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Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius 总被引:4,自引:1,他引:3
CIONI RAFFAELLO; CIVETTA LUCIA; MARIANELLI PAOLA; METRICH NICOLE; SANTACROCE ROBERTO; SBRANA ALESSANDRO 《Journal of Petrology》1995,36(3):739-776
A detailed study of the pyroclastic deposits of the AD 79 PompeiPlinian eruption of Vesuvius has allowed: (1) reconstructionof the thermal, compositional and isotopic (87Sr/86Sr) pre-eruptivelayering of the shallow magma chamber; (2) quantitative definitionof the syn-eruptive mixing between the different magmas occupyingthe chamber, and its relationships with eruption dynamics; (3)recognition of the variability of mafic magma batches supplyingthe chamber. During the different phases of the eruption 2530%of the magma was ejected as white K-phonolitic pumice, and 7075%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonoliticwhite magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor greyphono-tephritic magma (mafic end-member, MEM), which was nevererupted without first being mixed with white magma.Evidence is provided that mixing occurred within the chamberand was characterized by a transition with time from physicalmixing at a microscopic scale to chemical hybridization. TheMEM magma had a homogeneous composition and constant 87Sr86Srisotopic ratio, possibly as a result of sustained convection.No unambiguous liquidus phases were found, suggesting that theMEM magma was superheated (T = 10001100C); its verylow viscosity was a main cause in the establishment of a physicaldiscontinuity separating the white and the grey magmas. Thewhite-grey boundary layer possibly consisted of a multiply diffusiveinterface, periodically broken and recreated, supplying thephonolitic body through mixing of moderate amounts of fractionatedgrey melts with the overlying white magma. The presence of alarge overheated mass indicates the young, growing stage ofthe AD 79 chamber, whose main engine was the periodic arrivalof hot mafic magma batches. These were characterized by K-tephriticto K-basanitic compositions, high temperatures (>1150C),high volatile contents (2025% H2O +Cl+F+S), low viscosities[(1+2 102 poises)] and relatively low densities (25002600kg/m3). The birth of the Pompei chamber followed the repeatedarrival of these batches (on average characterized by 87Sr/86Sr070729)into a reservoir containing a tephriticphonolitic, crystal-enriched,magma, a residue from the preceding Avellino Plinianeruption (3400 BP).In fact, about half of magma ejected duringthe AD 79 eruption could have been inherited from pre-Avellinotimes. KEY WORDS: Vesuvius; magma chamber; magma mixing; compositional layering phonolites; magma supply; potassic magmas
*Correponding author 相似文献
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长白山天池火山千年大喷发岩浆含水量研究——熔融包裹体含水量的红外光谱测试 总被引:1,自引:0,他引:1
长白山天池火山在公元一千年左右曾发生过大规模喷发,其产物为大面积分布的灰白色碱流质浮岩和碎屑流,在其斑晶矿物橄榄石、钙铁辉石和碱性长石中均可见到熔融包裹体。在最主要的斑晶矿物——碱性长石中含有数量众多且个体也较大的熔融包裹体,多数含有一个以上的气泡,其中部分含有子晶,根据形貌特征的不同可分为截然不同的两组包裹体。这些熔融包裹体带有大量喷发前地下岩浆的信息,成为研究地下深部的岩浆在复杂的溢流-爆炸喷发中所发生变化的最好媒介。也是本文的研究对象,通过其中挥发份尤其是水的含量,可以推知天池火山发生大喷发的原因。经Nicolet Magna-IR 550红外光谱仪测定,这些熔融包裹体的含水量较高,达1.6%-3.6%,为当时天池火山发生了巨大规模爆炸喷发的原因提供了强有力的证据。但目前红外光谱仪的应用范围还比较有限,有待今后拓宽其应用领域。 相似文献
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Crystallization Driven by Decompression and Water Loss at Stromboli Volcano (Aeolian Islands, Italy) 总被引:11,自引:4,他引:11
METRICH NICOLE; BERTAGNINI ANTONELLA; LANDI PATRIZIA; ROSI MAURO 《Journal of Petrology》2001,42(8):1471-1490
Stromboli, in the Aeolian Archipelago, is famous for its persistentvolcanic activity. The normal activity, consistingof rhythmic explosions ejecting crystal-rich scoriae, is periodicallyinterspersed with more energetic explosions during which, inaddition to crystal-rich scoriae, crystal-poor pumices are alsoemitted. The scoriae contain 相似文献
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