Deep Magmatic Reservoir Research Articles

Source mechanisms of deep long period earthquakes beneath the Klyuchevskoy volcanic group (Kamchatka, Russia) inferred from S-to-P amplitude ratios

We study the long-period earthquakes that occur at the crust-mantle boundary beneath the Klyuchevskoy volcano group in Kamchatka in order to reconstruct their source mechanisms. These earthquakes are observed at frequencies between 1 and 4 Hz and the phases of their seismograms are strongly affected by the high-pass filtering required to remove the microseismic noise. Therefore, we decided to use an inversion method based on amplitude ratios between S- and P-waves. Considering the uncertainties associated with this method and potentially leading to non-uniqueness of the inversion, we decided not to explore the full space of the source parameters but to test a set of “elementary” mechanisms corresponding to processes possibly occurring within magmatic systems of volcanoes and their surroundings. Also, after measuring the raw amplitudes of P- and S-waves we corrected them for the site amplification effects. Based on the results of the inversion, the generation of the DLP earthquakes beneath Klyuchevskoy by shear faulting (as would be the case in thermomechanical stresses associated with cooling of deep intrusions) can be reasonably excluded. The observed signal amplitudes can be better explained with source mechanisms containing strong volumetric or single force components. The former can be associated with the pressure perturbation withing magmatic reservoirs or conduits and the latter with the sudden acceleration of the magma movement. The ensemble of our observations is compatible with the configuration when the magma is stored in nearly horizontal sills near the crust-mantle boundary and penetrates into the crust through conduits dipping south-southwest, in agreement with previously reported connection of the deep magmatic reservoir with the Bezymanny and Tolbachik volcanoes.

Genesis and ore-forming process of the Benqu mesothermal gold deposit in the Jiapigou ore cluster, NE China: Constraints from geology, geochronology, fluid inclusions, and whole-rock and isotope geochemistry

The Jiapigou ore cluster (JOC) located on the northeastern margin of the North China Craton (NCC) is a major producer of gold in China and is famous for its quartz vein-type gold deposits. It currently contains a dozen gold deposits with total proven reserves of >180 t Au, among which the Benqu deposit is the earliest discovered and most representative. The deposit is hosted within Neoarchean granitic gneisses and metamorphosed volcanic-sedimentary sequences. Gold orebodies mainly occur as gold-bearing quartz veins controlled by NE–NEE-trending ductile–brittle faults. Hydrothermal mineralization can be divided into early (quartz–pyrite), main (quartz–polymetallic sulfide–native gold), and late (quartz–carbonate) stages, with gold precipitation occurring in the main stage. Three types of primary fluid inclusions (FIs) are identified in quartz formed at different stages: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type) and pure CO2 (PC-type). The early-stage quartz contains C- and W-type FIs, which have homogenization temperatures of 269–298 °C and salinities of 8.8–16.7 wt% NaCl equiv. The main-stage quartz contains all three types of FIs, with homogenization temperatures of 197–268 °C and salinities of 5.1–15.2 wt% NaCl equiv. The late-stage quartz contains only W-type FIs with homogenization temperatures of 145–198 °C and salinities of 3.2–12.4 wt% NaCl equiv. The ore-forming fluid system evolved from a CO2–H2O–NaCl system in the early stage to a H2O-dominated system in the late stage. The addition of meteoric water led to fluid immiscibility, facilitating the precipitation of gold and associated sulfides. The H–O–S–Pb–Sr isotope results indicate that the ore-forming fluids were derived from magmatic water and that the ore-forming materials were mainly extracted from a deep magmatic reservoir sourced mostly from the lower crust, with trace mantle components. Rb–Sr dating of the gold-bearing sulfides yields an isochron age of 175.8 ± 4.6 Ma (MSWD = 1.9). Zircon U–Pb dating of the Benqu diorite yields a weighted mean 206Pb/238U age of 176.5 ± 2.4 Ma (MSWD = 1.2). The Benqu diorite is high-K calc-alkaline, with enriched LREEs and LILEs (e.g., Rb, Pb, and K) and depleted HFSEs (e.g., Nb, Ta, and Ti), which is typical of the geochemical signature of arc magmas. The zircons have negative εHf(t) values of −11.3 to −8.9, indicating strong involvement of old crustal materials. These data suggest that the primary magma originated mainly from the partial melting of ancient lower crust that mixed with minor underplated mantle-derived magma. The above results systematically reveal that the Benqu is a mesothermal magmatic–hydrothermal lode gold deposit that formed in an active continental margin environment in the late Early Jurassic. Comparative studies of Benqu and other gold deposits in the JOC confirm that a significant magmatic–hydrothermal event occurred during the conversion from compression to extension that followed the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

The Miocene Brahma porphyry Cu-Mo prospect in Central Chilean Andes (35°45′S): Geology, geochronology (U-Pb, Re-Os) and geochemistry

The Brahma prospect is a Neogene porphyry copper deposit located in the Chilean Andes at ~ 35°45′S, beyond the southerly extension of the previously defined Mio-Pliocene metallogenic belt of Central Chile. We present the results of a comprehensive study of this prospect based on drill hole logging, petrography, geochemistry, and geochronology, with the aim of constraining its paragenesis and formation within the metallogenic background of the region. The prospect is hosted by dioritic and granodioritic equigranular units mineralized by fluids from at least two hydrothermal pulses in turn related to the emplacement of porphyritic intrusives and hydrothermal breccias. Existing exploration drill holes only intercepted the phyllic and argillic zone of the system, consistent with the paucity of potassic alteration and the dominance of pyrite, sphalerite, chalcopyrite and molybdenite mineralization. Zircon U-Pb determinations of the main igneous units present yielded ages of ~ 17 Ma, 14 Ma, and 9 Ma. Mineralization ages of ~ 14 Ma, 12 Ma, and 10 Ma were obtained by Re-Os determinations on molybdenite from samples of different paragenetic stages. These data indicate that (i) the prospect is composed of at least three main magmatic events, (ii) early mineralization is associated with the ~ 14 Ma magmatic pulse, and (iii) economic mineralization occurred at ~ 10 Ma and lasted until at least 9 Ma, as indicated by the presence of a younger mineralized porphyritic unit of this age. The obtained ages are comparable to those reported for the Los Pelambres deposit, a Cu-Mo porphyry located at 31°43′S and belonging to the Mio-Pliocene Metallogenic Belt of Central Chile. For the main intrusive units recognized in the Brahma prospect, whole rock geochemical analyses reveal arc affinities, juvenile signatures, and the involvement of amphibole in the fractionating assemblages. In a whole, our results support the existence of a complex deep magmatic reservoir that fed the magmatic-hydrothermal system for a long period, of at least 4 to 5 m.y., a common characteristic in other Miocene-Pliocene deposits in Chile and also in porphyry Cu deposits worldwide. In addition, our results highlight an interesting exploratory potential for the Brahma prospect, a long-lived Cu-Mo porphyry deposit of early Miocene age which southerly expands until ~ 36°S the Mio-Pliocene Metallogenic Belt of Central Chile.

Progressive reactivation of the volcanic plumbing system beneath Tolbachik volcano (Kamchatka, Russia) revealed by long-period seismicity

After lying dormant for 36 yr, the Tolbachik volcano of the Klyuchevskoy group started to erupt on 27 November 2012. We investigate the preparatory phase of this eruption via a statistical analysis of the temporal behavior of long-period (LP) earthquakes that occurred beneath this volcanic system. The LP seismicity occurs close to the surface beneath the main volcanic edifices and at 30 km depth in the vicinity of a deep magmatic reservoir. The deep LP earthquakes and those beneath the Klyuchevskoy volcano occur quasi-periodically, while the LP earthquakes beneath Tolbachik are clustered in time. As the seismicity rate increased beneath Tolbachik days before the eruption, the level of the time clustering decreased. We interpret this as a manifestation of the evolution of the volcano plumbing system. We suggest that when a plumbing system awakes after quiescence, multiple cracks and channels are reactivated simultaneously and their interaction results in the strong time clustering of LP earthquakes. With time, this network of channels and cracks evolves into a more stable state with an overall increased permeability, where fluids flow uninhibited throughout the plumbing system except for a few remaining impediments that continue to generate seismic radiation. The inter-seismic source interaction and the level of earthquake time clustering in this latter state is weak. This scenario suggests that the observed evolution of the statistical behavior of the shallow LP seismicity beneath Tolbachik is an indicator of the reactivation and consolidation of the near-surface plumbing system prior to the Tolbachik eruption. The parts of the plumbing system above the deep magmatic reservoir and beneath the Klyuchevskoy volcano remain in nearly permanent activity, as demonstrated by the continuous occurrence of the deep LP earthquakes and very frequent Klyuchevskoy eruptions. This implies that these parts of the plumbing system remain in a stable permeable state and contain a few weakly interacting seismogenic sources. Our results provide new constraints on future mechanical models of the magmatic plumbing systems and demonstrate that the level of time clustering of LP earthquakes can be a useful parameter to infer information about the state of the plumbing system.

Deep and shallow long-period volcanic seismicity linked by fluid-pressure transfer

Volcanic long-period earthquakes are attributed to pressure fluctuations that result from unsteady mass transport in the plumbing system of volcanoes. Whereas most of the long-period seismicity is located close to the surface, the volcanic deep long-period earthquakes that occur in the lower crust and uppermost mantle reflect the activity in the deep parts of magmatic systems. Here, we present observations of long-period earthquakes that occurred in 2011–2012 within the Klyuchevskoy volcano group in Kamchatka, Russia. We show two distinct groups of long-period sources: events that occurred just below the active volcanoes, and deep long-period events at depths of ∼30 km in the vicinity of a deep magmatic reservoir. We report systematic increases of the long-period seismicity levels prior to volcanic eruptions with the initial activation of the deep long-period sources that reflects pressurization of the deep reservoir and consequent transfer of the activity towards the surface. The relatively fast migration of the long-period activity suggests that a hydraulic connection is maintained between deep and shallow magmatic reservoirs. The reported observations provide evidence for the pre-eruptive reload of the shallow magmatic reservoirs from depth, and suggest that the deep long-period earthquakes could be used as a reliable early precursor of eruptions. Shallow volcanic earthquakes can aid eruption forecasts. Analysis of seismicity beneath the Klyuchevskoy volcano group in Russia reveals much deeper magma-induced earthquakes that may serve as an early eruption indicator.

Large-volume lava flows fed by a deep magmatic reservoir at Ağrı Dağı (Ararat) volcano, Eastern Turkey

Agri Dagi (Ararat), whilst being the tallest volcano in Turkey, is largely understudied. Two predominant peaks, Greater and Lesser Agri, make up the main edifice, which has been built during four main phases. The most recent phase consisted of two volcanic eruptions. The respective surface area and volume of the first volcanic eruption were estimated at 96 km2 and 3.2 km3, whereas those of the second eruption were much smaller with the surface area and volume estimated at 25 km2 and 0.6 km3. It is unusual for stratovolcanoes to produce basaltic eruptions of more than 3 km3, although these and larger volumes are not uncommon in flood basalt-type eruptions. Large basaltic eruptions from stratovolcanoes normally require volcano-tectonic forcing (e.g. subsidence of collapse caldera and graben). However, there is no evidence for such volcano-tectonic forcing, during the most recent eruptions at Agri Dagi (Ararat), and therefore, their comparatively large volume basaltic lavas need to be explained in a different way. Here, we present an analytical method for calculating the source volume needed to supply magma to the eruptions at Agri Dagi. We found that the lava flow of 3.2 km3 was likely fed by a very large magma reservoir (∼13,000 km3), while the second flow of 0.6 km3 was fed by a reservoir of a much smaller effective size (or ∼2000 km3). ‘Effective size’ depends on what fraction of the reservoir participates in the eruption. We propose that the entire reservoir supplied magma to the larger eruption, but only one of its compartments (about one fifth of the total volume of the reservoir) supplied magma to the smaller eruption. Although seismic tomography indicates a magma reservoir at great depths (>20–30 km) below the Agri Dagi volcano, geochemical constraints on some of the later-formed rocks suggest an interaction between a shallow chamber (at 8–10-km depth) and the deep reservoir approximately 0.5 Ma. We provide numerical models whose results indicate that dykes injected from the lateral margins of the deep-seated reservoir are more likely to reach the surface directly rather than replenish the shallow magma chamber, suggesting also that the compartment for the second eruption was at the margin of the reservoir.

Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high‐frequency gas monitoring

Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high‐frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2‐rich gas (CO2/Stotal > 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur‐rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8–10 km deep, whereas the shallow magmatic gas source is at ~3–5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000 T/d SO2 and H2S/SO2 > 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 < 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high‐temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.

Eruptive activity of Planchón-Peteroa volcano for period 2010-2011, Southern Andean Volcanic Zone, Chile

Planchón-Peteroa volcano started a renewed eruptive period between January 2010 and July 2011. This eruptive period was characterized by the occurrence of 4 explosive eruptive phases, dominated by low-intensity phreatic activity, which produced almost permanent gas/steam columns (200-800 m height over the active crater). Those columns presented frequently scarce ash, and were interrupted by phreatic explosions that produced ash columns 1,000-3,000 m height in the more intense periods. Eruptive plumes were transported in several directions (NW, N, NE, E and SE), but more than half of the time the plume axis was 130-150° E, and reached a distance up to 638 km from the active crater. Tephra fall deposits identified in the NW, N, NE, E and SE flanks covered an area of 1,265 km2, thickness variable from 4 m (SE border of active crater) to ~0.5 cm 36.8 km SE and ~8 km NW from active crater, respectively, corresponding to a minimum volume of 0.0088 km3. Tephra fall deposit is exclusively constituted of no juvenile fragments including: lithics fragments as main component, quartz and plagioclase crystals, some oxidized lithics, and occasional presence of Fe oxide, and less frequently Cu minerals, as single fragments. We present new field-based measurements data of the geochemistry of gas/water from fumaroles and acid crater lakes, and fall deposit analysis, that integrated with the eruptive record and GOES satellite data, suggests that the eruptive period 2010-2011 has been related to an increasing of heat and mass transfer from hydrothermal-magmatic reservoirs, which would have been favoured by the formation and/or reactivation of cracks after 8.8 Mw Maule earthquake in February 2010. This process also allowed the ascent of fluids from a shallow hydrothermal source, dominated by reduced species as H2S and CH4, during the entire eruptive period, and the release of more oxidizing fluids from a deep magmatic reservoir, dominated by acid species as SO2, HCl and HF, increasing strongly after the end of the eruptive period, probably since October 2011. The eruptive period was scored with a magnitude of 3.36, corresponding to a VEI 1-2.

Gas and aerosol emissions from Lascar volcano (Northern Chile): Insights into the origin of gases and their links with the volcanic activity

This study focuses on the chemical compositions of volcanic gases and aerosols emitted by Lascar volcano (Northern Chile). The sustained volcanic plume was sampled in April 2009, April 2011 and November 2012 through filter packs and analyzed for major gaseous species (SO2, HCl, and HF) and trace elements. During fieldwork, SO2 flux measurements were also performed by UV spectrometry (DOAS). The Lascar volcano is a significant and sustained emitter of SO2 (between 150 and 940t/d), HCl (between 170t/d and 210t/d) and HF (up to 100t/d). Combining the SO2-normalized trace element concentrations and the SO2 emission rates, we evaluate that the quiescent degassing of Lascar is an important local source of trace metals to the atmosphere with contributions to global volcanic fluxes generally less than 2%.Our data were used to infer the origin of the gas emitted at Lascar. Two major degassing sources were identified: a deep magmatic reservoir and a shallow hydrothermal system. The variable interaction between these two sources is the most likely scenario for explaining the compositional ranges in acid gases but also in a volatile and fluid–mobile trace element such as B. These variations are related to changes in volcanic activity: an extensive interaction between the hot ascending magmatic gases and the shallow aquifer occurred in 2009, possibly due to a long period of quiescence, before magmatic activity renewed in 2011 and 2012. Our study highlights, therefore, that filter-pack measurements may be used to study changes in subsurface processes that probably play a key role in triggering volcanic eruptions.