Kálmán Török

Significance of silicate melt pockets in upper mantle xenoliths from the Bakony–Balaton Highland Volcanic Field, Western Hungary

Abstract Silicate melt pockets with or without carbonate occur in 10% of upper mantle xenoliths from the alkali basalts of the Bakony–Balaton Highland Volcanic Field (BBHVF), Western Hungary. Based on the estimated bulk composition of the melt pockets, both the carbonate-free and the carbonate-bearing ones are considered to be the result of the reaction between primary mantle clinopyroxene and/or amphibole and external CaO, Al2O3, alkali-rich and MgO-poor fluids/melts, as metasomatic agents, migrating in the upper mantle. The metasomatic melt that produced the carbonate-bearing melt pockets was extremely rich in volatiles, whereas metasomatic melt that contributed to the formation of the carbonate-free melt pockets was particularly rich in silica and relatively poor in volatiles. These metasomatizing melts could have originated from the melting of the previously metasomatized upper mantle due to Middle Miocene mantle diapirism.

Pre-eruptive volatile content, melt-inclusion chemistry, and microthermometry of interplinian Vesuvius lavas (pre-A.D. 1631)

Silicate-melt inclusions from lavas and pyroclastics from a selected suite of pre-A.D. 1631 interplinian Mt. Somma-Vesuvius lavas and scoria have been experimentally homogenized and studied by microthermometry, electron microprobe (EMPA) and secondary-ion mass spectrometry (SIMS) to examine pre-eruptive volatile content and magma evolution. The melt inclusions have a bubble about 0.06% their volume, uncommonly contain non-condensable gas but do not contain any dense fluid phases. Clinopyroxene-hosted inclusions yield homogenization temperatures (T h ) from 1170 to 1260°C, most between 1220 and 1240°C; plagioclase-hosted inclusions have T h from 1210 to 1230°C; these values are typical for the Vesuvius environment. The dominant factor controlling major element variability in the inclusions is clinopyroxene fractionation; MgO varies from 5 to 3 wt%, SiO 2 varies from 60 to 48 wt%, total alkalis vary from 15 to 4 wt%, and CaO varies from 13 to 5 wt%. H 2 O varies from 2.7 to 0.6 wt% and is decoupled from incompatible element evolution suggesting vapor saturation during trapping. Chlorine and F vary from 1.0 wt% to 0 and 0.63 to 0 wt%, respectively. Bulk rock and limited matrix glass analyses show that the lavas lost about half of their F and Cl content except for the A.D. 472-1631 lava which contains similar Cl abundances as the bulk rock. SO 3 varies from 0.5 to 0 wt% and compared with matrix glass and bulk rock demonstrate that the lavas have lost essentially all sulfur. The samples can be classified into three age groups, > 25,000 yr B.P., 25,000-17,000 yr B.P., and A.D. 472-1631. There is a systematic increase in some components, e.g., total alkalis, SO 3 , Cl, Li, B, and Sr with the youth of the sample and a decrease in others, e.g., Zr and Y. However, on average these samples seem less evolved than later A.D. 1631-1944 lavas.

Fluid inclusion evidence for magmatic silicate/saline/CO2 immiscibility and geochemistry of alkaline xenoliths from Ventotene Island, Italy

Abstract Fluid and melt inclusions and geochemical features of alkali syenite, mafic, and ultramafic cumulate xenoliths in the last ignimbritic event (volcanism up to 300 ky.b.P.) at the island of Ventotene in the Pontine archipelago (Gaeta Gulf) were investigated to establish the genesis and evolution of the fluids trapped in the inclusions. Ranges in lead isotopic compositions of the xenoliths as a group are narrow: 206Pb/204Pb:18.778–18.864; 207Pb/204Pb:15.641–15.701; 208Pb/204Pb:38.858–39.090; the values overlap among the groups, implying that the xenoliths are closely related. The xenoliths straddle the best-fit line describing the regional NW-SE variation of values for uranogenic Pb in volcanic rocks from the Roman alkaline province. The similarity between the xenoliths and volcanic rocks permits the interpretation that the xenoliths are representative of the source region of the volcanic rocks, residues after partial melting of the source region or fractional crystallization of the volcanic rocks, or even that the xenoliths represent assimilants obtained during the evolution of the magmas that produced the volcanic rocks. Xenoliths belonging to the ultramafic-mafic cumulate group contain only silicate melt inclusions ± vapor bubble ± droplets of an opaque phase and rarely some CO2 trapped in silicate melt inclusions. Xenoliths in the alkali syenite group have three types of fluid inclusions: (1) single phase vapor and silicate melt inclusions; (2) two-phase silicate melt + salt, silicate melt + CO2 (V), aqueous (L + V), and silicate melt + vapor inclusions, and (3) three-phase and multiphase inclusions: CO2 (L) + CO2 (V) + H2O; silicate melt + saline melt + H2O ± birefringent or opaque trapped minerals; H2O + salt + silicate glass ± birefringent trapped minerals. During heating experiments, melting of salt occurs at temperatures from 565 to 815°C, depending on the water content of the inclusions. Homogenization of the vapor bubble occurs from 850–1160°C, and complete melting of the silicate glass at about 950°C. The highly variable proportions of the individual phases in the silicate melt + salt + H2O inclusions and the coexistence of silicate melt + CO2 inclusions indicates immiscibility during the crystallization of the magma. Primary and secondary CO2 inclusions in the alkali syenite suite indicate lower densities (from 0.10 to 0.22 g/cm3) than those resulting from primary CO2 inclusions in the gabbroic suite (from 0.34 to 0.42 g/cm3). The P-T trajectory of the probable fluid evolution path shows that the crystallization of gabbro occurred between ∼1 and 1.4 kbars, whereas alkali syenite crystallized between ∼200 and 400 bars. The secondary H2O inclusions in alkali syenite were trapped in the later stages of the hydrothermal process and at much lower temperatures (130–290°C), but at pressures relatively close to those of alkali syenite crystallization. The almost isobaric conditions during the final stage of the fluid evolution path are explained by the very shallow emplacement of the alkali syenite intrusive body.

Sr-barite droplets associated with sulfide blebs in clinopyroxene megacrysts from basaltic tuff (Szentbékkálla, western Hungary)

Abstract A unique occurrence of strontian barite droplets associated with oriented planes of primary sulfide blebs in several clinopyroxene megacrysts was found in the Plio-Pleistocene basaltic tuff of Szentbekkalla (Bakony–Balaton Highland, western Hungary). The barite droplets contain 3.22–14.7 wt.% SrO, variable amount of FeO (up to 3.25 wt.%), minor CaO, CuO and NiO. Textural arrangements of oriented sulfide planes show that they were trapped as immiscible sulfide melt during crystallization of the clinopyroxene. Temperature and pressure estimation (978–1094 °C and 1.0–1.1 GPa) of the host clinopyroxenes indicate that formation of clinopyroxene megacrysts and entrapment of sulfide blebs happened under upper mantle conditions. A genetic relationship between the sulfate and sulfide blebs is doubtful. Textural features suggest that the barite droplets might have been trapped together with the sulfide blebs from immiscible sulfide melts. However, it cannot be excluded that the barite formed during metasomatic event following the sulfide formation.

Understanding Vesuvius magmatic processes: Evidence from primitive silicate-melt inclusions in medieval scoria clinopyroxenes (Terzigno formation)

SummaryMicrothermometric investigations of silicate-melt inclusions and electron microprobe analyses were conducted on experimentally homogenized silicate-melt inclusions and on the host clinopyroxenes from 4 scoria samples of different layers from the Mt. Somma-Vesuvius medieval eruption (Formazione di Terzigno, 893 A.D.). The temperature of homogenization, considered the minimum trapping temperature, ranges from 1190 to 1260 ± 5 °C for all clinopyroxene-hosted silicate melt inclusions.The major and minor-element compositional trends shown by Terzigno scoria and matrix glass chemical analysis are largely compatible with fractional crystallization of clinopyroxene and Fe-Ti oxides. Sulfur contents of the homogenized silicate-melt inclusions in clinopyroxene phenocrysts compared with that in the host scoria show that S has been significantly degassed in the erupted products; whereas, Cl has about the same abundance in the inclusions and in host scoria. Fluorine is low (infrequently up to 800 ppm) in the silicate-melt inclusions compared to 2400 ppm in the bulk scoria.Electron microprobe analyses of silicate-melt inclusions show that they have primitive magma compositions (Mg# = 75-91). The composition of the host clinopyroxene phenocrysts varies from typical plinian-related (Mg#≥85) to non plinian related (Mg#≤85). The mixed source of the host clinopyroxenes and primitive nature of the silicate-melt inclusions implies that these phenocrysts, in part, may be residual and/or have a polygenetic origin. The similar variation trends of major and minor-elements between homogenized silicate-melt inclusions from the Terzigno scoria, and silicate-melt inclusions in olivine and diopside phenocrysts from plinian eruptions (Marianelli et al., 1995) suggest that the trapped inclusions represent melts similar to those that supplied the plinian and sub-plinian magma chambers. These geochemical characteristics suggest that the Vesuvius magmatic system retained a vestige of the most recent plinian event.ZusammenfassungMikrothermometrische Untersuchungen von Silikatschmelz-Einschlussen und EMS Analysen wurden an experimentell homogenisierten Silikatschmelz-Einschlussen und an Klinopyroxenen von vier Schlackenproben durchgeführt. Die Proben stammen aus verschiedenen Lagen der mittelalterlichen Mt. Somma-Vesuv Eruption (Terzigno Formation, 893 n. chr.). Die Homogenisierungstemperaturen, sie werden als die minimalen Einschlußtemperaturen interpretiert, reichen von 1190–1260±5 °C.Die Trends der Haupt- and Spurenelemente der Terzigno Schlacken and des MatrixGlases sind weitgehend mit fraktionierter Kristallisation von Klinopyroxen und Fe-TiOxiden kompatibel. Der Vergleich der Schwefel-Gehalte der homogenisierten Einschlüsse in den Klinopyroxen-Phdnokristallen mit denen der Schlacken zeigt, daß beträchtliche Mengen an Schwefel in den Eruptionsprodukten durch Entgasung verlorengingen. Chlor hingegen zeigt in etwa die gleiche Verteilung in den Einschlüssen und in der Wirtsschlacke. Die Fluorgehalte in den Einschlüssen (selten bis zu 800 ppm) Bind im Vergleich zu den Schlacken (2400 ppm) niedrig.EMS-Analysen der Silikatschmelz-Einschlüsse belegen eine primitive Schmelzzusammensetzung (#Mg = 75–91). Die Zusammensetzung der Klinopyroxen-Phänokristalle variiert zwischen plinianischen Typen (#Mg ≥ 85) und nicht-plinianischen (#Mg≤85). Die heterogene Quelle der Klinpyroxene and die primitive Natur der Einschlüsse lassen vermuten, daß die Phänokristalle residualen und/oder polygenetischen Ursprungs sind. Ähnliche Haupt- und Spurenelementtrends der untersuchten Einschlüsse mit jenen von Einschlüssen in Olivin- und Diopsid-Phänokristallen aus plinianischen Eruptionen (Marianelli et al., 1995) legen die Vermutung nahe, daß die Einschlüsse ähnliche Schmelzen repräsentieren, wie jene, die die plinianischen und die sub-plinianischen Magmenkammern versorgt haben. Diese geochemischen Charakteristika belegen, daß das magmatische System des Vesuvs eine Spur des jüngsten plinianischen Ereignisses bewahrt hat.

Composition and origin of nodules from the ≈20 ka Pomici di Base (PB)-Sarno eruption of Mt. Somma — Vesuvius, Italy

Nodules (coarse-grain “plutonic” rocks) were collected from the ca. 20 ka Pomici di Base (PB)-Sarno eruption of Mt. Somma-Vesuvius, Italy. The nodules are classified as monzonite-monzogabbro based on their modal composition. The nodules have porphyrogranular texture, and consist of An-rich plagioclase, K-feldspar, clinopyroxene (ferroan-diopside), mica (phlogopite-biotite) ± olivine and amphibole. Aggregates of irregular intergrowths of mostly alkali feldspar and plagioclase, along with mica, Fe-Ti-oxides and clinopyroxene, in the nodules are interpreted as crystallized melt pockets.Crystallized silicate melt inclusions (MI) are common in the nodules, especially in clinopyroxenes. Two types of MI have been identified. Type I consists of mica, Fe-Ti-oxides and/or dark green spinel, clinopyroxene, feldspar and a vapor bubble. Volatiles (CO2, H2O) could not be detected in the vapor bubbles by Raman spectroscopy. Type II inclusions are generally lighter in color and contain subhedral feldspar and/or glass and several opaque phases, most of which are confirmed to be oxide minerals by SEM analysis. Some of the opaque-appearing phases that are below the surface may be tiny vapor bubbles. The two types of MI have different chemical compositions. Type I MI are classified as phono-tephrite — tephri-phonolite — basaltic trachy-andesite, while Type II MI have basaltic composition. The petrography and MI geochemistry led us to conclude that the nodules represent samples of the crystal mush zone in the active plumbing system of Mt. Somma-Vesuvius that were entrained into the upwelling magma during the PB-Sarno eruption.

Alpine P-T path of micaschists and related orthogneiss veins near Óbrennberg (W-Hungary, Eastern Alps)

High-pressure metapelites with the assemblage kyanite-staurolite-garnet ′ chloritoid-biotite-muscovite-plagioclase-quartz and crosscutting gneiss veins were studied in the Sopron Mountains near Obrennberg (Hungary, Eastern Alps). Different garnet generations were investigated by textural analysis and study of garnet zoning from different textural domains from both the micaschist and the gneiss veins. Thermobarometry of subsequent garnet generations and the surrounding mineral assemblage was used to reconstruct the Alpine P-T path of the metamorphic rocks during the Alpine orogenesis. Peak pressure conditions (1390-1470 MPa) are recorded by high-Si phengites (Si = 7.03 a.p.f.u.) and temperatures by white mica-garnet-albite-quartz assemblage at about 550-600 °C. This is in good agreement with the peak pressures obtained from the massive orthogneisses of the area. Gneiss veins crosscutting the micaschist show slightly lower peak pressures between 1230 and 1280 MPa at similar temperatures (575-590 °C). Subsequent garnet generations and associated mineral assemblages show decreasing pressure with slightly decreasing temperature. Textural observations, white mica chemistry and comparison of garnet zoning trends provided additional evidence on the formation of leucophyllites during retrograde metamorphism.