nai

Project Progress

Task 1. Chondrule Formation and Solar Nebula Chemistry (Desch)

Melting of chondrules by shocks is completely consistent with many aspects of their petrology, especially their cooling rates. No previous model had correctly predicted the cooling rates, which are constrained from the petrologic textures of chrondrules. A parameter study that will constrain the solar nebula environment in which chondrule-melting shocks had to take place was initiated by postdoctoral scientist Desch. It was shown that if chondrules were melted by shocks, then the nebula had to be very dense in order to match the frequencies of compound chondrules, the sizes of chondrules that are concentrated by turbulence, and the variations in chondrule textures. On the other hand, a dense nebula is consistent with the initiation of shocks by gravitational instabilities. So a self-consistent picture is developing in which shocks in the solar nebula melted chondrules. As a corollary, these same shocks could explain the annealing of silicate grains into crystalline form before the dust became part of long-period comets.

Task 2. Chemical Structure of Meteoritic Macroculture (Cody, Alexander)

The organic material in primitive chondritic meteorites has attracted considerable attention, not only because it retains a record of synthesis in the interstellar medium (ISM) and possibly in the Solar Nebula, but also because it may have been an important component of the prebiotic organic material on the early Earth. Complementary double- and single-resonance solid-state (¹H, ¹³C, and ¹⁵N) Nuclear Magnetic Resonance (NMR) experiments have been performed on solvent-extracted and demineralized organic residues from Orgueil (CI1), Murchison (CM2), and Tagish Lake. Combined, these NMR data provide a self-consistent picture of chemically complex solids composed of a wide range of organic (aromatic and aliphatic) functional groups, including numerous oxygen-containing functional groups, but with significant differences in the their relative abundances.

Employing a different set of NMR experiments, they have also been able to determine the fraction of aromatic carbon directly bonded to hydrogen in each residue and find that it is low and essentially identical in each residue, ~ 30 %. The low value of aromatic protonation indicates that the aromatic molecules in all three residues are highly substituted. The near constancy in aromatic H/C suggests similarity in aromatic structure and perhaps a similar origin. High-speed magic angle spinning ¹H NMR (25-30 kHz) is used to derive the fraction of aliphatic hydrogen to aromatic hydrogen, yielding an average CH_x (where x = 1 – 3 for CH, CH₂, and CH₃) that ranges from 2.0 (Orgueil), 1.9 (Murchison), to 1.7 (Tagish Lake). Tagish Lake, therefore, has a proportionally greater abundance of methine carbon relative to the other two residues.

It is possible that the all three meteoritic residues share a common origin but differ chemically because of either early or late processes in the solar nebula. If these chemical differences result from an alteration process within meteorite parent bodies, then the alteration must differ considerably from chemical evolutionary trends observed for terrestrial kerogens when subjected to thermal metamorphism.

Task 3. Interplanetary Dust Particles (Nittler, Mukhopadhyay)

Interplanetary dust particles (IDPs), collected in the stratosphere, are believed to have been an important contributor of organics to the early Earth. IDPs are known to have large and variable isotopic enrichments of deuterium and ¹⁵N, and they are believed to reflect incomplete processing of presolar organic material from the Sun’s parental molecular cloud. Nittler and Mukhopadhyay are using ion probe isotopic measurements of IDPs to constrain the characteristics and origins of organic material in the particles. A preliminary study of H and N isotopes in four IDPs has shown that like H, N is isotopically heterogeneous on a micron scale. Also, there is little correlation between H and N isotopic ratios in the particles, indicating that the isotope anomalies are carried by distinct organic materials. Correlated chemical studies (e.g., synchrotron x-ray techniques) are planned to determine the precise chemical makeup of the isotopically anomalous material. Recently, Mukhopadhyay and Nittler have also performed the first sulfur isotope measurements in IDPs. Such measurements should provide information on presolar carriers of S and the S isotopic evolution of the early Solar System.

Task 4. Hydrogen Isotope Signatures of Minerals and Melt Inclusions in Martian Meteorites (Alexander, Hauri, Boctor, Wang)

The CIW group has extended its investigation of H isotope signatures and water abundances in Martian meteorites to the nakhlites. Governador Valadares and Nakhla are clinopyroxene cumulates that show primary igneous textures. Among the SNC meteorites they are the least affected by shock metamorphism, a process that can alter the original D/H ratio trapped at the time of crystallization. The lack of pervasive alteration or shock melting in these meteorites suggests that their nominally anhydrous minerals and magmatic melt inclusions may provide clues to the magmatic H isotope signatures of parental Martian magmas.

The ion microprobe measurements of H isotopes in clinopyroxenes and melt inclusions show that these materials contain an extraterrestrial H component, identified by ɤD values higher than the Earth. The high ɤD values of clinopyroxenes from Nakhla (560 to 814‰) are similar to the whole rock value, while clinopyroxenes from Governador Valadares (142 to 874‰) and one melt inclusion (1408‰) are significantly higher than the whole rock value. The similarity of ɤD values for the nakhlite melt inclusion (low-shock) and shergottite melt inclusions (highly shocked) suggest that shock metamorphism may not have strongly influenced the D/H ratios of melt inclusions in Martian meteorites.

It is tempting to suggest that the elevated H isotope signatures of the clinopyroxenes were acquired by their crystallization from highly evolved melts with a high D/H ratio. The difference in ɤD values of clinopyroxenes and the melt inclusion precludes this explanation, however. The high D/H ratios in nakhlites suggest interaction with a highly fractionated water reservoir on Mars. The clinopyroxenes may have partially equilibrated with this fractionated water reservoir. Their ɤD values, therefore, may represent mixing of a Martian atmospheric H component with a low-D magmatic component or a terrestrial contaminant. If the low-ɤD component is indeed magmatic, then the pre-eruptive D/H ratios of Martian magmas may not be substantially different from those on the Earth.

Task 5. Iron Isotope Measurements of Terrestrial Rocks and Meteorites (Hauri, Emerson, Kehm)

Tracing biological activity using the stable isotope behavior of iron requires detailed understanding of the behavior of iron in a variety of settings, both biological and abiological. Refinement and calibration of inductively coupled plasma mass spectrometer (ICP-MS) techniques for iron isotope measurement over the last two years has culminated in measurements showing that the isotopic compositions of the igneous Earth, primitive chondrites, and iron meteorites are remarkably similar to those within our measurement precision (± 0.005%). However, chondrules extracted from one chondritic meteorite have compositions that are isotopically light (i.e., mass fractionation enriching the lighter isotopes) compared to the terrestrial/chondrite average by up to ~0.05%.

A new emphasis of ongoing work is the study of potential biological effects in biomineralized iron-rich sediments from hydrothermal vents. In particular, the CIW group is collaborating with David Emerson at George Mason University to determine whether iron precipitated from iron-rich thermal vent fluids at Loihi Seamount (offshore of Hawaii) by iron-metabolizing bacteria is isotopically distinct from normal terrestrial iron. Preliminary evidence suggests that there are resolvable isotopic differences between the iron present in the bacterial mats and iron in the surrounding igneous rocks. In addition to verifying this preliminary result, current work is attempting to clarify the source of this possible effect by examining the isotopic composition of iron in the vent water solution, as well as the potential fractionation caused by abiotic iron precipitation around these vents.