Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts

We document compositions of minerals and melts from 3 GPa partialmelting experiments on two carbonate-bearing natural lherzolitebulk compositions (PERC: MixKLB-1 + 2·5 wt% CO₂; PERC3:MixKLB-1 + 1 wt% CO₂) and discuss the compositions of partialmelts in relation to the genesis of alkalic to highly alkalicocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C;PERC3: 1050°C) carbonatitic partial melts with <10 wt%SiO₂ and 40 wt% CO₂ evolve continuously to carbonated silicatemelts with >25 wt% SiO₂ and <25 wt% CO₂ between 1325 and1350°C in the presence of residual olivine, orthopyroxene,clinopyroxene, and garnet. The first appearance of CO₂-bearingsilicate melt at 3 GPa is 150°C cooler than the solidusof CO₂-free peridotite. The compositions of carbonated silicatepartial melts between 1350 and 1600°C vary in the rangeof 28–46 wt% SiO₂, 1·6–0·5 wt% TiO₂,12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and42–48 wt% SiO₂, 1·9–0·5 wt% TiO₂,10·5–8·4 wt% FeO*, and 15–26 wt% MgOfor PERC3. The CaO/Al₂O₃ weight ratio of silicate melts rangesfrom 2·7 to 1·1 for PERC and from 1·7 to1·0 for PERC3. The SiO₂ contents of carbonated silicatemelts in equilibrium with residual peridotite diminish significantlywith increasing dissolved CO₂ in the melt, whereas the CaO contentsincrease markedly. Equilibrium constants for Fe*–Mg exchangebetween carbonated silicate liquid and olivine span a rangesimilar to those for CO₂-free liquids at 3 GPa, but diminishslightly with increasing dissolved CO₂ in the melt. The carbonatedsilicate partial melts of PERC3 at <20% melting and partialmelts of PERC at 15–33% melting have SiO₂ and Al₂O₃ contents,and CaO/Al₂O₃ values, similar to those of melilititic to basaniticalkali OIB, but compared with the natural lavas they are moreenriched in CaO and they lack the strong enrichments in TiO₂characteristic of highly alkalic OIB. If a primitive mantlesource is assumed, the TiO₂ contents of alkalic OIB, combinedwith bulk peridotite/melt partition coefficients of TiO₂ determinedin this study and in volatile-free studies of peridotite partialmelting, can be used to estimate that melilitites, nephelinites,and basanites from oceanic islands are produced from 0–6%partial melting. The SiO₂ and CaO contents of such small-degreepartial melts of peridotite with small amounts of total CO₂can be estimated from the SiO₂–CO₂ and CaO–CO₂ correlationsobserved in our higher-degree partial melting experiments. Thesesuggest that many compositional features of highly alkalic OIBmay be produced by 1–5% partial melting of a fertile peridotitesource with 0·1–0·25 wt% CO₂. Owing to verydeep solidi of carbonated mantle lithologies, generation ofcarbonated silicate melts in OIB source regions probably happensby reaction between peridotite and/or eclogite and migratingcarbonatitic melts produced at greater depths. KEY WORDS: alkali basalts; carbonated peridotite; experimental petrology; ocean island basalts; partial melting     

Early Indosinian Weiya Gabbro in Eastern Tianshan, China： Elemental and Sr-Nd-O Isotopic Geochemistry, and Its Tectonic Implications

The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.     

河南省栾川三道庄和黄背岭矿区合矿矽卡岩的对比研究

三道庄和黄背岭矿区矽卡岩、用岩的对比研究显示出，矽卡岩和角岩各元素丰度和配分型式主要取决于原岩的岩性，其次与交代热液性质和交代程度有关。不同矿区的砂卡岩具有不同的元素组合，这些元素组合对矿床形成和矿化规模有一定影响。     

川南德昌地区茨达碱性岩锆石SHRIMP U-Pb定年

对产于攀西古裂谷带内的德昌地区茨达碱性杂岩体中的碱长石英正长岩和钠铁闪石碱性花岗岩进行了锆石SHRIMP U-Pb年龄测定,年龄结果分别为221Ma±6Ma和225Ma±2Ma,表明该碱性岩的结晶年龄或成岩年龄相当于晚三叠世,同时暗示攀西古裂谷的张裂时间为三叠纪。     

鲁西青山组火山岩形成的构造背景及其成因探讨:主元素和微量元素证据

研究区内火山岩从基性—中性—中酸性都有出露，包括橄榄玄武岩、安山岩和英安岩，且都属于非碱性系列。通过对主元素和微量元素的研究，认为本区火山岩为滞后型弧(陆缘弧)火山作用的产物。源区由于存在大量的因俯冲作用进入地幔的陆壳物质以及流体的交代作用，从而出现富含金云母和不相容元素的交代富集型地幔源，并具有壳源的元素组成特征。火山岩的形成是富集地幔部分熔融的结果，但在成岩过程中可能存在单斜辉石、斜长石、橄榄石和Ti—Fe氧化物等矿物的分离结晶作用，以及橄榄石的堆晶作用。     

碱性玄武岩中钛金云母巨晶的研究

钛金云母巨晶产于安徽当涂釜山的碱性玄武岩中,在该岩石中还含有尖晶石二辉橄榄岩包体和辉石、石榴石和歪长石巨晶。 本文提供了钛金云母的光性特征、矿物化学和波谱学等方面的资料。其化学特征是富钛和铁。其折光率显著高于各种产状的金云母。X射线衍射谱的特征是基底奇数层反射强,而偶数层反射不明显。多型为3T型。这些反映了钛金云母在高压条件形成。     

中国东北地区中新生代岩浆岩中氮及其同位素组成

通过真空热解法分析中国东北地区中新生代以来形成的碱性玄武岩和基性侵入岩中氮含量及其同位素组成的差异，结果表明喷出岩和侵入岩中流体组分在岩浆的上升侵入过程中有不同的表现形式，喷出岩与围岩有较少的混染作用发生。碱性玄武岩包裹体中氮含量较低，为1．91－14．01μL/ g，δ^15N为－22．6‰～-1.8‰。侵入岩包裹体中氮含量为25．39－731.30μL/g,δ^15N为＋0．7‰～ 20.9‰。碱性玄武岩中N2含量和氮同位素组成可能部分代表了上地幔流体的特征，脱气作用造成同位素值有较大变化；而侵入岩中的N2含量和氮同位素组成更多地反映了地壳中含氮物质浓度及同位素组成特征。     

二连盆地北缘晚中生代火山岩Ar-Ar年代、地球化学及构造背景

中国东北二连盆地周缘分布有三组时代不同的晚中生代火山岩,其中早、中期为两套地球化学性质不同的流纹岩,晚期为玄武质火山岩。本文通过测定火山岩基质Ar-Ar同位素年龄,表明早期查干诺尔组流纹岩形成于142Ma,晚期不拉根哈达组基性火山岩形成于129Ma,可见二连盆地北缘晚中生代火山岩时代均为早白垩世。通过对主、微量元素地球化学特征和Sr-Nd-Pb同位素组成研究,以及与邻区同期满克头鄂博组英安岩和流纹岩、玛尼吐组英安岩、霍林河地区查干诺尔组英安岩、流纹岩对比,认为早期查干诺尔组流纹岩来源于新成下地壳,岩浆演化过程经历了强烈分异作用;中期流纹岩源区为中上地壳或下地壳岩浆经历了上地壳强烈同化混染作用;晚期不拉根哈达组基性火山岩则源于受俯冲洋壳流体交代的富集岩石圈地幔。结合早白垩世区域岩石圈减薄背景,本文认为研究区早白垩世火山岩形成于陆内伸展构造环境。     

硅谜与碱盲：近代热液成矿学的两大暗区

Si迁移和K,Na蚀变是了解全部热液成矿作用的两个关键问题,但却长期没有引起人们足够的重视。不研究去硅、加碱的机制要想真正认识热液成矿规律是不可能的。和一般习惯性思维不同,我们认为成矿元素在热液物质运动流中是微量的、被动的从属性成分,自然界不存在什么某某元素的成矿作用。除矿源问题受控于地幔和地壳岩石分布不均匀外,同一成矿作用几乎可以形成所有元素的矿床。热波物质系统的主活动成分是碱、硅和酸质挥发分。此3元素决定成矿元素释放、活化、迁移、沉淀和富集的命运。3元中各自起的成矿作用不同,有奇妙的职能分工。在这3者中主控因素是碱金属而硅和酸质是被碱制约的。酸质(包括硅)及成矿元素是碱金属活动的派生物。     

碱性玄武岩中的某些巨晶单斜辉石——Ⅰ:晶体结构和晶体化学

应用探针分析所获化学成分(加以一定限制的),对辽宁宽甸新生代碱性玄武岩中3个巨晶单斜辉石进行晶体结构测定与修正。最终R因子分别为0.0239,0.0213和0.0196。结构中M2分裂为M 2和M 2′,分别由Ca~(2+)、Na~+等较大半径阳离子和Mg~(2+)、Fe~(2+)等较小半径阳离子占据。通过键价分析,阐明了M2′相对于M2的位移趋向及其配位情形。此外,讨论了Al的占位及其成因意义。