Showing papers on "Phenocryst published in 2006"

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Abstract: Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630– 1690 � C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Agecorrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1. 3t oþ4.0]. The higher eNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have oceanisland-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-eNd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.

Magma heating by decompression-driven crystallization beneath andesite volcanoes

Abstract: Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic magma1. Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of magma as it ascends beneath a volcano2. This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in magma rising through the sub-volcanic system, as it loses gas and crystallizes en route3. Thermodynamic modelling of magma that degasses, but does not crystallize, indicates that both cooling and heating are possible4. Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals5 to develop a method for tracking pressure–temperature–crystallinity paths in magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding magma crystallinity and plagioclase–melt geothermometry to determine the temperature. These data are allied to ilmenite–magnetite geothermometry to show that the temperature of ascending magma increases by up to 100 °C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic magmas, which might otherwise be erroneously attributed to pre-eruptive magma mixing.

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Abstract: Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite, damtjernite) and subordinate dolomite-bearing carbonatite with U–Pb perovskite emplacement ages of 590–555Ma occur in the vicinity of Aillik Bay, coastal Labrador. The ultramafic lamprophyres principally consist of olivine and phlogopite phenocrysts in a carbonateor clinopyroxene-dominated groundmass. Ti-rich primary garnet (kimzeyite and Ti-andradite) typically occurs at the aillikite type locality and is considered diagnostic for ultramafic lamprophyre–carbonatite suites. Titanian aluminous phlogopite and clinopyroxene, as well as comparatively Al-enriched but Cr–Mg-poor spinel (Cr-number < 0.85), are compositionally distinct from analogous minerals in kimberlites, orangeites and olivine lamproites, indicating different magma geneses. The Aillik Bay ultramafic lamprophyres and carbonatites have variable but overlapping Sr/Sri ratios (0 70369–0 70662) and show a narrow range in initial «Nd (þ0 1 to þ1 9) implying that they are related to a common type of parental magma with variable isotopic characteristics. Aillikite is closest to this primary magma composition in terms of MgO ( 15–20wt %) and Ni ( 200–574 ppm) content; the abundant groundmass carbonate has dCPDB between 5 7 and 5‰, similar to primary mantle-derived carbonates, and dOSMOW from 9 4 to 11 6‰. Extensive melting of a garnet peridotite source region containing carbonateand phlogopite-rich veins at 4–7 GPa triggered by enhanced lithospheric extension can account for the volatile-bearing, potassic, incompatible element enriched and MgO-rich nature of the proto-aillikite magma. It is argued that low-degree potassic silicate to carbonatitic melts from upwelling asthenosphere infiltrated the cold base of the stretched lithosphere and solidified as veins, thereby crystallizing calcite and phlogopite that were not in equilibrium with peridotite. Continued Late Neoproterozoic lithospheric thinning, with progressive upwelling of the asthenosphere beneath a developing rift branch in this part of the North Atlantic craton, caused further veining and successive remelting of veins plus volatile-fluxed melting of the host fertile garnet peridotite, giving rise to long-lasting hybrid ultramafic lamprophyre magma production in conjunction with the break-up of the Rodinia supercontinent. Proto-aillikite magma reached the surface only after coating the uppermost mantle conduits with glimmeritic material, which caused minor alkali loss. At intrusion level, carbonate separation from this aillikite magma resulted in

Magma Evolution and Open-System Processes at Shiveluch Volcano: Insights from Phenocryst Zoning

Abstract: Phenocryst zoning patterns are used to identify open-system magmatic processes in the products of the 2001 eruption of Shiveluch Volcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclase andesites with average pre-eruptive temperatures of 840C and fO2 of 1.5–2.1 log units above nickel–nickel oxide (NNO). Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Apatite microphenocrysts display normal and reverse zoning of sulphur. The textural similarity of patchy hornblende and plagioclase, together with Ba–Sr systematics in patchy plagioclase, indicate that the cores of these crystals were derived from cumulate material. Plagioclase–liquid equilibria suggest that the patchy texture develops by resorption during H2O-undersaturated decompression. When H2O-saturated crystallization recommences at lower pressure, reduced pH2O results in lower XAn in plagioclase, causing more Al-rich hornblende to crystallize. Plagioclase cores with diffuse oscillatory zoning, and unzoned hornblende crystals, probably represent a population of crystals resident in the magma chamber for long periods of time. In contrast, oscillatory zoning in the rims of plagioclase phenocrysts may reflect eruption dynamics during decompression crystallization. Increasing Fe/Al in oscillatory zoned rims suggests oxidation as a result of degassing of H2O during decompression. A general lack of textural overlap between phenocryst types suggests that different phenocryst populations were spatially or temporally isolated during crystallization. We present evidence that the host andesite has mixed with both more felsic and more mafic magmas. Olivine and orthopyroxene xenocrysts with reaction or overgrowth rims and strong normal zoning indicate mixing with basalt. Sieve-textured plagioclase resulted from mixing of a more felsic magma with the host andesite. The mineralogy and mineral compositions of a mafic andesite enclave are identical to those of the host magma, which implies efficient thermal quenching, and thus small volumes of intruding magma. Mixing of this magma with the host andesite results in phenocryst zoning because of differences in dissolved volatile contents. We suggest that small magma pulses differentiated at depth and ascended intermittently into the growing magma chamber, producing incremental variations in whole-rock compositions.

The 26·5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

Abstract: The caldera-forming 26•5 ka Oruanui eruption (Taupo, New Zealand) erupted ~530 km3 of magma, >99% rhyolitic, <1% mafic. The rhyolite varies from 71•8 to 76•7 wt % SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt % SiO2. Average rhyolite compositions at each stratigraphic level do not change significantly through the eruption sequence. Oxide geothermometry, phase equilibria and volatile contents imply magma storage at 830–760°C, and 100–200 MPa. Most rhyolite compositional variations are explicable by ~28% crystal fractionation involving the phenocryst and accessory phases (plagioclase, orthopyroxene, hornblende, quartz, magnetite, ilmenite, apatite and zircon). However, scatter in some element concentrations and 87Sr/86Sr ratios, and the presence of non-equilibrium crystal compositions imply that mixing of liquids, phenocrysts and inherited crystals was also important in assembling the compositional spectrum of rhyolite. Mafic compositions comprise a tholeiitic group (52•3–63•3 wt % SiO2) formed by fractionation and crustal contamination of a contaminated tholeiitic basalt, and a calc-alkaline group (56•7–60•5 wt % SiO2) formed by mixing of a primitive olivine–plagioclase basalt with rhyolitic and tholeiitic mafic magmas. Both mafic groups are distinct from other Taupo Volcanic Zone eruptives of comparable SiO2 content. Development and destruction by eruption of the Oruanui magma body occurred within ~40 kyr and Oruanui compositions have not been replicated in vigorous younger activity. The Oruanui rhyolite did not form in a single stage of evolution from a more primitive forerunner but by rapid rejuvenation of a longer-lived polygenetic, multi-age ‘stockpile’ of silicic plutonic components in the Taupo magmatic system.

Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle

Abstract: The Martian meteorite Yamato (Y-) 980459 is an olivine-phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y-980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y-980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low-calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ±10 °C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ~100 km. Several lines of evidence suggest that the Y-980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y-980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y-980459 magmatic event suggests a significantly higher temperature at the core-mantle boundary than previously estimated.

Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands

Abstract: The volcanic history of Santo Antao, NW Cape Verde Islands, includes the eruption of basanite–phonolite series magmas between 7 5 and 0 3 Ma and (melilite) nephelinite–phonolite series magmas from 0 7 to 0 1 Ma. The most primitive volcanic rocks are olivine clinopyroxene-phyric, whereas the more evolved rocks have phenocrysts of clinopyroxene Fe–Ti oxide kaersutite hauyne titanite sanidine; plagioclase occurs in some intermediate rocks. The analysed samples span a range of 19–0 03% MgO; the most primitive have 37–46% SiO2, 2 5–7% TiO2 and are enriched 50–200 · primitive mantle in highly incompatible elements; the basanitic series is less enriched than the nephelinitic series. Geochemical trends in each series can be modelled by fractional crystallization of phenocryst assemblages from basanitic and nephelinitic parental magmas. There is little evidence for mineral–melt disequilibrium, and thus magma mixing is not of major importance in controlling bulk-rock compositions. Mantle melting processes are modelled using fractionation-corrected magma compositions; the models suggest 1–4% partial melting of a heterogeneous mantle peridotite source at depths of 90–125 km. Incompatible element enrichment among the most primitive magma types is typical of HIMU OIB. The Sr, Nd and Pb isotopic compositions of the Santo Antao volcanic sequence and geochemical character change systematically with time. The older volcanic rocks (7 5–2 Ma) vary between two main mantle source components, one of which is a young HIMU type with Pb/Pb 1⁄4 19 88, D7/4 1⁄4 5, D8/4 0, Sr/ Sr 1⁄4 0 7033 and Nd/Nd 1⁄4 0 51288, whereas the other has somewhat less radiogenic Sr and Pb and more radiogenic Nd. The intermediate age volcanic rocks (2–0 3 Ma) show a change of sources to two-component mixing between a carbonatite-related young HIMU-type source ( Pb/Pb 1⁄4 19 93, D7/4 1⁄4 5, D8/4 1⁄4 38, Sr/Sr 1⁄4 0 70304) and a DM-like source. A more incompatible element-enriched component with D7/4 > 0 (old HIMU type) is prominent in the young volcanic rocks (0 3– 0 1 Ma). The EM1 component that is important in the southern Cape Verde Islands appears to have played no role in the petrogenesis of the Santo Antao magmas. The primary magmas are argued to be derived by partial melting in the Cape Verde mantle plume; temporal changes in composition are suggested to reflect layering in the plume conduit.

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Abstract: Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km3 (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H2O, CO2 and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H2O and CO2 in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H2O and CO2 in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing.

Storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of Augustine Volcano, Alaska

Abstract: Compositional heterogeneity (56–64 wt% SiO2 whole-rock) in samples of tephra and lava from the 1986 eruption of Augustine Volcano, Alaska, raises questions about the physical nature of magma storage and interaction beneath this young and frequently active volcano. To determine conditions of magma storage and evolutionary histories of compositionally distinct magmas, we investigate physical and chemical characteristics of andesitic and dacitic magmas feeding the 1986 eruption. We calculate equilibrium temperatures and oxygen fugacities from Fe-Ti oxide compositions and find a continuous range in temperature from 877 to 947°C and high oxygen fugacities (ΔNNO=1–2) for all magmas. Melt inclusions in pyroxene phenocrysts analyzed by Fourier-transform infrared spectroscopy and electron probe microanalysis are dacitic to rhyolitic and have water contents ranging from <1 to ∼7 wt%. Matrix glass compositions are rhyolitic and remarkably similar (∼75.9–76.6 wt% SiO2) in all samples. All samples have ∼25% phenocrysts, but lower-silica samples have much higher microlite contents than higher-silica samples. Continuous ranges in temperature and whole-rock composition, as well as linear trends in Harker diagrams and disequilibrium mineral textures, indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter, more mafic magma. The dacitic endmember is probably residual magma from the previous (1976) eruption of Augustine, and we interpret the mafic endmember to have been intruded from depth. Mixing appears to have continued as magmas ascended towards the vent. We suggest that the physical structure of the magma storage system beneath Augustine contributed to the sustained compositional heterogeneity of this eruption, which is best explained by magma storage and interaction in a vertically extensive system of interconnected dikes rather than a single coherent magma chamber and/or conduit. The typically short repose period (∼10 years) between Augustine's recent eruptive pulses may also inhibit homogenization, as short repose periods and chemically heterogeneous magmas are observed at several volcanoes in the Cook Inlet region of Alaska.

Generation of Porphyritic and Equigranular Mafic Enclaves During Magma Recharge Events at Unzen Volcano, Japan

Abstract: Mafic to intermediate enclaves are evenly distributed throughout the dacitic 1991–1995 lava sequence of Unzen volcano, Japan, representing hundreds of mafic recharge events over the life of the volcano. This study documents the morphological, textural, chemical, and petrological characteristics of the enclaves and coexisting silicic host lavas. The eruptive products described in this study appear to be general products of magma mingling, as the same textural types are seen at many other volcanoes. Two types of magmatic enclaves, referred to as Porphyritic and Equigranular, are easily distinguished texturally. Porphyritic enclaves display a wide range in composition from basalt to andesite, are glass-rich, spherical and porphyritic, and contain large, resorbed, plagioclase phenocrysts in a matrix of acicular crystals and glass. Equigranular enclaves are andesitic, non-porphyritic, and consist of tabular, medium-grained microphenocrysts in a matrix glass that is in equilibrium with the host dacite magma. Porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface. Equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of the mafic intrusion.

U–Pb zircon geochronology of silicic tuffs from the Timber Mountain/Oasis Valley caldera complex, Nevada: rapid generation of large volume magmas by shallow-level remelting

Abstract: Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 km3, 12.8 Ma)–Tiva Canyon (TC, 1,000 km3, 12.7 Ma) and Rainier Mesa (RM, 1,200 km3, 11.6 Ma)–Ammonia Tanks (AT, 900 km3, 11.45 Ma; all cited ages are previously published 40Ar/39Ar sanidine ages). Within each pair, eruptions are separated by only 0.1–0.15 My and produced tuffs with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with δ18O values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-δ18O AT cycle were earlier determined to exhibit ∼1.5‰ core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5–2.5‰. The following U–Pb isochron ages were calculated after correcting for the initial U–Pb disequilibria: AT (zircon rims: 11.7 ± 0.2 Ma; cores: 12.0 ± 0.1 Ma); pre-AT rhyolite lava: (12.0 ± 0.3 Ma); RM: 12.4 ± 0.3); TC: (13.2 ± 0.15 Ma); TS: (13.5 ± 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar–Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70–0.35 Ma older than the respective Ar–Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4–4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5–16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-δ18O AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the 40Ar/39Ar eruption age compared to core ages that are on average ∼0.2–0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz–zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-δ18O AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-δ18O magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-δ18O magma presently known.

Short communication Magmatic origin of low-Ca olivine in subduction-related magmas: Co-existence of contrasting magmas

Abstract: Unravelling the origin of different components contributing to subduction-related magmas is a prerequisite to understanding the sources and processes involved in their origins. Mafic, high-Ca subduction-related magmas from geographically-diverse areas (Indonesia, Solomon Islands, Kamchatka, Valu Fa Ridge) contain two populations of olivine crystals, of which only the high-Ca population (CaO=0.3–0.5 wt.%) crystallized from the melt that dominantly contributed to the whole rock composition. Forsteriterich (Fo90–94), low-Ca (CaOb0.15 wt.%), high-Ni (NiON0.3 wt.%) olivine crystals, which constitute 16–37 vol.% of total olivine population, are generally interpreted as mantle or lithospheric xenocrysts. However, in these samples, the olivine shape and chemical zoning, the composition of included minerals (orthopyroxene, clinoenstatite and Cr-spinel) and presence of melt inclusions, are indications that these crystals are phenocrysts from a mafic magma with high silica and low calcium contents. The coexistence of contrasting magmas (mafic high-Ca silica-poor versus low-Ca silica-rich) within a number of arc systems and their mixing may not be a rare event, and should be taken into account when developing models of arc petrogenesis.

Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)

Abstract: A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km 2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000–900 °C for basaltic andesites to 700–600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals res ect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of

The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma

Abstract: The Oto-Zan lava in the Setouchi volcanic belt is composed of phenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids) and forms a composite lava flow. The phenocryst assemblages and element abundances change but Sr–Nd–Pb isotopic compositions are constant throughout the lava flow. The sanukitoid at the base is a high-Mg andesite (HMA) and contains Mg- and Ni-rich olivine and Cr-rich chromite, suggesting the emplacement of a mantle-derived hydrous (� 7w t %H 2O) HMA magma. However, Oto-Zan sanukitoids contain little H2O and are phenocryst-poor. The liquid lines of descent obtained for an Oto-Zan HMA at 0� 3GPa in the presence of 0� 7–2� 1w t %H 2O suggest that mixing of an HMA magma with a differentiated felsic melt can reasonably explain the petrographical and chemical characteristics of Oto-Zan sanukitoids. We propose a model whereby a hydrous HMA magma crystallizes extensively within the crust, resulting in the formation of an HMA pluton and causing liberation of H2O from the magma system. The HMA pluton, in which interstitial rhyolitic melts still remain, is then heated from the base by intrusion of a high-T basalt magma, forming an H2O-deficient HMA magma at the base of the pluton. During ascent, this secondary HMA magma entrains the overlying interstitial rhyolitic melt, resulting in variable self-mixing and formation of a zoned magma reservoir, comprising more felsic magmas upwards. More effective upwelling of more mafic, and hence less viscous, magmas through a propagated vent finally results in the emplacement of the composite lava flow.

The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy

Abstract: We report the occurrence of unusual, high-magnesium (Fo96) olivine phenocrysts in a basaltic lava and an ejected lithic block from the Upper Vancori period ( 13 ka) and the recent activity (2002–2003) of Stromboli volcano, Italy. The samples that contain this distinctive mineral chemistry are a shoshonitic basalt and a basaltic andesite with anomalous bulk-rock chemical characteristics in which the iron is highly oxidized (6–8 wt % Fe2O3 and <1 wt % FeO). In other respects these samples are similar to the majority of Stromboli basalts, characterized by the coexistence of olivine, clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts, and clinopyroxene, plagioclase and Fe–Ti oxides in the groundmass. In the high-magnesium olivine samples, Fe–Ti oxides (pseudobrookite) typically occur as symplectitic intergrowths with the olivine phenocrysts, indicating simultaneous growth of the two phases. We propose, as a paragenetic model, that the Fo96 olivine phenocrysts crystallized from a highly oxidized basaltic magma in which most of the iron was in the ferric state; hence, only magnesium was available to form olivine. The highly oxidized state of the magma reflects sudden degassing of volatile phases associated with instantaneous, irreversible, transient degassing of the magma chamber; this is postulated to occur during periods of sudden decompression induced by fracturing of the volcanic edifice associated with paroxysmic activity and edifice collapse.