Exhumation of high-pressure rocks of the Kokchetav massif: facts and models

The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100-120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds.Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°-80°) thin slices (few hundred metres) within granite-gneiss. Using geological, petrological and isotopic-geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models.Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle.We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift.     

高压下玄武岩浆的不混溶及其对双峰式火山岩的成因意义

我们在温度850～1300℃,压力1.0～3.0GPa和含约2%水的条件下的碱性玄武岩熔融实验产物中发现在高于2.0GPa压力下玄武岩熔融后的淬火产物中存在着两种不同成分的玻璃,其中一种呈褐色,另一种为浅色或无色.玻璃的电子探针分析结果表明,褐色玻璃具有相对高的MgO、FeO和相对低的SiO2、K2O、Na2O含量及Na2O＞K2O的特征.熔体的不混溶间隙与温度无明显关系,但与压力关系密切,即在大于2.0GPa的压力条件下两种玻璃的不混溶间隙突然增大.因此,由不混溶作用形成的双峰式火山岩应来自大于2.0GPa的上地幔,且是能够提供酸性端员物质以及快速上升的构造环境.这与双峰火山岩可以产生于大陆裂谷环境或主要产于俯冲带的弧后拉张环境而不是岛弧前缘一致.     

Grove Mountains (GRV) 020043: Insights into acapulcoite-lodranite genesis from the most primitive member

Although acapulcoites and lodranites played a key role in understanding partial differentiation of asteroids, the lack of samples of the chondritic precursor limits our understanding of the processes that formed these meteorites. Grove Mountains (GRV) 020043 is a type 4 chondrite, with abundant, well-delineated, pyroxene-rich chondrules with an average diameter of 690 μm, microcrystalline mesostasis, polysynthetically striated low-Ca pyroxene, and slightly heterogeneous plagioclase compositions. Similarities in mineralogy, mineral composition, and oxygen isotopic composition link GRV 020043 to the acapulcoite-lodranite clan. These features include a high low-Ca pyroxene to olivine ratio, high kamacite to taenite ratio, and relatively FeO-poor mafic silicates (Fa_10.3, Fs_10.4) relative to ordinary chondrites, as well as the presence of ubiquitous metal and sulfide inclusions in low-Ca pyroxene and ƒO₂ typical of acapulcoites. GRV 020043 shows that evidence of partial melting is not an essential feature for classification within the acapulcoite-lodranite clan. GRV 020043 experienced modest thermal metamorphism similar to type 4 ordinary chondrites. GRV 020043 suggests a range of peak temperatures on the acapulcoite-lodranite parent body similar to that of ordinary chondrites, but shifted to higher temperatures, perhaps consistent with earlier accretion. The mineralogy and mineral compositions of GRV 020043, despite modest thermal metamorphism, suggests that most features of acapulcoites previously attributed to reduction were, instead, inherited from the precursor chondrite. Although partial melting was widespread on the acapulcoite-lodranite parent body, ubiquitous Fe,Ni-FeS blebs in the cores of silicates were not implanted by shock or trapped during silicate melting, but were inherited from the precursor chondrite with subsequent overgrowths during metamorphism.     

江西银山矿床岩石稀土地球化学研究

银山矿区的三个旋回火山一次火山岩属同源产物,其原始物质的35%为沉积岩熔体和65%为玄武岩质熔依.是两者的混岩浆.因而.岩石的成因类型属幔壳混合型.根据矿床中蚀变围岩千枚岩的希土组成特征、认为不同矿化蚀变带稀土分配形式基本相似,从而揭示了各带成矿流体成因上的继承性和来源的一致性.     

地幔中的水与熔融

自二十世纪九十年代起,通过对天然样品的观察、矿物中水的溶解度的实验研究、以及地球物理的观测,人们逐步认识到地球内部以矿物晶格缺陷形式蕴含着至少与现今海洋水量相当的水。地球内部的水除了显著影响矿物的热导率、电导率、流变学、扩散等性质之外,还对不同构造背景下的地幔熔融产生了重要影响。水作为强不相容元素,在地幔发生熔融时会更倾向于进入到熔体相中,从而改变熔体的结构,影响与残留固相之间的平衡。总体上讲,在等温等压条件下,地幔中水的存在可以显著降低其固相线并增大熔融程度;而在绝热减压过程中,水的存在则可以加深初始熔融的深度,增大地幔整体熔融的区间,从而降低平均熔融程度。近年来,越来越多的证据表明,水对于从岩石圈-软流圈边界的低程度熔融到地幔超大规模熔融产物（大火成岩省）的形成都具有重要作用。本文综述了不同构造背景下（大洋中脊、岛弧、地幔过渡带、大火成岩省、大陆板内、洋岛等）水对地幔熔融影响的新进展,并提出现有研究的不足和未来的挑战。

     

动态高压下斜长石的熔融和玻璃化研究

斜长石在冲击波产生的动态高温高压作用下较易熔融和淬火为非晶态物质,其矿物学名为熔长石。在随州陨石冲击变质特征的研究中,发现橄榄石、辉石等矿物除出现微裂隙外均保存完好,而多数斜长石颗粒则已熔融和玻璃化。可区分出两种形态的熔长石,一种是保持了原有矿物颗粒外形的‘继形熔长石’,另一种则以熔池状和脉状体形式产出的‘改形熔长石’。两种熔长石的光学和物理性质相似,它们的化学成分与晶质斜长石也基本相同,说明斜长石在高压下熔融时没有与周围物质发生明显的组分交换。在吉林陨石的人工冲击实验产物中,斜长石是最先熔融的硅酸盐矿物,也多以填隙的他形颗粒或脉状体和熔池的形式产出。经与其它陨石的静态高压淬火实验研究结果的比较,推定继形和改形熔长石的形成条件为≈20GPa和≤1100℃。本研究结果对了解地幔橄榄岩和俯冲的大洋板片部分熔融作用,以及地球火山岩玻璃和超基性岩中铬尖晶石的玻璃相岩浆包襄体物理化学性质有一定参考价值。     

Melting and Sublimation of Planetary Ices Under Low Pressure Conditions: Laboratory Experiments with a Melting Probe Prototype

One possibility to explore the subsurface layers of icy bodies is to use a probe with a “hot tip", which is able to penetrate ice layers by melting. Such probes have been built and used in the past for the exploration of terrestrial polar ice sheets and may also become useful tools to explore other icy layers in the Solar System. Examples for such layers are the polar areas of Mars or the icy crust of Jupiter’s moon Europa. However, while on Earth a heated probe launched into an ice sheet always causes melting with subsequent refreezing, the behaviour of such a probe in a low pressure environment is quite different. We report on the results of some experiments with a simple “melting probe" prototype with two different kinds of hot tips in a vacuum environment. For one of the tips the probe moved into two types of ice samples: (i) compact water ice and (ii) porous water ice with a snow (firn) like texture. It was also found that the penetration behaviour was basically different for the two sample types even when the same kind of tip was used. While in the porous sample the ice was only subliming, the phase changes occurring during the interaction of the tip with the compact ice are much more complex. Here alternating phases of melting and sublimation occur. The absence of the liquid phase has severe consequences on the performance of a “melting probe" under vacuum conditions: In this environment we find a high thermal resistance between the probe surface and the underlying ice. Therefore, only a low percentage of the heat that is generated in the tip is used to melt or sublime the ice, the bulk of the power is transferred towards the rear end of the probe. This is particularly a problem in the initial phases of an ice penetration experiment, when the probe has not yet penetrated the ice over its whole length. In the compact ice sample, phases could be observed, where a high enough gas pressure had built up locally underneath the probe, so that melting becomes possible. Only during these melting periods the thermal contact between the probe and the ice is good and in consequence the melting probe works effectively.     

Study of a thermal drill head for the exploration of subsurface planetary ice layers

The recently discovered water vapor plumes on Saturn's moon Enceladus, the polar caps of planet Mars and the possible ice volcanism on the Jovian satellites call for suitable techniques to explore deep ice layers of the solar system bodies. This paper presents a novel approach to deliver scientific probes into deeper layers of planetary ice. Several existing locomotion concepts and techniques for such probes are presented. After studying the mathematical framework of the melting locomotion process, melting tests with different head forms were done to evaluate the influence of the head's geometry on the melting process. This work led to a novel concept of a thermal drill head, using heat and mechanical drill in combination to penetrate the ice. We compare the performance of such a hybrid concept versus the melting penetration alone by a mathematical model and tests in ice with a prototype of the melting drill head.     

吉林伊通-大屯地区晚中生代-新生代玄武岩的地球化学特征及其意义

吉林伊通-大屯地区有长达80Ma的火山喷发历史,为探讨该区深部地质演化提供了条件。从晚白垩长春大屯火山(92．5±0．5Ma)到伊舒地堑内第三纪伊通火山群(31Ma、9～15Ma),玄武岩碱性逐渐增强,Dy/Yb比值逐渐升高,根据地幔动态熔融模型计算获得的玄武岩最终形成深度由约50km变深至110km。根据岩石圈盖效应推测该区晚白垩纪以来岩石圈厚度逐渐增厚。伊通第三纪碱性玄武岩显示了均一的地球化学组成;类似洋岛玄武岩(OIB)的微量元素分配模式和Nb/U比值,以及低(~(87)Sr/~(86)Sr)_i、正ε_(Nd)(t)同位素特征暗示它主要来源于软流圈。与第三纪碱性玄武岩相比,晚白垩大屯拉斑玄武岩具有相对偏高的Ni、Cr和Sc,高Ba/Th、Rb/Nb、Ba/Nb比值,高(~(87)Sr/~(86)Sr)_i和低(~(143)Nd/~(144)Nd)_i。这些特征可能与软流圈熔体与古老富集地幔之间的相互反应有关。伊通-大屯玄武岩的演化特征反映了岩石圈在板内岩浆作用中所担当的不同角色:第三纪时,岩石圈并没有在物质上直接参与岩浆作用,但岩石圈对上涌软流圈起到了机械阻挡作用;而在晚白垩岩浆作用中,岩石圈的间接和直接作用都得到了体现。     

Fluid-bearing alkaline carbonate melts as the medium for the formation of diamonds in the Earth''s mantle: an experimental study

We have experimentally studied the formation of diamonds in alkaline carbonate–carbon and carbonate–fluid–carbon systems at 5.7–7.0 GPa and 1150–1700 °C, using a split-sphere multi-anvil apparatus (BARS). The starting carbonate and fluid-generating materials were placed into Pt and Au ampoules. The main specific feature of the studied systems is a long period of induction, which precedes the nucleation and growth of diamonds. The period of induction considerably increases with decreasing P and T, but decreases when adding a C–O–H fluid to the system. In the range of P and T corresponding to the formation of diamonds in nature, this period lasts for tens of hours. The reactivity of the studied systems with respect to the diamond nucleation and growth decreases in this sequence: Na₂CO₃–H₂C₂O₄·2H₂O–C>K₂CO₃–H₂C₂O₄·2H₂O–C>>Na₂CO₃–C>K₂CO₃–C. The diamond morphology is independent of P and T, and is mainly governed by the composition of the crystallization medium. The stable growth form is a cubo-octahedron in the Na₂CO₃ melt, and an octahedron in the K₂CO₃ melt. Regardless of the composition of the carbonate melt, only octahedral diamond crystals formed in the presence of the C–O–H fluid. The growth rates of diamond varied in the range from 1.7 μm/h at 1420 °C to 0.1–0.01 μm/h at 1150 °C, and were used to estimate, for the first time, the possible duration of the crystallization of natural diamonds. From the analysis of the experimental results and the petrological evidence for the formation of diamonds in nature, we suggest that fluid-bearing alkaline carbonate melts are, most likely, the medium for the nucleation and growth of diamonds in the Earth's upper mantle.