Phenocryst

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

Evidence for a copper-bearing fluid in magma erupted at the Valley of ten thousand smokes, Alaska

Abstract: Several lines of evidence point to the existence of a pre-eruptive fluid phase in the magmas erupted in 1912 at the Valley of Ten Thousand Smokes, Alaska. The discovery of rare, Cu-rich bubbles in some melt inclusions is best explained by the random entrapment of a low-density, fluid phase within growing phenocrysts. The H2O, S and Cl concentrations of the high-silica rhyolite are also consistent with fluid saturation in the rhyolite. Water contents, as determined by infrared spectroscopy on doubly polished melt inclusions, cluster between 3.5 and 4.5 wt% H2O, without any apparent differences between magmas vented explosively and effusively. These concentrations would be sufficient for H2O saturation at a pressure of ∼ 100 MPa, though the presence of other volatiles such as CO2 and SO2 could allow saturation with respect to a fluid phase at higher pressures. The S, Cu and Cl contents of the phenocryst assemblages, as determined by XRF analyses, are too low to account for the decrease in the concentrations of S and Cu in the melt, and only modest increase in Cl, with differentiation. Therefore, the behavior of these volatile elements was controlled either by eryptic fractionation if phenocryst phasese or, more likely, by partitioning into a coexisting fluid phase. The presence of this low-density phase apparently provided metals such as Cu with a volatile phase into which they could partition; the concentration of Cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0.05 wt% Cu. The distribution of Cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite, which cyrstallized directly from the silicate melt.

Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming

Abstract: Green, salitic pyroxenes occur as megacrysts and as cores in diopsidic pyroxene phenocrysts and microphenocrysts in a wyomingite lava from Hatcher Mesa, Leucite Hills, Wyoming. Al-rich phlogopite (16–21% Al2O3), apatite, Fe-Ti-oxide, Mg-rich olivine (Fo93) and orthopyroxene (En61) also occur as megacrysts or as inclusions in diopside phenocrysts. All of these phases are found in ultramafic xenoliths in the host lava, and petrographic and chemical evidence is presented that the megacrysts originate by the disaggregation of the xenoliths. It is concluded that the latter are accidental fragments of the wall rocks traversed by the wyomingite magma and it is suggested that the clinopyroxene-rich xenoliths, from which the green pyroxenes are derived, formed in the upper mantle as a result of local metasomatism or by crystallization from magmas of unknown composition during an earlier igneous event. The precise role of the clinopyroxene-rich xenoliths (which also contain apatite, Fe-Ti-oxide and amphibole) in the genesis of the Leucite Hills magmas cannot be elucidated on the basis of the available data, but it is unlikely that they represent the source material from which these magmas are derived.

Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii

Abstract: Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.