Aaron S. Bell¹, Paul V. Burger¹, Loan Le², Charles K. Shearer¹, James J. Papike¹, Steve R. Sutton³, Matthew Newville³ and John Jones⁴

¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²Jacobs, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁴NASA, Johnson Space Center, Houston, Texas, 77058, U.S.A

In this work we present a series of experiments that examine the relationship between oxygen fugacity and Cr valence ratio in olivine grown from a basaltic liquid. These experiments are specifically targeted for an olivine-rich martian basalt composition that was modeled after the bulk chemistry of the meteorite Yamato 980459 (i.e., Y-98). The chromium valence ratio in the olivine crystals was measured with X-ray absorption near edge spectroscopy (XANES) at the Advanced Photon Source, Argonne National Laboratory. Results from the XANES measurements indicate that the ratio of divalent to trivalent Cr in the olivine is not only systematically correlated with f_O2, but is also reflective of the molar Cr³⁺/Cr²⁺ in the silicate liquid from which it
grew. In this way, measurements of Cr valence in olivine phenocrysts can yield important information about the oxygen fugacity and molar Cr³⁺/Cr²⁺ of its parental liquid in the absence of a quenched melt phase. Although the results from the experiments presented in this work specifically apply to the Y-98 parental melt, the concepts and XANES analytical techniques discussed within the text present a novel, generalized methodology that may be applicable to any olivine-bearing basalt. Furthermore, the XANES-based measurements are made on a micrometer-scale, thus potential changes of the Cr³⁺/Cr²⁺ in the melt during crystallization could be examined with a great deal of spatial detail.

Reference
Bell AS, Burger PV, Le L, Shearer CK, Papike JJ, Sutton SR, Newville M and Jones J (2014) XANES measurements of Cr valence in olivine and their applications to planetary basalts. American Mineralogist 99:1404.
[doi:10.2138/am.2014.4646]
Copyright: The Mineralogical Society of America

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J. Drążkowska, F. Windmark and C. P. Dullemond

Heidelberg University, Center for Astronomy, Institute of Theoretical Astrophysics, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany

Context. Dust coagulation in protoplanetary disks is one of the initial steps toward planet formation. Simple toy models are often not sufficient to cover the complexity of the coagulation process, and a number of numerical approaches are therefore used, among which integration of the Smoluchowski equation and various versions of the Monte Carlo algorithm are the most popular.
Aims. Recent progress in understanding the processes involved in dust coagulation have caused a need for benchmarking and comparison of various physical aspects of the coagulation process. In this paper, we directly compare the Smoluchowski and Monte Carlo approaches to show their advantages and disadvantages.
Methods. We focus on the mechanism of planetesimal formation via sweep-up growth, which is a new and important aspect of the current planet formation theory. We use realistic test cases that implement a distribution in dust collision velocities. This allows a single collision between two grains to have a wide range of possible outcomes but also requires a very high numerical accuracy.
Results. For most coagulation problems, we find a general agreement between the two approaches. However, for the sweep-up growth driven by the “lucky” breakthrough mechanism, the methods exhibit very different resolution dependencies. With too few mass bins, the Smoluchowski algorithm tends to overestimate the growth rate and the probability of breakthrough. The Monte Carlo method is less dependent on the number of particles in the growth timescale aspect but tends to underestimate the breakthrough chance due to its limited dynamic mass range.
Conclusions. We find that the Smoluchowski approach, which is generally better for the breakthrough studies, is sensitive to low mass resolutions in the high-mass, low-number tail that is important in this scenario. To study the low number density features, a new modulation function has to be introduced to the interaction probabilities. As the minimum resolution needed for breakthrough studies depends strongly on setup, verification has to be performed on a case by case basis.

Reference
Drążkowska J, Windmark F and Dullemond CP(2014) Modeling dust growth in protoplanetary disks: The breakthrough case. Astronomy & Astrophysics 567:A38.
[doi:10.1051/0004-6361/201423708]
Reproduced with permission © ESO

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