Injection Of Mafic Magma Research Articles

Accumulation of rhyolite magma and triggers for a caldera-forming eruption of the Aira Caldera, Japan

The sudden destabilization of voluminous magma after stable accumulation in a crustal magma chamber is a key process in the sequence of a catastrophic caldera-forming eruption. We investigated the petrological characteristics of magma that has erupted before, during and after the caldera-forming eruption of the Aira Caldera, Japan. This provides an example of the evolution of a rhyolite magma system during a catastrophic caldera-forming eruption. Stratigraphic and petrological investigations on the pre-caldera eruptive materials showed accumulation of high-silica rhyolite 4–5 km below the ground surface, which commenced at least ~ 30,000 years prior to the caldera-forming eruption. A part of the accumulated rhyolite magma leaked from the chamber and caused several pre-caldera eruptions. Phase equilibrium relationships and water concentrations in glass inclusions in phenocrysts suggest that the vertical extension of rhyolite magma chamber was 2–2.5 km. The rhyolite magma maintained a temperature between 735 and 800 °C and crystal mush state with 20–50% of crystallinity during its accumulation. The injection of mafic magmas destabilized the stored magma. Further, the heating of the rhyolite magma to 780–840 °C by the heat transfer from the mafic magma decreased the crystallinity down to 10% and induced mobilization of the stored rhyolite magmas. The remobilized magma could erupt as the Plinian eruption at the beginning and produced the Osumi pumice-fall deposit. This deposit occupies approximately 10% of the total volume of the erupted magma during the caldera-forming eruption. Though the extraction of rhyolite magma was limited to part of the magma chamber, decompression of the magma chamber by the magma extraction was enough to induce the caldera collapse and resulted in the eruption of the main ignimbrite (Ito ignimbrite) from the rest of the magma chamber. The volume of the erupted magma during the caldera-forming eruption is comparable with that of the magma chamber. New rhyolite magma with discernible composition from the rhyolite magmas of the caldera-forming eruption started accumulating in the magma chamber following the caldera-forming eruption and was the source for the post-caldera eruptions.

Distinct sulfur saturation histories within the Palaeogene Magilligan Sill, Northern Ireland: implications for Ni – Cu – platinum group element mineralisation in the North Atlantic Igneous Province

The ∼60 m thick Magilligan Sill is part of the British Palaeogene Igneous Province in the North Atlantic. The sill comprises layers of dolerite and olivine gabbro, and it intrudes a thick sequence of Mesozoic mudstones and marls, which are locally baked at the sill margins. Since 2014, the sill has been an exploration target for orthomagmatic Ni – Cu – platinum group element (PGE) sulfide mineralisation analogous to the Noril’sk-Talnakh intrusion in Russia. We present new petrological, geochemical, and S isotope data to assess the prospectivity of the sill and the underlying magmatic plumbing system. Most sulfides in the dolerite portions of the sill are <50 μm in size and comprise only pyrite with PGE abundances below the detection limit. In the olivine gabbros, >150 μm size pentlandite, chalcopyrite, and pyrrhotite grains contain <4 ppm total PGE, 1460 ppm Co, and 88 ppm Ag. Pyrite from the dolerites have δ34S ranging from −10.0‰ to +3.4‰ and olivine gabbro sulfides range from −2.5‰ to −1.1‰, suggesting widespread crustal contamination. The S/Se ratios of sulfides in the dolerites and olivine gabbros range from 3500 to 19 500 and from 1970 to 3710, respectively, indicating that the latter may have come from upstream in the magma plumbing system. The Magilligan Sill records multiple injections of mafic magma into an inflating sill package, each with distinct mechanisms towards S saturation. Whilst the sulfide minerals in the sill do not constitute significant mineralisation themselves, detailed in situ studies highlight a divergence in S saturation histories and suggest that a larger volume of olivine gabbro sulfides at depth may be prospective.

Localised heating and intensive magmatic conditions prior to the 22\u201323 April 2015 Calbuco volcano eruption (Southern Chile)

Calbuco volcano is a Late Pleistocene composite stratovolcano and member of the Southern Volcanic Zone of the Chilean Andes (41°19′S, 72°36′W). It lies ~ 20 km west of the Liquiñe–Ofqui Fault Zone, but is not located directly upon any major regional structures. During April 2015, a sub-Plinian eruption occurred, with a bulk erupted volume of ~ 0.3–0.6 km3 (~ 0.1–0.2 km3 DRE). The eruption was a rapid-onset event that produced highly crystalline products (from 40 to 60 vol.%) including the mineral phases: plagioclase, clinopyroxene, orthopyroxene, amphibole, olivine, apatite, ilmenite, titanomagnetite and chalcopyrite. An upper-crustal reservoir is inferred using available geophysical data combined with amphibole geobarometry. Consideration of textural features, including high crystallinity, complex mineral zonation, crystal clots and interstitial glass between crystals from clots, suggests the presence of a mush zone within this reservoir. From the nine collected samples, whole-rock chemistry and an array of geothermometers (amphibole, amphibole-plagioclase, two-pyroxenes and Fe–Ti oxides) gave similar results for all samples possessing ~ 40 vol.% of crystals, with the exception of the sample Cal-160 (~ 60 vol.% crystallinity), which is slightly more evolved and yields lower temperatures for all geothermometers. By comparing temperatures calculated in sample Cal-160 using pairs of ilmenite-titanomagnetite core compositions with those calculated using rim compositions, we observe a late-stage temperature increase of between 70 and 200 °C. We suggest that this local-scale heating event was at least partly responsible for triggering the eruption. Our data suggest that the bulk of the erupted magma was derived from a relatively uniform (970–1000 °C), crystal-rich magma mass. Sample Cal-160 was derived from a cooler environment (910–970 °C), where it was subjected to pre-eruptive heating to temperatures considerably higher than those observed in associated, erupted magmas (up to 1070 °C). This requires the involvement of a hot, presumably mafic magma injection at the base of a shallow, crystal-rich reservoir, though the mafic magma was not itself erupted. The localised nature of interaction and rapidity of eruption onset have implications for potential future hazards at Calbuco volcano.

Structure of the shallow magma chamber of the active volcano Mt. Zao, NE Japan: Implications for its eruptive time scales

Zao volcano is one of the representative active stratovolcanoes in northeast Japan, having many historical eruption records, as the last one occurred in 1940. Since 2013 to present, precursory phenomena, such as volcanic tremors and deformation of the summit area, have been detected in this volcano; thus, it is urgently necessary to thoroughly characterize the magma feeding system beneath the volcano. Our aim is to understand the magma chamber structure and dynamics associated with the recent activity (ca. ~2 ka) of Zao volcano in NE Japan. We conducted petrological and geochemical studies in the representative pyroclastic products and youngest lavas of its recent activity. The products are divided into the GSN (Goshikidake-nanbu) pyroclastics, GST (Goshikidake-tobu) pyroclastics and GS lavas (Furikodaki lava and Goshikidake-nanpo lava and associated pyroclastic deposits).The rock samples are calc-alkaline olivine-bearing pyroxene andesites originated as the result of mixing between felsic (containing Mg-poor pyroxenes + anorthite (An)-poor plagioclases) and mafic (containing forsterite (Fo)-rich olivines + An-rich plagioclases magmas). Thermodynamic modelling indicates that the temperature (T)–pressure (P)–water activity (H2O) conditions of the felsic (ca. 60–61 wt% SiO2, 968–1000 °C, 1.6–2.0 kbar and 1.8–2.2 wt% H2O) and the mafic (ca. 48.0–50.5 wt% SiO2, 1113–1170 °C, <2.4 kbar and 1.0–2.2 wt% H2O) end–members are similar between the GSN, GST pyroclastics and GS lavas.Compositional zoning of orthopyroxene phenocrysts suggests the formation of two mixed magmas, namely the main mixed and the Mg-rich mixed ones. The mafic magma from a deeper source entered repeatedly into a shallower felsic magma chamber. As a result, the main mixed magma developed within the chamber. The Mg-rich-mixed magma tentatively developed by the injection and simultaneous mixing of both the main mixed magma and the injected mafic one, merging later on into the main mixed magma.The Fe-Mg diffusion modelling on orthopyroxenes reveal that the injections occurred from <60 days to 1.7 years for thin-darker-zone types, and multiple injections of mafic magma occurred from 2.5 years to 150 years for broad-darker-zone types until the eruptions. The thin-darker-zone type orthopyroxene phenocrysts are abundant in the GSN and GST pyroclastics but rare in the GS lavas. Instead, the broad-darker-zone type orthopyroxenes are rare in the GSN and GST pyroclastics and abundant in the GS lavas. Thus, the main mixed layer was more evolved for the GS lavas than for the GSN and GST pyroclastics. The injected mafic magmas for the GS lavas were merged more extensively than the GSN and GST pyroclastics, resulting in a larger release of volatile components that triggered the effusive episodes. Given that local precursory phenomena have been currently detected in Zao volcano, we are confident that any research to thoroughly characterize the magma feeding system under this active volcano is essential for society to better react to any potential volcanic hazard in the near future.

Timescales of magma mixing in the magma plumbing system of Bezymianny volcano: insights from diffusion chronometry

Zoned crystals record changes in magma evolution, such as injection of mafic magma into a shallow magma chamber, which often triggered to eruption of arc volcanoes. We applied diffusion chronometry for reverse zoned crystals of orthopyroxene from 6 eruptions of Bezymianny volcano during 2006–2012 years and showed correlation between time of injection of new magma and recorded seismicity. We descripted two type of Bezymianny volcano eruptions, which driven by different trigger mechanisms: 1) orthopyroxene rims formed during up to 3 years previous to eruption; 2) orthopyroxene rims formed during 0–2 months previous to eruption.

Volcanic crystals as time capsules of eruption history

Crystals formed prior to a volcanic event can provide evidence of processes leading to and timing of eruptions. Clinopyroxene is common in basaltic to intermediate volcanoes, however, its ability as a recorder of pre-eruptive histories has remained comparatively underexplored. Here we show that novel high-resolution trace element images of clinopyroxene track eruption triggers and timescales at Mount Etna (Sicily, Italy). Chromium (Cr) distribution in clinopyroxene from 1974 to 2014 eruptions reveals punctuated episodes of intrusion of primitive magma at depth. Magma mixing efficiently triggered volcanism (success rate up to 90%), within only 2 weeks of arrival of mafic intrusions. Clinopyroxene zonations distinguish between injections of mafic magma and regular recharges with more evolved magma, which often fail to tip the system to erupt. High Cr zonations can therefore be used to reconstruct past eruptions and inform responses to geophysical signals of volcano unrest, potentially offering an additional approach to volcano hazard monitoring.

Cogenetic origin of mafic microgranular enclaves in calc-alkaline granitoids: The Permian plutons in the northern North China Block

Mafic microgranular (microgranitoid) enclaves (MMEs) with fine-to medium-grain–size igneous microstructures are abundant in Permian granitoid plutons in the northern North China Block (NCB). They are mainly dioritic in composition with SiO 2 contents from 51.0 wt% to 62.7 wt% and are contiguous with their host granitoids (SiO 2 = 60.4–68.4 wt%). Their main mineral assemblage of plagioclase (oligoclase and andesine), hornblende, biotite, K-feldspar, and quartz is similar to that of their host granitoids but with different mineral proportions. Zircon laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb dating, hornblende-plagioclase thermobarometry, and Ti-in-zircon thermometry results show that the crystallization ages and pressure-temperature ( P-T ) conditions of the MMEs in each Permian granitoid pluton are very similar to those of their host granitoids, indicating simultaneous crystallization at the middle to upper crustal levels. Petrographical, geochemical, and Sr-Nd-Hf isotopic data show that the source areas of the Permian granitoid plutons in the northern NCB are temporally and spatially different and magma mixing was likely involved during formation of some granitoid plutons. However, the whole-rock Sr-Nd and zircon Hf isotopic compositions of the MMEs in each pluton are very similar to those of their host granitoids, indicating complete isotopic equilibration between the host granitoids and their enclaves. Combined with similar mineral assemblage and mineral chemistry between the MMEs and their host granitoids and absence of coeval mantle-derived rocks in or nearby the plutons, we confirm that the MMEs were crystallized from a coeval magma that gave rise to the host granitoids, and not by mixing and/or mingling between mantle-derived mafic and crustal-derived felsic magmas or by synplutonic injections of mafic magma into the host granitoid magma. We conclude that magma isotopic equilibration between host granitoids and their enclaves was achieved prior to emplacement and the magma mixing process during formation of some Permian granitoid plutons occurred mainly in the melting, assimilation, storage, and homogenization (MASH) zone in the lowermost crust or mantle-crust transition, not in the magma conduits or chambers at middle to upper crustal levels. A two-stage model that includes rapid cooling within the cogenetic host granitoid magma operating at pluton margins to form solid to sub-solid dioritic rocks and crystal accumulations of the host granitoid magma at the bottom of the pluton to form cumulates, and dioritic rocks and cumulates then fragmentated and interacted with progressive evolved host granitoid magma to change their shapes and orientations is proposed for origin of the MMEs in the Permian granitoid plutons in the northern NCB. This model may be more widely applicable to the generation of many MMEs in granitic rocks, especially where no direct evidence exists for the presence of mafic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves. Our new results on origin of the enclaves in the granitoid plutons in the northern NCB show that some MMEs previously used as evidence for magma mixing and/or mingling are “autoliths (cognate xenoliths)” crystallized from the same magma as their host granitoids. There is an essential need for caution in using the MMEs as evidence for crustal-mantle interaction or mixing and/or mingling between crustal-derived felsic and mantle-derived mafic magmas during formation of granitic rocks, especially where no independent evidence is found for such a process. The role of MMEs in evaluating the mixing and/or mingling processes between crustal-derived felsic and mantle-derived mafic magmas has been much more likely overestimated by some previous studies and should be reconsidered. Instead, the MMEs in this origin can provide valuable information on emplacement mechanism and incremental growth of granitoid plutons in continental crust.

A13 東伊豆小室山スコリアの斑晶メルト包有物の記載岩石学 : イオウに富むマフィックマグマの注入(浅部マグマ過程,口頭発表)

A13 東伊豆小室山スコリアの斑晶メルト包有物の記載岩石学 : イオウに富むマフィックマグマの注入(浅部マグマ過程,口頭発表)

A westward propagating slab tear model for Late Triassic Qinling Orogenic Belt geodynamic evolution: Insights from the petrogenesis of the Caoping and Shahewan intrusions, central China

Late Triassic granitoid intrusions are widespread in the South Qinling Belt (SQB), providing excellent subjects to understand the geodynamic evolution of the Qinling Orogenic Belt and the collision between the North China Craton (NCC) and Yangtze Craton (YZC). This study shows newly obtained geological, geochemical and zircon U–Pb–Hf isotopic data of the Caoping and Shahewan intrusions, revealing that the Caoping intrusion consists of ~215Ma fined-grained granites, and ~221–215Ma porphyritic and coarse to medium-grained tonalites, granodiorites and monzogranites, which assemble with coeval mafic magmatic enclaves (MMEs). The Shahewan intrusion is composed of ~215–210Ma porphyritic granodiorites and monzogranites, which also assemble with coeval MMEs. The fine-grained granites from the Caoping intrusion are characterized by high SiO2, Rb and (La/Yb)N ratio, but low MgO, CaO and Sc contents, with εHf(t) values of −8.6 to +4.3 and TDM2(Hf) ages of 883–1596Ma, suggesting that they are mainly derived from partial melting of the Meso- to Neoproterozoic metagreywackes. The porphyritic and coarse to medium-grained granitoid rocks from both Caoping and Shahewan intrusions are characterized by higher MgO, CaO, Sc, Mg# values, but low SiO2, Rb contents and (La/Yb)N ratio, with εHf(t) values of −0.7 to +2.8 and TDM2(Hf) values of 961–1158Ma, suggesting that they are mainly formed by magma mixing between melts that were derived from Meso- to Neoproterozoic basement rocks of the SQB and metasomatized lithospheric mantle. The MMEs from Caoping and Shahewan intrusions are characterized by low SiO2, Sr/Y ratio, high MgO, K2O, Rb, Sc, total REE contents, with εHf(t) values of +0.5 to +6.1 and TDM(Hf) values of 661–846Ma, suggesting that they are produced by partial melting of metasomatized lithospheric mantle. The rapakivi-like textures of the rocks from Shahewan intrusion may be caused by continued underplating and injection of mafic magma resulting in higher temperature (TZr=770–817°C, comparing with Caoping intrusion of TZr=727–773°C), together with the magma mixing and isothermal decompression processes. Integrated with previous regional data, the SQB shows that mantle-derived magmatic activations initiated at ~234Ma near Wudang in the east, and westward systematically decreased to ~222–219Ma near Caoping and to 212–208Ma near Yangba. We attribute this temporal–spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted oceanic slab. Combined with sedimentary studies on the foreland basins, paleomagnetic studies as well as numerical and seismic tomography models of slab break-off, we propose that the ~225–205Ma magmatism in the SQB resulted from a westward slab tear following the collision between the NCC and YZC.

Water‐rich and volatile‐undersaturated magmas at Hekla volcano, Iceland

AbstractOlivine‐hosted melt inclusions from four eruptions at Hekla volcano in Iceland were analyzed for their dissolved H2O, CO2, S, and Cl contents. A positive correlation among the repose interval, magmatic evolution, and volatile contents of magmas is revealed. H2O is the dominant volatile species; it behaves as an incompatible component, increasing in concentration over time as a result of fractional crystallization in the magma. The full suite of H2O contents ranges from a low of 0.80 wt % in basaltic andesites to a maximum of 5.67 wt % in rhyolites. Decreasing H2O/K2O at fixed major element compositions suggests that syneruptive degassing reduces H2O contents significantly. Hekla magmas are CO2 poor, with very low concentrations present only in the most evolved compositions (∼20–30 ppm or less). The decrease in S content from basaltic andesite to rhyolite demonstrates that sulfide saturation is attained when the melt composition reaches basaltic andesite, resulting in the precipitation of pyrrhotite. Low CO2/Nb ratios suggest that vapor saturation is most likely reached during an early period of cooling and solidification in the crust. Fresh injections of mafic magma interact with previously solidified intrusives, producing new melts that are volatile undersaturated. Vapor saturation pressures obtained using the most volatile‐rich melt inclusions suggest the presence of a magma chamber at a minimum depth of ∼7 km. This is in agreement with geophysical observations from recent small‐volume eruptions, but given the possibility of volatile‐undersaturated melts, some of the magmas may reside at greater depths.

From a long-lived upper-crustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina)

The formation of world class porphyry copper deposits reflect magmatic processes that take place in a deeper and much larger underlying magmatic system, which provides the source of porphyry magmas, as well as metal and sulphur-charged mineralising fluids. Reading the geochemical record of this large magmatic source region, as well as constraining the time-scales for creating a much smaller porphyry copper deposit, are critical in order to fully understand and quantify the processes that lead to metal concentration within these valuable mineral deposits. This study focuses on the Bajo de la Alumbrera porphyry copper deposit in Northwest Argentina. The deposit is centred on a dacitic porphyry intrusive stock that was mineralised by several pulses of porphyry magma emplacement and hydrothermal fluid injections. To constrain the duration of ore formation, we dated zircons from four porphyry intrusions, including pre-, syn- and post-mineralisation porphyries based on intersection relations between successive intrusion and vein generations, using high precision CA-ID-TIMS. Based on the youngest assemblages of zircon grains, which overlap within analytical error, all four intrusions were emplaced within 29 ka, which places an upper limit on the total duration of hydrothermal mineralisation. Re/Os dating of hydrothermal molybdenite fully overlaps with this high-precision age bracket. However, all four porphyries contain zircon antecrysts which record protracted zircon crystallisation during the ∼200 ka preceding the emplacement of the porphyries. Zircon trace element variations, Ti-in-zircon temperatures, and Hf isotopic compositions indicate that the four porphyry magmas record a common geochemical and thermal history, and that the four intrusions were derived from the same upper-crustal magma chamber. Trace element zoning within single zircon crystals confirms a fractional crystallisation trend dominated by titanite and apatite crystallisation. However, zircon cathodoluminescence imaging reveals the presence of intermediate low luminescent (dark) growth zones in many crystals from all intrusions, characterised by anomalously high Th, U and REE concentrations and transient excursions in trace element ratios. A return to the same fractionation trend after this excursion excludes external compositional forcing such as magma mixing. Instead we interpret the “dark-zones” to record zircon crystallisation during a transient event of rapid growth that resulted from mafic magma injection into the base of the magma chamber, releasing a CO2-rich vapour phase into the dacitic crystal mush. We propose that this vapour phase then migrated upwards to the apical part of the magma chamber from where it was expelled, together with successive batches of magma, to form the porphyry copper deposit within a short time-span of less than a few 10,000 years. The short duration of host rock emplacement, hydrothermal alteration and mineralisation presented in this study provides critical constraints on fluid storage in magma chambers and the genesis of large porphyry copper deposits.

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.

The Grizzly Lake complex (Yellowstone Volcano, USA): Mixing between basalt and rhyolite unraveled by microanalysis and X-ray microtomography

Magma mixing is a widespread petrogenetic process. It has long been suspected to operate in concert with fractional crystallization and assimilation to produce chemical and temperature gradients in magmas. In particular, the injection of mafic magmas into felsic magma chambers is widely regarded as a key driver in the sudden triggering of what often become highly explosive volcanic eruptions. Understanding the mechanistic event chain leading to such hazardous events is a scientific goal of high priority. Here we investigate a mingling event via the evidence preserved in mingled lavas using a combination of X-ray computed microtomographic and electron microprobe analyses, to unravel the complex textures and attendant chemical heterogeneities of the mixed basaltic and rhyolitic eruption of Grizzly Lake in the Norris-Mammoth corridor of the Yellowstone Plateau volcanic field (YVF). We observe evidence that both magmatic viscous inter-fingering of magmas and disequilibrium crystallization/dissolution processes occur. Furthermore, these processes constrain the timescale of interaction between the two magmatic components prior to their eruption. X-ray microtomography images show variegated textural features, involving vesicle and crystal distributions, filament morphology, the distribution of enclaves, and further textural features otherwise obscured in conventional 2D observations and analyses. Although our central effort was applied to the determination of mixing end members, analysis of the hybrid portion has led to the discovery that mixing in the Grizzly Lake system was also characterized by the disintegration and dissolution of mafic crystals in the rhyolitic magma. The presence of mineral phases in both end member, for example, forsteritic olivine, sanidine, and quartz and their transport throughout the magmatic mass, by a combination of both mixing dynamics and flow imposed by ascent of the magmatic mass and its eruption, might have acted as a “geometric perturbation” of flow fields further fuelling mass exchange between magmas in terms of both chemical diffusion and crystal transfer. These results illuminate the complexity of mixing in natural magmatic systems, identifying several reaction-related textural factors that must be understood more deeply in order to advance our understanding of this igneous process.

Links between arc volcanoes and porphyry-epithermal ore deposits

Porphyry and epithermal ore deposits, which are the products of magmatic hydrothermal fluids, are intimately associated with volcanoes in continental and island arcs above subduction zones, but the exact nature of this relationship has remained enigmatic. Although metal deposition is usually thought to occur during the waning stages of volcanism, numerous ore deposits have been demonstrated to be synvolcanic. Here we show how the formation of these deposits is tied to volcanic cycles. We relate the chemical variations in vapors from Merapi volcano, Indonesia, to different stages of its eruptive cycle. The chemical compositions of volcanic vapors from subduction zone volcanoes are then compared globally to those of fluid inclusions from porphyry-epithermal deposits. These data show that adiabatic decompression is the principal control on mineralization. The data also suggest that volcanic and subvolcanic magmatic-hydrothermal systems are under lithostatic pressure during quiescence but decompress rapidly during injections of mafic magma and explosive eruptions. During quiescence, the magma evolves through fractional crystallization and devolatilization, gradually becoming oxidized and enriched in gold and other incompatible metals. Upon the injection of sulfur-rich mafic magmas, subvolcanic intrusions brecciate the overlying rocks, the systems are depressurized, the volcanoes erupt explosively, supercritical fluids unmix into vapor and brine, and base metal sulfides precipitate.

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.

Multiphase melting, magma emplacement and P-T-time path in late-collisional context: the Velay example (Massif Central, France)

AbstractThe West European Variscan chain is a remarkable illustration of how partial melting marks out the geodynamic evolution of mountain belt through time. Here, we focus on the Late Carboniferous melting events reported in the southeastern French Massif Central (Velay dome), with emphasis on the modes of partial melting, relationships between partial melting and magma emplacement, transition between the melting episodes and related P-T-t path. Following nappe stacking events under medium pressure/temperature conditions (M1 and M2 events), three melting events are identified in the southern envelope of the Velay dome. A first melting episode (M3 event) occurred within the biotite stability field at 325–315 Ma (T ≈ 720°C and P = 0.5–0.6 GPa). It led to the complete disappearance of muscovite and to the formation of migmatites consisting of biotite ± sillimanite melanosome and of granitic/tonalitic leucosomes depending on protolith composition. It is interpreted as the result of internal heating mainly linked to decay of heat producing elements accumulated in a thickened crust. It resulted in the formation of a partially molten middle crust with decoupling between the lower and upper crust, late-collisional extension and crustal thinning.The second episode of melting (M4 event) occurred at ca. 304 Ma (T 800°C and P 0.4 GPa), synchronously with emplacement of the Velay granites and growth of the dome. It led to the breakdown of biotite and growth of cordierite (locally garnet or tourmaline), with formation of diatexites and heterogeneous granites. This high-T event synchronous with crustal extension is considered to result from intrusion of hot mantle-derived and lower crustal magmas triggering catastrophic melting in the middle crust. This event ends with local retrograde hydrous melting within the stability field of biotite close to the solidus in response to local input of water during temperature drop in the late stage of emplacement of the Velay dome.The last evidence of melting in this area (M5 event) corresponds to emplacement of late granites generated under conditions estimated at ≈850°C and 0.4–0.6 GPa. They may have been generated from melting of specific lithologies triggered by injection of mafic magmas. These granites emplaced in a partly cooled crust (medium-grade conditions). The emplacement age of these granites is not well constrained (305–295 Ma) though they clearly post-date the Velay granites.The melting episodes in the Velay area and generation of granites appear to correspond to the conjunction between (i) the effects of collision-related crust thickening and (ii) those related to slab break off and asthenospheric mantle decompression melting. The driving process is mainly the internal radiogenic heat in a first stage, relayed by the propagation of a thermal anomaly initially located in the lower crust (M3 event), but which subsequently rose to the middle and upper crustal levels through magma transfer (M4 event). Overall, the Velay example is a remarkable illustration of the progressive dehydration and sterilisation of a thickened crustal segment. It documents how large amounts of granitic magmas can be produced at shallow crustal levels in relation to the injection of mantle-derived magmas.

Dynamics of Magma Mixing in Partially Crystallized Magma Chambers: Textural and Petrological Constraints from the Basal Complex of the Austurhorn Intrusion (SE Iceland)

The Tertiary Austurhorn intrusive complex in SE Iceland represents an exhumed magma chamber that has recorded an extensive history of magma mixing and mingling. The basal part of the intrusion consists predominantly of granophyres that have been intensively and repeatedly intruded by more mafic magma. This association of granophyres, basic and hybrid rocks at Austurhorn is referred to in the literature as a ‘net-veined’ complex, but field relations suggest a much more complex emplacement history. Here we present petrological and physical constraints on the various processes that resulted in magma mixing and mingling and the formation of different generations of hybrid rocks at Austurhorn. The complexity of the mixing and mingling processes increases towards the inferred centre of the intrusion, where chaotic hybrid rocks dominate the exposed lithology. Complex cross-cutting relations between different hybrid generations strongly suggest multiple magma injection and reheating events in the basal part of the shallow magma chamber. Model calculations employing distribution coefficients based on rare earth element concentrations reveal that early stage hybrid magma generations formed by pure endmember mixing between felsic and mafic magma with about 10% mafic fraction in the hybrids. With repeated injections of mafic magma into the base of the magma chamber, the intruding magma interacted to a greater extent with pre-existing hybrids. This led to the formation of hybrid magma compositions that are shifted towards the mafic endmember over time, with up to 30% of the mafic fraction in the hybrids. These mixing processes are recorded in the zonation patterns of clinopyroxene and plagioclase phenocrysts; the latter have been divided into four main groups by cross-correlation analysis. Melt viscosity calculations were performed to constrain the possible conditions of magma mixing and the results indicate that the interaction of the contrasting magmas most probably occurred at temperatures of approximately 1000°C up to 1120°C. This suggests that the initiation of effective magma mixing requires local superheating of the felsic magmas, thereby confining the process to areas of localized, substantial mafic magma injection.

Mineral chemistry of high-Mg diorites and skarn in the Han-Xing Iron deposits of South Taihang Mountains, China: Constraints on mineralization process

The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Early Cretaceous high-Mg diorite complex, including gabbro diorite, hornblende diorite, diorite, diorite porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the clinopyroxenes in the diorite and monzonite are diopside. Amphiboles in the high-Mg diorites show compositional range from magnesiohornblende to magnesiohastingsite, with minor pargasite and tschermakite. Most plagioclase in the high-Mg diorite is andesine and oligoclase. The magnesio-biotite in gabbro diorites shows chemical characteristics of re-equilibrated primary biotites and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (<9wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly >25wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the early stage shows high temperature (~900°C–950°C), pressure (~300MPa–500MPa), relatively high logfO2 (NNO–NNO +2), and H2O content in melt (4%–8%). During the late stage, the magma temperature dropped to about 750°C, and pressure came down to less than 100MPa, with the logfO2 rising to NNO +1–NNO +2.The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection of mafic magma into the felsic magma chamber. The relatively high logfO2 and H2O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate wall rocks. The high logfO2, Fe3+ rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively.

Post-supereruption Magmatic Reconstruction of Taupo Volcano (New Zealand), as Reflected in Zircon Ages and Trace Elements

New zircon U–Th model-age and trace element datasets are presented from Taupo volcano (New Zealand), which are used to investigate the timescales and broad-scale magmatic processes involving zircon crystallization after the caldera-forming 25·4 ka Oruanui supereruption. Detailed 14C-based chronologies and controls on vent locations allow the timing and location of post-caldera eruptions to be spatially and temporally constrained to an extent not possible for any other supervolcano. After ∼5 kyr of post-Oruanui quiescence, Taupo erupted three dacitic units, followed by another ∼5 kyr break, and then a sequence of rhyolitic units in three subgroups (SG1–SG3) from 12 ka onwards. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U–Th zircon model ages in Taupo SG1 rhyolites (erupted from 12 to 10 ka) indicate only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains in some units. U–Pb dating of these older grains shows that both 300–450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units that were vented within the outline of the Whakamaru caldera. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolite (Units B and C) have notably high concentrations of U, Th, P, Y + (REE)3+ and Nb but with only minor variations in Hf and Ti. SG2 zircons typically have higher Sc contents, reflecting large-scale changes in melt chemistry and crystallizing mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U–Th ages from single post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallization, acting within a crustal protolith chemically independent of that which was dominant in the Oruanui system. These periods of cooling and crystallization alternate with times of rejuvenation and eruption, sometimes demonstrably accompanying syn-eruptive regional rifting and mafic magma injection. Not only were the processes that developed the supersized Oruanui magma body rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale.

Petrogenesis of Dongguashan skarn-porphyry Cu-Au deposit related intrusion in the Tongling district, eastern China: Geochronological, mineralogical, geochemical and Hf isotopic evidence

The Dongguashan skarn-porphyry Cu-Au deposit, located in the Tongling district of the Middle-Lower Yangtze River Valley metallogenic belt (MLYB), consists of skarn ore bodies in the upper part and porphyry ore bodies in the lower part, both of which are hosted in quartz diorite and quartz monzodiorite. Zircon U-Pb age and geochemical studies show that the quartz diorite of the Dongguashan intrusion formed at 140.3±2.0Ma (MSWD=0.19) and belongs to the high potassium calc-alkaline series. It is enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and has a slightly negative Eu anomaly. 176Hf/177Hf values of the rims of zircons show a variable range (0.282087–0.282391), corresponding with calculated εHf(t) values of −10.72 to −21.46. Plagioclases in the quartz diorite have unbalanced structure characterized by bright andesine and labradorite (An=37.0–65.5) cores with higher contents of Fe and Sr and are corroded by dark oligoclase (An=13.8–27.6) rim. Major elements, trace elements, Hf isotope, and the composition of plagioclases indicate that the parental magma of the Dongguashan intrusion was produced by the mixing of underplating mafic magma and felsic magma formed by remelting of Paleoproterozoic and Neoarchean crustal rocks, Neoproterozoic crust may also provide some material to the felsic magma. Mafic magma played a key role and made the parental magma rich in water, sulfur, metals (Cu, Au) and gave it a high oxygen fugacity. During its magmatic evolution, the parental magma underwent fractional crystallization of hornblende, apatite, sphene and other mafic minerals. Some quartz diorite and quartz monzodiorite samples that show adakitic signatures, may result from injection of mafic magma. Some inherited zircons of the quartz diorite in the Dongguashan intrusion gave ages of 2.40–2.50Ga, 1.95–2.05Ga and 0.74–0.81Ga, coming from ultramafic, mafic and andesitic igneous rocks, and this indicates that there may have been three periods (2.4, 2.0, and 0.8Ga) of magmatic activity in the Tongling district.