Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the approximately 60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of approximately 2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975-84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia-Arabia plate boundary.     

塔里木盆地火成岩对碳酸盐岩储层的改造作用

塔里木盆地形成与演化过程中出现过多期的岩浆侵入与喷发活动,形成了多种类型的火成岩体.岩浆活动形成的火成岩带来了地壳深部的热液流体,其中富含二氧化碳、硫化氢、硫、及少量一氧化碳、氢、HCl、NH3、NH4Cl、HF等的酸性流体对上部碳酸盐岩储层产生了多种多样的改造作用,包括大理岩化作用、萤石化作用、热液白云岩化作用、热液溶蚀作用等.多种改造作用的结果,使得原本致密的碳酸盐岩储层,形成了大量各种类型的孔洞,孔隙度大大增加,储集性能得到改善.勘探过程中,火成岩的出现可以预示附近地区良好储层的发育,为勘探油气起到了示踪作用.     

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems.     

Kimberlite ascent and eruption

Wilson and Head model kimberlite ascent and eruption by considering the propagation of a volatile-rich dyke. Wilson and Head's model has features in common with Sparks et al., but it is inconsistent with geological observations and constraints on volatile solubility. Here we show that this may be due to erroneous physical assumptions.     

Survival times of anomalous melt inclusions from element diffusion in olivine and chromite

The chemical composition of basaltic magma erupted at the Earth's surface is the end product of a complex series of processes, beginning with partial melting and melt extraction from a mantle source and ending with fractional crystallization and crustal assimilation at lower pressures. It has been proposed that studying inclusions of melt trapped in early crystallizing phenocrysts such as Mg-rich olivine and chromite may help petrologists to see beyond the later-stage processes and back to the origin of the partial melts in the mantle. Melt inclusion suites often span a much greater compositional range than associated erupted lavas, and a significant minority of inclusions carry distinct compositions that have been claimed to sample melts from earlier stages of melt production, preserving separate contributions from mantle heterogeneities. This hypothesis is underpinned by the assumption that melt inclusions, once trapped, remain chemically isolated from the external magma for all elements except those that are compatible in the host minerals. Here we show that the fluxes of rare-earth elements through olivine and chromite by lattice diffusion are sufficiently rapid at magmatic temperatures to re-equilibrate completely the rare-earth-element patterns of trapped melt inclusions in times that are short compared to those estimated for the production and ascent of mantle-derived magma or for magma residence in the crust. Phenocryst-hosted melt inclusions with anomalous trace-element signatures must therefore form shortly before magma eruption and cooling. We conclude that the assumption of chemical isolation of incompatible elements in olivine- and chromite-hosted melt inclusions is not valid, and we call for re-evaluation of the popular interpretation that anomalous melt inclusions represent preserved samples of unmodified mantle melts.     

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallaj?kull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18?years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5?mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ～0.05?km(3) magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma-ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallaj?kull's behaviour can be attributed to its off-rift setting with a 'cold' subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.     

The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile

Rhyolite is the most viscous of liquid magmas, so it was surprising that on 2?May 2008 at Chaitén Volcano, located in Chile's southern Andean volcanic zone, rhyolitic magma migrated from more than 5?km depth in less than 4?hours (ref.?1) and erupted explosively with only two days of detected precursory seismic activity. The last major rhyolite eruption before that at Chaitén was the largest volcanic eruption in the twentieth century, at Novarupta volcano, Alaska, in 1912. Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives. Here we present surface deformation data related to the Chaitén eruption based on radar interferometry observations from the Japan Aerospace Exploration Agency (JAXA) DAICHI (ALOS) satellite. The data on this explosive rhyolite eruption indicate that the rapid ascent of rhyolite occurred through dyking and that melt segregation and magma storage were controlled by existing faults.     

300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin

Except in regions of recent crustal extension, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter or mineral reactions. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO2/3He ratios and delta13C(CO2) values observed in CO2-rich natural gases in the west Texas Val Verde basin and also the mantle 3He/22Ne ratios observed in other basin systems. Regional changes in CO2/3He and CO2/CH4 ratios can be explained if the CO2 input pre-dates methane generation in the basin, which occurred about 280 Myr ago. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO2. Our identification of magmatic CO2 in a foreland basin indicates that the origin of CO2 in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for approximately 300 Myr is far longer than the residence time predicted by diffusion models.     

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85 degrees E, to acquire photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km(2)) area. At least 13.5 wt% CO(2) is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.     

The role of volatiles in magma chamber dynamics

Many andesitic volcanoes exhibit effusive eruption activity, with magma volumes as large as 10(7)-10(9) m(3) erupted at rates of 1-10 m(3) x s(-1) over periods of years or decades. During such eruptions, many complex cycles in eruption rates have been observed, with periods ranging from hours to years. Longer-term trends have also been observed, and are thought to be associated with the continuing recharge of magma from deep in the crust and with waning of overpressure in the magma reservoir. Here we present a model which incorporates effects due to compressibility of gas in magma. We show that the eruption duration and volume of erupted magma may increase by up to two orders of magnitude if the stored internal energy associated with dissolved volatiles can be released into the magma chamber. This mechanism would be favoured in shallow chambers or volatile-rich magmas and the cooling of magma by country rock may enhance this release of energy, leading to substantial increases in eruption rate and duration.     

五大连池尾山地区存在干热岩资源的可能性探讨

 干热岩被视为未来的清洁能源,是地热资源的重要组成部分,极具研究价值和开发前景。中国现阶段干热岩调查研究应遵循"面上开展,点上突破"的原则。基于"点上突破"原则,本文选取中国最新的火山群--五大连池火山群开展前期研究,通过搜集分析区域地质条件、岩石地球化学、深部CO₂气体,区域地球物理资料,探讨了五大连池尾山地区存在干热岩的可能性。研究结果表明,五大连池火山属于非深源火山,具备存在干热岩的可能,热源机制属于岩浆余热型,第四纪火山岩浆囊所处的深度上限值为3.3~3.4 km。区内深部CO₂气体来源于地幔岩浆冷却成岩过程中的脱排气作用,是幔源岩浆囊中岩浆结晶分异的结果。W8勘探孔及附近供水井发现高浓度CO₂的事实表明,在尾山与五大连池农场七分场之间的地下可能存在正在冷却成岩的岩浆囊。     

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.     

中条山西南段与变基性岩有关的铜矿地质特征及找矿远景

山西省中条山区是我国重要铜矿区之一,矿床类型多,成因复杂。但与变基性岩有关的铜矿床,是在中条山西南段发现的一种铜矿类型,其是在原始基性岩浆侵入或喷发作用形成的矿源基底和矿化体基础上,经过区域变质和后期岩浆热液等综合作用下改造加富形成。其成因属于与变基性岩有关的变质—热液矿床。探讨了中条山西南段与变基性岩有关的铜矿地质特征及找矿远景。     

甘肃北山地区同位素定年与构造岩浆热事件

在面积约100000Km~2的甘肃北山地区,共搜集同位素年龄数据216个,其中22个是本次测定的。依据这些资料做了年龄频谱图和地质环境一岩浆热事件综合示意图,从这些结果中可以得出以下看法:1.哈萨克斯坦板块的东端前寒武纪褶皱基底以微大陆的形式延入了本区,即本区有两大构造单元—塔里木板块和哈萨克斯坦板块。2.虽然有30多属燕山期的表现年龄,但从年龄频谱的骤然下落和野外复查表明,这些是后期岩脉和矿脉的年龄,而不是大规模岩浆活动的记录,即本区没有燕山期的大规模的岩浆活动。3.从加里东到印支期的每一构造岩浆旋回中,岩浆活动似有滞后现象,甚至延至下一个旋回,很可能这种滞后具有普遍意义。4.在每一构造单元内,大陆边缘区、过渡壳区和海洋区的喷发、侵入及变质作用也不尽相同。海洋区内喷发作用强于侵入和变质作用,而大陆边缘区侵入作用强于变质和喷发作用。     

山西临县紫金山碱性杂岩体的地球化学特征

通过对山西临县紫金山碱性杂岩体各类岩浆岩的岩石化学和稀土元素地球化学研究，认为在碱性岩形成过程中，源区岩浆经历了相对偏酸性向相对偏基性再向相对偏酸性的演化过程，K2O，Al2O3含量（质量分数）与SiO2含量（质量分数）呈正相关，TFe2O3，MgO,CaO与SiO2呈负相关关系。不同时期所形成的岩浆岩具有来自上地幔的同源性。     

佛坪地区印支期花岗岩的成因

通过地质、岩石学及地球化学研究,认为秦岭造山带佛坪地区印支期花岗岩是基性岩浆和酸性岩浆混合的产物,属岩浆混合型(H型)花岗岩;佛坪地区在秦岭主造山过程中曾发生强烈的壳慢相互作用,是一个深层岩浆活动中心。     

富宁基性岩区尾硐铜镍硫化物矿床矿化特征及矿床成因探讨

通过对尾硐铜镍硫化物矿床矿区地质特征、矿体分布特征、矿床矿化特征和S同位素分析,认为该矿区内各岩相带呈渐变过渡关系,为同期侵入岩体,存在深部岩浆熔离作用。橄榄苏长辉长岩和辉长苏长岩是矿区内主要赋矿岩体。S同位素分析显示其主要为幔源原生硫;但也可能有硫通过地壳混染作用加入。含矿岩浆深部熔离和岩体侵位后的岩浆分异结晶是导致金属硫化物富集成矿的主要过程。岩浆期后热液的叠加改造作用对部分地段的成矿物质富集有重要贡献。     

Widespread uplift and 'trapdoor' faulting on Galápagos volcanoes observed with radar interferometry

Volcanic uplift, caused by the accumulation of magma in subsurface reservoirs, is a common precursor to eruptions. But, for some volcanoes, uplift of metres or more has not yet led to an eruption. Here we present displacement maps of volcanoes in the Galápagos Islands, constructed using satellite radar interferometry, that might help explain this dichotomy. We show that all but one of the seven volcanoes on the islands of Isabela and Fernandina deformed during 1992-99. Cerro Azul and Fernandina erupted during the observation period and show evidence of inflation, co-eruptive deflation and shallow dyke intrusion. In contrast, the largest volcano, Sierra Negra, has not erupted, yet exhibits spatially and temporally variable deformation, with a maximum uplift of 2.7 m between 1992 and 1999, which can be modelled by a shallow inflating sill. Inflation during 1997-98, however, was accompanied by 'trapdoor' faulting on a steeply dipping fracture system within the caldera. Repeated trapdoor faulting over geological time has formed an arcuate intra-caldera ridge within Sierra Negra and may have acted to relax stresses above the magma chamber, inhibiting summit eruptions. Similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes.     

A phenomenological model for precursor volcanic eruptions

Intense explosions of relatively short duration frequently precede large explosive and effusive volcanic eruptions-by as much as weeks to months in the case of very viscous magmas. In some cases, such pre-eruption activity has served as a sufficient warning to those living in the vicinity to evacuate and avoid calamity. Precursor events seem to be related to the formation of a magma pathway to the surface, but their precise interpretation is a long-standing puzzle. It has been inferred from theoretical studies that exsolution of volatiles might create an almost separate gas pocket at the tip of a propagating dyke. Here we explain the role that such a process may have, using a laboratory study of the transient propagation of a liquid-filled crack with a gas pocket at its tip that grows with time. We show that once the gas pocket acquires sufficient buoyancy to overcome the fracture resistance of the host solid the dynamics of the gas pocket, rather than those of the liquid, determine the velocity of the crack tip. Furthermore, we find that the gas can ultimately separate from the liquid. We propose that fast-moving, gas-rich pockets reaching the surface ahead of the main liquid-filled fissure could be the origin of many precursor eruptions.     

Influence of the March 11, 2011 M w 9.0 Tohoku-oki earthquake on regional volcanic activities

Two days after the March 11,2011,Mw 9.0 Tohoku-oki earthquake,Shinmoedake volcano,located on the Japanese island of Honshu,erupted.Was this eruption triggered by the Tohoku-oki earthquake?Could Mount Fuji and Changbaishan volcanoes also be triggered to erupt?By calculating changes in the regional stress-strain field that resulted from the earthquake,we find that Mount Fuji,Shinmoedake and Changbaishan volcanoes are all located in regions of volumetric expansion.The volumetric expan-sions at a depth of 10 km are up to～220 nano-strain,～8 nano-strain,and～14 nano-strain,respectively,for the three volcanoes. The strain changes inferred from GPS co-seismic displacements also suggest that these three volcanoes are located in regions with surface areal expansion.Considering that the expansional stress may cause the opening of magma channels,exsolution of CO2 gases stored in magma,and a series of positive feedback effects,the Tohoku-oki earthquake may result in an increase in the activ-ity of these volcanoes.Attention should be paid to potential triggering of volcanic eruptions by stress changes induced by the Tohoku-oki earthquake.