Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan

Abstract: The Bonin archipelago represents an uplifted fore-arc terrain which exposes the products of Eocene supra-subduction zone magmatism. Chichijima, at the centre of the chain, represents the type locality for the high-Mg andesitic lava termed boninite. The range of extrusives which constitute the boninite series volcanics are present on Chichijima, and are disposed in the sequence boninite-andesite-dacite with increasing height in the volcano-stratigraphy. Progression to evolved compositions within the Chichijima boninite series is controlled by crystal fractionation from a boninite parental magma containing ? 15% MgO. Olivine and clinoenstatite are the initial liquidus phases, but extraction of enstatitic orthopyroxene, followed by clinopyroxene and plagioclase, is responsible for the general evolution from boninite, through andesite, to dacite. Some andesites within the overlying Mikazukiyama Formation are petrographically distinct from the main boninite series in containing magnetite phenocrysts and a high proportion of plagioclase. As such, these andesites have affinities with the calc-alkaline series. Major and trace element data for 74 boninitic series rocks from Chichijima are presented. Although major element variation is dominantly controlled by high-level crystal fractionation, the large variations in incompatiable trace element concentrations at high MgO compositions cannot be explained by this mechanism. Nd, Pb, and Sr isotopic data indicate the following: (1) a strong overprint on 87Sr/86Sr by seawater alteration; (2) Pb isotopes lie above the northern hemisphere reference line (NHRL) and are thus similar to the <30-Ma are and basin lavas of the Izu—Bonin system, and (3) ?Nd(40 Ma) ranges between 2.8 and 6.8 within the boninite series volcanics. Differences in rare-earth elements (REE), Zr, Ti, and 143Nd/144Nd at similar degrees of fractionation can be explained by the addition of a component of fixed composition from the down-going oceanic crustal slab to a variably depleted source region within the overlying wedge. Data presented for Sm/Zr and Ti/Zr indicate that boninite series volcanics are characterized by low values for both of these ratios. In particular, boninites appear to have uniquely low Sm/Zr ratios. These characteristics may be the result of slab melting in the presence of residual amphibole; the resultant melt could combine with typical slab dehydration fluids and infiltrate the overlying mantle wedge. Such a fluid—melt component could mix either with shallow mantle or directly with primitive melts from depleted mantle. Trace elements, REE, and isotope data thus point to a model for boninite genesis which requires tightly constrained pressure—temperature conditions in the slab combined with melting of a variably depleted source in the overlying wedge. Such constraints are rarely met except during the subduction of juvenile oceanic crust beneath a young, hot overriding plate.

A Complex Magma Mixing Origin for Rocks Erupted in 1915, Lassen Peak, California

Abstract: The eruption of Lassen Peak in May 1915 produced four volcanic INTRODUCTION rock types within 3 days, and in the following order: (1) hybrid Magma mixing is an important process among interblack dacite lava containing (2) undercooled andesitic inclusions, mediate magmas in volcanic arcs (e.g. Turner & (3) compositionally banded pumice with dark andesite and light Campbell, 1986; Philpotts, 1990, and references therein). dacite bands, and (4) unbanded light dacite. All types represent Laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption. They contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms (e.g. Cashman & Bergantz, 1991). However, use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite. The petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid–crystal mixtures at magmatic and partially hybridized with it. Phenocrysts from the dacite magma temperatures and pressures. If magmas mix incompletely, were reacted. Cooling, crystallization, and vesiculation of the hybrid i.e. mingle, features such as compositional banding or andesite magma converted it to a layer of mafic foam. The decreased undercooled inclusions are usually apparent, but if they density of the andesite magma destabilized and disrupted the foam. mix completely, mineralogical disequilibrium may be the Blobs of foam rose into and were further cooled by the overlying dacite magma, forming the andesitic inclusions. Disaggregation of only direct evidence for a mixing origin. andesitic inclusions in the host dacite produced the black dacite and If the thermal and compositional contrasts between light dacite magmas. Formation of foam was a dynamic process. two magmas are great and the ratio of silicic to mafic Removal of foam propagated the foam layer downward into the magma is large, there is little interaction between them, hybrid andesite magma. Eventually the thermal and compositional and the mafic magma is undercooled (e.g. Bacon, 1986). contrasts between the hybrid andesite and black dacite magmas were Many undercooled inclusions contain reacted phereduced. Then, they mixed directly, forming the dark andesite magma. nocrysts inherited from their host silicic magma (Heiken About 40–50% andesitic inclusions were disaggregated into the & Eichelberger, 1980), and Bacon (1986) pointed out host dacite to produce the hybrid black dacite. Thus, disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas. In general, the formation of undercooled an efficient magma-mixing process. Disaggregation of undercooled inclusions retards further mixing (Sakuyama, 1984; inclusions carrying reacted host-magma phenocrysts produces coThompson & Dungan, 1985; Sparks & Marshall, 1986; existing reacted and unreacted phenocryst populations. Koyaguchi & Blake, 1991). However, fragmentation and/ or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems (Thompson & Dungan, 1985; Coulon et al., 1986; Clynne & Christiansen, 1987; Clynne, 1989; Linneman & Myers,

Catastrophic isotopic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau Volcanic Field

Abstract: The Yellowstone Plateau volcanic field has undergone repeated eruption of rhyolitic magma strongly depleted in 18O. Large calderas subsided 2.0, 1.3, and 0.6 Ma ago, on eruption of ash flow sheets that represent at least 2500, 280, and 1000 km3 of zoned magma. More than 60 other rhyolite lavas and tuffs permit reconstruction of the long-term chemical and isotopic evolution of the silicic system. Narrow δ18O ranges in the ash flow sheets contrast with wide δ18O variations in postcaldera lavas of the first and third caldera cycles. Earliest postcollapse lavas are 3 to 6‰ lighter than the preceding ash flow sheets. The O18 depletions were short-lived events that immediately followed caldera subsidence; hundreds of cubic kilometers of magma were drastically 18O depleted and thousands were depleted by 1–2‰. Sequences of postcaldera lavas record partial recovery toward precaldera δ18O values; secular trends between collapse events thus reflect gradual reenrichment of the roofmost magma in δ18O. Much of the subcaldera reservoir was affected, because lavas that erupted as far apart as 115 km reflect the same pattern of depletion and partial recovery. Contemporaneous extracaldera rhyolites have the highest δ18O values in the volcanic field and show no effects of the repeated depletions. Sr and Pb isotope ratios of intracaldera rhyolites jump to more radiogenic values at times of caldera formation and show a longterm zigzag pattern like that of δ18O. Although some contamination by foundering roof rocks seenis to be required, water was probably the predominant contaminant. Even if roof rocks had been strongly depleted in O18 before engulfment, their assimilation would have been far from sufficient to account for the large O18 shift. The low- O18 lavas contain no xenocrysts and show no trace element or phenocryst evidence of massive contamination. Their Fe-Ti-oxide temperatures indicate no cooling relative to the caldera-forming ash flow magma, and their whole-rock, glass, and phenoeryst chemistry suggests compositional continuity with the ash flow sequence. Oxygen exchange between the magma and a mass of low-O18 water greatly exceeding solubility limits may require (1) recurrent explosive activity to sustain access and mixing of water with the magma and (2) convection of the magma reservoir to prevent local saturation.

Rapid decompression-driven crystallization recorded by melt inclusions from Mount St Helens volcano

Abstract: Crystals in hydrous magmas can form in response to falling temperature (magma cooling) or degassing (magma decompression). It remains unclear which process dominates beneath explosive silicic volcanoes. Because decompression and cooling operate on very different time scales, resolving the driving force behind crystallization is of fundamental importance for determining magma dynamics and eruption hazard. Here we use ion-microprobe measurements of dissolved H 2 O in phenocryst-hosted melt inclusions from pumices erupted between May and October 1980 at Mount St. Helens volcano to show that all microlites and a significant proportion of phenocrysts were formed by near isothermal decompression. Magmas erupted after 18 May show evidence for subsequent crystallization of both phenocrysts and microlites, indicating that the time scales of crystal nucleation and growth are on the order of months or less.