Phenocryst

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

A petrological study of the younger Tongan andesites and dacites, and the olivine tholeiites of Niua Fo'ou Island, S. W. Pacific

Abstract: Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An17-88), olivine (Fa18-63), titanomagnetite (usp. 59-73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites. Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008-1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210-1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990-1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.

The Alkalic Rock Suite of Bogoslof Island, Eastern Aleutian Arc, Alaska

Abstract: The historic eruptions of Bogoslof volcano were characterized by hornblende andesite in AD 1796, and nepheline-normative, hornblende basalt in AD 1883 and 1926-1927 Rare earth element (REE) abundances for the 1796 and 1926-1927 eruptions show enrichment of the light REE relative to the heavy REE, overall higher abundances in the andesite and a slight positive Eu anomaly in the basalt The $$^{87}Sr/^{86}Sr$$ ratios are $$070343 \pm 7$$ for the andesite and $$070296 \pm 7$$ for the 1927 basalt Lead isotopic abundances are similar for the two eruptions and the rocks are more radiogenic than mid-ocean ridge basalts Microprobe data for phenocrysts of potassio ferroan pargasite, calcic augite and plagioclase show a considerable compositional range and degree of compositional overlap Mafic inclusions in basalt and andesite contain the same phases as the host lavas and a similar compositional range, suggesting a syngenetic origin for inclusions and host One inclusion contains quartz and biotite which

Melt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation

Abstract: [1] A new method is presented to estimate the decompression rate of silicic magma prior to fragmentation and is applicable to rates of 1–100 kPa/s. This method uses the remaining concentrations of H2O and CO2 in bubble-free cylindrical melt (now glass) pockets in phenocrysts that are connected to the surrounding bubbly melt. These melt pockets are common in volcanic rocks. During magma ascent, bubbles grow in the surrounding melt, and the concentrations of dissolved H2O and CO2 decrease, establishing a boundary condition of low volatile concentration at the outlet of the melt pocket. In turn, volatiles diffuse from the melt pocket into the surrounding bubbly magma and form a gradient that is time dependent. The volatile concentration at the inner limit of the melt pocket likewise diminishes with time. Knowing the diffusivity and solubility of H2O and CO2 permits the remaining H2O and CO2 contents to be estimated for different decompression rates using diffusion equations. We apply this approach to the measured H2O and CO2 contents in quartz-hosted melt pockets in pumice from the phreatomagmatic 26.5 ka Oruanui eruption, Taupo, New Zealand. A decompression rate of 1–7 kPa/s (∼5–35 cm/s) was obtained for Oruanui magma.