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Center for Isotope Cosmochemistry and Geochronology

The Center for Isotope Cosmochemistry and Geochronology (CICG) led by Justin Simon is based in the Astromaterials Research and Exploration Directorate (ARES) at NASA Johnson Space Center. Our team measures a wide variety of elements and their isotopes to understand the origin of our Solar System, the processes that transformed nebular dust and gas into the building blocks of planets, and planet formation (accretion & differentiation). Investigations include the measurement of: (1) Short- and long-lived radionuclides for chronology studies, (2) Fractionation of middle mass stable isotopes to study the formation mechanisms of these planetary materials, and (3) Isotopic analyses that utilize the traditional ways that isotopic compositions can be used to study interaction among different reservoirs within the protoplanetary disk and terrestrial planets. We strive to integrate all of these isotopic measurements into a petrological context.

January 2013 Center for Isotope Cosmochemistry and Geochronology Group

January 2013 (Left to right). Chi-Yu Shih, Ryan Mills, Larry Nyquist, Mike Tappa, Kent Ross, and Justin Simon

Researchers
Current Research Highlights

Fine-scale Oxygen Isotope Records Contained in Refractory Inclusions and Their Implications for the Protoplanetary Disk
Recently our team (including collaborators at LLNL and the University of Chicago) performed oxygen isotope ion microprobe analysis of the core and outer layers of a pea-size meteorite fragment (a calcium- aluminum-rich inclusion, CAI) some 4.58 billion years old to reconstruct the history of its formation. The ion microprobe measurements were obtained as ~2 micrometers spot analyses spaced across the rim and the outer margin of the core. At this resolution, both the core and its rim exhibit a range that is close to the full range thought to exist among solids formed in the entire Solar System, varying more than 20‰ in Δ17Ο (a parameter used to indicate deviation from the terrestrial oxygen reservoir). These data imply that this CAI was transported among several different nebular oxygen isotopic reservoirs, potentially as it passed through and/or into various regions of the protoplanetary disk.

USE A DIFFERENT IMAGE HERE (Y98 Melt Inclusion) Compositional X-ray image of the Wark-Lovering rim and margin of a Type A Ca-, Al-rich inclusion (CAI) from the Allende CV3 meteorite. These features may record important information about protoplanetary disk evolution (Simon et al., 2005, 2011 & Dyl et al., 2011).












The evidence for transport of solid matter reported by the team supports the inference from theoretical studies that outward radial transport of solid matter is a basic consequence of protoplanetary disk evolution. Large-scale radial circulation of nebular solids is also consistent with the reports of crystalline material located in the outer reaches of our Solar System, and in the outer, cool regions of distant stars. The variable but largely 16O-rich composition found in the Wark-Lovering rim (above) suggests that after transport out of the inner Solar System, CAIs either continued to form within a region in the outer Solar System that varied in composition, or that they were returned back to the inner Solar System.

Interactions Among Water Reservoirs (mantle, crust, and atmosphere) on Mars and Their Implications for Planetary Differentiation
A study by our group (in collaboration with folks at DTM), led by former postdoctoral researcher Tomo Usui found that hydrogen in the martian interior accreted from planetary building blocks similar to those that formed Earth. This implies that terrestrial planets including Earth have similar water sources, which are chondritic meteorites, and not comets. We further found that, unlike on Earth, Mars rocks, containing atmospheric volatiles such as water, do not get recycled into the planet's deep interior. Otherwise, the primordial water would have been overprinted by recycled atmospheric water.

Figures

The signatures of martian surface water (green square) and primordial water (red triangle) have been observed in the martian meteorites LAR06319 and Yamato 980459, respectively (left). The distinct compositions of these reservoirs measured in tiny (<20 micrometer-sized) melt inclusions are shown in part per thousand differences in the hydrogen to deuterium ratio (δD). Several of the studied inclusions hosted by the mineral olivine in Yamato 980459 are shown in a scanning electron microscope image (right). (Usui et al., 2012; figure credit: T. Usui & T. Imai).

The melt inclusions contained in olivine crystals were studied for their D/H ratios and volatile abundances in two shergotites. One of these meteorites, Yamato 980459 (back scattered electron image shown above) appears to have changed little on its way to the surface of Mars from the martian mantle. It has a hydrogen isotopic composition similar to that of Earth. The other meteorite LAR (Larkman Nunatak) 06319 appears to have sampled martian crust that had been in contact with the martian atmosphere. Thus the meteorites represent two very different sources of water. One sampled water from the deep interior and represents the water that existed when Mars formed as a planet, whereas the other sampled the shallow crust and atmosphere.

Active Research

1. Coordinated isotopic, geochemical and mineralogical studies of primitive meteorite components as a means to address the early solar system and development of planetary systems

2. Ca and Nd isotopic tracer studies to study the origin of evolved magmas on Earth

3. K-Ca, Rb-Sr, Lu-Hf, and Sm-Nd studies to define the source and evolution of evolved rocks on the Moon and other extraterrestrial planetary bodies

4. D/H and Pb isotopes of shergottites to study interaction among different planetary reservoirs on Mars (mantle, crust, and atmosphere)

5. Coordinated zircon dating, Ar/Ar dating, and Hf, Nd, and Sr isotopic tracer studies of how young silicic magma bodies form and evolve on Earth

Our research utilizes both micro-analytical approaches (LA-MC-ICPMS and ion microprobe) and chemical purification methods for analyses by TIMS & MC-ICPMS to achieve high spatial resolution and higher precision measurements, respectively. We welcome outside collaborations. In particular we encourage and support the analytical needs of promising postdoctoral researchers and graduate students. Please contact us (Justin.I.Simon@NASA.gov) for additional information, including about the possibility of joining CICG.

Laboratories