Coupled Dissolution Reprecipitation Process Research Articles

Zirconium isotope tracing of the magmatic-hydrothermal transition

Zirconium (Zr) isotopes have significant potential for tracing magmatic processes, particularly late-stage fractionation involving fluids. However, the fractionation of Zr isotopes between magma and fluid is still unclear. In-situ Zr isotope analyses of zircons from three granite-pegmatite suites in the Tibetan plateau show a wide range of δ94/90Zr values (IPGP-Zr) between −0.23 ‰ and 1.38 ‰, with the highest values in hydrothermally altered zircons from pegmatites. A coupled dissolution-reprecipitation process of zircons in a fluid-rich magma is the most plausible mechanism to explain the strong enrichment of heavy Zr isotopes in hydrothermally altered zircons. During the formation of granitic pegmatite, exsolution of F-Na-Si-bearing fluids can enhance zircon solubility and is capable of significant removal of Zr from the melt. Faster diffusion of 90Zr into the fluid channel will result in preferential accumulation of light Zr isotope in the fluid, and drive heavy Zr isotope enrichment in the residual melt under disequilibrium conditions. Later reprecipitated zircons will inherit the heavy Zr isotope composition of the melt. Rayleigh fractionation models suggest fluid-melt fractionation factors for Zr isotope of about 0.99911–0.99930. The results show the major controls of fluid on Zr isotope fractionation during crystal-magma-fluid differentiation. Zr isotope can be a powerful tracer for the magmatic-hydrothermal transition.

Zircon in tin granite as tracer for fluid metasomatism and Sn mineralization

Most tin deposits in the world are genetically related to tin granite and form during complex magmatic-hydrothermal processes. Zircon is a common accessory mineral in granite and related ore systems and can host a number of ore metals, such as Sn, W, Nb, Ta, U and Th, in its crystal lattice. However, whether metal enrichment/depletion can trace ore-forming processes is still unclear. Here, we report that the metal concentrations in various types of zircons from the Mopanshan tin granites in the southern Great Xing'an Range (Northern China) can be used as good indicators of fluid metasomatism and Sn mineralization. Two lithological zones are developed in the Mopanshan pluton, including porphyritic syenogranite (PG) in the center and fine-grained syenogranite (FG) at the margin. Zircons in the PG (PGZ-1, PGZ-2, and PGZ-3) are all magmatic in origin, while zircons in the FG can be categorized into magmatic zircons (FGZ-1 and FGZ-2) and metasomatic zircons (FGZ-3). The PGZ-1 and FGZ-1 grains are transparent prismatic crystals with bright oscillatory zonation, whereas the PGZ-2 grains are murky crystals with dark oscillatory zonation. PGZ-3 and FGZ-2 grains occur as overgrowths of previously formed zircon (PGZ-1, PGZ-2, and FGZ-1) or as brown individual crystals, showing dark and homogeneous cathodoluminescence (CL) textures. The metasomatic FGZ-3 grains are translucent-opaque porous crystals with vermicular CL zonation and commonly replace FGZ-2. The trace element compositions of magmatic zircons are completely melt controlled, providing a record of magmatic evolution as a constant decrease in Zr/Hf ratios and a gradual increase in Th, U, Nb, and Ta contents. Moreover, the structure of magmatic zircons transforms from a crystalline state to an amorphous state as a consequence of radioactive decay of U and Th. A coupled dissolution-reprecipitation process is proposed for the formation of metasomatic FGZ-3. The reactive fluid is the magmatic fluid that exsolved from the melt in the late magmatic stage. The magmatic fluid replaced biotite and rare earth phosphates (mainly monazite and apatite) enclosed within biotite, resulting in significant amounts of Nb, Ta, Sn, P, Al, Ca, Fe, and REEs, which subsequently reacted with the FGZ-2 zircons to leach Th, U, Y, and HREEs. Eventually, REEs, Y, Th, and U in the fluid combined with P to form monazite and xenotime, while the other elements partially precipitated with the crystallization of the FGZ-3 zircons. Although the alteration of biotite only released approximately 190 ppm of Sn into the fluid, this is still a significant Sn source for the Sn deposits surrounding the Mopanshan pluton, taking the granite size (∼50 km2) and the volume proportion of biotite (∼5%) into account. Furthermore, based on previous studies on tourmaline from the Mopanshan granite and regional geochemistry, it may be inferred that the addition of wall rock components may also play an important role in Sn mineralization.

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

AbstractSecondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB‐TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron‐scale vein consisting of fluorapatite and an SiO2‐rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F‐bearing chlorapatite and the SiO2‐rich phase filled the center of the vein. The presence of nanoporosity and nanometer‐scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F‐, Si‐rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre‐terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low‐shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late‐stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.

Metasomatic alteration of zircon at lower crustal P-T conditions utilizing alkali- and F-bearing fluids: Trace element incorporation, depletion, and resetting the zircon geochronometer

Natural alteration of zircon takes place in melts or fluids either via dissolution coupled with overgrowth or via a coupled dissolution-reprecipitation process. The latter results in the zircon being partially or totally replaced by new, compositionally re-equilibrated zircon or a new mineral phase or both. In this study, fragments (50–300 μm) from a large, inclusion-free, clear, 520–530 Ma euhedral zircon with light radiation damage from a nepheline syenite pegmatite, Seiland Igneous Province, northern Norway, were experimentally reacted in 20 mg batches with 5 mg of ThO2 + ThSiO2 + SiO2 and a series of alkali-bearing fluids in sealed Pt capsules at 900 °C and 1000 MPa for 6–11 days in the piston cylinder press using a CaF2 setup with a cylindrical graphite oven. ThO2 + ThSiO2 + SiO2 was present at the end of the experiment. In experiments involving H2O, H2O + NaCl, H2O + KCl, and 2 N KOH, no reaction textures formed other than a slight dissolution of the zircon grain fragments. Experiments involving 2 N NaOH, Na2Si2O5 + H2O, and NaF + H2O resulted in zircon reaction textures with varying degrees of intensity, which took the form of partial replacement by compositionally modified zircon via a coupled dissolution-reprecipitation process. In the NaF + H2O experiment some overgrowth also occurred. Altered zircon is separated by sharp compositional boundaries from unaltered zircon. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis indicates that, relative to the unaltered zircon, the altered zircon is strongly enriched in Th, and heavily to moderately depleted in U and (Y + REE). In all the experiments, 206Pb (3–5 ppm in unaltered zircon) is depleted in the altered zircon to below the SIMS detection limit and to at or below the LA-ICP-MS detection limit. Hafnium and Ti concentrations in the altered zircon retained the same approximate value (within error) as the original zircon. The results from these experiments demonstrate that zircon can be compositionally modified by alkali-bearing and alkali-F-bearing fluids via a coupled dissolution-reprecipitation process. Near to total loss of radiogenic Pb via such processes under high-grade conditions resets the internal zircon geochronometer. Although the end result is the same as with zircon overgrowth, i.e. the production of new generation zircon at the time of a metamorphic/metasomatic event, such replacement processes can explain incomplete isotopic ‘resetting’; inclusion production through unmixing of solid solutions in metastable zircon compositions; and ‘ghost’ textures that preserve initial growth features but with isotopic disturbance. Diagnostic replacement features produced in experiments, such as interface geometries between altered and unaltered zircon, provide markers of the mechanism and aid in zircon interpretation. A major implication from this study is that if zircon with low radiation damage can be metasomatically altered under high-grade conditions, this would have important consequences with respect to zircons presumed role as an impregnable container for mineral inclusions. Namely the mineral inclusions contained within zircon could also be altered, reset as a geochronometer, or even replaced by another mineral.

Mechanism and age of gold remobilization in orogenic gold deposit from primary laminated ore to the high-grade ore: A case study from Cenozoic Bangbu gold deposit, Tethyan Himalaya

The formation of high-grade gold ore has always been attractive, mostly located in Precambrian gold deposits with various mechanisms. Yet, in the orogenic Bangbu gold deposit, Tibet, we discovered that the coupled dissolution-reprecipitation (CDR) process and low melting point chalcophile elements (LMCE) -related melting are responsible for the formation of high-grade ore at ca. 38 Ma constrained by in-situ U-Th-Pb dating of hydrothermal monazite. Three generations of pyrite have been recognized, metamorphic barren pyrite (Py I) in the host schist, hydrothermal pyrite (Py II) in the preceding primary laminated ore with an Au grade of ∼8 g/t, and the hydrothermal coarse pyrite (Py III) in later massive ore with a grade of up to 29 g/t. In the primary laminated ore, the Py II in the sulfide ribbons is intergrown with native gold and colloform quartz and carries abundant trace element contents, such as Au, As, Cu, Pb, Zn, Te, Se, and Bi. These suggest that the rapid precipitation and local supersaturation occurred during the episodic crack-seal process. In the later high-grade massive ore, Py III exhibits distinct zonation, ranging from Py III-1 in the core to Py III-3 in the rim. The Py III-1, porous and rich in inclusions, has sharp boundaries and successive lattice orientation with the mantle Py III-2 suggesting a CDR process. Higher Au and As in Py III-2 than the Py III-1, and the close affinity between visible gold and LMCE melt inclusions at the boundary suggest that gold was remobilized and facilitated by the LMCE during the CDR process. The electron backscatter diffraction (EBSD) mapping result shows little crystallographic misorientations of Py III, excluding the deformation process that has significantly contributed to the remobilization of gold. The decrease of the δ34S ratio of pyrite between Py II (δ34S = 2.0‰ to 4.3‰, mean 3.4‰) and Py III (δ34S = −2.5‰ to 3.8‰, mean −0.2‰) most likely reveals a more oxidized condition in the open space which may trigger the CDR process and gold remobilization of the pyrite in the primary ore. Within the zonation of Py III, an increasing trend of the δ34S ratio represent the evolution of the redox state of fluids towards relatively reducing conditions during the healing of quartz veins. The newly obtained ca. 38 Ma age is comparable to regional amphibolite- to granulite-facies metamorphism during the continental subduction, which was considered to trigger the modification of primary gold orebody to form high-grade ore.

Resetting of the U-Pb and Th-Pb systems in altered bastnäsite: Insight from the behavior of Pb at nanoscale

AbstractBastnäsite contains considerable amounts of U and Th and has been widely used for U-Th-Pb dating. Hydrothermal alteration of bastnäsite is common in nature but its effects on U-Th-Pb dating are not currently well constrained. Hence the significance of U-Th-Pb ages obtained from altered bastnäsite cannot be evaluated. Here, we present a detailed geochronologic as well as micro- and nano-scale mineralogical study of altered bastnäsite in a Mo-REE deposit, Central China. The original bastnäsite grains were confirmed to have crystalized at 208 Ma but were variably overprinted by a hydrothermal event at 150 Ma. They commonly exhibit typical replacement textures that appear to have formed from a coupled dissolution-reprecipitation process, i.e., a primary unaltered domain surrounded by a porous altered domain. Micro- and nano-scale mineralogical observations strongly suggest that during the coupled dissolution-reprecipitation process, non-radiogenic (common) Pb was incorporated into the altered domains in the form of nanoscale galena inclusions. Such incorporation (even minor) has significantly affected the 206Pb/238U and 207Pb/206Pb ratios due to the low contents of U and its daughter isotopes in bastnäsite, resulting in highly variable, discordant U-Pb dates for the altered domains. In contrast, incorporation of the non-radiogenic Pb has very limited effects (<5%) on the Th-Pb system due to the remarkably high contents of Th and radiogenic 208Pb in bastnäsite. Instead, the scattered 208Pb/232Th ages (208 to 150 Ma) of the altered domains were essentially affected by incomplete replacement, and thus can be used to approximate the lower age limit of the primary hydrothermal activity or the upper age limit of the secondary hydrothermal activity. The results from this study highlight that because of the different orders of magnitude between the U and Th contents in bastnäsite, the mobilization of radiogenic and non-radiogenic Pb during alteration may have significantly different impacts on the U-Pb and Th-Pb systems. Therefore, the two systems should be treated separately during the dating of bastnäsite resulting from secondary hydrothermal events.

Genesis of monazite-rich, orthopyroxene-bearing veins in the Kliphoog area of the Concordia Granite, Springbok, Namaqualand, South Africa: Sources, fluids, and the mobility of actinides and REE

A network of monazite-rich, coarse-grained veins and fine-grained veins permeating the mid-Proterozoic Concordia Granite in the Kliphoog area near Springbok, Namaqualand, North Cape Province, South Africa, are described with respect to their whole-rock chemistry, petrography, mineralogy, and mineral chemistry along with zircon and monazite geochronology. Chondrite-normalized trace element data indicate that the coarse-grained veins are enriched in LREE relative to the Concordia Granite with HREE enrichment being somewhat variable. HREE in altered Concordia Granite, at the contact with the coarse-grained vein, are heavily depleted compared to unaltered Concordia Granite while the LREE show the same approximate abundances. Chloritization is common in the coarse-grained veins along mineral grain boundaries with chlorite replacing most of the orthopyroxene megacrysts, though occasional remnants of orthopyroxene remain in the core. A traverse across one of the coarse-grained veins showed no obvious mineral chemical trends between the vein and surrounding granite. Relic Fe-rich megacryst orthopyroxene has an Al2O3 content of around 2.0 wt% suggesting formation at around 700 °C. The Fe-rich biotite has mean Ti values ranging from TiO2 = 4.1 to 1.2 wt% indicating formation at 700–800 °C. The coarse-grained veins and fine-grained veins are characterized by abundant accessory Th-rich monazite, heterogeneously distributed in the veins. Lesser amounts of monazite are found in the granite. Other accessory minerals in the veins and granite include zircon and rare fluorapatite. Back-scattered electron imaging shows that the anhedral to euhedral monazite grains (up to more than 1500 μm in size) tend to be complexly zoned with light and dark areas. Lighter areas are more enriched in Th and/or Ce than darker areas. This complex zoning can occur as magmatic/sector zoning and more commonly as a complex series of metasomatic alteration events. Metasomatic textures, which are due to variable fluid-induced mobility of Th, U, and REE, are the result of a coupled dissolution-reprecipitation process and have been reproduced experimentally using high pH alkali-bearing fluids. The analysis of seven zircon cores from the Concordia Granite yields a concordia age of 1173 ± 15 Ma suggesting that an age of ca. 1170 Ma most likely represents the true emplacement age of the Concordia Granite. The mean 207Pb/206Pb age of 1042 ± 9 Ma obtained on zircon from the main coarse-grained vein is interpreted as the crystallization age of the vein. 207Pb/235U monazite ages are highly variable in the veins and Concordia Granite host, ranging from ca. 1050 to 1000 Ma. This supports the petrographic and mineral chemistry evidence that the monazite U--Pb system was variably affected by a continuous or multi-pulse metasomatic/metamorphic event(s) occurring between 1050 and 1000 Ma. U--Pb analyses of deformed monazite crystals from a radioactive sinistral shear zone of the Kliphoog area yield a mean 207Pb/235U age of ca. 1006 ± 9 Ma, indicating that the late circulation of Th-LREE-rich fluids synchronous with strike-slip deformation occurred at ca. 1000 Ma. On a regional scale, the intrusion of the Kliphoog coarse-grained vein is contemporaneous with granulite-facies metamorphism as well as the A-type granitoid magmatism of the Spektakel Suite, mafic magmatism of the Koperberg Suite, and emplacement of the monazite vein-type deposit at Steenkampskraal. Therefore, a genetic relationship between the formation of monazite-rich Kliphoog veins and these metamorphic, magmatic, and metallogenic events is proposed.

Transformation of amorphous precursor to crystalline carbonate: Insights from Mg isotopes in the dolomite-analogue mineral norsethite [BaMg(CO3)2

Crystallization from an amorphous precursor is an important pathway of carbonate precipitation in nature. However, the mechanistic details of the transformation from an amorphous phase to a crystalline phase of carbonates remain a topic of intense debate. Two competing mechanisms, including solid-state transition and coupled dissolution-reprecipitation, have been proposed to explain this transformation process. Magnesium is a common element in carbonate crystal lattices and its isotopes may provide unique insights into this problem. In this study, we investigated the transformation of the amorphous carbonate (AC) precursor for norsethite [BaMg(CO3)2], a dolomite analogue mineral, by in situ XRD analysis and isotope exchange experiments using a 25Mg enriched tracer coupled with high precision isotope analyses of δ26Mg and δ25Mg values for aqueous and solid phases. In situ XRD experiments revealed that the AC can transformed to crystalline norsethite at various temperatures (25 °C, 50 °C and 70 °C) and no intermediate mineral formed during the AC transformation process. 25Mg tracers indicated that near-complete Mg isotope exchange occurred in all exchange experiments during AC transformation. More importantly, after the AC transformation, the system showed surprising apparent non-mass dependent fractionation relationship, that the δ25Mg value of solid phase became greater than that of aqueous solution from a lower value, producing positive Δ25Mgsolid-aq fractionation, whereas the Δ26Mgsolid-aq fractionation remained negative. We numerically modeled the behavior of Mg isotopes (in both δ26Mg and δ25Mg) for the experimental system according to the two competing mechanisms of AC transformation. The modeling results suggest that the apparent non-mass dependent isotope behavior can only be explained by the coupled dissolution-reprecipitation process. Therefore, this study does not support the solid-state transition mechanism for AC transformation. Further, this study rigorously proves that norsethite can form by precipitation from aqueous solution without replacement, and implies that Mg2+ in aqueous solutions can be efficiently dehydrated and incorporated into a well ordered dolomite-group mineral (norsethite) under abiotic, low temperature conditions, thus providing new insights for understanding dolomite precipitation in nature.

The formation of modified zircons in F-rich highly-evolved granites: An example from the Shuangji granites in Eastern Tianshan, China

Two populations of zircons are present in the F-rich highly evolved 300 Ma Shuangji granites, Xinjiang, eastern Tianshan, China. Zircon I comprise a very small proportion of the population. They show characteristics of primary igneous zircons and yield an age of ca. 300 Ma. Zircon II are the dominant type and are weakly luminescent under cathodoluminescence (CL) but show a range of complex textures in back-scattered electron (BSE) images. Zircon II are characterized by Th-U-rich mineral inclusions not seen in zircon I and can be divided into zircon IIa and zircon IIb. Autometasomatism during the magmatic-hydrothermal phase of the highly evolved Shuangji granites, fluids rich in F interacted with zircon I through a coupled dissolution-reprecipitation process forming pores and cracks in the zircon grains. The simultaneous alteration of U-rich euxenite, released high amounts of U and Th into the fluids. Interaction of these fluids with porous zircon I resulted in the precipitation of U- and Th-rich inclusions in the zircon I grains. Radiation damage resulted in the formation of metamict zircons. Later hydrothermal alteration affected the metamict zircons, forming nanoscale baddeleyite in the amorphous zone. Subsequent radiation damage accumulated continuously resulting in the final textures of zircon II.

In-situ U–Th–Pb geochronometry with submicron-scale resolution: low-voltage electron-beam dating of complexly zoned polygenetic uraninite microcrystals

Abstract Complexly zoned microcrystals of uraninite were encountered in orthogneiss from the central Tauern Window in Austria (K1 gneiss, Felbertal scheelite mine) and analysed in-situ for U, Th and Pb with state-of-the-art FE-SEM/EDX techniques. A three times finer spatial resolution was achieved using an acceleration voltage of 8 kV, compared to the classic 15–20 kV set-up of U–Th–total Pb electron microprobe dating. The lower voltage allows a spheroid of material with a diameter of only 0.3 µm to be selectively analysed. Careful tests on three uraninite reference materials show that the low-voltage method yields sufficient precision and accuracy for U–Th–total Pb uraninite dating, with errors on individual spot ages in the order of 10–30 Ma. By means of this innovative analysis technique, small-scale age zoning patterns could be resolved and dated in the uraninite microcrystals from the orthogneiss. Based on microstructures observed in backscattered electron images we interpret that an older uraninite generation in the rock, with a late Permian formation age (~260 Ma), was recycled two times through a coupled dissolution–reprecipitation process at ~210 Ma and at ~30 Ma. The younger dissolution–reprecipitation phase at ~30 Ma coincides with the Alpine regional metamorphism (lower amphibolite facies). The two older ages (~210 Ma and ~260 Ma) have been previously recognized in rocks from the Tauern Window by uraninite dating, but it is the first time here that both are recorded in the same rock and even the same uraninite grain. The present study shows that recrystallized accessory uraninite can provide a sensitive geological “hard disk” where several discrete thermal events of an area are stored. In addition, our work attests that the mineral uraninite has an unexpected geochronological robustness, even on the microcrystal scale.

Rutile alteration and authigenic growth in metasandstones of the Moeda Formation, Minas Gerais, Brazil – A result of Transamazonian fluid–rock interaction

Microstructures, trace elements and U–Pb ages of rutile grains are reported from two metasandstone samples of the Moeda Formation, which is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Minas Gerais, Brazil. The Moeda metasandstones, deposited between 2.68 and 2.65 Ga, are intercalated with Witwatersrand-like, pyritiferous and auriferous metaconglomerate layers. Rutile from both samples shows unusually high contents of Th (1–142 ppm) and rare-earth elements (ΣREE = 1–132 ppm), with chondrite-normalized middle REE humps, and reveals post-depositional U–Pb ages of ≤2.25 Ga; only three grains yielded significantly older ages of up to 2.72 Ga, pointing to a detrital origin. The combined data set suggests that rutile formation and alteration at 2.25 Ga was caused by distinct fluid-mediated processes, which erased nearly all provenance information. Detrital, rounded rutile grains in sample A were altered by a coupled dissolution–reprecipitation process (type 1), resulting in highly porous grains. These show variable contents of rutile-compatible and -incompatible trace elements: Th (1–48 ppm), Zr (25–796 ppm), U (12–167 ppm), Pb (9–52 ppm), Cr (900–12,878 ppm), W (143–3939 ppm), Nb (1148–6168 ppm), Sb (6–63 ppm), as well as variable ratios of Zr/Hf, Y/Ho and Th/U. Rutile grains in sample B are mostly authigenic (type 2), and formed after dissolution of pre-existing detrital rutile grains and subsequent transport and homogenization of trace elements. This interpretation is supported by the anhedral shape of some grains and the finding of numerous, randomly oriented muscovite laths. The authigenic population has significantly lower, and less variable contents of trace elements compared to type-1 rutile: Th (0.5–16 ppm), Zr (21–67 ppm), U (5.8–24 ppm), Pb (6-18 ppm), Cr (1379–3153 ppm), W (9.4–70 ppm), Nb (921–3058 ppm), Sb (3.3–10 ppm), and distinctively lower ratios of Zr/Hf (7.9) and Th/U (0.2), but variable Y/Ho ratios. Significant contents of Th and REE are explained by co-precipitation with TiO2 aggregates prior to structural ripening. The close association of rutile with muscovite, the rutile MREEN humps and the results of Zr-in-rutile thermometry suggest that rutile formation and alteration in the investigated sandstones was mediated by K(Na)–Si–P-bearing aqueous fluids at temperatures of about 490 °C. The rutile MREEN humps are interpreted to result from the tetrad effect during leaching of spatially associated detrital zircon grains. Uranium–Pb ages of ≤2.25 Ga indicate that the fluid overprint started at the onset of the Transamazonian orogeny, perhaps related to the opening of the Sabará foreland basin.

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

AbstractMetamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.

The Origin of Rapakivi Feldspar by a Fluid-induced Coupled Dissolution–Reprecipitation Process

The Origin of Rapakivi Feldspar by a Fluid-induced Coupled Dissolution–Reprecipitation Process

Geochemical and textural characterization of phosphate accessory phases in the vein assemblage and metasomatically altered Llallagua tin porphyry

Petrographic and geochemical characterization of phosphate accessory minerals represents a powerful tool in understanding the mineralization and metasomatic history of one of the world’s biggest tin deposits, the Siglo XX mine, Salvadora stock, Llallagua, Bolivia. The Llallagua tin deposit lies in a hydrothermally altered porphyry stock that is part of the subduction-related Bolivian tin belt. Despite numerous studies, there is still a debate over the timing and characteristics of mineralization history of the deposit. Primary igneous fluorapatite and monazite (for the first time) were recognized in the altered porphyry. The igneous monazite is enriched in Th, unlike the hydrothermal monazite that is recognized for its low Th concentration. Fluorapatite, monazite, and xenotime also coexist with cassiterite within the hydrothermal vein assemblage. Fluorapatite and xenotime are essentially pristine. Monazite, however, shows various degrees of alteration in the form of regenerative mineral replacement (RMR). This exemplifies differential reactivity and selective mineral replacement/alteration of three accessory phosphate minerals, that are all important geochemical tracers of magmatic and hydrothermal processes, and which can all be used as geochronometers. Mineral textures and composition in the altered porphyry and vein assemblages have been evaluated. Monazite-xenotime geothermometry indicates monazite crystallization beginning around 550 °C. Monazite continues to grow as temperatures gradually decrease to about 300 °C, when most of cassiterite precipitation occurred in the samples studied. The primary mechanism of phosphate alteration has been identified as a coupled dissolution-reprecipitation process, which led to REE exchange in the igneous fluorapatite and hydrothermal monazite. In Type I local alteration, La and Pr-Nd show continuity across the pre- and post- alteration concentric zones indicating that they were not affected by alteration. This is an example of a selective elemental exchange during coupled dissolution-precipitation. Type II, pervasive post-growth alteration, is evident by the presence of micro-porosity and the formation of secondary, reaction induced minerals. Release of HREE from the monazite goes into the formation of void filling xenotime inclusions; the first documentation of this metasomatic alteration product in monazite. A well-documented discrepancy exists among ages determined from the zircon, fluorapatite, monazite, and altered porphyry minerals. These observations, regarding selective alteration of fluorapatite and monazite, may help to elucidate the reasons for this discrepancy.

Recrystallization and hydrothermal growth of high U–Th zircon in the Weondong deposit, Korea: Record of post-magmatic alteration

In this study, we investigate zircons with high U and Th contents of 12,000–24,000 and 11,000–40,000ppm, respectively, from leucocratic granite in the Weondong region of South Korea. Hydrothermally epitaxial growth textures are observed in addition to four distinct textures with different backscattered electron (BSE) intensities within single zircon crystals. We describe the internal textures of the zircon crystals and define the chemical characteristics of each textural domain. The zircon crystals show internally recrystallized and externally crystallized textures, supporting the concept of post-magmatic fluid control. After crystallization of the primary zircon by late magmatism, four types of secondary textures were developed. The type-I domain shows patchy forms within the primary domain due to the structural and chemical recrystallization of self-irradiated zircon by a fluid-dominated diffusion reaction process. The type-II domain is characterized by a pure zircon composition, mineral inclusions (mainly thorite), and micropores due to chemical recrystallization by a coupled dissolution–reprecipitation process during interaction with aqueous fluids. The type-III domain is the purest zircon, is interconnected with type-II, and formed by a second coupled dissolution–reprecipitation process. The type-IV domain is the hydrothermal recrystallization/overgrowth texture formed by the direct crystallization process from fluorine-enriched, zircon-saturated aqueous fluid. These results indicate that the zircon crystals were formed sequentially or intermittently through multi-genetic processes by post-magmatic fluids. Furthermore, we conclude that F-enriched post-magmatic fluid migrated large amounts of zirconium, resulting in the typical overgrowth texture of zircon.

Discrimination of protolithic versus metamorphic zircon ages in eclogites: Constraints from the Erzgebirge metamorphic core complex (Germany)

Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin.

Different zircon recrystallization types in carbonatites caused by magma mixing: Evidence from U–Pb dating, trace element and isotope composition (Hf and O) of zircons from two Precambrian carbonatites from Fennoscandia

Zircon grains from two Precambrian carbonatites from Fennoscandia (Siilinjärvi and Tiksheozero) were studied by in-situ geochemical and isotope methods. Zircon domains which preserved primary mantle signatures were identified by a combination of microscopic investigations of thin sections of the carbonatite rocks and separated zircon grains (optical microscopy, optical microscopy combined with cathodoluminescence (OM–CL), and scanning electron microscopy). All studied samples show evidence for alteration processes caused by infiltration of late-stage carbonatite melt/s or fluid/s. This led to different changes in the geochemistry and isotope composition of altered zircon domains. For the 2.6Ga old Siilinjärvi carbonatite complex, zircon grains underwent solid state recrystallization mainly at their rims. Zircon cores often preserved primary mantle signatures and register high HREE/LREE ratios, undisturbed U–Pb ages, εHf values close to CHUR (chondritic uniform reservoir), and δ18O values typical for the mantle. The solid state recrystallization of zircon occurred mainly in contact with a late-stage carbonatite melt or fluid and led to (i) diffusion driven loss of HREE, Th, and U, (ii) partial disturbance of the U–Pb system, (iii) a small shift of the δ18O toward lower values, and (iv) higher CL intensities in such zircon domains. Contrarily, zircons from the 2.0Ga old Tiksheozero complex underwent a coupled dissolution–reprecipitation process. Dissolved–reprecipitated zircon domains typically have a distinctive patchy texture in BSE images and sometimes contained abundant micro-inclusions of calcite, phlogopite, apatite, and baddeleyite. The EDS spectra documented that these non-luminescent zircon regions have much higher concentrations of Ca (and sometimes Fe) indicating an “impure” zircon composition. In addition, many trace elements are highly enriched in such “impure” zircon. Enrichment of Th (and to a lower degree of U) resulted in high Th/U ratios (>1) and disturbance of the U/Pb ages. Hf and O isotope values varied widely even within a single zircon grain. Isotope signatures attest a complex crystallization history of carbonatites and can best be interpreted as melt mixing processes of different carbonatite melt pulses. The non-luminescent reprecipitated zircon domains in Tiksheozero are often rimmed by zircon with high CL-intensity recrystallized in a solid state as well as by new zircon overgrowths. These zircon regions consist of pure zircon without elevated concentrations of Ca and Fe. Solid state recrystallization was accompanied by annealing that caused volume loss resulting in fractures that are now frequently filled with calcite (often containing a minor proportion of apatite) and phlogopite which sampled the withdrawn elements from the recrystallized zircon. Co-crystallizing calcite may thus lead to HREE depletion in newly grown zircon. It is expected that interaction with carbonatite melts can lead to similar changes in zircon grains from other sub-continental mantle rocks. Then, the primary geochemical and isotope systems of zircon may be changed despite its chemical and physical robustness. Infiltration of late-stage carbonatite melts seems to be a common process within many carbonatitic rocks that may hamper to determine the primary geochemical and isotope signatures of carbonatitic zircons.

Alteration of plagioclase to nepheline in the Khariar alkaline complex, SE India: Constraints on metasomatic replacement reaction mechanisms

At the Khariar alkaline complex in southeastern India, some tholeiitic amphibolites inter-layered with syenite and nepheline syenite show microtextural evidence of fenitization. Typical metasomatic textures include the replacement of plagioclase by vein nepheline and albitic-plagioclase or by symplectitic nepheline+albite+Na–Al-rich clinopyroxene. Whole rock mass balance calculations indicate that the fenitized variants gained alkalis (Na+K), some large ion lithophile elements (Sr, Rb and Ba) and high field strength elements like Zr and Hf during the metasomatic exchange. The plagioclase replacement textures can be explained by a fluid-mediated coupled dissolution–reprecipitation process whereby the infiltration of alkali-bearing fluid along grain boundaries and deformation-induced intragranular fractures led to the dissolution of the host plagioclase along an inward moving reaction interface. Na+ ions were introduced and Ca2+ ions removed by the fluid phase. The Al and Si released from the breakdown of the anorthite component of the plagioclase combined with Na+ from the fluid to precipitate nepheline+albitic-plagioclase or Na–Al-rich clinopyroxene. Reaction enhanced cracking during metasomatism due to local volume changes in the alteration zone produced a hierarchical system of fractures. Mass-balance calculations using the volume proportion of phases in the plagioclase replacement zones show that SiO2 and Al2O3 were broadly conserved during the replacement; there was significant loss of CaO (ca. 73%) and huge gains of Na2O and K2O. These results indicate that alkali and alkaline elements (Na, K and Ca) were exchanged freely between the fluid phase and the reaction zone whereas Si and Al released during plagioclase dissolution were consumed in the replacement zone itself to form nepheline and albite-rich plagioclase or Na–Al-rich clinopyroxene. The trace element and Sr–Nd isotope signature of the rocks indicate that the fenitizing fluids were derived from the nepheline syenites.

Fluid-induced dissolution breakdown of monazite from Tso Morari complex, NW Himalayas: evidence for immobility of trace elements

Primary igneous monazite from the Polokongka La granite of the Tso Morari complex in the western Himalayas has been partially replaced by a three-layered corona of metamorphic fluor-apatite, allanite + U- and Th-bearing phases (huttonite + brabantite), and epidote. The alteration is related to high-pressure amphibolite-facies (10–11 kbar and 587–695 °C) fluid-induced retrogression of the ultra-high-pressure granite during exhumation after India–Asia collision. The corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid-mediated coupled dissolution–reprecipitation process. Mass balance calculations using the volume proportions and compositions of coronal minerals show that the REE, U, Th, Pb, Ba and P were conserved and not transported outside the alteration corona. The formation of fluor-apatite, allanite, huttonite and coffinite from monazite and the immobility of REE, U and Th require an influx of alkali- and F-bearing, Ca-rich fluid having high Ca/Na into the corona. We are aware of only two other occurrences of such alteration textures, and these have several similarities in terms of geodynamic setting and P–T histories of the host rocks. We suggest that there may be a common mechanism of exhumation style, and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high-P metamorphism in continental collision zones, which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks.

U–Pb–Hf-trace element systematics and geochronology of zircon from a granulite-facies metamorphosed mafic–ultramafic layered complex in Central Brazil

Zircon recrystallization is a common process during high-grade metamorphism and promotes partial or complete resetting of the original isotopic and chemical characteristics of the mineral and thus complicates U–Pb geochronological interpretation. In Central Brazil, this complexity may be illustrated by three composite mafic–ultramafic intrusions metamorphosed under amphibolite-to-granulite conditions. Their ages of emplacement and metamorphic ages have been a matter of controversy for the last thirty years. The Serra da Malacacheta and Barro Alto complexes make up the southernmost of these layered bodies and four samples from distinct rock types were investigated in order to verify the consequences of metamorphic alteration of zircon for U–Pb dating. Cathodoluminescent imaging reveals internal features which are typical of concomitant dissolution–reprecipitation processes, such as convolute zoning and inward-moving recrystallization fronts, even in samples in which partially preserved igneous textures are observed. Due to this extensive alteration, LA-ICPMS U–Pb isotopic analysis yielded inconclusive data. However, in situ Hf isotopic and trace-element analyses help to clarify the real meaning of the geochronological data. Low Lu/Hf (<0.004) and homogeneous 176Hf/ 177Hf t values imply that the zircon populations within individual samples have crystallized in a single episode, despite the observed variations in age values. Trace element signatures of zircon grains from garnet-bearing samples reveal that they were unreactive to the development of the peak metamorphism mineral assemblage and, thus, the main chemical feature in such grains is attributed to a coupled dissolution–reprecipitation process. However, in the Cafelândia amphibolite an additional alteration process is identified, probably related to the influx of late-stage fluids. Combined isotopic and geochemical investigation on zircon grains allowed the distinction of two magmatic events. The first corresponds to the crystallization of the Serra da Malacacheta Complex and characterizes a juvenile magmatism at ∼1.3 Ga. The younger episode, recognized in the Barro Alto Complex, is dated at ca. 800 Ma and is represented by mafic and ultramafic rocks showing intense contamination with continental crust, implying that the emplacement took place, most likely, in a continental back-arc setting. Altered zircon domains as well as titanite grains date the metamorphic event at ca. 760–750 Ma.