jueves, diciembre 29, 2005
The present report was published at http://www.astrobiology.com/, as a successful investigation carried out by B.S. Ann Nguyen and Dr. Ernst Zinner at the Washington University in St. Louis.
Résumé and Research Interests
Lan-Anh N. Nguyen obtained her B.S. in Chemistry at the University of North Carolina at Chapel Hill in 2000. As undergraduate, Ann was an intern at the Department of Terrestrial Magnetism, Carnegie Institution of Washington, under the supervision of Dr. Conel Alexander.
Currently, Ann is working with Drs. Ernst Zinner and Frank Podosek in studying the isotopic composition of presolar silicon carbide grains. These measurements will be performed mainly on the newly acquired NanoSIMS. This instrument, in comparison to conventional ion probes, has a smaller primary ion beam size, a higher transmission of secondary ions, and is capable of multidetection. These features allow for the study of smaller presolar grains, the study of more elements, and higher precision.
First silicate stardust found in a meteorite
Ann Nguyen chose a risky project for her graduate studies at Washington University in St. Louis. A university team had already sifted through 100,000 grains from a meteorite to look for a particular type of stardust -- without success.
In 2000, Nguyen decided to try again. About 59,000 grains later, her gutsy decision paid off. In the March 5 issue of Science, Ann Nguyen of Washington University in St. Louis and her advisor, Ernst K. Zinner, Ph.D., research professor of physics and of earth and planetary sciences, both in Arts & Sciences, describe nine specks of silicate stardust -- presolar silicate grains -- from one of the most primitive meteorites known.
"Finding presolar silicates in a meteorite tells us that the solar system formed from gas and dust, some of which never got very hot, rather than from a hot solar nebula," Zinner says. "Analyzing such grains provides information about their stellar sources, nuclear processes in stars, and the physical and chemical compositions of stellar atmospheres."
In 1987, Zinner and colleagues at Washington University and a group of scientists at the University of Chicago found the first stardust in a meteorite. Those presolar grains were specks of diamond and silicon carbide. Although other types have since been discovered in meteorites, none were made of silicate, a compound of silicon, oxygen and other elements such as magnesium and iron.
"This was quite a mystery because we know, from astronomical spectra, that silicate grains appear to be the most abundant type of oxygen-rich grain made in stars," Nguyen says. "But until now, presolar silicate grains have been isolated only from samples of interplanetary dust particles from comets."
Our solar system formed from a cloud of gas and dust that were spewed into space by exploding red giants and supernovae. Some of this dust formed asteroids, and meteorites are fragments knocked off asteroids. Most of the particles in meteorites resemble each other because dust from different stars became homogenized in the inferno that shaped the solar system. Pure samples of a few stars became trapped deep inside some meteorites, however. Those grains that are oxygen-rich can be recognized by their unusual ratios of oxygen isotopes.
Nguyen, a graduate student in earth and planetary sciences, analyzed about 59,000 grains from Acfer 094, a meteorite that was found in the Sahara in 1990. She separated the grains in water instead of with harsh chemicals, which can destroy silicates. She also used a new type of ion probe called the NanoSIMS (Secondary Ion Mass Spectrometer), which can resolve objects smaller than a micrometer (one millionth of a meter).
Zinner and Frank Stadermann, Ph.D., senior research scientist in the Laboratory for Space Sciences at the university, helped design and test the NanoSIMS, which is made by CAMECA in Paris. At a cost of $2 million, Washington University acquired the first instrument in the world in 2001.
Ion probes direct a beam of ions onto one spot on a sample. The beam dislodges some of the sample's own atoms, some of which become ionized. This secondary beam of ions enters a mass spectrometer that is set to detect a particular isotope. Thus, ion probes can identify grains that have an unusually high or low proportion of that isotope.
Unlike other ion probes, however, the NanoSIMS can detect five different isotopes simultaneously. The beam can also travel automatically from spot to spot so that many hundreds or thousands of grains can be analyzed in one experimental setup. "The NanoSIMS was essential for this discovery," Zinner says. "These presolar silicate grains are very small -- only a fraction of a micrometer. The instrument's high spatial resolution and high sensitivity made these measurements possible."
Using a primary beam of cesium ions, Nguyen painstakingly measured the amounts of three oxygen isotopes -- 16-O, 17-O and 18-O -- in each of the many grains she studied. Nine grains, with diameters from 0.1 to 0.5 micrometers, had unusual oxygen isotope ratios and were highly enriched in silicon. These presolar silicate grains fell into four groups. Five grains were enriched in 17-O and slightly depleted in 18-O, suggesting that deep mixing in red giant or asymptotic giant branch stars was responsible for their oxygen isotopic compositions.
One grain was very depleted in 18-O and therefore was likely produced in a low-mass star when surface material descended into areas hot enough to support nuclear reactions. Another was enriched in 16-O, which is typical of grains from stars that contain fewer elements heavier than helium than does our sun. The final two grains were enriched in both 17-O and 18-O and so could have come from supernovae or stars that are more enriched in elements heavier than helium compared with our sun.
By obtaining energy dispersive X-ray spectra, Nguyen determined the likely chemical composition of six of the presolar grains. There appear to be two olivines and two pyroxenes, which contain mostly oxygen, magnesium, iron and silicon but in differing ratios. The fifth is an aluminum-rich silicate, and the sixth is enriched in oxygen and iron and could be glass with embedded metal and sulfides.
The preponderance of iron-rich grains is surprising, Nguyen says, because astronomical spectra have detected more magnesium-rich grains than iron-rich grains in the atmospheres around stars. "It could be that iron was incorporated into these grains when the solar system was being formed," she explains.
This detailed information about stardust proves that space science can be done in the laboratory, Zinner says. "Analyzing these small specks can give us information, such as detailed isotopic ratios, that cannot be obtained by the traditional techniques of astronomy," he adds. Nguyen now plans to look at the ratios of silicon and magnesium isotopes in the nine grains. She also wants to analyze other types of meteorites. "Acfer 094 is one of the most primitive meteorites that has been found," she says. "So we would expect it to have the greatest abundance of presolar grains."
By looking at Meteorites that have undergone more processing, we can learn more about the events that can destroy those grains.
domingo, diciembre 25, 2005
Workshop on Parent-Body and Nebular Modification of Chondritic Materials
ALTERATION OF CAIs: TIMES AND PLACES.
S. S. Russell and G. J. MacPherson, Department of Mineral Sciences, MRC NHB-119, U.S. Museum of Natural History, Smithsonian Institution, Washington DC 20560, USA. E-mail: firstname.lastname@example.org
Calcium- Aluminium- rich inclusions (CAIs) commonly contain a distinctive suite of secondary minerals. The chemical and isotopic compositions of these minerals can be used to constrain the site and timing of the alteration event. The style of alteration in CAIs is strongly dependent on the meteorite group in which they are found.
CV meteorites: CAIs from the oxidised subgroup (e.g. Allende) show extensive signs of secondary alkali- and iron- enrichment. The fine grained secondary minerals (typically <10-20>monticellite, hedenbergite, andradite, and grossular; these typically embay primary minerals and fill cross- cutting veins within the CAIs. Some euhedral whiskers of wollastonite and nepheline are located within cavities. In addition, multi-layered Wark-Lovering rim sequences on CAIs clearly postdate the CAI interior, and in that sense can be considered secondary. Fine-grained inclusions are typically more altered than coarser grained ones: alteration in these inclusions consists of feldspathoid layers surrounding primary spinel. The temperature of melilite + anorthite breakdown to grossular + monticellite in type B Allende CAIs has been estimated to be 668 ºC . Hutcheon and Newton argued that the temperature must have remained around this value for a “prolonged period” to allow formation of large grossular grains, but the timing of the high temperature alteration event was probably minor to 100,000 years, otherwise Mg diffusion would ensure the CAI anorthite no longer retains a Mg-26 excess .
The location of the alkali-iron alteration has been widely debated. Most workers believe the alteration took place in a nebular setting. The sequence of alteration is compatible with equilibration with a cooling, oxidised solar nebula gas . Wark  argued for pre-accretionary alteration because of the presence of alkali-rich halos in the meteorite matrix surrounding some CAIs. Sodium mapping of Allende CAIs shows that the sodium is enriched in accretionary rims, suggesting CAIs became alkali- rich prior to incorporation in the parent body. Veins cross-cutting CAIs typically do not extend into the meteorite matrix, indicating they did not form in situ. Euhedral wollastonite whiskers, nepheline needles, and grossular in CAI cavities are indicative of condensation from a vapour, and these grains probably formed in the nebula. An alternative viewpoint, championed by Krot et al., argues that the alteration of CV CAIs can be explained by a parent body process of alteration by alkaline-rich fluids followed by dehydration . This process is postulated to have affected the more oxidised CV meteorites, such as Allende, more than the other CVs, a conclusion also reached by some other studies [e.g. 6].
CO meteorites: CAIs in CO3 chondrites have experienced considerable secondary alteration, both before and after accretion [7, 8]. The presence of altered CAIs in unmetamorphosed CO3s indicates some events occurred in the nebula: formation of Wark-Lovering rims, melilite and anorthite breakdown, and iron enrichment of spinels in hibonite-rich inclusions. In contrast, correlations between petrologic type of the host meteorite with iron content and melilite breakdown in Type A and spinel-pyroxene CAIs suggest some alteration occurred during parent body metamorphism . Hibonite seems to be unaffected by the metamorphism experienced by CO3s.
CM meteorites: CAIs in CM chondrites have suffered ubiquitous aqueous alteration. The CAI primary mineralogy has been altered to pyllosilicates (Fe- and Mg-serpentines) and tochilinite, calcite and calcium sulphate. Secondary minerals typically occur in a layer immediately beneath the rim sequence. Some CM CAIs have also suffered fragmentation and recrystallisation. It is not clear what the phyllosilicate is replacing: anorthite is a possibility. Greenwood et al.  suggest that nebula processes caused fragmentation of CAIs, whereas aqueous alteration took place over a protracted period of time on the parent body. In contrast, MacPherson and Davis  argued that the CAIs were not altered in the environment in which they are now found, and many are too fragile to have been moved to their current location by recycling in the regolith, so they favoured formation of hydrous secondary minerals in a nebula environment.
CR meteorites: CAIs in the CR chondrite Acfer 059 shows no evidence of alteration , whereas inclusions in Renazzo and Al Rais contain some secondary calcite .
CH meteorites: Some CAIs in the CH chondrite ALH 85085 show evidence of recrystallisation due to reheating . In contrast, inclusions from PCA 91467 and Acfer 182 appear unaltered .
Unequilibrated ordinary chondrites (UOC): CAIs in ordinary chondrites are rare. UOC CAIs are often rimmed, and secondary feldspathoids are occasionally present. In one Semarkona CAI, melilite has been partially replaced by sodalite .
Times: While most CAIs are believed to have formed at around the same time, their alteration was an on-going process that took place over several million years. I-Xe dating suggests that the alteration took place up to >10Myr after initial CAI formation . Al-Mg studies of grossular in CV CAIs also indicate formation > 2.4 Myr after CAI production , and a Al-Mg analysis of a recrystallised CAI from CH chondrite indicates a heating event > 2 Myr after CAI production . Chemical exchange between anorthite and melilite in Type B inclusions appears to have occurred >2-3 Myr after CAI formation . Similarly, sodalite in fine grained CV inclusions is postulated to have formed after Al-26 decay, ie., several Myr after CAI formation . In contrast, sodalite in a Semarkona (LL3.0) inclusion apparently formed very quickly after CAI formation .
Places: Many CAIs are pristine, but some underwent several heating events. The presence of altered plus pristine CAIs close together in some meteorites (e.g. CMs) suggests that some alteration occurred before they reached their current site in the parent body (although this may reflect post- accretionary brecciation). Many features of alteration appear to have occurred in the nebula. Wark-Lovering rims predate accretion into the present parent bodies. Some primary minerals exchanged with a nebula gas, and some secondary minerals condensed from a vapour. Sodalite in Semarkona  probably formed in the nebula, since Al-26 dating suggests it formed before the accretion of the asteroids. In contrast, the long time span of alteration suggested by I-Xe dating for Allende CAIs has been used to argue in favour of alteration in a parent body . In addition to nebula processes, metamorphism in parent bodies tended to equilibrate CAIs with their host rock. Aqueous processing in some meteorites may have affected CAIs in parent bodies. The location of the event responsible for incorporation of alkalis into CAIs, however, is still highly contentious.
 Hutcheon and Newton (1981), LPSC XII 491-493;
 LaTourette and Wasserburg (1997) LPSC 28 781-782;
 Hashimoto and Grossman (1987) GCA 51 1685-1704;
 Wark (1981) LPSC XII 1145-1147;
 Krot et al., (1995) Meteoritics 30 748-775;
 McSween (1977) GCA 41 1777-1790;
 Greenwood et al., (1992) Meteoritics 27 229;
 Russell et al., (1997) GCA, submitted;
 Greenwood et al., (1994) GCA 58 1913-1935;
 MacPherson and Davis (1994) GCA 58 5599-5625;
 Weber and Bischoff (1997) Chem. Erde 57 1-24;
 Weisburg et al., (1993) GCA 57, 1567-1586;
 Kimura et al., (1993) GCA 57 2329-2360;
 Weber et al., (1995) LPSC XXVI 1475-1476;
 Russell et al., (1997) LPSC XXVIII, 1209-1210;
 Swindle et al., (1998) GCA 52 2215-2227;
jueves, diciembre 08, 2005
Informe del Dr. Rodolfo Mannheim
El presente artículo muestra, casi en su totalidad, el informe escrito en septiembre de 2004 por el Dr. Rodolfo Mannheim, de la Universidad de Santiago de Chile (USACH), donde hace sus apreciaciones y conclusiones sobre el Acero que conforma la Roca Veas-01.
De acuerdo a las conclusiones del Dr. Mannheim el material de Veas-01 se trata en realidad de un Acero, y que de ninguna manera se trataría de un Fierro Fundido.
Respecto a las estructuras ovilladas, fotografiadas separadamente tanto por el Dr. Mannheim de la USACH como por el Dr. Mauricio Belmar del Departamento de Geología de la Universidad de Chile, y ampliamente mostradas y difundidas en este Blog, el profesor Rodolfo Mannheim dice: “en las figuras 17 a 20 aparecen elementos con una forma irregular que desde el punto de vista químico no tienen ninguna justificación.”. “De acuerdo a la composición química, los extremadamente bajos contenidos de silicio (Si), que están presentes en este Acero, indican que se le inyectó aire que posiblemente quemó una parte del carbono, la totalidad del silicio y algo de otros elementos.”
Finalmente, y bajo microscopía electrónica del Departamento de Metalurgica de la USACH, se probó que las estructuras en forma de ovillo contenían Fe, y que éste no se podía diferenciar del resto de la matriz.
El profesor Rodolfo L. Mannheim Cassierer es Ingeniero Civil Metalúrgico (1976) de la USACH, y Dr. en Ingeniería Metalúrgica y Materiales (1981) de la Universidad Técnica de Berlin, Alemania. Sus líneas más importantes de investigación son: Fundición de Metales, Solidificación de Metales, Pulvimetalurgia y Metalurgia Física.
Detalles de sus investigaciones pueden ser encontradas en el siguiente sitio web:
¿Magnetobacterias en el Meteorito ALH84001?
Durante febrero de 2001 los españoles, el Dr. Jacek Wierchos y la Dra. Carmen Ascaso, publicaron, en el PNAS (Proceedings of the National Academy of Science of the United States of America), un controvertido artículo titulado “Chains of magnetite crystals in the meteorite ALH84001: Evidence of biological origin”.
Sus análisis y publicaciones fueron hechos en base a un pequeño trozo, que les fuera entregado por NASA, del conocido meteorito marciano tipificado como ALH84001, del cual se comentó en 1996 podía contener restos de vida extraterrestre; versión que, al parecer, nunca habrían sido confirmados por los experimentos científicos posteriores.
Ambos doctores argumentan y extraen textualmente de su trabajo (ver foto oficial siguiente):
1.- “we confirmed the results that the magnetite crystals are mostly paralellepipedal and, less frequently, bullet or irregularly shaped, with a size range of 30-90 nm in length and 20-50 nm in width.”
2.- “No abiological process is known that would result in such sorting of crystals from a mixed pool of sizes and shapes”.-
Como ya ha sido comentado, tanto en los análisis efectuados en la Universidad de Chile como en los de la USACH, se encontraron estructuras curiosas, con formas ovilladas, formadas con base en Fe y posibles óxidos de Fe, cuyo orígen aún no se ha podido establecer. Lo concreto es que la dimensiones de los cristales de fierro que aparecen en las microfotos tomadas en Chile varían entre los 0.68 micras y 1.20 micrómetros; valores muy superiores a los diámetros de los cristales observados por los doctores Wierzchos y Ascaso.
Por lo anterior, nuestro equipo procedió a contactarse con el Dr. Jacek Wierzchos, y junto con enviarle a ellos copias de las microfotos adquiridas en Chile, se le formuló la siguiente pregunta técnica:
Pregunta formulada el 2 de noviembre de 2005:
¿es posible encontrar estructuras similares, es decir, cadenas de oxidos de fierro, como pudiese ser magnetita o maghemita, insertas en estructuras metalicas y/o mezcladas con piroxenos y plagioclasas; cadenas con un tamaño de cristales en un rango de entre 0.68 micras y 1.20 micras?. De ser asi, ¿no sería esta una prueba de algún comportamiento no biológico que pudiese haberlas formado?. Por otro lado, “entre las microfotos no publicadas que obran en su poder, ¿han podido hallar cristales gigantes, con diámetros como los descritos por nosotros?.
Hasta el momento no hemos recibido una respuesta técnica satisfactoria, y sus únicos correos recibidos por nosotros han sido del tenor siguiente:
(2 de noviembre de 2005) “Carmen Ascaso y yo tenemos otras fotos de las cadenas (unpublish) pero como estas no son publicadas deberiamos saber cuál es el objetivo por el cual Usted quisiera tener estas imagenes.”
(11 de noviembre de 2005). “He recibido correctamente todo el material y le pido perdón por la demora en la respuesta. En la semana que viene intentaré responder a su e-mail.”
Un cordial saludo,
email@example.com (Jacek Wierzchos)