Reaction‐induced nucleation and growth v. grain coarsening in contact metamorphic,impure carbonates

The understanding of the evolution of microstructures in a metamorphic rock requires insights into the nucleation and growth history of individual grains, as well as the coarsening processes of the entire aggregate. These two processes are compared in impure carbonates from the contact metamorphic aureole of the Adamello pluton (N‐Italy). As a function of increasing distance from the pluton contact, the investigated samples have peak metamorphic temperatures ranging from the stability field of diopside/tremolite down to diagenetic conditions. All samples consist of calcite as the dominant matrix phase, but additionally contain variable amounts of other minerals, the so‐called second phases. These second phases are mostly silicate minerals and can be described in a KCMASHC system (K₂O, CaO, MgO, Al₂O₃, SiO₂, H₂O, CO₂), but with variable K/Mg ratios. The modelled and observed metamorphic evolution of these samples are combined with the quantification of the microstructures, i.e. mean grain sizes and crystal size distributions. Growth of the matrix phase and second phases strongly depends on each other owing to coupled grain coarsening. The matrix phase is controlled by the interparticle distances between the second phases, while the second phases need the matrix grain boundary network for mass transfer processes during both grain coarsening and mineral reactions. Interestingly, similar final mean grain sizes of primary second phase and second phases newly formed by nucleation are observed, although the latter formed later but at higher temperatures. Moreover, different kinetic processes, attributed to different driving forces for growth of the newly nucleated grains in comparison with coarsening processes of the pre‐existing phases, must have been involved. Chemically induced driving forces of grain growth during reactions are orders of magnitudes larger compared to surface energy, allowing new reaction products subjected to fast growth rates to attain similar grain sizes as phases which underwent long‐term grain coarsening. In contrast, observed variations in grain size of the same mineral in samples with a similar T–t history indicate that transport properties depend not only on the growth and coarsening kinetics of the second phases but also on the microstructure of the dominant matrix phase during coupled grain coarsening. Resulting microstructural phenomena such as overgrowth and therefore preservation of former stable minerals by the matrix phase may provide new constraints on the temporal variation of microstructures and provide a unique source for the interpretation of the evolution of metamorphic microstructures.     

Isotopic constraints on age and duration of fluid-assisted high-pressure eclogite-facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen

Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.     

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway

Regionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M₁ metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.     

Modelling grain‐recycling zoning during metamorphism

Chemical zoning, recorded by grain growth during metamorphism, is a key source of information about P–T–t paths. Interpretation of these data must be carried out using appropriate models and recognizing their inherent assumptions. To assist with defining how zoned minerals form, a set of geometric criteria for three types of chemical zoning developed in minerals (diffusion, growth and grain recycling) is outlined. Re‐equilibration of minerals by lattice diffusion causes zoning if the re‐equilibration is incomplete. Growth of porphyroblasts is commonly considered in pelites, but in metagranitoids, large monophase domains undergo coarsening by recycling of material from one grain to another as grain boundaries migrate driven by surface energy. This type of grain size increase is termed here ‘grain recycling’. Zoning developed during grain recycling due to equilibration of the recycled material with grain‐boundary chemistry is termed ‘grain‐recycling zoning’. Furthermore, short lattice diffusion lengths relative to grain sizes cause metamorphic fractionation because material in the grain cores is not in communication thermodynamically with the rest of the rock. A new model is derived for this sort of grain size increase coupled with metamorphic reactions using Theriak–Domino. An example is given of plagioclase undergoing an increase in anorthite content as epidote breaks down during amphibolite facies metamorphism of a metagranitoid. Agreement between naturally occurring zoning profiles and those derived from modelled P–T–t paths shows that this model can be used to extract metamorphic conditions from rocks which are not accessible using conventional thermobarometry.     

Petrological and zircon evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

Petrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ¹⁸O values of ?0.6 to 0.1‰, but a few fall in the range ?5.2 to ?4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.     

A statistical analysis of the distribution of cordierite and biotite in hornfels from the Bugaboo contact aureole: implications for the kinetics of porphyroblast crystallization

The three‐dimensional disposition of cordierite and biotite crystals in a hornfels from the contact aureole of the Bugaboo Batholith is quantified using high‐resolution X‐ray micro‐computed tomography and global as well as scale‐dependent pattern statistics. The results demonstrate a random distribution of cordierite and biotite crystal sizes for all scales across the entire rock volume studied indicative of interface‐controlled prograde metamorphic reaction kinetics. The reaction considered responsible for the mineral assemblage and the formation of cordierite and biotite in the hornfels is Ms + Chl + Qtz = Crd + And + Bt + . Rock‐specific phase equilibria point to metamorphic conditions of ~520 –550 °C and 3 kbar for this reaction. The common approach to approximate the shape of crystals as spherical underestimates the influence of the Strauss hard‐core process on rock texture and may be misinterpreted to reflect ordering of crystal sizes by inhibition of nucleation and growth commonly associated with diffusion‐controlled reaction kinetics. According to our findings, Strauss hard‐core ordering develops at length scales equal to and less than the average major axis of the crystal population. This is significantly larger than what is obtained if a spherical crystal geometry would be assumed, and increases with deviation from sphericity. For the cordierite and biotite populations investigated in this research, Strauss hard‐core ordering developed at length scales of up to ～2.2 and 1.25 mm, respectively, which is almost 1 mm longer than the scales that would be obtained if a spherical geometry would have been assumed. Our results highlight the importance of a critical assessment of the geometrical model assumptions commonly applied in the three‐dimensional analysis of crystal size distributions, and underline the need for a quantitative understanding of interface processes in order to appreciate their role in the kinetics of contact metamorphic reactions and rock texture formation.     

Heterogeneous growth of cordierite in low P/T Tsukuba metamorphic rocks from central Japan

This paper examines the spatial statistics of matrix minerals and complex patterned cordierite porphyroblasts in the low‐pressure, high‐temperature (low P/T) Tsukuba metamorphic rocks from central Japan, using a density correlation function. The cordierite‐producing reaction is sillimanite + biotite + quartz = K‐feldspar + cordierite + water. The density correlation function shows that quartz is distributed randomly. However, the density correlation functions of biotite, plagioclase and K‐feldspar show that their spatial distributions are clearly affected by the formation of cordierite porphyroblasts. These observations suggest that cordierite growth occurred through a selective growth mechanism: quartz adjacent to cordierite has a tendency to prevent the growth of cordierite, whereas other matrix minerals adjacent to cordierite have a tendency to enhance the growth of cordierite. The density correlation functions of complex patterned cordierite porphyroblasts show power‐law behaviour. A selective growth mechanism alone cannot explain the origin of the power‐law behaviour. Comparison of the morphology and fractal dimension of cordierite with two‐dimensional sections from a three‐dimensional diffusion‐limited aggregation (DLA) suggests that the formation of cordierite porphyroblasts can be modelled as a DLA process. DLA is the simple statistical model for the universal fractal pattern developed in a macroscopic diffusion field. Diffusion‐controlled growth interacting with a random field is essential to the formation of a DLA‐like pattern. The selective growth mechanism will provide a random noise for the growth of cordierite due to random distribution of quartz. Therefore, a selective growth mechanism coupled with diffusion‐controlled growth is proposed to explain the power‐law behaviour of the density correlation function of complex patterned cordierite. The results in this paper suggest that not only the growth kinetics but also the spatial distribution of matrix minerals affect the progress of the metamorphic reaction and pattern formation of metamorphic rocks.     

超高压变质岩的放射性同位素体系及年代学方法

要深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对Sm—Nd,Rb—Sr同位素体系,只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明,与副变质岩互层的细粒榴辉岩的高压变质矿物之间,或者强退变质岩石的退变质矿物之间,其Nd,Sr同位素可以达到平衡;然而高压变质矿物与退变质矿物之间Nd,Sr同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物,因此石榴石＋全岩Sm—Nd法或多硅白云母＋全岩Rb—Sr法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd同位素不平衡。超高压变质岩多硅白云母所含过剩Ar主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因Ar。因此,不舍榴辉岩的花岗片麻岩多硅白云母基本不舍过剩Ar。对变质锆石成因的准确判断是正确理解锆石U-Pb年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结,并指出将阴极发光图形,锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一,是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。     

The significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization

The spatial disposition, compositional zoning profiles, and size distributions of garnet crystals in 11 specimens of pelitic schist from the Picuris Range of New Mexico (USA) demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks. An ordered disposition of garnet centers and a significant correlation between crystal radius and near-neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre-existing crystals. Compositional zoning profiles require diffusionally controlled growth, the rate of which increases exponentially as temperature increases with time; an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen. Crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process, but decline in the later stages because of suppression effects in the vicinity of earlier-formed nuclei. Simulations of porphyroblast crystallization, based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics, quantitatively replicate textural relations in the rocks. The simulations employ only two variable parameters, which are evaluated by fitting of crystal size distributions. Both have physical significance. The first is an acceleration factor for nucleation, with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock. The second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it. Crystal size distributions for the Picuris Range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals. The same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet, phlogopite, sphene, and pyroxene in rocks from both regional and contact metamorphic occurrences. These commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments.     

Zircon growth during subduction and exhumation metamorphism of mafic rocks,Aktyuz terrain,North Tiahshan,Kyrgyzstan

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Aktyuz metamorphic terrain, in the southern of Kokchetav‐North Tianshan belt, involving progressive metamorphic recrystallization of mafic rock to eclogite and associated behavior of zircon. Zircons in eclogites are mainly fine grains (5 to 20 μm), and preferentially concentrated with rutile/ilmenite. They also occur as individual grains or clusters in amphibole coronas of garnet. A few larger grains commonly preserve inherited cores and evidence of dissolution and metamorphic outgrowths. Zircon grains separated from amphibolites show inherited zircons with typically magmatic feature, although this become progressively blurred in response to resorption and recrystallization. Mineral inclusions represent epidote‐amphibolite facies in the prograde metamorphism, and the embayed boundary between recrystallized domains and inherited zircons suggest fluid/melt participation. The metamorphic domains are mainly simple overgrowth around the inherited cores or recrystallization domains. The absence of peak metamorphic mineral inclusions and steep pattern of MREE‐HREE indicate no sufficient garnet formed before the metamorphic zircon overgrowth. A tiny rim with homogeneously bright CL image can be distinguished in most zircons. Amphibole inclusions have similar compositions to those in the coronas of garnets, suggesting a retrograde metamorphic origin. The inherited zircon crystallized at 880‐730 Ma, revealing similar age range to the gneiss in Aktyuz area, whereas metamorphic zircon dates prograde metamorphism at 497.9 ±1.4 Ma. In this case, the bulk Zr budget in rocks will become locked into Zr‐bearing minerals during the mafic magma intrusion, when the inherited zircon melting and resorption. The texture shows that metamorphic zircon grew both in the prograde and retrograde stage, and Zr‐bearing magmatic minerals and rutile/ilmenite are by far the main source of Zr for the two stages, respectively.     

Geochronology of high-pressure mafic granulite dykes in SW Sweden: tracking the P–T–t path of metamorphism using Hf isotopes in zircon and baddeleyite

Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.     

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

Metamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ¹⁸O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.     

Fluid flow during exhumation of deeply subducted continental crust: zircon U-Pb age and O-isotope studies of a quartz vein within ultrahigh-pressure eclogite

Quartz veins in high‐pressure to ultrahigh‐pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U‐Pb dating of zircon from a kyanite‐quartz vein within ultrahigh‐pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA‐ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U‐Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O‐isotope analyses gave similar δ¹⁸O values for minerals from both vein and eclogite, indicating that the vein‐forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz–mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700–650 °C in a transition from ultrahigh‐pressure to high‐pressure eclogite‐facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ～500 °C without corresponding metamorphism. In either case, the kyanite–quartz vein formed later than the peak ultrahigh‐pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles.     

Th/U ratios in metamorphic zircon

The Th/U ratios of zircon crystals are routinely used to help understand their growth mechanism. Despite the wide application of Th/U ratios in understanding the geological significance of zircon U–Pb ages, the main controls on the Th/U ratio in metamorphic zircon are poorly understood. Here, phase equilibria modelling coupled with solubility expressions for accessory minerals are used to investigate the controls on the Th/U ratios of suprasolidus metamorphic zircon in an average amphibolite facies metapelite composition. We also present a new database of metamorphic Th/U ratios in zircon from Western Australia. Several factors affecting the Th/U ratio are investigated, including the bulk rock concentrations of Th and U, the amount of monazite in the system, and open v. closed system behaviour. Our modelling predicts that the main controls on the Th/U ratio of suprasolidus metamorphic zircon are the concentrations of Th and U in the system, and the breakdown and growth of monazite in equilibrium with zircon. Furthermore, the relative timing of zircon and monazite growth during cooling and melt crystallization has an important role in the Th/U ratio of zircon. Early grown zircon near the peak of metamorphism is expected to have elevated Th/U ratios whereas zircon that grew near the solidus is predicted to have relatively low Th/U ratios, which reflects the coeval growth of monazite during cooling and melt crystallization. Our modelling approach aims to provide an improved understanding of the main controls of Th/U in metamorphic zircon in migmatites and hence better apply this geochemical ratio as a tool to assist in interpretation of the genesis of metamorphic zircon.     

Grain coarsening in polymineralic contact metamorphic carbonate rocks: The role of different physical interactions during coarsening

The microstructural evolution of polymineralic contact metamorphic calcite marbles (Adamello contact aureole) with variable volume fractions of second-phase minerals were quantitatively analyzed in terms of changes in grain size and nearest neighbor relations, as well as the volume fractions, dispersion and occurrences of the second phases as a function of changing metamorphic conditions. In all samples, the calcite grain size is controlled by pinning of grain boundaries by second phases, which can be expressed by the Zener parameter (Z), i.e., the ratio between size and volume fraction of the second phases. With increasing peak metamorphic temperature, both the sizes of matrix grains and second phases increase in dependence on the second-phase volume fraction. Two distinct coarsening trends are revealed: trend I with coupled grain coarsening limited by the growth of the second phases is either characterized by large-sized or a large number of closely spaced-second phase particles, and results finally in a dramatic increase in the calcite grain size with Z. Trend II is manifest by matrix controlled grain growth, which is retarded by the presence of single second-phase particles that are located on calcite grain boundaries. It is supported by grain boundary pinning induced by triple junctions, and the calcite grain size increases moderately with Z. The two different grain coarsening trends manifest the transition between relatively pure polymineralic aggregates (trend II) and microstructures with considerable second-phase volume fractions of up to 0.5. The variations might be of general validity for any polymineralic rock, which undergoes grain coarsening during metamorphism. The new findings are important for a better understanding of the initiation of strain localization based on the activation of grain size dependent deformation mechanisms.     

Effects of matrix grain size on the kinetics of intergranular diffusion

A linear relationship exists between the mean volume of garnet porphyroblasts and the squared inverse of mean matrix grain diameter for six samples of garnetiferous mica quartzite with identical thermal histories and similar mineralogy and modes. This relationship accords with theoretical predictions of the dependence of intergranular diffusive fluxes on the volume fraction of grain edges that function as diffusional pathways during porphyroblast growth. The impact of matrix grain size is large: compared to a rock with a 1‐mm matrix, a rock with a 10‐μm matrix would experience rates of diffusion‐controlled porphyroblast growth that are 10 000 times faster, and characteristic length scales for chemical equilibration that are 100 times larger. Precursor grain sizes may therefore exert a major influence on crystallization kinetics. If matrix coarsening occurs during prograde reaction, a decrease in the volume fraction of diffusional pathways will tend to counteract the exponential thermal increase in diffusive fluxes. The impact of such matrix grain growth, although difficult to assess without firm knowledge of coarsening rates in polymineralic aggregates, might be significant for matrices finer than c. 100 μm at temperatures above c. 500–600 °C, but is likely negligible for coarser grain sizes and lower temperatures.     

Equilibration and disequilibration between monazite and garnet: indication from phase‐composition and quantitative texture analysis

The integration of information which can be gained from accessory [i.e. age (t)] and rock‐forming minerals [i.e. temperature (T) and pressure (P)] requires a more profound understanding of the equilibration kinetics during metamorphic processes. This paper presents an approach comparing conventional P–T estimate from equilibrated assemblages of rock‐forming minerals with temperature data derived from yttrium‐garnet‐monazite (YGM) and yttrium‐garnet‐xenotime (YGX) geothermometry. Such a comparison provides an initial indication on differences between equilibration of major and trace elements. Regarding this purpose, two migmatites, two polycyclic and one monocyclic gneiss from the Central Alps (Switzerland, northern Italy) were investigated. While the polycyclic samples exhibit trace‐element equilibration between monazite and garnet grains assigned to the same metamorphic event, there are relics of monazite and garnet obviously surviving independent of their textural position. These observations suggest that surface processes dominate transport processes during equilibration of those samples. The monocyclic gneiss, on the contrary, displays rare isolated monazite with equilibration of all elements, despite comparably large transport distances. With a nearly linear crystal‐size distribution of the garnet grain population, growth kinetics, related to the major elements, were likely surface‐controlled in this sample. In contrast to these completely equilibrated examples, the migmatites indicate disequilibrium between garnet and monazite with a change in REE patterns on garnet transects. The cause for this disequilibrium may be related to a potential disequilibrium initiated by a changing bulk chemistry during melt segregation. While migmatite environments are expected to support high transport rates (i.e. high temperatures and melt presence), the evolution of equilibration in migmatites is additionaly related to change in chemistry. As a key finding, surface‐controlled equilibration kinetics seem to dominate transport‐controlled processes in the investigated samples. This may be decisive information towards the understanding of age data derived from monazite.     

Testing the fidelity of thermometers at ultrahigh temperatures

A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H₂O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO₂ that is lower than the assumed value of unity.     

Contact metamorphic P–T–t paths from Sm–Nd garnet ages, phase equilibria modelling and thermobarometry: Garnet Ledge, south-eastern Alaska, USA

Sm–Nd garnet‐whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south‐eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single‐stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim‐matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid‐point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid‐point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim‐matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet‐in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet‐biotite‐sillimanite‐plagioclase‐quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure‐temperature‐time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.     

U–Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P–T–t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh‐pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction‐zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti‐in‐zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction‐zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti‐in‐zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr‐in‐titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr‐in‐titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction‐zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid‐T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.