Rim Growth Research Articles

Structural, metamorphic and geochronological constraints on Palaeozoic multi-stage geodynamic evolution of the Altai accretionary wedge system (Hovd Zone, western Mongolia)

A polyphase tectono-metamorphic evolution of the Mongolian tract of the Central Asian Orogenic Belt (CAOB) is inferred from a structural, petrological and geochronological study in the Hovd Zone at the NE periphery of the Altai Domain. Four tectonic events are recorded here. A relict sub-vertical fabric (S1) is associated with the growth of garnet cores and staurolite, typical of MP–MT Barrovian-type metamorphism (M1). This fabric was isoclinally folded and overprinted by sub-horizontal foliation (S2) associated with growth of garnet rims, sillimanite and cordierite, typical of LP–HT metamorphism (M2). The P–T path inferred from petrology and thermodynamic modelling shows that the peak conditions reached 590–610 °C and c. 5.8 kbar (M1) and were followed by decompression to 4–5 kbar associated with heating to 630–690 °C (M2). Subsequently, the studied region was strongly reworked by NW–SE trending upright folds (F3) locally associated with development of muscovite- and chlorite-bearing cleavage (S3). The region is weakly affected by post-metamorphic D4 overprint. Monazite in-situ electron-microprobe dating was complemented by LA–ICP-MS U–Pb dating, revealing three distinct texturally defined age groups at c. 455, 435 and 385 Ma. The ages at c. 455 Ma are related with monazite included in garnet cores, the age group at c. 435 Ma with monazite in garnet rims and in S2 foliation, while the ages of c. 385 Ma come mostly from monazite in chlorite–muscovite-bearing bands. Based on the monazite textural position and REE patterns, these ages are correlated with the tectono-metamorphic events D1–D3, reflecting short-lived Late Ordovician shortening, prolonged Middle Silurian–Early Devonian extension and Late Devonian–Early Carboniferous compression, respectively. Alternation of shortening and extension periods is interpreted as being caused by tectonic switching, i.e. repeated alternation of advancing and retreating subduction regimes. The latest Permian event resulted from the collision of the Mongolian orogenic collage with the Tarim and North China cratons in the south and caused rotation of previous fabrics to the current configuration.

Multigeneration garnets record the fluid evolution of volcanic-hosted skarn iron deposit: Study of the Yamansu deposit, NW China

Three generations of garnets are distinguished in the Yamansu volcanic-hosted skarn iron deposit in NW China, and their detailed textures and geochemical characteristics are presented to reveal the mechanisms controlling the distribution of trace elements in the garnets and to discuss the physicochemical conditions and evolution of the ore-forming fluid. First-generation garnets (Grt1) show well-preserved core–rim growth zonation and have considerable variations in composition (And25-99Gro0-73). Second-generation garnets (Grt2) show complex core–mantle–rim zonation textures with high andradite components (And54-99Gro0-44). Third-generation garnets (Grt3) occur in veins and exhibit nearly monotonic textures and geochemical components (And48-51Gro46-49). Garnet crystals in different generations have variable trace element compositions, such as REEs, U, and Y. We speculate that Grt1 cores were derived from a magmatic–hydrothermal fluid and crystallized in a closed system with low water/rock (W/R) ratios, low oxygen fugacity, and neutral pH, with the distribution of trace elements in Grt1 cores being largely controlled by crystal chemistry. However, during the crystallization of Grt1 rims and Grt2 cores, the hydrothermal fluid system is interpreted to have been open with high W/R ratios and with internal complexes (Cl complex) and external fluid (seawater-like) integrally controlling the fluid properties. The incorporation of trace elements into the Grt1 rims and Grt2 cores was controlled by not only crystal chemistry but also crystal-liquid equilibrium. The Grt2 mantles and rims are interpreted to have formed in a closed system as a result of crack-seal fluid flow. The inferred stable conditions associated with low W/R ratios, a reducing environment, and nearly neutral pH favored Fe reduction in the hydrothermal fluid and provided a material basis for later deposition of iron ores. Grt3 likely formed under near-equilibrium chemical conditions, with low W/R ratios during periods of slow fluid influx determined by episodic supply of hydrothermal fluids, and their distributions were largely controlled by crystal chemistry. The special environment of multistage hydrothermal fluid in the Yamansu deposit was the key factor controlling the multiple enrichment and precipitation of trace elements into garnet.

Vliv diagenetických procesů na asociaci těžkých minerálů v pískovcích z lokality Slivotín (ždánická jednotka, flyšové pásmo Vnějších Západních Karpat, Česká republika)

An electron microprobe study of polished sections prepared from a sample of fine-grained sandstone from the locality Slivotín (Ždánice-Hustopeče Formation, Ždánice Unit, Flysch Belt of the Outer Western Carpathians, Czech Republic) allowed to yield in addition to data on chemical composition also the detailed information on in situ textural relationships of individual minerals. During our study, emphasis was given to accessory phases belonging to the translucent heavy mineral fraction. The detrital garnet (Alm36-82Grs2-45Prp2-22Sps0-15) was extensively dissolved and replaced by calcite cement from its margins and along the cracks. Detrital fluorapatite was dissolved in a similar way, however, dissolution episode was followed by growth of authigenic rims composed of carbonate-fluorapatite. Other observed heavy minerals (zircon, chrome spinel, TiO2 phase, monazite, tourmaline) probably remained unaltered by diagenetic processes. The chemical composition of chrome spinels varies mostly between magnesiochromite and chromite, whereas spinel is very rare. The chemical composition of garnets and chrome spinels is comparable with published data from Czech, Polish and Slovak parts of the Flysch Belt of the Western Carpathians, and indicates the primary source of detrital material in rocks of deeper parts of orogen, characterized especially by the presence of catazonal metamorphites and almost lacking volcanic rocks. Redeposition of heavy minerals from older sediments cannot also be ruled out. The pronounced diagenetic alteration of garnet, if not very scarce in the area of Flysch Belt, could help to explain the earlier observations of wide fluctuations of contents of garnet in heavy mineral concentrates.

Repetitive Duality of Rhyolite Compositions, Timescales, and Storage and Extraction Conditions for Pleistocene Caldera-forming Eruptions, Hokkaido, Japan

Abstract During the Early Pleistocene, numerous caldera-forming eruptions occurred in the southernmost Kurile arc (central Hokkaido, Japan), building an extensive pyroclastic plateau with an area >1600 km2. The arc remains active today, and proximity to populations and infrastructure makes understanding these magmatic systems a critical endeavor. We investigate three major caldera-forming ignimbrite eruptions: Biei (c. 2·0 Ma), Tokachi (c. 1·2 Ma), and Tokachi–Mitsumata (c. 1·0 Ma), with an emphasis on constraining the pressures of magma extraction and storage and the timescales of crystallization. Although all pumice glass compositions from the three eruptions are high-silica rhyolites (77–78 wt% SiO2), hierarchical clustering analysis of major and trace element glass data indicates that the Tokachi and Tokachi–Mistumata ignimbrites each have two distinct pumice populations (Type-1F and Type-2F). We find that these two distinct pumice types record pre-eruptive temperatures, extraction pressures, and crystallization timescales that are strikingly similar between the two eruptions. Using the rhyolite-MELTS geobarometer, we estimate that although all magma types from all three eruptions had storage pressures of 50–150 MPa (∼2–6 km), Type-1F magma was extracted from a deeper mush reservoir (200–450 MPa) compared with Type-2F (100–200 MPa). Pre-eruptive temperatures, constrained by plagioclase–liquid equilibration thermometry and rhyolite-MELTS, suggest that Type-1F magma in both eruptions was hotter (800–820 °C) compared with Type-2F (780–800 °C), but that both reached thermal equilibrium upon eruption (760–780 °C). Because zircon is observed only as inclusions and rarely in contact with glass, we conclude that all magmas were zircon-undersaturated, and thus zircon saturation temperatures, which are 60–100 °C lower than those estimated by the other three independent thermometers, underestimate magmatic temperatures. Using these temperatures as minimum estimates, diffusional relaxation times of Ti zonation in quartz, as revealed by cathodoluminescence (CL), give absolute maximum quartz residence times of <1800 years for Type-2F samples and <600 years for Type-1F for all samples; residence times are <300 years for all samples if the more reasonable Fe–Ti oxide temperature is used instead (∼770 °C). Our modelling therefore suggests that the melt-dominated rhyolite magmas that fed these caldera-forming eruptions were ephemeral features that crystallized within the shallow crust for centuries to several millennia. Rapid rim growth occurred in all magma types in all three eruptions, with a majority of quartz rims (10–200 µm) having grown in less than 2 years. Using plagioclase textures and major and trace element data, we conclude that the bright-CL rims of quartz resulted from decompression and subsequent rapid growth, rather than by a recharge-driven heating event. Thus, decompression occurred within 2 years prior to eruption. Remarkably, the two distinct magma types are statistically similar in terms of composition, crystallization timescales, magma storage conditions, and extraction depths, despite being from eruptions that occurred 240 kyr apart, and from calderas that are separated by 35 km. This suggests magma assembly and storage processes that are spatiotemporally repetitive in this region of Hokkaido.

The initial structure of chondrule dust rims II: Charged grains

In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules, we simulate the growth of FGRs through direct accretion of monomers of various sizes onto the chondrule surfaces. Dust becomes charged to varying degrees in the radiative plasma environment of the solar nebula (SN), and the resulting electrostatic force alters the trajectories of colliding dust grains, influencing the structure of the dust rim as well as the time scale of rim formation. We compare the growth of FGRs in protoplanetary disks (PPD) with different turbulence strengths and plasma conditions to previous models which assumed neutral dust grains (Xiang, C., Carballido, A., Hanna, R.D., Matthews, L.S., Hyde, T.W., 2019). We use a combination of a Monte Carlo method and an N-body code to simulate the collision of dust monomers of radii 0.5–10μm with chondrules whose radii are between 500 and 1000μm: a Monte Carlo algorithm is used to randomly select dust particles that will collide with the chondrule as well as determine the elapsed time interval between collisions; at close approach, the detailed collision process is modeled using an N-body algorithm, Aggregate Builder (AB), to determine the collision outcome, as well as any restructuring of the chondrule rim. For computational expediency, we limit accretion of dust monomers to a small patch of the chondrule surface. The collisions are driven by Brownian motion and coupling to turbulent gas motion in the protoplanetary disk. The charge distribution of the dust rim is modeled, used to calculate the trajectories of dust grains, and then analyze the resulting morphology of the dust rim. In a weakly turbulent region, the decreased relative velocity between charged particles causes small grains to be repelled from the chondrule, causing dust rims to grow more slowly and be composed of larger monomers, which results in a more porous structure. In a highly turbulent region, the presence of charge mainly affects the porosity of the rim by causing dust particles to deviate from the extremities of the rim and reducing the amount of restructuring caused by high-velocity collisions.

Resubduction of lawsonite eclogite within a serpentinite-filled subduction channel

Translating burial and exhumation histories from the petrological and geochronological record of high-pressure assemblages in subduction channels is key to understanding subduction channel processes. Convective return flow, either serpentinite or sediment hosted, has been suggested as a potential mechanism to retrieve rocks from significant depths and exhume them. Numerical modelling predicts that during convective flow, subducted material can be cycled within a serpentinite-filled subduction channel. Geochronological and petrological evidences for such cycling during subduction are preserved in lawsonite eclogite from serpentinite melange in the Southern New England Orogen, eastern Australia. Ar–Ar, Rb–Sr phengite and U–Pb titanite geochronology, supported by phase equilibrium forward modelling and mineral zoning, suggest Cambro–Ordovician eclogite underwent two stages of burial separated by a stage of partial exhumation. The initial subduction of the eclogite at ca. 490 Ma formed porphyroblastic prograde-zoned garnet and lawsonite at approximate P–T conditions of at least 2.9 GPa and 600 °C. Partial exhumation to at least 2.0 GPa and 500 °C is recorded by garnet dissolution. Reburial of the eclogite resulted in growth of new Mg-rich garnet rims, growth of new prograde-zoned phengite and recrystallization of titanite at P–T conditions of approximately 2.7 GPa and 590 °C. U–Pb titanite, and Ar–Ar and Rb–Sr phengite ages constrain the timing of reburial to ca. 450 Ma. This was followed by a second exhumation event at approximately 1.9 GPa and 520 °C. These conditions fall along a cold approximate geotherm of 230 °C/GPa. The inferred changes in pressure suggest the lawsonite eclogite underwent depth cycling within the subduction channel. Geochronological data indicate that partial exhumation and reburial occurred over ca. 50 M y., providing some estimation on the timescales of material convective cycling in the subduction channel.

Petrochronological close-up on the thermal structure of a paleo-subduction zone (W. Alps)

We present the results from a study combining zoned geochronology, mineral trace element geochemistry and phase equilibria modeling of a mafic lithology from the Zermatt Saas Ophiolite in order to elucidate the tectonic rates and thermal structure of the Western Alps paleo-subduction interface. The Zermatt-Saas Ophiolite represents dismembered slices derived from an eclogitized, 60-km wide coherent fragment of Tethyan oceanic lithosphere. Two cm-sized garnet crystals from a pyrite-rich chlorite-talcschist (representative of sub-seafloor hydrothermally-altered basalts) from the Servette mine locality (St. Marcel Valley, Italy) were microdrilled to separate core and rim growth generations and dated using Sm-Nd geochronology to determine the overall duration of garnet growth. Garnet cores were dated to 46.9±1.6Ma, signifying the timing of the initiation of garnet growth. Garnet rims were dated to 43.5±1.3Ma, signifying the timing of burial to peak depths (constrained to ∼75 km). Major and trace element zoning in garnet suggest two distinct generations of garnet growth. Garnet crystal cores display evidence for rapid nucleation and growth, while garnet crystal rims suggest growth at relatively slow (tectonic) rates. The duration of garnet growth (3.4±2.1Ma), when coupled to constraints from phase equilibria modeling and Zr-in-rutile thermometry, provides estimates on the burial and heating rate of 4.7 (+7.9−2.1) km/Myr and 13.2 (+17.8−6.1)°C/Myr, respectively. Using the depth and temperature conditions recorded by laterally equivalent meta-ophiolites from across the Western Alps, a ‘snapshot’ of the geothermal gradient at the Alpine subduction interface is presented. This reveals a pressure-temperature trajectory highlighted by a significant decrease of the thermal gradient from ∼10°C/km down to ∼3°C/km at ∼45 Ma. We further present geometric calculations to identify the tectonic parameters that best fit this dual-segmented thermal gradient. Using convergence rates from paleogeographic reconstructions during Eocene times (on the order of 1.5-2 cm/yr), our calculations require a paleo-slab angle of 15-20°, in line with dip estimates from most modern subduction zones. This study highlights how a detailed petrochronology-based approach enables assessing the thermal structure and geometry of a paleo-subduction zone, with implications on seismicity, stress distribution, and devolatilization potential of deeper portions of the subduction interface.

Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

AbstractThe Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Magma Plumbing During the 2014–2015 Eruption of Fogo (Cape Verde Islands)

Phenocrysts in volcanic rocks are recorders of magmatic processes that have occurred at depth before and during a volcanic eruption. Our petrological investigations of stratigraphically controlled tephrite and phonotephrite samples from the latest eruption of Fogo (Cape Verde Islands) aimed to reconstructing magma storage and transport. The dates of sample emplacement have been determined using satellite instrument - derived high resolution thermal infrared maps. All samples are strongly phyric and commonly contain complexly zoned clinopyroxene crystals and cumulate fragments. Clinopyroxenes from all samples exhibit 10-50 µm wide rim zones, inferred to have grown in a few days to weeks during the ongoing eruption as a consequence of H2O loss from the melt. Clinopyroxene-melt thermobarometry using tephrite groundmass compositions suggests that the rims formed at upper mantle pressures of around 600 MPa (21 km depth). This level is interpreted to reflect temporary reduction in magma ascent velocity by near-isobaric movement through a complex storage system. Previously, the tephrite magma had accumulated at a deeper level, possibly between 700 and 900 MPa as indicated by clinopyroxene cores (Mata et al., 2017). The cause for H2O loss initiating rim growth could be degassing after rise of the magma from the deeper level, or CO2 flushing by a carbonic fluid phase released at depth. Corresponding data from phonotephrites indicate last equilibration at around 440 MPa (16 km); the phonotephrite magma is inferred to be a residuum from an earlier magmatic event that was entrained by advancing tephrite. Microthermometry of CO2-dominated fluid inclusions in tephrite clinopyroxenes results in pressures of around 330 MPa (12 km), indicating another short pause in magma ascent in the lowermost crust. Rim zonations of olivine phenocrysts indicate that after leaving this final stalling zone, the magma ascended to the surface in less than half a day. In strong contrast to these petrological equilibration depths, seismic events precursory to the eruption were located at <5 km below sea level, with only two exceptions at 17 and 21 km depth consistent with our barometry. Our results enhance the understanding of this potentially dangerous volcano, which helps to interpret future pre-eruptive unrest.

Genesis of metasomatic gold mineralization in the Pahardiha-Rungikocha gold deposit, eastern India: Constraints from trace element signatures in chromite-cored magnetite and bulk rock geochemistry

The Pahardiha- Rungikocha gold deposit is located south of the Dalma volcano-sedimentary belt within the North Singhbhum Mobile Belt (NSMB), in the eastern part of India. The rocks of this area have undergone three phases of deformation, viz. D1, D2, D3. While the D1 and D2 events signify progressive regional metamorphism, the D3 stage was a consequence of continued N-S compression resulting in E-W trending displacement fractures. This D3 deformation event was responsible for the mobilization of auriferous fluids within the host quartz-chlorite-schist and subsequent gold precipitation. Here, gold occurs as native grains either within the cracks or along grain boundaries of euhedral pyrite. Detailed mineral paragenesis reveals that euhedral pyrite associated with gold is coeval with abundant chromite-cored magnetite. The chromite-cored magnetite shows a complex paragenetic history, i.e., an initial chromite alteration event, followed by epitaxial growth of magnetite rims on the chromite cores, as revealed by LA-ICP-MS trace element data and EPMA X-ray compositional maps. The paragenetic stages of the chromite-cored magnetite is synchronous with the successive deformation events of the host rock. The final phase of deformation is responsible for the formation of the chromite-cored magnetite rims which is coeval with the gold mineralizing event. These magnetite rims formed as a result of metasomatism brought about by the interaction of hydrothermal fluids with the wall rock. The chromite-cored magnetite trace element signatures resemble the high temperature Ca-Fe alteration and high temperature K-Fe alteration facies, reported from various IOCG and IOA type deposits. There is a distinct enrichment of mafic elements such as Ni, Cr, V, Mg, V, Cu within magnetite rims. A similar pattern of enrichment in mafic elements is also seen in the bulk rock geochemical data. Major element discrimination diagrams reveal that the chlorite schists evolved from a mixed provenance of felsic and mafic/ultramafic rocks. Consequently, the D3 induced metasomatism most likely resulted in preferential stripping of the mafic elements, which is well reflected in the analyzed magnetite rims. Thus, we propound that the bulk rock chemical composition of the host chlorite-schist had a first order control on the chemistry of the chromite-cored magnetite, coeval with the gold mineralizing event. Given the fact that there are no causative igneous intrusions underlying the host chlorite schists, we infer that the gold mineralizing fluids were intrabasinal in nature and were mobilized due to reactivation of pre-existing faults and fractures during the D3 deformation event.

Neogene Metasomatism in the Subcontinental Lithosphere beneath SE Asia—Evidence from Modal and Cryptic Phosphorus Enrichment in Peridotites and Pyroxenites from Southern Laos

AbstractMetasomatism is the prime process to create compositional heterogeneity of the upper mantle. Mineralogical and mineral chemical changes of the mantle triggered by metasomatism can be used to deduce the nature of the metasomatic agent(s) and to constrain the timing of metasomatism. This information is vital for an understanding of the secular evolution of a given mantle segment and the magmatic processes occurring therein. For this study spinel-lherzolites and -websterites were collected from ∼16 Myr old alkali-basaltic lava flows that were extruded on the Bolaven Plateau in south–central Laos. These xenoliths are fragments of the shallow continental lithosphere of the SE Asian peninsula and originate from a mantle segment that acted as source for Cenozoic basaltic volcanism in the wake of the India–Asia collision. In both rock types modal metasomatism formed apatite ± whitlockite ± phlogopite ± calcic amphibole ± calcite ± orthopyroxene. The principal metasomatic phase is apatite, which appears in three varieties. Type-I apatite is ±inclusion-free and associated with phlogopite, calcic amphibole, calcite and lamellar orthopyroxene. It is high in Na and low in P and shows low analytical totals indicating a type-B carbonate–apatite component. Type-I apatite presumably precipitated from a P-alkali-rich mixed H2O–CO2 fluid with low large ion lithophile element (LILE)–light rare earth element (LREE) contents. Type-II apatite shows a spongy texture and has lower Na and higher P contents with higher analytical totals. Crosscutting discontinuous zones of type-II characteristics within type-I apatites indicate type-II formation through an exchange Na+ + CO32– = PO43– + Ca2+ by a later fluid with lower aCO2. REE-rich type-III apatite is the youngest type and formed by infiltration of basaltic melts as part of spongy rims around clinopyroxene. One lherzolite contains whitlockite in addition to apatite. Whitlockite formation is ascribed to a short-lived metasomatic event involving a fluid with extremely low aH2O. Disequilibrium between whitlockite and the bulk assemblage is indicated by hydrous silicates in the immediate vicinity of whitlockite and by substantial H2O contents of 250–370 µg g–1 in clinopyroxenes and 170–190 µg g–1 in orthopyroxenes. High-density (1·15–≥1·17 g m–3) CO2–fluid inclusions in the whitlockite-bearing sample provide evidence for the presence of low-aH2O fluids at mantle depths. The spinel-herzolites may also show cryptic metasomatism evidenced by P zoning in olivine, which is characterized by P-poor (&amp;lt;20–130 µg g–1) cores and P-rich (170–507 µg g–1) rims, the latter in part with oscillatory zoning on a µm scale. Element correlations indicate [4]Si4+ + [6](Mg, Fe)2+ = [4]P5+ + [6]Li+, 2 [4]Si4+ + 4 [6](Mg, Fe)2+ = 2 [4]P5+ + 3 [6](Mg, Fe)2+ + [6]vac and/or 5 [4]Si4+ = 4 [4]P5+ + [4]vac as major P incorporation mechanisms. High P–T experiments conducted at 2 GPa and 950–1050 °C yield apatite-saturated P contents of olivine in the range ∼360–470 µg g–1. Most P concentrations in olivines from the xenoliths including those in the P-rich rims, however, are significantly lower than the apatite-saturated values, which indicates disequilibrium uptake of P during growth of the P-rich rims by dissolution–reprecipitation. Diffusion modeling indicates that the P zoning must have formed within decades prior to the eruption of the host basalts. This is consistent with the preservation of Li disequilibrium partitioning between olivine and pyroxenes in some of the xenoliths. All metasomatic phenomena were assigned to two metasomatic events, both of which were in close temporal relation with the eruption of the xenolith host basalts: an older event-1 formed type-I apatite, hydrous silicates, calcite and orthopyroxene and caused the modification of type-I apatite composition towards that of type-II. It is also likely to be responsible for whitlockite formation and P zoning in olivine. A younger event-2 comprises all paragenetic, textural and compositional modifications of the xenolith assemblages associated with the infiltration of basaltic melts.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.

Evolution of surface morphology and optical transmittance of single crystal diamond film by epitaxial growth

The single crystal diamond (SCD) has great potential in the application of optical windows, photoelectric devices, semiconductors and other fields owing to its excellent performance in optics, mechanics, and thermotics. The SCD was homoepitaxially deposited on High Pressure and High Temperature (HPHT) seed substrate through microwave plasma chemical vapor deposition (MPCVD) method using CH4/H2 as the reaction gas. Hydrogen plasma treatment was proposed to pretreat the seed crystal. The top surface of the epitaxial layer of SCD has a creased morphology and no polycrystalline rim growth on the side. The results showed that the transmittance of the epitaxial SCD film was primarily affected by the surface roughness, which was mainly influenced by the growth time. The photoluminescence at 738 nm was attributed to the silicon color center in the grown SCD, suggesting the application in optoelectronic devices.

Two-stage garnet growth in coesite eclogite from the southeastern Papua New Guinea (U)HP terrane and its geodynamic significance

Mineral compositions and textures of Late Miocene coesite eclogite from Tomagabuna Island were investigated to constrain P–T conditions during UHP metamorphism and subsequent exhumation. Two stages of garnet growth (core and rim), at eclogite-facies conditions, were documented. In addition to core garnet (I), peak assemblages include omphacite, coesite, phengite, and rutile. Rim garnet (II), amphibole, paragonite, plagioclase, quartz, and accessory biotite and spinel were formed after peak pressure conditions. Although a compositional gradient is present at the core–rim garnet interface, the garnet core has a relatively flat compositional profile, suggesting its crystallization in the coesite stability field. Together with minerals formed subsequent to peak pressure (UHP) conditions, the composition of the garnet rim indicates heating of the eclogite during its decompression into the amphibolite facies mineral stability field. Based on compositional zoning in garnet and application of diffusion modelling, we propose that eclogite-facies metamorphism of the mafic protolith and its host lithologies occurred at, or near, UHP conditions. The core garnet (I) formed at 650 °C at 8 Myr in the coesite stability field and was partially resorbed during the onset of exhumation. We infer that garnet rim growth at < 7.1 Ma occurred at P–T conditions corresponding to the boundary between the eclogite and amphibolite facies fields. Diffusion modelling for the garnet core–rim boundary compositions suggests a transient heating event (0.3 Myr) occurred at 1.5 GPa that we infer resulted from heat transport within the Australian—Woodlark plate boundary zone as the coesite eclogite was exhumed. Results caution that apparent P–T paths documented for many UHP terranes may not result from isothermal decompression corresponding to peak temperatures. Instead, paths may link P–T points’ set during transient mineral growth events when the UHP rocks form within the subduction channel, and during subsequent heating events, when UHP rocks are exhumed from mantle depths.

S8A-08 SESSION 8A: METOPIC SYNOSTOSIS RADIATION-FREE 3D HEAD SHAPE AND VOLUME EVALUATION AFTER ENDOSCOPICALLY ASSISTED STRIP CRANIECTOMY FOLLOWED BY HELMET THERAPY FOR TRIGONOCEPHALY

Introduction: Radiation-free 3D post-operative sequential follow-up in craniosynostosis is hindered by the lack of consistent markers restricting evaluation to subjective comparison. However, using the computed cranial focal point (CCFP), it is possible to perform correct sequential image superposition and objective evaluation. We used this technique to evaluate the result of endoscopically assisted trigonocephaly surgery, looking at the mean volume and shape change of the head utilizing 3D photos. Methods: We performed a mean head shape and volume evaluation on age grouped 3D photos of 40 children who underwent endoscopically assisted strip craniectomy followed by helmet therapy at the Craniofacial team Nijmegen. We used CTscans of healthy children as reference. We performed a mean shape evolution analysis and calculated the anterior fossa to total volume ratio (A/T-ratio). The volume- and A/T-ratio pattern were compared with the reference group Results: The mean anterior fossa volume evolved from 336 ml (33.4% A/T-ratio) pre-surgery to 664 ml(36.0% A/T-ratio) at 37–48 months post-surgery. Both groups have a near similar volume- and A/T-ratio pattern over time. The first 18 months show a predominant growth around the resected metopic suture. Between 18 and 24 months we observed mostly anterior orbital rim growth. From 24 months till 36–48 months the head grows predominantly at the temporal area. The least outward growth was observed at the temporal bones. Conclusion: Using a novel technique we were able to objectively evaluate head shape and volume using stereophotogrammetry after endoscopically assisted strip craniectomy followed by helmet therapy. The A/T-ratio and volume growth pattern of endoscopically treated patients is near identical to that of the normal reference group.

Cockade structures as a paleo-earthquake proxy in upper crustal hydrothermal systems

Cockades are clasts completely surrounded by spheroidal hydrothermal overgrowth rims. They are observed inside hydrothermal fault breccias and can provide insights into fault dynamics. The formation of cockades with spheroidal hydrothermal overgrowth rims is related to fast fracturing and dilation, and requires primary clasts to be suspended in a fluid. The rim growth is driven by drops in fluid pressure and related oversaturation. We use descriptions of cockades, their rims and cements in a fault breccia. Geometrical data are combined with mechano-chemical calculations to gain insights into seismic processes and estimate seismic magnitudes. Fast rates for formation of cockade cores and first rim growth are interpreted to be the result of an earthquake’s main shock. Younger growth rims represent subsequent aftershocks, while cemented cockades record interseismic periods. We propose that by considering growth rates of hydrothermal precipitates and cements, paleo-earthquake cycles can be unraveled and a link between geophysics and fault structures can be established.

New evidence for the prograde and retrograde PT-path of high-pressure granulites, Moldanubian Zone, Lower Austria, by Zr-in-rutile thermometry and garnet diffusion modelling

Compositional zoning in garnet, mineral inclusions and the application of the Zr-in-rutile thermometry on rutile inclusions in garnet in combination with conventional geothermobarometry and thermodynamic modelling allows a reconstruction of the prograde pressure-temperature evolution in felsic and mafic high-pressure granulites from the Moldanubian Zone, Bohemian Massif, Lower Austria. Most garnets in these rocks show homogeneous core compositions with high grossular contents (~30 mol%), while their rim zones have a markedly reduced grossular content. Rutile inclusions in the grossular rich garnet cores have low Zr concentrations (400 to 1300 ppm) indicating a formation temperature of ~810–820 °C which implies that the garnet host grew at these temperature conditions as well. Based on numerous polycrystalline melt inclusions, high Ti-biotite relics and a generally high Ti concentration in garnet cores, the peritectic biotite breakdown reaction is considered to be responsible for a first garnet growth, now observed as high-grossular garnet cores. The corresponding pressure is estimated to be in the range of 1.6 to 2.5 GPa, based on experimentally determined biotite breakdown reactions, thermodynamic modelling and the occurrence of high-Ti biotite in garnet cores. Rutile inclusions in low-Ca garnet rims contain significantly higher Zr concentrations (1700 to 5800 ppm) resulting in ultrahigh temperatures of ~1030 °C. Similar temperature as well as corresponding pressure estimates of 1000 ± 50 °C and 1.60 ± 0.10 GPa were obtained by geothermobarometry and thermodynamic modelling using garnet rim and re-integrated ternary feldspar compositions. These high pressure and ultrahigh temperature conditions are well known from literature for these granulites. The proposed two-phase garnet growth is not only seen in different temperatures obtained from rutile inclusions in garnet core and rim areas, but also in discontinuous trace (Cr, Ga, P, Ti, V, Zr) and heavy rare earth element profiles across garnet porphyroblasts, implying a different reaction mechanism for garnet rim growth. This second phase of garnet growth must have occurred during near isobaric heating to the ultrahigh temperature peak, most likely even at slightly lower pressures compared to the garnet core growth.By applying a binary Fe-Mg diffusion model to strongly zoned garnet grains a maximum timescale of 5–6 million years was estimated for the exhumation and cooling process, assuming a linear cooling path from 1000 °C at 1.6 GPa to 760 °C at 0.8 GPa. This short-lived ultrahigh temperature event corresponds to cooling and exhumation rates of 40–50 °C Ma−1 and 5.3–6.6 mm y−1, respectively.

Disequilibrium REE compositions of garnet and zircon in migmatites reflecting different growth timings during single metamorphism (Aoyama area, Ryoke belt, Japan)

Chemical disequilibrium of coexisting garnet and zircon in pelitic migmatites (Aoyama area, Ryoke belt, SW Japan) is shown by microtextural evidence and their heavy rare earth element (HREE) patterns. In zircon, two stages of metamorphic rim growth is observed under cathodoluminescence image, although their SHRIMP UPb zircon ages are similar at ca. 92 Ma. Inner and outer rims of zircon tend to show steep HREE patterns irrespective of the UPb age. The inner rims tend to give higher U content than the outer rims; some rim analyses give various Th/U ratios of 0.02–0.07 compared to the very low (<0.02) values seen in the rest of rim analyses. The higher-Th/U values are ascribed to the mixed analyses between thin prograde domains and thick retrograde overgrowths. Zircon grains with inclusions similar to previously-reported melt inclusions are further enclosed in garnet, supporting the growth of thin zircon domains coexisting with garnet during the prograde metamorphism.Garnet rims are commonly replaced by biotite-plagioclase intergrowths, indicating a back reaction with partial melts. Garnet exhibits decrease in HREE and Y concentrations towards the rim, pointing to its prograde growth. The garnet cores have prograde xenotime inclusions, show steep HREE patterns, and yield growth temperature of ~530–570 °C by a YAG-xenotime thermometer. On the other hand, the garnet rims have no xenotime inclusion and show flat HREE patterns. Rare garnet domains including sillimanite needles also show flat HREE patterns and low Y concentrations, which is interpreted as a product of dehydration melting consuming biotite and sillimanite at near-peak P-T conditions (~800 °C and ~0.5 GPa). One such garnet domain gives nearly-equilibrium REE distribution pattern when paired with the matrix zircon rims.Retrograde xenotime is present in the cracks in garnet and in the biotite-plagioclase intergrowths, suggesting that retrograde breakdown of garnet released HREE and Y to form it. Considering the availability of HREE and Zr and presence of melt inclusions in zircon rims, most part of the zircon rims with positive HREE patterns likely grew during the melt crystallization stage, meaning that the zircon rims and presently-preserved garnet domains did not grow in equilibrium. The above scenario was tested by the array plot analysis and it gave a result consistent with microtextural and traditional REE distribution constraints. Combination of microtextural and the array plot analyses may become a powerful tool to reliably correlate the zircon ages to the P-T evolution of the high-grade metamorphic rocks.

Influence of differential stress on the growth of wet enstatite and enstatite-forsterite reaction rims

Reaction rim growth experiments provide insight into mass transport phenomena, which are important for metamorphic rock-forming processes and deformation mechanisms. We investigated the formation of enstatite single rims between quartz and forsterite and of enstatite-forsterite double rims between quartz and periclase using porous polycrystalline starting materials. About 3 wt% water was added, acting as a catalyst for reactions. Experiments of mainly 4 and 23 h duration were performed in a Paterson-type deformation apparatus at 1000 °C temperature, 400 MPa confining pressure and differential stresses between 0 and 46 MPa. The resulting reaction rim width varied between <1 μm and ≈ 23 μm, depending on duration and type of reaction product. At isostatic pressure conditions, our data indicate that rim growth is proportional to time, controlled by dissolution-precipitation at interfaces of interconnected fluid-filled pores. In contrast, under non-isostatic stress conditions the reaction rim thickness increases non-linearly with time, implying diffusion-controlled growth. The magnitude of differential stress has no systematic influence on the reaction rate. Microstructural observations suggest that deformation-induced reduction of interconnected porosity causes this change in rate-controlling mechanism. For a natural MgO-SiO2 system, the results infer that fast interface-controlled reaction in the presence of high amounts of water is easily suppressed by concurrent deformation.

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

AbstractMobilization and migration of the heat‐producing elements (HPE) during anatexis is a critical process in the development of orogenic systems, the evolution of continental crust and the stabilization of cratons. In many crustal rocks the accessory minerals are the dominant hosts of Th and U, and the behaviour of these minerals during partial melting controls the concentrations of these elements in draining melt and residue. We use phase equilibrium modelling to evaluate if loss of melt saturated in the essential structural constituents of the accessory minerals can explain the concentrations of Th and U in residual metasedimentary migmatites and granulites along two well‐characterized crustal transects in the Ivrea zone, Italy and at Mt Stafford, Australia. While an equilibrium model of accessory mineral breakdown and melt loss approximates the depletion of U in the residual crust along both transects, it does not explain the relative enrichment of Th. We propose that the high Th concentrations in residual crust may be explained by either inhibition of monazite dissolution by kinetic factors or near‐peak growth of new high Th grains and overgrowth rims on undissolved monazite due to migration of melt through the orogenic crust. Retention of the HPE in the middle and deep orogenic crust may allow metasedimentary granulites to overcome the enthalpy barrier of melting to achieve ultrahigh temperature conditions and may be partly responsible for the slow cooling of many granulite terranes. Lastly, although the mantle was warmer and crustal heat production was higher in the past, peak temperatures and apparent thermal gradients of high‐temperature (HT)–ultrahigh temperature (UHT) granulite terranes have not decreased significantly since the Neoarchean. However, the pressure of HP granulite facies metamorphism has increased gradually from the Archean to the Phanerozoic, which suggests that the lithosphere became stronger as secular cooling of the mantle enabled plate collisions to form thicker orogens. Thus, as the lithosphere became stronger, the proportion of HT–UHT metamorphism associated with thin lithosphere and mantle heat has decreased, whereas the proportion associated with the formation of thick crust and radiogenic heat has increased.