Showing papers on "Phenocryst published in 2019"

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

Abstract: Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere

Crustal controls on apparent mantle pyroxenite signals in ocean-island basalts

Abstract: Ocean-island basalts (OIBs) provide a unique insight into the extent of lithological heterogeneity (peridotite vs. pyroxenite) in Earth’s convecting mantle. However, crustal processing of these mantle melts significantly influences minor-element concentrations in olivine phenocrysts, challenging the suitability of this widely used approach to identify lithological variations in their mantle source. Using a numerical model of magma recharge, mixing, and diffusional reequilibration, we show that this type of crustal processing—which is widely observed in OIBs—results in elevated Ni and lower Ca contents of forsterite-rich olivine, causing erroneously high estimates of the proportion of pyroxenite-derived melt. We applied our model of magma recharge and mixing to several OIBs, including the Galapagos Islands, Canary Islands, and La Reunion. In particular, we critically examined olivine compositional variations in basalts from the eastern Galapagos, which display Sr- and Pb-isotope ratios similar to normal mid-ocean-ridge basalts. While previous interpretations (based on olivine chemistry) argued for a significant contribution from pyroxenite-derived melt, our results indicate that the postulated presence of pyroxenite in the eastern Galapagos mantle is an artifact of processing of magmas and their olivine cargo as they transition through the crust, consistent with major-element and isotopic evidence for a dominantly peridotitic source in this region. This new model for magma recharge and mixing may have important implications for our understanding of lithological heterogeneity beneath OIBs globally, and it highlights the importance of considering crustal processes when attempting to interpret olivine compositions with regard to mantle heterogeneity.

Decadal transition from quiescence to supereruption: petrologic investigation of the Lava Creek Tuff, Yellowstone Caldera, WY

Abstract: The magmatic processes responsible for triggering nature’s most destructive eruptions and their associated timescales remain poorly understood. Yellowstone Caldera is a large silicic volcanic system that has had three supereruptions in its 2.1-Ma history, the most recent of which produced the Lava Creek Tuff (LCT) ca. 631 ka. Here we present a petrologic study of the phenocrysts, specifically feldspar and quartz, in LCT ash in order to investigate the timing and potential trigger leading to the LCT eruption. The LCT phenocrysts have resorbed cores, with crystal rims that record slightly elevated temperatures and enrichments in magmaphile elements, such as Ba and Sr in sanidine and Ti in quartz, compared to their crystal cores. Chemical data in conjunction with mineral thermometry, geobarometry, and rhyolite-MELTS modeling suggest the chemical signatures observed in crystal rims were most likely created by the injection of more juvenile silicic magma into the LCT sub-volcanic reservoir, followed by decompression-driven crystal growth. Geothermometry and barometry suggest post-rejuvenation, pre-eruptive temperatures and pressures of 790–815 °C and 80–150 MPa for the LCT magma source. Diffusion modeling utilizing Ba and Sr in sanidine and Ti in quartz in conjunction with crystal growth rates yield conservative estimates of decades to years between rejuvenation and eruption. Thus, we propose rejuvenation as the most likely mechanism to produce the overpressure required to trigger the LCT supereruption in less than a decade.

Petrogenesis of a Hybrid Cluster of Evolved Kimberlites and Ultramafic Lamprophyres in the Kuusamo Area, Finland

Abstract: Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb ¼ 0·8 6 0·1; Th/ Nb ¼ 0·07 6 0·01; Nb/U ¼ 66 6 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe2þ/(Fe2þ þ Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 6 4·1 Ma (2r), which is considerably older than the ~750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ~100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors.

Petrogenesis of early cretaceous andesite dykes in the Sulu orogenic belt, eastern China

Abstract: This study presents data of in-situ mineral major and trace element chemistry, whole-rock major and trace element and Sr–Nd isotope geochemistry, as well as zircon U–Pb dating and Lu–Hf isotope studies of andesite dykes from the Sulu orogenic belt of eastern China to evaluate their petrogenesis and thus to provide insights into crust–mantle interactions in a tectonic terrane that underwent continental subduction and was then overprinted by oceanic subduction. The andesites mainly comprise plagioclase, hornblende, clinopyroxene and magnetite as phenocrysts that are embedded in a groundmass of fine-grained quartz and K-feldspar, with minor amounts of biotite, magnetite, titanite, apatite and zircon. They possess intermediate concentrations of SiO2 (54.97–62.24 wt.%), Na2O + K2O (6.35–7.24 wt.%) and MgO (3.37–7.12 wt.%) with high Mg# values [Mg# = 100 × Mg/(Mg + Fe2+) molar] of 54–64, and are characterized by arc-like trace element patterns that are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) but depleted in high field strength elements (HFSE). The hornblende and clinopyroxene phenocrysts exhibit enrichment in Th and U but an obvious depletion of HFSE. The andesites have high initial 87Sr/86Sr ratios of 0.7073–0.7086 and negative ɛNd(t) values of −15.7 to −14.4 for whole-rock, and newly crystallized magmatic zircons show negative ɛHf(t) values of −27.0 to −17.6. U–Pb dating on syn-magmatic zircons yields consistent ages of 124 ± 2 to 116 ± 1 Ma, indicating eruption of the andesitic lavas during the Early Cretaceous. Inherited zircons are present and yield Neoproterozoic, Paleoproterozoic and Archean U–Pb ages. Taking into account all these geochemical and geochronological data, and the compositional features of contemporaneously formed mafic–andesitic igneous rocks from the Sulu belt and the adjacent continental crust of the North China Craton, we propose that the andesites crystallized from primary andesitic magmas derived from partial melting of enriched metasomatites in the subcontinental lithospheric mantle. The enriched metasomatites are inferred to be formed by a two-step process: firstly by crust–mantle interactions during the Triassic collisional orogeny and secondly by a subsequent modification by fluids/melts that mainly derive from seafloor sediments during subduction of the paleo-Pacific plate in eastern Asia. Slab rollback of the subducted paleo-Pacific slab and concomitant asthenosphere upwelling are an appropriate geodynamic mechanism for the generation of extensive arc-like magmatism during the Early Cretaceous in the Sulu belt.

Fuzzy petrology in the origin of carbonatitic/pseudocarbonatitic Ca-rich ultrabasic magma at Polino (central Italy).

Abstract: The small upper Pleistocene diatreme of Polino (central Italy) is known in literature as one of the few monticellite alvikites (volcanic Ca-carbonatite) worldwide. This outcrop belongs to the Umbria-Latium Ultra-alkaline District (ULUD), an area characterized by scattered and small-volume strongly SiO2-undersaturated ultrabasic igneous rocks located in the axial sector of the Apennine Mts. in central Italy. Petrographic and mineralogical evidences indicate that Polino olivine and phlogopite are liquidus phases rather than mantle xenocrysts as instead reported in literature. The presence of monticellite as rim of olivine phenocrysts and as groundmass phase indicates its late appearance in magma chambers at shallow depths, as demonstrated by experimental studies too. The absence of plagioclase and clinopyroxene along with the extremely MgO-rich composition of olivine (Fo92–94) and phlogopite (average Mg# ~93) suggest for Polino magmas an origin from a carbonated H2O-bearing mantle source at depths at least of 90–100 km, in the magnesite stability field. In contrast with what reported in literature, the ultimate strongly ultrabasic Ca-rich whole-rock composition (~15–25 wt% SiO2, ~31–40 wt% CaO) and the abundant modal groundmass calcite are not pristine features of Polino magma. We propose that the observed mineral assemblage and whole-rock compositions result mostly from the assimilation of limestones by an ultrabasic melt at a depth of ~5 km. A reaction involving liquidus olivine + limestone producing monticellite + CO2 vapour + calcite is at the base of the origin of the Polino pseudocarbonatitic igneous rocks.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Abstract: Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2‰) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ∼650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ∼10-3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a common mineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions.

Forming olivine phenocrysts in basalt: a 3D characterization of growth rates in laboratory experiments

Abstract: Our current knowledge of the genesis, ascent, storage and eruption of mafic magmas is intimately linked with olivine, its primary crystal cargo. Recent claims that phenocryst-size crystals can grow rapidly and non-concentrically are challenging our perception of what olivine zoning represents (e.g. controlled by growth and/or diffusion), and whether accurate magma crystallization and diffusion timescales can be retrieved. A series of cooling experiments was carried out in order to quantify the kinetics of olivine growth as a function of the degree of undercooling, and to characterize morphological changes occurring with time. Growth rates estimated via 3D microtomography renderings of experimental crystals attain 10-7 m/s, and are found to be almost an order of magnitude higher than those calculated using 2D sections of the same experiments. We show that mm-sized crystals similar to those found in natural Kilauea samples can be produced after a few hours under moderate undercooling conditions (25-60°C). Growth rates decrease faintly with time, accompanying transitions between skeletal/hopper and more polyhedral morphologies. Growth rates generally increase up to -ΔT=40°C, and decrease slightly at -ΔT=60°C as rates of nucleation likely increase. The -ΔT=40°C vicinity may therefore represent a thermal ‘sweet spot’ for the formation of phenocrysts. Olivine overgrowths on crystals that survived initial dissolution grow slower than homogeneously-nucleated crystals, illustrating how new and old crystals in natural magmas likely respond differently to a thermal perturbation. We suggest that the main growth direction of natural olivine (a or c) may be a sensitive function of undercooling and the presence of a pre-existing growth substrate. Olivine grows faster along the a-axis under moderate to high undercooling conditions, while preferred development along the c-axis likely occurs under lower undercooling conditions and/or as rims grow around existing crystals. The early history of skeletal olivine crystals is controlled by diffusion in the melt (diffusion-controlled growth regime), while their long-term compositional zoning history is mainly controlled by diffusive reequilibration in the crystal.

Unravelling the complexity of magma plumbing at Mount St. Helens: a new trace element partitioning scheme for amphibole

Abstract: Volcanoes at subduction zones reside above complex magma plumbing systems, where individual magmatic components may originate and interact at a range of pressures. Because whole-rock compositions of subduction zone magmas are the integrated result of processes operating throughout the entire plumbing system, processes such as mixing, homogenisation and magma assembly during shallow storage can overprint the chemical signatures of deeper crustal processes. Whereas melt inclusions provide an effective way to study the uppermost 10–15 km of the plumbing system, challenges remain in understanding magma intrusion, fractionation and hybridisation processes in the middle to lower crust (15–30 km depth), which commonly involves amphibole crystallisation. Here, we present new insights into the mid-crustal plumbing system at Mount St. Helens, USA, using multiple regression methods to calculate trace element partition coefficients for amphibole phenocrysts, and thus infer the trace element compositions of their equilibrium melts. The results indicate vertically distributed crystal fractionation, dominated by amphibole at higher pressures and in intermediate melts, and by plagioclase at lower pressures. Variations in Nb, Zr and REE concentrations at intermediate SiO2 contents suggest repeated scavenging of partially remelted intrusive material in the mid-crust, and mixing with material from geochemically diverse sources. Amphibole is an effective probe for deep crustal magmatism worldwide, and this approach offers a new tool to explore the structure and chemistry of arc magmas, including those forming plutonic or cumulate materials that offer no other constraints on melt composition.

Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole-rock B–Mo isotopes

Abstract: Cenozoic continental basalts are widespread in central Asia. To explore their source nature and petrogenesis, this study presents an integrated study of olivine chemistry, bulk-rock 40Ar/39Ar age and geochemistry as well as Sr–Nd–Pb–B–Mo isotopes for the Miocene (ca. 15.5 Ma) Halaqiaola basalts in the Chinese Altai, central Asia. The Halaqiaola basalts mostly have basanite compositions with high total alkali (Na2O + K2O = 6.89–8.01 wt%) contents and high K2O/Na2O (0.87–1.39) ratios. Compared with partial melting products of mantle peridotite, the basaltic samples possess lower CaO and CaO/Al2O3 but higher TiO2, Zn/Mn and Zn/Fe values. Meanwhile, olivine phenocrysts from these basalts are characterized by lower Ca, Ni and Mn contents but higher Fe/Mn ratios than their counterparts in the peridotitic melts, suggesting a pyroxenite-rich source. Moreover, these rocks show OIB-like trace element patterns (e.g., spikes of Ba, Sr, Nb and Ta and troughs of Th and U), and constant Nd but variable Sr and EM1-like Pb isotopic compositions, and yield light δ11B (– 11.0 to – 8.1‰) and δ98Mo (– 0.40 to – 0.06‰) values. The above geochemical data suggest that secondary pyroxenite was likely produced by reaction of recycled oceanic crust with its ambient peridotite and subsequently became the main source for the basanite. Furthermore, their light and variable δ98Mo values probably reflect that recycled oceanic crust involved in such pyroxenite was altered with different degrees. In combination with available data from adjacent regions, we propose that the far-field effect of India–Eurasia collision was the first-order factor for the upwelling of dispersive asthenospheric mantle beneath central Asia, subsequent melting of which gave rise to the widespread Cenozoic volcanism.

Magmatic interactions as recorded in plagioclase phenocrysts of quaternary volcanics in SE Bam (SE Iran)

Abstract: In the southeast of Iran (SE Bam), there is a collection of volcanic rocks with andesite, basalt and trachyandesite composition. The textures of these rocks are often porphyritic with microlithic, porphyric cavity, and sometimes glomeroporphyritic, sore throat trachytic. Main minerals include olivine, clinopyroxene, plagioclase and secondary minerals including opaque minerals, Iddingsite, secondary biotite, chlorite and calcite. The analysis of magma textures gives us valuable information about magmatic processes. Micro textures in plagioclase of volcanic rocks in the region are divided into two groups: a) texture-linked to crystalline growth including: sieve texture, oscillatory zoning and degraded surfaces and b) Morphological textures such as glomerular crystals. Sieve texture and zoning in crystal represent processes such as magmatic mixing and abrupt reduction of pressure and, in general, unbalanced conditions in magmatic reservoirs. Based on electron microscope studies, plagioclase of igneous rocks of the region is within the boundaries of labradorite and bytownite. In the southeast of bam, there is a collection of volcanic rocks with andesite, basalt and trachy andesite composition. The texture of these rocks are often porphyritic with microlithic, porphyric cavity, and sometimes glomerulophorphuri, sore throat trachytic. Main minerals include Olivine, Clinopyroxene, Plagioclase and secondary minerals including opaque minerals, Iodenzyte, secondary Biotite, Chlorite and Calcite. Sieve texture and zoning in crystal represent processes such as magmatic mixing and abrupt reduction of pressure and, in general, unbalanced conditions in magmatic reservoirs.

The Influence of Phenocrysts on Degassing in Crystal-Bearing Magmas With Rhyolitic Groundmass Melts

Abstract: The porosity at which a magma becomes permeable (i.e., the percolation threshold; φc) is important for magma degassing; it is also poorly constrained in crystal‐bearing systems. To address this, we conduct high pressure‐temperature decompression experiments on water‐saturated rhyolitic melts with variable crystal contents. We find that crystal‐bearing run products become permeable at ~55‐vol.% vesicularity (crystal free), a value that is similar to that found in decompression‐crystallization experiments using basaltic andesite compositions. Our results provide insight into controls on the eruption styles of hydrous, crystal‐bearing magmas in general and controls on pulsatory Vulcanian behavior, in particular. Plain Language Summary Rates of gas escape from an ascending magma control volcanic eruption style. Gas transport along permeable connected bubble pathways is an important step in the degassing process as it allows pressurized gases to escape. The bubble concentration at which permeability develops (the percolation threshold) is poorly characterized in magmas with moderate to high crystallinities. In this study, we use decompression experiments with controlled crystal contents to address this knowledge gap. Our results show that the presence of at least 20‐vol.% crystals reduces the percolation threshold in crystal‐bearing magmas compared to crystal‐free magmas. This suggests that the presence of crystals will enhance gas escape during magma ascent and has implications for transitions between explosive and effusive eruption styles.

Submarine giant pumice: a window into the shallow conduit dynamics of a recent silicic eruption

Abstract: Meter-scale vesicular blocks, termed “giant pumice,” are characteristic primary products of many subaqueous silicic eruptions. The size of giant pumices allows us to describe meter-scale variations in textures and geochemistry with implications for shearing processes, ascent dynamics, and thermal histories within submarine conduits prior to eruption. The submarine eruption of Havre volcano, Kermadec Arc, in 2012, produced at least 0.1 km3 of rhyolitic giant pumice from a single 900-m-deep vent, with blocks up to 10 m in size transported to at least 6 km from source. We sampled and analyzed 29 giant pumices from the 2012 Havre eruption. Geochemical analyses of whole rock and matrix glass show no evidence for geochemical heterogeneities in parental magma; any textural variations can be attributed to crystallization of phenocrysts and microlites, and degassing. Extensive growth of microlites occurred near conduit walls where magma was then mingled with ascending microlite-poor, low viscosity rhyolite. Meter- to micron-scale textural analyses of giant pumices identify diversity throughout an individual block and between the exteriors of individual blocks. We identify evidence for post-disruption vesicle growth during pumice ascent in the water column above the submarine vent. A 2D cumulative strain model with a flared, shallow conduit may explain observed vesicularity contrasts (elongate tube vesicles vs spherical vesicles). Low vesicle number densities in these pumices from this high-intensity silicic eruption demonstrate the effect of hydrostatic pressure above a deep submarine vent in suppressing rapid late-stage bubble nucleation and inhibiting explosive fragmentation in the shallow conduit.

The prevalence of plagioclase antecrysts and xenocrysts in andesite magma, exemplified by lavas of the Tongariro volcanic complex, New Zealand

Abstract: Beneath subduction zone volcanoes, the parental magmas are thoroughly blended during temporary storage in the crust via repeated heating and mixing from new inputs of magma. To obtain a clearer picture of magma assembly, we employed a sub-crystal-scale Sr-isotope analysis, coupled with textural and geochemical data, on plagioclase phenocrysts in basaltic andesite and andesite erupted from the Tongariro volcanic complex, New Zealand. Deposits from two neighbouring vents were examined: (1) 0.3–3.4 ka lava flows of the polygenetic Red Crater volcano and (2) 26 ka pyroclastics of the monogenetic Pukeonake scoria cone. Despite the differences in eruptive styles and history of the two volcanoes, the plagioclase growth histories are similar. One common phenocryst type comprises of a resorbed core zone (variously, ~ An50–70) surrounded by a sieve-textured mantle/rim of higher An (~ 80), Fe and Mg contents. Other phenocrysts have resorbed calcic cores (~ An70–90). In situ 87Sr/86Sr analyses reveal a wide isotopic range (~ 0.7044–0.7060 for pre-1.8 ka Red Crater; ~ 0.7044–7057 for post-1.8 ka Red Crater; ~ 0.7047–0.7055 for Pukeonake). Most of the values are higher than that of the host bulk rock (87Sr/86Sr ~ 0.7049–0.7051 for pre-1.8 ka Red Crater; 0.7045–0.7046 for post-1.8 ka Red Crater; 0.7048 for Pukeonake). The variation within some individual phenocrysts is comparable to that of the entire population. The higher 87Sr/86Sr ratios are related relic phenocryst cores, indicating that their parental source was more radiogenic than the host magma. In contrast, their sieve-textured mantle/rim zones have lower 87Sr/86Sr ratios (~ 0.7045–0.7049), similar that of their host rock. These features record magma mixing. Less than 10% of phenocryst interiors are cognate (autocrysts) with the host rock based on their high 87Sr/86Sr ratio. Some phenocrysts are likely to be xenocrysts derived from the disintegration of deep crustal meta-igneous rocks. However, based on textures consistent with growth from a melt, the majority are antecrysts from intrusive forerunners and/or crystal mush zones, entrained during eruption or storage. Overall, the andesites are a product of crystal-poor magmas of mantle or lower crustal origin carrying phenocrysts from mostly crustal sources. The magma system likely comprised of a series of semi-isolated melt pods or crystal mush zones in a larger network of intrusions of various ages and states of solidification, possibly hosted in country rock of various compositions. Resorption of the sieve-textured mantle zones on the phenocrysts and growth of a normally zoned outermost rim represents a final period of fractional crystallization in the newly homogenised melt following the ascent and intrusion of mafic magma. Hence, the final tipping point to eruption may have involved crystallization-induced volatile exsolution and degassing.

Oxygen isotope systematics of chondrule olivine, pyroxene, and plagioclase in one of the most pristine CV3Red chondrites (Northwest Africa 8613)

Abstract: We performed in situ oxygen three-isotope measurements of chondrule olivine, pyroxenes, and plagioclase from the newly described CVRed chondrite NWA 8613. Additionally, oxygen isotope ratios of plagioclase in chondrules from the Kaba CV3OxB chondrite were determined to enable comparisons of isotope ratios and degree of alteration of chondrules in both CV lithologies. NWA 8613 was affected by only mild thermal metamorphism. The majority of oxygen isotope ratios of olivine and pyroxenes plot along a slope-1 line in the oxygen three-isotope diagram, except for a type II and a remolten barred olivine chondrule. When isotopic relict olivine is excluded, olivine, low- and high-Ca pyroxenes are indistinguishable regarding Δ17O values. Conversely, plagioclase in chondrules from NWA 8613 and Kaba plot along mass-dependent fractionation lines. Oxygen isotopic disequilibrium between phenocrysts and plagioclase was caused probably by exchange of plagioclase with 16O-poor fluids on the CV parent body. Based on an existing oxygen isotope mass balance model, possible dust enrichment and ice enhancement factors were estimated. Type I chondrules from NWA 8613 possibly formed at moderately high dust enrichment factors (50× to 150× CI dust relative to Solar abundances); estimates for water ice in the chondrule precursors range from 0.2 to 0.6× the nominal amount of ice in dust of CI composition. Findings agree with results from an earlier study on oxygen isotopes in chondrules of the Kaba CV chondrite, providing further evidence for a relatively dry and only moderately high dust-enriched disk in the CV chondrule-forming region.

Grain-scale dependency of metamorphic reaction on crystal plastic strain

Abstract: The Breaksea Orthogneiss in Fiordland, New Zealand preserves water‐poor intermediate and mafic igneous rocks that were partially recrystallized to omphacite granulite and eclogite, respectively, at P ≈ 1.8 GPa and T ≈ 850°C. Metamorphic reaction consumed plagioclase and produced grossular‐rich garnet, jadeite‐rich omphacite, clinozoisite and kyanite. The extent of metamorphic reaction, identified by major and trace element composition and microstructural features, is patchy on the grain and outcrop scale. Domains of re‐equilibration coincide with areas that exhibit higher strain suggesting a causal link between crystal plastic strain and metamorphic reaction. Quantitative orientation analysis (EBSD) identifies gradual and stepped changes in crystal lattice orientations of igneous phenocrysts that are surrounded by homophase areas of neoblasts, characterized by high grain boundary to volume ratios and little to no internal lattice distortion. The narrow, peripheral compositional modification of less deformed garnet and omphacite phenocrysts reflects limited lattice diffusion in areas that lacked three‐dimensional networks of interconnected low‐angle boundaries. Low‐angle boundaries acted as elemental pathways (pipe diffusion) that enhanced in‐grain element diffusion. The scale of pipe diffusion is pronounced in garnet relatively to clinopyroxene. Strain‐induced mineral transformation largely controlled the extent of high‐T metamorphic reaction under relatively fluid‐poor conditions.

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

Abstract: 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.

Petrological and experimental constraints on magma storage for large pumiceous eruptions in Dominica island (Lesser Antilles)

Abstract: The general question of the generation of large-volume silicic eruptions is here addressed through the experimental determination of the storage conditions of the primary magmas that generated ignimbritic eruptions at Dominica Island (Lesser Antilles) during the 24–51 ka period of time. The basal plinian fallouts and pumice pyroclastic flows from the large-volume (~ 5 km3 DRE) events of Layou, Roseau and Grand Fond were investigated, together with the smaller ignimbritic eruptions of Grand Bay and Grande Savane. All samples are dacitic (63–66 wt% SiO2) and contain ~ 30 vol% phenocrysts of plagioclase (~ 21 vol%), orthopyroxene (~ 5 vol%) and Fe-Ti oxides (< 1 vol%), in a rhyolitic matrix glass. The most differentiated samples contain additional amphibole (up to 5 vol%) and quartz. Crystallization experiments were performed starting from Layou and Roseau pumice samples at 800 to 900 °C, 200 to 400 MPa, ~ ΔNNO + 1 and for H2O-saturated and H2O-undersaturated conditions. The main phase contents, assemblages and compositions of both natural samples were reproduced experimentally at ~ 850 °C, ΔNNO + 0.6, 7–8 wt% melt H2O and ~ 400 MPa (~ 16 km depth) consistent with magma ponding at the mid-crustal discontinuity. There is also evidence of more differentiated magma batches that may reflect a plumbing system with a significant vertical extension. The relationships between the chamber depth, width and volume argue for eruptions that do not form collapse calderas, in agreement with field evidence. The erupted magma volumes in Dominica are more than five times larger than those emitted in the neighbouring islands (Martinique, Guadeloupe, Montserrat; < 1 km3), which may be explained by a locally extensional tectonic context that favoured assembly of large magma bodies, but also by the rarity of frequently draining upper crustal reservoirs (as evidenced on the neighbouring volcanic systems) that favoured deep accumulation of large volumes of magma during this period and time for differentiation to dacitic compositions.

Baossi–Warack monogenetic volcanoes, Adamawa Plateau, Cameroon: petrography, mineralogy and geochemistry

Abstract: Three monogenetic cones in the Baossi–Warack area, Ngaoundere, Adamawa Plateau forming part of the Cameroon Volcanic Line (CVL) are documented in this study. Basaltic lavas (< 1 km3) scattered around these vents and restricted volcaniclastic deposits were emplaced by Hawaiian and mild strombolian style eruptions. The lavas are porphyritic, mainly composed of olivine (chrysolite) and clinopyroxene (diopside and augite) phenocrysts and plagioclase (andesine) microphenocrysts. Accessory minerals include titano-magnetite and titano-hematite, nepheline, apatite and amphibole xenocrysts. Sanidine occurs in some samples and sodi-potassic albite in others. Some olivines and clinopyroxenes exhibit resorbed margins and thin reaction rims while plagioclase displays oscillatory zoning, and sieved textures as a result of magma mixing. Whole-rock geochemistry data indicates that the lavas are silica-undersaturated, composed of basanites and basalts, showing little compositional variations (SiO2: 39.20 wt.%–48.01 wt.%, MgO: 5.29 wt.%–9.70 wt.%). Trace elements patterns of these lavas suggest they are enriched in LILE including Pb, probably due to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes, and distinct sources below Warack volcano and nearby lavas. The lavas studied show affinity to high-µ (HIMU), enriched type I (EM1) and Oceanic Island Basalt (OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and polygenetic volcanoes along the CVL. Primary melts derived from low degrees of partial melting (0.5%–2%) and encountered low rates of fractionation, and crustal contamination coupled with magma mixing. These melts evolved independently through structural weaknesses in the basement.

High-resolution Fe–Ti oxide thermometry applied to single-clast pumices from the Bishop Tuff: a re-examination of compositional variations in phenocryst phases with temperature

Abstract: Numerous analyses of multiple ilmenite, titanomagnetite, sanidine, plagioclase, biotite, orthopyroxene, and clinopyroxene crystals (when present) in each of 24 single-clast pumices from the Bishop Tuff (BT), CA are presented, together with whole-rock compositions on the same single clasts. Data from multiple Fe–Ti oxide grains in each sample, coupled with an assessment of analytical uncertainty, show that there is a single population of ilmenite and titanomagnetite in each of 20 samples and no more than two populations of ilmenite and/or titanomagnetite in the remaining four. A statistical evaluation of microprobe uncertainty (for beam conditions of 15 kV and 10 nA) constrains the minimum number of ilmenite (≥ 21) and titanomagnetite (≥ 56) analyses required to obtain Fe–Ti two-oxide temperatures with propagated uncertainties ≤ ± 10 °C. Thermometry results confirm a continuous range (~ 700–800 °C) across the BT deposit. Plagioclase displays a bimodal compositional pattern, where a predominantly sodic (An14–15) vs. calcic (An22–23) population is found in single clasts where pyroxene is absent vs. present; the latter coincides with geochemical evidence for mixing between high-SiO2 rhyolite and less differentiated melts as previously documented in the literature. When sanidine rims are paired with plagioclase, calculated two-feldspar temperatures (~ 690–780 °C) are strongly correlated (R2 = 0.94) with Fe–Ti two-oxide temperatures. Application of plagioclase–liquid hygrometry reveals a systematic decrease in melt water content (~ 7.2–4.1 wt%) with increasing temperature across the BT deposit, consistent with experiments from the literature that show cotectic crystallization of quartz–sanidine–plagioclase in rhyolite melt requires a concomitant decrease of ~ 3 wt% H2O between 700 and 800 °C at 200 MPa. Single-clast, whole-rock concentrations of some elements (e.g., Rb) correlate strongly with Fe–Ti oxide temperature (R2 = 0.95). The Mg# in biotite also correlates strongly with temperature (R2 = 0.97), as do the Mg# values in orthopyroxene and clinopyroxene with whole-rock Mg# (R2 = 0.6 and 0.8, respectively) and melt water content (R2 = 0.7 for both). Evidence points to biotite crystallization in all BT samples from high-SiO2 rhyolite melt, which was already continuously zoned in composition and temperature prior to mixing with less differentiated melts, as previously noted in the literature. In contrast, several lines of evidence, including hydrous phase-equilibrium experiments on rhyolites from the literature, show that both pyroxenes in BT pumices co-crystallized with calcic plagioclase (~ An21–29), in equilibrium with both titanomagnetite and ilmenite, after mixing between less differentiated melt(s) and relatively hot (≥ 755 °C) high-SiO2 rhyolite.