腾冲火山区壳内岩浆囊现今温度:来自温泉逸出气体 CO2、CH4间碳同位素分馏的估计

温度是岩浆囊的重要物理参数,获取温度参数并监测其变化对更好地理解岩浆囊的物理化学性质和行为、评价火山的活动性和喷发危险性具有重要的理论和现实意义.本文通过对温泉逸出气体CO2和CH4碳同位素样品的采集、分析测试,利用Horita通过实验矫正的Richet的平衡分馏系数的理论计算数据,通过拟合得到的CO2-CH4碳同位素平衡分馏方程T(K)=3880.3(10001nα(CO2-CH4))-0.5984,计算了腾冲火山区现存3个岩浆囊的温度.结果表明:在整个腾冲火山区,由CO2和CH4的碳同位素分馏值计算的气体源区平衡温度最低397℃,最高1163℃,平均615℃.腾冲火山区的3个岩浆囊中,南部五合-龙江-浦川岩浆囊的现今温度在464～1163℃,平均773℃,温度最高;中部腾冲-和顺-热海岩浆囊的现今温度在438 ～773℃间,平均达566℃,温度次之;北部马站-曲石-永安岩浆囊的现今温度在397～651℃间,平均达524℃,温度最低.我们认为,腾冲火山区地下岩浆囊顶部气体富集区目前的温度变化范围为400～ 1200℃,岩浆囊的实际温度应高于平均值615℃.3个岩浆囊的边缘温度可能在400 ～600℃间,中心温度可能在700～1200℃间.3个岩浆囊中心的现今温度已达到流纹岩浆(600～900℃)、安山岩(800 ～ 1100℃)和玄武岩浆(1000～1250℃)的形成温度,进一步说明腾冲火山区目前3个岩浆囊是客观存在的.另外,我们发现在用δ-△图解法判断CO2-CH4间碳同位素分馏平衡的过程中,在保持2条拟合直线的斜率符号相反的条件下,无论如何剔除数据点,都不能使δ13CCO2-ΔCO2-CH4拟合直线的截距bCO2和δ13 CCH4-ΔCO2-CH14拟合直线的截距bCH4之差bΔ小于0.1245,说明2条拟合直线的交点不能落于δ轴上,而这一数值与Horita方程的常数项接近,这说明CO2-CH4间碳同位素分馏方程中确实存在常数项,CO2和CH4间在再高的温度下都存在碳同位素的分馏.δ-△图解法判断CO2-CH4间碳同位素分馏平衡准则应修正为:在保持2条拟合直线的斜率符号相反的条件下,δ13 CCO2-ΔCO2-CH4拟合直线和δ13CCH4-ΔCO2-CH4拟合直线应相交于δ轴附近截距差0.1245处.     

能量约束下的同化混染与分离结晶作用模型(EC-AFC)在超镁铁质岩浆演化过程中的应用——以含铜镍硫化物矿床的四川力马河岩体为例

除极少数情况外,岩浆的演化过程基本为开放体系.AFC模型(同化混染+分离结晶)是模拟岩浆演化过程的经典方法.事实上,岩浆演化过程不仅和围岩有物质交换还存在能量的交换,因此由Spera和Bohrson提出的能量约束下的开放系统岩浆演化过程的同化混染与分离结晶(EC-AFC)模型更加符合地质实际,本文介绍了该模型的方法,在此基础上,以含铜镍矿床的四川力马河岩体为例,运用EC-AFC模型模拟该岩体的岩浆演化过程.结果表明,EC-AFC模型能很好的模拟该岩体的开放系统中岩浆演化过程;Sr同位素的EC-AFC模拟曲线表明岩浆很可能在中下地壳发生混染,岩浆与发生混染的围岩在成分上都具有不均一性.     

Crustal Poisson’s ratio anomalies in the eastern part of North China and their origins

Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle.In addition,with proper computation,Poisson’s ratio images from the seismic velocities can be determined.However,it is unknown whether Poisson’s ratio images have any advantages when compared with the P-wave and S-wave velocity images.For the purposes of this study,high-resolution seismic tomography under the eastern part of North China region was used to determine detailed 3-D crustal P- and S-wave seismic velocities structure,as well as Poisson’s ratio images.Results of Poisson’s ratio imaging show high Poisson’s ratio(high-PR) anomalies located in the Hengshan-North Taihang-Zhangjiakou(H-NT-Z) region,demonstrating that Poisson’s ratio imaging can provide new geophysical constraints for regional tectonic evolution.The H-NT-Z region shows a prominent and continuous high-PR anomaly in the upper crust.Based on Poisson’s ratio images at different depths, we find that this high-PR anomaly is extending down to the middle crust with thickness up to about 26 km.According to rock physical property measurements and other geological data,this crustal Poisson’s ratio anomaly can be explained by Mesozoic partial melting of the upper mantle and basaltic magma underplating related to the lithospheric thinning of the North China craton.     

Petrologic investigations of the 1995 and 1996 eruptions of Ruapehu volcano, New Zealand: formation of discrete and small magma pockets and their intermittent discharge

 Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions. The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic magma by mixing between relatively high (1000–1200  °C)- and low (∼1000  °C)- temperature (T) end members. Oscillatory zoning in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature (high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions sequentially tapped these magma pockets. Received: 1 April 1998 / Accepted: 22 December 1998     

沉积盆地岩浆侵入的热模拟

沉积盆地中岩浆的侵入会导致盆地地温史的变化，影响烃类成熟度，本文对沉积盆地中岩浆侵入过程进行了热模拟，分析了岩浆侵入对沉积盆地温度结构和成油窗的影响，在计算中，既考虑了岩浆凝固过程释放的潜热，又考虑了岩石比热，热导率随温度的变化对热模拟结果的影响，通过计算认识到：沉积盆地中侵入岩浆在垂直方向上的热扩散比水平方向热扩散更快，几公里大小的侵入体其热影响可以在时间上持续数百万年，在空间上扩散至十数公里，     

华北板块北缘中段早中二叠世的构造属性：来自花岗岩类锆石U-Pb年代学及地球化学的制约

内蒙古温都尔庙南-铁沙盖黑云角闪石英闪长岩和角闪黑云花岗闪长岩位于华北板块北缘中段晚古生代构造岩浆活动带中;锆石LA-ICP-MS U-Pb定年结果显示,其形成时代分别为271.8±2.4Ma,271.4±1.3Ma、268.7±1.3Ma。主量元素SiO₂含量61.42%~71.73%,K₂O含量3.10%~5.11%,K₂O/Na₂O值介于0.93~1.96之间,Mg^#值介于42.54~49.93之间,A/CNK为0.90~1.03;副矿物为锆石、磷灰石和磁铁矿,属亚碱性高钾钙碱性系列、I型花岗岩类;富集轻稀土(LREEs)、大离子亲石元素(LILEs,Cs、Rb、K、Ba、Th和U),亏损重稀土元素(HREEs)及高场强元素(HFSEs,Nb、Ta和Ti),弱Eu负异常(δEu=0.55~0.95),形成于活动大陆边缘弧环境,是上地壳部分熔融的产物。黑云角闪石英闪长岩和角闪黑云花岗闪长岩中发育大量暗色微粒包体(黑云闪长岩),包体可能是岩石圈地幔部分熔融的产物,相对于寄主岩石贫Si、K富Fe、Mg、Ca、Ti、大离子亲石元素(LILEs,Rb、Cs、Sr和U)和高场强元素(HFSEs,Nb、Ta、Zr、Hf)。本文花岗岩类的侵位反映了早中二叠世古亚洲洋俯冲于华北板块北缘之下及岩浆混合作用的岩浆事件。     

Chalcophile element constraints on magma differentiation of Quaternary volcanoes in Tengchong,SW China

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.     

Crustal structure in the Tengchong volcanic area and position of the magma chambers

In this paper, we extract 1500 P receiver functions in the Tengchong volcanic area from 211 teleseismic events recorded at nine digital seismic stations. A common conversion point stacking technique is used to improve the signal-to-noise ratio and to get the time delays of the Ps, PpPs, PsPs + PpSs phases within grids of 10 km × 10 km. Finally, the crustal thickness and Poisson’s ratio are calculated. The results show that the crustal thickness ranges from 28 to 40 km and the Poisson’s ratio ranges from 0.28 to 0.36. There exist two mantle-uplifting sites each with a horizontal scale of about 30 km × 30 km, one in Mazhan–Tengchong–Maanshan and the other in Wuhe–Longjiang–Tuantian. The high Poisson’s ratio is consistently located within these two sites. Recorded shocks with Ms > 2.0 reveal that most of the shocks are distributed around the two sites and few are located at the centers. The shocks, the geothermal distribution, and the crustal structure suggest that the magma is still active, and the two mantle-uplifting sites detected may be the positions of two magma chambers in the crust.     

Settling of metal droplets in a terrestrial magma ocean: On the correction of the Stokes velocity

The formation of magma oceans on at least the major terrestrial planets is widely assumed even if the full accretion history and early evolution of terrestrial planets is discussed rather controversial. Various processes occur within these magma oceans, among them the settling of small iron drops and their chemical equilibration with the silicate environment. Different models were proposed to explain the differentiation within a magma ocean. In order to model a magma ocean and give constraints about settling time of iron droplets, a constant velocity is used in most models, which is calculated using Stokes’ famous formula. According to the other model parameters this assumption is invalid, since Stokes’ assumption of a creeping flow is violated. However, former models investigated the behaviour of a large number of droplets, which makes it is impossible to solve the flow around each single iron drop. We introduce a model, in which the flow around iron drops in a molten silicate environment is computed using the appropriate hydrodynamic equations. We investigate the terminal velocity of a single drop descending through a magma ocean and additionally study the effects of the presence of other obstacles in the neighbourhood. We determine a new mean velocity, which may serve as an input parameter for the existent models of magma oceans. Although we used a full fluid-dynamical approach, the velocity computed with our model is not very different from the simple Stokes case and thus proves that the former assumptions were legitimate and can be used in future as well as the new velocity presented here.