Phenocryst

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

40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States

Abstract: Laser probe Ar-40/Ar-39 dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 +/- 1 to 761 +/- 1 ha, Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 +/- 0.03 to 2.3 +/- 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts similar to 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

Geochemistry, mineralogy, and petrogenesis of basalt from the Gorda Ridge

Abstract: Basalt pillow lava with glassy rims was dredged from 17 sites along the Gorda Ridge between latitudes 43°N and 40.8°N. All samples are low-K2O mid-ocean ridge tholeiitic basalt having a narrow compositional range with Mg numbers clustered around 60–62; more primitive and evolved compositions are present but rare. None of the more fractionated samples is as evolved as ferrobasalt from the Juan de Fuca Ridge. Incompatible trace element ratios are typical of normal mid-ocean ridge basalt but show considerable scatter. The 87Sr/86Sr ratios, ranging from 0.70233 to 0.70267, are near the low end of the range for normal mid-ocean ridge basalt. The phenocrysts present are abundant plagioclase, less abundant olivine ± spinel, and traces of sulfide. Clinopyroxene is absent in the glasses. The mineral chemistry is typical of ocean floor basalt. However, unusually anorthitic plagioclase in some highly plagioclasephyric samples, complex zoning in plagioclase and olivine, and other disequilibrium features indicate complex petrogenetic processes involving magma mixing. Glass inclusions in olivine and plagioclase indicate primitive trapped melts. The chemical diversity of Gorda Ridge lava appears to have been primarily caused by variable degrees of partial melting, but crystal fractionation and magma mixing were also important processes. Compositional variations along the strike of the ridge indicate that lava from the segment south of the offset at latitude 41.6°N was generated by the smallest percentage of partial melting of a source less depleted than that of the two northern segments. Lava from the central and northern segments was generated by variable percentages of partial melting of a uniformly depleted source. Lava from the northern segment shows the greatest diversity with a large range in percentage of melting and both relatively primitive and evolved compositions in spatially and temporally closely associated lava. The diversity of lava from the northern segment may result from thermal perturbations caused by proximity to the Blanco Fracture Zone. Gorda Ridge lava is more similar to that from the Mid-Atlantic Ridge at 22°–25°N than to that from other spreading centers in the Pacific Ocean. Magma chambers under the Gorda Ridge appear to be small, discontinuous in time and space and possibly at greater depth than those beneath other Pacific spreading centers.

Preeruption conditions and timing of dacite‐andesite magma mixing in the 2.2 ka eruption at Mount Rainier

Abstract: Analytical, field, and experimental evidence demonstrate that the Mount Rainier tephra layer C (2.2 ka) preserves a magma mixing event between an andesitic magma (whole rock SiO 2 content of 57-60 wt %) and a dacitic magma (whole rock SiO 2 content of 65±1 wt %). The end-member andesite (a mix of an injected and chamber andesite) and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions. Many pumices, however, contain mixtures of the end-members. The end-member dacite contains a microlite-free matrix glass with 74-77 wt % SiO 2 , orthopyroxene rims of Mg 57-64 , clinopyroxene rims of Mg 66-74 , and plagioclase rim anorthite contents of An 45-65 . The temperature and oxygen fugacity, from Fe-Ti oxide compositions, are 930±10°C and 0.5-0.75 log units above NNO. The mixed andesite contains Mg 73-84 orthopyroxene rims, Mg 73-78 clinopyroxene rims, An 78-84 plagioclase rims, and Mg 67-74 amphibole rims. The temperature from Fe-Ti oxides, hornblendeplagioclase, and two-pyroxene geothermometry is 1060±15°C, and the oxygen fugacity is approximately one log unit above NNO for the injected andesite. The chamber andesite is estimated to be a magma with a ∼64-65 wt % SiO 2 melt at 980°C and a NNO oxygen fugacity. We conclude that the andesitic and dacitic magmas are from separate magma storage regions (at >7 km and ∼2.4 km) due to differences in the bimodal whole rock, matrix glass, and phenocryst compositions and the presence or absence of stable hornblende. The time involved from the mixing event through the eruption is limited to a period of 4-5 days based on Fe-Ti oxide reequilibration, phenocryst growth rates, and hornblende breakdown. The eruption sequence is interpreted as having been initiated by an injection of the 1060±15°C andesitic magma into the ∼980°C (>7 km) andesite storage region. The mixed andesitic magma then intersected a shallow,∼ 2.4 km, dacitic storage system on its way toward the surface. The eruption became more dacitic over time, and the final products some show evidence of partial reequilibration between the andesite and dacite.