Magma underplating and Hannuoba present crust-mantle transitional zone composition: Xenolith petrological and geochemical evidence

Studies on the deep-seated xenoliths from global volcanoes reveal that the present petrological crust-mantle boundary between the lower crust and the upper mantle actually is a transitional layer from mainly mafic granulites to ultramafic spinel lher-zolites[1,2], i.e. a transitional zone distinctive from the seismological Moho[3]. Oceanic lithosphere crust- mantle transitional zone can be established from the study on the exposed ophiolites. However, as for the continental lithosphere, since …     

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO₂), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO₂). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt     

The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel)

Plio-Pleistocene volcanism in the Golan and Galilee (northeasternIsrael) shows systematic variability with time and location:alkali basalts were erupted in the south during the Early Pliocene,whereas enriched basanitic lavas erupted in the north duringthe Late Pliocene (Galilee) and Pleistocene (Golan). The basaltsshow positive correlations in plots of ratios of highly to moderatelyincompatible elements versus the concentration of the highlyincompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and indiagrams of REE/HFSE (rare earth elements/high field strengthelements) vs REE concentration (e.g. La/Nb vs La). Some of thesecorrelations are not linear but upward convex. ⁸⁷Sr/⁸⁶Sr ratiosvary between 0·7031 and 0·7034 and correlate negativelywith incompatible element concentrations and positively withRb/Sr ratios. We interpret these observations as an indicationthat the main control on magma composition is binary mixingof melts derived from two end-member mantle source components.Based on the high Sr/Ba ratios and negative Rb anomalies inprimitive mantle normalized trace element diagrams and the moderateslopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalizeddiagrams, we suggest that the source for the alkali basalticend-member was a garnet-bearing amphibole peridotite that hadexperienced partial dehydration. The very high incompatibleelement concentrations, low K content, very low Rb contentsand steep MREE–HREE patterns in the basanites are attributedto derivation from amphibole- and garnet-bearing pyroxeniteveins. It is suggested that the veins were produced via partialmelting of amphibole peridotites, followed by complete solidificationand dehydration that effectively removed Rb and K. The requirementfor the presence of amphibole limits both sources to lithosphericdepths. The spatial geochemical variability of the basalts indicatesthat the lithosphere beneath the region is heterogeneous, composedof vein-rich and vein-poor domains. The relatively uniform ¹⁴³Nd/¹⁴⁴Nd(Nd = 4·0–5·2) suggests that the two mantlesources were formed by dehydration and partial melting of anoriginally isotopically uniform reservoir, probably as a resultof a Paleozoic thermal event. KEY WORDS: basanites; lithospheric heterogeneity; magma mixing; amphibole peridotite; pyroxenites     

山东大张矽卡岩型铁矿床中铁的富集机制 ——来自流体包裹体和氢、氧同位素的证据

大张铁矿是鲁西地区近年来新发现的一个重要的矽卡岩型矿床.矿体主要赋存于石英二长闪长岩与奥陶系马家沟组灰岩接触带及其附近.根据脉体穿插关系和交代蚀变特征,将大张矽卡岩型铁矿床成矿过程划分为矽卡岩阶段、氧化物阶段、硫化物阶段和碳酸盐阶段.通过对透辉石、绿帘石、石英和方解石等透明矿物显微观察发现,大张铁矿中流体包裹体类型主要...     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

The Moon 35 years after Apollo: What's left to learn?

With the cancellation of the Apollo program after Apollo 17 returned from the Moon in 1972, the focus of NASA switched to other areas of the Solar System. Study of the Moon did continue through analysis of the returned samples and remotely sensed data sets (both orbital and surface), as well as through Earth-based telescopic studies. In the 1990s, new orbital data were obtained from several missions (fly-by and orbital), the first being Galileo that allowed the lunar farside to be mapped, followed by global mapping by the Clementine and Lunar Prospector missions.Interest in the Moon started to increase at the beginning of the 21st century as other nations focused their space exploration programs on the Moon. The speech by President Bush in January 2004 put the Moon back into the critical exploration path for NASA, paving the way for humans to return to the lunar surface by 2020. This return will be critical for developing technologies and protocols for the eventual human exploration of other parts of the solar system. At the time of writing (June 2008), the SELENE/Kaguya mission (Japan and Chang’e-1 (China) are orbiting the Moon, with Chandrayaan-1 (India) and Lunar Reconnaissance Orbiter (USA) being scheduled to launch later in 2008.The past (and present) exploration of the Moon begs the question “what's left to be done?” With the renewed focus on the Moon, now that it is on the pathway for the exploration of Mars (and beyond) a similar question has been raised - what should the astronauts do on the Moon? The publication of the New Views of the Moon book [Jolliff et al., 2006. New Views of the Moon, Reviews in Mineralogy, vol. 60. American Mineralogical Society, 721pp] highlighted a number of important scientific questions that remain unanswered as well as posing many more on the basis of the currently available data. These questions resonated in three Lunar Exploration Analysis Group (LEAG) reports pertinent to this discussion, which were also published (on line) during 2006 (http://www.lpi.usra.edu/leag), and in the National Research Council of the National Academies [2007. The Scientific Context for Exploration of the Moon. National Academies Press, Washington, DC, 112pp] report entitled “The Scientific Context for Exploration of the Moon”. This paper synthesizes these recent studies, along with those from the 1980s and 1990s, to emphasize the lunar science questions that remain unanswered. In addition, it summarizes the missions already flown to the Moon along with those that are planned in order to give the reader an idea of exactly what lunar science has been and will be conducted in the hope that it will inspire proposals for missions to address the outstanding science questions.     

湖南上堡矿区钨锡矿成矿条件及找矿前景分析

上堡矿区出露地层有石炭系和二叠系,壶天群及栖霞组的灰岩、白云质灰岩化学性质活泼.W、Sn含量较高;区内的燕山早期黑云母花岗岩呈岩株或岩枝状产出,岩石具钠长石化和云英岩化,微量元素丰度值高;花岗岩体顶面的洼部构造很发育,规模大,是有利的成矿空间;围岩蚀变强烈,以矽卡岩化为主;W、Sn等元素的土壤异常围绕岩体分布,并且强度大,范围广;经钻探揭露,在接触带上有具工业意义的矽卡岩型钨锡矿体.通过分析,认为上堡矿区成矿地质条件优越,具有大型规模钨锡矿找矿潜力.     

内蒙古解放营子花岗闪长岩岩浆混合机制——来自斜长石和角闪石的矿物化学证据

为揭示华北克拉通北缘中晚三叠世解放营子花岗闪长岩的岩浆混合机制,对寄主岩石和镁铁质包体中斜长石和角闪石开展了电子探针分析.分析结果显示,多斑和少斑包体边部的斜长石斑晶发育An值增加的突变环带,环带的An值为32～46,明显高于核部和边部斜长石的An值(18～31),而核部的An值与寄主岩石中斜长石的An值一致,该特征指...     

安徽宣城荞麦山矿床成矿岩体及其暗色包体成因与成矿意义:锆石U-Pb年代学、地球化学、Sr-Nd-Hf-O同位素制约

宣城矿集区是长江中下游成矿带内新提出的矿集区,近年来取得了重大的找矿突破,尤其是大型斑岩型铜金矿的发现使宣城矿集区逐渐成为研究热点。宣城矿集区一系列的斑岩-矽卡岩矿床均与早白垩世侵入体密切相关。荞麦山铜钨矿床是区内典型的矽卡岩矿床,矿床形成与花岗闪长斑岩密切相关,且花岗闪长斑岩发育较多的暗色包体。确定暗色包体的成因有助于深入理解含矿岩体的岩浆演化过程及成矿意义。本次工作以荞麦山矿床花岗闪长斑岩和暗色包体作为研究对象进行U-Pb同位素年代学、岩石及矿物地球化学、全岩Sr-Nd、锆石Hf-O同位素地球化学分析,探讨暗色包体及成矿岩体的源区、岩浆演化过程及其对成矿的意义。暗色包体的形成时代为141Ma（MSWD=0.8）,（⁸⁷Sr/⁸⁶Sr）_i值为0.7059~0.7069,ε_Nd（t）值为-6.0~-9.2,ε_Hf（t）值为-12.7~-5.8,δ¹⁸O值为5.8‰~7.7‰;寄主岩石形成时代为140Ma（MSWD=0.3）,（⁸⁷Sr/⁸⁶Sr）_i值为0.7061~0.7064,ε_Nd（t）值为-8.7~-7.7,ε_Hf（t）值为-12.1~-8.1,δ¹⁸O值为5.6‰~7.4‰。此外,暗色包体与寄主岩石主要氧化物含量与SiO₂含量呈线性负相关趋势、微量元素特征相似和寄主岩石斜长石斑晶的不平衡结构及捕掳晶的发现指示荞麦山暗色包体为岩浆混合成因,花岗闪长斑岩为起源于富集地幔的岩浆与壳源岩浆混合成的母岩浆侵位而来,同时幔源岩浆和壳源岩浆的混合也分别提供了成矿金属元素铜和钨,最终形成了铜钨共生的荞麦山矿床。     

纳如松多矿集区电性结构特征及对成矿作用的约束

为了研究西藏纳如松多矿集区的电性结构特征和对成矿作用的约束,对覆盖矿集区的大地电磁测深数据进行全面的数据处理分析,得到了可靠的二维电性结构模型.研究结果表明,分别在深度为40~50 km,20~30 km和10 km处见高导体,推测这些高导体可能为部分熔融和水流体共同所致.由于纳如松多矿集区内矿床为岩浆-热液型,深部岩浆的上涌在成矿作用中起到关键作用,所以壳内高导体可能为与成矿有关岩浆房的电性痕迹,将这些高导体连起来可能代表着深部热液向上运移的古通道.电性结构主要体现了壳内高导体与区域成矿动力作用的关系,向上运移的富矿岩浆也可能通过局部的隐伏构造运移到Pb-Zn和Fe-Cu矿床的位置,再演化形成矿体.