Reverse Zoning Research Articles

Persistent gas emission originating from a deep basaltic magma reservoir of an active volcano: the case of Aso volcano, Japan

Volcanic gas emission is considered to reflect the degassing of magma beneath volcanoes. The combined observations of gas measurement and petrological study are expected to constrain the volatile concentrations and storage depths of the pre-eruptive and primitive magma. Aso volcano (Japan) is a constantly-monitored, persistently-degassing volcano, and an ideal site to acquire gas and petrologic data. We analyzed the melt inclusions and phenocryst minerals of Holocene basaltic eruption products, and reported their major and volatile element concentrations. The samples showed abundant evidence of magma mixing, such as reverse mineral zoning, and highly variable mineral and glass compositions. SiO2 measured in melt inclusions varied from 46.0–65.8 wt. %. High-volatile concentration, S up to 3750 ppm, was only found in mafic melt inclusions hosted by high-Fo olivine phenocrysts (~ Fo82). The pre-eruptive storage depths were determined from volatile concentrations: 2 and 4 km depth for Strombolian eruption and sub-Plinian eruption, respectively. The volatile-rich primitive magma, one end member of the mixed-magma, originated from a deeper level (> 10 km) than these magma reservoirs. Initial volatile concentrations of the primitive magma were determined using multiple constraints: > 4.68 wt. % H2O, 400—750 ppm CO2, 3750 ppm S, 716 ppm Cl, and 324 ppm F. The observed variation of volcanic gas composition was best explained by the mixing of the gas segregated from at least a depth of 10 km, with that from the shallow reservoirs. This study illustrated the method to identify the primitive mafic magma responsible for deep volatile flux in a mature volcano with complex magmatic processes.

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally. It has been exploited for nearly 100 years, yet its origin remains unresolved. In the present study, we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy. Our results indicate that the Merensky Reef formed through a range of diverse processes. Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth. The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow. Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation. The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts. Olivine-hosted melt inclusions containing Cl-rich apatite, sodic plagioclase, and phlogopite suggest the presence of highly evolved, volatile-rich melts. Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase, possibly triggered by flux of heat, acidic fluids, or hydrous melt. Together, these data suggest that the reef formed through a combination of magmatic, hydrodynamic and hydromagmatic processes.

Pre-eruptive storage, evolution, and ascent timescales of a high-Mg basaltic andesite in the southern Cascade Arc

Crustal storage and transit play a critical role in the compositional evolution of arc magmas; however, the enigmatic nature of lower crustal magma storage and early differentiation limit our understanding of the connections between the physical processes of subduction zones and the architecture of the arc crust. We present new geochemical data and applications of existing barometric and chronometric tools to interrogate the mantle source compositions, crustal storage depths, and ascent timescales of a primitive, high-Mg, clinopyroxene-bearing cinder cone, the basaltic andesite of Box Canyon, located in the vicinity of Lassen Volcanic National Park, CA, in the southern Cascades. Petrographic examination in addition to bulk and in situ geochemical analyses (XRF, LA-ICP-MS, and EPMA) of tephra and lava-derived samples reveals co-crystallization of clinopyroxene and olivine as phenocrysts and glomerocrysts with ~ Mg# 80 and > Fo85, respectively. Phase equilibria experiments of analogous Cascade Arc magma compositions estimate crustal storage of the observed phase assemblage at pressures in the lower crust > 700 MPa. Reverse zonation in olivine and clinopyroxene phenocryst interiors from core-rim analytical profiles record a lower crustal mafic mixing event. Results from one-dimensional, multi-elemental olivine and clinopyroxene diffusion models fit to these interior mixing zones provide an assessment of available trace element (Ti, La, Yb, Ce, and Nd) diffusion chronometers in clinopyroxene by considering multiple element profiles within two phases that experienced the same pre-eruptive conditions. While multi-elemental and multi-phase diffusion timescales span two to three orders of magnitude, Ni in olivine profiles provide the most-robust estimate of 19.1 ± 8.6 years from mixing-to-eruption. Our results provide new constraints on arc crustal differentiation processes, indicate rapid crustal transit timescales in agreement with a growing database of diffusion-based ascent timescales of basaltic magmas, and demonstrate significant, systematic deviation of diffusion timescales calculated from natural zonation of rare earth elements in clinopyroxene and Ti in clinopyroxene and olivine.

Negligible surface uplift following foundering of thickened central Tibetan lower crust

AbstractThis study used clinopyroxene (cpx) compositions and zircon Hf-O isotopes of Eocene adakitic rocks (EARs) from the Qiangtang block to resolve the mechanism(s) responsible for the formation of the central Tibetan Plateau. The two leading and opposing hypotheses for the origin of these rocks are (1) partially molten foundered lower crust, and (2) partial melting of continentally subducted upper crust. The consensus is that some crustal sources within the mantle have reached eclogite facies, while evidence remains insufficient. Reverse zonation for cpx in high Mg# andesitic samples shows a low Mg# core with lower Sr and Sr/Y than the high Mg# rim, suggesting derivation of parent magma by interaction between some eclogite-derived felsic melts and mantle peridotite. Overall, the mantle-like zircon δ18O (mean value of ∼5.9‰) and εHf(t) (up to +6.7) values argue for a mafic source rather than buried upper-crustal rocks. Given the EARs were formed within a short time span after the end of crustal shortening, the original felsic melts were most likely derived from the foundered and eclogitized lower crust. The foundering process explains the early Eocene low-relief topography and the intermediate, eclogite-free modern crustal composition of central Tibet. Surface uplift as a response to lithosphere removal, however, was likely negligible, based on various lines of evidence, including sediment provenance, isotope paleoaltimetry, and thermochronology, perhaps because the central Tibetan crust was weak.

Four Stages of the Thermal Evolution of Eclogites from the Maksyutov Complex (South Urals)

Abstract —Based on a detailed electron probe microanalysis of the composition and zoning of coexisting minerals (garnet, clinopyroxene, and plagioclase), we have studied the P–T conditions of formation of high-pressure (HP/UHP) rocks of the Maksyutov eclogite–blueschist complex in the South Urals. We have established its periodic evolution and have determined the specific thermodynamic parameters of mineral formation at each stage of its geodynamic history, from the conditions of crystallization of the protolith to the final stages of retrograde greenschist metamorphism. The new analytical data on the composition of coexisting phases confirm the high-pressure formation of eclogites present as numerous lenses, boudins, and interlayers among blueschist and feldspar–mica schists in the lower part of the complex. Thermobarometric calculations of metamorphism parameters were performed for the Grt + Cpx ± Pl + Qz paragenesis using a Grt–Cpx geothermometer and a Pl–Cpx–Qz geobarometer. Garnet in eclogites of the Maksyutov Complex, being conservative to changes in the P–T conditions, has a direct, reverse, or inverse chemical zoning. Under equilibrium of garnet with omphacitic clinopyroxene, this zoning records conjugate progressive and regressive P–T paths reflecting the thermodynamic conditions at the certain stages of the terrane evolution. Based on the data obtained, we have recognized at least four P–T stages of progressive metamorphic transformations of the Maksyutov Complex: (1) >800–910 °C/~2.5–3.5 GPa; (2) 540–790 °C/2.0–3.5 GPa; (3) 410–690 °C/1.1–2.5 GPa; and (4) 310–520 °C/1.0–1.2 GPa. The estimated P–T parameters of the conjugate regressive stages of metamorphism are as follows: (1) 870–625 °C/3.5–2.5 GPa; (2) 745–615 °C/3.5–2.0 GPa; (3) 690–550 °C/1.5–1.0 GPa; and (4) 590–460 °C/1.2–0.6 GPa, respectively. The age data for the certain stages, along with the parameters of metamorphism, form a single P–T–t path of the complex, which determines the position of the gradient of the metamorphic field during the complex exhumation.

Pre-eruptive magmatic processes associated with the historical (218 ± 14 aBP) explosive eruption of Tutupaca volcano (southern Peru)

Magma recharge into a differentiated reservoir is one of the main triggering mechanisms for explosive eruptions. Here we describe the petrology of the eruptive products of the last explosive eruption of Tutupaca volcano (southern Peru) in order to constrain the pre-eruptive physical conditions (P-T-XH2O) of the Tutupaca dacitic reservoir. We demonstrate that prior to the paroxysm, magma in the Tutupaca dacitic reservoir was at low temperature and high viscosity (735 ± 23 °C), with a mineral assemblage of plagioclase, low-Al amphibole, biotite, titanite, and Fe-Ti oxides, located at 8.8 ± 1.6 km depth (233 ± 43 MPa). The phenocrysts of the Tutupaca dacites show frequent disequilibrium textures such as reverse zonation, resorption zones, and overgrowth rims. These disequilibrium textures suggest a heating process induced by the recharge of a hotter magma into the dacitic reservoir. As a result, high-Al amphibole and relatively high-Ca plagioclase phenocryst rims and microlites were formed and record high temperatures from just before the eruption (840 ± 45 °C). Based on these data, we propose that the recent eruption of Tutupaca was triggered by the recharge of a hotter magma into a highly crystallized dacitic magma reservoir. As a result, the resident dacitic magma was reheated and remobilized by a self-mixing process. These magmatic processes induced an enhanced phase of dome growth that provoked destabilization of the NE flank, producing a debris avalanche and its accompanying pyroclastic density currents.

Zoned orthopyroxenes in the Ni-Co sulfide ore-bearing Xiarihamu mafic-ultramafic intrusion in northern Tibetan Plateau, China: Implications for multiple magma replenishments

The Xiarihamu mafic-ultramafic intrusion is situated in the East Kunlun Orogenic Belt (E-KOB), northern Tibetan Plateau, China, with Ni-Co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite. Whole-rock and mineral compositions show cyclic variations in drill-hole intersections through the Xiarihamu mafic-ultramafic intrusion. Microbeam X-ray fluorescence (XRF) mapping shows reverse zoning (Cr-poor core, Cr-rich rim) and oscillatory zoning (Cr-poor core, Cr-rich mantle and Cr-poor rim) in orthopyroxene of olivine orthopyroxenite and orthopyroxenite. Zoned orthopyroxenes are euhedral or subhedral, and there are relatively sharp interfaces between zones with distinct Cr contents. Crystal-size distribution (CSD) curves of orthopyroxenes in olivine orthopyroxenite show concave-upward curvature at size larger than 3.0 mm, and larger orthopyroxenes commonly display zoning, implying a secondary recrystallization and grain growth of larger orthopyroxenes. Quantitative analyses show that Cr and enstatite contents of cores are relatively lower, but incompatible element (e.g., Ti, Zr, Ga and Sr) contents of cores are higher than rims in reverse zoned orthopyroxenes, and chemical profiles of unzoned orthopyroxenes resemble those of rims (but different from those of cores) in reverse zoned orthopyroxenes.Chemical profiles of orthopyroxenes indicate that Cr zoning is not due to the change in Cr partitioning behavior between orthopyroxene and silicate melt, which shows significant temperature and redox dependence. Textural feature and crystal-size distribution of orthopyroxenes suggest the formation of Cr-rich zones attests to the multi-stage growth in the process of magma replenishment, rather than a reactive liquid flow model or in-situ trapped liquid overgrowth. Orthopyroxene zoning, together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the Xiarihamu mafic-ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition.

Magma chamber processes and dynamics beneath northwestern Anatolia: Insights from mineral chemistry and crystal size distributions (CSDs) of the Kepsut volcanic complex (NW Turkey)

Mineral chemistry, petrography and crystal size distributions of Early Miocene mafic and intermediate lavas from the Kepsut Volcanic Complex (KVC) have been studied to decipher magma chamber processes and dynamics beneath northwestern Anatolia. KVC which is a part of the Western Anatolian Volcanic Province comprises lavas represented by various compositions ranging from basaltic andesite to rhyodacite and associated pyroclastics. Basaltic andesite and andesite lavas of the KVC, the focus of this study, present distinct petrographical features such as three plagioclase generations, patchy zoning, oscillatory reverse zoning, sieve textured crystals and clinopyroxene-mantled amphibole that are clear indicators of disequilibrium conditions in magma chamber(s). Petrography, mineral chemistry and crystal size distribution studies of andesite and basaltic andesite lavas of the KVC indicate that magma evolution occurred in four stages: (I) Near-Equilibrium Stage, (II) Mafic Input Stage, (III) Mixing/Mingling Stage and (IV) Pre-eruption Stage. Thermobarometry calculations indicate that crystallization of andesite and basaltic lavas occurred at similar pressure (mid-to-deep crustal depths, 5–7 kbar) but distinct temperature conditions (920–952 °C and 921–1069 °C, respectively) suggesting that KVC magmas evolved either in the same or different magma chambers at similar depths and experienced open-system processes. Both of these hypotheses are compatible with multistage evolution of the KVC. Combined petrography, mineral chemistry and geothermometry studies on KVC rocks indicate that mixing/mingling processes and subsequent decompression-driven crystallization were the principal mechanisms for the origin of the textural and mineralogical diversity that is characteristic of KVC lavas. Disequilibrium crystallization was probably caused by the influx of a hotter basaltic magma into crustal magma chambers during the first major period of north-south extension in western Anatolia.

Magmatic Processes Associated with Oceanic Crustal Accretion at Slow-spreading Ridges: Evidence from Plagioclase in Mid-ocean Ridge Basalts from the South China Sea

AbstractMagmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

Origin and evolution of the early magmatism in the Oslo Rift (Southeast Norway): Evidence from multiple generations of clinopyroxene

This study presents in-situ major and trace element data on zoned clinopyroxenes in the oldest lavas (B1, ca. 300 Ma) in the Vestfold-Jeløya and Skien areas in the southern part of the Oslo Rift (southeast Norway), in addition to Sr-Nd data on clinopyroxene separates. The aim of the study is to throw light on the causal mechanism for the formation of the Oslo Rift, and the alkaline nature of the magmatism. Two different models have been proposed: (i) passive rifting caused by extension north of the Variscan Front, in which MORB-type melts became enriched through interaction with enriched sub-continental lithospheric mantle (SCLM), and (ii) active rifting in which the magmatic products were produced by partial melting of enriched sub-lithospheric plume material. Clinopyroxene in B1 basalts commonly consist of three types: (1) a resorbed, homogeneous Cr-Mg-rich diopside core (CrMgDi) strongly depleted in Zr-Hf and Yb-Lu, but enriched in MREE relative to LREE and HREE; (2) Cr-poor diopside (MgDi) that forms concentrically zoned overgrowth on the CrMgDi cores, and may form reaction rims on the CrMgDi; and (3) outer zones of augite. The concentrations of incompatible trace elements show a general increase from the CrMgDi, through the MgDi to the outer rims of augite (although reverse zoning is common). In the Vestfold-Jeløya area some CrMgDi cores have ɛNd300 that are significantly higher (≤+6.0) than found in any magmatic rock in the Oslo Rift (≤4.4), implying that the resorbed CrMgDi cores represent xenocrysts from a mantle source that was depleted up to the initiation of the Oslo rifting event: the subcontinental lithospheric mantle (SCLM). High LREE and MREE contents in the CrMgDi are caused by interaction with enriched host magma. The enriched trace element signatures of the B1 magmatism in the Vestfold-Jeløya area is thus a primary feature, as implied by the plume model (ii). In the Skien area, all clinopyroxene types, as well as the host B1 lavas, show HIMU Sr-Nd signatures, supporting former suggestions that the underlying SCLM has suffered local metasomatism during emplacement of the 580 Ma old Fen carbonatite complex. The MgDi in both areas formed by reaction between B1 magmas and CrMgDi xenocrysts, followed by crystallization from magnesian B1 melts undergoing fractional crystallization, magma mixing and assimilation of crustal melts. Progressive evolution of the B1 melts by these processes led to the crystallization of augite.

Insight on magma evolution and storage through the recent eruptive history of Cotopaxi Volcano, Ecuador

Cotopaxi is a stratovolcano in the Northern Volcanic Zone of the Andes, and has a history of bimodal volcanism, alternating between rhyolite and andesite. With Cotopaxi reawakening in 2015 after 138 years of no major eruptions (>VEI 1–2), the question of what is occurring beneath the surface becomes especially poignant in the recent past. This work characterizes the mineralogy and geochemistry of the recent eruptive products of Cotopaxi, with emphasis on the two pulses of the 1877 eruption. Over the past 500 years Cotopaxi has had five major eruptive events (VEI 3–4), which occurred in 1532, 1742, 1744, 1768, and 1877, and included regional tephra fallout, pyroclastic scoria flows and surges, as well as lahars. After the initial pulse of the 1877 eruption and the subsequent lahars, a second pulse of magma produced pyroclastic density currents, containing scoria clasts up to 1 m in diameter. All samples collected from these eruptions are basaltic-andesite to andesite (56–59 wt % SiO2), with a mineral assemblage composed of pl + opx + cpx + mag ± ol. Plagioclase from all samples range from An47 to An78 and show both normal and reverse zoning. Pyroxenes occur as augite and enstatite and do not exhibit significant zoning. The presence of disequilibrium textures such as zoned plagioclase and enclaves with fluidal margins with host indicate that magma mingling/mixing plays a role in magma genesis. The very similar petrologic characteristics of these deposits suggests that they were formed in a relatively long-lived magma system that experienced moderate differentiation and magma replenishment between eruptions. Fractional crystallization modeling shows that crystallization of plagioclase (11.7%) + pyroxene (3.4%) + amphibole (4.6%) + magnetite (1.1%) + apatite (0.2%) can produce the observed compositional variation. However, some of the variation can be explained by mixing, thus we conclude that both differentiation processes occurred. Thermobarometric data indicate that magma storage occurred at temperatures of ∼1017 °C and pressures ranging from 0.215 GPa (during the 1877 eruption) to 0.619 GPa (during the 1532 eruption), which is equivalent to depths of 7 and 21 km (±9 km), respectively.

Magma Replenishment as Revealed by Textural and Geochemical Features of Plagioclase Phenocrysts in Subduction‐Related Lavas

Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high‐temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element (REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end‐member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite (An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface (P1), or normal zoning with euhedral form and no resorption surface (P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin‐section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in P1, concentrations of FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre‐existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeOT and Sr. However, pre‐existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.

To be or not to be oxidized: A case study of olivine behavior in the fusion crust of ureilite A 09368 and H chondrites A 09004 and A 09502.

Meteorite fusion crusts are quenched melt layers formed during meteoroid atmospheric entry, mostly preserved as coating on the meteorite surface. Antarctic ureilite Asuka (A) 09368 and H chondrites A 09004 and A 09502 exhibit well preserved thick fusion crusts, characterized by extensive olivine crystallization. As olivine is one of the major components of most meteorites and its petrologic behavior is well constrained, it can be roughly considered as representative for the bulk meteorite. Thus, in this work, the evolution of olivine in fusion crusts of the above‐listed selected samples is investigated. The different shape and chemistry of olivine crystallized in the fusion crust, both as overgrown rim on relic olivine clasts and as new crystals, suggest a general temperature and cooling rate gradient. The occurrence of reverse and oscillatory zoning in individual olivine grains within the fusion crust suggests complex redox reactions. Overall, the investigated fusion crusts exhibit a general oxidation of the relatively reduced initial material. However, evidence of local reduction is preserved. Reduction is likely triggered by the presence of carbon in the ureilite or by overheating during the atmospheric entry. Constraining these processes provides a potential analog for interpreting features observed in cosmic spherules and micrometeorites and for calibrating experiments and numerical models on the formation of fusion crusts.

Magma Mixing Genesis of the Mafic Enclaves in the Qingshanbao Complex of Longshou Mountain, China: Evidence from Petrology, Geochemistry, and Zircon Chronology

The Qingshanbao complex, part of the uranium metallogenic belt of the Longshou-Qilian mountains, is located in the center of the Longshou Mountain next to the Jiling complex that hosts a number of U deposits. However, little research has been conducted in this area. In order to investigate the origin and formation of mafic enclaves observed in the Qingshanbao body and the implications for magmatic-tectonic dynamics, we systematically studied the mineralogy, petrography, and geochemistry of these enclaves. Our results showed that the enclaves contain plagioclase enwrapped by early dark minerals. These enclaves also showed round quartz crystals and acicular apatite in association with the plagioclase. Electron probe analyses showed that the plagioclase in the host rocks (such as K-feldspar granite, adamellite, granodiorite, etc.) show normal zoning, while the plagioclase in the mafic enclaves has a discontinuous rim composition and shows instances of reverse zoning. Major elemental geochemistry revealed that the mafic enclaves belong to the calc-alkaline rocks that are rich in titanium, iron, aluminum, and depleted in silica, while the host rocks are calc-alkaline to alkaline rocks with enrichment in silica. On Harker diagrams, SiO2 contents are negatively correlated with all major oxides but K2O. Both the mafic enclaves and host rock are rich in large ion lithophile elements such as Rb and K, as well as elements such as La, Nd, and Sm, and relatively poor in high field strength elements such as Nb, Ta, P, Ti, and U. Element ratios of Nb/La, Rb/Sr, and Nb/Ta indicate that the mafic enclaves were formed by the mixing of mafic and felsic magma. In terms of rare earth elements, both the mafic enclaves and the host rock show right-inclined trends with similar weak to medium degrees of negative Eu anomaly and with no obvious Ce anomaly. Zircon LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) U-Pb concordant ages of the mafic enclaves and host rock were determined to be 431.8 5.2 Ma (MSWD (mean standard weighted deviation)= 1.5, n = 14) and 432.8 4.2 Ma (MSWD = 1.7, n = 16), respectively, consistent with that for the zircon U-Pb ages of the granite and medium-coarse grained K-feldspar granites of the Qingshanbao complex. The estimated ages coincide with the timing of the late Caledonian collision of the Alashan Block. This comprehensive analysis allowed us to conclude that the mafic enclaves in the Qingshanbao complex were formed by the mixing of crust-mantle magma with mantle-derived magma due to underplating, which caused partial melting of the ancient basement crust during the collisional orogenesis between the Alashan Block and Qilian rock mass in the early Silurian Period.

Continental crust growth induced by slab breakoff in collisional orogens: Evidence from the Eocene Gangdese granitoids and their mafic enclaves, South Tibet

Collisional orogens are traditionally regarded as main regions related to crustal reworking with limited or without crust growth. However, our study on the granitoids and their mafic enclaves of the Ringqênzê plutonic complex, a representative syn-collisional intrusive complex of Gangdese batholith, shows huge mantle contributions and considerable continental crust growth. Zircon U-Pb dating yields Eocene ages of 50.2 ± 0.5 Ma, 46.4 ± 0.5 Ma, 50.6 ± 0.6 Ma and 50.4 ± 0.5 Ma for the quartz diorite, hornblende granodiorite, host pyroxene-bearing granodiorite and its dioritic enclave, respectively. Reverse zoning of plagioclase (spike zone), pyroxene and plagioclase relicts in both of the dioritic enclaves and the granitoids, as well as the mixing trends on plots of major and trace elements, indicate a petrogenesis of magmatic mixing. The relatively low SiO2 (53.43–56.23 wt%) contents, high Mg# values (48–56) and mineral compositions further suggest that precursor magmas of the enclaves are mantle-derived. The dioritic enclaves and granitoids have indistinguishable (87Sr/86Sr)i ratios (0.70460–0.70480) but slightly different εNd(t) values (+1.8 to +3.0 for the dioritic enclaves and +0.2 to +0.6 for the granitoids). The (87Sr/86Sr)i ratios and εNd(t) values plot on the mixing line defined by depleted mantle (DM) and lower crust xenolith from the Southern Lhasa subterrane and further simulation reveals a contribution of >55% from mantle. In the zircon εHf(t) versus δ18O diagram, the dioritic enclaves (εHf(t) = +3.7–+8.5, δ18O = 4.56–7.00‰) and the granitoids (εHf(t) = +1.2–+8.5, δ18O = 5.59–7.21‰) plot around the mixing line defined by zircons from granulite xenolith and a relatively depleted zircon from the dioritic enclaves. Meanwhile, the simple binary mixing calculation based on Hf-O isotopes suggests that the mantle contributions are not <30% (55–65% in average) to the magma of these granitoids. In addition, the synthesized data show the majority of the syn-collisional Gangdese granitoids were also generated by magma mixing processes with similar large quantity of mantle contributions to the Ringqênzê granitoids. Therefore, the extensive and significant mantle contributions in the magma of the collision-related Gangdese granitoids demonstrate considerable continental crust growth during the syn-collisional period. Furthermore, our data combined with previously published data suggest that the ca. 50 Ma samples have highest Ti-in-zircon temperatures (up to 900 °C) and high pyroxene crystallizing temperatures (>1000 °C), whereas other samples show lower Ti-in-zircon temperatures mostly below 750 °C. Because the Gangdese granitoids with unusual high temperatures had been produced dominantly at a very short time (~50 Ma) and the high temperature was related to underplating of mantle materials, we suggest that a slab breakoff was most likely responsible for the formation of the high-temperature magmas and the continental crust growth at the syn-collisional period in collisional orogens.

Petrology, geochemistry, and zircon U–Pb–Hf isotopes of Late Triassic enclaves and host granitoids at the southeastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China: Evidence for magma mixing during subduction of the Mudanjiang oceanic plate

We report zircon U–Pb ages and geochemistry (major and trace elements, mineral compositions, and Hf isotopic compositions) for the Hailin pluton in the southeastern Songnen–Zhangguangcai Range Massif, northeastern China, to constrain the role of magma mixing in its petrogenesis and to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon U–Pb dating yields ages of 215 ± 1 Ma, 217 ± 1 Ma, and 219 ± 1 Ma for the host granitoids of the Hailin pluton, and 221 ± 1 Ma for mafic microgranular enclaves (MMEs) within the granitoids, indicating they were emplaced coevally. The host granitoids are characterized by 63.18–70.44 wt% SiO2 and 0.69–1.94 wt% MgO, and are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs), and enriched in large ion lithophile elements (LILEs; e.g., Rb, Sr, and Ba) and light rare earth elements (LREEs). The zircons in the host granitoids yield two-stage model ages of 1058 Ma to 1770 Ma, together with εHf(t) values of −6.6 to +3.1. These results indicate that the host granitoid magmas were generated by partial melting of Mesoproterozoic lower crust contaminated by ancient crustal material. The MMEs have a restricted SiO2 range (47.61–57.4 wt%), high contents of Ni, Co, and Cr, and high Mg# values (42–54). They have higher Fe2O3T, MgO, Cr and Co contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. Their REE patterns are sub-parallel to those of the host granitoids, characterized by enrichments in LILEs and LREEs, and depletions in HFSEs and HREEs. Zircons from two samples of the MMEs yield single-stage Hf model ages of 666–1077 Ma and εHf(t) values of −5.4 to +5.2. The geochemical characteristics of the MMEs are similar to those of the host granitoids, indicating crust-mantle interaction during their formation. The mafic magmas were derived from melting of depleted lithospheric mantle that had been metasomatized by slab-derived fluids. MMEs are common in the host granitoids and are characterized by irregular spheroidal shapes, with sharp, rounded, or irregular transitional contacts, acicular apatites, and plagioclase reverse zoning, suggesting the importance of magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Late Triassic Hailin pluton formed by the mixing of felsic and mafic magmas in an active continental margin setting, closely related to the westward subduction of the Mudanjiang oceanic plate beneath the Songnen–Zhangguangcai Range Massif.

Magmatic crystal records in time, space, and process, causatively linked with volcanic unrest

How a volcano has behaved throughout its past is a guide to its future behaviour. Detailed knowledge of what preceded eruptions from specific volcanoes, and how this can be recognised in real-time, are pivotal questions of this field. Here, the physical history of the magma that erupted in 2010 from the flank of Eyjafjallajökull volcano, Iceland, is reconstructed in absolute time and space using only chemical records from erupted crystals. The details of this reconstruction include the number of magma bodies, their geometry, their depth, their relative inflation rate and changes to all of the aforementioned through time. Petrology and geodesy (data gathered in real-time) arrive at the same set of conclusions. As such, we report detailed agreement, which demonstrates a causative link between knowledge determined post-eruption via a physical–chemical perspective and knowledge gained syn-eruption from monitoring signals.The composition of olivine crystal cores (∼Fo74–87), and that of the chemical zonation around each core caused by disequilibrium processes, are shown to form systematic patterns at the population scale. Reverse zonation (toward Mg rich) exhibits a constant chemical offset from its crystal core (≤2 mol % Fo), while normal zonation (toward Fe rich) converges to a single composition (∼Fo75). Conventional petrological models — for instance multiple-magma-mixing across a range of crustal depths — can explain the presence of a range of crystal core composition in the erupted rocks, but cannot explain these patterns of crystal disequilibria.Instead, we describe how a single primitive melt produces crystals over a wide range in composition and generates systematic disequilibrium. Cooling causes crystal production from both roof and floor of a horizontal magma geometry. Crystal settling causes asymmetric thermal – and therefore compositional – stratification of the melt due to progressive insulation via development of a crystal mush at the floor, a process we term “Crystal Rain”. Crucially, each crystal's record is both a cause and effect of the internal process of simultaneous fractional crystallisation and settling; no external processes or materials are required.We then extract temporal information from our crystals using Fe–Mg interdiffusion modelling, and combine it with the composition and zonation data. The concept of Crystal Rain is applied, and resolves two thin (metres) sills which are staggered in time and depth, and exhibit different inflation rates. Since the approach of integrating crystal chronology within a causative physical framework may be applied to entire volcanic successions, it has potential to yield valuable insights to past, and by inference future, magmatic and volcanic behaviours by deterministic means.

Geochronology, geochemistry, and petrology of adakitic Pliocene–Quaternary volcanism in the Şebinkarahisar (Giresun) area, NE Turkey

ABSTRACTThe Pliocene–Quaternary volcanics in NE Turkey are mainly hornblende–phyric trachyandesites having a narrow range of SiO2 from 61.88 to 63.00 wt.% and exhibiting adakitic signatures with their Na2O (3.67–4.27 wt.%), Al2O3 (16.19–16.80 wt.%), Y (14.1–16.5 ppm) contents and K2O/Na2O (0.87–1.12), Sr/Y (44.24–54.90), and La/Yb (36.80–43.88) ratios. Plagioclases as the main mineral phases show a wide range of compositions, and weak normal and reverse zoning. Hornblendes are generally edenite and pargasite (Mg#: 0.39–0.74). Clinopyroxenes are augite (Mg#: 0.58–0.76). Biotites have Mg# ranging from 0.45 to 0.66. The textural and compositional variations indicate disequilibrium crystallization possibly arising from magma mixing. The U–Pb zircon dating of the adakitic volcanics yielded 3.4–1.9 Ma. The studied rocks display moderate light rare earth element /heavy rare earth element ratios and enrichment in the lithophile element and depletion in high field strength element, implying that the parental magmas were derived from mantle sources previously enriched by slab-derived fluids and/or subducted sediments. The crystallization temperature and pressure estimations based on the clinopyroxene thermobarometry range from 1144 to 1186°C and from 3.92 to 7.97 kbar, respectively. Hornblende thermobarometry, oxygen fugacity, and hygrometer calculations yielded results as 908–993°C at a pressure of 2.87–5.22 kbar, water content of 4.4–8.4 wt.%, and relative oxygen fugacity (ΔNNO log units) of −0.6 to 0.9, respectively. Biotite thermobarometry suggests relatively higher oxygen fugacity conditions (10–13.33 to 10–17.60) at temperatures of 676–819°C and at pressures from 1.15 to 1.76 kbar. In the light of the obtained data and modelling, it can be concluded that the magmas of the adakitic volcanics were derived from enriched mantle source through relatively higher partial melting and experienced magma mixing with melts at the crustal level. Additionally, the fractional crystallization and assimilation-fractional crystallization processes may have played an important role during the evolution of the studied volcanics.

The last subduction-related volcanism in the northern tip of the Arabian-Nubian Shield: A Neoproterozoic arc preceding the terminal collision of East and West Gondwana

The medium- to high-K calc-alkaline subduction-related Dokhan volcanic rocks of the Wadi Hamad area record two flare-up events during the history of a Neoproterozoic continental island arc, an early pulse dominated by andesite and a later pulse dominated by dacite. The trace element abundances and (La/Yb)n ratios of the basalts, andesites and dacites all overlap, contrary to the expectations of a single fractional crystallization trend, suggesting the need for three distinct parental magmas for the three groups. The parental magmas of the basalts and andesites were generated by variable degrees of partial melting of subduction-metasomatized mantle, whereas that of the dacites requires mixing of mantle and crustal melts. The andesites and dacites both evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe-Ti oxides in the more evolved dacitic rocks. Oscillatory and reverse zoning in clinopyroxene and plagioclase indicates that magma replenishment and mixing played a role in the genesis of the basalts and andesites. Depth-sensitive geochemical parameters show that the earlier andesites evolved at deeper levels in the arc crust compared with the later dacites, but not so deep as to stabilize garnet as a fractionating phase. Estimated values of the arc crust thickness indicate that the crust thickened from ∼35 km to ∼50 km from the time of basalt eruption to the time of andesite eruption, probably implying a high rate of crustal growth. The estimated arc crust thickness during the later subduction flare-up is slightly less than that of the earlier one despite ongoing magmatic addition, implying that arc crust delamination began before the production of the later dacites. The subduction-related geochemical characteristics of the Dokhan volcanic rocks suggest that the terminal collision between East and West Gondwana in the tip of the Nubian Shield occurred at ∼600 Ma.

Major and trace element, and Sr isotope compositions of clinopyroxene phenocrysts in mafic dykes on Jiaodong Peninsula, southeastern North China Craton: Insights into magma mixing and source metasomatism

Early Cretaceous mafic dyke swarms are widely developed on Jiaodong Peninsula in the southeastern part of the North China Craton (NCC), but their petrogenesis remains enigmatic. We have examined the in-situ major element, trace element and Sr isotope compositions of the clinopyroxene phenocrysts in these dykes in order to evaluate the extent of magma mixing and source metasomatism. Depending on the type of mineral zoning, the clinopyroxene phenocrysts in our samples can be classified into two groups: Group I (reverse zoning) and Group II (no zoning). Based on core compositions, the Group I phenocrysts with obvious reverse zoning can be divided into two subgroups: Groups IA and IB. The cores of Group IA clinopyroxenes have low values of Mg#, low Al2O3 contents, high Na2O contents, and high 87Sr/86Sr ratios, and they were probably derived from newly accreted lower crust that formed through the underplating of basaltic magma. In contrast, the cores of Group IB clinopyroxenes have lower Mg# values and lower contents of Al2O3, ΣREE (total rare earth elements), and incompatible elements, but they have similar 87Sr/86Sr ratios; these cores crystallised from crust-derived andesitic–dacitic magma. Group IA and IB clinopyroxene phenocryst rims (Group I rims) all have similar compositions with higher values of Mg# and higher Al2O3, Cr and Ni contents than the cores. The rims have high 87Sr/86Sr ratios, are enriched in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and are depleted in HFSEs (high field strength elements); these characteristics indicate that all the high-Mg rims were derived from a similar magma, possibly a relatively primitive magma derived from lithospheric mantle. We suggest, therefore, that the reversely-zoned clinopyroxene phenocrysts (Group I) in the Jiaodong mafic dykes provide evidence of magma mixing between a magma derived from lithospheric mantle and crust-derived andesitic–dacitic melt alongside with the newly accreted lower crust. The Group II clinopyroxene phenocrysts, which lack zoning, display major and trace element compositions and 87Sr/86Sr ratios that are similar to those of the Group I rims, which indicates that all the high-Mg clinopyroxenes were derived from a common source in the lithospheric mantle. These high-Mg clinopyroxenes exhibit high 87Sr/86Sr ratios, high Sr contents and remarkable depletions in HFSEs, reflecting metasomatism of the mantle source by aqueous fluids derived by dehydration of the subducting slab and its marine sediments. The metasomatism of the source reveals that the lithospheric mantle beneath Jiaodong Peninsula was metasomatised by fluids from the subducting Paleo-Pacific slab. Progressive thinning of the lithosphere mantle under the NCC was induced by continuous thermo-mechanical erosion, promoting the partial melting of lithospheric mantle and generating the mafic dykes at Jiaodong.