Hekla 1947, 1845, 1510 and 1158 tephra in Finland: challenges of tracing tephra from moderate eruptions

Several cryptotephra layers that originate from Icelandic volcanic eruptions with a volcanic explosivity index (VEI) of ≤ 4 and tephra volumes of < 1 km³ have previously been identified in Northern Europe, albeit within a restricted geographical area. One of these is the Hekla 1947 tephra that formed a visible fall-out in southern Finland. We searched for the Hekla 1947 tephra from peat archives within the previously inferred fall-out zone but found no evidence of its presence. Instead, we report the first identification of Hekla 1845 and Hekla 1510 cryptotephra layers outside of Iceland, the Faroe Islands, Ireland and the UK. Additionally, Hekla 1158 tephra was found in Finland for the first time. Our results confirm that Icelandic eruptions of moderate size can form cryptotephra deposits that are extensive enough to be used in inter-regional correlations of environmental archives and carry a great potential for refining regional tephrochronological frameworks. Our results also reveal that Icelandic tephra has been dispersed into Finnish airspace at least seven times during the past millennium and in addition to a direct eastward route the ash clouds can travel either via a northerly or a southerly transport pathway.     

Lithogeochemistry and genesis of basaltic lava in Raka ophiolite, Tibet

The Raka ophiolite is located in the middle section of the plate suture zone in the Yarlung Zangbo region, Tibet. It is suggested that the genesis of the ophiolite is similar to that of non-typic MORB in a marginal ocean basin through field geological investigation, lithogeochemical analysis and synthetical comparison. It is concluded that the ophiolite in this region may be relics of the subducted oceanic lithosphere in the Neo-Tethys period. This project was financially supported by the National Natural Science Foundation of China (No. 49772109, No. 49472100) and the Key Projects for the “Eighth-Five Year Plan” period in the Tibet Autonomous Region.     

Agronomic impact of tephra fallout from the 1995 and 1996 Ruapehu Volcano eruptions, New Zealand

 Eruptions from Ruapehu Volcano on 11 and 14 October 1995 and 17 June 1996 distributed at least 36×10⁶ m³ of sulphur(S)-rich tephra over the central and eastern North Island of New Zealand. The tephras added between 30–1500 kg ha^–1 S to at least 25 000 km² of land in primary production. Smaller but beneficial amounts of selenium (Se) and in some areas potassium and magnesium were also supplied. Addition of S to the soils in the form of sulphate and elemental S resulted in a drop in soil pH and an increase in pasture S contents within seven weeks of the eruptions. The soils affected by the tephra are naturally low in S and Se, but following the eruptions S was not required in fertilizer applications in many areas. The strongest and longest lasting effects of S and Se deposition were in high anion-retention soils particularly Hapludands (moist, moderately weathered soils, derived from volcanic ash). Soluble fluorine concentrations within the tephras were low compared to historic Icelandic and Chilean examples. However, pastoral livestock deaths were apparently caused by fluorosis in addition to starvation when tephra covered feed. The Ruapehu tephra contained very low concentrations of other soluble toxic elements. Received: 17 January 1997 · Accepted: 31 March 1997     

Cooling rates of basaltic hyaloclastites and pillow lava glasses from the HSDP2 drill core

Cooling rates have been determined for basaltic glasses from different depths of the submarine section of the drill core recovered in the 1999 phase of Hawaii Scientific Drilling Project (HSDP2). The glasses include degassed blocky hyaloclastite clasts and undegassed pillow rims. The degassed glassy clasts were generated in subaerial or shallow submarine environments, during explosive interactions between lava and seawater, before eventual deposition under water. The volatile contents of the glassy pillow rims are consistent with eruption and quenching in water several hundred metres deep. The cooling rates have been calculated from the calorimetric properties of the glass across the glass transition. The heat capacity (c_p) of each sample was measured during several cycles of heating from room temperature to temperatures above their glass transition using a differential scanning calorimeter (DSC). Their compositions did not change during the thermal treatment, a prerequisite for successful c_p measurements, although the glasses with higher H₂O contents became more opaque and their mid-IR spectra changed. Each c_p-T path exhibits the now classic features of the glass transition; glassy and liquid states separated by a hysteresis marking the transition. After experiencing the same experimental thermal history the glass transition occurs at lower temperatures in glasses with higher H₂O contents. Except for one sample, the c_p-T path measured on initial heating also releases energy stored during the natural quench, which is not recovered during subsequent experimental cooling. The energy stored in the HSDP2 glasses is much less than that observed in hyperquenched natural and synthetic glasses. Even so, the Tool-Narayanaswamy enthalpy relaxation geospeedometer, usually used to determine the cooling rates in volcanic glasses, is unable to deal with this energy release. For those samples that exhibit this feature an alternative method, developed for hyperquenched glasses, is applied. This uses the energy released to calculate T_f, from which the cooling rate is calculated. The degassed blocky hyaloclastite clasts exhibit cooling rates 0.1-72.2 K s⁻¹, while the undegassed pillow rims span 0.2-46.4 K s⁻¹. The fastest cooling rates are consistent with the cooling of lava bodies in seawater. The wide variation for both types of glass could reflect quenching at different distances from the basalt-seawater interface. However, for the degassed hyaloclastite clasts the range could indicate that the clasts were generated by different processes operating during the explosive interaction between lava and seawater in the littoral zone. In the undegassed pillow lavas, glassy rims may have been reheated, giving rise to more complex, slower, thermal histories, as a result of latent heat released during the crystallisation of pillow interiors, or flow replenishment. Both types of glass may also have experienced reheating from succeeding flows or deposits. Compared to deep-sea limu o Pele hyaloclastite fragments, whose hyperquench rates indicate simultaneous cooling and fragmentation, the shallow blocky hyaloclastite clasts may have formed during post-cooling brittle fragmentation.     

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     

Geochronology and genesis of subalkaline basaltic lava rivers at the Dzhavakheti highland,lesser Caucasus: K-Ar and Sr-Nd isotopic data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (⁸⁷Sr/⁸⁶Sr = 0.7039–0.7042; ∈_Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈_Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The ⁸⁷Sr/⁸⁶Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.     

Explosive eruptions of the Sarychev Peak volcano in the Holocene on Matua Island, the Central Kuriles: The tephra geochemistry

The first results of tephrochronological studies of the Sarychev Peak volcano on Matua Island in the Central Kuril Islands are presented and the eruptive evolution of the volcano in the Holocene is reconstructed. The volcanic products are typified and the general style of their geochemical evolution is reviewed on the basis of the distribution of the petrogenic oxides and trace elements in tephra samples taken bed-by-bed from the reference section of the soil-pyroclastic cover on Matua Island. The horizons of transit ashes are identified.     

Rubidium,strontium content and strontium isotopic composition of strongly alkalic basaltic rocks from the Cape Verde islands

The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr⁸⁷/Sr⁸⁶ ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K₂O/K₂O + Na₂O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part.     

云南会泽地区玄武粗安岩年代学、地球化学及构造演化

滇东北会泽地区发现峨眉山玄武岩中沿NNE向发育的玄武粗安岩,通过详细的野外地质调查、镜下岩相学分析、锆石U-Pb定年以及岩石地球化学测试等方法,研究了该玄武粗安岩形成年龄,地球化学特征及其指示的构造演化过程。LA-ICP-MS锆石U-Pb年代学确定玄武粗安岩形成的加权年龄为(251.6±4.2)Ma(MSWD=0.20),蚀变年龄分别为(143.5±1.9)Ma和(150.5±2.4)Ma。岩石地球化学主量、稀土和微量元素测试表明,玄武粗安岩的SiO_2含量在52.09%～54.10%,属于中基性火山岩;Mg~#介于19.88～40.86,在TAS图解上整体显示为碱性玄武岩粗安岩系列;稀土元素和微量元素特征显示出会泽玄武粗安岩来自交代的富集地幔源区,岩浆演化过程中发生了一定的单斜辉石、斜长石和磁铁矿的分离结晶作用。会泽玄武粗安岩是峨眉山玄武岩的一部分,推断其为峨眉山地幔柱活动消亡期的产物。     

不同降温速率约束下橄榄石晶体形态演变及其地质意义

基于控制降温速率的动力结晶实验,对五大连池老黑山钾质玄武质熔岩的冷却结晶过程中橄榄石晶体形态随降温速率变化动力学演变进行了研究。实验研究表明,老黑山钾质玄武岩的熔融温度发生在1 022 ℃～1 260 ℃之间,峰值为1 166.3 ℃～1 186.5 ℃;实验产物主要由橄榄石和玻璃质组成,新生的橄榄石晶体形态差异较大,有针状骸晶、铁轨状骸晶、羽毛状雏晶、细长条状和粒状微晶。橄榄石晶体的生长、尺寸和形态、分布和实验产物的结晶程度与降温速率之间存在着密切关联。随着降温速率的变慢,橄榄石从针状（0.075 ℃ /min～0.5 ℃ /min）、铁轨状（0.075 ℃ /min～0.5 ℃ /min）、放射状与羽毛状（0.046 8 ℃ /min～0.075 ℃ /min）、细长条状（0.062 5 ℃ /min）等晶体演变为粒状微晶（0.03 ℃ /min）;同时,岩石的结晶程度逐渐增强,橄榄石晶体尺寸逐渐变大、自形程度逐渐变高,玻璃相和气孔逐渐减少。这种新生橄榄石形态的变化与降温速率的内在关联性,对认识老黑山熔岩流固结的动力结晶机制、熔岩流的运动机制有一定科学意义。     

Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria,Austria

Summary The mineral chemistry of several Pliocene alkali basaltic rocks from Burgenland and Styria (Eastern Austria) have been investigated in order to determine the evolution path of the basalt magmas prior to eruption. With their wide range of substitutions, clinopyroxenes provide the best records of the evolution history of rocks. Pyroxene phenocrysts of the investigated basalts show both concentric and sector zoning. The investigation of sector zoned crystals shows, that not only Ti, Al and Fe contents are different in different sectors but there can be significant differences also in their Cr content. This fact apparently suggests that the distribution of Cr between clinopyroxene and melt could be influenced by crystallization kinetics.The depth of crystallization and differentiation of the basalts can be estimated from Ti and Al contents of clinopyroxene phenocrysts. From a combination of data on clinopyroxene composition, compatible trace element contents and mg-values of the rocks, it is concluded, that the alkali basalts of Pauliberg and Steinberg underwent slight olivine and clinopyroxene fractionation in shallow magma chambers prior to eruption, while the nephelinite of Stradnerkogel evolved mainly through clinopyroxene fractionation under high pressure conditions, probably in the upper mantle.

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Chemie der Pyroxene und Entwicklung von Alkalibasalten aus dem Burgenland und der Steiermark, Österreich

Zusammenfassung Einige pliozäne alkalibasaltische Gesteine aus dem Burgenland und der Steiermark wurden mineral-chemisch untersucht, um Aufschluß über ihre Evolution vor der Eruption zu erhalten. Klinopyroxene mit ihren vielfältigen Subtitutionsmöglichkeiten erlauben am besten eine Abschätzung der Evolution der Basalte. Pyroxen-Einsprenglinge der Basalte zeigen sowohl konzentrischen als auch sektoralen Zonarbau. Die von uns untersuchten Einsprenglinge zeigen in den verschiedenen Sektoren nicht nur unterschiedliche Gehalte an Ti, Al und Fe, sondern vielfach auch unterschiedliche Cr-Gehalte. Dies macht es wahrscheinlich, daß die Verteilung von Cr zwischen Klinopyroxen und Schmelze von der Kristallisations-Kinetik beeinflußt wird.Die Tiefe in der die Basalte kristallisierten und differenzierten kann von den Ti- und Al-Gehalten der Klinopyroxen- Einsprenglinge abgeschätzt werden. Die Zusammensetzung der Klinopyroxene im Verein mit den Gehalten an kompatiblen Spurenelementen und denmg-Werten der Gesteine erlauben den Schluß, daß die Alkalibasalte von Pauliberg und Steinberg vor ihrer Eruption eine geringfügige Olivinund Klinopyroxen-Fraktionierung in einer seichten Magmakammer erlebten. Der Nephelinit vorn Stradnerkogel hingegen erfuhr hauptsächlich eine Klinopyroxen Fraktionierung unter Hochdruck-Bedingungen, möglicherweise im oberen Erdmantel.

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With 8 Figures     

The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians,Alaska

Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H₂O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.     

Using hydrogeochemistry to understand inter-aquifer mixing in the on-shore part of the Gippsland Basin,southeast Australia

Groundwater in the Latrobe Valley in the Gippsland Basin of southeast Australia is important for domestic, agricultural and industrial uses. This sedimentary basin contains a number of aquifers that are used for water supply, dewatered for open pit coal mining, and which are potentially influenced by off-shore oil and gas production. Major ion chemistry together with stable and Sr isotope data imply that the main hydrogeochemical processes are evapotranspiration with minor silicate and carbonate weathering; methanogenesis and SO₄ reduction in reduced groundwater associated with coal deposits have also occurred. Groundwater has estimated ¹⁴C ages of up to 36 ka and is largely ³H free. Carbon-14 ages are irregularly distributed and poorly correlated with depth and distance from the basin margins. The observations that the geochemistry of groundwater in aquifers with different mineralogies are similar and the distribution of ¹⁴C ages is irregular implies that the aquifers are hydraulically connected and horizontal as well as vertical inter-aquifer mixing occurs. The connection of shallow and deeper aquifers poses a risk for the groundwater resources in Gippsland as contaminants can migrate across aquifers and dewatering of shallow units may impact deeper parts of the groundwater system.     

Hydrogeochemical evolution of the shallow and deep basaltic aquifers in Tamborine Mountain,Queensland (Australia)

Tamborine Mountain, Queensland (Australia), is a prime example of a basalt fractured-rock aquifer. Yet very little is known about the hydrochemistry of this groundwater system. Both analytical (major ions and stable isotopes) and multivariate (hierarchical cluster analysis, principal component analysis and factor analysis) analyses were used in this study to investigate the factors that interact within this aquifer system, in order to determine groundwater hydrogeochemistry. A new approach was applied to the data by classifying hydrographs by water type to clearly identify differing aquifer zones. Three distinct groundwater chemistry types were identified, and they were differentiated by variations in depth. Shallow bores were dominated by Na–Cl waters, deep bores were dominated by Na–HCO₃ and Ca–HCO₃ waters, and the two deepest bores were dominated by mixed water types. The evaluation of hydrogeochemical data has determined that both mineral weathering processes and groundwater/surface-water interaction had a strong influence on the hydrogeochemistry. Seasonal effects were minimal in the study area based on physicochemical parameters and ion chemistry. However, stable isotopic data show temporal trends. Increased rainfall events during the wet season produced a depletion in δ¹⁸O and increased d-excess values. The opposite is found during the dry season as a result of higher evaporation rates that are not hindered by intense rainfall events.

     

Halogen geochemistry of the McMurdo dry valleys lakes, Antarctica: Clues to the origin of solutes and lake evolution

We have determined the halogen and boron concentrations in the ice-covered lakes of Taylor Valley, Antarctica, to better establish the sources of salts and evolutionary histories of these unusual water bodies. In addition, we report on a series of ¹²⁹I measurements that were compared with previous ³⁶Cl data that also help constrain the source of solutes and histories of the lakes. The new data, when put into context of previous work on these systems over the past forty years, allow us to make the following conclusions. The primary source of solutes to Lake Hoare, the youngest of the lakes, is the dissolution of marine aerosols and aeolian salts and the chemical weathering of dust on the glaciers. The geochemistry of Lake Fryxell, the brackish water lake, is primarily dominated by the diffusion from a halite-saturated brine at the sediment-water interface and the recent infilling of the lake by glacier meltwater. These waters have chemical weathering and marine aerosols components. Lake Bonney has two distinct lobes whose hypersaline hypolimnia have different chemistries. Both of the lobes are remnants of ancient marine waters that have been modified by the input of weathering products. This lake has also been modified by periods of cryogenic concentration when solutes have been lost via mineral precipitation. Thus the geochemistry of Lake Bonney owes its unusual geochemistry, in part, to variations in the climate in the Taylor Valley over at least the past 300kyr. The ¹²⁹I data from the Taylor Valley are similar to those from fracture fluids in crystalline rocks from the Northern Hemisphere.     

准噶尔盆地西泉地区石炭系火山熔岩油气储层特征及成岩演化过程研究

火山熔岩油气藏是准噶尔盆地油气勘探和开发的重要目标之一。通过对准噶尔盆地西泉地区16口石炭系火山熔岩井的岩心观察、普通薄片和铸体薄片镜下鉴定和定量统计、扫描电镜和X-射线衍射等实验,研究玄武岩、安山岩、英安岩的岩石学特征、孔隙类型及储集特征及异同点,建立火山熔岩成岩演化序列,探讨成岩作用对火山熔岩孔隙演化的影响。研究表明,安山岩的储集空间最发育,孔隙类型及组合多样,多为气孔+斑晶溶孔+基质溶孔+溶蚀缝组合和气孔+斑晶溶孔+基质溶孔+溶蚀缝+构造裂缝组合。英安岩的孔隙组合为气孔+斑晶溶孔+基质溶孔和气孔+斑晶溶孔+基质溶孔+溶蚀缝。玄武岩孔隙类型及组合单一,为构造裂缝+溶蚀缝,构造裂缝+基质溶孔,气孔+溶蚀缝。挥发分逸出作用控制火山熔岩原生气孔的发育,岩浆期后热液作用和充填作用破坏了火山熔岩的储集空间,溶蚀作用、风化淋滤作用和构造破裂作用极大地改善了火山熔岩的储集性能。认为西泉地区石炭系顶部风化剥蚀淋滤带(距石炭系顶面0～20 m)的安山岩是油气聚集的最有利层段。     

Significance of assimilation and fractional crystallization (AFC) process in the generation of basaltic lava flows from Chhotaudepur area,Deccan Large Igneous Province,NW India

Basaltic and basaltic andesitic flows have been identified from the Chhotaudepur area in the Deccan Large Igneous Province. The \(\hbox {SiO}_{2}\) content of these flows varies from 46.7 to 53.7 wt%. None of the samples have primary magma signatures as they exhibit low Mg# (0.42–0.68), Ni (4.8–33.4 ppm) and Cr (33.08–143.06 ppm). Highly variable concentrations of the LILE [Rb (2–74 ppm), Ba (52–351 ppm), Cs (0.1–1 ppm) and Sr (273–745 ppm)] and relatively enriched LREE are also noticed. The coherent chondrite normalized REE diagram and primitive mantle normalized multi-element diagram suggests a consanguinity among the flows. Low (Nb/Th)\(_{\mathrm{PM}}\) (0.30–1.09), high (Th/Yb)\(_{\mathrm{PM}}\) (3.09–16.58) ratios and marked variations in Rb concentration (2.4–74.11 ppm) with variable La/Yb (5.5–23.7) ratios suggests that magmas in the Chhotaudepur region were assimilated by the crustal components. The elevated Th/Ta and La/Yb relation further indicate concurrent assimilation and fractional crystallization process was involved in the genesis of the flows. Assimilation and fractional crystallization modelling of the flows was carried out with 20% olivine, 25% clinopyroxene, 45% plagioclase and 10% titano-magnetite as fractionating minerals and upper continental crust as the assimilant. The results reveal that all the flows were modified by AFC process.     

Authigenic mineralogy, depositional environments and evolution of fault-bounded lakes of the Yunnan Plateau, south-western China

The Dianchi, Erhai and Fuxian lakes lie in faulted basins in a subtropical humid region of the Yunnan Plateau, China. Three groups of authigenic minerals have been recognized in their recent sediments - carbonate minerals, Fe-bearing minerals and silica minerals. The main authigenic minerals are goethite, calcite, aragonite, siderite and quartzine. Goethite is chemically precipitated from a colloidal suspension. Calcite is a widespread chemical precipitate that is present in deep parts of the lakes and in shallow areas associated with aquatic macrophytes. Aragonite is mainly biochemical in origin, and commonly associated with shallow benthos. Siderite forms in reducing environments, associated with pore waters with a high P_CO2 that resulted from microbial degradation of organic matter. It forms mainly in deep-water environments. Quartzine, which occurs mainly in delta front and prodeltaic sites, forms from diatom dissolution and dissolved silica introduced by streams. Six authigenic mineral associations are recognized, each of which can be related to depositional setting within the lake and the stage of lake development. The same associations can also be recognized in a 480-m-long core recovered from Dianchi Lake. Strong reducing environments and migrating pore fluids with high P_CO2 have led to the early diagenetic alteration of some of the initial authigenic minerals. Using the mineral associations from the modern lakes, the Pliocene to Recent history of Dianchi Lake has been interpreted, and is in general agreement with palaeoenvironmental reconstructions based upon palaeontological and other evidence.