The geochemical evolution of the Mt. Somma-Vesuvius volcano

Summary This work integrates new geochemical data with the numerous published analyses on rocks from the Mt. Somma-Vesuvius volcano. New quantitative models for the evolution of magma source regions and magma at different depths are proposed. The origin of the Somma-Vesuvius parental magma is modeled as 0.05–0.1 melt fractions of a MORB-type source composed of 54% olivine, 30% orthopyroxene, 10% clinopyroxene, 1% garnet, and 4% amphibole, and 1–5% sediment introduced through the adjacent arc system. The excess concentrations of Rb, Ba, K, and Sr are attributed to a subduction-related fluid phase. Major and trace element concentrations, coupled with Sr–Nd–Pb isotope signatures suggest that the bulk composition of sediments being subducted below southern Italy is similar to that of the carbonate rich sediment columns described by Plank and Langmuir (1998) and Vroon et al. (1995). Furthermore, it appears that the sediment contribution was introduced as a partial melt, which would account for some geochemical patterns, such as ¹⁴³Nd/¹⁴⁴Nd versus Th/Ce. The EC–AFC model (Spera and Bohrson, 2001) is then used to track the evolution of Somma-Vesuvius magmas. The results are consistent with the melting of crustal Hercynian basement at depths of 12 and >20 km (De Natale et al., 2001). Such a model is also consistent with the thermal model of Annen and Sparks (2002) for the evolution of magmatic provinces. Here, magmas from the upper mantle form a melt intrusion and storage zone at 12 to >20 km allowing for crustal melting to take place. At Vesuvius, Plinian eruptions involve the first magma withdrawn from a deep magma reservoir. Interplinian eruptions involve reduced volumes of magma stored over a larger depth range until the volcanic activity stops. This suggests that little magma is left in the melt intrusion and storage zone. A new cycle is started by a Plinian event when new magma rises from the upper mantle and is emplaced in the lower crust.     

Present condition of Mt. Elbrus volcano

The Freetown layered complex, located on the western coast of Sierra Leone, is a rift-related tholeiitic intrusion associated with the Jurassic (～193 Ma) opening of the Atlantic Ocean at midlatitude. The complex is ～ 60 km long, 14 km wide, and 7 km thick along a major E-W traverse extending from Waterloo to York. Gravity data and dips of laminations in the layered rocks suggest that the intrusive complex is lopolithic in shape, with some parts presently being submarine.

The exposed rocks consist of a rhythmically layered sequence of troctolite, olivine gabbro, gabbronorite, gabbro, and anorthosite. The complex has been divided into four zones delineated by (1) topographic expression, whereby the base of each zone forms a scarp, and the top forms dip slopes and strike valleys; and (2) cyclical repetition of rock types (Wells, 1962). A new detailed stratigraphic section along the Waterloo-York traverse is presented, in which Zone 3 is subdivided into an upper 2000-m-thick anorthosite-gabbro interval and a lower 1700-m-thick rhythmically layered subzone.

Inverted pigeonite first became a cumulus phase at the bottom of Zone 2, before disappearing near the middle of Zone 3 at the anorthosite-gabbro interval, only to reappear at the top of Zone 4 with cumulus titanomagnetite. Mineral compositions in the complex range from An₇₂ to An₇₂ plagioclase, Fo₅₆ to Fo₇₅ olivine, En_38.5 to En_44.8 augite, and En_54.9 to En_74.6 orthopyroxene. The compositions of plagioclase and olivine in Zone 2 vary irregularly, although the overall trend is toward reverse differentiation. By contrast, Zone 4 is characterized by a rapid decrease in Fo and An from the base of the zone upward, followed by an increase. Cryptic variation also is shown by the Ni content of olivine and Cr content of clinopyroxene.

The overall pattern of cryptic variation in the complex suggests continual leakage of fresh magma into the chamber, followed by oscillatory spikes in the rhythmically layered subzone of Zone 3, where major influxes of new magma occurred. The changes in mineral compositions and modal abundances as a function of stratigraphic height are the result of magma recharge, followed by mixing of new and evolved resident magmas in the Freetown magma chamber. This probably resulted in the expansion of the chamber and crystallization in situ without any discharge. The inferred crystallization sequence for each zone is different, reflecting different magma compositions and changes that occur in the magmas during crystallization. The alternative hypotheses that the Freetown Complex formed from a single parental magma, or that mineral layering was the result of the crystallization sequence Fe-Ti oxides→olivine→pyroxene→plagioclase, are not supported by the evidence.     

Genesis of alkalic rocks in Ural

As a rule, infiltrational metasomatism in axial parts of geosynclines terminates in the formation of a eutectic granitic magmatic solution. However, in case such solutions fail to develop while the accessions of the alkalic solutions continue, a miaskitic magmatic solution may arise, the parent of miaskites. Thus the crystallization of miaskites is contingent upon a progressive local metamorphism, in geologically rare and special environments in which the substratum acquires gradually a eutectoid, i.e. the miaskitic. The widely different series of intrusive alkalic rocks, from nephelinolites and alkalic gabbroids to plagioclase and corundum syenites, are the result of their reactions with their hosts, the mechanisms and the types of which depend upon the geological-structural conditions and other variables. – V. P. Sokoloff.     

The behavior of chlorine and sulfur during differentiation of the Mt. Somma-Vesuvius magmatic system

Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H₂O, S, Cl, F, B, and P₂O₅. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO₂, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P₂O₅, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H₂O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena. Received January 10, 2000 Revised version accepted July 17, 2000     

Tectonic conditions of origin of alkalic rocks in East Tuva

The following topics are discussed: 1) position of, alkalic intrusions in various structural and facies zones of East Tuva and its effect on the composition and form of the intrusions; 2) the close genetic relationship between the alkalic massifs and deep faults, including those approaching that rank. It is demonstrated that fault tectonics and its associated alkalic volcanism control the areal distribution, dimensions, form and structural complexity of the alkalic massifs; also the distribution of autometasomatism, namely albitization, associated with which is the rare-metal mineralization of East Tuva alkalic complex. — Author.     

Discussion of REE distribution patterns of carbonatites and alkalic rocks

Analytical data on REE, Sr, Ba, and Na of carbonatites, their calcite separates, and associated alkalic rocks from Fen/Norway, Alnö/Sweden and Kaiserstuhl/Germany are discussed. The REE distribution patterns of carbonatites suggest sympathetic behaviour with those of their associated alkalic rocks. This could be interpreted as the result of liquid/liquid separation. The typically steep patterns may be the result of Ca-metasomatism of alkaline-earth rich, but alkali-poor liquids with olivine-rich, alkali bearing, subcrustal rocks. Additionally, during this process incompatible elements could become concentrated in the carbonate rich liquid.     

Pleistocene to recent alkalic volcanism in the region of Sanganguey volcano,Nayarit, Mexico

Forty five cinder cones and associated lava flows have erupted within the last 300,000 years along five parallel lines through the calc-alkaline volcano, Sanganguey, in the northwestern segment of the Mexican Volcanic Belt. Lavas erupted from these cinder cones include ne- and hynormative alkali basalts, hawaiites, mugearites, and benmoreites. It is unusual that this suite has erupted in a calc-aikaline volcanic belt where volcanoes in the vicinity have been erupting calc-alkaline andesites, dacites and rhyodacites.Incompatible trace elements such Ba, Rb, Sr, and LREEs show little change with decreasing Mg, Ni, and Cr in the series alkali basalt to hawaiite, suggesting that simple crystal fractionation of observed phenocrysts has not been the dominant process in the derivation of the hawaiites from the alkali basalts. Petrographic evidence of magma mixing along with observed variation of trace element abundances suggests that the alkali basalts might represent mixtures of primitive magma with more evolved compositions.Crystal fractionation is capable of explaining major and most trace element trends in the series hawaiite — mugearite — benmoreite. However, such a process could only occur at pressure because of the requirement that clinopyroxene be a major crystallizing phase.The anomolous association of alkaline magmatism contemporaneously with calc-alkaline magmatism is probably related to the complex tectonic history associated with the rearrangement of plate boundaries in the vicinity of western Mexico.     

Some Xenoliths from the alkalic rocks of Teneriffe,Canary Islands

1. Xenoliths of ultrabasic, ultramafic, gabbroic or syenitic type occur in Teneriffe: dunites and clino-pyroxenites in the old alkalic basalt formations of Teno and Anaga peninsulas; gabbroic xenoliths in the Pedro Gil region; nepheline-syenite xenoliths in the Las Canadas and Vilaflor regions where intermediate and phonolitic lavas are abundant; ultramafic, clino-pyroxenite and syenitic xenoliths in the Anaga peninsula where there are many intrusions of nepheline-syenite and phonolitic syenite. Several xenoliths show signs of cataclasis, recrystallisation or reaction of their minerals with the host liquids.
2. The ultrabasic, ultramafic and anorthoclase-rich xenoliths appear to be of cumulus origin, subtracted from basic to intermediate alkalic liquids. Major cumulus phases are: magnesium-rich olivine, sub-silicic, aluminous pyroxene, titanomagnetite, sub-silicic potassic kaersutite, and anorthoclase. It is suggested that the xenoliths formed at depths between 11 km and 30 km, largely under wet conditions that helped suppress formation of cumulus plagioclase.
3. The subtraction of kaersutite from liquids of intermediate composition is thought to be a means of producing the gap in silica content between the Teneriffe trachybasalts and the more siliceous trachyphonolites and phonolites. It is also suggested that the subtraction of kaersutite and anorthoclase would considerably deplete residual liquids in alumina whilst enriching then in soda and this might be the means of producing peralkaline liquids.
4. The presence of the xenoliths supports the geophysical data that indicated that Teneriffe has a sub-crustal structure of plutonic rocks. Correlation of the Teneriffe plutonic xenoliths with exposed plutonic basement rocks from other Canary Islands, which are believed to have similar sub-crustal structures, is considered necessary.

     

望天鹅火山岩石地球化学特征及其成因

东北吉林望天鹅新生代火山岩（7．04—1．86Ma）主要由玄武粗安岩和碱性流纹岩组成。玄武粗安岩的SiO2介于49．38％-53．31％之间;K2O＋Na2O为4．27％-7．72％;∑REE为163．69×10^-6-258．55×10^-6,Eu异常不明显,δEu为0．72～1．17;碱性流纹岩具高SiO2（70．39％～71．49％）含量,高碱（K2O＋Na2O为9．28％～9．49％）,高稀土总量（∑REE介于309．30×10^-6～465．03×10^-6之间）,明显的Eu负异常,3Eu为0．52～0．71。碱性流纹岩的微量元素含量较玄武粗安岩高,且具有明显的Sr、P、Ti负异常。望天鹅火山岩的^87Sr／^86Sr比值在0．705156-0．709029之间,而^144Nd／^143Nd比值变化较小,介于0．512295～0．512602之间;放射性成因Pb同位素变化范围也较小,^206Ph／^204Pb为17．254～18．090,^207Ph／^204Pb为15．460～15．507,^208Pb／^204Ph为37．278—38．048。综合分析火山岩的主量元素、微量元素和Sr-Nd—Pb同位素特点,初步认为望天鹅火山的玄武粗安岩和碱性流纹岩具有相同的地幔源区。玄武粗安岩起源于微弱富集上地幔,在上升过程中结晶分异形成了碱性流纹岩。     

A Three—Stage Metallogenic Model for Gold Deposits in Metamorphosed Microclastic Rocks

Gld deposits occurring in metamorphosed microcelastic rocks are distributed extensively at home and abroad.Some deposits of this type are of superlarge tonnage.The formation of gold deposits in metamorphosed microclastic rocks involves three stages:the sedimentary stage,the regionally metamorphic stage,and the ore-forming stage.At the first stage,microclastic sedimentary source rocks were developed in a relatively semi-enclosed reducing sea basin and were enriched in carbon,sulfur and gold.At the second stage,the gold adsorbed on organic matter and clay minerals was relesed and poorly concentrated during the destruction of organic matter and the depletion of clay minerals by regional metamorphism with increase temperature and pressure.At the third stage,a tectono-hydrothermal event took place.As a result,gold was leached from metamorphosed microclastic rocks,transported to ore depositional locus and/or mixed with gold of other sources in the course of migration,and finally precipitared as ores.Gold deposits of this type were eventually formed at the third stage,and they also can be classified as the orogenic belt type and the activation zone type.The gold deposits occurring in metamorphosed microcalastic rocks are the products of reworking processes and the influence of magmatism should be taken into consideration in some cases.     

Major, trace element and isotope geochemistry (Sr-Nd-Pb) of interplinian magmas from Mt. Somma-Vesuvius (Southern Italy)

Summary Major, trace element and isotopic (Sr, Nd, Pb) data are reported for representative samples of interplinian (Protohistoric, Ancient Historic and Medieval Formations) activity of Mt. Somma-Vesuvius volcano during the last 3500 years. Tephra and lavas exhibit significant major, trace element and isotopic variations. Integration of these data with those obtained by previous studies on the older Somma suites and on the latest activity, allows to better trace a complete petrological and geochemical evolution of the Mt. Somma-Vesuvius magmatism. Three main groups of rocks are recognized. A first group is older than 12.000 yrs, and includes effusive-explosive activity of Mt. Somma. The second group (8000–2700 yrs B.P.) includes the products emitted by the Ottaviano (8000 yrs. B.P.) and Avellino (3550 yrs B.P.) plinian eruptions and the interplinian activity associated with the Protohistoric Formation. Ancient Historic Formation (79–472 A.D.), Medieval Formation (472–1139 A.D.) and Recent interplinian activity (1631–1944 A.D.) belong to the third group of activity (79–1944 A.D.). The three groups of rocks display distinct positive trends of alkalis vs. silica, which become increasingly steeper with age. In the first group there is an increase in silica and alkalis with time, whereas an opposite tendency is observed in the two younger groups. Systematic variations are also evident among the incompatible (Pb, Zr, Hf, Ta, Th, U, Nb, Rb, Cs, Ba) and compatible elements (Sr, Co, Cr). REE document variable degrees of fractionation, with recent activity displaying higher La/Yb ratios than Medieval and Ancient Historic products with the same degree of evolution. N-MORB normalized multi-element diagrams for interplinian rocks show enrichment in Rb, Th, Nb, Zr and Sm (> ^*10 N-MORB). Sr isotope ratios are variable, with Protohistoric rocks displaying ⁸⁷Sr/⁸⁶Sr =  0.70711–0.70810, Ancient Historic ⁸⁷Sr/⁸⁶Sr = 0.70665–0.70729, and Medieval ⁸⁷Sr/⁸⁶Sr = 0.70685–0.70803. Neodymium isotopic compositions in the interplinian rocks show a tendency to become slightly more radiogenic with age, from the Protohistoric (¹⁴³Nd/¹⁴⁴Nd = 0.51240–0.51247) to Ancient Historic (¹⁴³Nd/¹⁴⁴Nd = 0.51245–0.51251). Medieval interplinian activity (¹⁴³Nd/¹⁴⁴Nd: 0.51250–0.51241) lacks meaningful internal trends. All the interplinian rocks have virtually homogeneous compositions of ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb in acid-leached residues (²⁰⁷Pb/²⁰⁴Pb ∼15.633 to 15.687, ²⁰⁸Pb/²⁰⁴Pb ∼38.947 to 39.181). Values of ²⁰⁶Pb/²⁰⁴Pb are very distinctive, however, and discriminate among the three interplinian cycles of activity (Protohistoric: 18.929–18.971, Ancient Historic: 19.018–19.088, Medieval: 18.964–19.053). Compositional trends of major, trace element and isotopic compositions clearly demonstrate strong temporal variations of the magma types feeding the Somma-Vesuvius activity. These different trends are unlikely to be related only to low pressure evolutionary processes, and reveal variations of parental melt composition. Geochemical data suggest a three component mixing scheme for the interplinian activity. These involve HIMU-type and DMM-type mantle and Calabrian-type lower crust. Interaction between these components has taken place in the source; however, additional quantitative constraints must be acquired in order to better discriminate between magma characteristics inherited from the sources and those acquired during shallow level evolution. Received May 5, 2000; revised version accepted June 19, 2001     

Petrogenesis of alkalic and calcalkalic volcanic rocks of Mormon Mountain Volcanic Field,Arizona

The Cenozoic Mormon Mountain Volcanic Field (MMVF) of northern Arizona is situated in the transition zone between the Basin and Range and the Colorado Plateau. It is composed of alkalic to sub-alkalic basalts and calcalkalic andesites, dacites, and rhyodacites. Despite their spatial and temporal association, the basalts and the calcalkalic suite do not seem to be co-genetic. The petrogenesis of primitive MMVF basalts can be explained as the result of different degrees of partial melting of a relatively homogenous, incompatible element-enriched peridotitic source. The variety of evolved basalt types was the result of subsequent fractional crystallization of olivine, spinel, and clinopyroxene from the range of primitive basalts. Crustal contamination seems to have occurred, but affected only the highly incompatible element abundances. The formation of MMVF calcalkalic rocks is most readily explained by small to moderate amounts of partial melting of an amphibolitic lower crust. This source is LREE-enriched but depleted in Rb and relatively unradiogenic Sr (⁸⁷Sr/⁸⁶Sr 0.7040). Calcalkalic rhyodacites may also be derived from andesitic parents by fractional crystallization. The overall petrogenesis of the MMVF complex is the result of intra-plate volcanism where mantle-derived magmas intrude and pass through thick continental crust.     

Late Pleistocene glaciation of the Mt Giluwe volcano,Papua New Guinea

The Mt Giluwe shield volcano was the largest area glaciated in Papua New Guinea during the Pleistocene. Despite minimal cooling of the sea surface during the last glacial maximum, glaciers reached elevations as low as 3200 m. To investigate changes in the extent of ice through time we have re-mapped evidence for glaciation on the southwest flank of Mt Giluwe. We find that an ice cap has formed on the flanks of the mountain on at least three, and probably four, separate occasions. To constrain the ages of these glaciations we present 39 new cosmogenic ³⁶Cl exposure ages complemented by new radiocarbon dates. Direct dating of the moraines identifies that the maximum extent of glaciation on the mountain was not during the last glacial maximum as previously thought. In conjunction with existing potassium/argon and radiocarbon dating, we recognise four distinct glacial periods between 293–306 ka (Gogon Glaciation), 136–158 ka (Mengane Glaciation), centred at 62 ka (Komia Glaciation) and from >20.3–11.5 ka (Tongo Glaciation). The temperature difference relative to the present during the Tongo Glaciation is likely to be of the order of at least 5 °C which is a minimum difference for the previous glaciations. During the Tongo Glaciation, ice was briefly at its maximum for less than 1000 years, but stayed near maximum levels for nearly 4000 years, until about 15.4 ka. Over the next 4000 years there was more rapid retreat with ice free conditions by the early Holocene.     

Role of deep fractures in the formation of alkalic rocks genetically related to granite magma

Potentially favorable environments for the development of alkalic rocks, as products of infiltrational magmatic replacements of carbonates in upper part of the section, are indicated by analysis of processes in endo- and exo-contacts of a granitic intrusion, in zones of deep fractures, where the rocks were subjected to fragmentation and mylonitization. The phenomenon is contingent upon an enrichment of volatile and alkalic substances in the tops of the granitic intrusion and a development of advancing magmatic solutions in front of the intrusion. -- IGR Staff.     

Gas hazard assessment at the Monticchio crater lakes of Mt. Vulture, a volcano in Southern Italy

Geochemical investigations have shown that there is a considerable inflow of gas into both crater lakes of Monticchio, Southern Italy. These lakes are located in two maars that formed 140 000 years ago during Mt. Vulture volcano's last eruptive activity. Isotopic analyses suggest that CO₂ and helium are of magmatic origin; the latter displays ³He/⁴He isotope ratios similar to those measured in olivines of the maar ejecta. In spite of the fact that the amount of dissolved gases in the water is less than that found in Lake Nyos (Cameroon), both the results obtained and the historical reports studied indicate that these crater lakes could be highly hazardous sites, even though they are located in a region currently considered inactive. This could be of special significance in very popular tourist areas such as the Monticchio lakes, which are visited by about 30 000 people throughout the summer, for the most part on Sundays.     

A strontium isotopic study of the volcanic rocks of the Myoko volcano group,central Japan

Thirty-one selected volcanic rocks from the Myoko volcano group which comprises a volcanic chain of four independent volcanoes of Quaternary to Recent age are analyzed for ⁸⁷Sr/⁸⁶Sr ratios. The rocks of the lizuna volcano, the oldest among the Myoko volcano group, have higher ⁸⁷Sr/⁸⁶Sr ratios and show a larger scatter ranging from 0.7043₇ to 0.7055₆ than those of other volcanoes. The Kurohime volcanic rocks have a restricted range of ⁸⁷Sr/⁸⁶Sr ratios (0.7040₃∼0.7043₅). ⁸⁷Sr/⁸⁶Sr ratios of the Myoko volcanic rocks are almost the same in average to those of the Kurohime volcanic rocks, although somewhat varied ranging from 0.7037₈ to 0.7046₁. A single analysis of the Yakeyama volcanic rock yielded a ⁸⁷Sr/⁸⁶Sr ratio of 0.7042₇. A characteristic pattern in ⁸⁷Sr/⁸⁶Sr ratios is observed through the volcanic activity of the Myoko volcano group; ⁸⁷Sr/⁸⁶Sr ratios are high in the early stage of the volcanic activity and then decrease to low values, the late eruptives being characterized by constant ⁸⁷Sr/⁸⁶Sr ratios. The negative correlation between ⁸⁷Sr/⁸⁶Sr and Rb/Sr, and positive correlation between ⁸⁷Sr/⁸⁶Sr and Sr found in the rocks of the Iizuna volcano are interpreted to show the occurrence of contamination by materials with high ⁸⁷Sr/⁸⁶Sr ratios (>0.7056), low Rb/Sr ratios (<0.01) and high Sr contents (>300 ppm). Sialic crustal contamination may have played only a minor role.