Molybdenum in icelandic geothermal waters

Molybdenum concentrations in Icelandic geothermal waters lie in the range 1–70 ppb. Warm waters and dilute high-temperature waters which contain high concentrations of sulphide are lowest in molybdenum. No correlation is otherwise observed between molybdenum concentrations and temperature. Surface waters and cold ground waters do not contain detectable molybdenum (<1 ppb). It seems likely that leaching rate is the prime factor in limiting molybdenum levels in these waters. Within individual geothermal fields molybdenum concentrations are either approximately constant or they vary regularly across the field. This regular variation may often be correlated with variations in other solute concentrations and subsurface temperatures and is taken to indicate a control of molybdenum mobility by a temperature dependent equilibrium. The evidence suggests that the solubility of molybdenite is responsible. Molybdenite has not been found in active geothermal systems in Iceland but is known to occur in some New Zealand geothermal systems and it has been identified in hydrothermally altered Tertiary basalt formations at Reydarártindur in southeast Iceland. Boiling and mixing with cold water leads to molybdenite undersaturation and thus these processes favour leaching of molybdenum from the rock. On the other hand, conductive cooling leads to supersaturation which favours removal of molybdenum from solution.     

Origin of some magmas in oceanic and circum-oceanic regions

Partial melting experiments on spinel-lherzolite, a rock which probably occurs in relatively shallow parts of the oceanic upper mantle, demonstrate that alkali basaltic melt is formed at depths of at least 20 kbar whereas tholeiitic melt is formed at lower pressures (< 15 kbar) under anhydrous conditions. The specimen studied was a relatively iron-rich natural spinel-lherzolite (Fe/Mg+Fe=0.15) and the melts produced have ratios comparable to those obtained in basalts. Slight increase of degree of partial melting produces picritic melt over a wide pressure range. Under hydrous (water-excess) conditions, andesitic melt is produced by partial melting of the same natural spinel-lherzolite and a synthetic lherzolite. The melting experiments on two different abyssal tholeiites from the Mid-Atlantic Ridge suggest that the derivation of olivine tholeiite from a more mafic magma or a mantle peridotite (lherzolite) is possible, but is limited to depths shallower than 25 km under essentially anhydrous conditions, whereas plagioclase tholeiite may have been formed by fractional crystallization at depths of about 20 km in the presence of a small amount (～ 2 wt.%) of water.It is suggested that under mid-ocean ridges, partial melting of spinel-lherzolite at depths shallower than 60 km would produce olivine-tholeiitic magma, which differentiates at shallower levels (20–25 km) under either essentially anhydrous or hydrous (but vapor-absent) conditions to produce abyssal tholeiites of olivine-tholeiite type or plagioclase-tholeiite type. It may be also possible that the former olivine-tholeiite is generated by direct partial melting of plagioclase-lherzolite. Alkali basalts in the oceanic region may be generated at depths greater than 50 km by relatively small degree of partial melting. Along island arcs and continental margins, where the subduction zones probably exist, partial melting of lherzolite would take place in the presence of water that may be supplied by breakdown of hydrous minerals in the subducted oceanic crust, thereby producing andesitic magmas. High-alumina basalt magma could be produced by partial melting of the dehydrated oceanic crust in the subduction zone at depths between 40 and 60 km, where garnet is unstable above the solidus.     

SP-Mylonites: Origin of some mylonites by superplastic flow

Superplasticity in fine-grained materials is characterized by extensive grain boundary sliding. This phenomenon can take place only in special conditions. Six criteria are thus defined to determine when Superplasticity has been active. Applications of these criteria to several examples of mylonites are discussed and we conclude that superplasticity explains some types of mylonites and the tectonic banding that they exhibit.     

海南岛燕山晚期典型侵入岩成因矿物学研究及其地质意义

分析了海南岛燕山晚期典型的屯昌岩体、千家岩体和保城岩体中的钾长石、斜长石、黑云母和角闪石的化学成分和成因特点。结果表明:(1)多数钾长石晶体的Or含量<90,具岩浆结晶成因特征。大部分斜长石属奥长石-中长石,An含量与岩石的SiO2成负相关关系,显示岩浆发生了较高程度的结晶分异作用。(2)黑云母为镁质黑云母,主要为岩浆结晶成因,部分为耐熔残余。角闪石均为钙质角闪石类,既有岩浆成因角闪石,也有次生交代成因角闪石,以相对贫铝、富镁、(Na+K)A低为显著特点。(3)岩体最终定位深度可能≤3.3km,温度≥750°C,具深源、高温、浅侵位特征。     

Sulfur isotope systematics in icelandic geothermal systems and influence of seawater circulation at Reykjanes

Pyrite from altered basalts from Nàmafjall and Krafla high-temperature fields and deep zones at Reykir, Leira and other low-temperature fields, and aqueous sulfides from Nàmafjall, have δ³⁴S values of 0 to 2.6%. These values are close to those for postglacial basaltic lavas from the Reykjanes Peninsula. The major source of sulfur in these meteoric hydrothermal systems is the upper-mantle or basalt. At the low-temperature fields, however, the δ³⁴S values of sulfide decrease with decreasing depth, suggesting the presence of a light sulfur source in the shallower aquifers.In contrast, in the Reykjanes and Svartsengi geothermal fields, where seawater contributes to the hydrothermal systems, sulfide sulfur is distinctly enriched in ³⁴S at all depths except for one Reykjanes pyrite from 84 m depth. The enrichment is about 8%. at the deepest core (1734 m) of Reykjanes and decreases with decreasing depth. These enrichments are most likely due to seawater sulfate being involved in the hydrothermal systems. However, in the Reykjanes fluid, dissolved heavy sulfates are not in isotopic equilibrium with sulfide. Disequilibrium between sulfate and sulfide is also demonstrated in all other Icelandic geothermal systems studied.     

Boron geochemistry from some typical Tibetan hydrothermal systems: Origin and isotopic fractionation

The Tibetan plateau is characterized by intense hydrothermal activity and abnormal enrichment of trace elements in geothermal waters. Hydrochemistry and B isotope samples from geothermal waters in Tibet were systematically measured to describe the fractionation mechanisms and provide constraints on potential B reservoirs. B concentrations range from 0.35 to 171.90 mg/L, and isotopic values vary between −16.57 ‰ and +0.52 ‰. Geothermal fields along the Indus-Yarlung Zangbo suture zone and N–S rifts are observed with high B concentrations and temperatures. The similar hydrochemical compositions of high-B geothermal waters with magmatic fluid and consistent modeling of B isotopic compositions with present δ¹¹B values imply that the B in high-B geothermal waters is mainly contributed by magmatic sources, probably through magma degassing. In contrast, geothermal fields in other regions of the Lhasa block have relatively low B concentrations and temperatures. After considering the small fractionation factor and representative indicators of Na/Ca, Cl/HCO₃, Na + K and Si, the conformity between modeling results and the isotopic compositions of host rocks suggests that the B in low-temperature geothermal fields is mainly sourced from host rocks. According to simulated results, the B in some shallow geothermal waters not only originated from mixing of cold groundwater with deep thermal waters, but it was also contributed by equilibration with marine sedimentary rocks with an estimated proportion of 10%. It was anticipated that this study would provide useful insight into the sources and fractionation of B as well as further understanding of the relationships between B-rich salt lakes and geothermal activities in the Tibetan plateau.     

On the rate of lava- and tephra production and the upward migration of magma in four icelandic eruptions

The rate of the lava production of three Icelandic fissure eruptions (Lakagigar 1783, Hekla 1947, Askja 1961) is calculated and an attempt is made at a reasonable approximation of the rate of lava production per length and width of the feeding fissures and the rate of upward migration of the magma through the fissures. The results are summed up in Table 2. The figures for the maximum upward migration there presented should be regarded as minimum figures. As a comparison with the mainly lava producing fissure eruptions the tephra production of the acid and highly explosive Askja eruption of 1875 is discussed. It has hitherto been assumed that the enormous amount of tephra, about 2 km₃, which was produced by this eruption in 8 1/2 hours, came entirely from the Viti crater, but with regard to its small diameter it seems likely that the very fine grained tephra produced during the first hours of the eruption was partly expelled from nearby fissures now covered by Lake Öskjuvatn.

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Zusammenfassung Die Rate der Lavaförderung bei drei isländischen Spalteneruptionen (Lakagigar 1783, Hekla 1947, Askja 1961) wird errechnet, und es wird versucht, einen Annäherungswert zu gewinnen über die Rate der Lavaförderung, bezogen auf Länge und Weite der Zuführungsspalten und die Rate der Aufwärtsbewegung des Magma durch die Spalten. Die Ergebnisse sind in Tafel 2 zusammengefaßt. Die darin angegebenen Zahlen für die maximale Aufwärtsbewegung sollten als Minimum aufgefaßt werden. Als Vergleich mit den hauptsächlich Lava führenden Spalteneruptionen wird die Tephraproduktion der sauren und hochexplosiven Askja Eruption von 1875 behandelt. Bisher wurde angenommen, daß die enorme Menge Tephra — ungefähr 2 km₃, die diese Eruption in 8 1/2 Stunden hervorbrachte — ausschließlich aus dem Viti Krater kam; aber in Anbetracht seines geringen Durchmessers scheint es wahrscheinlich, daß die sehr feinkörnige Tephra, die während der ersten Stunden der Eruption entstand, teilweise von nahegelegenen Spalten ausgeworfen wurde, die jetzt vom Öskjuvatn-See bedeckt sind.

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Résumé La mesure de production de lave de trois éruptions fissurales de l'Islande (Lakagigar 1783, Hekla 1947, Askja 1961) est calculée et il est essayé de rapprocher la mesure de cette production à l'égard de longueur et largeur des fissures d'alimentation, et la mesure de la migration vers le haut du magma par les fissures. Les résultats sont présentés au tableau 2. Les chiffres de la migration maximum vers le haut doivent être entendus comme minimum. En comparaison avec l'éruption fissurale produisant principalement de la lave, la téphra production de l'acide et fort explosive éruption de l'Askja de 1875 est discutée. On a supposé jusqu'à présent que l'énorme masse de téphra, environ 2 km₃, qui fut produite lors de cette éruption en 8 1/2 heures, provenait seulement du cratére Viti; mais considérant son petit diamètre il est probable que la téphra de grain fin fut en partie lancée par des fissures voisines et qui à présent sont couvertes par le Lac Öskjuvatn.

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(Lakagígar 1783; Hekla 1947; Askja 1961). . Askja 1875 , . .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Origin of sandplains in Western Australia: A review of the debate and some recent findings

Views on the origin of sandplains in Western Australia remain controversial with debate focusing around three different models of formation. These are in situ, aeolian and in situ formation with local remobilisation by wind or colluvial transport. The only recent work on the subject to date espouses a dominantly aeolian origin. New work from a detailed study on the Victoria Plateau is described and demonstrates the applicability of utilising a range of evidence in understanding the origin of sandplains in Western Australia. Field investigations show a strong association of sandplain with sandstone and an absence of sand on non‐arenaceous geology in similar and adjacent topographic settings. Grainsize, mineral magnetic analysis and heavy‐mineral spectra show the Victoria Plateau to be a heterogeneous body of sand. These findings coupled with a lack of internal sedimentary structures are not consistent with an aeolian origin for the sandplain. Furthermore, scanning electron microscopy, grainsize and heavy minerals also demonstrate a clear link between bedrock and overlying sandplain. These data support the hypothesis that Western Australian sandplains are mostly the product of in situ weathering. Such findings question whether the origin of sandplains can be satisfactorily deduced without such a range of data.     

Origin and diagenetic evolution of Ca–Mg–Fe carbonates in some coalfields of Japan

Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I (< 500 m) strongly reflect the initial sedimentary facies, e.g. low Ca-Mg siderite in freshwater sediments which are initially rich in iron derived from lateritic soil on the nearby landmass, and Mg calcite and dolomite in brackish-marine sediments whose pore waters abound in Ca²⁺ and Mg²⁺ originating in seawater and calcareous shells. Carbonates formed during Stage II (500–2000 m) include high Ca-Mg siderite, ankerite, Fe dolomite and Fe–Mg calcite in freshwater sediments. The assemblage of Stage II carbonates in brackish-marine sediments in the coal measures is similar to that in freshwater sediments. This suggests similar diagenetic environments owing to an effective migration and mixing of pore water due to the compaction of host sediments. Carbonates formed during Stage III (> 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.     

Origin of CaCl2 brines by basalt-seawater interaction: Insights provided by some simple mass balance calculations

Modern rift zone hydrothermal brines are typically CaCl₂-bearing brines, an unusual chemical signature they share with certain oil field brines, fluid inclusions in ore minerals and a few uncommon saline lakes. Many origins have been suggested for such CaCl₂ brines but in the Reykjanes, Iceland, geothermal system a strong empirical case can be made for a basalt-seawater interaction origin. To examine this mechanism of CaCl₂ brine evolution some simple mass balance calculations were carried out. Average Reykjanes olivine tholeiite was “reacted” with average North Atlantic seawater to make an albite-chlorite-epidotesphene rock using Al₂O₃ as the conservative rock component and Cl as the conservative fluid component. The excess components released by the basalt to the fluid were “precipitated” at 275° C as quartz, calcite, anhydrite, magnetite and pyrite to complete the conversion to greenstone. The resulting fluid was a CaCl₂ brine of seawater chlorinity with a composition remarkably similar to the actual Reykjanes brine at 1750 m depth. Thus, the calculations strongly support the idea that the Reykjanes CaCl₂ brines result from “closed system” oceanic basalt-seawater interaction (albitization — chloritization mechanism) at greenschist facies temperatures. The calculation gives a seawater: basalt mass ratio of 3∶1 to 4∶1 (vol. ratio of 9∶1 to 12∶1), in keeping with experimental results, submarine vent data and with ocean crust cooling calculations. The brine becomes anoxic because there is insufficient dissolved or combined oxygen to balance all the Fe released from the basalt during alteration. Large excesses of Ca are released to the fluid and precipitate out in the form of anhydrite which essentially sweeps the brine free of sulfate leaving an elevated Ca concentration. The calculated rock-water interaction basically involves Na + Mg + SO₄ ? Ca + K, simulating chemical differences observed between oceanic basalts and greenstones from many mid-ocean ridges.     

Origin and formational history of some Pb-Zn deposits from Alborz and Central Iran: Pb isotope constraints

Several Pb-Zn deposits and occurrences within Iran are hosted by Mesozoic–Tertiary-aged sedimentary and igneous rocks. This study reports new Pb isotope analyses for galena from 14 Pb-Zn deposits in the Alborz and Central Iran structural zones. In general, Pb isotope ratios are extremely variable with data plotting between the upper crustal and orogenic curves in a plumbotectonic diagram. The latter may be attributed to Pb inputs from crustal and mantle end-members. Most of the galena samples are characterized by high ²⁰⁷Pb/²⁰⁴Pb ratios, suggesting significant input of Pb from old continental crust or pelagic sediment. Pb isotope data also indicate that some of the deposits, which are hosted by sedimentary rocks in Central Iran and Alborz, have similar Pb isotopic compositions and hence suggest similar source regions. Most of the galenas yield Pb model ‘ages’ that vary between ~140 and ~250 Ma, indicating that mineralization resulted from the extraction of ore-bearing fluids from Upper Triassic–Lower Jurassic sequences. The similarity in Pb isotope ratios for the Pb-Zn deposits located within these zones suggests analogous crustal evolution histories. Our preferred interpretation is that Pb-Zn mineralization within the sedimentary and igneous rocks of the Central Iran and Alborz tectonic regions occurred following a Late Cretaceous–Tertiary accretionary stage of crustal thickening in Iran.     

Origin of quartz cements in some sandstones from the Jurassic of the Inner Moray Firth (UK)

The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix-supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM-CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM-CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a-axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ¹⁸O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ¹⁸O composition of +20·0‰. Estimates of the δ¹⁸O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ¹⁸O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.     

大陆的起源

太阳系固体星球都有类似的核-幔-壳结构,但唯独人类居住的地球具有长英质组成的大陆壳。太古宙大陆克拉通主要由英云闪长岩(Tonalite)-奥长花岗岩(Trondhjemite)-花岗闪长岩(Granodiorite)为主的TTG深成侵入体变质而成的正片麻岩和由基性-超基性酸性火山岩及少量沉积岩变质的表壳岩(绿岩)组成。已有的资料显示这些太古宙大陆岩石组合起源于大洋壳的部分熔融。大洋壳分为大洋盆地、洋中脊、岛弧和洋底高原(大洋岛)。前两者地壳的平均厚度只有5～10km,不可能成为形成太古宙TTG深成侵入体的场所。因此,长英质大陆或起源于板块构造体制下的岛弧,或起源于地幔柱体制下的洋底高原。板块构造体制下的岛弧模式能够很好地解释太古宙克拉通TTG深成岩的成因,即俯冲大洋板片部分熔融所形成的埃达克岩相当于太古宙高压(高Al₂O₂)型TTG,而俯冲板片脱水导致地幔楔部分熔融形成的玄武质地壳再次熔融所形成的钙碱性花岗质岩石相当于太古宙低压(低Al₂O₂)型TTG。然而,板块构造体制下的岛弧模式不能令人满意地解...     

Origin of pegmatites

Pegmatites of several areas are described. It is contended that their properties and relationships support Fersman's idea that pegmatites are magmatic rather than hydrothermal or metasomatic. The principal arguments are as follows: 1) contacts of the pegmatite material are sharp where it occurs in the original bides or in xenolithic fragments; 2) later K-Na metasomatism affects pegmatites and enclosing granite the same way; 3) a pegmatite body contains the same minerals, including, accessories, whether the country rock is granite, diabase or limestone; 4) graphic texture occurs in some of the dikes; 5) there is strong differentiation in thick dikes; less, in thin ones (branches); 6) method of incorporation of xenoliths in pegmatites suggests viscous melt, not aqueous solution; 7) Xenoliths in pegmatite are'only slightly altered, if at all. -- E. Ingerson.     

关于天然放射性的成因

天然放射性衰变通常被认为是自发性的行为，并且这种衰变有一个恒定的半衰期，尽管有一些实验和经验性的证据对上述基本理论曾提出质疑。许靖华（1994）提出过这方面的问题－衰变作用是否是自发性行为？以及衰变作用是否是粒子与粒子之间相互作用的结果？为了检验β衰变是由中微子与中子相互作用的结果这一论点，对C－14衰变进行实验监测。我们的研究假设，β衰变速率的变化与中微子流的数量之间呈相关关系。在一个短实验周期里，具有较长半衰期的C－14样品的衰变速率应该是一个近似常数。然而我们的实验发现，检测到的衰变速率偏离理论值达1％。这种变化的信号强度超过本底噪声1000倍以上。曾经有一种假说提到，β衰变是由于中子与太阳发出的中微子碰撞产生的，而上述变化与太阳活动无关。经过一年半的实验检测，发现该衰变速率与在北京测量到的宇宙射线强度呈正相关关系。这一结果与作者提出的，β衰变是被宇宙射线的中微子激活的假说相符合。