¹M. B. Biren,²J.‐A. Wartho,¹M. C. VAN Soest,¹K. V. Hodges,^3,4H. Cathey,⁵B. P. Glass,^6,7C. Koeberl,⁸J. W. Horton Jr,⁹W. Hale
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13316]
¹Group 18 Laboratories, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 85287 USA
²GEOMAR Helmholtz Centre for Ocean Research Kiel, D‐24148 Kiel, Germany
³LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona, 84287 USA
⁴Central Analytical Research Facility, Queensland University of Technology, Brisbane, Queensland, 4000 Australia
⁵Department of Geological Sciences, University of Delaware, Newark, Delaware, 19716 USA
⁶Department of Lithospheric Research, University of Vienna, A‐1090 Vienna, Austria
⁷Natural History Museum, Burgring 7, A‐1010 Vienna, Austria
⁸U.S. Geological Survey, 926A National Center, Reston, Virginia, 20192 USA
⁹IODP Core Repository, Bremen, D‐28359 Germany
Published by arrangement with John Wiley & Sons

Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, ⁴⁰Ar/³⁹Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

¹Juulia-Gabrielle Moreau,^1,2Tomas Kohout,³Kai Wünnemann,⁴Patricie Halodova,^5,6Jakub Haloda
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.004]
¹Department of Geosciences and Geography, University of Helsinki, Finland
²Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
³Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
⁴Centrum výzkumu Řež, Husinec-Řež, Czech Republic
⁵Czech Geological Survey, Prague, Czech Republic
⁶Oxford Instruments NanoAnalysis, Bucks, United Kingdom
Copyright Elsevier

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40–50 GPa. This pressure range is at the transition between shock stage 5 (CS5) and 6 (CS6) of the shock metamorphism classification in ordinary and enstatite chondrites. To characterize CS5 and CS6 better with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3–6%. Transition from shock CS5 to CS6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of ~0.7–0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of ~0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from ~540 K at 30 GPa to ~1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (~800 K increase) and melt fractions (~0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10–15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stöffler et al. 2018, 2019).

^1,3Xiaojia Zeng,^1,2,3 Xiongyao Li,⁴ Dayl Martin,^1,2,3Hong Tang,^1,2,3Wen Yu,^5,6 Kang Yang,^5,6Zegui Wang,⁷Shijie Wang
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.06.011]
¹Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
²CAS Center for Excellence in Comparative Planetology, Hefei, China
³Key Laboratory of Space Manufacturing Technology, Chinese Academy of Sciences, Beijing 100094, China
⁴European Space Agency, Fermi Avenue, Harwell Campus, Didcot, Oxfordshire OX11 0FD, United Kingdom
⁵School of Mechanics and Civil Engineering, China University of Ming and Technology, Xuzhou 221116, China
⁶State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Ming and Technology, Xuzhou 221116, China
⁷State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Copyright Elsevier

Asteroid regolith simulants (i.e., substitute materials for asteroid surface regoliths) are useful for the preparation of asteroid landing and/or sample-return missions. In this study, we report a new Itokawa asteroid Regolith Simulant (called IRS-1) as an S-type asteroid surface analogue for China’s upcoming asteroid exploration. The IRS-1 simulant was developed from mixing terrestrial minerals with appropriate particle size distributions, based on the currently available mineralogy data of S-type asteroid 25143 Itokowa and a LL6 chondrite Sulagiri. Multiple properties of this simulant are well-characterized, including mineralogy, bulk chemistry, particle size, density, mechanical properties, reflectance spectra, thermal properties, thermogravimetry, and hygroscopicity. These results demonstrate that the IRS-1 simulant has similar mineralogy, bulk chemistry, and physical properties to the target materials (i.e., Itokowa samples and LL6 chondrite Sulagiri), making this simulant a reasonable surface analogue of S-type asteroids. Based on the investigation of mechanical properties of the IRS-1 simulant and two other prepared regolith samples (i.e., L-chondrite-like IRS-1 L and H-chondrite-like IRS1H), we found that the mineralogical variations on S-type asteroids have a relatively large influence on the mechanical properties of S-type asteroid regoliths. Our studies show that the IRS-1 simulant will be appropriate for a number of scientific and engineering-based investigations where a large amount (few kilograms to hundreds of kilograms) of sample is required (e.g., technology development, hardware testing, and drilling). This study also provides an effective production approach for the future development of asteroid regolith simulants for different types of asteroid regoliths and associated applications.

^1,2Zhuqing Xue,^1,3Long Xiao,²Clive R.Neal,⁴Yigang Xu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.022]
¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
²Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
³State Key Laboratory of Lunar and Planetary Science, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau, China
⁴State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China
Copyright Elsevier

We conducted a thorough analysis of the feldspathic breccia meteorite Dhofar 1428 with the aim of better understanding the composition and evolution of lunar crust. This sample comprises a heterogeneous array of lithic fragments including magnesian and ferroan anorthositic granulites, mafic granulites/granulitic breccia, basalts, and different kinds of impact melt rocks. In which, a high-Ti basalt clast comprising large zoned pyroxene was observed. Based on equilibrium melt calculations of mineral zonations from this basalt, Mg-pyroxene cores were interpreted to be formed from a light rare earth element (LREE) enriched liquid, whereas the Fe-pyroxene rims grew from an LREE-depleted magma. We propose that LREE-depleted signature of Fe-pyroxene results from co-crystallization with apatite. The Mg-pyroxenes suggest that enriched liquids with higher REE contents and different REE patterns relative to KREEP existed within lunar interior. Oscillating Ti/Al ratios across pyroxene in this basalt may indicate several magma recharge events or crystal movement within a zoned magma chamber. This feature illustrates that magmas were derived from a variety of sources around the time of formation of this basalt. In situ U-Pb dating was conducted on apatite grains within this basalt, the excellent consistence between the U-Pb Concordia age (3941±24 Ma, 2σ) and 207Pb/206Pb isochron age (3934±24Ma, 2σ) indicates the most likely crystallization age of this high-Ti basalt at ∼3940 Myr, making it one of the oldest high-Ti basalts formed on the Moon.
Magnesian anorthositic granulites are mineralogically and geochemically similar to those trace element-poor magnesian anorthositic granulites in many lunar meteorites. These magnesian granulites cannot form from simple mixing of pristine Ferroan Anorthosite and lithologies from the Mg-Suite, and do not have any affinities with KREEP or the Procellarum KREEP Terrane, and they could be important components of farside highlands.

¹Paolo A.Sossi,²Stephan Klemme,³Hugh St.C.O’Neill,²Jasper Berndt,^1,4Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.021]
¹Institut de Physique du Globe de Paris, 1 rue Jussieu, F-75005, Paris, France
²Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Correnst. 24, D48149 Münster, Germany
³Research School of Earth Sciences, Australian National University, 2601 Canberra, Australia
⁴Institut Universitaire de France, 75005, Paris, France
Copyright Elsevier

Moderately volatile elements (MVEs) are sensitive tracers of vaporisation in geological and cosmochemical processes owing to their balanced partitioning between vapour and condensed phases. Differences in their volatilities allows the thermodynamic conditions, particularly temperature and oxygen fugacity (fO₂), at which vaporisation occurred to be quantified. However, this exercise is hindered by a lack of experimental data relevant to the evaporation of MVEs from silicate melts. We report a series of experiments in which silicate liquids are evaporated in one-atmosphere (1-atm) gas-mixing furnaces under controlled fO₂s, from the Fe-“FeO” buffer (iron-wüstite, IW) to air (10^-0.68 bars), bracketing the range of most magmatic rocks. Time- (t) and temperature (T) series were conducted from 15 to 930 minutes and 1300-1550°C, at or above the liquidus for a synthetic ferrobasalt, to which 20 elements, each at 1000 ppm, were added. Refractory elements (e.g., Ca, Sc, V, Zr, REE) are quantitatively retained in the melt under all conditions. The MVEs show highly redox-dependent volatilities, where the extent of element loss as a function of fO₂ depends on the stoichiometry of the evaporation reaction(s), each of which has the general form M^x+nO_(x+n)/2 = M^xO_x/2 + n/4O₂. Where n is positive (as in most cases), the oxidation state of the element in the gas is more reduced than in the liquid, meaning lower oxygen fugacity promotes evaporation. We develop a general framework, by integrating element vaporisation stoichiometries with Hertz-Knudsen-Langmuir (HKL) theory, to quantify evaporative loss as a function of t, T and fO₂. Element volatilities from silicate melts differ from those during solar nebular condensation, and can thus constrain the conditions of volatile loss in post-nebular processes. Evaporation in a single event strongly discriminates between MVEs, producing a step-like abundance pattern in the residuum, similar to that observed in the Moon or Vesta. Contrastingly, the gradual depletion of MVEs according to their volatility in the Earth is inconsistent with their loss in a single evaporation event, and instead likely reflects accretion from many smaller bodies that had each experienced different degrees of volatilisation.

¹Jean Bollard et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.025]
¹Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Copenhagen DK-1350, Denmark
Copyright Elsevier

Chondrites are fragments of asteroids that avoided melting and, thus, provide a record of the material that accreted to form protoplanets. The dominant constituent of chondrites are millimeter-sized chondrules formed by transient heating events in the protoplanetary disk. Some chondritic components, including chondrules, contain evidence of the extinct short-lived radionuclide ²⁶Al (half-life of 0.73 Myr). The decay of ²⁶Al is postulated to have been an important heat source promoting asteroidal melting and differentiation. Thus, understanding the ²⁶Al inventory in the accretion regions of differentiated asteroids is critical to constrain the accretion timescales of protoplanets. The current paradigm asserts that the canonical ²⁶Al/²⁷Al ratio of ∼5 ×10⁻⁵ recorded by the oldest dated solids, calcium-aluminium refractory inclusions, represents that of the bulk Solar System. We report, for the first time, the ²⁶Al-²⁶Mg systematics of chondrules from the North West Africa (NWA) 5697 L 3.10 ordinary chondrite and Allende CV3_OxA (Vigarano type) carbonaceous chondrite that have been previously dated by U-corrected Pb-Pb dating. Eight chondrules, which record absolute ages ranging from 4567.57±0.56 to 4565.84±0.72 Ma, define statistically-significant internal isochron relationships corresponding to initial (²⁶Al/²⁷Al) ([²⁶Al/²⁷Al]₀) ratios in their precursors at the time of CAI formation at 4567.3±0.16 Ma ranging from (3.92+4.53-2.95) × 10⁻⁶ to (2.74+1.30-1.09) × 10⁻⁵. These initial ratios are much lower than those predicted by the Pb-Pb ages, corresponding to age mismatches between the Pb-Pb and ²⁶Al-²⁶Mg systems ranging from 0.69+0.54-0.44 to 2.71+0.66-0.59 Myr. All chondrules record ⁵⁴Cr/⁵²Cr compositions indicating an origin from inner Solar System precursor material and, as such, we interpret the age mismatch to reflect a reduced initial abundance of ²⁶Al in the chondrule precursors, similar to that proposed for the angrite parent body. In particular, the range of [²⁶Al/²⁷Al]₀ ratios essentially defines two groups, which are apparently correlated with the absolute ages of the chondrules. A first group, charactertized by chondrules with absolute Pb-Pb ages identical to CAIs, defines a mean [²⁶Al/²⁷Al]₀ value of (4.75+1.99-1.21) × 10⁻⁶, whereas a second group, with absolute ages ∼1 Myr younger than CAIs, record a mean mean [²⁶Al/²⁷Al]₀ of (1.82+0.57-0.40) × 10⁻⁵. We interpret this systematic variability in [²⁶Al/²⁷Al]₀ values as reflecting progressive inward transport and admixing of dust of solar composition and ²⁶Al content from the outer disk during chondrule recycling and remelting. Finally, a reduced [²⁶Al/²⁷Al]₀ ratio in chondrule precursors impacts our understanding of the accretion timescales of differentiated planetesimals if chondrules are indeed representative of inner disk material. Using the average [²⁶Al/²⁷Al]₀ ratio of (1.36±0.72) × 10⁻⁵ defined by the eight chondrules, thermal modelling constrains the accretion of differentiated planetesimals formed with this ²⁶Al inventory from ∼0.1 to ∼0.9 Myr after Solar System formation to ensure melting by ²⁶Al decay.

^1,2Travis J.Tenner,^1,3Daisuke Nakashima,^1,4Takayuki Ushikubo,⁴Naotaka Tomioka,^5,6Makoto Kimura,^7,8Michael K.Weisberg,¹Noriko T.Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.023]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
²Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, MSJ514, Los Alamos, NM 87545, USA
³Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
⁴Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe Otsu, Nankoku, Kochi 783-8502, Japan
⁵Faculty of Science, Ibaraki University, Mito 310-8512, Japan
⁶National Institute of Polar Research, Tokyo 190-8518, Japan
⁷Kingsborough Community College and Graduate Center, The City University of New York, 2001 Oriental Boulevard, Brooklyn, NY 11235-2398, USA
⁸American Museum of Natural History, Central Park West at 79th Street, New York, NY, 10024-5192, USA
Copyright Elsevier

Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#’s of 99.0 to 99.2 and Δ¹⁷O of −4.2‰ to −5.3‰ have resolvable excess ²⁶Mg. Their inferred (²⁶Al/²⁷Al)₀ values range from (3.5 ± 1.3) × 10⁻⁶ to (6.0 ± 3.9) × 10⁻⁶. This corresponds to formation times of 2.2 (–0.5/+1.1) Myr to 2.8 (−0.3/+0.5) Myr after CAIs, using a canonical (²⁶Al/²⁷Al)₀ of 5.23 × 10^-5, and assuming homogeneously distributed ²⁶Al that yielded a uniform initial ²⁶Al/²⁷Al in the Solar System. Seven chondrules lack resolvable excess ²⁶Mg. They have lower Mg#’s (94.2 to 98.7) and generally higher Δ¹⁷O (−0.9‰ to −4.9‰) than chondrules with resolvable excess ²⁶Mg. Their inferred (²⁶Al/²⁷Al)₀ upper limits range from 1.3 × 10⁻⁶ to 3.2 × 10⁻⁶, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS ²⁷Al/²⁴Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00-3.05 chondrites. We conclude that the resolvable (²⁶Al/²⁷Al)₀ variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr.

Using relationships between chondrule (²⁶Al/²⁷Al)₀, Mg#, and Δ¹⁷O, we interpret spatial and temporal features of dust, gas, and H₂O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ¹⁷O ∼−5‰ chondrules with resolvable excess ²⁶Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ∼100×. After these chondrules formed, we interpret a later influx of ¹⁶O-poor H₂O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher Δ¹⁷O (−5‰ to –1‰), with low (²⁶Al/²⁷Al)₀, and hence no resolvable excess ²⁶Mg.

We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (²⁶Al/²⁷Al)₀ and that Mg# < 99 chondrules generally have no resolvable excess ²⁶Mg, implying lower (²⁶Al/²⁷Al)₀. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.

¹T.J.Barrett,^1,2J.J.Barnes,^1,3M.Anand,¹I.A.Franchi,¹R.C.Greenwood,^1,4B.L.A.Charlier,¹X.Zhao,⁵F.Moynier,^1,3M.M.Grady
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.024]
¹School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA
³Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK
⁴Victoria University of Wellington, Wellington 6140, NZ
⁵Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univeristé Paris Diderot, 75005 Paris, France
Copyright Elsevier

Generally, terrestrial rocks, martian and chondritic meteorites exhibit a relatively narrow range in bulk and apatite Cl isotope compositions, with δ³⁷Cl (per mil deviation from standard mean ocean chloride) values between − 5.6 and + 3.8 ‰. Lunar rocks, however, have more variable bulk and apatite δ³⁷Cl values, ranging from ∼ − 4 to + 40 ‰. As the Howardite-Eucrite-Diogenite (HED) meteorites represent the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (4 Vesta), studying them for their volatiles may provide insights into planetary differentiation processes during the earliest Solar System history.

Here the abundance and isotopic composition of Cl in apatite were determined for seven eucrites representing a broad range of textural and petrological characteristics. Apatite Cl abundances range from ∼ 25 to 4900 ppm and the δ³⁷Cl values range from − 3.98 to + 39.2 ‰. Samples with lower apatite H₂O contents were typically also enriched in ³⁷Cl but no systematic correlation between δ³⁷Cl and δD values was observed across samples. Modelled Rayleigh fractionation and a strong positive correlation between bulk δ⁶⁶Zn and apatite δ³⁷Cl support the hypothesis that Cl degassed as metal chlorides from eucritic magmas, in a hydrogen-poor environment. In the case of lunar samples, it has been noted that δ³⁷Cl values of apatite positively correlate with bulk La/Yb ratio. Interestingly, most eucrites show a negative correlation with bulk La/Yb ratio. Recently, isotopically light Cl values have been suggested to record the primary solar nebular signature. If this is the case then 4 Vesta, which accreted rapidly and early in Solar System history, could also record this primary nebular signature corresponding to the lightest Cl values measured here. The significant variation in Cl isotope composition observed within the eucrites are likely related to degassing of metal chlorides.

¹Josh Wimpenny et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.016]
¹Lawrence Livermore National Laboratory, Livermore, CA, 94550
Copyright Elsevier

Zinc isotopes fractionate during evaporation, and thus can potentially be used to calculate the proportion of volatile elemental loss from objects such as tektites, nuclear fallout melt glasses formed from silicate soils, and rocks from the Moon. The utility of the Zn isotope system in constraining the magnitude of volatile loss depends on accurate knowledge of its fractionation behavior during evaporation, i.e., the Zn isotope fractionation factor. Here, we present new results of Zn isotope analyses of experimentally heated soil samples, together with analyses of environmentally derived fallout melt glasses, and australite tektites, to better constrain how and to what extent Zn isotopes fractionate during thermally-driven evaporation.

Major and trace element analyses of melt glass derived from the experimental heating of rhyolitic and arkosic soils demonstrates that elemental fractionation progressively becomes more important with increasing temperature and time. Initial Zn isotope ratios in the rhyolitic and arkosic soils are similar to upper continental crustal (UCC) values but become increasingly isotopically heavy as more Zn is lost from the heated glass, with a maximum δ⁶⁶Zn value of +1.49 ± 0.05 ‰. The progressive loss of Zn from the rhyolitic soil is consistent with a fractionation factor (α) of 0.99879 ± 13. After loss of a labile component from the arkosic soil further evaporative loss of Zn from the silicate rich residue is also consistent with an α of ∼0.9988. This fractionation factor is not sensitive to either the heating temperature or duration.

The fallout melt glass, derived from near surface nuclear weapons tests, and natural tektite samples are both relatively enriched in isotopically heavy Zn compared to the UCC. Although the fallout melt glass is largely comprised of the same rhyolitic soil used in our experiments the extent of Zn isotope fractionation is lower; consistent with an α of 0.9997. This is similar to estimated values of α associated with loss of Zn from Trinitite and lunar rocks. It is more difficult to constrain a value for α from australite tektites because the location of the impact site is not well constrained. Based on our analyses, we estimate a range of α values between 0.9988-0.9997, which is broadly consistent with experimental data and analyses of fallout melt glass.

In all cases the measured Zn isotope fractionation factors are much lower than would be expected from evaporative loss of Zn in a high vacuum. We hypothesize that, for Zn, the value of α is dependent on the localized gas pressure and composition, with the degree of isotopic fractionation decreasing from high vacuum (< 1×10^-9 bar) to atmospheric pressure. This would have consequences for the interpretation of Zn systematics on the Moon. Recent studies suggest that Zn loss from the Moon was dominantly caused by the Giant Impact and proceeded with an α of ∼0.9997. If correct, and taken together with our experimental data, this suggests that evaporative loss of Zn from the Moon is unlikely to have occurred in high vacuum and is more likely to have taken place at pressures similar to or higher than the current terrestrial atmosphere.

¹Pechersky,¹D.M., Markov, G.P.
Izvestiya, Physics of the Solid Earth 55, 287-297 Link to Article [DOI: 10.1134/S1069351319020083]
¹Schmidt Institute of Physics of the Earth, Russian Academy of Sciences (IPE RAS), Moscow, 123242, Russian Federation

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