Phenocryst

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

U–Th Disequilibrium and Rb–Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya

Abstract: Mildly peralkaline rhyolites of the Olkaria Volcanic Complex, minor, continental magma system of mildly peralkaline located in the Kenyan sector of the East African rift valley, have composition [molar (Na2O + K2O)/Al2O3 or agpaitic low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2 ppm; index, AI >1)]. The work was undertaken as a contrast Rb/Sr = 748–1769) that potentially allow the resolution of to far larger, prominent, caldera-forming peralkaline (e.g. time differences on the order of 1 ka by conventional Sr isotope Mahood, 1984) or metaluminous volcanic systems such determination. Because of their young eruption ages (Ζ20 ka), a as Long Valley (e.g. Halliday et al., 1989; Davies et al., chemically independent assessment of the Sr isotope results has been 1994). The goal is to assess how the rates of magmatic obtained by U-series dating. Rb–Sr isochron ages of pristine glasses processes vary with the size or composition of the system. and phenocrysts from the most chemically evolved rhyolites pre-date We utilize both the Rb–Sr and U–Th geochronometers the eruption ages and are best defined by a mineral isochron of 24 because they have distinct chemical behaviour and re± 1 ka. The glasses are in secular U–Th equilibrium so that no spond in different ways to magmatic processes. Comage information can be obtained. In contrast, glasses and minerals parison of the two isotope systems therefore may provide yield U–Th isochrons of 25 ± 10 ka and are probably controlled additional insights into the time scales of processes that by the Th-enriched accessory phase chevkinite. We therefore ascribe control the evolution of strongly differentiated silicic the pre-eruptive age information to crystallization of the observed magmas. phenocryst phases. Inferred high magma fractionation rates of up Peralkaline felsic magmas are most widespread in reto 2·5 × 10 km/yr are comparable with those for much gions of continental upwelling and/or rifting (Fitton & larger metaluminous silicic magma systems. Magma storage times Upton, 1987), but actually occur in a wide variety of (>22 ky), however, are much shorter and may best be accounted geodynamic settings, including oceanic islands (Caroff et for by the specific size, longevity and thermal gradient of the silicic al., 1993; Mungall & Martin, 1995), island arcs (Bevier magma system. et al., 1979) and extensional continental areas (Mahood, 1981; Civetta et al., 1984; Macdonald, 1987). There are various petrogenetic explanations for the generation of

Diversity and spatial zonation of volcanic rocks from the East Pacific Rise near 21° N

Abstract: A substantial range of petrologic rock types has erupted on the accreting plate boundary near 21° N on the East Pacific Rise (EPR). Young olivine basalts have Fo89-86 phenocrysts, low bulk TiO2 (1.1–1.3%), Ba (7–10 ppm), and high Ni contents (>100 ppm). Older plagioclase-olivine-pyroxene (POP) basalts have Fo86-81 phenocrysts, high TiO2 (1.4–1.7%), Ba (9–40 ppm), and low Ni (<100 ppm). The youngest olivine basalts erupt immediately around a segmented axial fissure system. Progressively older, more fractionated POP basalts have spread farther from the same fissure system, producing a stratigraphically-controlled zonal pattern of basalt type distribution around the eruptive fissures. A topographic and morphologic en echelon displacement of the ridge axis fissure of 1.7 km to the NW near 20°54′N offsets this zonal distribution pattern. Low pressure crystal fractionation of olivine, plagioclase, and clinopyroxene (2∶5∶1) from an olivine basalt parent would yield POP basalts of the observed Zr, Ti, Y, P and major element chemistry. However very incompatible elements Ba and K are too variably enriched in POP basalts for this model to be viable. Small, variable degrees of mantle partial melting is not a viable model either because of the substantial depletion of Ni which correlates with incompatible element enrichment and because of the precise low pressure cotectic character of POP basalts. The in situ fractionation model of Langmuir (1987) can explain these features. The relative abundance of fractionated lavas, their small-scale areal chemical zonation, the petrochemical correlation between types, and geophysical evidence point to the existence of shallow fractionating magma reservoirs beneath the EPR at 21° N.