A TEM study of the oriented orthopyroxene and forsterite inclusions in garnet from Otrøy garnet peridotite,WGR, Norway: new insights on crystallographic characteristics and growth energetics of exsolved pyroxene in relict majoritic garnet

Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otrøy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR‐I: <111>_grt//<001>_opx and {110}_grt～//～{100}_opx (～13° off) and COR‐II: <111>_grt//<011>_opx and {110}_grt～//～{100}_opx (～14° off), in four garnet grains analysed. Both variants of orthopyroxene have a blade‐like habit with one pair of broad crystal faces parallel/sub‐parallel to {110}_grt plane and the long axis of the crystal, <001>_opx for COR‐I and <011>_opx for COR‐II, along <111>_grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails/veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving–infiltrating–precipitating–healing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed/clarified. The exsolved orthopyroxene in a relict majoritic garnet follows COR‐III: {112}_grt//{100}_opx and <111>_grt//<001>_opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112}_grt//{100}_opx and the long crystal axis along <111>_grt//<001>_opx. Such a crystal habit can be rationalized by the differences in oxygen sub‐lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid‐state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid‐state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains/trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving‐infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid‐state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.     

Extremely magnesian olivine in igneous rocks

Published data on extremely magnesian olivine (> 96 mol.% forsterite) in igneous rocks were generalized and compared with data of new high-precision electron probe microanalyses of olivine from oxidized lavas of the Tolbachik Volcano (Kamchatka), chromitites from the Ray-Iz deposit (Russia), alkaline ultrabasic lavas from San Venanzo volcanoes (Italy), and skarns from the Kuh-i-Lal deposit (Tajikistan). All the found olivines resulted from low-temperature processes, such as subsurface oxidation, interaction with carbonates, and subsolidus re-equilibration. Low-temperature formation of olivine is reflected in its structure (hematite lamellae and abundance of inclusions of ore minerals) and abnormal contents of minor components (Mn, Ni, and Ca). The Mg content of olivine increases under the influence of postmagmatic processes and can be manifested in different rocks. This gives grounds to refine the genesis of olivine of exotic composition (93-96 mol.% forsterite) in some kimberlites, komatiites, and peridotites.     

Physical and numerical simulations of the presence of a forsterite transitional zone at high temperature and pressure: insight for scCO2 geological storage

Geological sequestration is one of the most effective ways to reduce greenhouse gases, such as carbon dioxide (CO₂). The deep oceanic crust dominated by ultrabasic rock could store CO₂ permanently. However, the storage mechanism has not been thoroughly understood because of the limited amount of research and experiments conducted. This study explored the reactive mechanisms of water–rock–gas in an ultrabasic system under different conditions. Forsterite, the most dominant mineral found in ultrabasic reservoirs, was used to conduct laboratory physical simulation experiments. Two experimental systems were designed including an scCO₂–forsterite–water system and an N₂–forsterite–water system. All experiments were performed for 1000 h at an experimental temperature of 150°C and a pressure of 150 bar, respectively, to mimic the geological conditions. The liquid products from the experiments were analysed by inductively coupled plasma-optical emission spectrometry, whereas the solid samples were analysed by scanning electron microscopy with energy disperse spectroscopy. Results showed that: (1) in the early stage during scCO₂/N₂–forsterite–water interaction, forsterite was dissolved with a reactive transitional zone forming on the surface, which caused H⁺ to enter into the silicate framework and accelerated the reaction; (2) in the N₂ system, the dissolution of forsterite was inhibited by the Mg²⁺ concentration after reaching its saturation in the late stage; and (3) in the scCO₂ system, magnesite was precipitated as a secondary mineral during the late stage, which promoted the dissolution of forsterite. As a result, the degree of dissolution of forsterite in the scCO₂ system was far higher than in the N₂ system. The experimental results are consistent with the numerical simulation using TOUGHREACT, a geochemical simulation procedure, which showed that CO₂ promotes the dissolution of forsterite greatly at high temperature and pressure.     

西藏罗布莎蛇绿岩豆荚状铬铁矿中镁橄榄石的晶体结构及其意义

在西藏罗布莎蛇绿岩块的豆荚状铬铁矿中，存在包裹体矿物镁橄榄石，这些镁橄榄石为无色透明多晶面形成的自形晶，Fo值可达到97－98，具富镁端员组分，选取一代表性颗粒的单晶进行X单晶和衍射分析，表明其为斜方晶系，空间群Pbnm，镁橄榄石晶胞参数,a,b,c以及M1－0和M2－0键长均小于地幔包裹体和一般地幔岩中橄缆中的对应值，测试出的晶胞参数与人工合成镁橄榄石的一致，镁橄榄石具有橄榄石族中已知最小的晶胞参数，据此推断它是在超高压环境下结晶的。     

湖北随州淅河镇陨石初步研究

作者对1986年4月15日降落在我国湖北省随州市淅河镇的陨石雨进行了光学性质、电子探针、扫描电镜、化学全分析;全岩稀土分析;X射线粉晶衍射、X射线单晶分析;红外吸收光谱、穆斯堡尔谱、同位素年龄测定,获得了大量有价值的数据。综合分析表明,主要组成矿物为贵橄榄石（Fa≈24）,古铜辉石（Fs≈27）,斜长石、白磷钙矿、磁黄铁矿、陨硫铁,镍纹石、铁纹石、铬铁矿和钛铁矿。已发现的最大球粒直径7.1mm,U—Pb等时线年龄为3.82±0.085×10~9年。     

橄榄石的蛇纹石化,滑石化,水镁石化的热力学讨论

本文运用热力学的计算和分析方法,对橄榄石、斜顽辉石的蛇纹石化、滑石化、水镁石化进行研究,以期对其成矿理论和矿山开采时的综合利用提供一定依据。     

云南元谋超基性岩群地质特征及综合开发探讨

云南元谋超基性岩群中 ,朱布及黑泥坡以辉石橄榄岩为主 ,黑泥坡岩群中还有部分纯橄榄岩 ;白泥湾白铜厂为含镍蛇纹菱镁片岩。朱布、黑泥坡开发镁橄榄石砂、镁橄榄石砖与白铜厂开发水溶性农肥并副产菱镁矿均具良好的开发前景。     

3D-FEM modeling of the microscopic stress field of forsterite aggregate under hydrostatic pressure: Significance of the crystal orientation

Three Dimensional Finite Element Method(3D-FEM)has been used to model the deviatoric stress field in a forsterite aggregate with a sandwich geometry:two forsterite cubes aligned in the same crystallographic orientation(the"breads")sandwich a third forsterite cube(the"filling"),which might have an identical or different crystallographic orientation.The results show that there is no von Mises stress in the forsterite sandwich if the sandwiching and sandwiched forsterite cubes are aligned in the same crystallographic orientation.If the crystallographic orientations are different,however,von Mises stress and heterogeneous stress distribution occur both along the boundary and in the forsterite cubes.For the investigated P-T conditions(up to 6.4GPa and 500°C),the resulted deviatoric stress is much lower than the yield strength of forsterite,so that higher P,higher T,or other means to create higher deviatoric stress is necessary,in order to constrain the yielding behavior of forsterite.     

辽宁宽甸大西岔乡庙沟硼矿的成矿规律及对该地区找矿的指导意义

辽宁宽甸大西岔乡庙沟硼矿赋存于下元古界辽河群含硼变粒岩层位。通过对矿床的研究认为在褶皱构造翼部蛇纹石化镁橄岩的厚大部位,蛇纹石化、金云母化、透闪石化强烈地段,有利于赋存有工业价值的硼矿体。此外,相伴的磁异常应在找矿工作中引起重视。     

Experimental investigation of Ostwald-ripening rates of forsterite in the haplobasaltic system

The rate of Ostwald ripening of forsterite is experimentally measured in a haplobasaltic melt (SiO₂–Al₂O₃–CaO–MgO). The goal of the experiments is to determine if the rate of Ostwald ripening is sufficient to affect textural development in crystallizing magmas of a basaltic composition. With preequilibrated charges, experiments were run isothermally so that all textural changes could be ascribed to Ostwald ripening. An increase of crystal size from 3.5 μm to 14 μm (in average) has been observed during 10 days. Glass compositions were identical in all charges, indicating that the observed increase in average crystal size is not the results of chemical disequilibrium but the results of Ostwald ripening. Due to the constraints on experimental duration, the rate-limiting process (i.e., diffusion control or reaction control) could not be determined. Our experimental results, however, demonstrate that the rate of Ostwald ripening in basaltic rocks is sufficient to be of fundamental importance in the development of textures in igneous rocks. These results strongly suggest that Ostwald ripening, as well as nucleation and crystal growth, should be considered in analyses of textures in igneous rocks.