Fountains in Magma Chambers   总被引：5，自引：3，他引：5  

CAMPBELL  IAN H.; TURNER  J. S. 《Journal of Petrology》1989,30(4):885-923

Cyclic layering is a common feature of the ultramafic zone oflayered intrusions and is usually attributed to the entry ofnew pulses of dense magma into the chamber. Since the crystallizationof olivine and bronzite lowers the density of the magma, a newpulse of the parent magma will be denser than the fractionatedmagma in the chamber. If the new pulse enters with excess momentumit will initially rise up into the host magma to form a fountain,then fall back around the feeder when negative buoyancy forcesovercome the initial momentum of the pulse. Laboratory experimentsusing aqueous solutions with both point and line sources havebeen conducted to obtain a quantitative understanding of thefluid-dynamical processes that are important in fountains. Itis observed that convection within the fountain is highly turbulent,resulting in appreciable entrainment of the host magma. A gravity-stratifiedhybrid layer develops at the floor and this breaks up into aseries of double-diffusive convecting layers if the new pulseis hotter than the host magma. The number of layers that formdepends on a number of factors, especially R, the ratio of thecontributions of composition and heat to the total density differencebetween the host magma and the new pulse. Raising the valueof R, results in the formation of more, thinner layers. The thickness of the hybrid layer at any time t is given byH = h₀+(V₀/A)t where V₀ is the volume flux through the feederand A is the horizontal area of the chamber. h₀ is related tothe initial steady-state height of the fountain and, for a linesource, is given by h₀=CU₀^4/3 d^–1(g/)^–2/3 whereU₀ is the volume flux per unit length, g is the accelerationdue to gravity, d is the width of the feeder, is the densityof the host magma, is the density difference between the magmasand C is a constant. Calculations based on these results and the consideration ofthe flow in the feeder dykes below the chamber indicate thata fountain will rise at least 350 m in a continental magma chamberif the feeder width is greater than 10 m. This will lead toextensive mixing between the new pulse and the fractionatedmagma in the chamber, producing a zoned hybrid layer at thefloor that is commonly over 1000 m thick. If the chamber receivesmany pulses of dense magma, the resulting zoning may persistthroughout much of the life of the chamber, especially if thefirst pulse to enter becomes contaminated by light magma releasedby melting at the margins. The highest Mg/Fe ratio for olivineand pyroxenes from cyclic units from the ultramafic zones oflayered intrusions is often well below the value expected forminerals crystallizing from a melt derived directly from themantle, supporting the hypothesis that new pulses of dense magmacan mix extensively with the fractionated magma in the chamber. The feeder dykes to some oceanic magma chambers, such as theBay of Islands Ophiolite, are believed to be narrower, so thatfountains do not rise more than a few metres above the floorof the chamber. This restricts mixing between the input magmaand the host magma and can result in the formation of a hybridzone that is only a few metres thick.  相似文献   

The anatomy of an early Dinantian terraced floodplain: palaeo-environment and early diagenesis     

JULIAN E. ANDREWS  MICHAEL S. TURNER  GHULAM NABI  BARUCH SPIRO 《Sedimentology》1991,38(2):271-287

The lowermost Carboniferous rocks in the Cockburnspath area of east Berwickshire (southern Scotland) are interpreted as coastal floodplain sediments. A lower mudstone-dominated unit is composed of silty mudstones and shales with subordinate sandstones and argillaceous ferroan dolomites (cementstones). These are interpreted as distal floodplain sediments with periodic crevasse-splay deposition. The dark grey colour of the mudrocks suggests deposition in reducing conditions, probably in floodplain lakes. Most of the cementstones are concretionary, some with septarian cracks, suggesting an early diagenetic origin. An immature palaeosol suggests periodic pedogenesis under improved drainage. A synsedimentary erosion surface indicates incision of a valley into the floodplain, presumably in response to base-level lowering. An upper sandstone-dominated unit starts with fine-grained rippled sandstones, cut by small channel sandstones. These are interpreted as floodplain lake deposits fed by crevasse channels. A distinctive conglomerate with cementstone clasts, wood fragments and fish remains is interpreted as a major overbank deposit, dumped into a pre-existing floodplain lake. A bivalve fauna was established in the overlying mudstones, followed by a thin limestone with a restricted marine fossil assemblage, showing that seawater flooding of the lakes occurred at times. Mudrocks throughout the sequence contain no pyrite, except for the marine band which has an organic-carbon/sulphur ratio and degree of pyritization value typical of marine sediments. The concretionary cementstones have δ¹³C values around —4 to —6%^0/00 PDB which are interpreted as indicative of anaerobic oxidation of organic matter. The combined geochemical data suggest a significant involvement of iron reduction in cementstone formation, although the δ¹³C values are ambiguous in assessing the relative involvement of methanogenesis and methane oxidation. Limited seawater inundation of the floodplains might have supplied magnesium and calcium ions for dolomite formation assuming that any H₂S derived as a result of sulphate reduction was oxidized by iron reduction. Alternatively a weathering source for solutes might have been involved.  相似文献   

Ocean fertilization with iron: effects on climate and air quality   总被引：1，自引：0，他引：1  

PETER LISS  ADELE CHUCK  DOROTHEE BAKKER  SUZANNE TURNER 《Tellus. Series B, Chemical and physical meteorology》2005,57(3):269-271

  相似文献   

Aragonite in California Glaucophane Schists, and the Kinetics of the Aragonite--Calcite Transformation   总被引：2，自引：0，他引：2  

BROWN  W. H.; FYFE  W. S.; TURNER  F. J. 《Journal of Petrology》1962,3(3):566-582

Coleman & Lee's discovery that aragonite is a widespreadmetamorphic mineral in California glaucophane schists is confirmedand amplified. Methods of microscopic distinction between aragonite,calcite, and dolomite, including a universal-stage technique,are described. Further data are recorded regarding the paragenesisof aragonite-bearing glaucophane and lawsonite schists. Carbonatesin the greenschist facies are found to be exclusively calciteand dolomite. In many Californian metamorphic aragonites partialinversion to calcite has been observed. This appears to be anequal-volume replacement in which an a axis and an edge [f:f] of calcite—both directions of Closest spacing of Ca⁺⁺ions—commonlyare parallel to of the crystal axes a, b, and c of aragonite. The problem of survival of metamorphic aragonite through a necessarilylong period of post-metamorphic unloading is approached by experimentalexploration of the kinetics of the aragonite calcite transformation.It is found that Californian aragonite could survive unloadingfrom a depth of 20 km if the linear temperature gradient were10? C per km, but not appreciably higher. Available experimentaldata are consistent with crystallization of aragonite, jadeite,and lawsonite at depths of 20–30 km if a gradient of 10?C per km is assumed. The corresponding conditions of the glaucophane-schistfacies (T = 2OO?-3OO? C, P = 6000-9000 bars) are attributedessentially to deep burial in regions (subsiding geosyn-clines)of unusually low temperature gradient (10? C per km).  相似文献   

Oxygen isotopes in forsterite grains from Julesburg and Allende: Oxygen-16-rich material in an ordinary chondrite     

J. M. SAXTON  I. C. LYON  G. TURNER 《Meteoritics & planetary science》1998,33(5):1017-1027

Abstract— We have used the Manchester ISOLAB 54 ion microprobe to make in situ measurements of the ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios of olivine grains in the Julesburg (L3.6) and Allende (CV3) chondrites. We have discovered a population of olivines in Julesburg characterised by (1) the most ¹⁶O-rich compositions yet reported for olivine from an ordinary chondrite; (2) cores of low-Fa olivine, which frequently shows blue cathodoluminesce; (3) thick coats of more Fa-rich (Fa ～20) olivine, which is also ¹⁶O-enriched. In an O isotopic plot, the Julesburg ¹⁶O-rich grains form a roughly linear array that is offset from the Allende mixing line. The presence of very low Fa olivine and, sometimes, well-defined Fa-rich coats indicates that these grains experienced significantly less thermal metamorphism than most of the olivine in the meteorite. Some ¹⁶O-rich Julesburg grains are associated with minor feldspar or pyroxene and are probably chondrule fragments. They are isotopically indistinguishable from forsterite in Allende; however, Allende forsterite grains do not have the thick Fa20 coats typical of those in Julesburg. These ¹⁶O-rich forsterite grains appear to be related to the “blue olivine” of Steele (1986). Both cores and coats of ¹⁶O-rich grains in Julesburg are isotopically distinct from olivine in Semarkona group A and group B chondrules.  相似文献   

Significant Increase of Oil and Natural Gas Reserves throughout 2013     

HAO Ziguo  FEI Hongcai  HAO Qingqing  Susan TURNER 《《地质学报》英文版》2014,88(2):718-718

正China’s Ministry of Land and Resources announced on 27th February 2014 that proven reserves of oil,natural gas and coal continued to increase significantly,and that their production shows a steady increase.This greatly improves China’s support capacity of fossil fuel resources.Oil resources:In 2013,China newly discovered oil geological reserves yielded 10.84×108 t,and  相似文献   

Comparison of a Manual and an Automated Tracking Method for Tibetan Plateau Vortices     

Julia CURIO  Yongren CHEN  Reinhard SCHIEMANN  ANDrew G.TURNER  Kai Chi WONG  Kevin HODGES  Yueqing LI 《大气科学进展》2018,35(8):965-980

Tibetan Plateau vortices(TPVs) are mesoscale cyclones originating over the Tibetan Plateau(TP) during the extended summer season(April–September).Most TPVs stay on the TP,but a small number can move off the TP to the east.TPVs are known to be one of the main precipitation-bearing systems on the TP and moving-off TPVs have been associated with heavy precipitation and flooding downstream of the TP(e.g.,in Sichuan province or over the Yangtze River Valley).Identifying and tracking TPVs is difficult because of their comparatively small horizontal extent(400–800 km) and the limited availability of soundings over the TP,which in turn constitutes a challenge for short-term predictions of TPV-related impacts and for the climatological study of TPVs.In this study,(i) manual tracking(MT) results using radiosonde data from a network over and downstream of the TP are compared with(ii) results obtained by an automated tracking(AT) algorithm applied to ERA-Interim data.Ten MT-TPV cases are selected based on method(i) and matched to and compared with the corresponding AT-TPVs identified with method(ii).Conversely,ten AT-TPVs are selected and compared with the corresponding MT-TPVs.In general,the comparison shows good results in cases where the underlying data are in good agreement,but considerable differences are also seen in some cases and explained in terms of differences in the tracking methods,data availability/coverage and disagreement between sounding and ERA-Interim data.Recommendations are given for future efforts in TPV detection and tracking,including in an operational weather forecasting context.  相似文献   

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models     

Kai Chi WONG  Senfeng LIU  Andrew G.TURNER  Reinhard K.SCHIEMANN 《大气科学进展》2018,35(8):1049-1062

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.  相似文献   

Braidplain deposition of the Upper Triassic Molteno Formation in the main Karoo (Gondwana) Basin, South Africa   总被引：1，自引：0，他引：1  

BRIAN R. TURNER 《Sedimentology》1983,30(1):77-89

ABSTRACT The Upper Triassic (Carnian?) Molteno Formation in the main Karoo (Gondwana) Basin, South Africa forms a northerly thinning, intracratonic clastic wedge comprising sandstones, shales and coals occurring within thick (up to 140 m) laterally persistent fining-upward sequences. These sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type. Geometric relations between sequences indicate three phases of tectonic activity. The lowermost fining-upward sequence in the south accumulated against a rising mountain front; cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining-upward sequence adjacent to a fault-block granite terrain located close to the present south-east coastline of South Africa. This is believed to be the granite at the eastern end of the Falkland Island Plateau, an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face. Faulting is attributed to the first phase of extension prior to continental breakup. The sourceward recession and lack of gross fining-upward trends shown by the uppermost fining-upward sequences is accounted for by limited back-faulting of the still active basin margin. Cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits (Elliot Formation) which were distal equivalents of the braided alluvial plain.  相似文献   

The Influence of Viscosity on Fountains in Magma Chambers   总被引：5，自引：4，他引：5  

CAMPBELL  IAN H.; TURNER  J. STEWART 《Journal of Petrology》1986,27(1):1-30

Geological observations suggest that basaltic magmas mix readilybut that rhyolites and basalts can erupt through the same volcanicvent without mixing. The implication is that viscosity may havean important influence on the mixing of magmas. This paper examines the influence of viscosity on magma mixingwhen a magma of low viscosity is injected as a turbulent fountaininto a chamber containing a magma of higher viscosity. Threeseries of experiments were carried out. In the first, the lowviscosity fluid was injected into the tank at a Reynolds number(Re₁) of about 1000 to ensure that flow within the fountainwas fully turbulent. Both fluids were maintained at the sametemperature and the viscosity of the host fluid was varied systematically.If the viscosities of the two fluids are similar, turbulencein the fountain leads to extensive mixing and to formation ofa density-stratified layer which collects at the bottom of thetank. However, if the viscosity ratio of the fluids used isgreater than 400, no detectable mixing occurs. Motion withinthe fountain is again fully turbulent but the high momentumof the input fluid is not transmitted to the more viscous hostfluid. Experiments carried out at intermediate viscosity ratiosresulted in intermediate amounts of mixing. Theoretical considerations suggest that the criterion for mixingin turbulent fountains, when a fluid of low viscosity v₁ isinjected into a fluid of much higher viscosity v₂ is wd>kv₂ where w is the flow velocity through an input pipe of diameterd and k is a constant. This relationship is believed to be ageneral one, applying equally to jets and plumes, although thevalues of k may vary with the nature of the flow. The secondseries of experiments was designed to test this relationshipand determine values of k for fountains. The results show that,if wd/v₂ > 70, the inflowing fluid mixes with the host fluidas there was no viscosity difference between them. However,if wd/v₂ < 7, little or no mixing occurs even if motion withinthe fountain is fully turbulent. In the third series of experiments the temperature of the inputfluid was 70?C above that of the host fluid. This led to heatingofa thin boundary layer of the host fluid, lowering the viscosityof the host fluid adjacent to the fountain and thus allowingadditional mixing between the fluids. This effect was most obviousin the experiment with the highest viscosity ratio (1229) wherea small amount of mixing was detected, compared with no mixingin the equivalent experiment in the first series. The criterion for mixing, wd> kv₂ can be applied to naturalmagmas. The results show that if a primitive basaltic magmais injected, as a turbulent fountain or plume, into a chambercontaining fractionated basaltic magma, the two magmas willmix readily. However, if basaltic magma is injected into a chambercontaining granitic melt, little or no mixing occurs.  相似文献