Yasuhiro Hasegawa¹, Bradley M. S. Hansen², and Gautam Vasisht¹
Astrophysical Journal Letters 876, L32 Link to Article [DOI: 10.3847/2041-8213/ab1b5a]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Mani L. Bhaumik Institute for Theoretical Physics, Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

Identification of the main planet formation site is fundamental to understanding how planets form and migrate to their current locations. We consider the heavy-element content trend of observed exoplanets, derived from improved measurements of mass and radius, and explore how this trend can be used as a tracer of their formation sites. Using gas accretion recipes obtained from hydrodynamical simulations, we confirm that the disk-limited gas accretion regime is most important for reproducing the trend. Given that such a regime is specified by two characteristic masses of planets, we compute these masses as a function of the distance (r) from the central star, and then examine how the regime appears in the mass–semimajor axis diagram. Our results show that a plausible solid accretion region emerges at r  0.6 au and expands with increasing r, using the conventional disk model. Given that exoplanets that possess the heavy-element content trend distribute currently near their central stars, our results imply the importance of planetary migration that would occur after solid accretion onto planets might be nearly completed at r ≥ 0.6 au. Self-consistent simulations would be needed to verify the predictions herein.

¹Robert M. Hazen
American Mineralogist 104, 810-816 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/open_access/AM104P0810.pdf]
¹Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A. Orcid: 0000-0003-4163-8644
Copyright: The Mineralogical Society of America

Minerals reveal the nature of the co-evolving geosphere and biosphere through billions of years of Earth history. Mineral classification systems have the potential to elucidate this rich evolutionary story; however, the present mineral taxonomy, based as it is on idealized major element chemistry and crystal structure, lacks a temporal aspect, and thus cannot reflect planetary evolution. A complementary evolutionary system of mineralogy based on the quantitative recognition of “natural kind clustering” for a wide range of condensed planetary materials with different paragenetic origins has the potential to amplify, though not supersede, the present classification system.

¹Bernard J. Wood,¹Duane J. Smythe,¹Thomas Harrison
American Mineralogist 104. 844-856 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/open_access/AM104P0844.pdf]
¹Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, U.K.
Copyright: The Mineralogical Society of America

As part of a project to investigate the volatilities of so-called “moderately volatile elements” such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl during planetary formation, we began by re-calculating the condensation temperatures of these elements from a solar gas at 10-4 bar. Our calculations highlighted three areas where currently available estimates of condensation temperature could be improved. One of these is the nature of mixing behavior of many important trace elements when dissolved in major condensates such as silicates, Fe-rich metals, and sulfides. Nonideal solution of the trace elements can alter (generally lower) condensation temperatures by up to 500 K. Second, recent measurements of the halogen contents of CI chondrites (Clay et al. 2017) indicate that the solar system abundance of chlorine is significantly overestimated, and this affects the stabilities of gaseous complexes of many elements of interest. Finally, we have attempted to improve on previous estimates of the free energies of chlorine-bearing solids since the temperature of chlorine condensation has an important control on the condensation temperatures of many trace elements. Our result for the 50% condensation temperature of chlorine, 472 K is nearly 500 K lower than the result of Lodders (2003), and this means that the HCl content of the solar gas at temperatures <900 K is higher than previously estimated. We based our calculations on the program PHEQ (Wood and Hashimoto 1993), which we modified to perform condensation calculations for the elements H, O, C, S, Na, Ca, Mg, Al, Si, Fe, F, Cl, P, N, Ni, and K by free energy minimization. Condensation calculations for minor elements were then performed using the output from PHEQ in conjunction with relevant thermodynamic data. We made explicit provision for nonidealities using information from phase diagrams, heat of solution measurements, partitioning data and by using the lattice strain model for FeS and ionic solids and the Miedema model for solutions in solid Fe. We computed the relative stabilities of gaseous chloride, sulfide, oxide, and hydroxide species of the trace elements of interest and used these, as appropriate in our condensation calculations. In general, our new 50% condensation temperatures are similar to or, because of the modifications noted above, lower than those of Lodders (2003).

¹James J. Papike,¹Steven B. Simon,¹Charles K. Shearer
American Mineralogist 104, 838-843 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/Abstracts/AM104P0838.pdf]
¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A
Copyright: The Mineralogical Society of America

The usefulness of the Mn/Fe ratios of olivine and pyroxene to identify a sample’s host parent body is well established. Although there is an overarching, defining slope for each planetary body, there is some scatter, or “dispersion” around the defining slope. This dispersion reveals important facts relating to the planetary body. The source regions of the three main types of lunar basalts (very high-Ti, lowTi, and very low-Ti) have fO2 values near IW-1 or below, and all iron is either ferrous or metallic. The dispersion in the Mn/Fe ratios of pyroxene from the Moon is largely caused by differences in the Ti and Al concentrations in the mantle source regions and the resulting differences in Ti activity of the primary basaltic melts derived from those sources. Ti displaces ferrous iron in the pyroxene M1 site (in a coupled substitution with Al for Si in the tetrahedral site), and therefore, with increasing Ti activity the Mn/Fe ratio in pyroxene increases in all three suites studied. For lunar mare basalts, the effect of Ti activity on the occupancy of the pyroxene M1 site, and crystallization sequence differences among high-Ti, low-Ti, and VLT basalts account for almost all of the observed dispersion in the Mn/Fe ratios.

¹Charles K.Shearer,¹Aaron S.Bell,²Christopher D.K.Herd,¹Paul V.Burger,¹Paula Provencio,¹Zachary D.Sharp,¹James J.Papike
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.034]
¹Institute of Meteoritics, Department of Earth and Planetary Science, University of New Mexico (UNM), Albuquerque, New Mexico 87131
²Department of Earth and Atmospheric Sciences University of Alberta, Edmonton, AB T6G 2E3
Copyright Elsevier

In part 1 of our examination of Martian meteorite Northwest Africa 8159 (NWA 8159) we illustrated many interesting mineralogical and textural attributes that make this martian basalt unique. Unlike the shergottites that illustrate a clear relationship between the extent of trace element and isotopic characteristics and oxygen fugacity (reduced, depleted magmas; oxidized, enriched magmas), NWA 8159 illustrates a decoupling of this relationship as it has oxidized and depleted signatures. In part 2, using a series of new observations and measurements (Cl isotopes, XANES, TEM, empirical modeling) we use NWA 8159 to explore the interaction between mantle-derived magmas and the martian crust. The magnetite-orthopyroxene intergrowths associated with olivine are a product of a martian subsolidus oxidation event near the QFM buffer and not a magmatic reaction in an oxidizing magma (>QFM+3). This subsolidus event is further supported by Cr valence in the olivine, alteration of P-rich olivine, and end-member magnetite in the matrix of the meteorite. Although this subsolidus alteration makes it extremely difficult to determine the original fO₂ of the parental magma for NWA 8159, there is evidence that during the initial stages of crystallization the fO₂ was modestly reducing (∼IW+1). Potential manifestations of more reducing magmatic conditions include P-rich cores in the olivine and low Fe³⁺ in silicates (plagioclase, pyroxene). Further, if analogous to all other depleted shergottites, NWA 8159 initially crystallized under reducing conditions. This decoupling between oxygen fugacity and isotopic-trace element characteristics suggests that basalts derived from the martian mantle interacted with the crust in ways that significantly influenced redox history and volatile element isotopic composition (Cl, S), without dramatically modifying many of its radiogenic isotope and trace element mantle fingerprints.

¹Kent C.Condie,²Charles K.Shearer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.027]
¹Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.
Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (> 4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (< 3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.
Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.

Richard C. HUGO¹, Alex M. RUZICKA¹, and Alan E. RUBIN^2,3,
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13304]
¹Department of Geology and Cascadia Meteorite Laboratory, Portland State University, Portland, Oregon 97201, USA
²Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, California 90095-1567, USA
³Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME 04217, USA
Published by arrangement with John Wiley & Sons

Saint‐Séverin and Elbert, two LL6 chondrite breccias, were systematically studied to evaluate multiple deformation effects on spatial scales ranging from thin section (mesoscale) to micron‐submicron (microscale) using optical microscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The different techniques provide consistent results but have complementary strengths, together providing a powerful approach to unravel even complex impact histories. Both meteorites have an S4 conventional shock stage, but interclast areas are more deformed, and clasts are more deformed in Elbert than in Saint‐Séverin. TEM and EBSD data provide compelling evidence that Saint‐Séverin experienced significant shock deformation while already hot, and cooled rapidly afterward, as a result of a major, possibly disruptive impact on the LL chondrite parent body ~4.4 Ga ago. In contrast, Elbert was shocked from a cold initial state but was heated significantly during shock, and cooled in a localized hot impact deposit on the LL asteroid. Both meteorites probably were shocked at least twice; data for Saint‐Séverin are best reconciled with a three‐impact model.

Fred JOURDAN¹, Sebastien NOMADE², Michael T. D. WINGATE³, Ela EROGLU⁴, and AlDEINO⁵
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13305]
¹Western Australian Argon Isotope Facility, JdL Centre & School of Earth and Planetary Sciences, Curtin University, GPOBox U1987, Perth, Western Australia 6845, Australia
²Laboratoire des Sciences du Climat et de L’Environnement, UMR 8212, LSCE/IPSL, CEA-CNRS-UVSQ, Universite Paris-Saclay, Gif-Sur-Yvette, France
³Dept of Mines, Industry Regulation and Safety, Geological Survey of Western Australia, East Perth, Western Australia 6004,Australia
⁴Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia
⁵Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, California 94709, USA
Published by arrangement with John Wiley & Sons

The Australasian tektites are quench melt glass ejecta particles distributed over the Asian, Australian, and Antarctic regions, the source crater of which is currently elusive. New ⁴⁰Ar/³⁹Ar age data from four tektites: one each from Thailand, China, Vietnam, and Australia measured using three different instruments from two different laboratories and combined with published ⁴⁰Ar/³⁹Ar data yield a weighted mean age of 788.1 ± 2.8 ka (±3.0 ka, including all sources of uncertainties) (P = 0.54). This age is five times more precise compared to previous results thanks, in part, to the multicollection capabilities of the ARGUS VI noble gas mass spectrometer, which allows an improvement of almost fourfold on a single plateau age measurement. Diffusion experiments on tektites combined with synthetic age spectra and Monte Carlo diffusion models suggest that the minimum temperature of formation of the Thai tektite is between 2350 °C and 3950 °C, hence a strict minimum value of 2350 °C.