Description: Melt inclusions in olivine Fo83-72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6-8 wt%) formed by ~46 wt% fractional crystallization of olivine (~6 wt%), plagioclase (~27 wt%), pyroxene (~13 wt%) and magnetite (〈1 wt%) from primitive basaltic melt (average SiO2=49 wt%, MgO=7.6 wt%, H2O=6 wt%) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1050 to 1090 oC in the pressure range from 400 to 50 MPa and was induced by release of mixed H2O-CO2 fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low MgO (3-10 wt%) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H2O content, occurrence of low-Mg to high-Mg basalts reflect the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.

Description: Over the last decade there has been renewed interest in determining the water contents of basaltic magmas. A commonly applied method is analysis of H2O from melt inclusions in olivine. However, it is also well known that these can rapidly lose (or gain) H2O by diffusion. An alternative is to measure the H2O contents of clinopyroxene phenocrysts and use a partition coefficient (D) to estimate the original H2O content of the host magma. This approach is not without complications and several recent studies have attempted to assess the effects of diffusive loss of H2O from magmatic clinopyroxenes. In the ideal case, these crystals should be taken from rapidly cooled tephra or lapilli but such materials are not always available. In order to further assess the potential of using 5-10mm clinopyroxenes from lavas we undertook a detailed, multi-analytical investigation of clinopyroxenes from an ankaramite flow on Pico Island in the Azores. We conclude that these can be trusted to preserve (probably minimum) magmatic H2O contents if the H2O concentrations of multiple clinopyroxenes from a single sample form a linear correlation with the AlIV content that demonstrates a coupled substitution with little or no H2O loss. Conversely, if H2O contents decrease from core to rim whereas AlIV contents remain relatively constant then it is likely that those clinopyroxenes lost H2O during differentiation and/or cooling. We suspect that the olivine melt inclusions we analysed from Pico and São Miguel Islands also underwent diffusive loss of H2O. Using these criteria, we present clinopyroxene-derived magmatic H2O estimates for Corvo, Flores Faial, Pico and São Miguel Islands that range from 0.28 to 2.2wt%. When combined with published data these show that H2O contents often extend to higher values on the islands than along the adjacent mid-Atlantic ridge. These localised, elevated H2O contents can explain why the islands are emergent despite being situated away from the ridge and perhaps also the asymmetric nature of the bathymetry of the archipelago. It is possible that this H2O was recycled from material subducted very early on in Earth's history.

Description: Highlights • First comprehensive dataset of spinel inclusions in high-Mg olivine from Kamchatka • Oxidation state of parental magmas of Kamchatka ranging from ΔQFM+0.7 to +3.7 • ΔQFM correlates with Ba/La and La/Nb for back-arc magmas of Kamchatka • Decoupling of Cr# and TiO2 in primitive Cr-Spinel suggests slab melt contribution Abstract The Kamchatka volcanic arc (Russia) is one of well-studied but complex tectonic margins on Earth, with an extensive geologic history stretching as far back as the Late Cretaceous. Unlike many other subduction zones, primitive basalts with Mg# 〉 65 are abundant in Kamchatka, thereby allowing characterization of the mantle source through compositional analyses of near-liquidus minerals in the rocks. In this paper, we present a comprehensive dataset on the composition of Cr-spinel inclusions in olivine for all main Late Quaternary volcanic zones in Kamchatka, comprising of analyses of 1604 spinel inclusions and their host-olivine from 104 samples representing 30 volcanoes and volcanic fields. The studied rocks are basalts, basaltic andesites and high-Mg andesites, which cover the whole compositional range the Late Quaternary primitive volcanic rocks in Kamchatka. The composition of spinel shows large variability. Spinel inclusions with the lowest Cr# and Fe3+/Fe2+ ratios were found in basalts from Sredinny Range and Northern Kamchatka, whereas the most Cr-rich and oxidized spinel inclusions occur in basalts and high-Mg andesites from the Central Kamchatka Depression. Intermediate Cr-spinel compositions characterize the Eastern Volcanic Belt of Kamchatka. The compositions of olivine-spinel pairs were used to quantify the oxidation state of parental Kamchatka magmas and the degree of partial mantle melting. The redox conditions recorded in spinel compositions range from ΔQFM = +0.7 to +3.7. ΔQFM for samples from the Sredinny Range and Northern Kamchatka correlates with a number of proxies of the involvement of slab-derived components incorporated in the composition of their host-rocks (e.g., La/Nb and Ba/La), which suggests a coupling between the mantle oxidation and metasomatism by slab-derived fluids or melts. These correlations were not observed for frontal Kamchatka volcanoes with the highest estimated ΔQFM, which possibly indicates a buffering of the mantle oxidation state by sulfur. The estimated degrees of partial mantle melting range from 8 to 〉20% for Kamchatka volcanoes. Spinel from the Central Kamchatka Depression has the highest Cr# and could crystallize from magmas generated from the most depleted sources. In contrast to the Eastern Volcanic Belt, spinel Cr# and the inferred degrees of melting in the Central Kamchatka Depression do not correlate with spinel TiO2 content. The apparent decoupling between the proxies of mantle depletion in the CKD spinel is interpreted to reflect refertilization of the CKD mantle by oxidized Ti-rich slab- or mantle lithosphere-derived melts near the northern edge of the subducting Pacific Plate. This study demonstrates that the composition of Cr-spinel in volcanic rocks in combination with bulk-rock compositions can be a powerful tool to map regional variations of the mantle source depletion, oxidation state, and involvement of various slab derived components in island-arc magmatism.

Description: Silicic Icelandic magmas are widely believed to contain low to moderate H2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO2=57–72 wt%, K2O=1.3–2.6 wt%) in Fe-rich olivines (Fo2–42) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H2O contents (3.3–6.2 wt%). The silicic Hekla melts originate primarily by extensive (∼90%) crystal fractionation of H2O-poor (∼0.6 wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO2 (ΔQFM ∼−0.1 to −0.4) and have as high H2O contents as significantly more oxidized island-arc magmas (ΔQFM≥1). This demonstrates that H2O and ΔQFM do not correlate in silicic magmas from different tectonic settings, and that fO2, not H2O content, shows a major difference between silicic ocean-island (e.g., Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H2O activity and low fO2 expand the field of olivine stability in silicic melts. Low fO2 and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H2O-rich silicic parental magmas crystallized at low fO2. Finally, the high H2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma–ice interaction.

Description: Evaluation of short and long-term effects of volcanism on the global climate requires quantitative estimates of the volcanic emission of volatiles. One cannot directly measure the amount of volatiles emitted by volcanoes in the past but can estimate it using petrologic methods based on study of melt inclusions. In this work we estimate emission of volatiles resulted from basaltic volcanism in Kamchatka since the last Ice Age using data on volatiles in olivine-hosted melt inclusions. We studied about 900 glassy and experimentally homogenized olivine-hosted (Fo92-65) melt inclusions from 10 volcanic centers representative for 3 volcanic zones of the Eastern Volcanic Belt of Kamchatka: volcanic front (Ksudach, Zheltovsky, Vysoky, Krasheninnikov, Karymsky and Zhupanovsky volcanoes), rear-arc (Zavaritsky volcano and Tolmachev Dol) and the southern segment of the Central Kamchatka Depression (SCKD) (Klyuchevskoy volcano and Tolbachinskiy Dol). The compositions of rocks studied range from low- to high-K basalts and basaltic andesites and are representative for major magma types of the Eastern Volcanic Belt. Inclusions were analyzed for volatiles (S, Cl, H2O, F), major and trace elements using electron and ion microprobes. H2O content in the most primitive inclusions is 2-3.5 wt% for EVF, 2.5-3.5 wt% for SCKD volcanoes and ~1.5 wt% for rear-arc Zavaritsky volcano. This difference in water content between frontal and rear-arc EVF volcanoes can be explained by decreasing water concentrations in parental melts and their sources with increasing depth from volcano to the subducting plate (Portnyagin et al. 2007). H2O concentrations in EVF melts decrease with increasing K2O and indicate degassing of water during crystallization. The rate of water degassing is much slower than that of sulfur. No more than ~ 50% of the initial water content is lost from magmas at 70% of crystallization. H2O content in SCKD melts increases up to 5-5.5 wt.% during first 30-35 % of fractionation (Fo82-83) and then decreases due to degassing at shallow pressure. olivines is 2500-3000 ppm from Zheltovsky and Zhupanovsky volcanoes from EVF, Zavaritsky and Tolmachev Dol from rear-arc zone and slightly less in olivines Fo81-73 from Ksudach (1700 ppm), and in olivines Fo78-75 from Vysoky and Krasheninnikov (1500 ppm). SCKD melts also have high S concentrations ranging from 1700 to 4000 ppm. Sulfur content correlates inversely with K2O in all samples and decreases to less than 200 ppm in groundmass glasses. Fast depletion of fractionating melts in sulfur and high proportion of sulfate species (measured S6+/STotal is 0.40±0.16 on average) dissolved in melts suggest that sulfur preferentially partitions into fluid phases during magmatic evolution. We estimated that magmas in Kamchatka lose more than 90% of sulfur after 70% crystallization. Average chlorine and fluorine content in primitive magmas of Kamchatka volcanoes are shown at the Table 1 [Table 1: Average amounts of volatiles in mafic magmas of Kamchatka and their long-term fluxes]. Concentrations of these components slightly increase during crystallization but Cl/K2O and F/K2O ratios decrease that indicates partial loss of chlorine and fluorine into fluid phase. Volcanic fluxes of volatiles to the exosphere (total flux to atmosphere, crust and hydrosphere) were estimated from published data on productivity of all EVF volcanoes during the Holocene (80×106 t/y) and CKD (Klyuchevskoy-Tolbachik: 107 t per year) (Ponomareva et al. 2007) and data on the volatile content in parental melts. The estimated minimum total and normalized to the length of the arc segments volatile fluxes are shown in Table 1. The flux estimated for the EVF is comparable to the average global flux from island arcs (Sadofsky et al. 2008). Significantly larger fluxes of volatiles from the SCKD volcanoes on the regional scale and globally reflect exceptionally high volcanic productivity of this region hosting Klyuchevskoy Volcano, the most productive volcano in the Pacific Ring of Fire. The estimated long-term sulphur flux for Kamchatka is at least 5 times higher than COSPEC measurements for this region (Hilton et al. 2002). The difference indicates that results of short period measurements cannot be representative for the long-term flux. Large eruption occurred during the period of satellite monitoring of gas emission from volcanic area can in turn lead to large overestimate of the long-term flux. In summary, we conclude that primitive magmas of Kamchatka are very rich in volatiles and particularly in sulfur which concentrations in primitive Kamchatkan magmas are among the highest measured so far in island arcs (3000-6000 ppm). Given the large productivity of Kamchatkan volcanism during the last post-glacial period, its contribution to the volcanic forcing of the Earth climate should be discernable on the global scale. This work was supported by the KALMAR project (BMBF grant 03G0640A) and the RFBR grant # 05-09-01234a. References Hilton DR, Fischer TP, Marty B (2002) Noble gases and volatile recycling in subduction zones. In: Porcelli D, Ballentine C, Weiler R (eds) Noblegases in geochemistry and cosmochemistry, reviews in mineralogy and geochemistry. Mineralogical Society of America, Washington, DC, 47: 319–370 Ponomareva VV, Melekestsev IV, Braitseva OA, Pevzner MM, Sulerzhitsky LD (2007) Late Pleistocene- Holocene Volcanism on the Kamchatka Peninsula, Northwest Pacific region. In: Eichelberger J, Gordeev E, Kasahara M, Izbekov P, Lees J ( Eds) “Volcanism and Tectonics of the Kamchatka Peninsula and Adjacent Arcs”. Geophysical Monograph Series, 172: 169- 202 Geophysical Monograph Series, 172: 169-202 Portnyagin, MV, Hoernle, K, Plechov, PY, Mironov NL, Khubunaya, SA (2007) Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles (H2O, S, Cl, F) and trace elements in melt inclusions from the Kamchatka Arc. Earth and Planetary Science Letters, 255 (1-2): 53-69 Sadofsky S, Portnyagin M., Hoernle K, van den Bogaard P (2008) Subduction Cycling of Volatiles and Trace Elements Through the Central American Volcanic Arc: Evidence from Melt Inclusions. Contributions to Mineralogy and Petrology, 155(4): 433-456

Description: Arc magmas are generated by a number of mantle and crustal processes. Our multidisciplinary, long-term research is aimed at deciphering these processes for a single arc volcano, Kliuchevskoy volcano in Kamchatka. Some key results of the study follow: 1) Modeling of trace element and H2O contents in melt inclusions suggests that the primary magmas originate via hydrous flux-melting of the mantle wedge at temperatures close to the dry peridotite solidus. The role of decompression melting is minor or absent at Kliuchevskoy and other arc volcanoes built on relatively thick crust. 2) Geochemistry of high-Mg olivine suggests that primary Kliuchevskoy magmas have substantial contribution from olivine-free pyroxenite (up to 30 %), which could be formed by reaction of slab melts (or supercritical fluids) with mantle wedge peridotite. 3) Parental Kliuchevskoy melts start to crystallize as deep as the Moho boundary, and the erupted magmas reflect multistage and complex processes of crystallization, magma mixing and crustal assimilation. None of the Kliuchevskoy rocks analyzed thus far represent true primary melt compositions. 4) The Kliuchevskoy Holocene eruptive history is not steady-state in terms of eruption rate and geochemistry. There are two millenial cycles with major and trace element and OSr- Nd-Pb and U-series isotope compositions of the magmas changing gradually from more to less affected by crustal (?) assimilation. The onset of the cycles correlates with periods of enhanced volcanic activity in Kamchatka, suggesting that the extent of magma-crust interaction is inversely related to magma production rate and thus magma flux from the mantle.