The Allende Blast
Helps prove our solar system was born in the blast of a dying star.
At 1:05 AM on February 8, 1969, the Allende meteorite created a huge fireball which lighted up thousands of square miles of Northern Mexico and Southwestern United States. When it shattered, it rained fragments over an estimated 50 square miles, and created several pits – the biggest one two+ feet (60 cm) across and 6 inches (15 cm) deep.
Its explosion and break up produced thousands of fusion crusted individuals. This is typical of falls of large stones through the atmosphere and is due to the sudden braking effect of air resistance.
Figure 1.
As a result, these Allende stones became one of the most widely distributed meteorites, and their shear number has provided a large amount of material to study.
As they fell through the atmosphere at great speed, the exterior of the stones became very hot, melting it, and forming a glassy fusion crust.
Here's a drawing of the Allende strewn field where these crusted stones fell:
Figure 2.
And while several tons were originally collected, specimens are still being found.
Here's some of the first samples from the fall:
Figure 3.
What's It Made Of
When a scientist cuts an Allende stone into slices and polishes the cut surfaces, the structure in the interior can be examined.
Using the sample in our collection, let's take a look:
Figure 4.
What we see is a dark stone background, a matrix, embedded throughout with mm-sized, lighter-colored chondrules. If you remember, these tiny stone balls are only found in meteorites and not in earth rock. (Thus, making Allende a chondritic meteorite. See Specs.)
The matrix of the Allende is primarily iron rich olivine. The total iron content is around 24 percent, but flecks of iron-nickel are rarely found.
Also seen are white inclusions, up to several cm in size, ranging in shape from spherical to highly irregular or "clumps of snow flakes." These are known as calcium-aluminum-rich inclusions or "CAIs," so named because they are dominantly composed of Ca- and Al-rich silicate and oxide minerals.
Like many chondrites, Allende is also a breccia, and contains many dark-colored clasts or "dark inclusions" which have a chondritic structure that is distinct from the rest of the meteorite.
Allende also contains small amounts of carbon (including graphite and diamond), and many organic compounds, including amino acids – some not known on Earth.
What else has been discovered?
When all of its elements were analyzed, Allende's chondrules and CAIs were stunning. The CAIs (Calcium-Aluminum Inclusions) are white, millimeter-sized objects found, often with chondrules, in the most primitive carbonaceous chondrites, notably CV and CO. They consist of high-temperature minerals, including silicates and oxides of Ca, Al, and Ti.
In 2002, an international team of scientists accurately dated CAIs at 4.567 Ga, making them the oldest known objects in the solar system, 30 million years older than the Earth, and 700 million years older than the oldest rock known on Earth.
The same team found that chondrules, another of the earliest relics of the solar system, are 2-3 Ma younger than CAIs.
Both types of object formed when dusty regions of the solar nebula were heated to high temperatures by shock waves. The dust melted and then crystallized, forming first CAIs and then chondrules.
Figure 5.
Thus, the Allende meteorite reveals information about conditions prevailing during the early formation of our solar system. Such primitive matter has undergone the least mixing and remelting since the early stages of solar system formation.
Because of this, the age of such matter is frequently taken as the "age of the solar system.
Figure 6.
A Side Trip – Continuing Research
In 1971, a team from Case Western Reserve University, closely examined the condrules and discovered tiny black markings, up to 10 trillion per square centimeter, which were absent from the matrix and interpreted as evidence of radiation damage.
Similar structures have turned up in lunar basalts but not in their Earth equivalent which would have been screened from cosmic radiation by the Earth's atmosphere and geomagnetic field.
Figure 7.
Science considers this irradiation of the chondrules happening after they had solidified -- but before the cold accretion of matter that took place during the early stages of formation of the solar system, when the parent meteorite came together.
Here's an illustration of how chondrules were formed:
Figure 8.
In 1977, the California Institute of Technology found new forms of the elements calcium, barium and neodymium in the meteorite. The researchers believed these new forms came from some source outside the early clouds of gas and dust that formed our solar system – from perhaps a supernova.
Figure 9.
These new forms support the theory that shock waves from a supernova - the explosion of an aging star - may have triggered the formation of, or contributed to the formation of our solar system. As further evidence, the Caltech group said the meteorite contained Aluminum 26, a rare form of aluminum.
This acts as a "clock" in the meteorite, dating the explosion of the supernova to within less than 2 million years before the solar system was formed.
Figure 10.
Subsequent studies have found isotopic ratios of krypton, xenon, nitrogen and other elements that are also unknown in our solar system.
The conclusion, from many studies with similar findings, is that there were a lot of substances in the pre-solar disc that were introduced as fine "dust" from nearby stars, including novas, supernovas, and red giants.
These specks persist to this day in meteorites like Allende, and are known as presolar grains.
In 1997, at the Technical University of Denmark – researches A. C. Andersen, K. Glejbl, and U. G. Jrgensen – studied Presolar diamonds within presolar Silicon carbide (SiC) grains in the Allende meteorite.
They discovered that each diamond contained a few thousand carbon atoms, and that these diamonds grew.
Various mechanisms have been proposed to account for the production of diamond grains in space but the most likely scenario seems to be that they have condensed directly from stellar out-flows.
The conditions in such cool stellar out-flows are remarkably similar to those employed in industry to produce diamonds by chemical vapor deposition (CVD).
Once the diamonds were extracted, they were studied using transmission electron microscopy and electron diffraction: Both presolar and CVD diamonds were examined to gain a deeper insight into their structures. The researchers also examined possible presolar SiC grains.
Just The Specs
Carbonaceous Chondrite CV3
Village of Pueblito de Allende, Chihuahua, Mexico
Fall - February 8, 1969
Matrix or fine grained part is primarily iron rich olivine.
Iron content is around 24 percent, but flecks of iron-nickel are rarely found.
From the Smithsonian:
Three distinct components can be recognized:
- finegrained black matrix (~60%),
- chondrules (~30%), and
- irregular white aggregates (^10%).
The matrix consists almost entirely of iron-rich olivine (average 50% Fe2SiO4), with minor amounts of troilite, pentlandite, and taenite, rendered opaque by dispersed carbonaceous material.
Most of the chondrules are magnesium-rich, and consist of olivine (average 9% Fe2SiO4) with minor amounts of clinoenstatite and some glass; a few chondrules are rich in calcium and aluminum, and are made up largely of anorthite, gehlenite, augite, and spinel.
The irregular aggregates are also rich in calcium and aluminum, and contain anorthite, gehlenite, augite, spinel, nepheline, grossular, and sodalite (the last two minerals have not previously been recorded from meteorites).
Here's a link to The Meteoritical Society database:
tin.er.usgs.gov
Terms
As we have seen with other meteorites, Allende is a historic chondrite with some unique features. Here's some of the terms used to describe the meteorite.
Definition of Breccia
- n. A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.
Here's an example:
Photo from www.lexic.us
Definition of Carbonaceous
- Adj. Relating to or consisting of or yielding carbon.
Photo from the Wolf Research Group – Murchison Chondrite, 1969
carbon.indstate.edu
The average Carbonaceous Chondrite contains:
Element % by Weight
-----------------------
Carbon 2.0
Metals 1.8
Nitrogen 0.2
Silicates 83.0
Water 11.0
-----------------------
At most, they can be 20% water and can contain as much as 4% carbon.
Definition of Chondrule
- n. A peculiar rounded granule of some mineral, usually enstatite or chrysolite, found embedded more or less abundantly in the mass of many meteoric stones, which are hence called chondrites.
Photo of an exceptional NWA 2785 LL3 Chondrule filled w/ Green Olivine Crystals.
From www.meteorite-pictures.org
Definition of Clast ( -s)
- n. A fragment of rock.
Here's a picture of a clast:
www.lexic.us
Links & PodCasts
Here's two dictionaries for reference:
www.science-dictionary.org/
Lexic also includes excellent pictures:
www.lexic.us
See "Fluffy" Type "A" Calcium Aluminum Inclusions by Russell Kempton, New England Meteoritical Services
www.meteorlab.com
Using Aluminum-26 as a Clock for Early Solar System Events
www.psrd.hawaii.edu
Here's a PodCast from Brain's Matter: Dr Maria Lugaro on the topic of pre-solar grains
brainsmatter
The Story of a Blast Witness -- A short article by Geoffrey Notkin.
It's a short story about Vicki Allison, who at 5-years old, was living with her American missionary parents in an old adobe home in Chihuahua, Mexico, on the eastern edge of the Sierra Madre mountains.
The family witnessed the Allende fall: meteoriteblog.org
To visit Geoffrey's excellent site, click here: www.aerolite.org
Figures & Acknowledgments
Figures
Figure 1. Photo from www.phys.ncku.edu.tw
Figure 2. Illustration by Ben Roesch. Wikipedia.
Figure 3. Photograph courtesy of Brian Mason, National Museum of Natural History, Smithsonian Institution.
Figure 4. Photo by New England Meteoritical Services. www.meteorlab.com
Figure 5. Image source: publicaffairs.llnl.gov
Figure 6. Photo by New England Meteoritical Services. www.meteorlab.com
Figure 7. Photo by NASA.
Figure 8. Image from www.meteoritemarket.com
Figure 9. Photo by Hubble - The Crab Nebula supernova remnant.
Figure 10. www.corbisimages.com © 2002-2009 by Corbis Corporation.
Notes
A number of researchers have proposed that the CARC's (Calcium-aluminium-rich clast) in the Allende Carbonaceous chondrite represent high-temperature condensates from a gas of solar composition during the very earliest stages in the formation of the solar system.
|
