Phenocryst Assemblages Research Articles

Shallow storage of the explosive Earthquake Flat Pyroclastics magma body, Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand: evidence from phase-equilibria experiments

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km3 of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An30, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H2O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H2O and at an fO2 of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and fO2s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.

Textures and Chemistry of Crystal Cargo of the Pleiades Volcanic Field, Antarctica: Potential Influence of Ice Load in Modulating the Plumbing System

Abstract The Pleiades Volcanic Field (PVF) of northern Victoria Land (Antarctica) is made up of a dozen scoria cones whose erupted products present an unusually complete evolutionary trend from alkali-basalt to trachyte. With the aim of reconstructing the evolution of the PVF plumbing system, we have investigated the petrography and chemistry of main mineral phases using scanning electron microscopy (SEM-EDS) coupled with major element analyses using an electron probe microanalyser (EPMA-WDS). We further focussed on clinopyroxene phenocrysts obtaining a more detailed characterization by means of trace laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) element analyses coupled with machine learning thermobarometry. The results indicate that fractional crystallization and magma mixing are the major processes determining the development of the complete evolution trend. While fractional crystallization is a persistently active process in all parts of the plumbing system, mixing among differently evolved magma batches pertaining to the same association is responsible for the formation of intermediate compositions in the differentiation lineage at a specific pressure range (0.4–0.5 GPa). These processes are compatible with significant residence time of magmas at depth, resulting in multiple episodes of magma mixing, as testified by resorption and overgrowth textures in phenocryst assemblage occurring under isobaric conditions. The prolonged residence time likely increased the efficiency of the mixing process, leading to the formation of magmas with intermediate composition. In turn, the build-up of volatiles during the magma differentiation at depth could have favoured the eruption of these (variably differentiated) magmas. Considering that the PVF is situated in a glacial region, a process forcing long magma residence time can be envisaged associated with increased ice loading during glacial stages. This study specifically considers the ice fluctuations in the last 100 ka, theorizing the possibility of a climate-controlled volcano plumbing system.

Petrological and geochemical evidence for a hot crystallization path and a recharge filtering bypass at Antimilos, Milos volcanic field, Greece

Antimilos volcano in the South Aegean Volcanic Arc, Greece, comprises an andesite–dacite suite that follows a distinct evolutionary path than the main edifice of the Milos volcanic field, despite their proximity. Petrographic and geochemical analyses reveal that basaltic andesite to low-Si dacite lavas have similar phenocryst assemblages that indicate crystallization from hot, relatively dry magmas in an upper crustal storage region. Rare antecrystic high-Mg# clinopyroxene cores with low Y, low Dy, and high Sr contents record the cryptic involvement of amphibole, a phase nominally absent from the erupted products, in the deeper parts of the plumbing system. Low temperature antecrysts with textures recording various degrees of disequilibrium suggest a protracted history of interaction between the upper crustal reservoir and deeper mafic melts, forming mobile hybrid magmas that consequently erupt as highly mingled, crystal-rich lava domes. Antimilos magmas seem to have escaped recharge filtering in the upper crust and prolonged stalling, which is the process that is probably responsible for the paucity of mafic eruptions in the rest of the Milos volcanic system. Large extensional structures offshore of Antimilos promote rapid ascent of mafic melts, inhibiting prolonged stalling and interaction with the arc crust. This model highlights the dominant role of the regional stress field in generating petrologically distinct suites in the marginal parts of some volcanic fields.

Silicic frothy xenoliths (xeno-pumice) in recent volcanics from Gran Canaria, Canary Islands

The Quaternary small-volume alkaline magmatic episode on Gran Canaria erupted dominantly basanite and nephelinite lavas and scoria deposits that contain a range of mantle and crustal xenoliths. These xenoliths comprise peridotite nodules, partially melted plutonic and volcanic rock fragments, and a group of light colored, felsic, and commonly frothy quartz-bearing rock fragments (xeno-pumice) that show evidence for intense interaction with their host magmas. Here we study a selection of these felsic and, in part, glassy and vesicular xenoliths from North and North-East Gran Canaria, with the aim to unravel their ultimate origin and learn more about magma storage and ascent within and below the island. Inspection of textures, mineral assemblages and glass compositions reveal one group of felsic xenoliths with fresh to partly altered igneous phenocryst assemblages and relict magmatic textures in addition to δ18O values of 3.6 to 6.6‰. This group is interpreted to be of igneous origin. A second group of frothy felsic xenoliths displays mineralogy and textural characteristics more similar to sedimentary rocks with frequent occurrence of quartz, a mineral usually not present as phenocrysts in magmatic rocks from the Canary Islands. This second group displays relatively high δ18O values (8.1 to 16.8‰), more typical for sedimentary lithologies, and is thus interpreted to represent material derived from the extensive pre-island sedimentary part of the ocean crust. The investigated xenoliths from North Gran Canaria thus provide a snapshot of pre-island sedimentary geology as well as the island's “magmatic” interior. These new data help constrain the available subsurface compositional variations within and below the Canary Islands and will hence be useful in interpreting magma evolution trends and magma storage levels.

A Global Assessment of the Controls on the Fractionation of Arc Magmas

AbstractDuring the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.

Sulfur in apatite from the Nakhla meteorite record a late-stage oxidation event

Estimates of the oxygen fugacity (fO2) recorded by the Martian nakhlite meteorites from direct observations of the main igneous phenocryst assemblages range from values similar to that recorded by the quartz-fayalite-magnetite oxygen buffer to ∼two orders of magnitude lower. Inferences of changes in fO2 during the late stages of crystallization, volcanic degassing, and emplacement of the nakhlite cumulate pile have been made based on variable sulfide and apatite chemistry. We present S-XANES measurements of the oxidation state of sulfur in apatite and associated mesostasis glass in Nakhla to place direct constraints on the magnitude of changes in fO2 experienced by the Nakhla portion of the nakhlite cumulate pile during apatite crystallization. Nakhla apatites range from containing dominantly S2− to containing dominantly S6+. This, together with correlations between S2−, Cl, and FeO in the mesostasis glass near these apatites, suggest that our measurements capture directly the oxidation of the interstitial late-stage Nakhla magmas as the result of Cl-saturation and degassing. As the result of this degassing, at least part of the nakhlite cumulate pile experienced an increase in fO2 of ∼1.5–2.5 orders of magnitude during apatite crystallization and final mesostasis cooling. Based on these measurements, the sulfur oxidation states of apatites in the other nakhlite meteorites are predicted to range from exclusively S2−-bearing to exclusively S6+-bearing.

Development of major element proxies for magmatic H2O content in oceanic basalts

Analysis of H2O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H2O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H2O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H2O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H2O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H2O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al2O3/FeO*(7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H2O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H2O contents. H2O(7.0) positively correlates with Al2O3/FeO*(7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H2O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al2O3/FeO*(7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H2O(7.0) = 1.109Al2O3/FeO∗(7.0) − 1.111. This model enables the estimation of magmatic H2O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages.

Petrology and Sr–Nd isotope geochemistry of Mosonik: a polygenetic phonolitic nephelinite–phonolite volcano located in the North Tanzanian Divergence of the East African Rift

AbstractMosonik, a 3.25 Ma extensively dissected stratovolcano located in the North Tanzanian Divergence of the East African Rift, consists predominantly of phonolite and three types of phonolitic nephelinite distinguished by the presence or absence of amphibole or garnet antecrysts and differing populations of complexly zoned antecrystal and phenocrystal pyroxenes. The antecryst–phenocryst assemblage is typical of hybrid lavas derived by magma mixing. Compositional data are given for all major minerals. Owing to the high modal proportions (30–60 vol. %) of antecrysts and phenocrysts of pyroxene and nepheline plus the hybrid character of the lavas, bulk-rock compositions do not represent those of the parental liquids. Thus, assimilation–fractional crystallization modelling of the bulk-rock major- and trace-element abundances is inappropriate and an unevolved parental magma cannot as yet be defined. Sr–Nd isotopic data for Mosonik and other Older Extrusive Series rocks suggest derivation by partial melting of ancient metasomatized lithospheric mantle with mixing of Sr and Nd from two sources coupled with minor lower crustal contamination, melting being induced by the plume currently impinging on the Tanzanian craton, and representing the initial interaction of the plume with the cratonic lithosphere. In contrast, the Younger Extrusives, as exemplified by Oldoinyo Lengai nephelinite–carbonatite volcanism, could be derived from this ancient metasomatized lithospheric mantle plus a recent plume-derived asthenospheric component and no contamination by crustal material. The isotopically and genetically distinct Natron–Engaruka melilitites are considered to represent direct adiabatic melting of the Tanzanian plume without lithospheric contributions. Carbonatites and melilite-bearing nephelinites also occur at Mosonik but are not considered in this study as they are only a very minor volumetric component of the volcano.

Connected volcanic and plutonic association by crystal-melt segregation in the Daiyunshan volcanic field, SE China

The Daiyunshan volcanic field is one of the largest volcanic fields in the volcanic-plutonic complex belt along the coast of SE China, which records a long volcanic activity history from the Late Jurassic to the Late Cretaceous. In this paper, we conduct comprehensive petrological, geochemical, isotopic and zircon trace element analyses for two representative calderas (Shiniushan and Yunshan) to reveal the crystal-melt segregation and magma recharge processes of the Daiyunshan volcanic field. Systematic LA-ICP-MS zircon U-Pb dating and isotope analyses reveal that the porphyritic quartz monzonite, porphyritic granite and rhyolite of the Shiniushan caldera have identical crystallization ages (97–94 Ma) and consistent Nd-Hf isotope compositions [εNd(t) = −3.6 to −2.8; zircon εHf(t) = −2.7 to 1.8], which are consistent with those of the porphyritic quartz monzonite and rhyolites in the nearby Yunshan caldera, indicating that the magmas of the two calderas were derived from a common magma source region. Volcanic-plutonic rocks from two calderas display metaluminous to peraluminous features and have low MgO, FeOT, Ni and Cr contents. We thus suggest that their parental magmas were derived by remelting of pre-existing crust with contributions of juvenile components. The Shiniushan porphyritic granite and rhyolite and the Yunshan rhyolites are characterized by distinctly negative Eu anomalies and depletions of Ba, Sr, P and Ti, while the porphyritic quartz monzonites from both calderas show complementary geochemical signatures, such as positive Ba anomalies and neglectable Eu anomalies. The back-scattered electron (BSE) images and energy-dispersive X-ray spectrometry (EDS) phase mapping of the Shiniushan porphyritic quartz monzonite reveal textural indicators of crystal accumulation, such as imbrication and synneusis of plagioclase phenocrysts. Their interstitial assemblages of K-feldspar, quartz and plagioclase commonly resemble the phenocryst assemblages of porphyritic granite. We believe that the Shiniushan porphyritic granite and rhyolite and Yunshan rhyolites represent extracted melts from the mush reservoir, while porphyritic quartz monzonites in both calderas are inherent cumulates that have lost interstitial melts. Therefore, our study shows that the Late Cretaceous volcanic-plutonic rocks in the Daiyunshan volcanic field should be connected by crystal-melt segregation processes. Furthermore, a typical zigzag variation in zircon εHf(t) values is shown by the Late Jurassic to Late Cretaceous volcanic-plutonic rocks from the Daiyunshan volcanic field, indicating variable contributions of asthenospheric mantle-derived melts and the multistage slab rollback of the subducting paleo-Pacific plate beneath SE China.

Polymagmatic Glaciovolcanism: Cracked Mountain Tuya, Canadian Cascades

Monogenetic volcanoes are characterized as having no temporal break in eruptive activity and are often assumed to have a simple (singular) magmatic plumbing system. However, recent studies on monogenetic systems have started to recover evidence of complexities within the magma-crustal dynamics. Here we investigate Cracked Mountain (CM), a 401 ± 38 ka glaciovolcanic basaltic landform in southwest, British Columbia, Canada. The volcano covers an area of ∼1.5 km2, has an eruptive volume of ∼0.18 km3, and comprises lapilli tuff, breccia, peperite, pillow and sheet lava, and dykes with no erosional surfaces present between the stratigraphic successions. The paleomagnetic signature of all volcanic lithofacies records a single-pole direction and, in conjunction with stratigraphic evidence, implies a monogenetic eruption. We establish that the Cracked Mountain volcano was fed by two separate crustally-stored magmas (i.e., polymagmatic), each characterized by a unique phenocryst assemblage indicative of different pre-eruptive storage conditions. The first mineral assemblage is an olivine-and-plagioclase phyric (OP) suite, and the second is an olivine-plagioclase-and-augite phyric (OPA) suite. The major-element geochemical compositions of the two petrographic suites vary slightly, with OPA samples higher in SiO2 and total-alkali contents than OP. The two magmas have similar rare earth (REE) trace element signatures, suggesting the same mantle source. We use thermodynamic modeling (Rhyolite-MELTS) to show that the OP suite derives from magma stored at depths &amp;lt;6 km (&amp;lt; 2 kbar) and temperatures of 1240–1155°C. In contrast, the OPA magmas crystallized at depths between 7–9 km (∼2–2.5 kbar) at 1,250–1,150°C prior to eruption. Both magmas are shown to be nearly “dry” having less than 0.5 H2O wt% in their respective systems. We use Pearce Element Ratios (PER) to show that the chemical variations within and between the two CM magmas are controlled solely by the crystal fractionation of two phenocryst assemblages that underwent syn-eruptive mixing. This study concludes that the polymagmatic plumbing system at Cracked Mountain shows similar complexities to other global investigations of monogenetic volcanoes. Lastly, we propose a causal link between the crustal dynamics of magma systems and the impact of crustal loading and unloading during cycles of glaciation.

Plumbing System Architecture of Late-Stage Hotspot Volcanoes in Eastern Australia

Abstract Eastern Australia encompasses the longest track (~2000 km) of age-progressive continental volcanoes on Earth. These so-called ‘central volcanoes’ are shield volcanoes considered as surficial expressions of Cenozoic mantle plume activity under the northward moving Australian continent. Here, we investigate three central volcanoes located in the southern, younger part of the volcanic track (Ebor, Nandewar, and Canobolas) with the aim of unraveling the plumbing system architecture during waning hotspot activity. We explore the duration of volcanic activity and compare long-term evolution of magmatic processes via 40Ar/39Ar geochronology, mineral and groundmass chemistry, mineral-melt thermobarometry, and Rhyolite-MELTS thermodynamic simulations. 40Ar/39Ar geochronology on groundmass and mineral separates indicates that Ebor is the oldest of the three volcanoes, with duration of at least ~1 Ma (20.4 ± 0.09 to 19.4 ± 0.07 Ma). Nandewar also lasted ~1 Ma (19.4 ± 0.03 to 18.5 ± 0.03 Ma). The Canobolas volcanic complex was younger and shorter lived at ~0.5 Ma (12.0 ± 0.02 to 11.55 ± 0.05 Ma). Interestingly, all three volcanoes share a repetitive tempo of ~0.1 Ma between eruptions. The volcanoes produced porphyritic to aphyric lavas with basalt to trachyte compositions. The phenocryst assemblage includes plagioclase and K-feldspar, pink and green clinopyroxene, rare olivine, and titanomagnetite. Textural and compositional zoning of phenocrysts reveals successive events of mafic replenishment and magma transport prior to eruption. Dissolution textures in plagioclase, coupled with increasing An and FeOt and decreasing Ba and Ce from crystal cores to mantles, indicate recharge with mafic, oxidised melt. Increasing Mg# and Cr from clinopyroxene cores to rims also supports primitive magma replenishment. Mineral–melt thermobarometry and Rhyolite–MELTS simulations indicate a main level of magma storage in the three volcanoes in the middle crust (18–25-km depth; ~1100°C), repeatedly replenished by undegassed, primitive melts. Green clinopyroxene cores crystallised in isolated pockets where magmas underwent extensive fractionation at depths of 15 to 30 km and ~800°C. The shallow level plumbing system was volumetrically minor and dominated by crystallisation of low-An plagioclase with large melt inclusions, possibly crystallised from degassed, reduced and evolved magma, as suggested by plagioclase hygrometry and fO2 modelling. Our combined geochronological and geochemical approach reveals that the three spatially separated but genetically linked volcanoes had comparable, complex plumbing system architectures. Fractionation and repeated magma rejuvenation were critical processes throughout the lifespans of volcanism, and eruptive tempos were controlled by recurrent mafic influx. The maficity of lavas and their crystal cargo correlate with the volume fraction of phenocrysts, suggesting mafic recharge was a key driver of mush remobilisation and eruption. Other volcanoes active during the late stages of plume activity in eastern Australia share similar textural and geochemical features, suggesting that waning hotspot activity may result in increased complexity in magma transport and storage.

Magmatic origins and storage conditions for the historic eruption of Tseax Volcano, British Columbia, Canada

The Northern Cordilleran Volcanic Province (NCVP) is the most active volcanic belt in Canada, yet there are few detailed petrologic studies of the individual volcanoes. Tseax volcano is the southernmost volcanic centre of the NCVP, the second youngest (~1700 CE) eruption in Canada, and led to one of Canada's worst natural disasters killing up to ~2000 Nisga'a First Nation people. We present a conceptual model for the origins, evolution and pre-eruptive storage of the Tseax magma which erupted in ~0.5 km3 of volcanic material, mainly in the form of valley-filling lava flows. All Tseax products are alkali Fe- Ti- rich basanite-to-tephrite having trace element distributions similar to the other mafic NCVP lavas. Phenocrysts consist of plagioclase, olivine and titanomagnetite, often forming glomerocrysts suggesting co-crystallisation of these 3 mineral phases. Clinopyroxene is never observed as a phenocryst phase. The lavas have low Mg#’s implying they are fractionated relative to a mantle-derived ‘parent’ magma. Trace elements indicate the magma derives from melting of a fertile mantle source (i.e., (Nb/K)n ~ 1), most likely the upper asthenosphere. There is no evidence in the Tseax volcanic rocks for magma mixing or lithospheric contamination during ascent. The phenocryst assemblage suggests rapid ascent of a low viscosity magma to <5.5 km where titanomagnetite becomes the first phase on the liquidus (~ 1133 °C). There the magma stalled for a very short period of time under PT conditions where clinopyroxene did not reach saturation. Based on the size of the plagioclase phenocrysts, the magma stalled for less than a year and cooled down to 1094–1087 °C prior to eruption.

Trachyte-phonolite transition at Dunedin Volcano: Fingerprints of magma plumbing system maturity and mush evolution

Phonolite-trachyte associations are a common feature of alkaline volcanoes in intraplate settings, and their coexistence challenges closed-system magmatic differentiation scenarios. Here we have investigated the mineralogical and petrochemical features of dikes, lavas, pyroclastic deposits, and comagmatic crystal-rich enclaves outcropping at Dunedin Volcano (Otago region, southern New Zealand). These alkaline magmatic products show both highly and mildly alkaline affinities, trending towards phonolitic and trachytic end-members, respectively. Intermediate rocks are phonotephrites + tephriphonolites (highly alkaline series) and mugearites + benmoreites (mildly alkaline series) with a phenocryst assemblage of clinopyroxene + plagioclase ± amphibole formed at low to mid-crustal levels (i.e., ~29–16 km). Phonolites are porphyritic rocks characterized by alkali feldspar ± amphibole ± clinopyroxene. Their whole-rock compositions are highly enriched in incompatible elements, with variable Ba + Sr contents. A weak negative to slightly positive Eu anomaly is also associated with 87Sr/86Sr ratios of 0.7028–0.7031, which are comparable to those of parental magmas. Geochemical models indicate that phonolites originate as interstitial melts that are generated via abundant alkali feldspar crystallization from a shallow crystalline mush (i.e., ~14–5 km). Strong melt differentiation and extraction is testified by crystal-rich enclaves, as remnants of the mush region. On the other hand, trachytes are phenocryst-poor products strongly depleted in Ba + Sr and with a marked negative Eu anomaly. Trachytes are characterized by 87Sr/86Sr ratios of 0.7040–0.7060, which are different from intermediate rocks and phonolites, and trend towards crustal isotopic compositions. Integrated mass balance, trace element, and energy-constrained modeling confirm that trachytes originate from mildly alkaline magmas interacting with the country rock during feldspar fractionation. We interpret the transition from trachyte to phonolite formation and eruption resulting from the maturation of the plumbing system through accumulation, cooling, and degassing of both highly and mildly alkaline magmas.

Conditions of magma generation at the Me-akan volcano, northern Japan

The Me-akan volcano is one of the most active volcanoes in Japan. In this study, the conditions of magma generation were estimated for basaltic products derived from Akan-fuji, the youngest mafic volcanic edifice at Me-akan, to elucidate the magmatic system. The volcanic products show significant variations in their whole-rock compositions (50.1 to 52.6 wt% SiO2), and can be divided into a low-SiO2 group (<50.4 wt% SiO2) and a main group (>50.7 wt% SiO2) based on Pb isotopic variations. The rocks exhibit the following two groups of phenocryst assemblages: one group contains high-Fo [100 × Mg/(Mg + Fe)] olivine and high-An [100 × Ca/(Ca + Na)] plagioclase phenocrysts (A group), and the other consists of low-Fo olivine, pyroxene, and low-An plagioclase phenocrysts (B group). Our study demonstrates that both the low-SiO2 group and the main group magmas were produced by assimilation and fractional crystallization of distinct basaltic magmas and via subsequent mixing with more primitive magmas in lower crustal magma chambers. It is suggested that the A-group phenocrysts were derived from the primitive magmas, whereas the B-group phenocrysts crystallized in situ before and/or after the occurrence of magma mixing. Multi-component thermodynamic analyses suggest that the primary magmas were segregated from the mantle at 1210–1230 °C and 0.9–1.0 GPa. The observations on estimated melt segregation depths of ~30 km are consistent with seismological observations that low-frequency micro-earthquakes (M 0.0–2.5) occur actively in the uppermost mantle. The generation of primary magmas involved slab-derived fluids of similar composition for both groups. The primary magma for the main group was generated by a greater contribution of the slab-derived fluid relative to the mantle peridotite than that of the low-SiO2 group.

Geochemical Features of Magmas of the Largest Holocene Eruption of Khangar Volcano (Sredinny Range, Kamchatka): Melt Inclusion Evidence

Melt inclusions were investigated in the minerals of dacite tephra of the largest Holocene eruption (7900 years) of Khangar volcano, Sredinny Range of Kamchatka. Melt compositions correspond to rhyolite (SiO2 = 70–77 wt %, Na2O+K2O = 6–7 wt %) with ~5 wt % H2O. The melts show a minor negative Nb anomaly and lesser HREE depletion (La/Yb is ~7.1) compared to those of Ichinsky Volcano, another active volcano of the Sredinny Range of Kamchatka. It was determined that different phenocryst assemblages were formed within temperature ranges of 750–785 and 830–870°C. Evidence for the assimilation of granite-gneiss basement by magmas of Khangar volcano are discussed.

Hekla Revisited: Fractionation of a Magma Body at Historical Timescales

AbstractHekla is an elongate volcano that lies at the intersection of the South Iceland Seismic Zone and the Eastern Volcanic Zone. We report major and trace element, oxygen isotopic, and H2O analyses on rocks, glass, melt inclusions, and minerals from almost all of the historical lavas and tephra deposits. This new dataset confirms the remarkable observation that not only are many eruptions compositionally zoned from felsic to mafic, but the extent of zoning relates directly to the length of repose since the previous eruption. Compositional data are consistent with the origin of the basaltic andesites and andesites by fractional crystallization, with no measurable crustal interaction once basaltic andesite has been produced. Although the 1104 CE Plinian rhyolite and 1158 CE dacite are also created by fractional crystallization, uranium–thorium isotopic disequilibria measured by others require that they evolved in a separate body, where magma is stored in a molten state for &amp;gt;104 years. Consistent trace element trends and ratios, as well as oxygenisotopic data, preclude significant crustal input into the evolving magma. The phenocryst assemblages are dominated by crystals that formed from their host melt; an exception is the 1158 CE dacite, which contains abundant crystals that formed from the 1104 CE rhyolite melt. A suite of thermobarometers indicates that most crystals formed in the lower crust at temperatures ranging from ∼1010 to 850 °C. Hekla’s unique and systematic petrological time series and geophysical activity are attributed to the unusual geometry of the magma body, which we propose to be a tabular, vertically elongate macrodike, extending from the lower to the upper crust. The vertical body is recharged with basaltic andesite magma at the end of each eruption, which then undergoes cooling and crystallization until the subsequent eruption. The entire system is supplied by a lower-crustal body of basaltic andesite, which is produced by fractional crystallization of basaltic magma in a reservoir that is thermochemically buffered to ∼1010 °C. Cooling and crystallization of recharged basaltic andesite magma in a background geothermal gradient from the lower to the shallow crust accounts for the systematic relationship between repose and composition.

Emplacement processes of proto‐arc basalt in the Izu–Bonin–Mariana arc system

AbstractAscertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra‐subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km‐thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast‐spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene‐phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome‐spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation.

Generation of Calc-Alkaline Magmas during Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA

Abstract Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H2O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO2 buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO2 buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO2) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeOT observed in the tephra whole rock compositions. The continuous depletion in FeOT is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeOT via crystallization of spinel, and (2) elevated H2O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Formation of crystal-rich, mixed, intermediate lavas at Pouakai Volcano and the evolution of the Taranaki volcanic lineament, western North Island, New Zealand

Over the past ~2 Ma, volcanic activity in the western North Island of New Zealand has been focused along the NW to SE-younging Taranaki volcanic lineament (TVL). Geophysical imaging of the convergent boundary between the Pacific and Indo-Australian plates shows that the subducted slab lies at least ~200 km below the present TVL and the <200 ka andesitic Mt. Taranaki. Pouakai is the second youngest (~0.25 Ma) volcanic edifice of the TVL and lies immediately NW of Mt. Taranaki. It is deeply eroded with limited exposure, but it has been sampled by a major collapse event that generated the 0.21–0.24 Ma Maitahi debris avalanche deposit. The Pouakai lavas sampled by the Maitahi deposit are medium-K, pyroxene-bearing basalts and amphibole- and pyroxene- bearing basaltic andesites and andesites. Plagioclase, clinopyroxene and amphibole are the dominant phenocryst phases. Olivine and orthopyroxene are rare. Compared to younger Mt. Taranaki lavas, the Pouakai volcanic rocks have lower K, P, Ba, Rb, Sr and Zr and higher Sc and V abundances, and they define subtly different trends on TiO2, MgO and K2O versus SiO2 variation diagrams. Most Pouakai eruptives are similar to the older ring plain debris avalanche deposits of Mt. Taranaki, but Pouakai summit lavas are compositionally similar to younger Mt. Taranaki eruptives. For the Pouakai sample suite, the geochemical trends and patterns are broadly consistent with control by fractional crystallisation (FC), or assimilation fractional crystallisation (AFC). However, textural and geochemical evidence indicates that the rocks are not representative of simple magmas related by a common liquid line of descent. Pouakai basaltic andesites and andesites are highly porphyritic and phenocrysts show complex zoning and disequilibrium reaction textures. Mg# [mol. MgO/(MgO + FeO)] of amphibole and clinopyroxene compositions vary beyond the range expected if the minerals had equilibrated with melts having bulk whole rock compositions. Furthermore, geothermobarometry based on amphibole and pyroxene compositions indicates that mafic phenocrysts in all rock types, as well as in individual samples, equilibrated with comparatively felsic melts over a wide range of pressures. Therefore, each lava sample represents a mixed magma, comprising felsic melts derived from a range of crustal and mantle sources hosting a complex, polybaric phenocryst assemblage. The whole rock compositions were derived through varying degrees of bulk fractionation of phenocryst phases with net removal of amphibole and clinopyroxene and consequent concentration of plagioclase phenocrysts.

Anatomy of a Recharge Magma: Hornblende Dacite Pumice from the rhyolitic Tshirege Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA

The 1.26 Ma Tshirege Member of the Bandelier Tuff is the second of two major (~ 400 km3, dense rock equivalent) compositionally zoned rhyolitic eruptions from the Valles caldera. Here we analyze 25 samples of a minor component of compositionally and texturally variable silicic dacite pumice (~ 67 to 72% SiO2) that is widely distributed through the unit. The dacite has a phenocryst assemblage dominated by feldspar and hornblende and is presumed to be a recharge magma. Quenching of dacite against cooler rhyolite, melting of rhyolitic crystal mush, and mixing between dacite and rhyolite contributed to textural complexity. The dacite can be broken into three petrographic pumice types resulting from different degrees of dacite–rhyolite interaction. The earliest stage in the history of the dacite discernable from mineral chemistry, thermobarometry and hygrometry is mid-crustal storage at temperatures close to 900 °C and water content ~ 5 wt%. Plagioclase zoning suggests that the dacite was subject to more mafic recharge at this stage. The dacite was then injected into rhyolitic crystal mush at temperatures between 700 and 800 °C and pressures ~ 0.3 GPa. Consequences of mixing with mushy rhyolite include the following: (1) cooling and partial crystallization of dacite; (2) growth of large, dendritic feldspars with ternary compositions; (3) ingestion of and melting of feldspar and quartz from the rhyolitic mush; (4) enrichment in fluorine due to melting of biotite in the mush; (5) enrichment in light REE contents in some samples due to melting of chevkinite-rich domain(s) in the mush; (6) second boiling of quenched dacite rendering it buoyant and distributing dacite ‘enclaves’ through the zoned rhyolite magma column. The dacite was likely injected into the rhyolite over a protracted period and eventually triggered the Tshirege eruption.