METEORITE RESEARCH AT UCLA:

To those browsing this document on the web: What follows is the text of an exhibit of meteorites and books about meteorites that is occurring at the Powell Library at UCLA on March 11-27, 1998. The exhibit is in the rotunda of this striking "romanesque" building.

Cosmochemistry at UCLA:
The UCLA Collection of Meteorites

The UCLA collection of meteorites is a major research resource serving the needs of the international meteorite-research community. It includes about 620 different meteorites including the main masses of a number of meteorites. Our collection is the fifth largest in the United States, and the largest on the West Coast.

Numerous publications in cosmochemistry (as meteorite research is commonly designated) have resulted from the study of these (and other) meteorites using state-of-the-art analytical instrumentation available on the UCLA campus. This includes an ion microprobe which can measure trace elements and isotopic ratios in individual mineral grains, an electron microprobe which yields accurate mineral compositions and neutron-activation equipment that can measure up to three dozen elements at concentrations as low as 10^-12 grams per gram of sample. Current topics of interest to UCLA researchers include the origin of chondrules and refractory inclusions in chondritic meteorites, the collisional history of asteroids (the parent bodies of most meteorites), and the crystallization of the molten cores of asteroids.

This exhibit was organized to celebrate the purchase of the new Anoka iron meteorite by UCLA and a consortium that included five other institutions* [*the Field Museum, the Natural History Museum of London, the Smithsonian Institution, Harvard University and the University of Minnesota]. This beautiful new 123-lb (56-kg) iron is a member of an uncommon class, the IIICD iron meteorites. It was uncovered when a trench for a water line was dug in a town near Minneapolis.

The history of the UCLA collection. The nucleus of the collection was the private collection of Frederick C. Leonard, the UCLA professor who founded the Department of Astronomy. Following Leonard’s death, the collection, numbering about 180 different meteorites, was purchased by the Institute of Geophysics and Planetary Physics. It has grown to its present size mainly due to the efforts of John Wasson and Alan Rubin, with key financial support from the Institute of Geophysics and Planetary Physics and several Deans of Physical Sciences. Occasional financial support from NASA has helped with key purchases.

Case 1: Chondritic meteorites; origin of chondrules & refractory inclusions

Chondrites formed as individual particles that agglomerated together in the solar nebula, the primitive gas and dust cloud that surrounded the forming Sun and gave birth to the planets. Chondritic abundances of the condensable elements (all elements except noble gases such as He and Ar and the common light elements C, N and O) are essentially the same as in the Sun. The inner rocky planets including the Earth and Venus are believed to have chondritic compositions.

The name chondrite refers to the fact that a large fraction of these rocks consists of millimeter-size silicate grains called chondrules, after the Greek word for grains.

The Allende specimen is a carbonaceous chondrite containing numerous millimeter-size dark chondrules and larger white particles known as refractory inclusions or calcium-aluminum-rich inclusions (sometimes called CAIs).

In some chondrites, the chondrule shapes have been obscured by recrystallization. Although chondrules are poorly defined in the Faucett specimen, one easily sees shiny grains of iron-nickel metal in a matrix of silicate minerals. Such metal-silicate rocks inherited these phases from the solar nebula. Chondrites have never been melted and, hence, are examples of undifferentiated rocks. Melting would have caused the metal and silicate to separate.

Chondrules formed very early in solar system history by the flash-melting of dust aggregates in the solar nebula. The occurrence of some chondrules within other chondrules (i.e., compound chondrules) indicates that the flash-melting mechanism was a repeatable process. Chondrules constitute about 70% of the volume of common chondritic meteorites. Chondritic meteorites themselves make up about 80% of all meteorites observed to fall. These observations indicate that the process of chondrule formation was an important one in the solar nebula. Lightning is widely discussed as a possible mechanism for chondrule formation.

Refractory inclusions are comprised of minerals rich in calcium and aluminum which crystallize at high temperatures. These inclusions are considered to be the first solids to have formed in the solar nebula. Many contain unusual ratios of oxygen and magnesium isotopes which provide important clues to the origin of the solar system. Recent UCLA work on these objects suggests that all may have been formed in one region and then dispersed throughout the nebula.

Faucett Missouri USA	H5 ordinary chondrite. Several stones were recovered totaling more than 100 kg. The abundant flakes of metallic iron-nickel mixed in with the silicate indicate that this rock never melted.
Allende Mexico	CV3 carbonaceous chondrite. The small dark spherules are chondrules; the large white chips are refractory inclusions.
Murchison Victoria Australia	CM2 carbonaceous chondrite . This dark stone contains significant amounts of carbon including many pre-terrestrial amino acids and a moderately large content of hydrous minerals.

Case 2: Chondritic meteorites; heating, melting and brecciation

The structures of most meteorites were modified on their parent asteroids by a variety of processes including thermal metamorphism (this mainly consists of crystal growth and chemical equilibration of minerals) and shock metamorphism (high-pressure alteration including formation of new phases). A major puzzle is the identification of the mechanism(s) responsible for heating the asteroids. Leading contenders are the decay of short-lived radio-isotopes, heating by electrical currents induced by ionized particles streaming away from the Sun and collisional heating. Recent work at UCLA has found that there is a correlation between the degree to which individual chondrites have been shocked and the extent to which they have been heated. This suggests that collisional heating played an important role.

Some chondrites have been partly melted by the energy released when a meteorite projectile impacts the surface of another asteroid. Metal and silicate liquids are immiscible and tend to separate during impact-melting events. In many impact-melted chondrites, the silicate liquid forms thick veins that cut through unmelted material; the molten metal appears to have plated out against the walls of the veins. The specimen of Chico contains a linear vein of silicate melt between the blue arrows and a broad irregular vein between the red arrows. The metal nodules in the Faucett chondrite (case 1) also represent impact-melted metal.

In some cases, chondrites have been crushed (i.e., brecciated), but not melted, by the energy of an impact on its parent asteroid. In some cases a subsequent impact caused minor melting along grain boundaries to fuse the meteorite back together. Meteorites made up of fragments bonded together to form a new rock are called breccias. The sample of Naryilco shows such a brecciation history.

La Criolla Entre Rios Argentina	L6 ordinary chondrite. Purchase of this 7.0 kg (15.4-lb) stone, the largest mass recovered from a shower of stones, was made possible by a gift from Professor Henry Bruman. The dark, matte fusion crust and light-colored "concrete-like" interior is typical of ordinary chondrites, the most common meteorites to fall.
Chico New Mexico USA	L6 ordinary chondrite impact-melt breccia. A straight narrow band of impact-generated melt is outlined by blue arrow tips and a broad irregular region lies between the red arrows. The round hole in the specimen is where research material was removed. You may have to look at the specimen from different angles to recognize the slightly darker melted zones.
Naryilco Queensland Australia	L/LL6 ordinary chondrite breccia. The specimen contains numerous angular, centimeter-size light and dark clasts that have been fused together. Such structures are typical of impact products observed in stony meteorites.

Case 3: Differentiated meteorites; volcanic silicates, core-mantle samples and mixtures of volcanic silicates and droplets splashed from a molten core

Initially all asteroids consisted of chondritic materials. At some later time some of them were extensively melted. In those cases, the dense metallic liquid separated from the silicate liquid and settled in the center of the asteroid to form an iron core. Such cores are the source of most iron meteorites (case 4). The overlying mantle of these bodies is thought to consist mainly of coarse crystals of the magnesium silicate, olivine. Some meteorites (known as pallasites) formed at the core-mantle boundary of differentiated asteroids; they consist of about 60 % (by volume) olivine crystals from the asteroid mantle and 40% metal.

Partial melting of chondritic asteroids produced a dark, fine-grained low-density volcanic rock called basalt. Basaltic meteorites are called eucrites and they are believed to be from the crust of differentiated asteroids.

Mesosiderites are enigmatic because they consist mainly of basaltic meteorite fragments derived from the crust of a differentiated body and finely disseminated metallic iron-nickel from the core, but relatively little olivine from the mantle. Numerous models have been proposed to account for mesosiderites, but none gained wide acceptance. Recent research at UCLA has led to the most successful model of mesosiderite origin: the low-velocity collision of a molten core of a differentiated asteroid with the basaltic surface of another asteroid early in solar-system history.

Imilac Atacama Chile	Main-group pallasite. The angular olivine is typical of pallasite, the angularity is indicative of the fragmentation that occurred at the time molten metal from the core was injected into these mantle silicates.
Millbillillie W. Austr. Australia	Eucrite. This basalt contains crystals of variable size; they are relatively coarse in this specimen, indicating that it was buried deep enough to cool slowly. The fusion crust on eucrites has a glassy sheen.
Emery S. Dakota USA	Mesosiderite. This meteorite consists of dark clasts of basaltic and related volcanic rocks and shiny metallic iron-nickel grains, an unlikely mixture of core materials with those formed on the surface of a differentiated asteroid.

Case 4: Iron Meteorites; Crystallization of metallic melts

Iron meteorites formed by melting (and subsequent crystallization) of asteroids that originally consisted of chondritic materials. After melting, the immiscible metal and silicate liquids separated, the metal crystallizing to form the Widmanstätten structure now visible in polished and etched sections of iron meteorites. These coarse structures require extremely low cooling rates of about 18°F (10°C) per million years, too slow to be reproducible in terrestrial labs.

There are about 700 different iron meteorites in the world's collections. About 98% of these have been analyzed at UCLA using an analytical technique known as neutron activation analysis. Small samples weighing about a quarter of a gram are sent to a nuclear reactor where they are bombarded by neutrons. This process creates unstable, neutron-rich isotopes which emit gamma rays of characteristic energies. The gamma rays are counted and the concentrations of about 14 elements determined.

The iron meteorites constitute 13 major groups (with the number of members ranging from 5 to 200) and many minor grouplets. Altogether the iron meteorites seem to have been derived from more than 60 different asteroids. Most irons seem to have formed in the cores of differentiated asteroids by slow crystallization of a metallic melt. This process, called fractional crystallization, has led to huge fractionations of some elements; the Ir content in the first solid crystallized from iron-meteorite-group IIAB is 5000 times higher than that in the last solid that formed.

Three groups of irons (IAB, IIE and IIICD) seem to have formed at the floors of impact craters at the surfaces of chondritic parent bodies; the new Anoka iron belongs to IIICD, one of these three groups.

Canyon Diablo Arizona USA	IAB iron meteorite. The impact of this meteorite produced Meteor Crater, a bowl-shaped depression about 1-km in diameter, about 50000 years ago. The coarse banding indicates that it cooled at a rate of about 10°C per million years.
Henbury N. Territory Australia	IIIAB iron meteorite. Numerous 50-200 m craters were produced by the fall of this meteorite. IIIAB irons formed by fractional crystallization. Henbury, one of the first of these irons to crystallize, has a high concentration of Ir (15 parts per million), an element that concentrates in the crystallizing solid.
Buenaventura Chihuahua Mexico	IIIAB iron meteorite. The bronze-colored round inclusions are made up of the mineral iron sulfide. Buenaventura was one of the last irons to crystallize from the IIIAB core, and has a very low content of Ir (15 parts per billion), one thousand times lower than that in Henbury. This specimen, originally sawed from the main mass of Buenaventura in the UCLA collection, is on loan from Harvard University.
Anoka Minnesota USA	IIICD iron meteorite. This small polished slab is from the original 1.1-kg specimen found in 1961. The distribution of Ni between the metallic phases is shown in the diagram in the book by Burke.

Books by UCLA authors:

Shown in case 4 is a book on the history of meteorite research by the late John Burke; its title is Cosmic Debris, and it was published by the University of California Press in 1986. Burke was a history professor at UCLA and also Dean of Humanities.

John Wasson, professor of geochemistry and chemistry and the organizer of this exhibit, has written two books: (in case 1) Meteorites, their Classification and Properties, Springer (1974) and (in case 3) Meteorites: their Record of Early Solar-System History, Freeman (1985). The former is more technical, the latter more general; both are out-of-print.

The most comprehensive book on meteorite research is Meteorites and the Early Solar System, Univ. Arizona (1989); it was edited by John Kerridge of UCLA and Mildred Matthews of Univ. of Arizona. It is shown in case 2.