ALBERT

Description: Changbaishan volcano (China/North Korea; last eruption in 1903 AD) was responsible for a Volcanic Explosivity Index (VEI) 7 eruption in 946 AD. Approximately 186,000 people live around Changbaishan and 2,000,000 tourists/year visit the volcano. An unrest occurred between 2002 and 2006. Despite the relevant hazard, the eruptive history is poorly known, a condition common to many volcanoes worldwide. Here, we investigate the extension of the areas potentially a ected by pyroclastic density currents (PDCs) in case of future eruptions following a scenario-based approach. We perform energy cone runs referred to four scenarios from columns of height 3, 10, 20 and 30 km at di erent vents. By using global datasets on PDCs, we produce spatial probability maps of PDCs invasion. Empirical laws between covered areas, PDC travelled distances, and heights of collapse are provided. In scenarios 3 and 4, PDCs expand at distances up to 42 km and 85 km, respectively. In scenarios 1 and 2, PDCs invade the touristic area and few main roads. Severe e ects emerge from scenarios 3 and 4 with the interruption of the China–North Korea land and aerial connections and PDC. Our approach may serve as guide for the rapid evaluation of the PDC-related hazard at poorly known volcanoes.

Description: Published

Description: 2622

Description: 6V. Pericolosità vulcanica e contributi alla stima del rischio

Description: JCR Journal

Description: The geodynamic significance of continental volcanoes located far from the plate boundaries remains highly controversial as exemplified by contrasting models that favor either a deep mantle plume rooted from the base of the mantle or, alternatively, the shallower subduction or lithospheric-related processes. The Changbaishan (also referred to as Paektusan or Baekdusan) volcanic field (CHVF), located in the interior of eastern Eurasian continent, provides a good opportunity to constrain the magma origin and geodynamic mechanism governing continental intraplate volcanism. Here, we review the volcanic geology, eruptive history, geochemical data on volcanic rocks and released gases and geophysical observations of the Changbaishan volcanoes with the aim to (a) reconstruct temporal and spatial evolution of eruptive activities, (b) identify source of the primary magmas, (c) delineate magma evolution in the crust, (d) highlight geodynamic significance of the CHVF volcanism, (e) characterize crustal magmatic structure, and (f) analyze recent dynamics with a focus on the 2002–2005 unrest episode at Tianchi caldera, the only active volcano in the area (last eruption in 1903 CE). The eruptive activities of the Changbaishan volcanoes can be divided into three main stages: (1) central vent and fissure eruptions of basaltic magmas started approximately in Pliocene and culminated in Early Pleistocene (ca. 5–1 Ma), forming a shield-like lava plateau; (2) multi-stage eruptions of voluminous silicic (and minor intermediate) magmas constructed cones of the polygenetic volcanoes (e.g., Tianchi, Wangtian'e and Namphothe) between Late Pliocene and Pleistocene (3.14–0.01 Ma); and (3) explosive silicic eruptions [e.g., the Millennium eruption (ME) in 946 CE] during Holocene dominated the Tianchi volcano and led to the formation of its summit caldera. Small-scale eruptions of basaltic magmas from monogenetic scoria cones (and minor fissures) were coeval with the Tianchi cone-construction stage (ca. 1–0.01 Ma). The elemental and Sr-Nd-Pb isotopic characteristics of the Changbaishan basalts indicate an enriched, heterogeneous mantle source with components from depleted mantle (DM), enriched mantle 1 (EM1) and subduction-related materials (e.g., recycled oceanic crust and sediments). The interaction between the DM-like peridotite and carbonatite melts released by subducted oceanic slab in the mantle transition zone (MTZ) led to the formation of carbonated peridotite characterized by low δ26Mg values. By contrast, origin of the EM1-like components remains highly debated. The alkaline basalts and intermediate to silicic volcanic rocks from the polygenetic volcanoes constitute an integrated spectrum of magma composition controlled by closed system fractionation according to their element co-variations and uniform Sr-Nd-Pb isotopic compositions. Subordinate mingling between trachyte and comendite has been reported only for the ME at Tianchi caldera. The occurrence of a big mantle wedge (BMW) with a continuous stagnant Pacific slab in the MTZ is responsible for origin of the Changbaishan volcanoes. On the basis of subduction dynamics of the Pacific plate, we present a Late Cenozoic geodynamic framework of NE Asia, which can account for formation of the present-day BMW system via: (a) shallow-angle subduction (55–25 Ma), (b) slab rollback and sinking into the MTZ together with trench retreat (25–15 Ma), and (c) slab bottoming, thickening and flattening in the MTZ (15–0 Ma). Constraints from reconstructed plate motion history, numerical simulation and present-day geophysical observation of the BMW lend support to our geodynamic model, which reconciles well with the Izanagi slab breakoff, development of the Japan Sea and Late Cenozoic continental intraplate volcanism in NE China. In response to the Rayleigh-Taylor instability, a MTZ-derived plume incorporating fragments of carbonated peridotite, EM1- like components and the Pacific slab-derived materials ascended and experienced decompression partial melting at shallow depths to feed the Changbaishan volcanism. From the perspective of magma origin and geodynamic mechanism, the Changbaishan volcanoes can shed light on the potential relationships between origin of continental intraplate volcanism and deep subduction of oceanic lithosphere. The spatial distribution of the Changbaishan volcanoes shows that the magmas ascended along a NW-SE trending, strike-slip fault oriented perpendicularly to the major faults delimiting the Songliao Basin, NE China. This interpretation is consistent with the 2009–2013 epicenters of tectonic earthquakes, also suggesting a NW-SE trending, buried and seismically active deep fault in the crust. Geophysical and petrological constraints indicate the presence of magma reservoirs at crustal depth beneath the active Tianchi volcano, which are likely to have high thermal state and act as the source of heat and material for shallow hydrothermal system. In consideration of magma origin from the MTZ-derived plume, the volatile outgassing from the Tianchi volcano associated with deep subduction of the Pacific plate represents an important mechanism for liberating volatile elements (especially carbon) from Earth's interior to the exosphere. Tianchi caldera suffered an unrest episode between 2002 and 2005, as evidenced by increased shallow seismicity, surface uplift and changes in chemical and isotopic composition of the hydrothermal gases. Such volcanic unrest was triggered by pressurization of a 2–6 km depth magma reservoir, from which magmatic volatiles were released into shallow hydrothermal system. Tianchi caldera shows different types of hazards related to volcanic, tectonic, geomorphological and hydrological processes. Further monitoring and additional volcanological data, especially those on eruptive dynamics of the past eruptions, should be collected to better constrain the potential hazards of future eruptions and to improve early warning management.

Description: Published

Description: 19-52

Description: 1V. Storia eruttiva

Description: JCR Journal

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zhang, M., Guo, Z., Xu, S., Barry, P. H., Sano, Y., Zhang, L., Halldorsson, S. A., Chen, A.-T., Cheng, Z., Liu, C.-Q., Li, S.-L., Lang, Y.-C., Zheng, G., Li, Z., Li, L., & Li, Y. Linking deeply-sourced volatile emissions to plateau growth dynamics in southeastern Tibetan Plateau. Nature Communications, 12(1), (2021): 4157, https://doi.org/10.1038/s41467-021-24415-y.

Description: The episodic growth of high-elevation orogenic plateaux is controlled by a series of geodynamic processes. However, determining the underlying mechanisms that drive plateau growth dynamics over geological history and constraining the depths at which growth originates, remains challenging. Here we present He-CO2-N2 systematics of hydrothermal fluids that reveal the existence of a lithospheric-scale fault system in the southeastern Tibetan Plateau, whereby multi-stage plateau growth occurred in the geological past and continues to the present. He isotopes provide unambiguous evidence for the involvement of mantle-scale dynamics in lateral expansion and localized surface uplift of the Tibetan Plateau. The excellent correlation between 3He/4He values and strain rates, along the strike of Indian indentation into Asia, suggests non-uniform distribution of stresses between the plateau boundary and interior, which modulate southeastward growth of the Tibetan Plateau within the context of India-Asia convergence. Our results demonstrate that deeply-sourced volatile geochemistry can be used to constrain deep dynamic processes involved in orogenic plateau growth.

Description: This work was supported by China Seismic Experimental Site (CSES) (2019CSES0104), the Strategic Priority Research Program (B) of Chinese Academy of Sciences (XDB26000000), the National Key Research and Development Program of China (2020YFA0607700), the National Natural Science Foundation of China (41930642, 41602341, 41772355, and 41702361), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (2019QZKK0702), and the United Laboratory of High-Pressure Physics and Earthquake Science (2019HPPES02). P.H.B. was supported by the US National Science Foundation EAR Grant 1144559 during a portion of this work.

Description: The impact of large-scale caldera-forming eruptions on our society and climate can be considerable. The triggering mechanisms of these eruptions and the instability of their magmatic systems are still elusive. Here we use X-ray tomographic microscopy, glass geochemistry and volatile element concentration data on the products of the 946 CE ‘Millennium’ eruption (ME) of Changbaishan volcano (China/North Korea) with the aim to identify the triggering mechanism of the eruption. ME emitted rhyolites and trachytes whose textural parameters suggest vesiculation events related to crystallization and magma ascent in the conduit, and to the arrival of new gas in the magmatic system. Solubility models show that the CO2 and H2O dissolved in the glass are consistent with a pressure of 100–200 MPa. Literature data from fluid inclusions in minerals indicate that the residing magma was CO2 free before the eruption, whereas the CO2 content in the glass reaches 600 ppm at the flash of the ME event. We find that a single, shallow magma reservoir localized between 7.5 and 3.7 km depth in which rhyolites occupies the top and trachytes the bottom is fully destabilized by the arrival of external CO2-rich fluids. Such fluids are released from a deeper, carbonate-rich mantle source. Our results and those of independent geophysical data show that the ME magmatic system is still active, and the continuous upraising of fluids from depth may drive unrest episodes like that recorded in 2002–2006. Our findings elucidate the role of deep, mantle-derived fluids in driving large-scale explosive eruptions. We provide evidence that volcanic unrests may not mirror the internal dynamics of magmatic reservoirs.

Description: Published

Description: 706-719

Description: 4V. Processi pre-eruttivi

Description: JCR Journal

Description: Mantle degassing transect across different tectonic units within a plate convergent setting has been well documented for oceanic convergent margins by systematic changes in geochemistry (e.g., 3He/4He, d13C, and CO2/3He) of hydrothermal gases. However, little is known about spatial variations in volatile geochemistry across a continental convergent margin. In this study, we identify a mantle degassing transect in the southeastern Tibetan Plateau using He-CO2 systematics of hydrothermal gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. d13C-CO2 ( 11.8‰ to 3.1‰) and CO2/3He (1.7 108 to 7.1 1011) values of hydrothermal gases show large variations that are consistent with modification by secondary physico-chemical processes, such as multi-component mixing, hydrothermal degassing, and calcite precipitation. Three levels of He degassing can be recognized based on 3He/4He dataset (0.01–5.87 RA) of the hydrothermal gas samples and their distances to volcanic centers. A magmatic level He degassing (35–74% mantle He) is found near active and/or Quaternary volcanoes fed by mantle-derived magmas. With increasing distance to volcanic centers, the outgassed magmatic volatiles are gradually diluted by crustal components (e.g., radiogenic 4He), defining a transitional level He degassing (13–33% mantle He). The 3He/4He values (8.16–8.48 RA) of olivine phenocrysts indicate a MORB (mid-ocean ridge basalts)-type mantle source for the magmatic and transitional levels of He degassing that are localized in Quaternary volcanic fields. In contrast, a background level He degassing (〈12% mantle He) dominates the entire study area, and can be attributed to (i) degassing of sub-continental lithospheric mantle, and/or (ii) higher degrees of crustal contamination than those of the transitional level He degassing near volcanic centers. Combined with olivine 3He/4He data reported in this study and whole-rock 87Sr/86Sr data of host basalts from literature, source components of the mantle-derived magmas are suggested to include the MORB-type convective mantle, subducted Indian slab materials, and less degassed mantle materials, which can account for the possible decoupling between He and Sr isotope systematics. These findings delineate the origin and outgassing of mantle volatiles controlled by the India-Asia continental convergence, and would contribute to a better understanding of the deeply-sourced volatile emissions in these tectonic settings.

Description: Published

Description: 61-78

Description: 9T. Geochimica dei fluidi applicata allo studio e al monitoraggio di aree sismiche

Description: JCR Journal

Description: Changbaishan volcano (China/North Korea; last eruption in 1903 AD) was responsible for a Volcanic Explosivity Index (VEI) 7 eruption in 946 AD. Approximately 186,000 people live around Changbaishan and 2,000,000 tourists/year visit the volcano. An unrest occurred between 2002 and 2006. Despite the relevant hazard, the eruptive history is poorly known, a condition common to many volcanoes worldwide. Here, we investigate the extension of the areas potentially affected by pyroclastic density currents (PDCs) in case of future eruptions following a scenario-based approach. We perform energy cone runs referred to four scenarios from columns of height 3, 10, 20 and 30 km at different vents. By using global datasets on PDCs, we produce spatial probability maps of PDCs invasion. Empirical laws between covered areas, PDC travelled distances, and heights of collapse are provided. In scenarios 3 and 4, PDCs expand at distances up to 42 km and 85 km, respectively. In scenarios 1 and 2, PDCs invade the touristic area and few main roads. Severe effects emerge from scenarios 3 and 4 with the interruption of the China–North Korea land and aerial connections and PDC. Our approach may serve as guide for the rapid evaluation of the PDC-related hazard at poorly known volcanoes.