mineralized granitoid. In the MG, composition of magmatic epidotes (pistachite – Ps: 21.6–31.1mole%) and hydrothermal epidotes (Ps: 22.6–31.1mole%) are overlapping in terms of mole percent of pistachite component. It does not provide any significant discrimination between the two varieties. Presence of oscillatory zoning in magmatic epidotes indicates that there was cyclic change in the oxygen fugacity or bulk composition of granitic magma during crystallization.Al-in-hornblende barometry indicates that the MG crystallized under 2–5.6kbar pressure and high oxidation state (FMQ-HM) conditions, inferred from Fe/(Fe+Mg) ratio in hornblende (0.36–0.51) over wide range of temperature (800–650°C). Partial dissolution of epidote indicates an average 6 years time that corresponds to upward transport rates 0.45km/year for magma migration in the crust. Rapid upward magma migration in most cases was probably through dyke mechanism, which is also the most appropriate model to understand the emplacement of granitic magma responsible for the formation of Malanjkhand pluton.In the Malanjkhand ore deposit, hydrothermal epidotes associated with major sulfide phases (chalcopyrite and pyrite) suggest that they equilibrated with the mineralizing ore fluid. Hydrothermal epidotes were formed over a wide range of temperature (147–424°C). From mineral–fluid equilibria modeling it was inferred that low to moderate temperature, moderate to high fO2 (>HM buffer) and low fS2 conditions were favorable for formation of hydrothermal epidotes. Interaction between hydrothermal epidote with mineralizing ore fluid in the wall rock would raise the log(aCa2+/aH+2) ratio that brings a fall in pH values, followed by potassic alteration, which promotes the deposition of sulfide ores at Malanjkhand. Sulfide mineralization in the MG represents a unique Paleoproterozoic granite ore system.

Contrasting sources and P-T crystallization conditions of epidote-bearing granitic rocks, northeastern Brazil: O, Sr, and Nd isotopes

The 618Ma Curral de Cima tonalite and 577Ma Lourenço monzodiorite, northeastern Brazil, are magmatic epidote-bearing plutons that carry ferrohornblende, biotite, titanite, and epidote. Major, trace, and isotope chemistry suggests that the major magmas of the two plutons followed similar differentiation trends but derived from source rocks that differed in age and isotopic composition. The mineral phases of the Curral de Cima tonalite, the presence of amphibole-rich clots, and juvenile component (average εNd=−3.55) point to an I-type source for these rocks. These data and high calculated δ18O(w.r.) (10.0‰) for the tonalite and high δ18O value for a clot (9.3‰) argue that the clots are fragments of a metabasaltic source rock that has been hydrothermally altered at a low temperature. In contrast, average calculated δ18O(w.r.) for the Lourenço monzodiorite =7.8‰, 87Sr/86Sr=0.7083, εNd=−14.6, and TDM=1.92Ga. These data are compatible with a long crustal residence time of lower crust amphibolites source. Epidote in the Curral de Cima pluton crystallized close to the NNO buffer, and hornblende chemistry, due to Al reequilibration, yielded sub-solidus temperature and pressure. In contrast, in the Lourenço pluton epidote crystallized close to the HM buffer and Al-in-hornblende points to near-solidus solidification (685°C) around 4.4Kbar. This study confirms that magmatic epidote in granitic plutons can crystallize at pressures lower than 5.5Kbar under higher fO2 as experimentally foreseen. Rapid magma transportation through hot continental crust during the peak of metamorphism in early stages of an orogenic cycle prevents epidote dissolution.

Zeolites in fissures of granites and gneisses of the Central Alps

AbstractSix different Ca‐zeolite minerals are widespread in various assemblages in late fissures and fractures in granites and gneisses of the Swiss Alps. The zeolites formed as a result of water–rock interaction at relatively low temperatures (&lt;250 °C) in the continental upper crust. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as monomineralic fissure fillings. They represent the youngest fissure minerals formed during uplift and exhumation of the Alpine orogen. A systematic study of zeolite samples showed that the majority of finds originate from three regions particularity rich in zeolite‐bearing fissures: (i) in the central and eastern part of the Aar‐ and Gotthard Massifs; (2) Gibelsbach/Fiesch, in a fissure breccia located at the boundary of Aar Massif and Permian sedimentary rocks; and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca). Rail and road tunnel construction across the Aar‐ and Gotthard Massif provided excellent data on zeolite frequency in Alpine fissures. It was found that 32% (Gotthard NEAT rail base tunnel, Amsteg section) and 18% (Gotthard road tunnel) of all studied fissures are filled with zeolites. The number of different zeolites is limited to six species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Calcium is the dominant extra‐framework cation, with minor K and Na. Heulandite and chabazite contain Sr up to 29 and 10 mol.% extra‐framework cations respectively. Na and K contents in zeolites tend to increase during growth as a result of changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compared to subsurface samples may indicate late stage cation exchange with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations show that the Ca‐dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite. The necessary components for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite fissures occur in areas of H2O‐dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature.

Geochronological and mineralogical constraints on depth of emplacement and ascencion rates of epidote-bearing magmas from northeastern Brazil

Calc-alkalic to high-K calc-alkalic granitoid plutons in the Borborema province, northeastern Brazil, have been studied to constrain depth of emplacement by mineralogical and geological methods and to estimate upward magma transport rate based on partial dissolution of magmatic epidote. Laser-probe incremental heating 40Ar/ 39Ar dating of biotite and hornblende single crystals from the Neoproterozoic Tavares and Brejinho high-K calc-alkalic magmatic epidote (mEp)-bearing plutons reveals age differences of around 60 M.y. between these two minerals in each of these two intrusions. These data suggest solidification at relatively great depth followed by prolonged cooling interval between the closure temperatures of biotite and hornblende. Al-in-hornblende barometry indicates that hornblende in several mEp-bearing plutons in the Transversal Domain of the Borborema province solidified at 5 to 7 kbar, whereas in the Seridó and Macururé terranes, solidification pressures range from 3 to 4 kbar. Partial dissolution of epidote indicates very rapid upward transport. Partial corrosion occurred during 15–35 years (Cachoerinha–Salgueiro terrane), 10–30 years (Alto Pajeú), 15 years (Seridó), and 10 years (Macururé) corresponding to upward transport rates of 450–1300, 650–1050, 1200, and 1800 m/year respectively in these four terranes. Rapid upward magma migration in most cases was probably facilitated by diking simultaneous with regional shearing.

Emplacement, petrological and magnetic susceptibility characteristics of diverse magmatic epidote-bearing granitoid rocks in Brazil, Argentina and Chile

Magmatic epidote (mEp)-bearing granitoids from five Neoproterozoic tectonostratigraphic terranes in Northeastern (NE) Brazil, Early Palaeozoic calc-alkalic granitoids in Northwestern (NW) Argentina and from three batholiths in Chile have been studied. The elongated shape of some of these plutons suggests that magmas filled fractures and that dyking was probably the major mechanism of emplacement. Textures reveal that, in many cases, epidote underwent partial dissolution by host magma and, in these cases, may have survived dissolution by relatively rapid upward transport by the host magma. In plutons where such a mechanism is not evident, unevenly distributed epidote at outcrop scale is armoured by biotite or near-solidus K-feldspar aggregates, which probably grew much faster than epidote dissolution, preventing complete resorption of epidote by the melt. Al-in-hornblende barometry indicates that, in most cases, amphibole crystallized at P≥5 kbar. Kyanite-bearing thermal aureoles surrounding plutons that intruded low-grade metamorphic rocks in NE Brazil support pluton emplacement at intermediate to high pressure. mEp show overall chemical variation from 20 to 30 mol% (mole percent) pistacite (Ps) and can be grouped into two compositional ranges: Ps 20–24 and Ps 27–30. The highest Ps contents are in epidotes of plutons in which hornblende solidified under P<5 kbar. The percentage of corrosion of individual epidote crystals included in plagioclase in high-K calc-alkalic granitoids in NE Brazil, emplaced at 5–7 kbar pressure, yielded estimates of magma transport rate from 70 to 350 m year −1. Most of these plutons lack Fe–Ti oxide minerals and Fe +3 is mostly associated with the epidote structure. Consequently, magnetic susceptibility (MS) in the Neoproterozoic granitoids in NE Brazil, as well as Early Palaeozoic plutons in Argentina and Late Palaeozoic plutons in Chile, is usually low (<0.50×10 −3 SI), which is typical behavior of plutons which crystallized under low fO 2 (ilmenite-series granitoids), although Fe/(Fe+Mg) ratios in hornblende (0.40–0.65) indicate crystallization under high fO 2. Mesozoic to Tertiary calc-alkalic plutons in Chile, however, exhibit iron oxide minerals and MS values >3.0×10 −3 SI, typical of magnetite-series granitoids crystallized under higher oxygen fugacity. In NE Brazil, Argentina and Chile, it seems that mEp is more common in Precambrian to Palaeozoic ilmenite-series granitoids, while its occurrence in magnetite-series granitoids is more restricted to Mesozoic to Tertiary granitoids.

Dissolution kinetics and alteration of epidote in acidic solutions at 25°C

The dissolution kinetics of epidote [Ca 1.83Al 2.00(Al 0.11Fe 0.74 3+Fe 0.10 2+Mn 0.001Mg 0.19Ti 0.05 4+)Si 3.00O 12OH] were studied over the pH range 1–6.5 at 25°C using flow-through reactors. Ca and Si were found to be the most rapidly released elements in the structure, while Al and Fe tend to be retained. The Si–O–Al bonds are readily broken when protonated, resulting in release of Si-dimers and Si-tetrahedra, while the Al polymers are more resistant towards dissolution. Selective leaching of Ca produces a Ca-depleted rim with a thickness of 60–90 nm after 50–1000 h leaching depending on pH, whereas, Al and Fe are enriched in this zone. The calculated thickness of a Si-depleted rim is approximately 5–20 nm at pH≥2. After an initial growth period the thickness of the altered rim remains constant. However, Fe is selectively leached out together with Si and Ca in the pH 1 and 2 experiments, and as a result, Al alone constitutes the enriched layers at those conditions. Leaching of both Ca and Si is slowed down by the formation of this Al enriched layer, thus decreasing the dissolution process. Epidote dissolution rates based on Si release for one of the experimental series, are closely comparable with those of K-feldspar and micas. The reaction order with respect to H + activity, was found to be 0.15 for grain size 5–20 μm and 0.44 for the 125–250 μm fraction. However, the dissolution stoichiometry is reversed for epidote compared with the silicates mentioned above, since Si is preferentially released over Al.

Dissolution Rates of Epidote at pH 2 in Aqueous Solution

The dissolution rates for minerals are a important factor in determining the susceptibility of landscapes to acid deposition. In soil models such as PROFILE and SAFE the dissolution rate coefficients are of great importance in calculating the critical loads for different types of soils. However, minerals such as epidote, amphiboles and pyroxenes which are important contributors of base cations in forest soil solution, have incomplete kinetic data. This study investigates the mechanisms involved in dissolution of epidote. The dissolution of epidote in aqueous solution in the presence of Ca and A1 was studied under controlled pH and temperature conditions using both a continuously stirred batch reactor and a flow-through reactor based on the fluidized bed technique. Steady state Si, Ca and Fe release data thus obtained were used to achieve dissolution rates of epidote under conditions similar to forest soils. Dissolution rates from the continuous stirred batch reactor were significantly different from results obtained with the flow-through reactor technique. The rate dependence on base cation and aluminium activity in solution was investigated and determined. The activation energy for epidote dissolution at pH 2 was 18.9 kcal tool -1. The results indicate that the Transition State Theory can be applied to the dissolution of epidote and that the rate is surface controlled. This is consistent with the rate equation:

Constraints on Rates of Granitic Magma Transport from Epidote Dissolution Kinetics

Epidote is stable only at high pressure in granitic magmas but is found in magmas crystallized at shallow crustal levels. This observation suggests qualitatively that magma transport is sufficiently rapid to prevent complete epidote resorption. Experimental measurements of epidote dissolution showed that the presence of epidote implies that magma transport from the deep crust in these systems was rapid and that the transport was by dikes rather than diapirs.

Dissolution rates of prehnite, epidote, and albite

Dissolution rates of prehnite and epidote in aqueous solutions were measured in the temperature range 25 to 90°C, and as a function of pH. The dissolution rate of albite was measured at pH 1.4 at temperatures between 25 and 90°C. Batch experiments using low ionic strength pH buffers in constant temperature water baths or ovens provided data on cumulative element release as a function of time. Steady state Si, Ca, Na, and Al release data obtained from these experiments were used to obtain limiting dissolution rates, where the term limiting rate denotes dissolution of a bulk mineral under conditions where it is far from equilibrium with the fluid. At 90°C and at pH 1.4 to 6, prehnite and epidote dissolution rates decrease and are proportional to approximately —0.3 pH for prehnite and —0.2 pH for epidote. Above pH 6, prehnite dissolution becomes pH independent, but epidote dissolution increases with rates that are proportional to between +0.3 and +0.6 pH. Prehnite and epidote dissolution is linear and stoichiometric at low pH. At pH greater than 7, both minerals initially display preferential release of Si and Al relative to Ca; however, with increasing reaction dissolution becomes stoichiometric. This suggests that a Ca-enriched layer forms but reaches a steady state thickness which does not impede subsequent linear stoichiometric release. At pH 1.4, the limiting dissolution rate for albite is linear and stoichiometric. At pH 1.4, the activation energies are 18.12 ± 0.81 kcal mol −1 for prehnite, 19.76 ± 1.2 kcal mol −1 for epidote and 17.07 ± 1.6 kcal mol −1 for albite. At pH 6.5, the activation energy for prehnite dissolution is 20.73 ± 3.2 kcal mol −1.

Burial Diagenetic Geothermometer and the Classifying Characteristics of the Burial Diagenetic Zones of Tertiary Clastic Rocks in Shengli Oil Field, East China

It is very important to study burial diagenesis for reservoir assessment. During the Cenozoic Era, the studied area was a downfaulted depression basin with continuous subsidence that was filled by thick clastic sediments. Based on the petrological data from 66 wells in this basin, the present geotemperature corresponding to the occurring depth of the burial diagenetic event approximates or is equal to the initial temperature of the diagenetic event. Therefore, the initial temperature of the burial diagenetic event can be calculated by this equation: T = G(H {minus} h) + 14.5C, where T is the initial temperature of the burial diagenetic even; G is geothermal gradient; H is the shallowest depth for this diagenesis occurrence; and h is the depth of constant temperature zone. The annual average surface temperature of the studied area is 14.5C. The different burial diagenetic geothermometers calculated by the equation are as follows: 65 {plus minus} 5C for minor quartz overgrowth; 90 {plus minus} 5C for intensive quartz overgrowth; 75 {plus minus} 5C for epidote dissolution; 92 {plus minus} 5C for authigenic garnet; 95 {plus minus} 5C for late carbonate cement; 140 {plus minus} 10C for transformation from kaolinite to illite. Based on all the burialmore » diagenetic geothermometers, the burial diagenetic sequence can be divided into shallow ( 130C) zones. It is demonstrated that these zones can be used to classify and assess the reservoir rocks and source rocks.« less