Jens Hopp¹, Mario Trieloff¹, Uli Ott², Ekaterina V. Korochantseva³, Alexey I. Buykin³

¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Max-Planck-Institut für Chemie, Mainz, Germany
³Vernadsky Institute for Geochemistry, Moscow, Russia

Ar-Ar isochron ages of EL chondrites suggest closure of the K-Ar system at 4.49 ± 0.01 Ga for EL5 and 6 chondrites, and 4.45 ± 0.01 Ga for EL3 MAC 88136. The high-temperature release regimes contain a mixture of radiogenic ⁴⁰Ar* and trapped primordial argon (solar or Q-type) with ⁴⁰Ar/³⁶Ar_TR ~ 0, which does not affect the ⁴⁰Ar budget. The low-temperature extractions show evidence of an excess ⁴⁰Ar component. The ⁴⁰Ar/³⁶Ar is 180–270; it is defined by intercept values of isochron regression. Excess ⁴⁰Ar is only detectable in petrologic types >4/5. These lost most of their primordial ³⁶Ar from low-temperature phases during metamorphism and retrapped excess ⁴⁰Ar. The origin of this excess ⁴⁰Ar component is probably related to metamorphic Ar mobilization, homogenization of primordial and in situ radiogenic Ar, and trapping of Ar by distinct low-temperature phases. Ar-Ar ages of EH chondrites are more variable and show clear evidence of a major impact-induced partial resetting at about 2.2 Ga ago or alternatively, prolonged metamorphic decomposition of major K carrier phases. EH impact melt LAP 02225 displayed the highest Ar-Ar isochron age of 4.53 ± 0.01 Ga. This age sets a limit of about 25–45 Ma for the age bias between the K-Ar and U-Pb decay systems.

Reference
Hopp J, Trieloff M, Ott U, Korochantseva EV and Buykin AI (in press) 39Ar-40Ar chronology of the enstatite chondrite parent bodies. Meteoritics & Planetary Science
[doi:10.1111/maps.12243]
Published by arrangement with John Wiley & Sons

Link to Article

Michel Nuevo^1,2, Scott A. Sandford¹, George J. Flynn³, Susan Wirick⁴

¹NASA Ames Research Center, MS 245-6, Moffett Field, California, USA
²SETI Institute, Mountain View, California, USA
³Department of Physics, SUNY-Plattsburgh, Plattsburgh, New York, USA
⁴Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA

The Sutter’s Mill meteorite fell in northern California on April 22, 2012. Several fragments of the meteorite were recovered, some of them shortly after the fall, others several days later after a heavy rainstorm. In this work, we analyzed several samples of four fragments―SM2, SM12, SM20, and SM30―from the Sutter’s Mill meteorite with two infrared (IR) microscopes operating in the 4000–650 cm⁻¹ (2.5–15.4 μm) range. Spectra show absorption features associated with minerals such as olivines, phyllosilicates, carbonates, and possibly pyroxenes, as well as organics. Spectra of specific minerals vary from one particle to another within a given stone, and even within a single particle, indicating a nonuniform mineral composition. Infrared features associated with aliphatic CH₂ and CH₃ groups associated with organics are also seen in several spectra. However, the presence of organics in the samples studied is not clear because these features overlap with carbonate overtone bands. Finally, other samples collected within days after the rainstorm show evidence for bacterial terrestrial contamination, which indicates how quickly meteorites can be contaminated on such small scales.

Reference
Nuevo M, Sandford SA, Flynn GJ and Wirick S (in press) Mid-infrared study of stones from the Sutter’s Mill meteorite. Meteoritics & Planetary Science
[doi:10.1111/maps.12269]
Published by arrangement with John Wiley & Sons

Link to Article

Rita K. Mann¹, James Di Francesco^1,2, Doug Johnstone^1,2,3, Sean M. Andrews⁴, Jonathan P. Williams⁵, John Bally⁶, Luca Ricci⁷, A. Meredith Hughes⁸, and Brenda C. Matthews^1,2

¹National Research Council Canada, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
²Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
³Joint Astronomy Centre, 660 North A’ohoku Place, University Park, Hilo, HI 96720, USA
⁴Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 USA
⁶CASA, University of Colorado, CB 389, Boulder, CO 80309, USA
⁷Department of Astronomy, California Institute of Technology, MC 249-17, Pasadena, CA 91125, USA
⁸Van Vleck Observatory, Astronomy Department, Wesleyan University, 96 Foss Hill Drive, Middletown, CT 06459, USA

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of protoplanetary disks (“proplyds”) in the Orion Nebula Cluster. We imaged five individual fields at 856 μm containing 22 Hubble Space Telescope (HST)-identified proplyds and detected 21 of them. Eight of those disks were detected for the first time at submillimeter wavelengths, including the most prominent, well-known proplyd in the entire Orion Nebula, 114-426. Thermal dust emission in excess of any free-free component was measured in all but one of the detected disks, and ranged between 1 and 163 mJy, with resulting disk masses of 0.3-79 M _jup. An additional 26 stars with no prior evidence of associated disks in HST observations were also imaged within the 5 fields, but only 2 were detected. The disk mass upper limits for the undetected targets, which include OB stars, θ¹ Ori C, and θ¹ Ori F, range from 0.1 to 0.6 M _jup. Combining these ALMA data with previous Submillimeter Array observations, we find a lack of massive (3 M _jup) disks in the extreme-UV-dominated region of Orion, within 0.03 pc of θ¹ Ori C. At larger separations from θ¹ Ori C, in the far-UV-dominated region, there is a wide range of disk masses, similar to what is found in low-mass star forming regions. Taken together, these results suggest that a rapid dissipation of disk masses likely inhibits potential planet formation in the extreme-UV-dominated regions of OB associations, but leaves disks in the far-UV-dominated regions relatively unaffected.

Reference
Mann RK, Di Francesco J, Johnstone D, Andrews SM, Williams JP, Bally J, Ricci L, Hughes AM and Matthews BC (2014) ALMA Observations of the Orion Proplyds. The Astrophysical Journal 784:82
[doi:10.1088/0004-637X/784/1/82]

Link to Article

T. Albertsson, D. Semenov, and Th. Henning

Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Formation and evolution of water in the solar system and the origin of water on Earth constitute one of the most interesting questions in astronomy. The prevailing hypothesis for the origin of water on Earth is by delivery through water-rich small solar system bodies. In this paper, the isotopic and chemical evolution of water during the early history of the solar nebula, before the onset of planetesimal formation, is studied. A gas-grain chemical model that includes multiply deuterated species and nuclear spin-states is combined with a steady-state solar nebula model. To calculate initial abundances, we simulated 1 Myr of evolution of a cold and dark TMC-1-like prestellar core. Two time-dependent chemical models of the solar nebula are calculated over 1 Myr: (1) a laminar model and (2) a model with two-dimensional (2D) turbulent mixing. We find that the radial outward increase of the H2O D/H ratio is shallower in the chemodynamical nebular model than in the laminar model. This is related to more efficient defractionation of HDO via rapid gas-phase processes because the 2D mixing model allows the water ice to be transported either inward and thermally evaporated or upward and photodesorbed. The laminar model shows the Earth water D/H ratio at r  2.5 AU, whereas for the 2D chemodynamical model this zone is larger, r  9 AU. Similarly, the water D/H ratios representative of the Oort-family comets, ~2.5-10 × 10^–4, are achieved within ~2-6 AU and ~2-20 AU in the laminar and the 2D model, respectively. We find that with regards to the water isotopic composition and the origin of the comets, the mixing model seems to be favored over the laminar model.

Reference
Albertsson T, Semenov D and Henning Th (2014) Chemodynamical Deuterium Fractionation in the Early Solar Nebula: The Origin of Water on Earth and in Asteroids and Comets. The Astrophysical Journal 784:39
[doi:10.1088/0004-637X/784/1/39]

Link to Article