An Intelligent Method for Geochemical Discrimination of Tectonic Settings of Basalt Based on Olivine Composition: GWO-SVM Method and its VerificationEI北大核心CSCD

Geochemical discrimination of tectonic settings of basalts has been an important research direction of geochemistry for decades. Olivine is one of the earliest crystallized minerals of basaltic magma, which records a lot of hidden information of the formation and evolution of the magma. Therefore, basic elements in olivine are used to discriminate three tectonic settings, including the mid-ocean ridge basalt (MORB), ocean island basalt (OIB) and island arc basalt (IAB). However, it is still difficult to accurately discriminate the tectonic settings by using these diagrams. The machine learning algorithm is introduced to solve the aforementioned problem. The classification performance of the machine learning discrimination method largely depends on the rationality of parameter determination. To this end, the paper proposes a coupling intelligent method for geochemical discrimination of tectonic settings using olivine composition of the basalts based on the grey wolf optimizer (GWO)-optimized support vector machine (SVM), or GWO-SVM for short. GWO is used to seek the optimal parameter combination of SVM to form the optimal mapping relationship between basic elements in olivine and basalt tectonic settings, so as to realize the accurate discrimination of MORB, OIB and IAB. In addition, according to the published geochemical data of basalt samples, the discrimination performance of GWO-SVM is evaluated by means of the simulation experiment, hold-out validation and k-fold cross-validation. The evaluation results are represented by the confusion matrix and its derived evaluation indicators. The results show that GWO-SVM can discriminate the tectonic settings of the basalts based on olivine compositions with overall classification accuracy of up to 85%. Thus, in comparison with the traditional discrimination diagram method, the machine learning discrimination method based on multi-algorithm fusion can significantly improve the discrimination accuracy of basalt tectonic settings. © 2020, Science Press. All right reserved.     

橄榄石成核动力学方程的修正及其对动力学数据的约束

本文从成核动力学理论出发,考虑新相晶核的表面能对其形状的影响,修正了晶面成核情形的成核动力学方程.新的成核率方程还原了传统成核率方程中通常简化为常数的指前系数.结合修正的相变体积分数计算公式,新的成核率方程可以确定出成核过程中的两个物理参量:新相晶粒与反应相晶粒的接触角度及新相物质与反应相物质之间的表面能.分析结果表明,Ni2SiO4的接触角度和表面能分别为0.57°,32.33 J·m-2,而Mg2SiO4和(Mg,Fe)2SiO4的数据质量不足以确定表面能与形状因子. 结合对加温加载路径在高温高压试验中影响的研究,我们认为亟需补充高质量的成核率数据来确定橄榄石的成核率参数.     

Trace Elements in Olivines from the Giant Jinchuan Ni-Cu-(PGE) Deposit,NW China,and Its Geological Implication

The giant Jinchuan magmatic sulfide deposit in China is the third largest mining deposits in the world. Although many research have been done, there still exist some debates in the genesis of deposit. This study using the LA-IC-MS to obtain the trace elements concentrations of the olivine in order to discuss the mechanism of influence the element variability and illustrate the process of magmatism and ore-forming. The analytical results show that Ni, Co correlate negatively with Fo in the olivine, Mn/Fe is positively correlate with Fo, while Mn/Zn and Zn/Fe show no obvious correlation with Fo. The primitive mantle olivine-normalized trace element patterns of the Jinchuan olivine show that Jinchuan Ⅰ, Ⅱ intrusions have the same trace elements characteristics, which display negative Cr, V, Ni, Co and Ti anomalies and enrichment of Zr, Y, Ti, Sc and Ca. The multi-element patterns of the Jinchuan olivine imply same parental magma in the intrusion Ⅰ and Ⅱ. The spinel which cocrystallization with the olivine make it display negative Cr, V and Ti anomalies. The contents of Ni and Co in olivine are influenced by the process of sulfide segregation and the reaction between sulfide and olivine. The lower content of Cr and V in olivine of the intrusion Ⅱ compared with the intrusion Ⅰ imply that the parental magma of the intrusion Ⅱ was more evolved. Higher Mn/Zn (>13) ratios and lower Zn/Fe (<11) ratios indicate that the magma of Jinchuan intrusion likely originate from partial melting of peridotite mantle possibly, instead of pyroxene mantle sources.     

Bulk modulus of Fe-rich olivines corrected for non-hydrostaticity

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (K_T0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were ${({\mathrm{Fe}}_x,{\mathrm{Mg}}_{1?x})}_2\mathrm{Si}{\mathrm{O}}_4$ with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch–Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: $K_{T0}^{\mathrm{Fa45}}=131.4\pm 2.6$ GPa, $K_{T0}^{\mathrm{Fa64}}=132.1\pm 3.1$ GPa, $K_{T0}^{\mathrm{Fa82}}=136.3\pm 1.7$ GPa and $K_{T0}^{\mathrm{Fa100}}=134.8\pm 1.4$ GPa. These values combined with data available in the literature show that the K_T0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend.     

我国某些基性和超基性岩中橄榄石环带的研究

利用电子探针对我国不同地质环境的某些基性岩和超基性岩中橄榄石的成分进行了系统测定,并画分出三类晶体:Ⅰ.无环带均匀晶体;Ⅱ.对称带状晶体;Ⅲ.具均一核心和高梯度环带边缘的晶体。Ⅰ类橄榄石产于深成基性岩、阿尔卑斯型超基性岩及碱性玄武岩的橄榄岩包体中;Ⅱ和Ⅲ类橄榄石出现在碱性玄武岩内橄榄石斑晶中。文中对各类橄榄石成分特点及形成机理作了较详细讨论。     

塔里木东北缘坡十Ni矿化侵入体中橄榄石和铬尖晶石对成岩成矿及源区特征的约束

坡十Ni矿化超镁铁侵入体的矿化岩相主要为第二侵入期次的（斜长）单辉橄榄岩、（斜长）二辉橄榄岩、 纯橄岩等岩相。坡十超镁铁岩的橄榄石成分变化范围较大, 橄榄石的Fo值在76.8～89.6之间, Ni含量为767×10^-6～4 580×10^-6。铬尖晶石的Mg^#值和Cr^#值变化范围分别为19.4～41.9和49.8～64.8, 原生铬尖晶石中Cr₂O₃和Al₂O₃表现为负相关, 蚀变改造的铬尖晶石则表现为正相关。橄榄石成分剖面显示坡十母岩浆处于一个动态的岩浆系统中, 成分稳定的新鲜岩浆的补给、 持续向上的动力及浅部橄榄石快速分离结晶,造成了不同深度橄榄石成分的不同变化。坡十侵入体母岩浆估算结果为MgO=14.49％, FeO=10.01％,模拟结果显示橄榄石中Ni含量的变化主要受橄榄石结晶分异和硫化物不混溶作用共同控制,其中橄榄石与硫化物熔体发生明显的Fe-Ni交换反应。坡十母岩浆中橄榄石分离结晶造成的硫饱和,是坡十硫化物熔离的重要因素。橄榄石高Fo值、母岩浆高MgO、超镁铁岩中斜长石发育、矿物高结晶温度和铬尖晶石成分的弧岩浆特征显示,塔里木东北缘坡十侵入体是俯冲交代的岩石圈地幔部分熔融形成的母岩浆的产物,表现出低压高温的演化特征,其中源区熔融机制可能与塔里木二叠纪地幔柱提供的热源或该区大规模拆沉作用造成的软流圈上涌有关。     

The origin of hydrous,high-δ<Superscript>18</Superscript>O voluminous volcanism: diverse oxygen isotope values and high magmatic water contents within the volcanic record of Klyuchevskoy volcano,Kamchatka, Russia

Klyuchevskoy volcano, in Kamchatka’s subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest δ¹⁸O values for olivines and basalts. We present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of ¹⁴C- and tephrochronologically-dated tephra layers and lavas spanning the eruptive history of Klyuchevskoy. Whole-rock and groundmass analyses of tephra layers and lava samples demonstrate that both high-Mg (7–12.5 wt% MgO) and high-Al (17–19 wt% Al₂O₃, 3–6.5 wt% MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine δ¹⁸O range from normal MORB values of 5.1‰ to values as high as 7.6‰. Likewise, tephra and lava matrix glass have high-δ¹⁸O values of 5.8–8.1‰. High-Al basalts dominate volumetrically in Klyuchevskoy’s volcanic record and are mostly high in δ¹⁸O. High-δ¹⁸O olivines and more normal-δ¹⁸O olivines occur in both high-Mg and high-Al samples. Most olivines in either high-Al or high-Mg basalts are not in oxygen isotopic equilibrium with their host glasses, and Δ¹⁸O_{olivine–glass} values are out of equilibrium by up to 1.5‰. Olivines are also out of Fe–Mg equilibrium with the host glasses, but to a lesser extent. Water concentrations in olivine-hosted melt inclusions from five tephra samples range from 0.4 to 7.1 wt%. Melt inclusion CO₂ concentrations vary from below detection (<50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to ~17 km (5 kbar). The variable H₂O and CO₂ concentrations likely reflect crystallization of olivine and entrapment of inclusions in ascending and degassing magma. Oxygen isotope and Fe–Mg disequilibria together with melt inclusion data indicate that olivine was mixed and recycled between high-Al and high-Mg basaltic melts and cumulates, and Fe–Mg and δ¹⁸O re-equilibration processes were incomplete. Major and trace elements in the variably high-δ¹⁸O olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to δ¹⁸O, and hydrous basic volcanism in Klyuchevskoy and other Central Kamchatka depression volcanoes is explained by a model in which the ascending primitive melts that resulted from the hydrous melt fluxing of mantle wedge peridotite, interacted with the shallow high-δ¹⁸O lithospheric mantle that had been extensively hydrated during earlier times when it was part of the Kamchatka forearc. Following accretion of the Eastern Peninsula terrains several million years ago, a trench jump eastward caused the old forearc mantle to be beneath the presently active arc. Variable interaction of ascending flux-melting-derived melts with this older, high-δ¹⁸O lithospheric mantle has produced mafic parental magmas with a spectrum of δ¹⁸O values. Differentiation of the higher δ¹⁸O parental magmas has created the volumetrically dominant high-Al basalt series. Both basalt types incessantly rise and mix between themselves and with variable in δ¹⁸O cumulates within dynamic Klyuchevskoy magma plumbing system, causing biannual eruptions and heterogeneous magma products. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

吉林省敦化意气松南山橄榄石宝石矿基建探矿

在地质勘探的基础,采用加密法对矿体进行基建探矿。进一步查明矿体底板标高、产状、厚度、包裹体含量、宝石粒度等特征,进行储量升级。并对地质勘探和基建探矿进行矿体面积、厚度对比分析。     

Ferropericlase—a lower mantle phase in the upper mantle

Experiments on compositions along the join MgO–NaA³⁺Si₂O₆ (A=Al, Cr, Fe³⁺) show that sodium can be incorporated into ferropericlase at upper mantle pressures in amounts commonly found in natural diamond inclusions. These results, combined with the observed mineral parageneses of several diamond inclusion suites, establish firmly that ferropericlase exists in the upper mantle in regions with low silica activity. Such regions may be carbonated dunite or stalled and degassed carbonatitic melts. Ferropericlase as an inclusion in diamond on its own is not indicative of a lower mantle origin or of a deep mantle plume. Coexisting phases have to be taken into consideration to decide on the depth of origin. The composition of olivine will indicate an origin from the upper mantle or border of the transition zone to the lower mantle and whether it coexisted with ferropericlase in the upper mantle or as ringwoodite. The narrow and flat three phase loop at the border transition zone—lower mantle together with hybrid peridotite plus eclogite/sediments provides an explanation for the varying and Fe-rich nature of the diamond inclusion suite from Sao Luiz, Brazil.     

Petrogenesis of the Pt–Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan Large Igneous Province,SW China

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.