岩浆补给作用对硅质火山岩浆系统演化的制约

硅质火山喷发作为大陆地壳岩浆活动的重要表现，在研究大陆地壳形成与演化、探讨岩浆过程与动力学机制等方面具有重要的价值，其通常所表现的强烈爆炸式喷发，甚至可以导致全球性的环境和气候变迁。硅质岩浆系统在开放体系中不同来源岩浆的贡献和相互作用是目前研究的热点问题。持续的岩浆补给可以延长岩浆存储的时间，促进岩浆房的对流、岩浆的分异演化以及晶体 熔体的分离和晶粥的再活化，同时也是触发火山喷发的重要机制之一。此外，岩浆补给以及硅质岩浆的晶体 熔体演化过程也是火山喷发产物多样性的原因，导致同一火山在其活动过程中喷发产物规律性的变化，如富晶体火山岩、贫晶体火山岩、火山岩成分分层、以及复活岩穹和中央侵入体等。因此，岩浆补给作用是制约硅质火山岩浆系统演化和火山岩成分多样性的重要因素，也是活动火山监测和灾害评估的重要依据。岩石学、岩石地球化学、矿物(长石、石英、石榴子石、锆石等)同位素及成分变化，以及模拟实验、地震层析成像等研究为揭示硅质岩浆系统中的岩浆补给作用和复杂岩浆过程提供了多种视角。     

基于ArcGis建模分析圈定腾冲火山潜在喷发危险区域

火山是基于充足的岩浆源、通畅的岩浆通道及较高的岩浆温度形成的,同时受地形、岩性及历史火山喷发条件等因素的控制。结合前人的研究成果,以岩浆房、断层和岩浆温度为主要因素,同时以岩性、应力、应变及火山分布为辅助因素,利用Arc Gis中的Modelbuilder工具进行建模分析,并将每种因素按危险程度分9个等级,通过加权叠加,得出腾冲地区火山喷发危险区域等级图,从而圈定了该地区潜在的火山喷发危险区域,并且根据图中颜色的变化直观地显示出各地区的喷发危险程度。建模研究结果表明,目前最危险的地区是荷花—马鞍山—上庄一带的断层附近;其次分别是曲石—焦山—大坡头、腾冲—清水—荷花、五合—团田一带的断层附近。为预测未来火山喷发可能性提供了理论依据。     

Spatial and temporal distribution of large volcanic eruptions from 1750 to 2010

Using the dataset provided by the Smithsonian Institution＇s Global Volcanism Program, we have extracted the large volcanic eruptions（volcanic explosivity index ≥ 4） from the period 1750–2010 and have then analyzed the main characteristics of large volcanic eruptions since 1750 according to their geographic latitudes, their elevations, and the years and months in which they occurred. The results show that most large volcanic eruptions were located around the margins of the Pacific Ocean and the islands of Sumatra and Java, especially in the equatorial regions（10°N–10°S）. Large volcanic eruptions were concentrated at 1000–2000 m elevations and in the months of January and April. There were more eruptions in the summer half-year（from April to September） than in the winter half-year（from October to the next March）. Large volcanic eruptions have interdecadal fluctuations, including cycles of 15–25 years and 35–50 years, which were detected by Morlet wavelet analysis, with the fluctuations being more frequent after 1870 than before. During the periods 1750–1760, 1776–1795, 1811–1830, 1871–1890, 1911–1920 and 1981–1995, there were relatively many large volcanic eruptions.     

Paleo-redox conditions across the Permian-Triassic boundary in shallow carbonate platform of the Nanpanjiang Basin,South China

Ocean anoxia has been widely implicated in the Permian-Triassic extinction.However,the duration and distribution of the ocean anoxia remains controversial.In this study,the detailed redox changes across the Permian-Triassic boundary(PTB)in the shallow platform interior at Great Bank of Guizhou(GBG)has been reconstructed based on the high-resolution microfossil composition and multiple paleo-redox proxies.The shallow platform is characterized by low sulfur(total sulfur(TS)and pyrite sulfur(Spy))concentrations,low Spy/TOC ratios,and low DOP values before the mass extinction,representing oxic conditions well.Following the mass extinction,the shift of multiple geochemical proxies,including high Spy/TOC ratios and DOP values,indicates dysoxic-anoxic conditions in shallow ocean.Furthermore,we reconstruct the transition of the redox conditions of Nanpanjiang Basin:the intense volcanic eruptions,which release huge CO2 and SO2 before the mass extinction,provoke the temperature rising and the collapse of terrestrial ecosystem.As a result,the increased weathering influx causes the carbon isotopic negative excursion and the expansion of the ocean oxygen minimum zone(OMZ).When the OMZ expanded into the photic zone,the episodic H2S release events enhance the pyrite burial at Dajiang section.Thus,intense volcanic eruptions,temperature increase,and oceanic hypoxia together lead to the PTB extinction.Recent studies show high temperature might be the key mechanism of the PTB extinction.In addition,this study confirms that the microbialites were formed in the dysoxicanoxic shallow water.     

Peperites in the Permian Tarim large igneous province in Northwest China and their constraints on the local eruption environments

Peperites are special kinds of volcaniclastic materials generated by mingling of magma and unconsolidated sediments.They directly demonstrate the contemporaneity of volcanism and sedimentation,and hence they can be used to constrain the local paleoenvironments during volcanic eruptions.We identified peperites in the lower sequence of the northwest outcrops(Inggan-Kalpin area)of Permian Tarim large igneous province(TLIP),Northwest China.In Inggan,blocky peperites were observed at the base of lava flows generated in the second eruption phase.This kind of peperites is generated by quenching of magma in a brittle fragmentation mechanism.While in Kalpin,both the second and the fourth eruption phases preserved peperites in the base of lava flows.Not only blocky but also fluidal peperites can be observed in Kalpin.The fluidal peperites were generated in vapor films,which insulated the magmas from cold sediments and avoided direct thermal shock,and therefore kept the fluidal forms of magma.All of these peperites are hosted by submarine carbonates.In lava sequences generated in the same eruption phases but located in Kaipaizileike,~15 km east to Inggan,terrestrial flood basalts developed while peperites are absent,implying a paleoenvironmental transition between Kaipaizileike and Inggan-Kalpin area.Gathering information from observed peperites,TLIP lava flows,and the Lower Permian sedimentary strata,we precisely constrained the spatial distribution and temporal evolution of sedimentary facies of the early stage of TLIP.As a result,two marine transgressions were identified.The first transgression occurred contemporaneous with the second eruption phase.The transition from submarine to subaerial is located between Kaipaizileike and Inggan.The second transgression occurred contemporaneous with the forth eruption phase,and the transition from submarine to subaerial occurred between Inggan and Kalpin.     

Holocene vertical deformation along the coastal sector of Mt. Etna volcano (eastern Sicily,Italy): Implications on the time–space constrains of the volcano lateral sliding

A detailed survey of morphological and biological markers of paleo-shorelines has been carried out along the coastal sector of Mt. Etna volcano (eastern Sicily, Italy), in order to better define causes and timing of vertical deformation. We have mapped markers of raised Holocene shorelines, which are represented by beach rocks, wave-cut platforms, balanid, vermetid and algal rims. The timing of coastal uplift has been determined by radiocarbon dating of shells collected from the raised paleo-shorelines and, to correctly assess the total amount of tectonic uplift of the coast during the Late Holocene, we have compared the elevation-age data of sampled shells to the local curve of Holocene sea-level rise. Taking into account the nominal elevation of the associated paleo-shorelines, an uplift rate of 2.5–3.0 mm/year has been estimated for the last 6–7 ka. This general process of uplifting is only locally interrupted by subsidence related to flank sliding of the volcanic edifice, measured at docks and other manmade structures, and by acceleration along the hinge of an active anticline and at the footwall of an active fault. Based on this new data we suggest more precise time–space constraints for the dynamics of the lower eastern flank of Mt. Etna volcano.     

Potential Hazards of Eruptions around the Tianchi Caldera Lake, China

Since the eruption of the Tianchi volcano about 1000 years ago, there have been at least 3 to 5 eruptions of small to moderate size. In addition, hazardous avalanches, rock falls and debris flows have occurred during periods between eruptions. A future eruption of the Tianchi volcano is likely to involve explosive interaction between magma and the caldera lake. The volume of erupted magma is almost in a range of 0.1-0.5 km3. Tephra fallout may damage agriculture in a large area near the volcano. If only 1% of the lake water were ejected during an eruption and then precipitated over an area of 200 km2, the average rainfall would be 100 mm. Moreover, lahars are likely to occur as both tephra and water ejected from the caldera lake fall onto flanks of the volcano. Rocks avalanching into the caldera lake also would bring about grave hazards because seiches would be triggered and lake water with the volume equal to that of the landslide would spill out of the existing breach in the caldera and cause flooding     

宁-芜矿集区及其西缘深部结构初探

为了探查宁(南京)-芜(湖)矿集区及其西缘地区上地壳结构及其与区域成矿作用的关系,在宁芜矿集区及其西缘布设两条大地电磁测深剖面(江宁区-姑山,巢湖-当涂),通过数据处理、分析和反演,获得了宁芜矿集区10 km以内地壳电性结构模型.大地电磁测深及重力多尺度边缘检测结果表明:(1)宁芜矿集区自南向北表现为低-高-低的电性结构特征,南段深部存在低阻体,向上延伸至浅部,中段深部发育巨型高阻体上隆至浅部,倾向南西,推测可能是巨型花岗岩体,北段低阻体发育深度大,向上延伸至浅部时分叉并上升至地表;(2)揭示了滁河断裂和长江断裂带的存在; 3线剖面东段与2线南段的低阻体位置较为一致,推测为岩浆喷发通道,在含山附近发现两条电性梯度带,3线剖面的电性结构特征可能反映了区域经历了先挤压、后拉伸的演变过程.     

Sumisu volcano,Izu-Bonin arc,Japan: site of a silicic caldera-forming eruption from a small open-ocean island

Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km³ of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano’s surface.     

Persistent activity and violent strombolian eruptions at Vesuvius between 1631 and 1944

During the period 1631–1944, Vesuvius was in persistent activity with alternating mild strombolian explosions, quiet effusive eruptions, and violent strombolian eruptions. The major difference between the predominant style of activity and the violent strombolian stages is the effusion rate. The lava effusion rate during major eruptions was in the range 20–100 m³/s, higher than during mild activity and quiet effusion (0.1–1 m³/s). The products erupted during the mild activity and major paroxysms have different degree of crystallization. Highly porphyritic lava flows are slowly erupted during years-long period of mild activity. This activity is fed by a magma accumulating at shallow depth within the volcanic edifice. Conversely, during the major paroxysms, a fast lava flow precedes the eruption of a volatile-rich, crystal-poor magma. We show that the more energetic eruptions are fed by episodic, multiple arrival of discrete batches of magma rising faster and not degassing during the ascent. The rapidly ascending magma pushes up the liquid residing in the shallow reservoir and eventually reaches the surface with its full complement of volatiles, producing kilometer-high lava fountains. Rapid drainage of the shallow reservoir occasionally caused small caldera collapses. The major eruptions act to unplug the upper part of the feeding system, erupting the cooling and crystallizing magma. This pattern of activity lasted for 313 y, but with a progressive decrease in the number of more energetic eruptions. As a consequence, a cooling plug blocked the volcano until it eventually prevented the eruption of new magma. The yearly probability of having at least one violent strombolian eruption has decreased from 0.12 to 0.10 from 1944 to 2007, but episodic seismic crises since 1979 may be indicative of new episodic intrusions of magma batches.