Pliocene clays from Aegina (Greece): Reference material for chemical provenance studies on bronze age pottery from the Island

Normally, the use of clays as reference materials in chemical provenance studies of ancient ceramics is complicated due to the original clay paste processing. The primary mixing and/or refining of raw materials during pottery production makes a straightforward comparison of archaeological ceramics with extant geological materials difficult if not impossible in many cases. However, in the case of Pliocene clays from Aegina (Greece), which were examined chemically and mineralogically and compared with Bronze Age pottery produced on the island, a successful exception can be presented. The chemical composition of a large group of Aeginetan pottery resembles the chemical composition of clays from a deposit in close vicinity to the main Bronze Age settlement of the island. Clays from specific outcrops exhibit considerable chemical and mineralogical homogeneity, and the suitability of those clays for pottery production apparently made substantial clay paste processing unnecessary. © 2004 Wiley Periodicals, Inc.     

Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy)

The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions (MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H₂O, CO₂, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H₂O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection of H₂O, CO₂ and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO₂ degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition of the residual liquid towards higher Cl solubilities. Received: 28 October 1999 / Accepted: 21 April 2000     

Archaeology,geochemistry, and trade of Igneous Rock millstones in Cyprus during the late bronze age to Roman periods

Millstones used in Cyprus between the Late Bronze Age and Roman periods (saddle querns, reciprocally operated hopper-rubber mills, cylindrical rotary querns, and hour-glass-shaped Pompeian style mills) were frequently manufactured from igneous rocks including vesicular lavas clearly imported to the island. Potential sources include volcanics of mainland Greece, the Aegean, Anatolia, Egypt, and the Levant (Syria, Israel, and Jordan). Geochemical analysis using X-ray fluorescence of thirty-seven Cypriot millstones showed that sources were used as follows: Late Bronze Age saddle querns were made of Levant lavas and of local Troodos rocks, the later hopper-rubbers were imported from the Aegean island of Nisyros, a rotary mill was of Santorini lava, and the Roman Pompeian style mills were manufactured from Levant lavas, including sources in north Syria and probably around Lake Tiberias. Millstones were probably exported from the Levant ports of Akko, Tell Abu Hawam, and Laodicea, and imported to the Cypriot towns of Salamis and Nea Paphos.     

Geological and archaeological chronology of a late Holocene coastal enclosure: The Sanguinet lake (SW France)

Sanguinet lake is separated from the Atlantic Ocean by a wide Holocene coastal dunes system in SW France. The present day lake level is 21 m above mean sea level (msl). It formed when aeolian sand closed the mouth of the small La Gourgue river which gradually became a lagoon and then a lake. Dated sub‐lacustrine archaeological remains (human settlements, canoes, and wooden architectural structures), as well as paleoenvironmental evidence (drowned tree stumps and lagoonal deposits exposed on the beach) are used to interpret the formation and chronology of lake level rise during the past 4000 years. Around 2000–1650 B.C., the river flowed into a lagoon or an estuary which connected with the ocean west of the present Sanguinet Lake. Its level was affected by the tide, which ranged between 2 m below and 3 m above msl. The accumulation of aeolian sand before 1500–1000 B.C. began to close the connection with the sea. At this time, the elevation of the surface of the lake water was approximately 5 m above msl, but it still remained connected to the ocean. Around 1000 B.C., the lake level rose quickly by 1 to 2 m during a period of renewed mobility of the coastal aeolian sand, and continued to rise slowly until about 100 A.D. when there was a gradual closure of the lake outlet. This rise forced people who were living on the lake shore and along the rivers to move to higher land along the valley. The nearby Gallo‐Roman site of Losa was settled at the end of the 1st century B.C.; then the final blocking of the outlet occurred because of spit growth as a result of north‐south littoral drift accompanied by the deposit of aeolian sand. This led to the lake level rising rapidly. Consequently, Losa was abandoned in the 3rd century A.D. and ruins of its temple (at 17 m above msl) were submerged in the 6th century. Further oscillations of the lake level probably correspond to water table fluctuations before it became stable at around 1000 A.D. The highest lake level (23.35 m) was reached during the 18th century as a consequence of modern dune formation, and thus was artificially reduced to 21 m in 1840 by construction of an overflow channel. © 2008 Wiley Periodicals, Inc.     

Deposits and physical properties of pyroclastic density currents during complex Subplinian eruptions: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy

Small-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using differaent methods for massive and stratified deposits. The calculated mobility of the flows was 0·2 to 0·3, in the expected range for small-scale PDCs.     

The AD 365 Crete earthquake and possible seismic clustering during the fourth to sixth centuries AD in the Eastern Mediterranean: a review of historical and archaeological data

Historical and archaeological data are used to test geological claims that, in the fourth to sixth centuries AD, the Eastern Mediterranean experienced an unusual clustering of destructive earthquakes (the ‘Early Byzantine Tectonic Paroxsym’). A review of historical accounts of a notable earthquake at this time, that of 21 July AD 365, indicates that this event destroyed nearly all the towns in Crete and was followed by a tsunami which devastated the Nile Delta. The AD 365 event was also probably responsible for reported or observed destruction in ancient towns of west Cyprus and Libya. This earthquake is most likely to be identified with a Hellenic Arc subduction-zone event of ‘great’ (M>8) magnitude, as testified by up to 9 m of uplift in western Crete dated by previous geological studies to around this time. Historical and archaeological data also support the hypothesis that the fourth to sixth centuries AD was a period of abnormally high seismicity in the Eastern Mediterranean. The high seismicity rates of this period may reflect a reactivation of all plate boundaries in the region (Dead Sea Transform, East Anatolian Fault, North Anatolian Fault, Hellenic Arc, Cyprus Arc Fault).     

Landscape response to the deposition of airfall pyroclastics from large explosive eruptions: An example from the campanian area (Southern Italy)

We present in this work data about a peculiar type of alluvial fans formed exclusively by volcaniclastic material from large explosive eruptions. Each alluvial fan results formed by the superimposition of several sedimentary bodies, each of them formed by quite homogeneous volcaniclastic material. Lithological analyses allow us to correlate each sedimentary body with the emplacement of pyroclastics from Phlegrean Fields or Somma-Vesuvius. The development of these alluvial fans is controlled by three main factors: area of dispersion of pyroclastics, morphological features of the supplying basin (i.e. mean slopes and area) and climate. Finally, we present an assessment of the minimum time needed for the complete remobilization of pyroclastics of Pomici di Base and Greenish eruptions.     

Middle to late Pleistocene record of explosive volcanic eruptions in marine sediments offshore Kamchatka (Meiji Rise,NW Pacific)

This paper presents the first detailed study of a late Pleistocene marine tephra sequence from the NW Pacific, downwind from the Kamchatka volcanic arc. Sediment core SO201-2-40, located on the Meiji Rise ~400 km offshore the peninsula, includes 25 tephras deposited within the last 215 ka. Volcanic glass from the tephras was characterized using single-shard electron microprobe analysis and laser ablation inductively coupled mass spectrometry. The age of tephras was derived from a new age model based on paleomagnetic and paleoclimate studies. Geochemical correlation of distal tephras to Kamchatkan pyroclastic deposits allowed the identification of tephras from the Karymsky, Gorely, Opala and Shiveluch eruptive centers. Three of these tephras were also correlated to other marine and terrestrial sites and hence are identified as the best markers for the north-west Pacific region. These are an early Holocene tephra from the Karymsky caldera (~8.7 ka) and two tephras falling into the Marine Isotope Stage (MIS) 6 glacial time: an MIS 6.4 tephra from Shiveluch (~141 ka) and the MIS 6.5 Rauchua tephra (~175 ka) from Karymsky. The data presented in this study can be used in paleovolcanological and paleoceanographic reconstructions.     

A new late glacial to early Holocene palaeobotanical and archaeological record in the Eastern Pre‐Alps: the Palughetto basin (Cansiglio Plateau,Italy)

A late glacial to early Holocene lacustrine and peat succession, rich in conifer remains and including some palaeolithic flint artefacts, has been investigated in the Palughetto intermorainic basin (Venetian Pre‐Alps). The geomorphological and stratigraphical relationships, ¹⁴C dates and pollen analyses allow a reconstruction of the environmental history of the basin and provide significant insights into the reforestation and peopling of the Pre‐Alps. The onset of peat accumulation is dated to 14.4–14.1 kyr cal. BP, coinciding with reforestation at middle altitudes that immediately post‐dates the immigration of Larix decidua and Picea abies subsp. europaea. Plant macrofossils point to the expansion of spruce about 14.3 kyr cal. BP, so far one of the earliest directly dated in the late glacial period of southern Europe. The previous hypothesis of an early Holocene spruce immigration in the Southern Alps from Slovenia needs reconsideration. Organic sedimentation stopped at the end of the Younger Dryas and was followed by the evolution of hydromorphic soils containing lithic artefacts, anthropic structures and wood charcoal. The typological features of the flint implements refer human occupation of the site to the end of the recent Epigravettian. Charcoals yielded dates either consistent with, or younger than, the archaeological chronology, in the early and middle Holocene. Copyright © 2000 John Wiley & Sons, Ltd.     

Brunovistulian terrane (Bohemian Massif, Central Europe) from late Proterozoic to late Paleozoic: a review

The Brunovistulian terrane represents a microcontinent of enigmatic Proterozoic provenance that was located at the southern margin of Baltica in the early Paleozoic. During the Variscan orogeny, it represented the lower plate at the southern margin of Laurussia, involved in the collision with the Armorican terrane assemblage. In this respect, it resembles the Avalonian terrane in the west and the Istanbul Zone in the east. There is a growing evidence about the presence of a Devonian back-arc at the margin of the Brunovistulian terrane. The early Variscan phase was characterized by the formation of Devonian extensional basins with the within-plate volcanic activity and formation of narrow segments of oceanic crust. The oldest Viséan flysch of the Rheic/Rhenohercynian remnant basin (Protivanov, Andelska Hora and Horní Benesov formations) forms the highest allochthonous units and contains, together with slices of Silurian Bohemian facies, clastic micas from early Paleozoic crystalline rocks that are presumably derived from terranes of Armorican affinity although provenance from an active Brunovistulian margin cannot be fully excluded either. The development of the Moravo–Silesian late Paleozoic basin was terminated by coal-bearing paralic and limnic sediments. The progressive Carboniferous stacking of nappes and their impingement on the Laurussian foreland led to crustal thickening and shortening and a number of distinct deformational and folding events. The postorogenic extension led to the formation of the terminal Carboniferous-early Permian Boskovice Graben located in the eastern part of the Brunovistulian terrane, in front of the crystalline nappes. The highest, allochthonous westernmost flysch units, locally with the basal slices of the Devonian and Silurian rocks thrusted over the Silesicum in the NW part of the Brunovistulian terrane, may share a similar tectonic position with the Giessen–Harz nappes. The Silesicum represents the outermost margin of the Brunovistulian terrane with many features in common with the Northern Phyllite Zone at the Avalonia–Armorica interface in Germany.     

Deconvolution of pyroclastic grain-size spectra for interpretation of transport mechanisms: an application to the AD 79 Vesuvio deposits

A computer code using sequential fragmentation/transport theory was used to deconvolute and characterize a large grain-size data set taken from the AD 79 Vesuvio deposits. The results allow us to interpret transport and deposition processes. Four principal morphological classes of grain-size spectra were recognized in the AD 79 deposits: 1 unimodal distributions with coarse modes and very good sorting; 2 polymodal distributions in which relative fractions of each subpopulation are considerably variable; 3 polymodal distributions, but with one mode greatly prevailing over the other ones; 4 flat spectra in which a large number of size classes show the same loading. Because different eruptive, transport and deposition conditions may have operated on pyroclasts which occur in the same bed, we have assigned grain-size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the textures of the beds. The fragmentation/transport processes considered here occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high-concentration flows (as a fluidized system or one with an extremely high sedimentation rate). Variation in strength and position of modes throughout the entire vertical section of AD 79 products illustrates changes in transport and deposition processes with time. Size spectra from Vesuvio quantitatively demonstrate contemporaneous deposition from fall and surge mechanisms as well as contributions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flow deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the top. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margins. This study supports the model of continuous modification in loadings of several discrete subpopulations during deposition from a single explosive cloud.     

若干地区出土部分商周青铜器的矿料来源研究

本工作采用ICP-AES方法,测试分析了若干地区出土青铜器残片的微量元素,初步探索了它们的矿料来源.将青铜器与铜绿山、铜陵、南陵、中条山、照壁山等先秦矿冶遗址铜锭或铜块的特征微量元素进行比较研究后,发现安徽境内青铜器的铜矿料主要来自长江中下游的古铜矿,而辽西地区和侯马青铜器的铜料则可能来自大井铜矿或其周边铜矿.同时,研...     

A thick dacitic debris flow sequence, Lake of the Woods greenstone terrane, central Canada: resedimented products of Archean vulcanian, plinian and dome-building eruptions

A 4-km thick, vertically dipping, south-facing, homoclinal, intermediate to felsic, volcaniclastic sequence in central Lake of the Woods, is underlain and overlain by subaqueous, tholeiitic basalt flows. Volcaniclastic lithologies comprise a lower tuff-breccia and lapilli-tuff assemblage, an upper turbiditic greywacke formation, and three lenticular, upward-fining, conglomerate formations interspersed within other units. Tuff-breccia and lapilli-tuff are characterized by very thick, poorly defined beds, pronounced heterogeneity, mostly angular blocks and lapilli, and a plagioclase+quartz crystal component that, in places, is incompatible with phenocryst populations in fragments. These appear to be subaqueous debris flow deposits that resulted from multistage mixing, during downslope mass transport, of fragments produced by subaerial phreatomagmatic vulcanian-type eruptions. Anomalous crystal contents represent intermixing of subaerial subplinian or plinian ash. Conglomerate formations comprise very thick, poorly defined beds composed almost entirely of well rounded, fine- to medium-grained trondhjemite. They are also subaqueous debris flow deposits but were apparently derived from wave erosion of large, flank dome complexes that periodically blocked normal transport of volcanic debris from higher on the volcano. The upper greywacke represents a shift in eruptive processes to largely plinian or subplinian eruptions.     

Soil,archaeological, biotic,and climatic relationships for the late holocene of the Wyoming basin: The case of the Garrett Allen (Elk Mountain) site (48CR301)

Soils at the Garrett Allen (Elk Mountain) site (48CR301) along Quealy Spring in Carbon County, Wyoming, indicate nearly continuous, although episodic, meadow soil formation during much of Neoglacial or post-Altithermal time at a mountain-basin edge (elev. 2237 m) of the Wyoming Basin. Seven relict and buried meadow soils now under sagebrush (Haplaquents or Fluvaquents, Haplaquolls, and Argiaquolls) of the T₁ terrace date less than 200 B.P. as well as around 200, 600, 900, 1700, and two soils at greater than 3100 B.P. Composite-superposed soils of two or more of the individual soils are common. Some soils have E horizons and the older ones have intensely gleyed cambic or argillic subsoils. Development of meadow soils and deposition of their respective parent materials seem to coincide with glacial episodes in the region when the climate may have been somewhat humid. Occupations of the meadow by Archaic and younger cultural groups, whose artifacts occur mainly in A(O?) horizons of meadow soils, appear to coincide with these humid–glacial climates when the meadow grew and flourished. Changes in subsistence and settlement of these peoples may have been partly influenced by humid climates during glacial episodes when rangeland and habitat were able to support large human and animal populations. However, four major erosional unconformities separate some soils. These unconformities may coincide with times of lessened glacial activity when the climate may have become drier and the meadow decreased in size. During these times, human groups may have abandoned the meadow, left the region, or only sparsely occupied the plains and basins. The youngest unconformity developed when Quealy Spring cut from T₁ to the T₀ level in the last century. This appears to coincide with sagebrush invasion across portions of the meadow, although a small relict meadow still exists. Sagebrush invasion and downcutting of the drainage may relate to climatic drying in the last century. Fluvents are now forming in alluvium of the T₀ level along the Quealy Spring drainage.     

Geochemical Features of Chondrules from Orlovka Meteorite (H5): Evidence of Melting Precursors

Doklady Earth Sciences - The results of studies of the trace-element contents in olivine and plagioclase of chondrules from equilibrated ordinary chondrite Orlovka (H5) are reported. Olivine is...     

Modelling Geophysical Precursors to the Prehistoric c. AD1305 Kaharoa Rhyolite Eruption of Tarawera Volcano, New Zealand

The 1305 Kaharoa rhyolite eruptive episode is the largest volcanic event(4 km³ magma) to have occurred in New Zealand during the last 1000 years. Proximal areas were devastated by pyroclastic flows, and tephra fell over much of the northern North Island. No eyewitness observations are recorded, but ejecta analyses show that the rhyolite eruptions were primed and triggered by basalt intrusions. This key finding, combined with observations of similar modern eruptions, has allowed construction of a conceptual scenario of the seismic and other activity that likely preceded the Kaharoa episode.The precursory scenario begins at -5 years (before the first eruption). Rising basalt magma intrusions generate deep long-period earthquakes in the lower crust, before intersecting and heating a rhyolite magma body at 6 km depth beneath Tarawera. By -1 year, increased heat flux from the rhyolite magma body had raised temperatures and pressures in the overlying hydrothermal system; generating shallow long-period earthquakes and increased heat flow at the surface. At -2 months, shallow volcano-tectonic earthquake activity intensified, driven by inflation of the rhyolite magma body, with magmatic gas appearing in fumarole discharges. Rapidly accelerating seismicity, ground deformation and surface heat flow occurred in the last few weeks and days, before the initial vent-opening explosions intensified into major plinian eruptions.Effectiveness of the present volcano monitoring system at Tarawera can be evaluated against this scenario. The precursory seismic activity, including the critical deep long-period earthquakes, would be recorded but not accurately located. Similarly, the existing ground deformation monitoring systems would detect early magma chamber inflation, but discrimination from the background tectonic tilting signal would be difficult. Continuous telemetering of geodetic data from existing and additional instruments would be required for any useful monitoring of rapid ground deformation in the final precursory phases.     

Soft-shelled turtles (Trionychidae) from the Bissekty Formation (Late Cretaceous: late Turonian) of Uzbekistan: Shell-based taxa

In this paper we describe previously unpublished trionychid turtle material, consisting of numerous shell fragments, from the Late Cretaceous (late Turonian) Bissekty Formation of the Dzharakuduk locality in Uzbekistan. This material is assigned to two shell-based taxa: Aspideretoides cf. riabinini and “Trionyx” cf. kansaiensis. The material which cannot be confidently attributed to these two taxa is identified as Trionychidae indet. In addition to these shell-based trionychid taxa, the Dzharakuduk turtle assemblage includes two skull-based taxa of trionychids (Khunnuchelys kizylkumensis and Trionychini indet.). The trionychids from the Bissekty Formation are most similar to trionychids from the younger (Santonian – early Campanian) Bostobe Formation of Kazakhstan, represented by three shell-based taxa (Aspideretoides riabinini, Paleotrionyx riabinini and “Trionyx” kansaiensis), and one skull-based taxon (Khunnuchelys sp.). We provide an improved understanding of the subtle similarities and differences between four closely related Cretaceous turtle assemblages of Middle Asia and Kazakhstan.     

Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius

A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’Plinian 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 25–30%of the magma was ejected as white K-phonolitic pumice, and 70–75%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850–900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonolitic‘white’ magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor ‘grey’phono-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 = 1000–1100C); 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 (20–25% H₂O +Cl+F+S), low viscosities[(1+2 10² poises)] and relatively low densities (2500–2600kg/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     

Chemical zoning and crystallization mechanisms in the magma chamber of the Pomici di Base plinian eruption of Somma-Vesuvius (Italy)

Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO₂ = 62.5 wt%) to latite (SiO₂ ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO₂ ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1). Received: 23 March 1998 / Accepted: 11 December 1998