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.     

Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust

At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting. However, there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust.     

Zircon U-Pb SHRIMP dating for the volcanic rocks of the Xiong’er Group: Constraints on the initial formation age of the cover of the North China Craton

The volcanic rocks of the Xiong‘er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong‘er Group formed in 1.80--1.75 Ga of Paleo-Proterozoic. Since the Xiong‘er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong‘er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ～2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ～2.20 Ga crust, or from a mantle magma with significant contamination of the ～2.20 Ga crust.     

Redox evolution of a degassing magma rising to the surface

Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma dynamics and the composition of the Earth's atmosphere. In particular, the redox state of volcanic gases emanating at the Earth's surface is widely believed to mirror that of the magma source, and is thought to have exerted a first-order control on the secular evolution of atmospheric oxygen. Oxygen fugacity (f(O2) estimated from lava or related gas chemistry, however, may vary by as much as one log unit, and the reason for such differences remains obscure. Here we use a coupled chemical-physical model of conduit flow to show that the redox state evolution of an ascending magma, and thus of its coexisting gas phase, is strongly dependent on both the composition and the amount of gas in the reservoir. Magmas with no sulphur show a systematic f(O2) increase during ascent, by as much as 2 log units. Magmas with sulphur show also a change of redox state during ascent, but the direction of change depends on the initial f(O2) in the reservoir. Our calculations closely reproduce the H2S/SO2 ratios of volcanic gases observed at convergent settings, yet the difference between f(O2) in the reservoir and that at the exit of the volcanic conduit may be as much as 1.5 log units. Thus, the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Our findings may require re-evaluation of models aimed at quantifying the role of magmatic volatiles in geological processes.     

Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas

Lava fountains are spectacular continuous gas jets, propelling lava fragments to heights of several hundred metres, which occasionally occur during eruptions of low-viscosity magmas. Whether they are generated by the effervescent disruption of fast-rising bubbly melt or by the separate ascent of a bubble foam layer accumulated at depth still remains a matter of debate. No field measurement has yet allowed firm discrimination between these two models. A key insight into the origin of lava fountains may be gained by measuring the chemical composition of the driving gas phase. This composition should differ markedly depending on whether the magma degassing occurs before or during eruption. Here we report the analysis of magmatic gas during a powerful (250-600 m high) lava fountain, measured with Fourier transform infrared spectroscopy on Mount Etna, Sicily. The abundances of volcanic gas species, determined from absorption spectra of lava radiation, reveal a fountain gas having higher CO2/S and S/Cl ratios than other etnean emissions, and which cannot derive from syn-eruptive bulk degassing of Etna basalt. Instead, its composition suggests violent emptying of a gas bubble layer previously accumulated at about 1.5 km depth below the erupting crater.     

马朗凹陷哈尔加乌组火山岩相研究

根据岩心观察、岩石薄片鉴定、测井识别得出,马朗凹陷哈尔加乌组火山岩存在火山熔岩和火山碎屑岩两大类共23种具体岩石类型,岩性包括玄武岩类、安山岩类和部分中-基性过渡岩类,以玄武岩类为主。由单井相结合地震相分析得出,本区火山岩岩相存在5相8亚相类型,以喷溢相发育最为广泛。剖面上,火山岩相存在爆发-溢流-沉积、溢流-沉积、溢流-爆发-沉积三种相模式。平面相分析表明,爆发相沿主要断裂带呈串珠状分布,并且主要集中于近火山口附近,喷溢相呈沿爆发相外侧和沿裂隙两种形式广泛分布,火山沉积相主要分布在洼槽和火山机构之间的低洼地带。     

Magma heating by decompression-driven crystallization beneath andesite volcanoes

Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic magma. Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of magma as it ascends beneath a volcano. This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in magma rising through the sub-volcanic system, as it loses gas and crystallizes en route. Thermodynamic modelling of magma that degasses, but does not crystallize, indicates that both cooling and heating are possible. Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals to develop a method for tracking pressure-temperature-crystallinity paths in magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding magma crystallinity and plagioclase-melt geothermometry to determine the temperature. These data are allied to ilmenite-magnetite geothermometry to show that the temperature of ascending magma increases by up to 100 degrees C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic magmas, which might otherwise be erroneously attributed to pre-eruptive magma mixing.     

中国东南部晚中生代伸展应力体制的岩石学标志

在中国东南部晚中生代时期（J2－K2），发育着大规模岩墙群、广泛的岩浆混合作用（包括侵入杂岩中的岩浆混合和火山岩中的复合岩流／双峰式火山岩）、大范围玄武岩浆的底侵、巨型链状火山岩带、晶洞花岗岩－A型花岗岩带、变质核杂岩，以及玄武岩的无斑隐晶－细粒结构和花岗岩的均匀块状结构。对它们的研究表明，它们形成于伸展应力体制，是伸展构造的岩石学标志，发生在晚中生代整个地质时期，并认为这一伸展构造由古太平洋库拉板块对欧亚板块的消减作用诱发。     

大平掌铜多金属矿床火山岩构造环境的地球化学证据

大平掌铜多金属矿床是滇西目前发现的最重要海相火山岩块状硫化物矿床,其主要含矿建造为大凹子组火山岩系。大凹子组火山岩系主要岩性为中酸性火山熔岩及其火山碎屑岩夹细碧岩,其次还含多层硅质岩、凝灰质粉砂岩、沉凝灰岩、碳质或钙质页岩及碳酸盐岩等。通过结合已有的地球化学数据和最新的地球化学数据,对该区域的火山岩构造环境进行比较系统的研究。矿区细碧岩、英安岩等明显地Na2O>K2O;而凝灰岩和蚀变流纹岩则相反。矿区主要火山岩均以相对富集U、Th、La、Nd、Hf、Sm和相对亏损Rb、Ba、Nb、Sr、Ti、P为特征。火山岩的稀土元素总体上具有LREE>HREE的特点,LREE略微富集。综合大平掌火山岩主量、痕量元素岩石地球化学特征,表明大凹子组火山岩系形成于消减作用环境下的火山岛弧与弧后裂陷盆地的过渡带,并具有裂陷带双峰系列火山岩组合的特点。     

Kimberlite ascent by assimilation-fuelled buoyancy

Kimberlite magmas have the deepest origin of all terrestrial magmas and are exclusively associated with cratons. During ascent, they travel through about 150 kilometres of cratonic mantle lithosphere and entrain seemingly prohibitive loads (more than 25 per cent by volume) of mantle-derived xenoliths and xenocrysts (including diamond). Kimberlite magmas also reputedly have higher ascent rates than other xenolith-bearing magmas. Exsolution of dissolved volatiles (carbon dioxide and water) is thought to be essential to provide sufficient buoyancy for the rapid ascent of these dense, crystal-rich magmas. The cause and nature of such exsolution, however, remains elusive and is rarely specified. Here we use a series of high-temperature experiments to demonstrate a mechanism for the spontaneous, efficient and continuous production of this volatile phase. This mechanism requires parental melts of kimberlite to originate as carbonatite-like melts. In transit through the mantle lithosphere, these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions, and causing a marked drop in carbon dioxide solubility. The solubility drop manifests itself immediately in a continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the increasingly kimberlitic magma. Our model provides an explanation for continuous ascent of magmas laden with high volumes of dense mantle cargo, an explanation for the chemical diversity of kimberlite, and a connection between kimberlites and cratons.     

准噶尔盆地西缘车排子凸起石炭系火山岩储层成因

通过岩心观察、薄片鉴定、元素测试、物性分析等技术手段,对准噶尔西缘车排子凸起石炭系火山岩岩性、岩相、储集空间类型、物性特征及储层成因进行详细研究。结果表明:石炭系火山岩以玄武岩、安山岩、玄武安山岩等火山熔岩和中基性火山角砾岩、凝灰岩等火山碎屑岩为主;火山岩储层储集空间主要为次生溶蚀孔隙和裂缝,属于低孔-特低渗储层;石炭系火山岩储层的形成经历了复杂的演化过程,火山岩先后经历了冷凝固结、构造抬升、表生风化淋滤、埋藏流体充填、溶蚀等后期改造作用;火山岩储集性能呈现先提高后降低再提高的演化过程。     

念青唐古拉弧背断隆东段古近纪大规模火山作用及找矿意义

探讨念青唐古拉弧背断隆东段古近纪火山岩的地质地球化学特征、成因机制、动力学背景以及找矿意义.通过对火山岩进行岩石学特征研究及全岩地球化学分析,结果表明该套火山岩相对富集不相容元素K、Rb、Th、Ce,而Sr、Ba、Ta、Nb、Ti呈现低谷,LREE富集且分馏明显,而HREE相对亏损且分馏不明显,稀土配分模式图呈以Eu为...     

古龙断陷营城组一段火山岩特征与火山喷发模式

 利用钻、测井及地震等资料和技术,对松辽盆地古龙断陷营城组一段火山岩特征及火山喷发模式进行研究。结果表明,研究区发现有火山熔岩、火山碎屑熔岩、火山碎屑岩和沉火山碎屑岩4类火山岩石类型,主要发育爆发相、喷溢相和火山沉积相3种火山岩相。平面上,爆发相火山岩主要发育在研究区北部,沿各级控陷断裂呈串珠状分布;喷溢相火山岩沿断裂向构造低部位运移,在研究区大部分地区都有分布;火山沉积相主要分布在断陷的构造低部位。根据研究结果建立了研究区火山岩沿各级控陷断裂喷发的裂隙-中心式喷发模式。     

牛圈湖-马北地区石炭系火山岩相分析

牛圈湖-马北地区石炭系火山岩主要以基性-中基性的碱性-钙碱性系列岩石组合类型为特征,存在火山熔岩和火山碎屑岩两大类共38种具体岩石类型。岩性以玄武岩、火山角砾岩为主。本区火山岩岩相存在5相10亚相类型,剖面上以溢流相占主导,其次是爆发相。存在两种相模式:爆发—溢流—沉积,溢流—沉积。本区火山活动是由弱变强再变弱的一个过程。火山口分布主要受断裂控制,沿断裂两侧呈串珠状分布,走向以东南向为主。以裂隙式喷发为主,中心式喷发为次。     

阿北油田火山岩岩相及其储集性能

火山岩是一种不多见的储集岩类型。本文根据岩心和薄片分析讨论了流状熔岩(即熔岩流)的岩相特征。并且指出在一典型的流状熔岩层中,顶部岩流自碎角砾状熔岩、上部和下部气孔状、杏仁状熔岩三个微相是有利储集层,而致密块状熔岩微相不能作为储层。     

辽河盆地西部凹陷南区火山岩地球化学特征

辽河盆地西部凹陷南区的火山岩主要分布于中生界和下第三系,岩石类型多种多样。在岩石取心和岩石薄片不多的情况下,为了对本区火山岩准确定名,了解本区火山岩的地球化学特征以及岩浆的结晶分异等岩浆作用的形成和演化,对本区１２块火山岩的主元素分布（包括铬、钴、镍等微量元素分析）和７块火山岩的稀土元素进行了分析。本区中生界岩石类型主要为安山岩和碱流岩,而房身泡组主要是碱性玄武岩和玄武岩类,这表明在岩浆的演化过程中曾发生过橄榄石和辉石的结晶分异作用。本区火山岩应主要来自上地幔,而房身泡组源区相对较浅,可能有下地壳物质混人。     

金湖凹陷闵北地区阜一、二段火山岩地层划分与对比

根据火山岩的岩性、电性以及岩石化学等特征 ,将闵北地区阜一、二段火山岩地层划分为岩流单元、岩流组、亚旋回等不同的单元。以岩流组作为该区火山岩地层划分的基本单位 ,对该区火山岩地层进行了划分与对比。该区火山岩分布的规律是 :岩流组的厚度、数目由东向西呈衰减趋势 ,而在南北方向上则相对比较稳定 ;岩流组的厚度分布受断层活动的控制 ;岩流组的可对比性 ,南北方向略好于东西方向 ;火山岩岩体呈东厚西薄楔状分布的格局。     

辽河盆地新生界火山喷发旋回和期次及其油气地质意义

基于300口钻遇火山岩层系的岩心及岩屑薄片、测井和地震等资料,依据火山地层学方法及地震火山地层学理论,通过火山地层界面识别,研究辽河盆地新生界火山喷发旋回和期次,对各个喷发旋回、期次火山地层进行平面成图,揭示火山活动时空规律;进一步结合火山地层分布与油气的关系,探究火山喷发旋回、期次的油气地质意义。结果表明:辽河盆地新生界火山岩系可划分为4旋回15期;底部旋回一和顶部旋回四为玄武岩,显示火山喷发以基性岩开始,又以基性岩结束;中部的旋回二和三均为玄武岩→粗面岩→玄武岩的岩性序列,构成本区火山地层的主体。火山岩受北东向主干断裂控制,最大厚度位于断裂带附近;早期(期次1至期次2)全区分布,末期(期次15)分布局限;中期(期次3至期次14)表现为喷发中心由北向南依次迁移特征。该结果提高了火山岩序列的纵向刻画精度,厘清了有利火山岩层系的空间分布,有助于寻找火山岩勘探新层系。以往辽河盆地火山岩勘探主要集中于沙三段中亚段粗面岩(期次5),本次研究发现与粗面岩相邻的期次4、期次6、期次7玄武岩亦展现出良好的油气勘探前景,为下一步勘探指明了方向。     

New moon from an old hand

The outcome of the Apollo lunar exploration program 1969-1974 was a model of lunar petrogenesis which ignored the probability of selective volatilisation of gases and alkalis from fire-fountaining lunar basalts. It overlooked the fact that the average compositions of the erupted basalts were those of plagioclase-saturated and low-pressure cotectic liquids on eruption, clearly indicating that they were not primary magmas. The basalts may have undergone evolution by gabbro fractionation within the lunar crust, perhaps in large lava lakes now solidified as the lunar maria. The existing model involving a global magma ocean and plagioclase flotation to form the lunar crust arises solely from the incorrect assumption that the basalts are primary magmas. The plagioclase flotation hypothesis requires the presence of a significant positive europium anomaly in the lunar highland crust, and was founded upon its alleged presence. However, that positive anomaly is neither established by the original data set nor by the much broader current data set. Critical observations, and experiments which will advance the debate are suggested, and alternative interpretations outlined.