P. Yu. Plechov

Opacitization conditions of hornblende in Bezymyannyi volcano andesites (March 30, 1956 eruption)

The paper is devoted to the conditions under which opacite rims developed around hornblende grains in andesite of the catastrophic eruption (March 30, 1956) of Bezymyannyi volcano, Kamchatka. The opacite rims were produced by a bimetasomatic reaction between hornblende and melt with the development of the following zoning: hornblende → Px + Pl + Ti-Mag → Px + Pl → Px → melt. Biometasomatic reaction was accompanied by the active removal of CaO from the rim, addition of SiO2, and more complicated behavior of other components. The hornblende also shows reactions of its volumetric decomposition under near-isochemical conditions. The opacite rims developed under isobaric conditions, at a pressure of approximately 6 kbar. The main reason for the instability of the hornblende was the heating of the magma chamber from 890 to 1005°C due to new hot magma portion injection. The time interval between the injection and the start of eruption was estimated from the thickness of the opacite rims and did not exceed 37 days. Hence, the March 30, 1956, eruption was not related to the volcanic activity in November of 1955 but to the injection of a fresh magma portion in February–March of 1956.

Formation conditions of allivalites, olivine-anorthite crystal enclaves, in the volcanics of the Kuril-Kamchatka arc

In this paper we address allivalites, coarse-and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka arc. New data are reported on the petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples from Ksudach, Il’inskii, Zavaritskii, Kudryavyi, and Golovnin volcanoes. The crystallization temperatures of allivalite minerals were estimated as 970–1080°C at a melt water content of 3.0–3.5 wt % and oxygen fugacity NNO = 1–2. A genetic link was established between the compositions of melt inclusions and interstitial glasses in allivalites and volcano rocks. The cumulate nature of allivalites was demonstrated. Using mass balance calculations, the degree of fractionation of primary melts during the formation of cumulate layers was estimated for various volcanoes as 22–46%.

Evolution of melt composition during intrusion of basalts into a silicic magma chamber

The article describes heat exchange between basaltic and rhyolite melts accompanied by fractional crystallization of phases in a basaltic melt. A numerical model has been developed for the homogenization mechanism of magma composition during intrusion of basaltic magma batches into felsic magma chambers. The results of numerical modeling demonstrate that the time needed for cooling the basalts and their fractionation to rhyolite melts is much shorter than the time required for chemical interaction based on diffusive mechanisms.

The petrology of the Upper Albian tuffites from the Bakhchysarai district, southwestern Crimea

We present the first data on the composition of plagioclase and quartz-hosted melt inclusions, plagioclase primary magmatic zoning and mineral compositions for the Upper Albian tuffites from the Bakhchysarai district. The estimated composition of the magmatic melts corresponds to the high potassium rhyolites. The measured water content in the melt inclusions reaches 4 wt %. The phenocrysts crystallized at the temperature within the range 820–860°C and under the pressure not less than 0.1 GPa. The Upper Albian tuffites from the Bakhchysarai district were formed in the active continental margin environment.

The Chenka Sandstone Sequence (Lower Jurassic) of the Crimean Mountains: Stratigraphy and Depositional Environments

The position of the Chenka Sequence in the section, original data on its clay minerals, facies and structural parageneses with consideration of volcanic and climatic events, as well as the available published information provide grounds for accepting it as Pliensbachian-Toarcian in age. It is shown that the Chenka sandstones are a facies or group of facies deposited in deltaic-plain or delta-front environments and that it cannot be ranked as an autonomous formation or other stratigraphic unit.

The character of the volcanic activity in southwestern Crimea during the Late Albian

For the first time data on the composition of melt inclusions in plagioclases were obtained. The mineral composition of tuffites in the area of Balaklava and the types of primary-igneous zoning of plagioclases are examined in detail. The eruptions were located at the active continental margin; material transport during the eruption and accumulation took place under submarine coastal conditions. The eruption center was located no more than a few tens of kilometers south of the present-day shoreline. The volcanic activity of the Lower Cretaceous paleovolcanic center in southwestern Crimea was reconstructed.

Magma degassing during 7600 14C Kurile Lake caldera-forming eruption and its climatic impact

The main goal of this investigation is estimating volume of volatile emission, atmospheric and climatic impact of the Kurile Lake caldera-forming eruption, one of the Earth’s largest Holocene explosive eruptions. The volatile content of magma before the eruption was estimated by comparing H2O, S, Cl and F contents in natural quenched glassy melt inclusions trapped by plagioclase phenocrysts. The volatile content of igneous rocks after eruption was estimated by comparing concentrations of degassed matrix glasses. As a result of KO-eruption not more than (3.7–4.2) × 1012 kg of water, (4.3–4.9) × 1010 kg of chlorine, (8.6–9.8) × 109 kg of fluorine and (2.6–2.9) × 1010 kg of sulphur were injected into the atmosphere. This eruption had to produce an important climatic impact.

Composition and conditions of formation of the parental melts of Jurassic dolerites of southwestern Crimea: Evidence from melt inclusions in olivine phenocrysts

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus.

Petrology of mafic enclaves in the 2006–2012 eruptive products of Bezymianny Volcano, Kamchatka

This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir (T > 1090°C and P > 9 kbar) are much less common.

Test determinations of paleointensity in historical lavas of Kamchatka

The reliability of the Thellier method for determining the paleointensity of a geomagnetic field is explored on recent igneous rocks of Kamchatka. The main magnetic mineral in the studied rocks is titanomagnetite with different degree of oxidation. It is obtained that the reliability of the results can be assessed based on the deviations of the check points of the partial thermoremanent magnetization (pTRM) during the Thellier experiment. Besides, for different rocks, it is found that the stability of titanomagnetites to heating during the experiments can be insufficient for validating the reliability of the results of paleointensity determination; however, at the same time, the reliability may depend on the initial (oxidation) state of the magnetic minerals of the studied rocks.