Formation of IIAB iron meteorites

Group IIAB is the third largest group of iron meteorites and the second largest group that formed by fractional crystallization; many of these irons formed from the P-rich portion of a magma consisting of two-immiscible liquids. We report neutron-activation data for 78 IIAB irons. These confirm earlier studies showing that the group has the largest known range in Ir concentrations (a factor of 4000) and that slopes are steeply negative on plots of Ir vs. Au or As (or Ni). High negative slopes imply relatively high distribution coefficients for Ir, Au, and As (but, with rare exceptions, remaining less than unity for the latter). IIAB appears to have had the highest S contents of any magmatic group of iron meteorites, consistent with its high contents of other volatile siderophiles, particularly Ga and Ge. Large fractions of trapped melt were present in the IIAB irons with the highest Au and As and lowest Ir contents. As a result, when these irons crystallized, the D_Au and D_As values can, with moderate accuracy, be estimated to have been roughly 0.53 and 0.46, respectively. These low values imply that the initial nonmetal (S + P) content of the magma was much lower than 170 mg/g, as estimated in earlier studies; our estimate is 75 mg/g. Our results are consistent with an initial P/S ratio of 0.25, similar to the ratio estimated for other magmatic groups. There is little doubt that incompatible S-rich and P-rich metallic liquids were involved during the formation of group IIAB. After 20% crystallization of our assumed starting composition the two-liquid boundary is encountered (at 72 mg/g S and 18 mg/g P). Initially the volume of S-rich liquid is very small, but continued crystallization increased the volume of this phase and decreased its P/S ratio while increasing this ratio in the P-rich liquid. Most crystallization of the IIAB magma would have occurred in the lower, P-rich portion of the core. However, metal was still a liquidus phase at the top of the core and, because both the immiscible liquids would have convected, they may have approached equilibrium throughout the very limited crystallization of the magma recorded in group IIAB. All IIAB irons contain trapped melt, and this melt will have had very different compositions depending on whether the liquid is S-rich (at the outer solid/liquid interface) or P-rich (at the inner interface). The P/S ratio in the melt trapped in the Santa Luzia iron is about 0.6 g/g, consistent with our modeling of Ir-Au and Ir-As trends implying that Santa Luzia formed in the lower, P-rich portion of the core after about 48% crystallization of the magma. Because the liquids were in equilibrium, the point at which immiscibility first occurred is not recorded by a dramatic change in the trends on element-Au diagrams; the main compositional effect is recorded in the P/S ratio of the trapped melt. The high-Au (>0.8 μg/g) irons for which large sections are available all contain skeletal schreibersite implying a relatively high (>0.3 g/g) P/S ratio; none of these irons could have crystallized from the S-rich upper layer of the core.     

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

Ferric-ferrous ratios have been measured in 22 experiments on three natural compositions equilibrated at known temperature (950°–1100° C) and oxygen fugacity, and at water-saturated conditions over a pressure range from 0.05 to 0.2 GPa. There does not appear to be any reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these compositions has also been determined at 1 bar as a function of temperature and oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per-alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This result parallels the effect of water on sulfide speciation in basaltic melts, and confirms published results on experimental hydrous basalts.     

Mineralogy of silicate inclusions in the Elga IIE iron meteorite

The petrography, mineral modal data and major and trace element compositions of 15 silicate inclusions in the Elga iron meteorite (chemical group IIE) show that these inclusions represent chemically homogeneous zoned objects with highly variable structures, reflecting the sequence of crystallization of a silicate melt during cooling of the metal host. The outer zones of inclusions at the interface with their metal host have a relatively medium-grained hypocrystalline texture formed mainly by Cr-diopside and merrillite crystals embedded in high-silica glass, whereas the central zones have a fine-grained hypocrystalline texture. Merrillite appears first on the liquidus in the outer zones of the silicate inclusions. Na and REE concentrations in merrillite from the outer zones of inclusions suggest that it may have crystallized as α-merrillite in the temperature range of 1300–1700°С. Merrillite tends to preferentially accumulate Eu without Sr. Therefore, strongly fractionated REE patterns are not associated with prolonged differentiation of the silicate melt source but depend on crystallization conditions of Н-chondrite droplets in a metallic matrix. The systematic decrease in Mg# with increasing Fe/Mn in bronzite may indicate partial reduction of iron during crystallization of the inclusion melt. The modal and bulk compositions of silicate inclusions in the Elga meteorite, as well as the chemical composition of phases are consistent with the model equilibrium crystallization of a melt, corresponding to 25% partial melting of H-chondrite, and the crystallizing liquidus phase, merrillite, and subsequent quenching at about 1090°С. Despite a high alkali content of the average weighted bulk inclusion composition, La/Hf and Rb/Th fall within the field of H chondrites, suggesting their common source. Our results reveal that silicate inclusions in the Elga (IIE) iron meteorite originated by mixing of two impact melts, ordinary chondrite and Ni-rich iron with а IIE composition, which were produced by impact event under near-surface conditions on a partially differentiated parent asteroid.     

An experimental study of the formation of metallic iron in chondrules

The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (P_H2 = 1.3 × 10⁻⁵ atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at P_H2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H₂) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.     

Solubility of palladium in picritic melts: 1. The effect of iron

In order to improve our understanding of HSE geochemistry, we evaluate the effect of Fe on the solubility of Pd in silicate melts. To date, experimentally determined Pd solubilities in silicate melt are only available for Fe-free anorthite-diopside eutectic compositions. Here we report experiments to study the solubility of Pd in a natural picritic melt as a function of pO₂ at 1300 °C in a one atm furnace. Palladium concentrations in the run products were determined by laser-ablation-ICP-MS. Palladium increases from 1.07 ± 0.26 ppm at FMQ-2, to 306 ± 19 ppm at FMQ+6.6. At a relative pO₂ of FMQ the slope in log Pd concentration vs. log pO₂ space increases considerably, and Pd concentrations are elevated over those established for AnDi melt compositions. In the same pO₂ range, ferric iron significantly increases relative to ferrous iron. Furthermore, at constant pO₂ (FMQ+0.5) Pd concentrations significantly increase with increasing X_FeO-total in the melt. Therefore, we consider ferric Fe to promote the formation of Pd²⁺ enhancing the solubility of Pd in the picrite melt significantly.The presence of FeO in the silicate melt has proven to be an important melt compositional parameter, and should be included and systematically investigated in future experimental studies, since most natural compositions have substantial FeO contents.     

Sulfur contents of the parental metallic cores of magmatic iron meteorites

Magmatic iron meteorites are thought to be samples of the central metallic cores of asteroid-sized parent bodies. Sulfur is believed to have been an important constituent of these parental cores, but due to the low solubility of S in solid metal, initial S-contents for the magmatic groups cannot be determined through direct measurements of the iron meteorites. However, experimental solid metal-liquid metal partition coefficients show a strong dependence on the S-content of the metallic liquid. Thus, by using the experimental partition coefficients to model the fractional crystallization trends within magmatic iron meteorite groups, the S-contents of the parental cores can be indirectly estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of the largest magmatic iron meteorite groups leads to best estimates for the S-contents of the parental cores of 12 ± 1.5 wt% for the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1 ± 1 wt% for the IVB group. The IVA elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial S-content. These S-content estimates are much higher than those recently inferred from crystallization models involving trapped melt. The discrepancy is due largely to the different partition coefficients that are used by the two models. When only partition coefficients that are consistent with the experimental data are used, the trapped melt model, and the low S-contents it advocates, cannot match the Ge and Ir fractionations that are observed in IIIAB iron meteorites.     

The role of S in the evolution of the parental cores of the iron meteorites

Most iron meteorites presumably formed from the cores of parent bodies having more or less chondritic bulk compositions. Consideration of the behavior of S during condensation and core formation indicates that these cores, at least in the case of groups having high or moderate volatile contents (IIAB, IIIAB), contained a substantial amount of S. When elemental fractionations observed in these iron meteorite groups are compared to model calculations of fractional crystallization it becomes evident that at least the IIAB parent melt, and very likely the IIIAB parent melt as well, did not contain the full S complement of the parent body. We consider three possible scenarios to account for the S depletion: (1) Outgassing of S during parent body differentiation; this was probably only possible if the parent body contained organic material, which is improbable for IIIAB. (2) Liquid immiscibility. Our fractional crystallization model would predict curved log Xvs. log Ni relationships in this case, which for many elements are not observed. (3) Formation of metastable liquid layers by episodic melting during core formation. This is based on the fact that the difference in melting temperature between a FeFeS eutectic and FeNi metals is ～500 K. Two melting episodes would tend to form distinct liquid layers that maintain their identities over the crystallization lifetime of the core.Solidification of the cores parental to the main iron meteorite groups should also produce a significant number of sulfide meteorites. The scarcity of sulfide-rich meteorites can be attributed to their lower mechanical resistance to space attrition, higher ablation during atmospheric passage, and faster weathering on earth.     

The mechanism of iron removal in estuaries

A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water.     

磁铁石英岩铁运输沉淀的pH值条件约束:以朝鲜半岛龙渊铁矿床为例

前寒武纪时期铁矿形成过程中铁物质如何迁移的研究,已取得了众多科研成果,但仍有些问题未得到很好的解决。尤其是对陆壳来源性铁矿床(非Algoma类型)的成因,仍然具有诸多争议,焦点主要集中在铁迁移问题。笔者以朝鲜半岛龙渊铁矿床为例研究了铁介质形成和运输的问题。首先通过铁矿石的地球化学研究和前人研究结果的考察,发现此铁矿床不属于Algoma类型,而是在强酸性介质条件下陆壳物质风化、移动和沉积而形成的。那么为什么会出现如此强大的酸性环境条件呢？为了揭示这一点,对在当时环境下把水的性质能够变成酸性的物质进行了热力学计算。研究结果表明,当硫化物如黄铁矿风化时,形成了能够使铁源物质风化和迁移的介质。这些结果也符合这样一个事实,即目前从富含硫化物地层淋沥的水的pH值小于3.5,并且铁含量远高于非硫化物类型的地层。本次研究结果表明,陆壳来源的铁矿床形成过程中,不能忽视硫化物的风化作用。     

The geochemistry of iron in puget sound

The distribution of dissolved iron, ferrous iron and acid-reducing agent soluble iron has been investigated in the main basin of Puget Sound and its tributaries. Essentially all measurable iron in Puget Sound can be removed by filtration through 0.8 μm membrane filters, thus dissolved iron must be less than about 20 nM (our limit of detection). We could also detect no ferrous iron in Puget Sound. This is due to the rapid kinetics of oxidation of Fe(II). Our measured rate constant for the kinetics of oxidation suggests that strong ferrous-organic matter complexes do not exist in Puget Sound pore waters.The distribution of acid-soluble iron is low at mid-depth and increases toward the air-water and sediment-water interface and appears to be controlled by inputs from the rivers and the sediments. Only a small fraction of the river load appears to make it through the estuaries because the magnitude of the surface concentrations is smaller than predicted based on the calculated river flux. The “dissolved” iron in the rivers appears to be actually fine colloidal particles that coagulate before the salinity exceeds 5%. The increase towards the sediments is probably due to resuspension of botton sediments perhaps augmented by iron diffusing out of the sediments to form new iron oxide particles.     

新疆蒙库铁矿床稀土元素地球化学及对铁成矿作用的指示

新疆富蕴县蒙库大型铁矿呈层状、似层状、透镜状赋存于下泥盆统康布铁堡组变质火山-沉积岩系中.矿体中发育矽卡岩,但矽卡岩并不产在侵入岩接触带上.绿帘石、石榴石和矿石的稀土配分模式具有相似性,均为轻稀土富集,正铕异常,基本上无铈异常,暗示它们之间存在成因联系.石榴石稀土配分模式呈折线型,具有明显的正铕异常,石榴石流体包裹体中熔融包裹体、熔流包裹体和气液包裹体共存,表明石榴石矽卡岩具有岩浆成因和热液成因的特征,形成于晶体 熔体 流体三相共存的岩浆-热液过渡阶段.矿床地质特征、矽卡岩矿物和矿石稀土特征表明蒙库铁矿为矽卡岩型矿床.     

大冶铁矿床夕卡岩矿物中熔融包裹体的发现及其地质地球化学意义

以显微照片方式报道了对大冶铁矿床夕卡岩矿物中熔融包裹体、熔－流包裹体和流体包裹体的观察结果和对包裹体相态特征的研究结果。结果表明,在所研究的包裹体中,矿物熔融包裹体最为常见,偶见熔－流包裹体和流体包裹体。熔融包裹体在大冶铁矿床夕卡岩矿物中不仅存在而且广泛分布,在偏光显微镜下很容易找到。熔融包裹体的存在对夕卡岩传统的接触交代成因观点具有挑战性,有助于夕卡岩和夕卡岩矿床成因理论研究水平的提高。     

The effect of phosphorus on the iron redox ratio,viscosity, and density of an evolved ferro-basalt

Despite the abundant evidence for the enrichment of phosphorus during the petrogenesis of natural ferro-basalts, the effect of phosphorus on the physical properties of these melts is poorly understood. The effects of phosphorus on the viscosity, density and redox ratio of a ferro-basaltic melt have been determined experimentally. The viscosity measurements were obtained using the concentric cylinder method on a ferro-basaltic melt above its liquidus, at 1 atm, in equilibrium with air and with CO₂. The density measurements were performed using the double Pt-bob Archimedean method at superliquidus conditions under 1 atm of air. The redox ratio was obtained by wet chemical analysis of samples collected during physical property measurements. Phosphorus pentoxide reduces ferric iron in ferro-basaltic melt. The reduction due to P₂O₅ is much larger than that for most other oxide components in basaltic melts. A coefficient for the reduction of ferric iron has been generated for inclusion in calculation schemes. The effect of P₂O₅ on the viscosity is shown to be complex. The initial reduction of ferric iron with the addition of P₂O₅ results in a relatively small change in viscosity, while further addition of P₂O₅ results in a strong increase. The addition of phosphorus to a ferro-basaltic melt also reduces the density. A partial molar volume of 64.5±0.7 cm³/mol for P₂O₅ in this melt has been obtained at 1300° C, yielding a volume of 12.9 cm³/mol per oxygen, consistent with a tetrahedral coordination for this high field strength cation. The effects of P₂O₅ on redox state, density and viscosity provide constraints on the structural role of phosphorus in these melts. The results suggest a complex interaction of phosphorus with the aluminosilicate network, and tetrahedral ferric iron. In light of the significant effects of phosphorus on the physical and chemical properties of ferro-basaltic liquids, and the extreme enrichments possible in these liquids in nature, the role of phosphorus in these melts should, in future, be considered more carefully.     

The IIG iron meteorites: Probable formation in the IIAB core

The addition of two meteorites to the iron meteorite grouplet originally known as the Bellsbank trio brings the population to five, the minimum number for group status. With Ga and Ge contents in the general “II” range, the new group has been designated IIG. The members of this group have low-Ni contents in the metal and large amounts of coarse schreibersite ((Fe,NI)₃P); their bulk P contents are 17-21 mg/g, the highest known in iron meteorites. Their S contents are exceptionally low, ranging from 0.2 to 2 mg/g. We report neutron-activation-analysis data for metal samples; the data generally show smooth trends on element-Au diagrams. The low Ir and high Au contents suggest formation during the late crystallization of a magma.Because on element-Au or element-Ni diagrams the IIG fields of the important taxonomic elements Ni, Ga, Ge and As are offset from those of the IIAB irons, past researchers have concluded that the IIG irons could not have formed from the same magma, and thus that the two groups originated on separate parent bodies. However, on most element-Au diagrams the IIG fields plot close to extensions of IIAB trends to higher Au concentrations.There is general agreement that immiscibility led to the formation of an upper S-rich and a lower P-rich magma in the IIAB core. We suggest that the IIG irons formed from the P-rich magma, and that schreibersite was a liquidus phase during the final stages of crystallization. The offsets in Ni and As (and possibly other elements) may result from solid-state elemental redistribution between metal and schreibersite during slow cooling. For example, it is well established that the equilibrium Ni content is >2× higher in late-formed relative to early-formed schreibersite. It is plausible that As substitutes nearly ideally for P in schreibersite at eutectic temperatures but becomes incompatible at low temperatures.[Wasson J. T., Huber, H. and Malvin, D. J. (2007) Formation of IIAB iron meteorites. Geochim. Cosmochim. Acta71, 760-781] argued that, in the most evolved IIAB irons, the amount of trapped melt was high. The high P contents of IIG irons also require high contents of trapped melt but the local geometry seems to have allowed the S-rich immiscible melt to escape as it formed. The escaping melt may have selectively depleted elements such as Au and Ge.     

Trace elements in natural metallic iron from Disko Island,Greenland

The largest occurrence of natural metallic iron on Earth is on the island of Disko, Greenland. Metallic iron is found there in a variety of different types, from small metal particles in basalts to large meter-sized blocks. We have studied three different types of metallic iron: small metal spherules (< 300 m) in basaltic magma; larger metal grains (300 m-3 mm), often composed of aggregates of smaller particles, in similar host rocks; and massive iron lumps (up to several tons). Analytical data for 13 siderophile elements in samples from these three types are presented. All metals analysed have a distinctly crustal pattern of siderophile elements. High Co/Ni, Re/Ir or W/Ir ratios clearly demonstrate that a meteoritic origin for the metallic iron must be excluded. Since the Co/Ni and Re/Ir ratios are approximately chondritic in the upper mantle of the Earth, a mantle origin for the Disko metals can also be ruled out. This supports earlier petrological and geological evidence that the metallic iron was formed through reduction of basaltic magma by carbon derived from Tertiary shales and coals. Significant differences in absolute and relative abundances of siderophile elements occur among the three kinds of metals. The strongly siderophile elements (e.g. Ir, Re, Ni) increase in concentration from the small metal spherules through the larger grains to the massive iron lumps. The contents of less strongly siderophile elements (P, W, Ga) decrease in the same sequence. Evidence is presented that the small metal spherules are formed by in situ reduction. Larger iron metal grains and massive iron lumps are composed of small spherules, accumulated by gravitational settling in a magma reservoir. These metal cumulates have extracted highly siderophile elements from a larger volume of basaltic melt.Part of a Ph.D. thesis by W. Klöck     

Geology of the Gushan iron oxide deposit associated with dioritic porphyries,eastern Yangtze craton,SE China

The major Gushan iron oxide deposit, typical of the Middle‐Lower Yangtze River Valley, is located in the eastern Yangtze craton. Such deposits are generally considered to be genetically related to Yanshanian subvolcanic‐volcanic rocks and are temporally‐spatially associated with ca. 129.3–137.5 Ma dioritic porphyries. The latter have a very narrow ⁸⁷Sr/⁸⁶Sr range of 0.7064 to 0.7066 and low ?_Nd(t) values of ?5.8 to ?5.7, suggesting that the porphyries were produced by mantle‐derived magmas that were crustally contaminated during magma ascent. The ore bodies occur mainly along the contact zone between dioritic porphyries and the sedimentary country rocks. The most important ore types are massive and brecciated ores which together make up 90 vol.‐% of the deposit. The massive type generally occurs as large veins consisting predominantly of magnetite (hematite) with minor apatite. The brecciated type is characterized by angular fragments of wall‐rocks that are cemented by fine‐grained magnetite. Stockwork iron ores occur as irregular veins and networks, especially with pectinate structure; they are composed of low‐temperature minerals (e.g. calcite), which indicate a hydrothermal process. The similar rare earth element patterns of apatite from the massive ores, brecciated ores and the porphyries, coupled with high‐temperature fluids (1000°C) suggest that they are magmatic in origin. Furthermore, melt flow structure commonly developed in massive ores and the absence of silicate minerals and cumulate textures suggest that the iron ores formed by the separation of an immiscible oxide melt from the silicate melt rather than by crystal fractionation. Combined with theoretical and experimental studies, we propose that the introduction of phosphorus due to crustal contamination during mantle‐derived magma ascent could have been a crucial factor that led to the formation of an immiscible oxide melt from the silicate magma.     

新疆西天山晚古生代铁矿床的地质特征、矿化类型及形成环境

西天山成矿带是我国重要的铁多金属成矿带之一,以阿吾拉勒铁成矿带为主体,近年来铁矿勘查工作取得重大进展,相继勘查或发现了查岗诺尔、备战、智博、敦德、松湖、雾岭及尼新塔格-阿克萨依等多个铁矿床,使该地区成为新疆重要的大型铁矿开发基地。这些新发现的铁矿床普遍赋存于安山质熔岩及火山碎屑岩中,规模多数达到大中型,品位较高。总体上,对铁矿床的研究程度普遍很低,成矿环境和成矿规律认识不清,缺乏综合性的总体研究。文章在已有的研究成果基础上,结合笔者研究小组近三年的大量野外调查工作和室内的整理研究,综述了新疆西天山主要铁矿床的地质特征、分布规律、矿化类型。将西天山的铁矿床划分为海相火山岩型和矽卡岩型2个大类,根据矿化类型将海相火山岩型细分为火山沉积型、火山岩浆-热液型、类矽卡岩型3个亚类。初步讨论了西天山铁矿床的成矿地质背景,认为石炭纪晚期可能属于碰撞造山晚期阶段的陆缘弧环境,局部存在挤压-伸展的构造转变,是铁矿形成的有利环境。通过区域铁矿床特征的对比以及与国内外火山岩有关的典型矿床特征的对比研究,认为铁矿床的形成与火山-侵入活动有密切的成因联系,形成时间接近或稍晚于火山活动期;早期阶段以富铁流体(熔体)充填-交代作用成矿为主,晚期热液交代富集成矿,整个成矿过程伴随大量的热液围岩蚀变;成矿物质来源可能以岛弧岩浆作用所携带的深部铁质为主,并含有少量火山-次火山气液交代围岩所萃取的铁质;富铁流体(熔体)可能是由俯冲过程中形成的基性岩浆分异形成的,但具体的形成机制、岩浆起源和演化过程是今后研究的重点。     

Biogeochemistry and bacteriology of ferrous iron oxidation in geothermal habitats

A survey of hot, acid springs in Yellowstone Park has shown that high concentrations of ferrous and ferric iron are often present. Total ionic iron concentrations in different springs ranged from less than 1 ppm to greater than 200 ppm, and up to 50% of the ionic iron was in the ferrous form. Some of these springs also have high concentrations of reduced sulfur species (S^2? and S⁰). Significant populations of the bacterium Sulfolobus, acidocaldarius, an autotrophic organism able to live and oxidize sulfur compounds at low pH and high temperature, were present in most of these springs. The role of this organism in the oxidation of ferrous iron was investigated by incubating natural samples of water and assaying for disappearance of ferrous iron. Controls in which bacterial activity was inhibited by addition of 10% NaCl were also run. Bacterial oxidation of ferrous iron occurred in most but not all of the spring waters. The temperature optimum for oxidation varied from spring to spring, but significant oxidation occurred at temperatures of 80–85°C, but not at 90°C. Thus, 85–90°C is the upper temperature at which bacterial iron oxidation occurs; a similar upper limit has previously been reported for sulfur oxidation in the same kinds of springs. The steady-state concentrations of ferrous and ferric iron are determined by the rate at which these ions move into the spring pools with the ground water (flow rate), by the rate at which ferric iron is reduced to the ferrous state by sulfide, and by the rate of bacterial oxidation. The bacterial oxidation rate is faster than the flow rate, so that the rate of reduction of ferric iron is probably the rate-controlling reaction. In several springs, no decrease in ferrous iron occurred, even though high bacterial populations were present. It was shown that in these springs, ferrous iron oxidation occurred but the ferric iron formed was reduced back to the ferrous state again. These springs were all high in suspended sediment and the reductant was shown to be present in the sediment. X-ray diffraction revealed that the sediment contained three major ingredients, elemental sulfur, natroalunite, and quartz. Chemical analyses showed a small amount of sulfide, too little to reduce the ferric iron. Elemental sulfur itself did not reduce ferric iron but when elemental sulfur was removed from the sediment by CS₂ extraction, the activity of the sediment was abolished. It is hypothesized that the sulfide present in the sediment (possibly bound to natroalunite) reacts with elemental sulfur to form a reductant for ferric iron. The results show that bacteria can have a profound influence on the ferrous/ferric ratios of geothermal systems, but that temperature and mineral composition of the water may significantly influence the overall result.     

Penetration of molten iron alloy into the lower mantle phase

The core–mantle boundary is the only interface where the metallic core and the silicate mantle interact physically and chemically. Many geophysical anomalies such as low shear velocity and high electrical conductivity have been observed at the bottom of the mantle. Perturbations in the Earth's rotation rate at decadal time periods require the existence of a thin conductive layer with a conductance of 10⁸ S. Substantial additions of molten iron from the outer core into the mantle may produce these geophysical anomalies. Although iron enrichment by penetration has only been observed in (Mg,Fe)O, the second dominant mineral in the lower mantle, the penetration process leading to iron enrichment in the silicate mantle has not been experimentally confirmed. In this study, high-pressure and high-temperature experiments were conducted to investigate the penetration of molten iron alloy into lower mantle phases; postspinel, polycrystalline bridgmanite and polycrystalline (Mg,Fe)O. At the interface between (Mg,Fe)O aggregate and molten iron alloy, liquid metal penetrated the (Mg,Fe)O aggregate along grain boundaries and formed a thin layer containing metal-rich blobs. In contrast, no penetration of molten iron alloy was observed at the interface between molten iron alloy and silicate phases. Penetration of liquid iron alloy into the (Mg,Fe)O aggregate is caused by the capillarity phenomenon or Mullins–Sekerka instability. Neither mechanism occurs at the boundary of pure polycrystalline MgO, indicating that the FeO in (Mg,Fe)O plays an essential role in this phenomenon. Infiltration of molten iron alloy along grain boundaries (capillarity phenomenon) is the dominant process and precedes penetration due to the Mullins–Sekerka instability. The capillarity phenomenon is governed by the balance of forces between surface tension and gravity. In the case where the ultralow velocity zone (ULVZ) with a low shear velocity is composed of Fe-rich (Mg,Fe)O, the maximum penetration distance of molten iron alloy by capillary rise is limited to 20 m. The addition of iron-rich melt to the base of the mantle is therefore unlikely to be the main cause of the high conductance of the CMB region predicted from decadal variation of the length of day. Furthermore, the absence of molten iron alloy penetration into silicate phases does not allow an extensive modification of the chemical composition of the mantle by core–mantle interaction.     

Effect of surface oxidation and iron contents on xanthate ions adsorption of synthetic sphalerites

Abstraction of xanthate ions from solution by copper-activated synthetic sphalerites with various iron contents was measured. It was found that raising the iron content in the samples caused a decrease in xanthate ions abstraction. For sphalerite samples characterized by the same percentage of iron deoxidized surfaces took up more xanthate ions than oxidized surfaces.Surface coverage by EtX^? ions was determined to be less than one monolayer. These values were compared with copper ion coverage at sphalerite surface. The amount of EtX^? ions abstracted from solution was noted to be independent of the pH of the solution in the range from 6 to 10 pH units, above $\text{pH}\text{=10}$ a decrease in the abstraction was observed.