Hugo Moreno

Calbuco Volcano and minor eruptive centers distributed along the Liquiñe-Ofqui Fault Zone, Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes

Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20′ S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO2 contents in the 55–60% range, and have comparatively low K2O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but 87Sr/86Sr and 143Nd/144Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. 87Sr/86Sr and 143Nd/144Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO2 contents, low 143Nd/144Nd and high 87Sr/86Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ.

Standard stars for photometry of comets

Abstract A set of standard stars for photometric observations of comets has been established. Magnitudes for 63 stars in 10 bandpasses are presented. The bandpasses are those of the nine filters recommended by the International Astronomical Union for comet photometry, which measure the C 2 , C 3 , CN, CO + , H 2 O + and OH emission and three continuum points, plus a 10th filter which measures NH emission. The stars observed include those recommended by the International Halley Watch for use in photometric observations of Comet Halley during the 1985–1986 apparition.

STUDIES OF SOUTHERN PLANETARY NEBULAE. I. FLUXES FROM BRIGHT PLANETARY NEBULAE.

A study of spectrophotometric observations of some southern planetary nebulae, made during 1980 and 1981, to increase and improve the absolute data for some of them and to detect objects of special interest, is discussed. Observed H-beta fluxes, logarithmic reddening correctitons, and intrinsic forbidden-line intensities relative to H-beta are given for 14 bright southern planetary nebulae. The results of the study, as well as those obtained from other authors, are listed in tables and compared.

Lanín volcano (39.5°S), Southern Andes: geology and morphostructural evolution

El volcan Lanin es un estratovolcan compuesto, predominantemente efusivo, para el que se han definido cuatro unidades segun criterios geomorfologicos. La primera de ellas representa restos de una estructura ancestral; las tres siguientes constituyen el edificio volcanico actual construido esencialmente desde el Pleistoceno Medio-Superior. Composicionalmente, las rocas volcanicas del volcan Lanin corresponden, principalmente, a basaltos/andesitas basalticas y dacitas subordinadas con escasas variedades intermedias. Los depositos piroclasticos post glaciales muestran tambien composiciones siliceas y confirman una estricta bimodalidad composicional de los magmas. Los patrones de tierras raras y elementos mayores indican una evolucion magmatica de baja presion controlada por fraccionamiento de plagioclasa y ortopiroxeno con extraccion de olivino, clinopiroxeno y magnetita, sin interacciones complejas. Los ciclos volcanicos efusivos serian controlados por un reducido tiempo de residencia en una camara magmatica superficial con evacuacion rapida y simultanea de dacitas y basaltos. En erupciones recientes, el sellamiento del conducto central con magma viscoso induciria el drenaje lateral de los basaltos y, eventualmente, el colapso parcial de la zona apical del cono. Sin embargo, los procesos de degradacion mas activos de la estructura volcanica actual se relacionan mas bien con procesos exogenos ligados a la cobertura de hielo. La singular evolucion del volcan Lanin, composicional y morfologicamente intermedio entre los conos monogenicos y los estratovolcanes de la cadena Villarrica-Lanin, se asociaria a su posicion distal de la fosa que origina bajos grados de fusion en la astenosfera y reducidos pulsos de magma ascendente que se almacenarian en una camara de actividad efimera.

A STUDY OF ATMOSPHERIC EXTINCTION AT CERRO TOLOLO INTER-AMERICAN OBSERVATORY. II

The behavior of the atmospheric extinction at CTIO during 1984 is analyzed. Rayleigh scattering, aerosol extinction, and ozone absorption parameters are determined. The behavior of the extinction during different months of the year is discussed.

Photometry at Cerro Tololo, Chile: Effects of Mount Agung Eruption

Extinction effects of the volcanic dust injected into the stratosphere by the violent eruption of Mount Agung, Bali, 17 March 1963, were studied. Visual extinction coefficients were measured photoelectrically for 153 nights during the period from March 1963 to September 1964. The data indicate that the dust is now widely dispersed and that its effects may persist for several more years. A study of the wavelength dependence of the extinction shows that the dust is a neutral scattering agent. The total global amount of airborne volcanic material is crudely estimated at 1012 grams as of September 1964.

New Spectroscopic Observations of the Planetary Nebula PC 11

New observations of the planetary nebula PC 11, made in 1991, are presented and discussed. The object is a compact planetary nebula, small and dense, with a low mass of ionized gas. It is losing matter through one or more jets, formed mainly by O iii. The central star is a binary, with a cool component of spectral type F0-F1 V. The hot component has an energy balance temperature of 105,000 K, a mass of 0.638 M,, and very low luminosity, estimated as 11 L,, with a radius of the order of 0.01 R,. The position of PC 11 in the H-R diagram is peculiar, since it seems incompatible with its being a very young planetary nebula, as suggested by its small size and high density.

Ultraviolet and Optical Observations of Hen 1213, Hen 1341, and Hen 1761

We present UV observations of the symbiotic stars Hen 1213, Hen 1341, and Hen 1761, made almost simultaneously with optical observations during 1995; previous optical observations were made during 1987 and 1991. The UV spectrum of Hen 1213 was badly affected by background radiation, and only a rough description of the spectrum could be given here; for this reason, only the observations of Hen 1341 and Hen 1761 are discussed. They have the typical spectra of symbiotic stars, with a late giant component, with many emission lines superimposed. It is found that for both objects the reddening determined from He II λ1640/He II λ4686 is smaller than that obtained from the Balmer decrement, including optical depth effects. Hen 1341 does not show important variations for the three observing periods. Hen 1761 shows P Cygni profiles for He II λ4686 and He I λ4471, with expansion velocities of the order of 900 and 600 km s-1, respectively. The 1991 observations give the lower temperature of the hot component with the larger size of the nebula and the lowest He contents. Rough estimates indicate that the hot component of Hen 1761 is smaller and less luminous than that of Hen 1341.

SPECTROSCOPIC OBSERVATIONS OF THE PLANETARY NEBULAE ME 2-1

Observations of the planetary nebula Me2-1 were obtained with different set-ups, in order to analyze some effects of contamination produced by the second order ultraviolet spectrum in the first order red, for wavelengths longer than approximately 6000 A. This contamination problem will be discussed elsewhere. Here we present the observations of Me2-1, which include a wide wavelength range, from about 3100 to 10200 A. A comparison with previous results is shown for the wavelength intervals in common. From these data, the most relevant nebular parameters are derived, using lines in the optical and near IR regions. Some parameters corresponding to the central star are also determined. The results are compared with previously obtained values.

STUDIES OF SOUTHERN PLANETARY NEBULAE. II. ELECTRON TEMPERATURES AND DENSITIES.

Electron temperatures and densities for 14 bright southern planetary nebulae, obtained from data previously published, are presented here. It is found that the difference between the logarithmic electron densities obtained from the ratio of nebular lines and from the ratio of auroral or transauroral lines to nebular lines is linearly correlated with the electron densities. It is also found that the ratio of the temperatures in the O III and N II forbidden line regions, TH/TL, is linearly correlated with the degree of excitation, measured through the intensity of He II 4686A. The relation is double valued, with values of TH/TL both larger and smaller than 1 for the same degree of excitation. Up to this moment, no physical parameter giving a unique correlation with TH/TL has been found.