辽西阜新地区义县组的喷发旋回特征

通过581.64km的路线地质调查和对20.16km的实测剖面的研究,将辽西阜新地区义县组自下而上划分为喷发旋回Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ;每个喷发旋回的下部为火山碎屑岩组成的爆发相、中上部为熔岩组成的溢流相;熔岩类型主要是玄武岩、玄武安山岩、安山岩及部分英安岩、流纹岩、珍珠岩、黑曜岩、粗安岩;各喷发旋回内部及不同喷发旋回中,具有熔岩的酸性或碱性程度向上逐渐增强的总体变化规律。义县组在紫都台—七家子地区较老河土—十家子、后新秋—苇子沟地区的火山碎屑岩的总厚度偏大,而玄武岩的发育厚度偏小,且老河土—十家子地区缺乏喷发旋回Ⅴ等,空间分布有差异性。     

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island,the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12 wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40 Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO^*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.     

A Plinian treatment of fallout from Hawaiian lava fountains

A new model is presented which simulates the dispersal and deposition of material from a Hawaiian eruption column. The model treats the Hawaiian column as a coarse-grained Plinian column and uses a modified version of the Wilson and Walker [Wilson, L., Walker, G.P.L., 1987. Explosive volcanic eruptions: VI. Ejecta dispersal in Plinian eruptions: the control of eruption conditions and atmospheric properties. Geophys. J. R. Astron. Soc. 89, 657–679.] Plinian pyroclast dispersal model to simulate the fall out of material during a Hawaiian eruption. The model results are found to be in good agreement with independent estimates of various parameters made for the 1959 Kilauea Iki eruption of Kilauea volcano. The close agreement between the model results and these independent estimates shows that, dynamically, Hawaiian eruptions are indistinguishable from Plinian eruptions. The major differences in the styles and deposits of these two types of eruptions are accounted for by differences in the mass fluxes and gas contents of the erupting magmas and, most fundamentally, by differences in the grainsize distribution of the erupted clasts. Plume heights predicted by the model are greater than those found for previous models of Hawaiian eruptions. This is because previous models did not allow for the progressive fall out of particles from the plume and, more importantly, made no correction for the velocity disequilibrium between gas and clasts when the grainsize distribution is coarse.     

Was the lethal eruption of Lake Nyos related to a double CO2/H2O density inversion?

The 1986 lethal eruption of Lake Nyos (Cameroon) was caused by a sudden inversion between deep, CO₂-loaded bottom lake waters and denser, gas-free surface waters. A deep CO₂ source has been found in fluid inclusions which occur predominantly in clinopyroxenes from lherzolitic mantle xenoliths, brought to the surface by the last erupted alkali basalts. P–T conditions of CO₂ trapping correspond to a gas density equal (or higher) than that of liquid water. It is suggested that this dense CO₂, found in many ultrabasic mantle xenoliths worldwide, has accumulated at km depth, below a column of descending lake water. It may remain in a stable state for a long period, as long as the temperature is above the density inversion temperature for pure H₂O/CO₂ systems. At an estimated depth of about 3 km, cooling by descending waters (to about 30 °C) induces a density inversion for the upper part of the CO₂ reservoir. This causes a constant, regular upstream of low-density CO₂ which, in its turn, feeds the shallower lake density inversion.     

长白山天池火山双岩浆房岩浆作用与互动式喷发

广义的长白山火山在我国境内包括天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。长白山各个火山区的火山活动具有此起彼伏的穿时性特征,天池火山之下地壳和地幔两个岩浆房具有上下呼应、互动式喷发之特点。一方面来自地幔的钾质粗面玄武岩浆直接喷出地表,在天池火山锥体内外形成诸多小火山渣锥;另一方面钾质粗面玄武岩浆持续补给地壳岩浆房,发生岩浆分离结晶作用和混合作用,形成双峰式火山岩特征并触发千年大喷发。西太平洋板块俯冲-东北亚大陆弧后引张是长白山天池火山喷发的动力学机制。