¹Putirka, K.D.,¹Rarick, J.C.
American Mineralogist 104, 817-829 Link to Article [DOI: 10.2138/am-2019-6787]
¹Department of Earth and Environmental Sciences, Fresno State, 2345 E. San Ramon Avenue, MS/MH24, Fresno, CA 93720, United States
Copyright: The Mineralogical Society of America

Combining occurrence rates of rocky exoplanets about sun-like stars, with the number of such stars that occupy possibly hospitable regions of the Milky Way, we estimate that at least 1.4 × 108 near-Earth-sized planets occupy habitable orbits about habitable stars. This number is highly imprecise to be sure, and it is likely much higher, but it illustrates that such planets are common, not rare. To test whether such rocky exoplanets might be geologically similar to Earth, we survey >4000 star compositions from the Hypatia Catalog – the most compositionally broad of such collections. We find that rocky exoplanets will have silicate mantles dominated by olivine and/or orthopyroxene, depending upon Fe partitioning during core formation. Some exoplanets may be magnesiowüstite- or quartz-saturated, and we present a new classification scheme based on the weight percent ratio (FeO+MgO)/SiO2, to differentiate rock types. But wholly exotic mantle mineralogies should be rare to absent; many exo-planets will have a peridotite mantle like Earth, but pyroxenite planets should also be quite common. In addition, we find that half or more of the range of exoplanet mantle mineralogy is possibly controlled by core formation, which we model using αFe = FeBSP/FeBP, where FeBSP is Fe in a Bulk Silicate Planet (bulk planet, minus core), on a cation weight percent basis (elemental weight proportions, absent anions) and FeBP is the cation weight percent of Fe for a Bulk Planet. This ratio expresses, in this case for Fe, the fraction of an element that is partitioned into the silicate mantle relative to the total amount available upon accretion. In our solar system, αFe varies from close to 0 (Mercury) to about 0.54 (Mars). Remaining variations in theoretical exoplanet mantle mineralogy result from non-trivial variations in star compositions. But we also find that Earth is decidedly non-solar (non-chondritic); this is not a new result, but appears worth re-emphasizing, given that current discussions often still use carbonaceous or enstatite chondrites as models of Bulk Earth. While some studies emphasize the close overlap of some isotope ratios between certain meteoritic and terrestrial (Earth-derived) samples, we find that major oxides of chondritic meteorites do not precisely explain bulk Earth. To allow Earth to be chondritic (or solar), there is the possibility that Earth contains a hidden component that, added to known reservoirs, would yield a solar/chondritic bulk Earth. We test that idea using a mass balance of major oxides using known reservoirs, so that the sum of upper mantle, metallic core, and crust, plus a hidden component, yields a solar bulk composition. In this approach, the fractions of crust and core are fixed and the hidden mantle component, F h, is some unknown fraction of the entire mantle (so if FDM is the fraction of depleted mantle, then F h + F DM = 1). Such mass balance shows that if a hidden mantle component were to exist, it must comprise >28% of Earth’s mantle, otherwise it would have negative abundances of TiO2 and Al2O3. There is no clear upper limit for such a component, so it could comprise the entire mantle. But all estimates from Fh = 0.28 to Fh = 1.0 yield a hidden fraction that does not match the inferred sources of ocean island or mid-ocean ridge basalts, and would be geologically unusual, having higher Na2O, Cr2O3, and FeO and lower CaO, MgO, and Al2O3 compared to familiar mantle components. We conclude that such a hidden component does not exist. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.

¹Menon, A.,¹Karakas, A.I.,²Lugaro, M.,^1,2Doherty, C.L.,^3,4,5Ritter, C.
Monthly Notices of the Royal Astronomical Society 482, 2320-2335 Link to Article [DOI: 10.1093/mnras/sty2606]
¹Monash Centre for Astrophysics (MoCA), School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
²Konkoly Observatory, Hungarian Academy of Sciences, Konkoly-Thege Miklos ut 15-17, Budapest, 1121, Hungary
³Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, United Kingdom

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¹Josiah B. Lewis,¹Christine Floss,²Dieter Isheim,^1,3Tyrone L. Daulton,²David N. Seidman,¹Ryan Ogliore
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13373]
¹Laboratory for Space Sciences, Washington University, St. Louis, MO, 63130 USA
²Northwestern University Center for Atom‐Probe Tomography, Evanston, IL, 60208 USA
³Institute for Materials Science and Engineering, Washington University, St. Louis, MO, 63130 USA
Published by arrangement with John Wiley & Sons

To constrain the origins of meteoritic nanodiamonds, the abundance ratios of stable C isotopes in acid residues from the carbonaceous chondritic meteorite Allende CV3 were measured using coordinated atom‐probe tomography (APT) and transmission electron microscopy (TEM). We combined our data with previously published APT data. A statistical analysis of this combined data set suggests an upper bound of 1 in 10² on the subpopulation that could have a large isotopic enrichment in ¹³C relative to ¹²C, consistent with the possible detection by secondary ion mass spectrometry of a similar enrichment in a 1 in 10⁵ fraction, abundant enough to account for the Xe‐HL anomalous isotopic component carried by the acid residues. Supernovae are believed to be the source of Xe‐HL, leading to the mystery of why all other supernova minerals do not carry Xe‐HL. The lack of Xe‐HL in low‐density disordered supernova graphite suggests that the isotopically anomalous component is the nanodiamonds, but the disordered C in the residue is not ruled out. We discuss possible origins of the disordered C and implications of our results for proposed formation scenarios for nanodiamonds. At least 99% of the meteoritic acid residue exhibits no unambiguous evidence of presolar formation, although production with solar isotope ratios in asymptotic giant branch stars is not ruled out. Comparison of TEM and APT results indicates that a minority of the APT reconstructions may preferentially sample disordered C rather than nanodiamonds. If this is the case, a presolar origin for a larger fraction of the nanodiamonds remains possible.

^1,2R.H. Hewins,^1,3B. Zanda,¹S. Pont,⁴P.‐M. Zanetta
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13374]
¹Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France
²Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, 08854 USA
³IMCCE, Observatoire de Paris, CNRS UMR 8028, 77 Av. Denfert Rochereau, 75014
Paris, France
⁴Université Lille, CNRS, UMR 8207, UMET, 59000 Lille, France
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si‐rich mesostasis, 2% merrillite, and minor chromite‐ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En_68‐43Fs_27‐48Wo_5‐15) and maskelynite An_~54‐36, more sodic than most maskelynite in shergottites. Chromite‐ulvöspinel composition plots between the earliest and most fractionated spinel‐group minerals in olivine‐phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg‐rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine‐phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine‐phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.