METEORITES RESEARCH

We have begun several studies of meteorites, with research that falls into two broad categories: 1) processes that affected primitive solar-system materials; and 2) the timing and nature of differentiation in asteroidal and planetary objects. Included in the first broad category are petrologic-chemical studies of (a) large, igneous-textured objects in ordinary chondrites and (b) olivine aggregate inclusions in carbonaceous chondrites. Included in the second category are various studies of (a) martian meteorites, and (b) the search for evidences of life on Mars, (c) HED meteorites that are believed to have originated in the differentiated asteroid 4-Vesta, (d) enigmatic meteorites known as ureilites, and (e) IIE silicated iron meteorites.

Processes affecting primitive solar-system materials

• Large igneous objects in ordinary chondrites.
  Igenous-textured inclusions much larger than typical-sized chondrules are present in many ordinary chondrites. Although
  comparatively rare, these objects may shed light on the origin of chondrules, which have perplexed researchers for over a century.

  Sodium-poor and -rich inclusions form different chemical trends that provide clues as to how these inclusions formed. The compositions of Na-rich inclusions fall mainly on a mixing line between whole-rock ordinary chondrites and Na-feldspar; they could have formed by incipient shock-melting of ordinary chondrites. In contrast, Na-poor inclusions may have formed by the melting of precursors similar to CI-chondrites that contained variable proportions of a refractory (CAI-like) component (Trend A) or that contained variable proportions of an olivine-rich component (Trend B). The precursors to Na-poor inclusions could have formed by the melting of condensates or vaporization residues in the solar nebula.

  Our studies suggest that the large inclusions can be subdivided into two chemical types (Na-rich and Na-poor) that may have different origins. Na-rich inclusions appear to have been derived from ordinary chondrite protoliths by shock-melting, often accompanied by enrichment in a Na-feldspar component. Na-poor inclusions appear to have been derived from precursors that experienced vapor-fractionation (volatility-controlled) processes.

• Sulfur isotopes in chondritic meteorites
  Using the ion micoprobe at ORNL, we have measured the sulfur isotopic compositions of individual grains of sulfides(troilite and pyrrhotite) in ordinary chondrites and in the Kaidun carbonaceous chondrite. The absence of isotopic fractionation of sulfur in most chondrites is discinct from Kaidun, which has lost isotopically heavy sulfur dioxide.
  
  
• Olivine aggregate inclusions.
  Olivine aggregate inclusions are one of the most volumetrically important inclusion types in some carbonaceous chondrites. Unlike chondrules, which show clear evidence for having been melted, olivine aggregate inclusions appear to have formed by the aggregation of individual, tiny olivine grains and other material. These inclusions may have been a fundamental building block out of which much of the rocky material in the solar system was originally constructed. We are characterizing the major- and trace-element composition of this inclusion type to serve as a baseline for other work, and to evaluate possible relationships between these and other inclusions in chondrites
  
  
• Chondrule glasses and melt inclusions.
  Chondrules are spherical glassy objects and are the main components of ordinary chondrites. Chondrules show evidence of having formed in short-lived, high-temperature events of unknown origin in the early solar nebula. The behavior of moderately volatile (under nebular conditions) elements may hold keys for understanding the nature of the high-temperature events that formed chondrules. We have begun a study of sodium contents of melt inclusions and mesostases in an attempt to determine whether chondrules still preserve a nebular signature, or have been affected by later processes during incorporation in early planetesimals
  
  

Timing and nature of differentiation in asteroidal objects

• Kr and Xe Isotope Measurements from Few-Micron Sized Samples

  Recent studies have focused on the separation of highly refractory microscopic grains from the much more abundant carbonaceous matrix in primitive meteorites. A remarkable isotropic composition diversity was found in a small fraction of single grains from the same meteorite, implying multiple stellar sources. Studies of noble gases in SiC, graphite, diamonds, individual olivine grains, ilmenite grains and phosphate grains have led to unique grain-specific isotope data. Although conventional noble gas mass spectrometers do not have the sensitivity to measure the heavy (Kr, Xe) noble gases from single grains, the low detection limit for krypton of the mass spectrometer under development at IRIM makes it ideal for this field.

  The unique time-of-flight mass spectrometer has been equipped with a new extraction chamber designed to hold mapped thin sections or multiple individual grains of the study sample. A focused (~3mm dia.) fourth harmonic (266 nm) pulsed Nd:YAG laser beam is used to vaporize selected individual grains. The gases liberated can be purified by exposure to various getters, but the ionization specificity of the lasers may permit release of the gas directly into the system for simultaneous Kr and Xe analysis. Initial studies will look at thin sections from meteorite groups, such as the Nakhlites, looking for any isotopic diversity within the meteorite sample itself and obtaining cosmic-ray exposure ages of individual grains. Later, the Kr and Xe isotopic composition of single refractory microscopic grains will be studied.

• Martian Meteorites
  The SNC meteorites, thought to be from Mars, provide important constraints on the geologic processes and history of tht body. We have studied the pertrology of these samples to understand the compositions of their parent magmas and crystallization histories. Modeling of the solidification of trapped melt inclusions in cumulus minerals has led to an estinmate of the planet's volatile inventory and outgassing history.
  
  
• Life on Mars ?
  The recently reported finding of "evidence" for past life in a martian meteorite, ALH84001, has triggered a frenzy of work by scientists around the world, attempting to prove or refute NASA's findings. Using the ion microprobe facility at ORNL, we have measured the isotopic abundances of carbon and sulfur in the supposed biogenic carbonates of ALH84001. The results of our present studies are not consistent with bacterial activity on Mars. Instead, the results point towards a high-temperature, inorganic origin for the formation of the carbonates, such as formation in a shock event, or as fumarolic condensates

• Evidence for life in a Martian Meteorite.
  Electron microscope studies of carbonates in ALH84001 reveals tiny whiskers of magnetic with screw dislocations. The morphologies and growth mechanisms of these magnetites suggest formation from a hot vapor and appear to be inconsistent with life. These whiskers are apparently the nanofossils previously reported from this meteorite. Ion microprobe measurements of sulfur isotopes in sulfides from SNC meteorites are inconsistent with their formation by sulfate-respiring bacteria.

• The Vesta-HED connection.
  Meteorites in the HED (howardite, eucrite, and diogenite) clan represent igneous rocks and their brecciated equivalents that appear to have been derived from the crust of the same, differentiated asteroid, probably the asteroid 4-Vesta.
  
  Geochemical and petrologic constraints provided by these meteorites and astronomical constraints on the size and density of Vesta can be used together to model the differentiation and internal structure of the asteroid.
  
  

  This sketch illustrates two possible interior models for Vesta that are consistent with petrologic and bulk density constraints. These models show what the interior structure of Vesta could like like if it fully differentiated. The olivine-rich case on the left assumes that the silicate portion of Vesta originated as a carbonaceous chondrite protolith; the olivine-poor case on the right assumes that the silicate portion of Vesta originated as an enstatite chondrite protolith. The real situation for Vesta probably lies somewhere between these two extremes.

  The data are compatible with the idea that a magma ocean formed on Vesta, and that diogenites and eucrites represent magma ocean differentiates.
  
  Observations of Vesta by the Hubble Space Telescope reveal an unsually large crater near the south pole that may be mineralogically stratified in a fashion consistent with these models.
  
  
  
• Ureilites.
  Ureilites are enigmatic meteorites that show conflicting evidence for significant igneous processing on the one hand, and for minimal igneous processing on the other. We are attempting to resolve this conflict through isotopic studies of ureilite mineral separates. Some of the confusion generated by studies of ureilites may be caused by the late admixture into ureilites of a distinctive, petrologically and isotopically unrelated component, and our isotopic studies are being designed to test this hypothesis
  
  
• Crystal -bearing lunar shperules.
  Partly crystalline lunar shpeurles share many important similarities to chondrules, found ubiquitously in most chondrites. Despite nearly a century of study, the origin of meteoritic chondrules remains hotly contested, but there is several agreement that the crystalline spherules in lunar samples formed by hypervelosity impacts. We are studying the lunar spherules in more detail to elucidate the similarties and differences between them and chondrules, with the good of further constraining the origin of chondrules. Althugh crystalline shperules tend to be rare in lunar samples, they are found abundantly in some samples, such as Apollo 14 "sherule-rich breccia" 14315 (see photo).

• Silicate inclusions in IIE iron meteorites.

  IIE iron meteorites contain silicate inclusions that appear to record different stages in the heating, melting, and differentiation of chondritic material. Some IIE meteorites contain inclusions with fractionated (i.e., rhyolitic) compositions.

  In the Weekeroo Station IIE iron meteorite, neither phase nor bulk compositions of inclusions vary in a manner consistent with terrestrial-style igneous differentiation, despite large changes in the proportions of mineral phases and bulk chemistry. For example, inclusions range from rhyolitic (WS1A), to quasi-peritectic (WS7), to orthopyroxenite (WS4). However, as the figure shows, trace-element compositions of the same phases in these different inclusions are similar. This and other evidence suggests that the inclusions formed in part by the physical mixing of pre-existing phases.

  
  Our geochemical and isotopic studies of these inclusions suggest that their petrogenesis was complex. Inclusions were affected by differentiation, remelting, FeO-reduction, and physical mixing of pre-existing phases, over a time period extending from 4.6 Ga to 0.8 Ga. Impact processes almost certainly were important in the formation of the inclusions.

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