Geochemistry and mineralogy of the phonolite lava lake,Erebus volcano,Antarctica: 1972–2004 and comparison with older lavas     

Peter J. Kelly  Philip R. Kyle  Nelia W. Dunbar  Kenneth W.W. Sims 《Journal of Volcanology and Geothermal Research》2008

Mount Erebus, Antarctica, is a large (3794 m) alkaline open-conduit stratovolcano that hosts a vigorously convecting and persistently degassing lake of anorthoclase phonolite magma. The composition of the lake was investigated by analyzing glass and mineral compositions in lava bombs erupted between 1972 and 2004. Matrix glass, titanomagnetite, olivine, clinopyroxene, and fluor-apatite compositions are invariant and show that the magmatic temperature (∼ 1000°C) and oxygen fugacity (ΔlogFMQ = − 0.9) have been stable. Large temperature variations at the lake surface (~ 400–500°C) are not reflected in mineral compositions. Anorthoclase phenocrysts up to 10 cm in length feature a restricted compositional range (An_10.3–22.9Ab_62.8–68.1Or_11.4–27.2) with complex textural and compositional zoning. Anorthoclase textures and compositions indicate crystallization occurs at low degrees of effective undercooling. We propose shallow water exsolution causes crystallization and shallow convection cycles the anorthoclase crystals through many episodes of growth resulting in their exceptional size. Minor variations in eruptive activity from 1972 to 2004 are decoupled from magma compositions. The variations probably relate to changes in conduit geometry within the volcano and/or variable input of CO₂-rich volatiles into the upper-level magma chamber from deeper in the system.  相似文献   

Thermal–mechanical modeling of cooling history and fracture development in inflationary basalt lava flows     

Simon A. Kattenhorn  Conrad J. Schaefer 《Journal of Volcanology and Geothermal Research》2008,170(3-4):181-197

Thermal–mechanical analyses of isotherms in low-volume basalt flows having a range of aspect ratios agree with inferred isotherm patterns deduced from cooling fracture patterns in field examples on the eastern Snake River Plain, Idaho, and highlight the caveats of analytical models of sheet flow cooling when considering low-volume flows. Our field observations show that low-volume lava flows have low aspect ratios (width divided by thickness), typically < 5. Four fracture types typically develop: column-bounding, column-normal, entablature (all of which are cooling fractures), and inflation fractures. Cooling fractures provide a proxy for isotherms during cooling and produce patterns that are strongly influenced by flow aspect ratio. Inflation fractures are induced by lava pressure-driven inflationary events and introduce a thermal perturbation to the flow interior that is clearly evidenced by fracture patterns around them. Inflation fracture growth occurs incrementally due to blunting of the lower tip within viscoelastic basalt, allowing the inflation fracture to pivot open. The final stage of growth involves propagation beyond the blunted tip towards the stress concentration at the tapered tip of a lava core, resulting in penetration of the core that causes quenching of the lava and the formation of a densely fractured entablature. We present numerical models that include the effects of inflation fractures on lava cooling and which support field-based inferences that inflation fractures depress the isotherms in the vicinity of the fracture, cause a subdivision of the lava core, control the location of the final portion of the lava flow to solidify, and cause significant changes in the local cooling fracture orientations. In addition to perturbing isotherms, inflation fractures cause a lava flow to completely solidify in a shorter amount of time than an identically shaped flow that does not contain an inflation fracture.  相似文献   

Volcanic facies analysis of a subaqueous basalt lava-flow complex at Hruškovec,NW Croatia — Evidence of advanced rifting in the Tethyan domain     

Ladislav A. Palinkaš  Vladimir Bermanec  Sibila Borojević Šoštarić  Tea Kolar-Jurkovšek  Sabina Strmić Palinkaš  Ferenc Molnar  Goran Kniewald 《Journal of Volcanology and Geothermal Research》2008

The Hruškovec quarry of basaltoid rocks is situated on the northwestern slopes of Mt. Kalnik, within the Zagorje–Mid-Transdanubian zone, a part of the North-western Dinarides. The basaltoids are inter-bedded with radiolarites of the Middle and Upper Triassic age (Langobardian, Carnian–Norian). Spilites, altered diabases and meta-basalts form part of Triassic volcanic-sedimentary sequence, made of sandstones, shales, micritic limestone, altered vitric tuffs and radiolarian cherts, incorporated tectonically into the Jurassic–Cretaceous mélange.  相似文献   

长白山天池火山气象站期晚期碱流质熔岩的岩石学研究   总被引：3，自引：0，他引：3  

刘永顺  聂保锋  孙善平  穆大卫  史延升 《现代地质》2007,21(2):296-306

通过野外火山堆积物的相分析、岩石显微鉴定和造岩矿物化学分析,可以确认气象站期晚期碱流质熔岩是典型的溢流熔浆冷却固结形成的,不是碎成熔岩。熔岩流的顶部和底部是黑曜质碱流岩的表壳相,中部是致密厚层与薄层相间产出的、具有塑性变形的碱流岩的内部相。组成熔岩流的岩石具有斑状和聚斑结构,从表壳相向内部相基质依次发育玻璃质结构、（微）球粒结构和霏细结构。斑晶矿物钠透长石、钙铁辉石、铁橄榄石和少量石英出现在熔岩流的所有岩相中,而钠铁闪石、铁钠透闪石和钛铁矿则作为内部相基质中的显微斑晶出现。气象站期晚期碱流质熔岩是过铝质、过碱性的钠闪碱流质熔岩,属于碱性系列,与意大利潘泰莱里亚岛碱流岩相比,富Si、Al、K、Mg、Ca和P而贫Fe、Na、Mn和Ti。  相似文献   

Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms     

A. Garner  J. McPhie 《Bulletin of Volcanology》1999,61(6):396-410

 Lithic megablocks ranging from <1 to 50 m in diameter occur in the Yardea Dacite, a widespread (12,000 km²), thick (>200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30–40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted. Received: 11 November 1998 / Accepted: 18 April 1999  相似文献   

Magnetic field intensity study of the 1960 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique   总被引：1，自引：0，他引：1  

Mimi J. Hill  & John Shaw 《Geophysical Journal International》2000,142(2):487-504

It is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known. In this study, two vertical sections, 16 m apart, have been sampled from the approximately 1 m thick 1960 Kilauea lava flow, Hawaii. Variations are seen in the rock-magnetic and palaeomagnetic properties between and within the two sections, indicating that there are small-scale lateral and vertical variations in the lava flow. The two sections showed different responses to microwave palaeointensity analysis. Section H6001 generally gave ideal linear behaviour on plots of natural remanent magnetization (NRM) lost against microwave-induced thermoremanent magnetization (T_M RM) gained, whilst the majority of samples from H6002 showed anomalous two-slope behaviour. When all plots were interpreted by taking the best-fitting line through all points, the flow mean intensity for H6001 was 31.6 ± 3.6 μT and that for H6002 was 37.1 ± 6.4 μT, compared with the expected intensity of 36 μT. Additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava, and whether it is best to always interpret NRM lost/T_M RM gained plots by taking the line of best fit regardless of shape.  相似文献   

Historical eruptions of Merapi Volcano, Central Java, Indonesia, 1768–1998     

B. Voight  E. K. Constantine  S. Siswowidjoyo  R. Torley 《Journal of Volcanology and Geothermal Research》2000,100(1-4)

  相似文献   

大兴安岭北段斯木科流纹质碎斑熔岩特征及成因   总被引：1，自引：0，他引：1  

赵焕利  韩彦东  李仰春  柳震  于荣文 《地质与资源》2004,13(4):207-210

在大兴安岭北段斯木科地区发育一种具有独特结构特征的火山岩石类型——流纹质碎斑熔岩,其具有熔岩和熔结凝灰岩的双重结构.岩石富Si、碱,Na₂O/K₂O=0.61~0.82,轻稀土富集,Eu亏损强烈,富集Rb、Th,亏损Ba、K,与同期流纹岩的地球化学特征相同,其区别主要在岩相学上.二者均形成于造山后伸展环境,属于由于岩浆向上运移、定位过程的不同而形成的"同质异相体".  相似文献   

A new interpretation of the sedimentary environment before and during eruption of the Emeishan LIP,Southwest China     

《International Geology Review》2012,54(10):1295-1313

The environment where the Permian Emeishan large igneous province (LIP) of Southwest China erupted remains controversial, especially regarding whether it was terrestrial, involving a 1 km scale domal uplift, or submarine. Slightly younger Daqiao conglomerate and Binchuan pillow lavas suggest that the Emeishan LIP erupted in a submarine environment. We show that at Binchuan, sandstone and rhyolite lie beneath the pillow lavas. In the Daqiao cross-section, there is an eastwards-verging syncline that reverses the succession of basalt and conglomerate. The conglomerate is not a basal conglomerate, and it does not contain any magmatic hydrovolcanic deposits. The basalt underlying the conglomerate is not the first of the LIP eruptions; that first eruption is found ~420 m below, on top of the Permian Maokou limestone. All together, these observations show that the deposits, including the conglomerate and pillow lava, do not represent the environment at the very start of the LIP volcanic eruptions, but represent conditions that existed before and possibly during the Emeishan LIP eruptions. Based on field investigations, the petrology of the rocks, and structural features, we conclude that submarine sedimentation and subaerial basalt eruptions coexisted in time and space in the region during or prior to the Emeishan LIP basalt eruptions.  相似文献   

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth     

《International Geology Review》2012,54(13):1569-1595

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.  相似文献