¹Heather C. Watson, ²Frank Richter, ²Ankun Liu, ³Gary R. Huss
Earth and Planetary Science Letters 451, 159–167 Link to Article [doi:10.1016/j.epsl.2016.06.030]
¹Union College, Schenectady, NY, United States
²University of Chicago, United States
³University of Hawai‘i at Mānoa, United States
Copyright Elsevier

Mass-dependent, kinetic fractionation of isotopes through processes such as diffusion can result in measurable isotopic signatures. When these signatures are retained in geologic materials, they can be used to help interpret their thermal histories. The mass dependence of the diffusion coefficient of isotopes 1 and 2 can be written as (D1/D2)=(m2/m1)β(D1/D2)=(m2/m1)β, where D1D1 and D2D2 are the diffusion coefficients of m1m1 and m2m2 respectively, and β is an empirical coefficient that relates the two ratios. Experiments have been performed to measure β in the Fe–Ni alloy system. Diffusion couple experiments between pure Fe and Ni metals were run in a piston cylinder at 1300–1400 °C and 1 GPa. Concentration and isotopic profiles were measured by electron microprobe and ion microprobe respectively. We find that a single β coefficient of β=0.32±0.04β=0.32±0.04 can describe the isotopic effect in all experiments. This result is comparable to the isotope effect determined in many other similar alloy systems. The new β coefficient is used in a model of the isotopic profiles to be expected during the Widmanstätten pattern formation in iron meteorites. The results are consistent with previous estimates of the cooling rate of the iron meteorite Toluca. The application of isotopic constraints based on these results in addition to conventional cooling rate models could provide a more robust picture of the thermal history of these early planetary bodies.

¹J. Mortimer, ¹A.B. Verchovsky, ^1,2M. Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.08.006]
¹Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
²Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Copyright Elsevier

Simultaneous static-mode mass spectrometric measurements of nitrogen, carbon, helium, neon, and argon, extracted from the same aliquot of sample by high-resolution stepped combustion, have been made for a suite of five lunar soils.

Noble gas isotope ratios show that the majority of noble gases are derived from a solar wind source; for example, at peak release temperatures of 500-600 °C, 21Ne/22Ne = 0.0313 ± 0.0007 to 0.0333 ± 0.0007, and 20Ne/22Ne = 11.48 ± 0.05 to 12.43 ± 0.07, with values at the lowest temperature steps less fractionated during implantation from, and therefore even closer to, solar values (21Ne/22NeSW = 0.03361 ± 0.00018 and 20Ne/22NeSW = 14.001 ± 0.042 (Pepin et al., 2012)). Despite the co-release of nitrogen and solar wind argon, measured nitrogen isotopic signatures at each temperature step, whilst variable, are significantly more enriched in 15N compared to the measured solar wind nitrogen value from the Genesis mission. Therefore, mixing between a 15N-enriched non-solar planetary nitrogen source with solar wind nitrogen is required to explain the measured isotopic values from the stepped combustion analysis of lunar soils. Binary mixing calculations, made under different assumptions about the degree of loss of solar wind 36Ar, reveal that the majority (up to 98%) of the nitrogen released is derived from a non-solar source. The range of modelled non-solar end-member nitrogen compositions required to satisfy the measured δ15N values varies between samples and temperature steps from +5 ‰ up to +300 ‰, or between +87 ‰ and +160 ‰ for bulk samples. This range of modelled isotopic compositions for the non-solar source of nitrogen encompasses measured values for several different groups of carbonaceous chondrite, as well as IDPs.

¹D.J. Cherniak, ¹E.B. Watson, ²R. Trappisch, ³J.B. Thomas, ⁴D. Chaussende
Geochimica et Cosmochmica Acta (in Press) Link to Article
[doi:10.1016/j.gca.2016.08.007]
¹Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180 USA
²Department of the Geophysical Sciences, The University of Chicago, and Chicago Center for Cosmochemistry, Chicago, IL 60637 USA
³Department of Earth Sciences, Syracuse University, Syracuse, NY 13244 USA
⁴Laboratoire des Matériaux et du Génie Physique, CNRS – Grenoble INP, 3 parvis Louis Néel, BP, 257, 38016 Grenoble, France
Copyright Elsevier

Diffusion of helium has been characterized in silicon carbide of cubic and hexagonal (4H and 6H) forms. Polished sections of SiC were implanted with 3He at 100 keV at a dose of 1×1015/cm2. The implanted SiC samples were sealed under vacuum in silica glass ampoules, and annealed in 1-atm furnaces. 3He distributions following all experiments were measured with Nuclear Reaction Analysis using the reaction 3He(d,p)4He. For He diffusion in cubic SiC and 4H hexagonal SiC we obtain the following Arrhenius relations:

Dcubic=1.83×10-6exp(-254±10kJmol-1/RT)m2sec-1Dcubic=1.83×10-6exp(-254±10kJmol-1/RT)m2sec-1

D4H=4.78×10-7exp(-255±29kJmol-1/RT)m2sec-1D4H=4.78×10-7exp(-255±29kJmol-1/RT)m2sec-1

While He diffusion is considerably slower in SiC than in many silicate phases, He retentivity may be limited under some conditions. For example, helium will be lost from SiC grains over much shorter timescales than potential survival times of SiC presolar grains in the solar nebula. When exposed to impact heating followed by slow cooling, nearly complete loss of He from SiC grains near the site of impact will occur within several hours to a few days. For SiC grains at greater distance from impact sites, He would be better retained, depending on the rapidity of cooling. At tens of km away from a large impactor, where peak T would be ∼800K, SiC grains would lose about 50% of their He if the grains cooled within a few thousand years, and 5% if they cooled within a few tens of years. At greater distances where heating is more modest (500K and lower), SiC grains would be quite retentive of He even for cases of very slow cooling. Helium would also be retained in cases of impact heating followed by very rapid cooling. For these short heating pulses, 10 μm diameter SiC grains would retain more than 50% of their He for peak heating temperatures of 2173, 1973 and 1773K for durations of 3, 10 and 60 seconds, respectively.