Stony-iron meteorites

Stony-iron meteorites are intermediate in composition between stony meteorites and iron meteorites. The two major types of stony-iron meteorites are the pallasites and the mesosiderites. The study of pallasites provides evidence for constraints on planetary differentiation processes. The mesosiderites record a history of repeated impacts of projectiles on the basaltic surfaces of their parent body.

Overview

Meteorites are divided into three broad categories: stony meteorites (or stones), nickel-iron meteorites (or irons), and a group called the stony-irons that have both stone and iron components. These rare stony-iron meteorites constitute only about 1 percent of all the meteorites recovered soon after their fall to Earth was observed. However, the stony-irons are more critical than their low abundance suggests since they provide a link between the stones and the irons and serve as probes of certain planetary processes. There are four distinct types of stony-iron meteorites: pallasites, mesosiderites, siderophores, and lodranites. The pallasites and mesosiderites are the most common stony-irons. The siderophyres and lodranites are rare, represented by only a few specimens.

Pallasite meteorites comprise millimeter- to centimeter-sized angular or rounded fragments of magnesium-rich olivine set in a continuous nickel-iron matrix. In these meteorites, the olivine content ranges from 37 to 85 percent by volume, with nickel-iron metal accounting for almost all the remaining material. The minerals troilite, schreibersite, and chromite are sometimes found in small amounts.

The detailed process by which the pallasites formed is still a scientific debate. Still, they appear to sample a boundary region where nickel-rich iron was in contact with silicate crystals, an environment analogous to the Earth’s core-mantle boundary. One mechanism for forming pallasites could have been the heating and consequent differentiation of a chondritic parent body. The high-density iron-nickel-sulfur liquid settled to the center, creating a molten core and leaving a silicate-rich mantle. As the mantle cooled, olivine, generally the first silicate mineral to crystallize out of cooling silicate liquids of various compositions, formed and settled to the core-mantle boundary.

The mechanism by which molten metal from the core surrounded the olivine crystals to produce the pallasite structure is not yet understood. It has been proposed that perhaps the mantle shrank as it cooled, squeezing molten metal out of the core and into the olivine-rich layer. Alternatively, the core may have contracted during cooling, causing the olivine layer to collapse into the void, giving rise to the mixing. Further cooling would have resulted in the solid pallasite material, which later was excavated from the parent body by major impacts. Therefore, the pallasites are thought to provide samples similar to the core-mantle boundary region on Earth.

Comparison of the chemistry of the pallasites with that of the Earth provides some constraints on the Earth’s formation and differentiation process. The Earth is assumed to have formed with the same chondritic composition as the pallasite parent body. After differentiation, the concentration of nickel in the Earth’s upper mantle remained at about 0.2 percent. The silicates in the pallasites are much more depleted in nickel, having a concentration of only 0.002 percent. One possible explanation for the additional nickel in the Earth’s outer layers is that after differentiation, additional chondritic material was added to the surface, presumably by impacting objects.

Constraints on the size of the pallasite parent body come from a study of how fast these objects cooled after differentiation. If two objects start at the same temperature and are allowed to cool, the smaller object will cool more rapidly since it has a larger ratio of surface area to volume than the larger object. The cooling rates determined for the pallasites and the iron meteorites related to them are consistent, with the formation in an object much smaller than the Earth’s Moon, perhaps no larger than ten kilometers in diameter. The texture, composition, and cooling rate of typical pallasites are consistent, with their metal being related to a group of iron meteorites called the IIIAB irons. If so, then samples of the pure core material of the pallasite parent body are also available as the IIIAB irons.

The differentiation process believed to have occurred in the Earth's early history and of the pallasite parent body has been simulated in the laboratory by heating chondritic meteorites. As the temperature increases, the meteorites melt in stages. The first liquid to appear is composed mainly of iron, nickel, sulfur, and trace elements with an affinity for these significant elements. Because this liquid is twice as dense as the remaining silicates, it sinks to the bottom. Further melting yields basaltic composition liquids and mostly olivine solid residue. The basaltic liquid, less dense than the solids, floats to the top. When cooled, the resulting structure has metal at the bottom, an olivine layer in the middle, and basaltic material on top. For the Earth, this process would give rise to a dense metal core surrounded by an olivine-rich mantle and covered with a basaltic crust. However, the absence of samples from the Earth’s deep interior prevents direct verification of this structure.

Examination of the pallasite meteorites strongly suggests that the pallasite parent body formed with a chondritic composition was heated and melted, differentiated into a metallic core and silicate mantle, and cooled and solidified. Thus, the pallasites confirm that planetary differentiation took place on the pallasite parent body in the same manner as proposed for the Earth.

The mesosiderite meteorites are quite different from the pallasites. They are composed of angular chunks of basaltic rocks and rounded masses of metal. The metal phases constitute 17 to 80 percent of the mesosiderites by weight. The major silicate minerals are plagioclase feldspar, calcium-rich pyroxene, and olivine. The mesosiderites are polymictic breccias; they are composed of fragments of unrelated rocks. They contain pyroxene-rich fragments, like the diogenite achondrite meteorites, and fine-grained pieces of eucrite achondrite meteorites. The eucrites and diogenites are composed of magmatic rocks similar to terrestrial basalts and cumulates.

The mesosiderites appear to have formed from repeated impacts on an asteroidal surface, which brought together at least three distinct types of material: diogenitic and eucritic rocks from the surface of the asteroid and a nonindigenous metallic component, possibly from the impacting objects. Suppose the metal fragments in the mesosiderites are projectile material from the core of a previously fragmented asteroid. In that case, these fragments must have struck the surface of the diogenite-eucrite parent body at a very low velocity. Impacts at velocities higher than about one kilometer per second lead to very low concentrations of the projectile material in the resulting breccias. This low-impact velocity would suggest that the parent body exerted a minimal gravitational attraction on the falling metal, indicating that the diogenite-eucrite parent was a relatively small asteroid, not a planet-sized object.

The mesosiderites are similar to lunar surface breccias, also formed by multiple impacts into basaltic rock. They allow the processes of basaltic volcanism and impact brecciation to be examined in a different solar system region and at an earlier time than occurred on the Moon.

A few other meteorites contain mixtures of metal and silicate phases, but they are otherwise dissimilar to pallasites and mesosiderites. The siderophore type, represented only by the single meteorite Steinback, comprises the silicate mineral bronzite (an iron-magnesium pyroxene) and metal. The lodranites are composed of olivine, calcium-poor pyroxene, and metal. These two rare types of stony-iron meteorites have not been as well studied as the pallasites and mesosiderites.

Methods of Study

The pallasite meteorites have been well studied by various techniques because, along with the iron meteorites, they provide a window on the processes and conditions in the deep interior of their parent bodies and clues to similar processes thought to have occurred on Earth. Much of the evidence concerning these processes comes from detailed analyses of the chemical abundances of major and trace elements in individual minerals from each meteorite. Detailed modeling of the differentiation process suggests that certain metal-seeking trace elements will concentrate in the metallic core while other trace elements concentrate in the silicate mantle.

Early studies of the metal phases in iron and stony-iron meteorites were done by examining their textures because the abundances of the trace elements were difficult to determine. In the twenty-first century, however, the abundances of these trace elements, present at the level of no more than a few atoms in every million atoms of bulk material, have been measured by neutron activation, X-ray fluorescence, and electron microprobe analysis.

Detailed measurements of the abundance of trace elements, emphasizing the elements gallium, germanium, and iridium in iron meteorites, show twelve to sixteen distinct compositional clusters, indicating that the irons sample a minimum of twelve different parent bodies. Almost all the pallasites have metal compositions and textures, suggesting they are related to a single group of iron meteorites, the IIIAB irons. This relationship suggests that the metal portion of the pallasites samples the core of the same parent body as the IIIAB irons.

Chemical analysis of the olivine grains in pallasites indicates that the olivine has a very narrow range of compositions. Within each meteorite, the olivine crystals are homogeneous; that is, they show no significant compositional variation from grain to grain. This narrow range of olivine compositions suggests that the grains formed from a silicate liquid of uniform composition. Most of the pallasites, then, appear to sample the core-mantle boundary of a single-parent body. A few pallasites differ from the majority because they contain olivine richer in iron than the common pallasites. A few of these pallasites are also enriched in nickel and the trace elements germanium and iridium and depleted in gallium relative to the common pallasites. The trace element abundances suggest that these pallasites sample a parent body different from the common pallasites; however, these metals cannot be identified with any iron meteorite group.

The cooling rates of pallasite meteorites, from which the size of the parent bodies can be inferred, are determined by examination of the metal. A meteoritic metal consists of two distinct nickel-iron alloys: kamacite, which can be no more than 7 percent nickel, and taenite, which frequently has more than 20 percent nickel. Metallic liquid cores are generally thought to have a higher nickel content than can be accommodated in the kamacite structure alone. As the metal cools, kamacite increases and nickel atoms diffuse from the newly formed kamacite into the nearby taenite. Since nickel diffuses more rapidly in kamacite than in taenite, the nickel will build up at the kamacite-taenite boundaries. There will, therefore, be more nickel at the edges of the taenite than near the center. This distribution of nickel in the taenite varies with the cooling rate.

Electron microprobe analysis of the taenite grains gives the abundance of nickel as a function of distance from the edge, which allows the cooling rate to be estimated. The nickel distribution in the metal of the normal pallasites is consistent with a very rapid cooling rate, implying an extremely small parent body (less than ten kilometers in diameter). The same technique suggests that the cooling rate of the IIIAB iron meteorites, with which the pallasites are associated, was somewhat slower, implying a parent body of 200 to 300 kilometers in diameter. The reason for this difference has yet to be understood.

Although the mesosiderites are also mixtures of stone and metal, they are quite different from the pallasites. The silicate portion of the mesosiderites is rich in the minerals plagioclase feldspar and calcium-rich pyroxene. These minerals melt at relatively low temperatures and are common on Earth's and the Moon's surface. Unlike the olivine found in the pallasites, the basaltic minerals found in the mesosiderites were probably never in direct contact with the metal in the core of the parent body.

Detailed examination of the mineralogy of the mesosiderites shows that they consist of a mixture of three distinct components, each represented by a specific type of meteorite. The silicates are fragments of both eucrite and diogenite achondrite stony meteorites; the metal phases resemble the iron meteorites. The eucrites are basalt-like meteorites composed mainly of calcium-poor pyroxene and plagioclase feldspar thought to have crystallized on or near the surface of their parent body. The diogenites primarily comprise bronzite (an iron-magnesium pyroxene), which resembles the pyroxene cumulates found in the Stillwater complex and other layered terrestrial intrusions. This combination of eucrite and diogenite fragments in the stony-iron mesosiderites and in the howardite achondrite stony meteorites is evidence that the eucrites and diogenites formed on the same parent body. The metal in the mesosiderites occurs mainly in nuggets, sometimes up to nine centimeters in diameter, or fragments. They are quite distinct from the continuous metal veins throughout the pallasites. Trace element analysis of the metal in the mesosiderites suggests a similarity with the IIE iron meteorites; however, the link is much weaker than that of the common pallasites with the IIIAB iron meteorites.

Context

The process of differentiation (the melting of a primitive parent body and the concentration of iron-nickel-sulfur in the core and the lighter silicate minerals in the mantle) is believed to have occurred on Earth. The study of the pallasite meteorites, composed of a mixture of iron-nickel core material and olivine mantle material, confirms that this differentiation process occurred early in solar-system history, at least on the pallasite parent body. Though the chemical compositions of most pallasites are consistent with a single parent body, the few pallasites of unusual composition suggest that the pallasite meteorites sample core-mantle boundaries of several parent bodies. That indicates that the differentiation process was relatively common. The good match between the postulated chemical compositions of the core and mantle and the compositions seen in the pallasites' metal and olivine phases confirms the model of chemical segregation developed for the differentiation of the Earth. It is believed, however, that the common pallasites sample the core-mantle boundary of a much smaller body than the Earth.

Although the mesosiderites also consist of a mixture of metal and silicates, they are quite different from the pallasites. The silicates in the mesosiderites sample basaltic material similar to that found on the lunar surface, in the Earth’s crust, and, as implied by chemical measurements taken by spacecraft, on the surfaces of Mars and Venus. The iron fragments may be projectiles that struck the rocky surface of the mesosiderite parent body and were incorporated into the rock produced by the impact. The mesosiderites demonstrate that basaltic rocks similar to those on Earth occur on the surfaces of some asteroids and that the impact processes that dominate the lunar landscape also occurred in the solar system's early history.

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