Magma Conduit System Research Articles

Mechanisms of Co enrichment in the Zhubu Ni–Co–PGE deposit in the Emeishan LIP, SW China: Constraints from mineral geochemistry and Fe–S isotopes of sulfides

The Zhubu Ni–Co–PGE sulfide deposit in the Permian Emeishan Large Igneous Province (LIP), SW China is hosted by a mafic–ultramafic intrusion that comprises two morphologically distinct ore-forming spaces: a central, layered sequence that represents a magma chamber surrounded by a marginal sub-vertical conduit zone. The marginal conduit zone contains significant Ni–Co–PGE sulfide mineralization, suggesting that high volumes of sulfide-bearing silicate melt flowed through the magma conduit system. Determining the hosts of Co is key to understanding the Co enrichment process in the Zhubu deposit, as well as in the Emeishan LIP as a whole. We assess this here using the geochemistry and Fe–S isotope compositions of sulfides in the Zhubu deposit. Cobalt is mainly concentrated in pentlandite sulfide in the websterite, with Co contents increasing in the order pyrrhotite < chalcopyrite < pyrite < pentlandite < violarite; the abundance of violarite, however, is notably lower than that of the other sulfides. The Co contents of pentlandite, pyrite, and chalcopyrite are negatively correlated with their Ni and Fe contents, suggesting that Co substitutes isomorphically for Ni and Fe in the sulfide structure. The δ56Fe values of sulfides increase in the order pyrrhotite < chalcopyrite < pentlandite < pyrite resulted from kinetic fractionation of Fe isotopes. The δ56Fe values of pentlandite and chalcopyrite in lherzolite are higher than those in websterite, implying that lherzolite and websterite formed in compositionally distinct magmas, Fe isotope compositions become heavier with crystal fractionation. The δ34S values of sulfides increase in the order pyrrhotite < pentlandite < chalcopyrite < pyrite, similar to the fractionation sequence of sulfide melts, suggesting that externally derived S was added to the magma and contributed to its sulfide saturation. The Co contents of chalcopyrite and pyrite vary positively with δ56Fe values, and negatively with δ34S values, indicating that the addition of externally derived S and resulting sulfide segregation were critical to Co enrichment.

Vulcanian eruptions at Sakurajima Volcano: geophysical data, numerical modelling, and petrological evidence

We investigated the magma conduit system beneath three active craters that have repeatedly generated Vulcanian eruptions at Sakurajima Volcano, Japan, by analysing seismic, infrasound, and tilt data. The hypocentres of the earthquakes associated with the Vulcanian eruptions are distributed separately at shallow depths of approximately 0.5 km beneath the craters. Infrasound indicated that the swelling of the crater floor starts approximately 0.2 s after the occurrence of earthquakes and that the eruption starts about 0.3 to 0.5 s later. During an eruption, tilt vectors at a station approximately 2.7 km far from the active craters indicated a deflationary trend directed toward one of the three active craters. A 1-D conduit flow simulation indicated pressure build-up at a depth of 0.4–1.0 km beneath the crater, consistent with previously reported pressure sources during eruptions detected by geodesy. Volcanic lapilli emitted from the three craters have the same chemical composition; hence, Vulcanian eruptions of all three studied craters originate from a common magma source.

Hidden Markov model for spatial analysis of three-dimensional mineralization distribution: Insights into magma flow and mineral exploration targets in the Jinchuan Ni-Cu-(PGE) sulfide deposit, China

The spatial distribution of mineralization includes significant clues about the magma/fluid flow process for ore formation. However, given the complex nature of the ore-forming process and its weak signature in the mineralization data, revealing the magma/fluid flow process in their mineralization footprint presents a considerable challenge. In this paper, we present a hidden Markov model (HMM) for the analysis of three-dimensional (3D) spatial distribution of mineralization. The HMM assumes the magma flow is a Markov process that is “hidden” in the “observable” mineralization features. Under this setting, the HMM quantifies the states of magma flow and links them as a whole in a probabilistic manner. This allows for the comprehensive combination of mineralization information, facilitating holistic inference of magma flow hidden in the mineralization signal. Based on the maximum likelihood principle, the magma trajectories and the associated injection points are inferred from the HMM. The proposed model is applied to spatial analysis of the mineralization in the world-class Jinchuan Ni-Cu-(PGE) magmatic sulfide deposit in NW China. Based on the 3D mineralization data interpolated from drill assays, the HMM infers magma trajectories and the associated injection points at Jinchuan, which indicate the Jinchuan deposit was formed by a multiple magma conduit system that comprises four independent injection points of magma ingress. According to the identified magma trajectories and injection points, four sites beneath the injection points and along the extension of magma trajectories are considered promising for future mineral exploration. With the insights gained from this case study, the proposed HMM can serve as an effective tool in the exploration information system to unveil magma/fluid flow history and guide exploration efforts toward concealed, deep-seated mineralization.

Controls on the metal tenors of sulfide ores in the Jinchuan Ni–Cu–PGE sulfide deposit, NW China: Implications for the formation of distinct textural types of sulfide ores in magma conduits

The giant Jinchuan Ni–Cu–platinum-group element (PGE) sulfide deposit is a magma conduit system comprising four main intrusive units termed segments III, I, II-W, and II-E. The deposit comprises disseminated, net-textured, massive sulfide, and Cu-rich sulfide ores, with variations in metal tenors occurring among these different ore types and among the segments. Controls on these metal tenor variations must be linked to metallogenic processes, but this remains poorly constrained. To asses this, we systematically compared metal tenors (Ni, Cu, and the PGE) of ores from two perspectives — i) different ore types in each segment and ii) a single ore type across all four segments. Two major styles of metal tenor variations are documented. Style 1: Segments III and I have higher metal tenors than segments II-W and II-E for any given type of ore. Based on sulfide segregation models, this variation is interpreted to be related to variable degrees of early removal of sulfide liquid from the magmas that formed the four segments. Style 2: Disseminated ores in segments III and I have higher metal tenors than the other ore types, whereas all of the ore types in segments II-W and II-E have similar metal tenors. This is interpreted to be the result of the disseminated ores in segments III and I having formed from sulfide liquids that experienced higher R factors than the sulfide liquids that formed the other ore types, whereas all of the ores in segments II-W and II-E formed from sulfide liquids that experienced similar R factors. This difference suggests that the different ore types in segments III and I formed via early percolation and accumulation of sulfide liquids (the net-textured and massive ores) followed by the capture of disseminated, high R factor sulfides (the disseminated ore), whereas the different ore types in segments II-W and II-E likely formed by the physical percolation of originally disseminated sulfide liquid through the inter-crystal pore space. This study demonstrates that both sulfide ore texture and their metal-tenor variations are critical to characterizing ore-forming processes in dynamic magma conduits.

Imaging of a shallow magma conduit system based on a high-power frequency-domain controlled-source electromagnetic survey

Mantle-derived magma may undergo lateral flow and emplacement through horizontal magma conduits; this occurs after magma from a deep source ascends through a vertical conduit. Regardless of this understanding, the characteristics of such shallow magma conduit systems remain unclear. We have developed an image of a shallow (5 km deep) magma conduit system based on the resistivity results of a high-power frequency-domain controlled-source electromagnetic survey in the Kalatongke magmatic-type Ni-Cu deposit. The magma upwelled and ascended along the vertical fault. At shallow depths, the magma moved horizontally along the tuff layer, laterally propagating in a horizontal magma conduit. Magma extending horizontally in the horizontal tuff layer could not break through the upper, conductive, and low-density carbonaceous layer. This image provides a conduit model with vertical magma ascent and lateral magma propagation. The model provides a basis for studying shallow magma conduit systems in orogenic belts on a global scale.

Mixing of heterogeneous, high-MgO, plume-derived magmas at the base of the crust in the Central Iapetus Magmatic Province (Ma 610-550): Origin of parental magmas to a global LIP event

High-MgO (>12 wt%) magmas represent some of the most primary and high temperature melts from mantle plumes. The compositional diversity of high-MgO magmas gained by fractional melting within inhomogeneous mantle sources, is often overprinted by wall rock assimilation, magma mixing and fractional crystallisation within magma chambers at various depths within the crust. The deepest intrusions, at the base of the crust, are the first stop for magmas as they leave the mantle and such localities offer vital insights into the crustal processes that modify high-MgO melts. The Seiland Igneous Province (SIP) represents a rare exposure of a deep crustal magma conduit system that transported large volumes of mantle-derived melts through the lower continental crust. In this work, trace element compositions of clinopyroxenes in dunite, wehrlite and olivine clinopyroxenite samples from the Reinfjord intrusion were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The melts calculated to be in equilibrium with the clinopyroxenes represent derivatives of mantle-derived magmas and have steep rare earth element (REE) profiles with La/YbPM of 8.4–14., Sm/YbPM of 3.5–4.8, and negative anomalies in Nd, Zr and Hf. Assimilation, recharge and fractional crystallisation modelling shows that the ultramafic cumulates in the Reinfjord intrusion formed through concurrent fractional crystallisation and repetitive recharge (and mixing) of new primitive magmas from the mantle with very limited input from crustal sources. The recharge and mixing model is strongly supported by field and petrographic data. Two end-member ultramafic magmas are identified; the first end-member melt composition (MELTCPX0) is similar to the melt compositions that are in equilibrium with the most trace element depleted clinopyroxenes from Reinfjord. The second end-member melt composition (MELTPIC0) is similar to a number of picrite dykes found throughout the SIP, which have less steep LREE/HREE and lack strong negative anomalies in Zr and Hf. The REEBOX PRO melting model (Brown and Lesher, 2016) was used to forward model adiabatic decompression melting of lithologically homogeneous and heterogeneous sources containing anhydrous/hydrous peridotite ± pyroxenite. It was found that MELTPIC0 formed by melting of a peridotite source with up to 10% pyroxenite component and mantle potential temperatures of up to 1450 °C. MELTCPX0, however, formed by melting of a carbonate- metasomatised peridotite with potential temperatures of up to 1630 °C. These results indicate a compositionally and thermally inhomogeneous mantle plume beneath the SIP.

Eruptive Activity on the Western Flank of Piton de la Fournaise (La Réunion Island, Indian Ocean): Insights on Magma Transfer, Storage and Evolution at an Oceanic Volcanic Island

AbstractPetrological and geochemical (major element, trace element, Sr–Nd isotope) data for recent (&amp;lt;5 kyr old) basalts that sporadically erupt on the western flank of Piton de la Fournaise (PdF), one of the most active volcanoes on Earth, allow the tracking of magma transfer and evolution from mantle to crustal depths. In the western peripheral area of PdF we document the broadly synchronous eruptions of (1) primitive olivine and olivine–clinopyroxene transitional basalts with tholeiitic affinity that are closely associated in space with (2) transitional olivine basalts with alkaline affinity, and (3) hybrid lavas, intermediate between the ‘alkaline’ and the ‘tholeiitic’ end-members. The composition of the latter overlaps with that of the lavas frequently erupted from the conduit system feeding the main summit cone. AlphaMELTS modelling, and fluid inclusion and clinopyroxene barometry, constrain the conditions of magma storage at 10–30 km, and the ascent of magma from the upper mantle to the shallow crustal plumbing system. Variable degrees of mantle melting, together with minor source heterogeneity and contamination with cumulate-derived partial melts, contribute to the diversity of PdF magmas. However, all these processes do not represent the dominant factors that produce the large variability we found in major element composition. Indeed, the composition of basalts erupted from PdF peripheral centers is strongly controlled by polybaric olivine–clinopyroxene fractionation at pressures higher than 3 kbar. Crystal textures and geochemical modelling suggest that fast magma ascent is critical to prevent clinopyroxene dissolution. Conversely, long-lasting magma stagnation promotes pyroxene resorption and magma differentiation. ‘Central’ eruptions occurring close to the PdF summit cone emit variably more evolved melts, which result from olivine–clinopyroxene–plagioclase differentiation at intermediate–shallow pressure (&amp;lt;3 kbar and in most cases &amp;lt;1 kbar). Deep and extensive magma mixing before injection into the crustal magma conduit system, located below the summit region, results in the apparent homogeneity of basalts erupted from the central area. As regards ‘peripheral’ eruptions, deep-seated stagnation of basaltic melts and differentiation at the mantle–crust transition zone (c. 4 kbar) produces a range of magma compositions. We demonstrate that rapid magma ascent from deep-seated reservoirs can bypass the central plumbing system. The eruptions of these magmas both in the central area and on the densely populated flanks have major consequences in terms of volcanic hazard at PdF.

Evolution of a Conduit System at the Marathon PGE–Cu Deposit: Insights from Silicate Mineral Textures and Chemistry

Abstract The Marathon platinum group element (PGE)–Cu deposit is hosted by the Two Duck Lake Gabbro of the Mesoproterozoic Coldwell Complex, Canada, and comprises three zones of mineralization, which have different textural, mineralogical and geochemical characteristics. The Footwall Zone occurs at the base of the Two Duck Lake Gabbro, at the contact with the Archean country rocks. The Main Zone occurs within the Two Duck Lake Gabbro, above the Footwall Zone. The W Horizon is an extraordinarily PGE-enriched zone that is characterized by very low Cu/Pd values, less pyrrhotite, and appreciably more bornite than the Footwall and Main zones. It occurs above Main Zone-equivalent mineralization in the southern half of the Two Duck Lake intrusion. Silicate mineral textures and chemistry reveal that the Marathon deposit formed in a dynamic magma conduit system with a complex magma evolution history. The deposit developed into three zones as a result of the multiple pulses of compositionally different magmas. Plagioclase commonly shows resorption surfaces with overgrowths and the anorthite content and concentrations of trace elements such as Fe, Sr, Ba, and rare earth elements in plagioclase change significantly across these surfaces. In rocks that contain sulfide minerals, plagioclase crystals have been partly resorbed and the resorbed rims are Ca-enriched and intergrown with sulfide mineral inclusions. These rims contain higher base-metal and sulfur contents and are light rare earth element-enriched compared with the primary plagioclase. These characteristics indicate input of a separate sulfide-enriched melt with a different composition compared with that from which the primary plagioclase crystallized. Additional evidence for the involvement of slightly different magmas during evolution of the three mineralized zones is derived from variations in pyroxene chemistry (e.g. higher Fe, lower Mg, V, and Sc) between the Footwall Zone and the other two zones and the absence of inverted pigeonite from the W Horizon. The PGE enrichment occurred at depth prior to intrusion and late-stage PGE-rich sulfide-bearing magmas intruded and formed the W Horizon. Therefore, the W Horizon is a zone of mineralization that formed late in the evolution of the Two Duck Lake Gabbro.

Multiple Magma Conduits Model of the Jinchuan Ni-Cu-(PGE) Deposit, Northwestern China: Constraints from the Geochemistry of Platinum-Group Elements

The giant Jinchuan nickel-copper-platinum-group element (PGE) deposit is hosted by two individual sub-vertical intrusions, referred to as the western and eastern intrusions (including segment II-W and segment II-E). Exactly how the Jinchuan deposit was formed by a system of sub-vertical magma conduits is still not well understood. This paper reports new major elements, trace elements and PGEs data from the Jinchuan deposit to study the formation mechanism of sulfide ores with different textures and their relationship with the magma conduit system. Our study shows that the PGE tenors of disseminated and net-textured sulfide in segment II-E are significantly lower than segment II-W and the western intrusion, but the Cu/Pd ratios are opposite. In addition, net-textured sulfides in segment II-W show a negative correlation between IPGE (Ir, Ru and Rh) and PPGE (Pt and Pd) in contrast to the positive correlation in segment II-E and the western intrusion. These features indicate the parental magma sources of the western intrusion, segment II-W and segment II-E were originally three different surges of PGE-depleted magma. Modeling of parental magma in the western intrusion, segment II-W and segment II-E suggests that they were formed by the same initial picritic basalt (100 ppm Cu, 1 ppb Ir and 10 ppb Pd) with different prior sulfide segregations (0.0075%, 0.0085% and 0.011%). The three parts of Jinchuan sulfides show that the Pt/Pd and (Pt + Pd)/(Ir + Ru + Rh) ratios decrease from section III-5 toward both sides in the western intrusion and decrease from section II-14 toward all sides, whereas no regular spatial variations occur in segment II-E, showing the different fractionation processes of sulfide melt. The massive sulfides in the western intrusion and segment II-E experienced a ~20% to 40% and ~40% to 60% fractionation of sulfide melt, respectively. We propose that the Jinchuan deposit was generated in a metallogenic system of multiple magma conduits, where sulfides entrained in parental magma experienced different amounts of prior removal.

SBAS interferometric analysis for volcanic eruption of Hawaii island

The Kilauea volcano on the Hawaii island started erupting on 3rd May 2018 after an earthquake of magnitude 5.0. The initial volcano eruption and the subsequent earthquakes resulted in the opening of new volcanic fissures, continuous lava flow at the lower east rift zone and the collapse of the summit caldera floor. All these activities continued until the 1st week of August 2018. The Small Baseline Subset (SBAS) technique is an advanced form of Differential Interferometry (DInSAR) which is capable of monitoring surface deformations with millimetre level accuracy. SBAS technique was utilised here for estimating the surface deformation on Hawaii island due to the volcanic episode. InSAR coherence technique was also utilised in this study for localised analysis at the summit and at the lower east rift zone to detect the time series deformation information and lava flow patterns. 14 interferometric datasets acquired by the ESA's Sentinel-1 C-band satellite from April 2018 to September 2018 were used in this study. The data acquired by the seismometers installed at Kilauea by Hawaii Volcano Observatory, the SWIR band of Sentinel-2 and the thermal band of Landsat-8 were used to correlate and verify the results obtained through the SBAS and InSAR coherence analysis. The combined interpretation of results obtained from all the sources gave insights about the underground magma conduit system, surface deformations, lava flow channels and the correlation between the magmatic and seismic activity happened there which are discussed in detail in this paper.

Mineralogical and Geochemical Controls on the Formation of the Woods Point Dike Swarm, Victoria, Australia: Evidence from the Morning Star Dike and Implications for Sourcing of Au Within Orogenic Gold Systems

The Woods Point dike swarm of eastern Victoria, Australia, has produced over 133 tonnes (t) of Au. Here, as in a number of other orogenic Au provinces, a close spatial relationship is observed between orogenic Au mineralization and intrusive rocks; this study focuses on the well-endowed Morning Star dike, a gabbro to gabbro-diorite dike that has produced ~28 t of hydrothermal quartz-carbonate-pyrite vein-hosted Au and a further 24 t of placer gold derived from primary rocks in the area immediately around Morning Star. The fractionated magma that formed the dikes within the Woods Point dike swarm was sourced from a staging magma chamber at a depth of 16 to 23 km; here, mantle-derived magmas were driven to S saturation by assimilation of crustal material, leading to the formation of Cu-Ni-platinum group element (PGE)-Au–bearing sulfides, most of which segregated and settled toward the bottom of the staging chamber. Emplacement of the dike initiated with tapping of the staging chamber and release of the magmas within the chamber, forming the Woods Point dike swarm; flow differentiation and the formation of dike bulges enabled the segregation of sulfide-bearing mafic units and intermediate sulfide-barren units within these dikes. This is exemplified by primitive amphibole-rich, sulfide-bearing gabbroic sections and more evolved sulfide-barren gabbro-diorite portions of the Morning Star dike. We suggest that the source of Au within the Morning Star dike as well as that in other members of the Woods Point dike swarm was the Cu-PGE-Au sulfides left at depth within the magma conduit system, for example within the Thomson River Copper Mine dike. Mass balance calculations undertaken suggest that ~104 million tonnes (Mt) of magmatic sulfide may exist within the magma conduit system; these magmatic sulfides, if they are the equivalent of the sulfides preserved in the Morning Star dike, probably contain around 3% Ni and 10% Cu. During postmagmatic metamorphism, we infer that hydrothermal fluids flowed along dikes, interacting with magmatic sulfides and leaching some of the Au, which was subsequently deposited in quartz-carbonate veins at higher levels in the system.

Seismic tremors and magma wagging during explosive volcanism

Volcanic tremor is a ubiquitous feature of explosive eruptions. This oscillation persists for minutes to weeks and is characterized by a remarkably narrow band of frequencies from about 0.5 Hz to 7 Hz (refs 1-4). Before major eruptions, tremor can occur in concert with increased gas flux and related ground deformation. Volcanic tremor is thus of particular value for eruption forecasting. Most models for volcanic tremor rely on specific properties of the geometry, structure and constitution of volcanic conduits as well as the gas content of the erupting magma. Because neither the initial structure nor the evolution of the magma-conduit system will be the same from one volcano to the next, it is surprising that tremor characteristics are so consistent among different volcanoes. Indeed, this universality of tremor properties remains a major enigma. Here we employ the contemporary view that silicic magma rises in the conduit as a columnar plug surrounded by a highly vesicular annulus of sheared bubbles. We demonstrate that, for most geologically relevant conditions, the magma column will oscillate or 'wag' against the restoring 'gas-spring' force of the annulus at observed tremor frequencies. In contrast to previous models, the magma-wagging oscillation is relatively insensitive to the conduit structure and geometry, which explains the narrow band of tremor frequencies observed around the world. Moreover, the model predicts that as an eruption proceeds there will be an upward drift in both the maximum frequency and the total signal frequency bandwidth, the nature of which depends on the explosivity of the eruption, as is often observed.

Origin of extremely PGE-rich mafic magma system: An example from the Jinbaoshan ultramafic sill, Emeishan large igneous province, SW China

The ~ 260 Ma Jinbaoshan sill is part of the Permian Emeishan large igneous province in SW China and is mainly composed of wehrlite and hosts a PGE deposit containing ~ 45 tonnes of Pt and Pd with ore grades ranging from 1 to 5 ppm Pt + Pd. The sill is composed of several PGE-rich horizons at different stratigraphic heights. A ~ 5 m-thick PGE-rich horizon at the base of the sill, which is underlain by a chromite-rich, PGE-poor layer, accounts for 44% of the total ore reserve. The Jinbaoshan sill is rich in PGEs with Ir ranging from 1.7 to 138 ppb, Ru from 0.9 to 39 ppb, Rh from 3.1 to 133 ppb, and a total Pt and Pd from 76 to 5371 ppb. All the rocks from the sill display primitive mantle-normalized chacophile element patterns enriched in PGEs relative to Ni and Cu. The low Cu/Pd ratios (20 to 1938) of the rocks relative to the primitive mantle (Cu/Pd = 7000) imply that there was no sulfide fractionation of the magmas. The PGE enrichment of the sill indicates that it formed from extremely PGE-rich magmas. Modeling results show that such PGE-rich magmas likely resulted from a multistage-dissolution upgrading process in an open magma conduit system. Large amounts of chromite crystallization triggered sulfide saturation when PGE-rich magmas emplaced into the upper magma chamber of the system. Settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest PGE-rich horizon at the base of the sill. The rest minor sulfide liquid accumulated to form PGE-rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber.

Siderophile and chalcophile elemental constraints on the origin of the Jinchuan Ni-Cu-(PGE) sulfide deposit, NW China

The Jinchuan deposit, NW China, is one of the world’s most important Ni-Cu-(PGE) sulfide deposits related to a magma conduit system and is hosted in an ultramafic intrusion. The intrusion is composed of lherzolite and dunite with the two largest sulfide ore bodies (named as ore body 1 and 2) in its middle portion. The sulfide ores may be disseminated, net-textured, or massive. The disseminated and net-textured sulfide ores are characterized by variable but generally low PGE concentrations: 10–3200 ppb Pt, 240–9800 ppb Pd, 17–800 ppb Ir, 25–1500 ppb Ru, and 15–400 ppb Rh in 100% sulfides. The massive sulfide ores are extremely low in Pt (<30 ppb) on a 100% sulfides and have very high Cu/Pd ratios, ranging from 10 4 to 4.5 × 10 5. The low PGE contents suggest that the sulfide ores formed from the silicate magmas that had already experienced prior-sulfide separation. Our calculations indicate that if the first stage basaltic magmas had contained 6.3 ppb Pt, 6.2 ppb Pd, and 0.1 ppb Ir, 0.008% sulfide removal would result in PGE-depletion in the residual magma with 0.57 ppb Pt, 0.25 ppb Pd, and 0.009 ppb Ir. The Jinchuan sulfides were formed by a second stage of sulfide segregation from a PGE-depleted magma under silicate/sulfide liquid ratios (R-factor) ranging from 10 3 to 10 4 in a deep-seated staging chamber. The massive sulfide ores and some of the net-textured sulfide ores exhibit strong negative Pt-anomalies that cannot be explained by sulfide segregation under variable R-factors. Instead, the sulfide melts that formed the massive ores were segregated from magmas experienced prior fractionation of Pt–Fe alloy. Alternatively, the Pt may have been selectively leached by hydrothermal fluids during remobilization of the sulfide melts that produced the massive sulfides, which occur in cross-cutting veins. We propose that the Jinchuan intrusion and ore bodies were formed by injections of sulfide-free and sulfide-bearing olivine mushes from a deep-seated staging chamber.

Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse

Research Article| May 01, 2004 Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse Kohei Kazahaya; Kohei Kazahaya 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Hiroshi Shinohara; Hiroshi Shinohara 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Kozo Uto; Kozo Uto 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Masanobu Odai; Masanobu Odai 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Yasuhiro Nakahori; Yasuhiro Nakahori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hirokazu Mori; Hirokazu Mori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hideki Iino; Hideki Iino 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Makoto Miyashita; Makoto Miyashita 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Junichi Hirabayashi Junichi Hirabayashi 3Volcano Fluid Research Centre, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-0033, Japan Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (5): 425–428. https://doi.org/10.1130/G20399.1 Article history received: 10 Dec 2003 rev-recd: 10 Jan 2004 accepted: 12 Jan 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kohei Kazahaya, Hiroshi Shinohara, Kozo Uto, Masanobu Odai, Yasuhiro Nakahori, Hirokazu Mori, Hideki Iino, Makoto Miyashita, Junichi Hirabayashi; Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse. Geology 2004;; 32 (5): 425–428. doi: https://doi.org/10.1130/G20399.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO2 emission rate from nonerupting volcanoes evaluated before this activity. The SO2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cross section of a magma conduit system at the margin of the Colorado Plateau

We present crystallization depth vs. temperature estimates for clinopyroxene phenocrysts from the Springerville volcanic field, Arizona. These calculations reveal several intriguing patterns that have considerable implications for magma transport and genesis. First, partial crystallization occurs over a wide depth range (0–60 km), but most partial crystallization occurs between 0 and 30 km. Second, low crystallization temperatures, low-density magmas, and evolved liquid compositions derive exclusively from two depth intervals, 0–12 and 23–30 km. These intervals coincide with a density contrast in the upper crust and a rheology contrast at the base of the middle crust. They also coincide with two highly seismically reflective depth intervals. These relationships indicate that (1) the Moho is not a staging area for volcanic eruptions; (2) density contrasts in the upper crust, and a rheology contrast in the middle crust, control magma transport and liquid evolution; (3) magma conduits are probably magma mush columns, with a preponderance of sills within the 0–12 and 23–30 km intervals; and (4) seismically reflective layers are sills related to Tertiary–Holocene volcanic activity. Moreover, these sills appear to represent the principal sites of magma evolution.

Three-dimensional seismic velocity structure beneath Aso Volcano, Kyushu, Japan

Tomographic results for P- and S-wave velocity structure beneath the active Aso Volcano, Kyushu, Japan, using 800 well-recorded earthquakes and ten shots recorded by an eight-station seismic network, are presented. A 68% variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure. Well-resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26% slower and 18% faster in P velocity, and up to 31% slower and 22% faster in S velocity, than the one-dimensional model. The largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones. Two low-velocity regions are imaged. The first region, a 10×15-km region encompassing the upper 3 km centered near the caldera wall at Tateno Valley, is characterized by P velocities up to 19% slower (20% for S). The second low-velocity region is associated with the central cones and active magma conduit system at 6 km depth. Velocities as low as 4.3 km/s (up to 26%) in P and 2 km/s (31% slower) in S characterize the 7-km-wide volume. The magma chamber is roughly spherical in shape, centered at 6 km depth, flattens at 10 km depth, and is located between Mt. Kishima, Mt. Eboshi, and Mt. Naka, the present focus of magmatism. A sharp velocity contrast at the depth of 3 km, with high velocities to the southwest and lower velocities to the northeast, characterizes different abutting structures associated with the Oita-Kumamoto Tectonic Line.

Magma transport at Hawaii: Inferences based on igneous thermobarometry

Research Article| January 01, 1997 Magma transport at Hawaii: Inferences based on igneous thermobarometry Keith Putirka Keith Putirka 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (1): 69–72. https://doi.org/10.1130/0091-7613(1997)025<0069:MTAHIB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Keith Putirka; Magma transport at Hawaii: Inferences based on igneous thermobarometry. Geology 1997;; 25 (1): 69–72. doi: https://doi.org/10.1130/0091-7613(1997)025<0069:MTAHIB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Pyroxene + liquid equilibrium in Hawaiian lavas occurs at a range of pressures for each volcano. Ranges are systematic and may be related to the stage of development of the magma conduit system. Kilauea, which is in its shield-building phase, yields relatively shallow storage estimates. Loihi and Mauna Kea, which are in the early and late stages of volcano growth, respectively, yield deeper storage estimates. Shallowest depth estimates at Loihi and Mauna Kea are similar to estimates of elastic plate thickness, suggesting that the mechanical behavior of the lithosphere, rather than density contrasts at the Moho, regulates magma delivery. Apparently, a large increase in fracture energy below the brittle-ductile transition inhibits transport at depth, whereas magma transport by fracture propagation is rapid through the brittle lithosphere. Some shallow depth estimates at Kilauea support the hypothesis that the strength of the unbuttressed southeast flank influences magma storage. Kilauea transport depths correlate with an eruption sequence, which illustrates a top-to-bottom emptying of the conduit system. Successively deeper reservoirs at Kilauea were tapped within 300 days, indicating that magma is stored at a range of depths, including in the mantle, rather than at a single level within the lithosphere. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fluctuations in surface height of active lava lakes during 1972–1974 Mauna Ulu Eruption, Kilauea Volcano, Hawaii

An “active” lava lake is one that forms directly on top of a magma column, as opposed to an “inactive” one formed by passive ponding of lava within a preexisting crater. Two active lava lakes operating within summit craters of newly constructed volcanic shields at Mauna Ulu and Alae were the longest‐lived of any observed in a Hawaiian rift zone to date. Small‐scale fluctuations (a few to 20 m) in the height of lake surfaces are typically associated with degassing (“gas piston”) activity. Larger fluctuations, as great as 200 m, reflect major changes in eruptive activity. Measurements of surface height fluctuations of Mauna Ulu lake show gross positive correlation with variation in summit tilt, which in turn reflects the inflation‐deflation state of Kilauea's summit magma reservoir. A more tenuous conclusion is that a sympathetic variation possibly exists between surface heights of the Mauna Ulu and Alae lakes. Collectively, these observations (1) require the existence of a fully engorged, connected, open, and hydraulically efficient magma conduit system linking the summit reservoir with the lava lakes, (2) provide additional evidence for the postulated regime of “quasi‐steady‐state” magma transfer that characterized much of the eruption, (3) imply that fluctuations in lake surface reflect variation in magma supply rate to the reservoir and/or outflow rate from the Mauna Ulu‐Alae vents, and (4) suggest undetected episodes of magma intrusion or storage in Kilauea's east rift zone during 1973.

Cryptic mineral-chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the North Arm Mountain massif of the Bay of Islands ophiolite, Newfoundland

Summary The compositions of cumulus olivine, orthopyroxene, and clinopyroxene from a detailed traverse through the upper portion of the cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite complex have been determined. The compositions of olivine (Fo 91.5–83.6 ), clinopyroxene (100 × Mg/(Mg + Fe) = 94.2–87.5) and orthopyroxene (100 × Mg/(Mg + Fe) = 90.8–85.1) within this traverse, when compared to the compositions of minerals that crystallize from oceanic basalts at various pressures, suggest that these cumulate ultramafic rocks crystallized from basaltic liquids at high pressures (&gt;10 kb) in a subaxial magma conduit system. The mineral compositions, when plotted against stratigraphic height within the complex, define eleven different cryptic units over a ‘stratigraphic’ thickness of 136 m. The boundary of each of these cryptic units is marked by an abrupt discontinuity in mineral chemistry from somewhat fractionated to very primitive compositions, suggesting that batches of magma passing through the conduit system do not intermix with other batches of magma undergoing crystallization in the system.