Plagioclase Rims Research Articles

Late-stage microstructures in Chang’E-5 basalt and implications for the evolution of lunar ferrobasalt

Abstract This study investigates silicate liquid immiscibility (SLI) microstructures in the Chang’E-5 (CE-5) lunar ferrobasalt sample, the youngest recovered mare basalt (ca. ~2.0 Ga). Employing advanced high-resolution imaging techniques and chemical analysis, we examined a subophitic fragment, revealing two distinct types of microstructures indicative of multi-stage SLI events. The first type is observed in the mesostasis pockets and exhibits both “sieve” and “maze” textures, where the Si-K rich glassy phases are interconnected with Fe-rich minerals, e.g., fayalite. This type of microstructure, similar to previous observations in Apollo and Luna samples, is the product of a stable SLI event. The second type is characterized by K-free but high-Si melt inclusions occurring as emulsions in the rims of plagioclase. The entrapment of these emulsions followed a metastable SLI event, with the Fe-rich liquids serving as precursors to subsequent stable SLI processes. Additionally, the Fe-rich droplets within the emulsions underwent coarsening via Ostwald ripening, a phenomenon in which smaller particles in solution dissolve and deposit on larger particles. Our simulation of this coarsening process suggests a duration of at least 15-32 days for the SLI processes, alongside a slow cooling rate (< 0.3 °C/hour) of the late-stage CE-5 lava. We propose that metastable SLI may have influenced the effusive signature of the CE-5 lava flow during its late-stage evolution. The metastable SLI process can potentially lead to the formation of various phases during the late-stage evolution of lunar ferrobasaltic magmas, thereby contributing to the diversity of lunar rock types.

Fluid–Melt–Rock Interaction during the Transition from Magmatism to HT-UHT-Granulite- and Amphibolite-Facies Metamorphism in the Ediacaran Adrar–Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid–melt–rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar–Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P–T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole–plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 ± 0.28 GPa and 975 ± 50°C, and ca. 0.82 ± 0.15 GPa and 894 ± 50°C, to amphibole-facies conditions at ca. 0.28 ± 0.28 GPa and 787 ± 45°C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th–U–Pb zircon ages of the igneous protoliths. An amphibole 40Ar–39Ar cooling age of 499 ± 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole 40Ar–39Ar closure temperatures of 520–555°C were obtained for an age range of 604–499 Ma and an average constant cooling rate of 4.2°C/My, suggesting that the lower-crustal blocks cooled down to the greenschist–amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O–CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

Magma Chamber Process of Post‐collisional Magmatism: Insight from Textural and Elemental Characteristics of Plagioclase from the Tatun Volcanic Group, Northern Taiwan Volcanic Zone

AbstractThe Taiwan mountain belt, one of the youngest orogenies in the world, is caused by the collision of the Luzon arc with the Eurasian margin, which leads to post‐collisional extension and magmatism in the Northern Taiwan Volcanic Zone (NTVZ). The magma chamber process in this region has not previously been elucidated in detail. In this paper, the textural and compositional features of plagioclase phenocrysts in basalt from the Tatun Volcanic Group (TTVG) were studied to restrict the dynamics of magma system. Results show that the magma melts in TTVG are mainly sourced from the underlying MORB‐like mantle wedge but influenced by incorporation of subduction components, causing the elevated Sr/Y and Ba/Y ratios in magma melts. The subduction components are mainly transported in the form of sediment melt. The plagioclase phenocrysts in the TTVG volcanic rocks are generally coarsely core‐sieved with a clear rim. The An contents in the rims of plagioclase are much lower than those of cores, and elevated FeO concentrations are detected in the plagioclase rims. We propose there exists a double‐layer magma chamber in this region. The core of the plagioclase was crystalized in the deeper quiescent magma chamber (∼21 km), which was subsequently partially dissolved during the ascent of magma melt under H2O‐undersaturated condition, forming the typical coarsely sieved texture and synneusis. When this crystal‐rich melt migrates into the shallower chamber, water saturation is reached and more sodic plagioclase formed as the rim of phenocryst. Due to the considerably higher fO2 in the shallow chamber than in the deeper one, and the distribution of Fe between plagioclase and melt positively correlates with fO2, the FeO content in the plagioclase rim elevates in conjunction with increasing fO2.

The Shallow Magmatic Plumbing System of the Deccan Traps, Evidence from Plagioclase Megacrysts and Their Host Lavas

AbstractWe investigate the shallow plumbing system of the Deccan Traps Large Igneous Province using rock and mineral data from Giant Plagioclase Basalt (GPB) lava flows from around the entire province, but with a focus on the Saurashtra Peninsula, the Malwa Plateau, and the base and top of the Western Ghats (WG) lava pile. GPB lavas in the WG typically occur at the transition between chemically distinct basalt formations. Most GPB samples are evolved basalts, with high Fe and Ti contents, and show major and trace elements and Sr-Nd-Pb isotopic compositions generally similar to those of previously studied Deccan basalts. Major element modeling suggests that high-Fe, evolved melts typical of GPB basalts may derive from less evolved Deccan basalts by low-pressure fractional crystallization in a generally dry magmatic plumbing system. The basalts are strongly porphyritic, with 6–25% of mm- to cm-sized plagioclase megacrysts, frequently occurring as crystal clots, plus relatively rare olivine and clinopyroxene. The plagioclase crystals are mostly labradoritic, but some show bytownitic cores (general range of anorthite mol%: 78–55). A common feature is a strong Fe enrichment at the plagioclase rims, indicating interaction with an Fe-rich melt similar to that represented by the matrix compositions (FeOt up to 16–17 wt%). Plagioclase minor and trace elements and Sr isotopic compositions analyzed by laser ablation inductively coupled plasma mass spectrometry show evidence of a hybrid and magma mixing origin. In particular, several plagioclase crystals show variable 87Sr/86Sri, which only partially overlaps with the 87Sr/86Sri of the surrounding matrix. Diffusion modeling suggests residence times of decades to centuries for most plagioclase megacrysts. Notably, some plagioclase crystal clots show textural evidence of deformation as recorded by electron back-scatter diffraction analyses and chemical maps, which suggest that the plagioclase megacrysts were deformed in a crystal-rich environment in the presence of melt. We interpret the plagioclase megacrysts as remnants of a crystal mush originally formed in the shallow plumbing system of the Deccan basalts. In this environment, plagioclase acquired a zoned composition due to the arrival of chemically distinct basaltic magmas. Prior to eruption, a rapidly rising but dense Fe-rich magma was capable of disrupting the shallow level crystal mush, remobilizing part of it and carrying a cargo of buoyant plagioclase megacrysts. Our findings suggest that basaltic magmas from the Deccan Traps, and possibly from LIPs in general, are produced within complex transcrustal magmatic plumbing systems with widespread crystal mushes developed in the shallow crust.

Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.

Crystallographic orientation relationships between quartz and feldspar in myrmekite: a case study in monzodiorite from Meichuan pluton, China

AbstractMyrmekites occurring in monzodiorite from the Meichuan pluton in the Dabie ultrahigh-pressure metamorphic belt were investigated. The petrographic evidence demonstrates a metasomatic origin for myrmekite formation at the scale of individual alkali feldspar grains, and that the myrmekitic quartz and plagioclase matrix are generated simultaneously replacing precursor feldspar. Energy-dispersive X-ray spectroscopy and electron microprobe analysis indicate a low anorthite content in the narrow rim of host plagioclase near the myrmekite–alkali-feldspar interface. The Ca2+, Na+proportion of hydrothermal fluids replacing precursor alkali feldspar is 1:5.4, calculated from the anorthite content of the inner part of the host plagioclase and the neighbouring alkali feldspar. Electron back-scattered diffraction was used to identify the crystallographic orientation of the myrmekitic quartz, plagioclase matrix and the precursor alkali feldspar. The crystallographic orientation relationships (110)Kfs//(11$\bar{2}\bar{1}$)Qtz, (20$\bar{1}$)Kfs//(11$\bar{2}$1)Qtzand [11$\bar{2}3]$Qtz//[001]Kfsbetween myrmekitic quartz and adjacent alkali feldspar were obtained from statistical analysis. No clear crystallographic orientation relationship between quartz and plagioclase was found. The growth of myrmekitic quartz is constrained by the precursor alkali feldspar rather than the simultaneously crystallised plagioclase. This research is helpful for understanding the intergrowth mechanism during metasomatism.

Metamorphic evolution of mafic granulites in the Kuruksayi area of the South Altyn Orogen, West China: Insights from petrography, phase equilibria modeling and geochronology

The South Altyn high-pressure–ultrahigh-pressure (HP–UHP) metamorphic belt in West China records the processes of deep continental subduction (>150–300 km). The HP mafic granulite pods hosted by massive granitic gneiss were for the first time discovered in the Kuruksayi area of the South Altyn Orogen, and they consist mainly of garnet, clinopyroxene, plagioclase, hornblende, biotite, and quartz, with minor rutile, ilmenite, and K-feldspar. The garnets vary in grain size and chemical zoning, and can be divided into three types: subhedral porphyroblastic (g1), anhedral porphyroblastic (g2), and neoblastic fine (g3) grains. Four generations of mineral assemblage (M1–M4) are recognized, and are used to constrain the P–T evolution of the mafic granulites using phase equilibria modeling calculated by THERMOCALC. The first generation (M1) is represented by the core domains of g1 (g1C) that have inclusions of rutile and quartz and conspicuous chemical zoning. A pre-peak prograde stage of metamorphism from ~ 15.8 kbar/600 °C to ~ 19.2 kbar/655 °C is recorded by g1C. The second generation (M2) is inferred to consist of garnet + omphacite + phengite + rutile + quartz and is recorded in the mantle domains of g1 (g1M). Modeling suggests that this is representative of conditions that reached a maximum P–T of ~ 36.2 kbar/920 °C. The third generation (M3) is characterized by intergrowths of clinopyroxene + plagioclase and biotite + plagioclase, and this stage is recorded by the compositions of g2 with modeling suggesting an evolution from ~ 17.8 kbar/820 °C to ~ 11.5 kbar/826 °C, consistent with near-isothermal decompression. The last generation (M4) is characterized by kelyphitic rims of plagioclase + hornblende around garnet and the hornblende coronas that encompass symplectitic clinopyroxene, and is recorded by the compositions of g3. Modeling suggests evolution from ~ 11.5 kbar/826 °C to ~ 8.7 kbar/735 °C, compatible with near-isobaric cooling. These four stages of metamorphism together establish a clockwise P–T path, which indicates a series of tectonic processes that took the continental slab from subduction to exhumation. Zircon U–Pb dating by LA–ICP–MS yielded two distinct clusters of ages: c. 897 Ma magmatic protolith ages and c. 494 Ma metamorphic ages. The metamorphic ages are interpreted to represent the timing of the eclogite-facies metamorphism, and are similar to the metamorphic ages of other HP/UHP rocks elsewhere in the South Altyn Orogen. Accordingly, we propose that the Kuruksayi mafic granulites underwent UHP metamorphism similar to that recorded in other South Altyn eclogites exposed in Jianggalesayi and Danshuiquan to the west and Yinggelisayi to the east.

The Role of Crystal Accumulation and Cumulate Remobilization in the Formation of Large Zoned Ignimbrites: Insights From the Aso-4 Caldera-forming Eruption, Kyushu, Japan

The Aso-4 caldera-forming event (86.4 ± 1.1 ka, VEI-8) is the second largest volcanic eruption Earth experienced in the past 100 ka. The ignimbrite sheets produced during this event are some of the first ever described compositionally zoned pyroclastic flow deposits exhibiting clear compositional, mineralogical and thermal gradients with stratigraphic position. Large quantities of the deposits are composed of crystal-poor, highly evolved juvenile pumices, while late-erupted pyroclastic flows are in many cases dominated by crystal-rich and less silicic scoria. These petrological gradients in the Aso-4 deposits have been linked to extensive magma mixing of two compositionally distinct magmas in a complex upper crustal reservoir. However, new studies on several other zoned ignimbrites suggest that magma mixing alone is not sufficient to fully explain such strong compositional gradients in the deposits. These gradients are expected to be dominantly caused by the recharge-induced reactivation of extracted melt caps and their complementary cumulate in the upper crust. Here, we investigate bulk rock and matrix glass data with detailed analyses of mineral chemistry in order to re-evaluate the Aso-4 deposits in light of these latest developments. Reverse chemical zoning in phenocrysts, Sr enrichment in euhedral rims of plagioclase and the presence of mafic minerals (clinopyroxene, olivine) indicate recharge of hot, mafic magmas shortly prior to eruption, inducing a mixing signature. However, the marked enrichment in some elements in bulk-rock analyses and the presence of highly evolved minerals (some in the form of glomerocrysts) in the late-erupted, crystal-rich units, provide clear evidence for crystal accumulation in these scoria. Mass balance modeling of P2O5, Sr and SiO2 supports the extraction of melt-rich lenses within an upper crustal mush zone, leaving a partly cumulative evolved crystal residue. We therefore propose an origin of the compositionally zoned Aso-4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt caps within its associated cumulate mush. This complex reservoir was reactivated by mafic recharge shortly prior to eruption, imparting an additional mixing signature to the deposits.

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Element and Sr isotope zoning in plagioclase in the dacites from the southwestern Okinawa Trough: Insights into magma mixing processes and time scales

The processes and time scales of magma mixing in shallow silicic magma chambers prior to eruptions in the Okinawa Trough (OT) are still poorly understood. Here, we employed in situ major-trace elements and Sr isotopic analyses, coupled with textural investigations, on plagioclase phenocrysts in the mafic magmatic enclaves (MMEs) and their host dacites from the southwestern OT to address this issue. The plagioclase phenocrysts in the MMEs have high An% contents (82–88) and Sr/Ba ratios (7.2–17.2) and relatively unradiogenic 87Sr/86Sr ratios (0.70536–0.70595), suggesting that they crystallized in the chilled basaltic andesitic magma. In contrast, plagioclase phenocrysts in the host dacites show a wide composition range, with An% contents of An47 to An93, Sr/Ba ratios of 3.4–18.0, and 87Sr/86Sr ratios of 0.70535–0.70791. The plagioclase phenocrysts are zoned, with high-An% cores characterized by resorption textures and mantled by euhedral low-An% rims. The transition between cores and rims is very abrupt, with a decrease of 30 to 40 mol% An. This shift coincides with large and abrupt changes in Sr/Ba ratios. The plagioclase phenocryst rims have low Sr/Ba values (3.4–5.1) and 87Sr/86Sr ratios (0.70632–0.70675) that are similar to those of the whole-rock dacites, suggesting that the plagioclase rims grew from the host dacitic magma. In contrast, Sr isotopes in the plagioclase cores are either radiogenic (0.70724–0.70791) or unradiogenic (0.70535–0.70595) compared with rims, combined with glomerocryst textures, consistent with their derivation from distinct deep mafic crystal mush zones, and were then entrained by ascending melts and transported rapidly to the shallow silicic magma chamber and underwent decompression during ascent, leading to plagioclase core resorption. Mg diffusion calculations indicate that incorporation of the calcic cores into the shallow silicic magma chamber experienced a long pre-eruptive storage (~600 years). The distinct Sr isotopes of the plagioclase, particularly the different compositions of titanomagnetite in the MMEs and host dacites, reflect the mafic magma injection into the silicic magma chamber and formation of the MMEs must have occurred over very short time scales before eruption to inhibit complete re-equilibration, which likely triggered the dacitic magma to finally erupt. Hence, the silicic magma in the southwestern OT contains a crystal cargo with a complex, open-system crystallization history and that magma mixing/mingling is an important process for controlling the chemical and textural diversity of these silicic magmas as well as a likely eruption trigger.

Petrology of the April 2015 Eruption of Calbuco Volcano, Southern Chile

Abstract Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constraining the mechanisms of continental crust formation. We performed a detailed mineralogical and petrological study of the eruptive products from the April 2015 eruption of Calbuco volcano, Chile, a three-phase sub-Plinian eruption that produced pyroclastic deposits of andesitic composition. The eruptive products comprise a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, and orthopyroxene, with minor olivine, amphibole, and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims, and no intermediate compositions between them. Based on thermodynamic calculations and published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite melt containing 3·5–4·5 wt% H2O. The bulk-rock major and trace element variability at Calbuco is best explained by the accumulation of a variable amount of minerals (in relative proportion 72 % plagioclase, 28 % pyroxene) in a dacitic melt. These minerals most probably formed in the crystal mush zone of the magma chamber, at 200–300 MPa (8–11 km depth) according to pyroxene and amphibole compositions. A few weeks to months before the eruption, the crystal mush was disaggregated, perhaps owing to magmatic underplating, and a crystal-bearing dacitic melt migrated into a subsurface storage region where the albitic plagioclase rims crystallized. The eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

Scapolite pegmatite from the Nordøyane ultra-high pressure domain, Western Gneiss Region, Norway: Partial melting driven by infiltration of mantle-derived fluid

Scapolite, a volatile-bearing framework silicate, is found in pegmatites and leucosomes spatially associated with (ultra)-high pressure ((U)HP) eclogites in the Nordøyane domain of the Western Gneiss Region (WGR), Norway. The field relationships, petrology, and stable isotope geochemistry of scapolite-bearing rocks offer insights into the role of fluids in partial melting of WGR gneisses during the Scandian orogeny. Three types of scapolite pegmatite are distinguished based on mineralogy, texture, and scapolite chemistry. The most abundant Type 1 pegmatites are associated with dioritic to granodioritic migmatites, including local diatexite, developed in dioritic to granitoid orthogneisses. They are zoned from scapolite+quartz cores to hornblende+plagioclase rims, and concentrated in low-strain areas including boudin necks between disaggregated eclogite bodies. They are interpreted as late-stage differentiates of the (grano)dioritic migmatites, which also contain accessory scapolite in leucosomes. Type 2 and 3 pegmatites also contain coarse-grained scapolite but lack hornblende-rich selvages; Type 2 scapolites are texturally and chemically distinct from other varities. Type 1 and 3 pegmatites and host rock migmatites contain meionitic scapolite with high S- and C– contents, whereas Type 2 pegmatites contain marialitic scapolite with high Cl-contents. The pegmatites crystallised at amphibolite-facies conditions, ca. 750–800 °C and 1 GPa. Scapolites in Type 1, 2, and 3 pegmatites and host rock leucosomes share similar δ13C (−7.9 to −2.2‰) and δ34S (−1.1 to 5.7‰) compositions, interpreted to indicate a common source of volatile components. Comparison with literature data suggests a mantle source for the S and C in the scapolite-forming fluids. We propose that infiltration of mantle-derived fluids into the Baltican continental slab during subduction or the early stages of exhumation drove partial melting in subducted Baltican crust.

Testing for Pb isotopic differences between minerals in the Kiglapait layered intrusion by LA-ICP-MS

Mineral-scale Pb isotopic variations in cumulates of the Kiglapait intrusion, the largest troctolitic intrusion of the Proterozoic Nain Plutonic Suite in Labrador (Canada), are evaluated directly in thin section by single spot LA-ICP-MS analysis of minerals with low Pb concentrations (e.g., plagioclase = 0.31–3.3 ppm, clinopyroxene = 0.06–1.1 ppm) at high spatial resolution (<100 μm). Analysis of reference materials (BCR-2G, NIST 612, KL2-G, MASS-1) demonstrates that the results are accurate despite the very low amounts of Pb measured (0.04–18 pg) to preserve high spatial resolution. The Pb isotopic ratios of all plagioclase spot analyses overlap within uncertainty and with co-existing clinopyroxene. There are no significant differences between the cores and rims of plagioclase or clinopyroxene, between interstitial or cumulus augite, and with the analyses of interstitial sulfide, biotite, and symplectites (late-stage reactive microstructures). The LA-ICP-MS results support closed-system crystallization of the constituent phases (primocrysts, interstitial minerals) in the Kiglapait cumulates. The results contrast with published Pb isotopic ratios of mineral separates (unleached, leached, leachates) from the Kiglapait intrusion analyzed in solution by MC-ICP-MS where only the relatively unradiogenic leached plagioclase, taken as representative of the initial isotopic composition at 1307 Ma, has the same isotopic composition as that defined by the LA-ICP-MS analyses. A radiogenic Pb component, volumetrically minor and not detected by the LA-ICP-MS analyses (n = 483), appears to have been introduced by a cryptic alteration event from high-temperature fluids during the earliest stages of cooling of the Kiglapait intrusion. Determination of the Pb isotope compositions of common rock-forming minerals, characterized by low Pb concentrations (a few ppm), in mafic layered intrusions can be achieved rapidly by LA-ICP-MS and has the potential to yield significant new insights related to the petrogenesis of plutonic rocks, and phenocryst-bearing volcanic rocks, in the Earth's crust.

The ion microprobe as a tool for obtaining strontium isotopes in magmatic plagioclase: A case study at Okataina Volcanic Centre, New Zealand

We investigated the potential of multi-collector secondary ion mass spectrometry (MC-SIMS) as a tool for obtaining Sr isotopic compositions in plagioclase, a ubiquitous mineral in igneous rocks that serves as a recorder of crystallization history. MC-SIMS allows for high spatial resolution analysis (~12 μm in this study) of isotopes, and therefore improves the temporal scale at which fluctuations in crystallization conditions can be recognized, ultimately improving our understanding of rates of magmatic processes. Plagioclase crystals from two young rhyolitic deposits from two major eruptive complexes, Tarawera and Haroharo, of the Okataina Volcanic Centre in New Zealand were analysed. Results were corrected for matrix effects using linear modelling of MC-SIMS data versus An contents, as well as 87Sr/86Sr ratios acquired via laser ablation inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS). Corrected MC-SIMS Sr isotopic ratios had an average 2σ uncertainty of ±0.0008 per spot, and were homogeneous in Okataina plagioclase at high spatial resolutions. Average LA-MC-ICP-MS 87Sr/86Sr ratios of plagioclase from both intra-caldera volcanic complexes (Tarawera 87Sr/86Sr = 0.7056 and Haroharo 87Sr/86Sr = 0.7054) suggest similar magma sources and similar assimilation and fractional crystallization processes for the two complexes. Overall homogeneity of plagioclase (excluding relict cores) indicates no significant changes in contributions (i.e., crustal assimilation, mafic influx) to the system during the majority of plagioclase crystal growth. Furthermore, lack of 87Sr/86Sr ratio fluctuations in plagioclase rims suggest interaction between the resident silicic magma and the intruding mafic magma that triggered the eruptions was largely limited to volatile and heat transfer. Using appropriate standards and analysis, this MC-SIMS method can be used to detect short-lived, open-system events in magma reservoirs where differences in 87Sr/86Sr isotopic ratios are significant.

Shallow magma pre-charge during repeated Plinian eruptions at Sakurajima volcano

Vigorous explosive eruptions that produce continuous high eruption plumes (Plinian eruptions) are generally assumed to tap a magma reservoir. The 1914 Plinian eruption at the Sakurajima volcano located on the Aira caldera rim is one such case, where the main magma reservoir was assumed to be located approximately 10 km beneath the caldera. However, we report that estimated magma storage depths immediately prior to the eruption were much shallower (0.9–3.2 km) on the basis of pressure at which volatiles within the phenocryst melt inclusions and plagioclase rims were finally equilibrated. The same is observed for two historic Plinian eruptions in 1471 and 1779. This depth is even shallower than the shallowest magma reservoir estimated from the pressure source for geodetic deformation during recent Vulcanian explosions (4 km beneath the crater). We propose that the magmas were fed from a thick conduit pre-charged from deeper reservoirs. The ground subsidence observed after 1914 within the Aira caldera may have been caused by conduit recharge following the eruption. Voluminous conduit recharge could be key to forecasting the next possible large eruption at the Sakurajima volcano.

Abiotic formation of condensed carbonaceous matter in the hydrating oceanic crust

Thermodynamic modeling has recently suggested that condensed carbonaceous matter should be the dominant product of abiotic organic synthesis during serpentinization, although it has not yet been described in natural serpentinites. Here we report evidence for three distinct types of abiotic condensed carbonaceous matter in paragenetic equilibrium with low-temperature mineralogical assemblages hosted by magma-impregnated, mantle-derived, serpentinites of the Ligurian Tethyan ophiolite. The first type coats hydroandraditic garnets in bastitized pyroxenes and bears mainly aliphatic chains. The second type forms small aggregates (~2 µm) associated with the alteration rims of spinel and plagioclase. The third type appears as large aggregates (~100–200 µm), bearing aromatic carbon and short aliphatic chains associated with saponite and hematite assemblage after plagioclase. These assemblages result from successive alteration at decreasing temperature and increasing oxygen fugacity. They affect a hybrid mafic-ultramafic paragenesis commonly occurring in the lower oceanic crust, pointing to ubiquity of the highlighted process during serpentinization.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Sr isotope zoning in plagioclase from andesites at Cabo De Gata, Spain: Evidence for shallow and deep contamination

Plagioclase crystals in andesites from the Cabo De Gata region show generally radiogenic Sr isotope compositions and consistent core to rim increases in 87Sr/86Sr that are indicative of open system processes in the lithosphere and crustal contamination during crystallization. High-grade metamorphic rocks of the Alpujárride and Nevado-Filábride complexes represent the most likely crustal contaminants. The plagioclases are characterized by subtly zoned and resorbed calcic cores (An73–86). These cores also have radiogenic 87Sr/86Sr (0.7127–0.7129), although typically less radiogenic than plagioclase rims, groundmass plagioclase and whole rock compositions (up to 87Sr/86Sr = 0.7135). These cores are interpreted to represent early crystallization of plagioclase from hydrous melts emplaced into the lower crust. The parental melts to these andesites must therefore have already inherited their radiogenic Sr isotope compositions prior to entering the lower crust and before the onset of crystallization of plagioclase, which is inconsistent with previous models suggesting that the generally radiogenic nature of Sr in these volcanics reflects large amounts of crustal contamination. Instead, the isotope systematics are consistent with models invoked significant addition of a subducted sediment component to the mantle source. The high-An% plagioclase cores are characterized by resorption textures, which are consistent with dissolution during rapid decompression and/or devolatisation during magma migration from the lower crust into upper crustal magma chambers.

Vein-type graphite deposits in Sri Lanka: The ultimate fate of granulite fluids

Hydrothermal graphite vein deposits hosted in granulite facies metamorphic rocks in Sri Lanka are unique because of their large size and high crystallinity. In this paper, we present a review of the structural-metamorphic setting of the graphite veins and of the graphite stable carbon isotope data, and we present fluid inclusion data from quartz in the graphite veins and in the host rocks from several graphite deposits in Sri Lanka. The studied host rocks show decompression rims of plagioclase and orthopyroxene after garnet. The quartz in these decompression rim textures comprise, in order of abundance, high-salinity brine, H2O-CO2, and low-salinity aqueous fluid inclusions. The brine fluid is responsible for metasomatic features observed in garnet decompression rims, including feldspar leaching and re-precipitation. Quartz cogenetic with vein graphite comprises, in order of abundance, low-salinity aqueous, CO2, high-salinity brine, and H2O-CO2 fluid inclusions. Published graphite carbon isotope data indicates a dominant mantle source, mixed with small amounts of carbon-bearing fluids of supracrustal origin. We propose that large quantities of mantle-derived CO2 fluid are temporarily stored in the lower crust during the final stage of Gondwana supercontinent amalgamation. The CO2 is subsequently released from the lower crustal rocks during decompression associated with fast uplift. The graphite veins in Sri Lanka were formed during this uplift stage and represent as such paleofluid channels. In this respect, they are comparable to the quartz‑carbonates mega-shear zones found in other granulite terranes. Depending on the redox conditions, mantle CO2 and brines may either result in the formation of graphite or quartz‑carbonate veins.

Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland

New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ, equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re-equilibration during a period of 0.1-0.25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.