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1.
The White Trachytic Tuff (WTT) is a compositionally-zoned, trachytic, pyroclastic-flow deposit which erupted from Roccamonfina volcano about 300,000 years ago. It was principally emplaced as unwelded, pumice-rich flow units with an estimated volume of 10 km3. These now cover the flanks of the volcano on all sides except the west, behind the highest rim of Roccamonfina's summit caldera; the caldera was probably in existence prior to the WTT activity. Eruption of the WTT generally initiated the leucite-free, second stage of Roccamonfina's development, following a long history of leucite-bearing volcanism, but minor leucite-bearing lavas and pyroclastics overlie the WTT as well. The WTT was in turn followed by progressively more basic, leucite-free magmas (latite, trachybasalt, and basalt). During the course of the eruption, the WTT evolved from white, crystal-poor pumices containing 66% SiO2 and 1.2% CaO, to grey pumices containing higher crystal contents, 60% SiO2, and 3% CaO. Early pumices are also relatively enriched in Mn, Na, Zn, Ga, Rb, Y, Zr, Nb, Cs, La, Ce, Yb, Lu, W, Hf, Th, and U, and depleted in Ti, Fe, Mg, K, Sc, V, Cr, Co, Sr, Ba, Nd, Sm, Eu, Tb, Dy, and Ta. The pumices are essentially bimodal in composition, with several minor intermediate types including megascopic, physical mixtures of the white and grey varieties. Certain WTT pumices, including all analyzed intra-caldera samples, are relatively enriched in Pb, Th, Zr, Rb, Ga, Zn, and Cs compared to the rest of the suite. These enrichments may reflect local assimilation of carbonates or more complex exchange processes at the magma chamber margin. All WTT pumices contain the phases sanidine, plagioclase, clinopyroxene, biotite, titanomagnetite, sphene, and apatite; grey varieties also contain magnesian olivine crystals which are probably xenocrysts. The white, crystal-poor types show relatively simple mineralogies with little compositional variability or zoning among crystals of a single phase. Other pumice types, and dark, trachyandesitic inclusions separated from white pumices, show a large compositional spectrum of individually homogeneous crystals. These compositionally diverse crystals and inclusions are interpreted as a result of widespread mixing between the trachytic magmas and more basic magmas prior to or during the WTT eruption. Major-element crystal-fractionation models can successfully derive the early trachytes from the late trachytes by 50–85% separation of a syenitic assemblage of all phases. The predicted phase proportions and compositions closely match cumulate syenite nodules found at Roccamonfina. Trace element models are permissive of syenitic fractionation within the large uncertainties allowed by published partition coefficients.  相似文献   

2.
S. Signorelli  B. Capaccioni   《Lithos》1999,46(4):171-730
This study deals with the distribution of chlorine in glassy mesostases and whole-pumices from the 79 A.D. Plinian eruption (Somma–Vesuvius volcanic complex, Italy). This explosive event produced a prominent Plinian fall deposit followed by flows and surges. The fall deposit can be divided into two sub-units on the basis of an abrupt change in colour at approximately mid-level: a phonolitic white pumice layer at the base and a tephriphonolitic grey pumice layer at the top. Due to its hybrid nature (a mixture of k-tephritic, tephriphonolitic and phonolitic magmas), information on chlorine behaviour in tephriphonolitic magma has been inferred only by means of mass balance calculations. In the white pumices, chlorine concentrations show constant whole-pumice values, whereas glassy mesostases display significant compositional variations. These variations have been linked to the cryptocrystallisation of leucite in glassy mesostases, which affected the original melt compositions just before and during the eruptive event. In this framework, whole-pumice appears to better represent the pre-eruptive melt compositions. Using chlorine concentration in whole-pumices, a three-stage model of chlorine behaviour prior and during the eruptive event is predicted: (i) free variation during Rayleigh fractionation, according to a system with variance greater than zero; (ii) exsolution of volatile chlorine compounds (e.g., metal chlorides), when chlorine reaches its solubility limit in silicate melt, in coexisting hyper-saline and in dilute immiscible fluids; the variance of the system is zero at a given temperature and pressure; (iii) residual syn-eruptive variable enrichment of chlorine in the melt due to cryptocrystallization of leucite, suggesting a very minor loss of chlorine in the gas phase during the eruption. Although chlorine does not behave as a volatile element during the eruption, it is present in the volcanic plume. This is due to the postulated ‘excess' fluid phase containing chlorine that formed in the magma chamber prior to the eruption. The homogeneous distribution of chlorine in whole pumices, in contrast with a well-established chemical and isotopic layering in Vesuvian magmas prior to Pompei eruption, suggests that the trace element zonation is not directly linked to chlorine distribution in silicic melts.  相似文献   

3.
This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO2, high to moderate contents of Al2O3, alkalis, and P2O5, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO2, K2O, and Na2O contents and in the frequent presence of normative nepheline and leucite. K2O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents.  相似文献   

4.
爆炸式喷发过程中,火山碎屑物气孔记录了挥发分出溶、膨胀和合并等信息,其大小、形态、数量密度、空间分布等局域特征是推断火山喷发动力条件的重要参考。文章基于天池火山三期喷发(50 000年前大喷发的黄色浮岩、千年大喷发的灰白色浮岩和1668年八卦庙期喷发的黑色浮岩)野外地质工作,以非线性火山喷发动力学为指导,开展了火山通道内气泡生长的流体动力学研究,揭示出岩浆流体黏性力和界面张力的共同作用对于岩浆减压和气泡生长过程的约束。在浮岩气孔结构的定量化分析基础上,进一步研究了天池火山三期喷发的浮岩气孔参数,通过气泡生长流体动力学方程得到了千年大喷发灰白色浮岩毛细管数Ca值为253, 明显高于50 000年前大喷发黄色浮岩(Ca值为94)和八卦庙期喷发黑色浮岩(Ca值为111),表明了千年大喷发曾发生过明显的成分变化,推测可能与幔源基性岩浆注入有关;而50 000年前大喷发黄色浮岩气孔不规则形态参数(1-Ω)值为0.098,大于后两期喷发(分别为0.052和0.064),可能意味着天池火山系统动力学平衡的弛豫周期变小或浮岩气泡生长受动力学、流变学改造过程减弱,这可为进一步研究天池火山活动规律提供参考。三期浮岩毛细管数Ca量级为102,气孔不规则形态参数(1-Ω)量级为10-1,从动力学上首次证实了天池火山属于普林尼型或超普林尼型喷发。  相似文献   

5.
本文在长白山天池火山野外地质工作基础上,对天池火山黄色浮岩(50000年前大喷发)、灰白色浮岩(千年大喷发)和黑色浮岩(八卦庙期喷发)等三期空降浮岩开展了扫描电子显微镜高精度结构分析以及浮岩气孔结构的局域化定量研究。三期浮岩洛伦茨分析表明:黄色浮岩和黑色浮岩气孔均一化程度基本一致,而灰白色浮岩均一化程度更高,反映了千年大喷发在岩浆演化程度和喷发动力学行为上的特殊性,推测可能与幔源岩浆注入地壳岩浆房的动力过程有关。气孔大小幂律分布以及气孔形态分形特征的复杂性分析表明,50000年前大喷发的浮岩幂指数和分形维数最高,揭示了其自组织临界态程度最高,对外界环境触发最为敏感,而另外两期浮岩幂指数和分形维数呈现出标准的幂律衰减,据此我们推测三期喷发可能均处在天池火山系统逐渐稳态恢复的大周期中。  相似文献   

6.
Sea-rafted Loisels Pumice is one of the few stratigraphic markers used to correlate late Holocene coastal deposits in New Zealand. Along with underlying sea-rafted products of the local Taupo eruption of ca. 1800 yr B.P., these events have been used to bracket the first arrival of humans at New Zealand. Loisels Pumice is dacitic to rhyolitic (SiO263–78 wt%) in composition, but individual clasts are homogeneous (SiO2range ± 1 wt%). Characteristics include very low K2O (0.5–1.75 wt%) and Rb (<25 ppm) and a mineralogy dominated by calcic and mafic xenocrysts. Similar features are shared by pumices of the Tonga–Kermadec arc, suggesting a common tholeiitic oceanic source. Interclast diversity of Loisels Pumice suggests that it is the product of several eruptive events from different volcanoes. The differences in glass and mineral compositions found at various sites can be explained if the deposits are from different events. A multisource origin can also partially explain the discrepancy in reported14C ages (ca. 1500–600 yr B.P.) from different localities. Therefore, the value of Loisels Pumice as a stratigraphic marker is questionable, and it does not constrain the arrival of humans. The predominant westward drift of historic Tonga–Kermadec arc pumices and prevailing ocean currents suggest a long anticlockwise semicircular transport route into the Tasman Sea before sea-rafted pumice arrival in New Zealand. The diversity of the pumices indicates that silicic eruptions frequently occur from the predominantly basic oceanic volcanoes.  相似文献   

7.
The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO2 and lower incompatible element concentrations in the low-SiO2 PIV/PV, than the high-SiO2 PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI- and PIII-dominant), where associated ignimbrites isolated earlier (PIV- and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.  相似文献   

8.
The Aeolian Islands are an arcuate chain of submarine seamounts and volcanic islands, lying just north of Sicily in southern Italy. The second largest of the islands, Salina, exhibits a wide range of compositional variation in its erupted products, from basaltic lavas to rhyolitic pumice. The Monte dei Porri eruptions occurred between 60 ka and 30 ka, following a period of approximately 60,000 years of repose. The bulk rock composition of the Monte dei Porri products range from basaltic-andesite scoria to andesitic pumice in the Grey Porri Tuff (GPT), with the Monte dei Porri lavas having basaltic-andesite compositions. The typical mineral assemblage of the GPT is calcic plagioclase, clinopyroxene (augite), olivine (Fo72?84) and orthopyroxene (enstatite) ± amphibole and Ti-Fe oxides. The lava units show a similar mineral assemblage, but contain lower Fo olivines (Fo57?78). The lava units also contain numerous glomerocrysts, including an unusual variety that contains quartz, K-feldspar and mica. Melt inclusions (MI) are ubiquitous in all mineral phases from all units of the Monte dei Porri eruptions; however, only data from olivine-hosted MI in the GPT are reported here. Compositions of MI in the GPT are typically basaltic (average SiO2 of 49.8 wt %) in the pumices and basaltic-andesite (average SiO2 of 55.6 wt %) in the scoriae and show a bimodal distribution in most compositional discrimination plots. The compositions of most of the MI in the scoriae overlap with bulk rock compositions of the lavas. Petrological and geochemical evidence suggest that mixing of one or more magmas and/or crustal assimilation played a role in the evolution of the Monte dei Porri magmatic system, especially the GPT. Analyses of the more evolved mineral phases are required to better constrain the evolution of the magma.  相似文献   

9.
Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   

10.
黑龙江镜泊湖西北的蛤蟆塘地区发现了一种特殊类型的白榴石碱玄岩,其岩石化学成分说明其为偏钠质岩石,并且含有钛角闪石、金云母和歪长石巨晶。其地球化学特征与离它直距只有15km的“火山口森林”地区的碱性橄榄玄武岩具有明显的不同,白榴石碱玄岩总体上以高的Al2O3(16.73%～18.09%)、Na2O(5.15%～6.36%)、K2O(4.24%～4.92%)含量、强烈富集轻稀土和大离子亲石元素和低的MgO(4.19%～4.59%)、CaO(5.66%～5.85%)、Mg#(0.43～0.50)值为特征。结合其地质背景和Sr、Nd、Pb同位素,认为白榴石碱玄岩是由被富含水、氟及其它挥发性组分的地幔柱物质强烈交代富集后的岩石圈地幔经低部分熔融后形成的原始岩浆在中深部经多级分离结晶后的产物。  相似文献   

11.
Experimente zur Zeolithbildung durch hydrothermale Umwandlung   总被引：1，自引：0，他引：1  
The formation of zeolites by hydrothermal alteration has been investigated by taking trass from the Laach volcanic area as a sample. Zeolites to be found are chabazite, phillipsite and analcime, all of which originated from the same phonolitic glass. This paper aims at explaining the formation of zeolites by means of experimental alteration of the pumice with various solutions. NaOH and KOH solutions were used in the experiments, these limited the formation conditions of chabazite, phillipsite, analcime in alkaline environments. Moreover, experiments were carried out with H2O dist and with solutions that formed during the alteration of pumice by reacting with H2O. These experiments were conducted to supply clues concerning the formation of zeolites in the Laach volcanic area. With NaOH solutions zeolites were formed from pumice within a temperature range of 70 to 250° C and a concentration range of 0.001 to 1.0 n; with KOH solutions they were formed in the same concentration range within a temperature range of 100 to 250° C. The formation of zeolites proved to depend much on temperature and concentration: At low temperatures high concentrations are necessary, higher temperatures need lower concentrations. With NaOH solutions the zeolites are formed in the succession, chabazite, phillipsite, analcime at increasing temperatures and increasing concentrations. With KOH solutions the same succession is to be found with rising temperature. With rising concentration, however, this succession is changed at higher temperatures: Phillipsite appears in place of analcime at high temperatures and concentrations. By the alteration of pumice with H2O, zeolites are formed from 200° C onward. With solutions that had been formed during the alteration of pumice by reacting with H2O, the minimum temperature for the formation of zeolites was 180° C. Concerning the genesis of zeolites in the Laach volcanic area, the experiments showed that the temperature for the formation of analcime was about 250° C; for the formation of chabazite and phillipsite it was between 150 and 190° C. The pH of the reacting solutions may have been between 7 and 8. Thus the experiments proved that from the same material different zeolites, chabazite, phillipsite, analcime may be formed by temperature change of the reacting solutions (H2O to slightly alkaline solutions).  相似文献   

12.
The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO3, Cl, and H2O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.  相似文献   

13.
The paper describes a suite of volcanic and intrusive rocks from Ua Pu, one of the Marquesas Islands, situated in the central Pacific Ocean. The rocks comprise alkali olivine basalts, hawaiites, mugearites, trachytes, and phonolites. Their petrographic characters are briefly described and 24 new chemical analyses presented. The rocks fall into a sodic and a potassic series, since some rocks are richer in K2O than Na2O, whilst the phonolites have exceptionally high total alkali contents.The differentiation of the series is discussed and special attention is paid to the phonolites in which icositetrahedral pseudomorphs occur. It is concluded that these pseudomorphs are probably after analcime rather than leucite.  相似文献   

14.
At least 8 km3 of felsic ignimbrites and high-K silica-undersaturatedlavas and tephra were erupted from the Latera caldera between250 and 150 ka. Four distinct periods of explosive eruptions(at about 232, 206, 195, and 156 ka) produced ignimbrite sequencesthat show an upward compositional progression from trachyteand differentiated phonolite to less evolved phonolite. Duringthe last two of these periods, the tuffs grade upward from phonoliteto tephriphonolite. The stratigraphy indicates that eruptionssampled magmas that were stratified downward from trachyte andphonolite to tephriphonolite, and the compositional cyclicitysuggested by the timing of the eruptions implies an unusualcontemporaneity of silica-saturated and -undersaturated compositions. At Latera, pumice fragments in the same deposit can exhibitup to 10-fold differences in vesicularity and crystal content(from <5 to >50 vol.% phenocrysts). These clasts, in conjunctionwith glassbearing syenite and skarn xenoliths, represent a rangeof progressively crystallized magmas that were quenched at theinstant of their eruptive entrainment. The syenites compriseeutectic mineral assemblages with high percentages of titanite,apatite, and melanite garnet as accessory minerals. Least-squaresmodels based on major element and compatible trace element (e.g.,Ba and Sr) abundances of the pumices and syenite indicate thatthe fractionation of plagioclase and sanidine largely controlledthe liquid lines of descent for phonolite and trachyte, respectively.Additional mineral phases that may have contributed to magmaticdifferentiation include fassaitic diopside, leucite, biotite,apatite, and alkali amphibole. Models further imply that tephriphonoliticliquids required roughly 70% crystallization of tephritic orbasanitic parent magmas, whereas the evolved phonolitic liquidswere obtained after the removal of >85% of the above mineralassemblage. The commonly aphyric trachytic tuffs represent themost evolved derivatives. Despite the limited range in major element contents, trace elementsvary considerably among the different pumice types and syenites.Large ranges in Rb/Sr, Nb/Ta, Zr/Hf, La/Yb, and Ba/Th reflectthe selective partitioning of some elements into accessory phases.However, the variations of B, Sc, Rb, Nb, Hf, Y, and Yb cannotbe explained completely by crystal fractionation. Syenite compositions,for example, bracket the range of most elements in all pumicetypes, and chemical models demonstrate that processes operatingalong the chamber margins could have greatly influenced thebehavior of trace elements in the evolved liquids. Plausiblemechanisms that might have accompanied crystal fractionationin these magmas include the mixing of several magma batches,and the possible dilution of central reservoir magmas by back-mixingwith fractionated liquids or with CO2-rich fluids released fromskarns. *Present Address: Bureau of Economic Geology, Mineral Studies Laboratory, University of Texas, Austin, Texas 78713-7508  相似文献   

15.
The latest eruption of Haruna volcano at Futatsudake took placein the middle of the sixth century, starting with a Plinianfall, followed by pyroclastic flows, and ending with lava domeformation. Gray pumices found in the first Plinian phase (lowerfall) and the dome lavas are the products of mixing betweenfelsic (andesitic) magma having 50 vol. % phenocrysts and maficmagma. The mafic magma was aphyric in the initial phase, whereasit was relatively phyric during the final phase. The aphyricmagma is chemically equivalent to the melt part of the phyricmafic magma and probably resulted from the separation of phenocrystsat their storage depth of 15 km. The major part of the felsicmagma erupted as white pumice, without mixing and heating priorto the eruption, after the mixed magma (gray pumice) and heatedfelsic magma (white pumice) of the lower fall deposit. Althoughthe mafic magma was injected into the felsic magma reservoir(at 7 km depth), part of the product (lower fall ejecta) precedederuption of the felsic reservoir magma, as a consequence ofupward dragging by the convecting reservoir of felsic magma.The mafic magma injection made the nearly rigid felsic magmaerupt, letting low-viscosity mixed and heated magmas open theconduit and vent. Indeed the lower fall white pumices preservea record of syneruptive slow ascent of magma to 2 km depth,probably associated with conduit formation. KEY WORDS: high-crystallinity felsic magma; magma plumbing system; multistage magma mixing; upward dragging of injected magma; vent opening by low-viscosity magma  相似文献   

16.
Peralkaline magma evolution and the tephra record in the Ethiopian Rift   总被引：3，自引：3，他引：0  
The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.  相似文献   

17.
To investigate the origin of compositional zonation in the Bishop Tuff magma body, we have analyzed trace elements in the matrix glass of pumice clasts and in quartz-hosted melt inclusions. Our results show contrasting patterns for quartz in different parts of the Bishop Tuff. In all samples from the early part of the eruption, trace element compositions of matrix glasses are similar to but slightly more evolved than quartz-hosted melt inclusions. This indicates a cogenetic relationship between quartz crystals and their surrounding matrix glass, consistent with in situ crystallization. The range of incompatible element concentrations in melt inclusions and matrix glass from single pumice clasts requires 16–20 wt% in situ crystallization. This is greater than the actual crystal content of the pumices (<15 % crystals). In contrast to the pattern for the early pumices, pyroclastic flow samples from the middle part of the eruption show contrasting trends: In some clasts, the matrix is more evolved than the inclusions, whereas in other clasts, the matrix is less evolved. In the late Bishop Tuff, all crystal-rich samples have matrix glasses that are less evolved than the melt inclusions. Trace element abundances indicate that the cores of quartz in the late Bishop Tuff crystallized from more differentiated rhyolitic magma that was similar in many ways, yet distinct from the early-erupted Bishop Tuff. Our results are compatible with a model of secular incremental zoning (Hildreth and Wilson in Compositional zoning of the Bishop Tuff. J Petrol 48(5):951–999, 2007), in which melt batches from underlying crystal mush rise to various levels in a growing magma body according to their buoyancy. Early- and middle-erupted quartz crystallized from highly evolved rhyolitic melt, but then some parts of the middle-erupted magma were invaded by less differentiated rhyolite such that the matrix melt at the time of eruption was less evolved than the melt inclusions. A similar process occurred but to a greater extent in magma that erupted to form the late Bishop Tuff. In addition, there was a final, major magma mixing event in the late magma that formed Ti-rich rims on quartz and Ba-rich rims on sanidine, trapped less evolved rhyolitic melt inclusions, and resulted in dark and swirly crystal-poor pumice that is a rare type throughout much of the Bishop Tuff.  相似文献   

18.
Replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales. The underlying mechanisms are, in general, poorly understood, but both mechanical and chemical processes are thought to be important. Replacement of leucite (KAlSi2O6) by analcime (NaAlSi2O6 .H2O) is common in silica-poor igneous rocks. A 10% increase in volume is associated with the replacement process, and this generates stresses that eventually cause fracturing of the reacting leucite. Experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric ‘onion-skin’-like layers near the reacting interface and the formation of cross-cutting, often hierarchically arranged, sets of fractures that divide the remaining leucite into progressively smaller domains. These structures may explain the ‘patchy’ alteration patterns observed in natural leucite samples and similar, so-called, mesh-textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks. They are also strikingly similar to larger scale patterns formed during spheroidal weathering processes. A simple discrete element model illustrates the mechanics that control the formation of such systems, and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes.  相似文献   

19.
The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been studied on two silica-undersaturated joins of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F− = O2−). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine, spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join that intersects the Forsterite – Diopside – KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl, and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join, but is preserved in a part of the Forsterite – Diopside – KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure. Received: 3 June 1997 / Accepted: 5 January 1998  相似文献   

20.
Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km3 in a total magma volume of ~530 km3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from 87Sr/86Sr = 0.70547 to 0.70657, with compositions of An43 to An78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87Sr/86Sr = 0.70764 and An78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction.  相似文献   

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