Mafic Injection Research Articles

Nucleation delay controlling the formation of mafic enclaves and banded pumice

The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.

Modelling ancient magma plumbing systems through clinopyroxene populations: a case study from Middle Triassic volcanics (Dolomites, Italy)

Modelling plumbing systems dynamics of active volcanoes through textural and chemical studies of mineral phases is crucial to unravel their eruptive behaviour, but it is rarely applied in ancient volcanic and volcano-plutonic systems. Here, we present an investigation of the architecture, magma dynamics and pre-eruptive timing of Middle Triassic plumbing systems in the Dolomites area (Southern Alps) through a detailed investigation of textures and compositional zoning of clinopyroxenes in lavas and dykes from Predazzo, Mt. Monzoni, Cima Pape and Sciliar volcano-plutonic complexes. The clinopyroxene composition varies between low-Mg# (67–78), low-Cr augite and high-Mg# (77–91), Cr-rich diopside. Diopside is less frequent and appears as variably thick single or multiple bands between augitic cores and rims or as resorbed homogeneous, or patchy-zoned and mottled cores. Rims are homogeneous or oscillatory zoned, with augitic composition. The mid- to low-crustal plumbing systems of all volcanic centres were characterised by the presence of a mildly evolved trachyandesitic magma (Mg# 45; T = 1044–1118 °C), where augitic clinopyroxene formed. Periodic mafic injections of more primitive and hotter trachybasaltic magma (Mg# 56; T = 1056–1170 °C) caused frequent crystallisation of diopsidic bands around augitic cores. The presence of resorbed or patchy-zoned mottled diopsidic cores in clinopyroxene phenocrysts, as well as of rare clinopyroxenitic xenoliths with analogous diopsidic composition, indicates the recycling and remobilisation of antecrysts from the deeper part of the plumbing system, which was probably located at a depth of 10–17 km. Diffusion chronometry models based on Fe–Mg interdiffusion in clinopyroxene revealed that the time elapsed from the mafic injection into the shallow portion of the plumbing systems to the eruption ranges from decades to < 1 year. Our findings enabled us to resolve the different plumbing system dynamics acting at the local scale beneath each volcanic centre. Our data indicate a striking similarity with magma dynamics and timing of pre-eruptive processes at active volcanoes: therefore, we posit that this work sets a starting point to deepen our knowledge of the volcano-plutonic links and, in turn improves our ability to interpret the main processes acting in active plumbing systems.

Explosive volcanism of Piton des Neiges (Reunion Island) and excess age dispersion in sanidine: Insights into magma chamber processes in a hotspot setting

Explosive eruptions of trachytic to trachyandesitic composition produced thick and widespread pyroclastic deposits during the recent volcanic activity of Piton des Neiges (PdN), the largest volcano of La Réunion Island (France). However, without precise geochronology for these eruptions, the future behavior of the dormant volcano is difficult to assess. New high-resolution single crystal 40Ar/39Ar dates on alkali feldspar, together with petrochemical data on juvenile clasts, indicate that instead of multiple and small eruptions, PdN produced a single large-volume event (“Dalle Soudée” eruption) at 198.8 ± 2.5 kyrs in which its entire crustal plumbing system was reactivated by mafic injections in the silicic reservoir. After 60 kyrs of quiescence, PdN activity resumed with several small silicic eruptions. From a dome-forming trachytic event (Belouve eruption) represented by some of the most recent block-and-ash flow deposits near the present-day summit we report in-situ 238U/230Th disequilibrium dates on pristine zircons. Sanidine and zircon crystalized at ca. 875 °C in the trachytic magma. For this event, the corresponding 238U/230Th isochron age of the zircons overlaps with the youngest 40Ar/39Ar dates on single sanidine crystals and yields an age of 42.7 ± 3.8 kyr. The variability in trace elements of zircon argues for a prolonged crystallization during this phase prior to eruption. Similarly, the 40Ar/39Ar sanidine dates show a significant spread in both eruptions, covering ca. 80 kyrs, uncommon for a method with a predicted low closure temperature. Such a pattern of age distribution argues for the presence of inherited argon in sanidines and for long residence times in the magma reservoir. The recent eruptive history of Piton des Neiges, involving evolved magma compositions, long-living silicic reservoirs and millennia-long quiescence in volcanic activity, suggests that future potentially hazardous eruptions from this volcano cannot be excluded.

The role of crystal mush in porphyry systems: A case study from the Baishiya ore field, East Kunlun orogenic belt, northern Qinghai-Tibet plateau

Since a cold storage crystal mush reservoir has been gradually used to describe magmas in shallow crust, traditional mechanisms, such as wall-rock assimilation, melt-crystal separation, as well as magma mixing and mingling, must be used with caution because recycled antecrysts can play a significant role. As a result, magmatic processes accounted for texture, geochemistry, and mineralization diversity among ore-related porphyry, overlying (sub)volcanic rocks, and underlying pluton in porphyry systems remain ambiguous. The Baishiya igneous suite (East Kunlun orogenic belt, China) with porphyry-related skarn mineralization provides an opportunity to resolve this issue. Within the crystal mush concept, we propose a comprehensive model based on the subtle changes. First and foremost, large K-feldspar grains characterized by high Sr/Y ratios (52–134) in the top area of granodiorite have a slightly enriched lead isotopic signature, indicating a previously existed crystal mush equilibrated with K-feldspar. The locked crystal mush was injected by mantle-derived mafic melts, leading to a transfer in heat and volatiles from hot melts to cold crystal mush. The buoyant volatiles and loaded metals introduced by episodic mafic replenishments concentrated on the roof, providing a fluid-rich and thermal cycling condition for K-feldspar coarse graining. Some enlarged K-feldspar grains sunk into the bottom area due to gravity, and subsequently reacted with upwelling hot melts to form rapakivi textures. As a gradually waning of injection of mafic melts, crystallization drove fluid exsolution, forming magmatic-hydrothermal events. The finally solidified granodiorite acted as a host rock for a newly produced magma reservoir, but the following development was different. Catastrophic eruptions induced by tectonic stress transition led to a strong depletion of volatiles and associated metal endowment in the underlying magma reservoir, and subsequently left a barren magma source, terminating potential mineralization events. The residual felsic melts migrated upwards along previously formed magma conduits, and thus emplaced as barren porphyry dike. Our work reveals that recycling of K-feldspar antecryst induced by mafic injection contributes to high Sr/Y ratio in ore-related porphyritic granodiorite. Within a long-lived replenishment of mantle-derived magma, a crystal mush reservoir at shallow crustal levels can act as a ‘cooling pond’ to quench these hot melts, but accept volatiles and loaded ore-forming elements, concentrating metals by volatile buoyancy with the help of a compressional stress, which is presumably essential for late fluid exsolution from the reservoir’s roof.

Stretching, Shaking, Inflating: Volcanic-Tectonic Interactions at a Rifting Silicic Caldera

Silicic caldera volcanoes are frequently situated in regions of tectonic extension, such as continental rifts, and are subject to periods of unrest and/or eruption that can be triggered by the interplay between magmatic and tectonic processes. Modern (instrumental) observations of deformation patterns associated with magmatic and tectonic unrest in the lead up to eruptive events at silicic calderas are sparse. Therefore, our understanding of the magmatic-tectonic processes associated with volcanic unrest at silicic calderas is largely dependent on historical and geological observations. Here we utilize existing instrumental, historical and geological data to provide an overview of the magmatic-tectonic deformation patterns operating over annual to 104 year timescales at Taupō volcano, now largely submerged beneath Lake Taupō, in the rifted-arc of the Taupō Volcanic Zone. Short-term deformation patterns observed from seismicity, lake level recordings and historical records are characterized by decadal-scale uplift and subsidence with accompanying seismic swarms, ground shaking and surface ruptures, many of which may reflect magma injections into and around the magma reservoir. The decadal-scale frequency at which intense seismic events occur shows that ground shaking, rather than volcanic eruptions, is the primary short-term local hazard in the Taupō District. Deformation trends near and in the caldera on 101–104 yr timescales are atypical of the longer-term behavior of a continental rift, with magma influx within the crust suppressing axial subsidence of the rift basin within ∼10 km of the caldera margin. Examination of exposed faults and fissures reveals that silicic volcanic eruptions from Taupō volcano are characterized by intense syn-eruptive deformation that can occasionally extend up to 50 km outside the caldera structure, including ground shaking, fissuring and triggered fault movements. We conclude that eruption and unrest scenarios at Taupō volcano depend on the three-way coupling between the mafic-silicic-tectonic systems, with eruption and/or unrest events leading to six possible outcomes initially triggered by mafic injection either into or outside the magma mush system, or by changes to the tectonic stress state.

Decoding multiple zoning patterns in clinopyroxene phenocrysts at Vulcano Island: A record of dynamic crystallization through interconnected reservoirs

Here we document how the different growth features and intracrystalline distributions of both major and trace cations in clinopyroxene phenocrysts are important recorders of the intricate magma dynamics at Vulcano Island (Aeolian Arc, Italy). The compositions of clinopyroxene phenocrysts from products erupted over the last ~54 ka cluster at different degrees of evolution, paralleling the polybaric-polythermal differentiation of mantle-derived mafic magmas into more evolved silicic melts. The hotter lower crust is the most favorable location for the storage of mafic magmas and the early crystallization of diopsidic (Mg#91) clinopyroxene (Pmax ≈ 750 MPa and Tmax ≈ 1220 °C). Diopsidic phenocrysts are depleted in both rare earth elements (REE) and high field strength elements (HFSE) but are enriched in transition elements (TE). The transfer and accumulation of primitive magmas in the colder upper crustal regions lead to the formation of an interconnected series of more differentiated magmatic reservoirs (P ≈ 100–450 MPa and T ≈ 1100–1180 °C) hosting discrete populations of clinopyroxene (Mg#84–85) with a broad spectrum of zonations and dissolution features. Recharge bands in clinopyroxene are markers of multiple inputs of primitive REE-HFSE-poor, TE-rich magmas from depth. Augitic phenocrysts (Mg#82) with strong negative Eu anomaly and REE + HFSE enrichments crystallizes from highly differentiated trachytic and rhyolitic melts stored at very shallow crustal conditions (P ≤ 50 MPa and T ≤ 1100 °C). These silicic reservoirs represent residual melts trapped-extracted from crystal-dominated mush regions in the uppermost part of the plumbing system. The residence time of clinopyroxene increases from ∼0.1 to ∼44 years from basalt to rhyolite, together with an increasing number of recharge bands. The mineral assemblage in more silicic and viscous mush melts is sufficiently resilient to record numerous mafic injections and high degrees of magma mixing, hybridization, and crystallization before eruption. Overall, the compositional zoning pattern of clinopyroxene presents a picture of plumbing system that extends through the crust and is characterized by distributions of melts and crystals which are progressively more evolved and heterogeneous in both space and time.

Interactions of plutons and detachments: a comparison of Aegean and Tyrrhenian granitoids

Abstract. Back-arc extension superimposed on mountain belts leads to distributed normal faults and shear zones interacting with magma emplacement within the crust. The composition of granitic magmas emplaced at this stage often involves a large component of crustal melting. The Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCCs) below crustal-scale low-angle normal faults and ductile shear zones. Intrusion processes interact with extension and shear along detachments, from the hot magmatic flow within the pluton root zone to the colder ductile and brittle deformation below and along the detachment. A comparison of the Aegean plutons with the island of Elba MCC in the back-arc region of the Apennine subduction shows that these processes are characteristic of pluton–detachment interactions in general. We discuss a conceptual emplacement model, tested by numerical models. Mafic injections within the partially molten lower crust above the hot asthenosphere trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain localization on persistent crustal-scale shear zones at the top that guide the ascent until the brittle ductile transition. Once the system definitely enters the brittle regime, the detachment and the upper crust are intruded, while new detachments migrate upward and in the direction of shearing.

Mush remobilisation and mafic recharge: A study of the crystal cargo of the 2013–17 eruption at Volcán de Colima, Mexico

Volcán de Colima is a highly active stratovolcano at the western end of the Trans-Mexican Volcanic Belt. Present-day activity consists of lava dome growth and destruction cycles, lava flows, small explosions, and larger explosive Vulcanian eruptions. It has been postulated that an increased frequency of more mafic eruptions signals the run-up to the end of c. 100-year eruptive ‘cycles’, terminating with a Plinian eruption such as those in 1818 and 1913. It is therefore important to understand the role played by mafic recharge during interplinian activity. We present new petrological and geochemical data for lava and ash from the 2013–17 phase of eruption. The uniform paragenesis and geochemical homogeneity of bulk rocks indicate efficient long-term homogenisation of magmas within the plumbing system, similar to the previous 1998–2005 eruptive products. Mineral chemistry however preserves complex patterns of magma recharge and mixing. Chemical and textural information support the interpretation of two magmatic end-members – an evolved end-member, saturated with respect to Fe--Ti oxides and apatite and crystallising low-An plagioclase and pyroxenes in the Mg# 69–75 range; and a more primitive, mafic end-member, crystallising high-An plagioclase and pyroxenes in the Mg# 77–88 range. Pyroxene textures and zoning patterns suggest mixing of the mafic melts with the evolved magma and remobilisation of the crystal mush. Two-pyroxene geothermometry constrains magmatic temperatures to c. 980–1000 °C for the evolved end-member, and c. 1020–1080 °C for the mafic end-member. Pressure estimates suggest crystallisation at 4–6 kbar, or c. 12–18 km depth. We interpret this to reflect periodic injections of mafic melts and remobilised crystals into evolved reservoirs in a mushy magma storage system in the mid-crust, in agreement with geophysical data suggesting a semi-molten, partially crystallised body at this depth. An increase in reverse zoned crystals, indicative of mafic injection, from mid-2015 onwards suggests that these melts were injected into the system following the large eruption in July 2015. Our findings suggest that the intense July 2015 eruption may be linked to increased input of mafic magmas into the shallow system, indicating that mafic injections may be a key process governing the timing and style of interplinian eruption at Volcán de Colima.

Magmatic Evolution of Zoned and Unzoned Ignimbrites: Evidence for a Complex Crustal Architecture Feeding four Rapid-sequence, Caldera-forming Eruptions in the San Juan Mountains, Colorado

Abstract The last four caldera-forming ignimbrites in the central San Juan caldera cluster, Colorado, erupted 1400 km3 in ≤80 kyr and alternated between zoned crystal-poor rhyolite to crystal-rich dacite and unzoned, crystal-rich dacite. The zoned 150 km3 Rat Creek Tuff (26·91 Ma), unzoned 250 km3 Cebolla Creek Tuff, and zoned 500 km3 Nelson Mountain Tuff (26·90 Ma) formed the nested San Luis caldera complex with slightly offset calderas, and the unzoned 500 km3 Snowshoe Mountain Tuff (26·87 Ma) formed the Creede caldera to the south. The Rat Creek Tuff, Nelson Mountain Tuff, and Snowshoe Mountain Tuff have similar mineral assemblages of plagioclase, sanidine, quartz, biotite, hornblende, clinopyroxene, Fe–Ti oxides, and accessory zircon, titanite, and apatite. The Cebolla Creek Tuff differs from the other three ignimbrites with more abundant hornblende and a lack of quartz and sanidine. Trace element compositions of interstitial glass are unique to each ignimbrite, correlating with mineral assemblages and inferred crystallization depths. Glass, feldspar, hornblende, and clinopyroxene thermobarometry calculations provide evidence for vertically extensive crustal magma reservoirs with a common magmatic zone at ∼435–470 MPa (∼16–17 km) showing a transition into shallow pre-eruptive reservoirs between ∼110 and 340 MPa (∼4–13 km), similar to the estimated magma reservoir architecture of the Altiplano Puna Volcanic Complex. The upper portions of the eruptible parts of the magma reservoirs of the Rat Creek Tuff (215 ± 50 MPa, ∼810–820 °C), Cebolla Creek Tuff (340 ± 20 MPa, ∼860–880 °C), Nelson Mountain Tuff (215 ± 20 MPa, ∼745–800 °C), and Snowshoe Mountain Tuff (110 ± 40 MPa, 825 ± 10 °C) occupied shallow levels in the crust similar to other magma reservoirs of the central San Juan caldera cluster. Trace element modelling correlates with a deep crystallization signature in the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff, typified by a flat trend in Ba versus Sr whole-rock and glass chemistry. The zoned Rat Creek Tuff and Nelson Mountain Tuff are typified by a steep trend in Ba versus Sr chemistry interpreted as a shallower crystallization signature. Similarly, the unzoned Cebolla Creek Tuff and Snowshoe Mountain Tuff have flatter slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant deep plagioclase crystallization and a difficulty in physically mixing with Fe-rich mafic recharge magma owing to higher viscosity. The zoned Rat Creek Tuff and Nelson Mountain Tuff have higher slopes in FeO versus An space of plagioclase chemistry interpreted as more abundant shallow plagioclase crystallization and more feasible mixing with Fe-rich mafic recharge magma owing to lower viscosity. The eruption of the Rat Creek Tuff was probably triggered by mafic injection, but the other three ignimbrites lack mingling textures in pumice, suggesting that other mechanisms were important in causing such large eruptions. After a prolonged period of mantle-derived magma injection and crustal heating (∼25 000 km3 Conejos Formation erupted during ∼35–29 Ma), the San Juan magmatic body became a robust and versatile producer of diverse eruptible magmas in short time periods during its Oligocene ignimbrite flare-up.

U-series histories of magmatic volatile phase and enclave development at Soufrière Hills Volcano, Montserrat

Injection of volatile-rich mafic magma prior to an eruption may trigger episodes of volcanism and can act to transfer metals from depth. However, petrologic knowledge of the timescales from mafic injection to eruption have thus far been focussed on mineral-scale studies of chemical zoning patterns. The study of mafic enclaves dispersed within eruption products can provide insights into the interaction between deep and shallow reservoirs. We combine 238U-230Th-226Ra-210Pb isotope data with trace element concentrations across the interface of two contrasting mafic enclaves in contact with their host andesite from the 2010 eruption at Soufrière Hills Volcano (SHV), Montserrat to investigate the history of mass exchange between the mafic enclave and the andesite host. The application of these time-sensitive isotopes highlights complexities in the transfer of volatiles and metal elements between magmas and the enclaves' potential as eruption triggers. The enclaves exhibit (210Pb/226Ra)0 ratios >1 consistent with volatile input to the subsurface plumbing system a few decades prior to eruption. Samples of the andesitic host, however, which make up the bulk of the eruptive products, have (210Pb/226Ra)0 ≤ 1 suggesting no net volatile gain in the decades leading up to eruption, or that melt-volatile interaction is on a timescale unresolvable by 210Pb226Ra systematics (i.e. <2 years). Variations in trace elements such as Cu, Pb and Ba show loss of a magmatic volatile phase and transport of metals within the deeper part of the plumbing system during differentiation of magmas feeding SHV. Our results do not support that volatile transfer into the andesite via enclaves is a direct trigger of explosive eruptions although the enclaves are likely syn-eruptively formed. 238U-230Th-226Ra-210Pb and trace element systematics at SHV support a role for fresh magma influx during periods of unrest, but long-term accumulation of the andesite.

Mush cannibalism and disruption recorded by clinopyroxene phenocrysts at Stromboli volcano: New insights from recent 2003–2017 activity

The Present-day (<1.2 kyr) activity of Stromboli (Aeolian Islands, Southern Italy) is fed by a vertically-extended mush column with an open-conduit configuration. The eruptive products are the result of periodic supply of mafic magma (low porphyritic or Lp-magma) from depth into a homogeneous shallow reservoir (highly porphyritic or Hp-magma). Clinopyroxene phenocrysts from the 2003–2017 activity exhibit marked diopside-augite heterogeneities caused by continuous Lp-Hp magma mixing and antecryst recycling. Diopsidic bands record Lp-recharge injected into the shallow Hp-reservoir, whereas resorbed diopsidic cores testify to the continuous disruption and cannibalism of relic antecrysts from the mush. The transition between diopside (∼1175 °C) and augite (∼1130 °C) takes place at comparable P (∼190 MPa) and H2O (0.5–2.4 wt%) conditions. Short timescales (∼1 year) for diopsidic bands from the 2003 paroxysm document restricted temporal intervals between mafic injection, magma mixing and homogenization in the Hp-reservoir. Longer timescales (∼4–182 years) for diopsidic cores indicate protracted antecryst remobilization times. By comparing clinopyroxenes from the Present-day and Post-Pizzo eruptions, we argue a distinct phase in the life of Stromboli volcano is evident from the 2003 paroxysm onwards. More efficient mechanisms of mush disruption and cannibalism involve diopsidic antecrysts remobilized and transported by Lp-magmas permeating the mush, in concert with gravitational instability of the solidification front and melt migration within the shallow Hp-reservoir. Magmatic injections feeding the persistent Present-day activity are more intensively mixed and homogenized prior to eruption, reflecting small recharge volumes and/or a more mafic system in which the mafic inputs are less pronounced.

Seismic tomography of compressional wave velocity and attenuation structure for Makushin Volcano, Alaska

Located in the central portion of the Alaska-Aleutian subduction zone, Makushin Volcano is among the most active volcanoes in the United States and has been classified as a very high threat by the National Volcano Early Warning System based on its eruptive history and proximity to the City of Unalaska and international air routes. In 2015, five standalone seismic stations and three mini seismic arrays of 15 stations each were deployed on Unalaska Island to supplement the Alaska Volcano Observatory permanent seismic network. This temporary array was operational for one year. Taking advantage of the increased azimuthal coverage and the array's increased earthquake detection capability, we developed body-wave Vp and Vp/Vs seismic images of the velocity structure beneath the volcano. Our results show a complex structure with the upper 4 km of the crust. The shallow high-Vp features possibly delineate remnant magma pathways or conduits. Low-Vp regions are found east of the caldera at approximately 4–8 km depth. This is in agreement with previous seismic tomographic work and InSAR-based geodetic models, which had identified this region as a possible long-term source of magma. A three-dimensional compressional wave attenuation model complemented the velocity models. In general, we observe regions of corresponding low P-wave velocity and high attenuation east of the summit caldera between 5 and 7 km depth, supporting the presence of a melt-rich zone. The absolute Vp values (5.5–6.0 km/s) in this region are consistent with the presence of a magma body with a mafic signature. We suggest that a triggering mechanism that involves mafic injections into a more evolved shallow magma chamber is representative of the current magmatic system below the volcano, similarly to what petrological studies have inferred for past eruptions.

From magma to mush to lava: Crystal history of voluminous felsic lavas in the Gawler Range Volcanics, South Australia

The Gawler Range Volcanics (GRV) of South Australia are important extrusive analogues to the Mesoproterozoic Hiltaba Suite intrusive rocks of the Gawler Craton, which both host and are implicated in the formation of iron oxide-copper-gold deposits in the Olympic Province. We study three voluminous lavas in the GRV that were emplaced in less than one Ma and contain abundant glomerocrysts of pyroxene, titanomagnetite, plagioclase, K-feldspar, fayalite, apatite, and zircon. Within individual glomerocrysts, pigeonite can exhibit compositional zonation and a range of Y and heavy rare earth element compositions. Compositions of some pigeonite cores from within the same glomerocrysts vary, and crystal zonation does not have a systematic orientation with respect to the crystal's position within its host cluster. This suggests some phenocrysts did not grow adjacent to the other crystals found in individual clusters in the solidified rock, but rather aggregated or settled with other crystals following a stage of growth as independent, melt-bound phenocrysts. The aggregation of phenocrysts is also consistent with the multi-phase nature and diversity of grain-sizes present in the GRV glomerocrysts, rather than accumulations of crystals formed through winnowing of low-density phases or other mechanical sorting processes. In this scenario, glomerocrysts were formed partly by flocculation of independently nucleated crystals, and intergrew after aggregation during magma flow and/or as magma crystallinity increased in the magma reservoir prior to the emplacement of the lavas and the eruption trigger events. The last-erupted lava contains the highest proportion of glomerocrysts comprising complex and anhedral intergrowths. This is consistent with a normally zoned reservoir where the crystal:melt ratio and crystal concentration is greatest at depth. Loosely packed frameworks and free crystals are interpreted to occur in the middle and upper sections of the reservoir, respectively, represented by the first erupted Upper GRV lava and quartz-phyric portions of the lavas. Plagioclase is the most abundant phenocryst throughout the lavas but is present in only a subset of glomerocrysts. The low abundance and absence of feldspar and quartz in glomerocrysts is likely due to mineral resorption and subsequent separation and loss of these components from other glomerocrysts components prior to and during the eruption of the studied lavas. Once solid-solid crystal contacts were dissolved, the separation of the cluster components was driven by the stress placed on them by melt flow. Mafic magmatic enclaves recognised throughout the Upper GRV lavas likely represent the replenishing mafic magmas that provided the thermal and volatile input that drove crystal resorption. Continuing mafic replenishment and injection of sufficient volumes of magma into the Upper GRV reservoir may have also driven the eruption of the lavas. Prolonged fractionation and interaction with mafic magmas could have potentially conveyed or enhanced the distinctive A-type signature of these rocks, negating the requirement of a specialised precursor composition.

Stratigraphy, distribution, and evidence for mafic triggering of the ca. 8.5 ka Driftwood Pumice eruption, Makushin Volcano, Alaska, U.S.A

Makushin Volcano on Unalaska Island, Alaska, threatens the Aleutian's largest population centers (Unalaska and Dutch Harbor), yet its eruption mechanisms are poorly known. This study presents a detailed stratigraphic and geochemical investigation of Makushin's most recent highly explosive event: the ca. 8.5 ka Driftwood Pumice eruption.The Driftwood Pumice has measured thicknesses of over 2.5 m, and isopach reconstructions estimate a total deposit volume of 0.3 to 1.6 km3, indicating a VEI 4–5 eruption. Proximal deposits consist of normally-graded, tan, dacitic to andesitic pumice, capped by a thinner dark layer of lower-silica andesitic scoria mixed with abundant lithic fragments. This stratigraphy is interpreted as an initial vent-clearing eruption that strengthened into a climactic ejection of pumice and ash and concluded with vent destabilization and the eruption of somewhat more mafic, gas-poor magma.Within the pumice, geochemical trends, disequilibrium mineral populations, and mineral zonation patterns show evidence of magma mixing between a bulk silicic magma and a mafic melt. Euhedral high-Ca plagioclase (An68–91) and high-Mg olivine (Fo69–77) phenocrysts are in disequilibrium with trachydacitic glass (65–68 wt% SiO2) and more abundant sodic plagioclase (An34–55), indicating the former originally crystallized in a more mafic melt. Tephra whole rock compositions become more mafic upwards through the deposit, ranging from a basal low-silica dacite to an andesite (total range: 60.8–63.3 wt% SiO2). Collectively, these compositional variations suggest magma mixing in the Driftwood Pumice (DWP) magma reservoir, with a systematic increase in the amount of a mafic component (up to 25%) upward through the deposit. Olivine-liquid and liquid-only thermometry indicate the mafic magma intruded at temperatures ~140–200 °C hotter than the silicic magma. Diffusion rates calculated for 5–7 μm thick, lower-Mg rims on the olivine phenocrysts (Fo60 rim vs Fo76 bulk) suggest that the eruption occurred several days to weeks following the mafic injection into a dacitic reservoir. Based on this timing, we infer that the mafic intrusion provided a thermal pulse that initiated convection and volatile exsolution, and ultimately resulted in the DWP eruption. Unalaska's Holocene stratigraphy includes multiple light-dark ashfall couplets with physical and geochemical similarities to the DWP, suggesting that magma mixing may be a common eruptive trigger at Makushin Volcano.

Intermittent generation of mafic enclaves in the 1991–1995 dacite of Unzen Volcano recorded in mineral chemistry

Mafic enclaves in the 1991–1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and hornblende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemistry. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO2 contents (56–60 wt%) when compared with the type-III mafic enclaves (SiO2 = 53–59 wt%), but the overall bulk enclave compositions are within the trend of the basalt–dacite eruptive products of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of plagioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystallized at high temperature and were subsequently re-equilibrated at low-temperature conditions. Compositional profiles of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991–1995 Unzen eruption. Presence of different types of mafic enclaves suggests that the 1991–1995 dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas.

MECHANISMS OF MANTLE‐CRUST INTERACTION AT DEEP LEVELS OF COLLISION OROGENS (CASE OF THE OLKHON REGION, WEST PRIBAIKALIE)

In the Chernorud granulite zone in the Olkhon region of West Pribaikalie, we studied gabbro‐pyroxenites composing tectonic plates (Chernorud, Tonta) and synmetamorphic intrusive bodies (Ulan‐Khargana), as well as nu‐ merous disintegrated boudins and inclusions embedded in the metamorphic matrix. Based on the results of compara‐ tive analysis of the chemical compositions, the gabbro‐pyroxenites are classified into a single island‐arc tholeiitic se‐ ries. The COMAGMAT software was used to simulate this series and to estimate the initial composition of the parent magma (magnesian basalt: SiO2=46.0 wt. %, TiO2=0.8 wt. %, Al2O3=15.3 wt. %, ΣFeO=9.0 wt. %, MnO=0.15 wt. %, MgO=10.5 wt. %, CaO=17.0 wt. %, Na2O=1.0 wt. %, K2O=0.2 wt. %, P2O5=0.05 wt. %, total = 100.0 %, Mg# = 67.5 %). It is concluded that the granulite metamorphism (P=7.7 to 8.6 kbar, T=770 to 820 °C) was due not only to dipping of the initial sedimentary‐volcanic series to a depth of 25–28 km, but also to the presence of a deep chamber of magnesian basalt magma. In our estimations, garnet‐pyroxenites (i.e. mafic rocks of the top facies in the above‐mentioned cham‐ ber) originated at P=8.0–8.3 kbar and T=900–930 °C. Considering petrology, the deep mafic chamber under the layer of granulite facies is evidenced by metamorphic magma mingling, as well as pipe‐shaped intrusions characterized by the specific morphology, internal structure and bulk rock compositions. Based on the data on the Ulan‐Khargana mas‐ sif and gabbro‐pyroxenite bodies involved in the structure of the marble melange, we propose a petrological model showing two stages of mafic injection – Stage 1: hydraulic fracturing of granulite series and the emergence of tubular structures and bodies, which are similar to kimberlite pipes or channels of different shapes; Stage 2: rising of the flu‐ idized residual alkaline melt through the emerging ‘pipes’ and fractures armored by hardened zones, which is fol‐ lowed by metamorphic magma mingling under viscous deformation conditions. The mafic magmas intruding to the level of the granulite facies facilitated the deep anatexis and formation of synmetamorphic hypersthene plagiogranites (U‐Pb isotope dating: 500–490 Ma) and high‐K stress granites. In the Chernorud granulite zone, intense ductile‐plastic and brittle‐plastic deformations accompanied the processes of metamorphism, intrusion and formation of gabbro‐ pyroxenites and the anatexis of the crustal substance. As a result, the intrusive bodies were fragmented, and specific tectonic structures termed ‘metamorphic magma‐mingling’ were formed. All the tectonic and magmatic structures were subsequently ‘sealed up’ by K‐Na synkinematic granites at the regressive stage under conditions of the amphibo‐ lite‐facies metamorphism (U‐Pb and Ar‐Ar isotope dating: 470–460 Ma).

Evolution of the 120 ka caldera-forming eruption of Kutcharo volcano, eastern Hokkaido, Japan: Geologic and petrologic evidence for multiple vent systems and rapid generation of pyroclastic flow

We investigated the eruptive sequence and temporal evolution of juvenile materials during the 120ka Kutcharo pumice flow IV (Kp IV) eruption, which was the most voluminous (175km3: bulk volume) caldera-forming eruption of Kutcharo volcano. The eruptive deposits are divided into four units in ascending order. Unit 1 is widely dispersed and consists of silt-sized, cohesive ash. Unit 2 is a thin, moderately sorted pumice fall deposit with a restricted distribution and small volume (<0.2km3). Unit 3, consisting of widely distributed ignimbrite, is the most voluminous. Unit 4 is also composed of pyroclastic flow deposits, but its distribution is limited to the northwest side of the caldera. Juvenile materials consist mainly of rhyolite pumice (74%–78% SiO2) associated with a minor amount of scoria (52%–73% SiO2) that are found only northwest of the caldera in Unit 3 and Unit 4. These scoriae can be classified on the basis of the P2O5 contents of their matrix glass into low-P, medium-P, and high-P types, which are almost entirely restricted to the lower part of Unit 3, Unit 4, and the upper part of Unit 3, respectively. These three types display distinct mixing trends with the rhyolitic compositions in SiO2–P2O5 variation diagrams. This evidence indicates that three distinct mafic magmas were independently and intermittently injected into the main body of silicic magma to erupt from the northwestern part of the magma system. Mafic injections did not occur in the southern part of the magma system. This petrologic evidence implies that the northwestern and southeastern flows of Unit 3 are heterotopic, contemporaneous products derived from multiple vent systems. Although Unit 2 was derived from an eruptive column, its volume is very small compared to Plinian fall deposits of typical caldera-forming eruptions. In our interpretation, the activity of the Kp IV eruption reached its climax rapidly, depositing Unit 3, without first producing a stable Plinian column. The presence of multiple vent systems could have allowed the system to bypass an initial eruptive stage with a stable Plinian column and begin its climactic stage, represented by Unit 3, rapidly. Multiple vents could have been the result of sequential injections of mafic magma in the early stages of the Kp IV eruption.

Evolution and Eruption Processes of a Highly Porphyritic Silicic Magma System: Petrology of the Historical Eruptive Stage of Hokkaido-Komagatake Volcano, Japan

After about 5000 years of dormancy, the Hokkaido-Komagatake volcano started its historical eruptive stage with a plinian eruption in AD 1640. During this eruption, a zoned magma chamber was formed with upper mafic and lower silicic layers. This study focuses on the temporal evolution and eruption processes of the magma chamber based on petrological investigations of subsequent eruptions in AD 1694, 1856, 1929 and 1942. In all the eruptive products the majority of the clasts are strongly porphyritic white pumice (silicic andesite), which was derived from the silicic end-member magma. These are associated with small amounts of mafic ejecta (mafic to intermediate andesite). Based on density calculations for the magmas, the layered structure of the zoned magma chamber is likely to have been stable throughout the historical eruptive stage. In each eruption since AD 1694, relatively mafic magma erupted initially followed by more silicic magma. In addition, although there is petrological evidence for a mafic magma injection into a resident silicic magma chamber prior to the AD 1640 eruption, all eruptions from AD 1694 onwards appear to have taken place without new mafic injections. This suggests that since the AD 1694 eruption magmas have been erupted sequentially from the upper mafic part of the magma chamber. The composition of scoria, the most mafic ejecta, changed from relatively mafic magma in the AD 1694 eruption to gray pumice in later eruptions, which represent a hybrid magma. This temporal change is interpreted to indicate that mafic magma mixed gradually with more silicic magmas during repeated eruptions. In addition, the volume of the mafic and hybrid magmas decreased because of repeated eruptions without new mafic magma injections, so that the AD 1942 eruption was dominated by the eruption of silicic magma. We consider the present chamber to be filled with highly porphyritic, viscous silicic magma.

Coeval Felsic and Mafic Magmas in Neoarchean Calc-alkaline Magmatic Arcs, Dharwar Craton, Southern India: Field and Petrographic Evidence from Mafic to Hybrid Magmatic Enclaves and Synplutonic Mafic Dykes

Abstract We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30-40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (∼70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.

Chapter 18 Characterization of mafic enclaves in the erupted products of Soufrière Hills Volcano, Montserrat, 2009 to 2010

Abstract Lavas from the current eruption of the Soufrière Hills Volcano (SHV), Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. We present detailed field, petrological, textural and geochemical descriptions of mafic enclaves in andesite erupted during 2009–2010, and subdivide the enclaves into three distinct types: type A are mafic, glassy with chilled margins and few inherited phenocrysts; type B are more evolved with high inherited phenocryst content and little glass, and are interpreted as significantly hybridized; type C are composite, with a mafic interior (type A) and a hybrid exterior (type B). All enclaves define tight linear compositional trends, interpreted as mixing between a mafic end member (type A) and host andesite. Enclave glasses are rhyolitic, owing to extensive crystallization during quenching. Type A quench crystallization is driven by rapid thermal equilibration during injection into the andesite. Conversely, type B enclaves form in a hybridized melt layer, which ponded near the base of the chamber and cooled more slowly. Vesiculation near the mafic–silicic interface resulted in disruption of the hybridized layer and the formation of the type B enclaves. The composite enclaves represent an interface between types A and B, suggesting multiple episodes of mafic injection.