Phenocryst

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

Zircon crystallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

Abstract: In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ∼60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; [Smith et al., 1996][1]) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). [1]: #ref-32

DSDP site 334: Magmatic cumulates from oceanic layer 3

Abstract: Coarse-grained lherzolitic peridotites and two-pyroxene gabbros recovered at Deep Sea Drilling Project site 334 beneath a thin layer of fine-grained basalt are primary magmatic cumulates similar to the upper cumulate portion of ophiolite complexes. Mineral chemistry indicates that they are the crystallization product of oceanic tholeiite magma. Long cooling and annealing times indicated by exsolution and equilibration in pyroxenes indicate emplacement near the Mid-Atlantic Ridge and long residence time in oceanic layer 3, prior to tectonic emplacement at shallow crustal levels. Slow depletion of the anorthite component in cumulate plagioclase relative to increase of Fe/Fe + Mg in coexisting mafic phases indicates more efficient fractionation of sodium into residual liquid than implied by fractionation of plagioclase phenocryst found in oceanic magmas. Strong depletion of incompatible minor elements and lack of intercumulate residual phases may indicate the operation of a diffusion mechanism that would allow interaction of pyroxene and magma prior to the crystallization of pyroxene as a liquidus phase.

Akahoya Ash-A Holocene Widespread Tephra Erupted from the Kikai Caldera, South Kyushu, Japan

Abstract: A Holocene volcanic ash layer comprising abundant glass shards occurs as near-surface, soil-forming parent materials in south to north Kyushu and in Shikoku. This layer has been given several local names such as “Akahoya”, “Imogo”, “Onji”, etc. by farmers and pedologists. Its remarkable characteristics as a parent material of soil stimulated the interest of many pedologists to study its source, pedological features, distribution, etc. However, opinions on its source and proper identification varied considerably from one author to another.Detailed petrographic observation and accurate determinations of the refractive indices of the glass and several phenocryst phases in the tephra, together with extensive field work, have led to the conclusion that the Akahoya ash is the product of a single major eruption of the Kikai caldera.The ash is dacitic in composition and contains abundant bubble-walled glass shards and plagioclase, hypersthene, augite and opaque minerals as phenocrysts. The refractive index of the glass ranges from 1.505 to 1.514, and that of the hypersthene, from 1.705 to 1.714. The thickness contour of the ash layer and its grain-size distribution clearly indicate that this ash represents ejecta from the Kikai caldera, which is one of the largest calderas in Japan with an approximate diameter of 20km and largely submerged beneath the sea.The formation associated with this widespread tephra consists of three members; (1) a pumice-fall deposit as the earliest stage, (2) pyroclastic-flow deposits as the middle to the latest stages, and (3) an ash-fall deposit approximately contemporaneous with the pyroclastic flow. The 3rd member is assigned to the Akahoya ash and has the most extensive lobe with an axis length of over 1, 000km, covering most of southwest to central Japan and northwest Pacific Ocean. The volumes of the Akahoya ash-fall deposits must be greater than those of the pyroclastic flows.More than twenty-seven radiocarbon dates of the ash have been obtained so far, ranging rather widely from ca. 3, 000y.B.P. to ca. 9, 000y.B.P. However, the average value of the carbonated woods and peaty materials containing in the layer and the stratigraphical relationships with human remains give a probable age of the ash between 6, 000y.B.P. and 6, 500y.B.P. This marker-tephra is thus extremely significant for studies of Holocene climatic changes and sea levels, as well as for the correlation of archaeological sites.