Experimental Investigation of Melt Inclusions in Phenocrysts from Klyuchevskoy Volcano Basalts, Kamchatka, Russia

Nikita L. Mironov (stup@geo.web.ru) & Pavel Y. Pletchov (pavel@cs.ru)

Room A-402, Geological Department, Moscow State University, Vorobievi Gori, Moscow,119899, Russia

Detailed study of melt inclusions in olivine (Fo_71-79), clinopyroxene (#Mg_70-81) and plagioclase (An_47-84) phenocrysts from Klyuchevskoy volcano high-Al₂O₃ basalts was carried out. The aim of the research was to determine P-T conditions and melts composition at the final stage of Klyuchevskoy volcano magma evolution. Experimental study of melt inclusions is represented by series of microthermometric experiments. For this purpose it was used high-temperature (up to 1600°C) Sobolev-Slutsky microthermometric stage with visual (microscopic) control (Sobolev, Slutsky, 1984). All the experiments were carried out within pure helium atmosphere. Major elements in glasses were analyzed by EPMA, water content was estimated using calculation methods and by SIMS.

Estimates of trapping temperatures and calculated temperatures and pressures by mineral-melt equilibria suggest that crystallization proceeded within the interval of T = 1145-1030°C and P = 1-2 kbar. Water content ranged from 0.2 to 5 wt.% and shows active degassing processes during crystallization. Significant part of melts doesn't correspond to Klyuchevskoy volcano bulk rock analyses. Most of measured melts (in comparison with high-Al₂O₃ basalts) have similar or lower MgO contents (5.5-3 wt.%), higher SiO₂ (52-67 wt.%), TiO₂ (1.1-1.7) FeO (8.5-11), Na₂O (3.5-4.5) and lower Al₂O₃ (19-14.5 wt.%) contents (Mironov et al., 2000). Prevalent part of melts entrapped by plagioclases is silica-rich (SiO₂ = 72-74 wt.%) and MgO-poor (0.2 wt.%) (Pletchov et al., 2000). Water content variation in melts was the major factor that controlled cotectic phase relations. Increasing of water content during the evolution from high-MgO melts was the main reason for high-Al₂O₃ melts generation due to suppression of plagioclase crystallization. Following crystallization of plagioclase affected by water content reducing (and active melt degassing) determined further compositional evolution of high-Al₂O₃ melts.

Mironov NL, Portnyagin MV, Pletchov PY & Khubunaya SA, Petrology, (2000).

Pletchov PY, Mironov NL, Pletchova AA & Khubunaya SA, Geochemistry, N 1, (2000).

Sobolev AV & Slutsky AB, Geology and Geophysics, (1984).

The Effect of Cation Exchange Na⁺, Ca²⁺-NH⁺₄, on the Analcime and Chabazite Structures

Yulia Miroshnichenko (yulia@uiggm.nsc.ru)

Inst. of Miner. and Petr., prosp. Acad. Koptyug, 3, Novosibirsk, 630090, Russia

Ammonium-exchanged forms of natural analcime (East Siberia, Nidym-river) Na_15.04[Al_15.04Si_32.96O₉₆]·16H₂O and chabazite (Far East, cape Povorotnyi) Ca_1.86K_0.28Na_0.06[Al_3.98Si₈O₂₄] ·12.46H₂O were used in comparative analysis of their physicochemical properties by X-ray diffraction, IR spectroscopy and thermal analysis methods. The degree of structural distortion caused by the NH⁺₄-ion entering depends on the structure type of studied zeolite. Analcime presents a structure type ANA with dense and rigid framework, whereas the chabazite framework is more porous and contains large inner cavities. Our calculations of the unit cell parameters showed strong anisotropic distortion of the analcime pseudocubic unit cell and formation of tetragonal leucite-like structure. The chabazite unit cell was only slightly distorted at NH⁺₄ entering, the structure remaining trigonal. NH₄-analcime is anhydrous (Moroz et al., 1998), and NH₄-chabazite contains 80% of the initial water molecules amount. According to DTA analysis both deammination and dehydroxylation proceed in a single step. H-form seems to exist in a minor temperature interval, which is related to the structural instability and gradual destruction of the framework at 600°C. Deammination and dehydroxylation seem to overlap in chabazite framework as shown by complex shape of the DTA curve. A part of NH⁺₄-ions in the chabazite structure appear to occupy the low energy sites and are removed right away after the removal of the last water molecules.

Leucite-like structure was found to arise at NH⁺₄-Na⁺ exchange, judging as by the IR spectra of leucite, original and NH₄- analcime, so by both NH₄-analcime and leucite belonging to tetragonal symmetry. The chabazite framework was found to be less distorted by NH⁺₄-ion entering, as both the IR spectra and calculated unit cell parameters of original and NH₄-chabazite differ less than in the case of analcime.

Moroz NK, Seryotkin YV, Afanasiev IS & Belitskii IA, Zhurnal strukturnoi khimii, 2, 342-346, (1998).

Neogene Intrusive Magmatism of the Zone of Transform Faulting Dislocations in Kamchatka Island Arc System: Melt Inclusions, Mineralogy, Geochemistry

Michail A. Mitichkin (region@igc.irk.ru), Alexandre B. Perepelov & Vsevolod Yu. Prokofyev

Favorsky Str., 1A, Irkutsk, 664033, Institute of Geochemistry SB RAS, Russia

Malko-Petrpavlovsk Transform Faulting Zone (MPTFZ) of Kamchatka was characterized in Neogene by forming of different petrogenetic types of intrusive rock complexes. MPTFZ is located between Eastern and Southern volcanic belts and having NW direction.

Results of our researches on material composition and geologic position of MPTFZ intrusive massifs led us differ two age complexes among them: N₁² (14,7 - 11,4 Ma) and N₂ (5,1 - 3,7 Ma). First ones are localized in cores of large anticline structures and composed of rock association "gabbro - quartz diorite - tonalite - granite" (Pl-Fsp-Px-Amph-Bi-Q-Mt). Intrusions of the second group are included in volcano-plutonic complexes and consist of rocks with higher kalium alkalinity (monzo-gabbro-diorites - quartz monzodiorites - monzogranites; anorthoclase, magnesial Bi and Phl). Rare-element compositions of both groups rocks correspond with island-arc geochemical types (moderate LILE and low HFSE concentrations). The difference between two gropes of granitoides is apparently confirmed by the data on melt inclusions in their Pl and Q. The higher temperatures of melting (780-890°C) and homogenization (970-1310°C) characterize Miocene granitoides; Pliocene ones are characterized by low values of these parameters (610-830°C and 760-970°C accordingly).

Miocene complexes were connected with the latest evolution stages of Oligocene-Miocene Kamchatka island-arc; their standing taken place at the relatively deep horizons of the earth crust and was determined by transporting up of magmatic melts during the lowering of active subduction velocity and, accordingly, the lowering of fluid flow intensity. Tectonic reconstruction of the island-arc system in Late-Miocene-Pliocene epoch and the start of development of recent subduction zone led to activation of magma generation processes. The evolution of structure and magmatism within MPTFZ at this period was determined significantly by the processes connected with fluid flow of high intensity above subducted Avacha Transform Fault. The study was supported by 97-05-65671 from the Russian Foundation of Basic Research.