^1,2,3Marc Neveu,⁴Christopher H.House,^2,3,5Scott T.Wieman
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113714]
¹Department of Astronomy, University of Maryland, 4296 Stadium Dr., College Park, MD 20742, USA
²Planetary Environments Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20770, USA
³Center for Research in Space Science and Technology, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt, MD 20770, USA
⁴Department of Geosciences, Pennsylvania State University, 503 Deike Building, University Park, PA 16802, USA
⁵Center for Space Sciences and Technology, University of Maryland, Baltimore County, 1000 Hilltop Cir., Baltimore, MD 21250, USA
Copyright Elsevier

Clark et al. [Clark, R.N., Brown, R.H., Cruikshank, D.P., Swayze, G.A., 2019. Icarus, 321, 791–802] reported an extremely low value of the ¹²C/¹³C ratio in CO₂ ice on Phoebe, a likely captured moon of Saturn. Unless Phoebe did not form in the solar system, we interpret this value as indication that Phoebe accreted surface carbon from a region of the protosolar nebula where ¹³C was enriched due to self-shielding of ¹²CO from photodissociation. This could imply that Phoebe is also enriched in ^17,18O relative to most solar system objects sampled to date. Phoebe and other objects that may have sampled these ¹³C-rich regions, such as Pallas or Triton, may provide the opportunity to directly measure isotopic fractionations in endmembers of the self-shielded solar nebula.

¹Lukáš Ackerman,¹Karel Žák,¹Roman Skála,¹Jan Rejšek,¹Šárka Křížová,²Josh Wimpenny,³Tomáš Magna
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.025]
¹Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, CZ-165 00 Praha 6, Czech Republic
²Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
³Czech Geological Survey, Klárov 3, CZ-118 21 Praha 1, Czech Republic
Copyright Elsevier

The Australasian tektite (AAT) strewn field is the largest strewn field on the Earth with about ∼10–30% coverage, both land and ocean, but a clearly identified source impact crater is absent despite the young age of AAT of ca. 790 ka. A genetic link between the Australasian tektites and their unequivocal parental materials is therefore largely impossible to establish. Nevertheless, the nature of the parental materials and the extent of volatilization can be constrained using the splash form tektites, carrying the chemical signatures of high-temperature processes, and the layered (so-called Muong Nong-type) tektites, which are less chemically homogenized and exceptionally abundant in the AAT field. New high-precision Sr, Nd and Pb isotopic measurements were obtained for a chemically and petrographically well-characterized suite of AAT, which included the Muong Nong-type (MN-AAT) with precisely known field locations in Laos and splash forms (SF-AAT) from different parts of the strewn field. In addition, optically dark and light zones of the MN-AAT were also separately analyzed. Homogeneous ε_Nd values from −11.8 to −11.2, combined with a narrow range of two-stage Nd model ages from 1.67 to 1.72 Ga for the entire AAT suite, point to a well-mixed source, in terms of REE, of the crustal segment from which the sedimentary material for tektites was ultimately derived. The Sr isotopic data largely overlap for SF-AAT and MN-AAT (⁸⁷Sr/⁸⁶Sr = 0.71636–0.72021) and indicate Paleozoic to Mesozoic sedimentary parentage. However, late Neogene to early Quaternary re-deposition and formation of a thick silt-sized sedimentary section with vertical stratification is required to comply with ¹⁰Be data. Lead isotope systematics documents at least three different components which can perhaps be represented by different mineral phases, such as feldspar, zircon, organic matter adsorbed on young sediments etc., sorted during fluvial transport and final deposition. In addition, the SF-AAT have systematically lower Pb contents than the MN-AAT, and generally show isotopically heavier Pb isotopic ratios. This is theoretically consistent with a preferential volatilization of lighter Pb isotopes during evaporation and considerably larger Pb loss from SF-AAT when compared to MN-AAT. Nevertheless, further experimental work would be necessary to unambiguously distinguish kinetic fractionation from source mixing.

^1,2Sergey N. Britvin,¹Michail N. Murashko,³Yevgeny Vapnik,¹Yury S. Polekhovsky,^1,2Sergey V. Krivovichev,¹Maria G. Krzhizhanovskaya,¹Oleg S. Vereshchagin,^1,4Vladimir V. Shilovskikh,¹Natalia S. Vlasenko
American Mineralogist 105, 428 – 436 Link to Article [https://doi.org/10.2138/am-2020-7275]
¹St. Petersburg State University, Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia
²Kola Science Center, Russian Academy of Sciences, Fersman Str. 14, 184200 Apatity, Russia
³Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel
⁴Institute of Mineralogy, Urals Branch of Russian Academy of Science, Miass 456317, Russia
Copyright: The Mineralogical Society of America

This paper is a first detailed report of natural hexagonal solid solutions along the join Fe₂P–Ni₂P. Transjordanite, Ni₂P, a Ni-dominant counterpart of barringerite (a low-pressure polymorph of Fe₂P), is a new mineral. It was discovered in the pyrometamorphic phosphide assemblages of the Hatrurim Formation (the Dead Sea area, Southern Levant) and was named for the occurrence on the Transjordan Plateau, West Jordan. Later on, the mineral was confirmed in the Cambria meteorite (iron ungrouped, fine octahedrite), and it likely occurs in CM2 carbonaceous chondrites (Mighei group). Under reflected light, transjordanite is white with a beige tint. It is non-pleochroic and weakly anisotropic. Reflectance values for four COM recommended wavelengths are [R_max/R_min, % (λ, nm)]: 45.1/44.2 (470), 49.9/48.5 (546), 52.1/50.3 (589), 54.3/52.1 (650). Transjordanite is hexagonal, space group P62m; unit-cell parameters for the holotype specimen, (Ni_1.72Fe_0.27)_1.99P_1.02, are: a = 5.8897(3), c = 3.3547(2) Å, V = 100.78(1) ų, Z = 3. D_calc = 7.30 g/cm³. The crystal structure of holotype transjordanite was solved and refined to R₁ = 0.013 based on 190 independent observed [I > 2σ(I)] reflections. The crystal structure represents a framework composed of two types of infinite rods propagated along the c-axis: (1) edge-sharing tetrahedra [M(1)P₄] and (2) edge-sharing [M(2)P₅] square pyramids. Determination of unit-cell parameters for 12 members of the Fe₂P–Ni₂P solid-solution series demonstrates that substitution of Ni for Fe in transjordanite and vice versa in barringerite does not obey Vegard’s law, indicative of preferential incorporation of minor substituent into M(1) position. Terrestrial transjordanite may contain up to 3 wt% Mo, whereas meteoritic mineral bears up to 0.2 wt% S.

¹Patricia L. Clay,¹Katherine H. Joy,¹Brian O’Driscoll,¹Henner Busemann,¹Lorraine Ruzié-Hamilton,¹Ray Burgess,¹Jonathan Fellowes,²Bastian Joachim-Mrosko,¹John Pernet-Fisher,^3,4Stanislav Strekopytov, ⁵Christopher J. Ballentine
American Mineralogist 105, 289 – 306 Link to Article [https://doi.org/10.2138/am-2020-7237]
¹Department of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, U.K.
²Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria
³Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London, SW7 5BD, U.K.
⁴National Measurement Laboratory, LGC Ltd, Queens Road, Teddington, TW11 0LY, U.K.
⁵Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, U.K.
Copyright: The Mineralogical Society of America

Volatile elements (e.g., H, C, N) have a strong influence on the physical and chemical evolution of planets and are essential for the development of habitable conditions. Measurement of the volatile and incompatible heavy halogens, Cl, Br, and I, can provide insight into volatile distribution and transport processes, due to their hydrophilic nature. However, information on the bulk halogen composition of martian meteorites is limited, particularly for Br and I, largely due to the difficulty in measuring ppb-level Br and I abundances in small samples. In this study, we address this challenge by using the neutron irradiation noble gas mass spectrometry (NI-NGMS) method to measure the heavy halogen composition of five olivine-phyric shergottite meteorites, including the enriched (Larkman Nunatak LAR 06319 and LAR 12011) and depleted (LAR 12095, LAR 12240, and Tissint) compositional end-members. Distinct differences in the absolute abundances and halogen ratios exist between enriched (74 to136 ppm Cl, 1303 to 3061 ppb Br, and 4 to 1423 ppb I) and depleted (10 to 26 ppm Cl, 46 to 136 ppb Br, and 3 to 329 ppb I) samples. All halogen measurements are within the ranges previously reported for martian shergottite, nakhlite, and chassignite (SNC) meteorites. Enriched shergottites show variable and generally high Br and I absolute abundances. Halogen ratios (Br/Cl and I/Cl) are in proportions that exceed those of both carbonaceous chondrites and the martian surface. This may be linked to a volatile-rich martian mantle source, be related to shock processes or could represent a small degree of heavy halogen contamination (a feature of some Antarctic meteorites, for example). The differences observed in halogen abundances and ratios between enriched and depleted compositions, however, are consistent with previous suggestions of a heterogeneous distribution of volatiles in the martian mantle. Depleted shergottites have lower halogen abundances and Br and Cl in similar proportions to bulk silicate Earth and carbonaceous chondrites. Tissint in particular, as an uncontaminated fall, allows an estimate of the depleted shergottite mantle source composition to be made: 1.2 ppm Cl, 7.0 ppb Br, and 0.2 ppb I. The resultant bulk silicate Mars (BSM) estimate (22 ppm Cl, 74 ppb Br, and 6 ppb I), including the martian crust and depleted shergottite mantle, is similar to estimates of the bulk silicate earth (BSE) halogen composition.