Shift and Rotation of Composition Trends by Magma Mixing: 1983 Eruption at Miyake-jima Volcano, Japan

Pre-eruption processes are investigated for magmas erupted in1983 from Miyake-jima volcano, which is one of the most activevolcanoes in Japan. The whole-rock compositional trends of theeruptive products are principally smooth and linear. Magmaserupted from some fissures have compositions that deviate fromthe main linear trend. Phenocryst contents of samples displacedfrom the linear compositional trends are significantly lowerthan those of samples on the main trends. Anorthite-rich plagioclasephenocrysts, present throughout the 1983 products, are too calcicto have crystallized from the erupted magma composition, andwere derived from a basaltic magma through magma mixing. Althoughthe linear whole-rock composition trends favor simple two-componentmagma mixing, this cannot explain the presence of samples thatdeviate from the main trend. Instead, the observed compositiontrends were formed by mixing of a homogeneous basaltic magmawith andesitic magmas exhibiting compositional diversity. Theoriginal linear composition trends of the andesitic end-membermagma were rotated and shifted to the direction of the basalticend-member magma by magma mixing. The samples out of the maintrends represent magmas with less basaltic component than thoseon the trend. The density and viscosity of the basaltic end-membermagma were comparable with those of the andesitic end-membermagmas. The basaltic magma, discharged from one magma chamberat 2 kbar pressure, was injected into a magma chamber at lowerpressure occupied by the chemically zoned andesite magma (1kbar), and possibly as a fountain. To establish the characteristicmixing trend of the 1983 magma, the basaltic component musthave been distributed systematically in the zoned andesite magma.A requirement is that the basaltic magma spread laterally andmixed with the andesite magma at various levels of ascent ofthe fountain in the host andesite magma. Analysis of compositionalzoning in titanomagnetite crystals revealed that the eruptionof the 1983 magmas was initiated soon after the replenishmentof the basaltic magma in the 1 kbar magma chamber. KEY WORDS: compositional trend; liquid–liquid blending; magma chamber; magma mixing; Miyake-jima Volcano     

甚低纬哨声低电离层透射过程的数值模拟

利用全波解算法模拟哨声波束在甚低纬地区黎明前低电离层透射的三维能量分布,依据波场能量和偏振分布及其对波参量和电子浓度剖面的依赖特征,分析了哨声透射、反射及与大地－电离层波导耦合过程．结果表明,哨声模波存在于９０ｋｍ以上高度,吸收、反射、波束扩展及波模转换主要发生于电离层底部８０－９０ｋｍ区间;到达地面的透射能量密度衰减２０ｄＢ以上,透射衰减随频率变化不大,但随波入射角呈不对称变化;距透射区１５０ｋｍ以外区域的测向方位角有很大偏差;入射波能量的很少一部分（对５ｋＨｚ约为－２５ｄＢ）被反射并激发起哨声模波,反射波束能量集中于入射波束附近,并随频率下降而迅速增强．计算也表明,地面接收到的甚低纬哨声回波可能与使回波向极侧偏移的电离纬向梯度有关．     

Oxygen isotopic zoning of reversely zoned melilite crystals in a fluffy type A Ca‐Al‐rich inclusions from the Vigarano meteorite

Abstract– The oxygen isotopic microdistributions within melilite measured using in situ secondary ion mass spectrometry correspond to the chemical zoning profiles in single melilite crystals of a fluffy type A Ca‐Al‐rich inclusions (CAIs) of reduced CV3 Vigarano meteorite. The melilite crystals show chemical reverse zoning within an individual single crystal from the åkermanite‐rich core to the åkermanite‐poor rim. The composition changes continuously with the crystal growth. The zoning structures suggest that the melilite grew in a hot nebular gas by condensation with decreasing pressure. The oxygen isotopic composition of melilite also changes continuously from ¹⁶O‐poor to ¹⁶O‐rich with the crystal growth. These observations suggest that the melilite condensation proceeded with change consistent with an astrophysical setting around the inner edge of a protoplanetary disk where both ¹⁶O‐rich solar coronal gas and ¹⁶O‐poor dense protoplanetary disk gas could coexist. Fluffy type A CAIs could have been formed around the inner edge of the protoplanetary disk surrounding the early sun.     

Mixed-water and hydrothermal dolomitization of the Pliocene Shirahama Limestone, Izu Peninsula, central Japan

The early Pliocene Shirahama Limestone is a grainstone-packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore-filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (?2 to ? 3.5%₀ PDB) and a light oxygen group (?5.5 to ? 7.5%₀ PDB). Calcite associated with heavy oxygen dolomite has δ¹⁸O of ? 6.5 to ?8.5%₀ PDB, whereas those associated with light oxygen dolomite have a wide range from ?7.5 to ?14%₀ PDB. Calcite in dolomite-free limestone has an oxygen isotopic composition of ?2 to ?8.5%₀ PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric-marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C. Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric-marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric-hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores (Stage IV).     

苏鲁地体超高压和非超高压花岗质片麻岩的判别标志——来自锆石中矿物包裹体的证据

中国大陆科学钻探工程预先导孔CCSD－PP1中存在两类不同性质的花岗质片麻岩，它们的岩相学性质，地球化学特征，锆石中的包体矿物组合和分布规律以及阴极发光图像均存在明显的判别，其中第一类花岗质片麻岩中的锆石呈完好的自形晶，不含任何超高压矿物包体，自晶体中心至边缘均具有典型的岩浆结晶生长环带，表明该类花岗质片麻岩未经历超高压变质作用，第二类花岗质征麻岩中的锆石呈半自形晶一它形晶，晶体核部保存典型的岩浆结晶生长环带，且无超高压矿物包体，具有继承性锆石的特点，而在晶体的过渡带及其与边缘的交界部位，普遍存在以柯石英粉 包体矿物，表明第二类花岗质片麻岩曾经历了超高压变质作用，且锆石在此变质过程进一步结晶生长，形成新的生长环带，该项成果对于深入探讨苏鲁超高压变质带的俯冲－折返机制及其与岩浆作用的相互关系以及对中国大陆科学钻探工程的选址和实施有着重要的科学意义。