¹Kevin Zahnle,²James F. Kasting
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2023.09.023]
¹NASA Ames Research Center, Mails Stop 245-3, Moffett Field, 94043, CA, USA
²The Pennsylvania State University, State College,, PA, USA
Copyright Elsevier

We develop a new model of diffusively modulated hydrodynamic escape to predict oxygen isotopic fractionations caused by the loss of water from a steam atmosphere of Venus. The chief technical advance over previous work is including CO₂ as a major species. We find that ordinary (�18O) and mass-independent (Δ17O) fractionations depend mostly on the extent of lithospheric buffering and the ferocity of EUV heating when escape took place, and relatively little on the size of the lost ocean(s). It is likely that Δ17O evolved significantly from its birth state, not only in the atmosphere but also in the silicates of the crust and upper mantle. If both �18O and Δ17O of Venus are identical to Earth and Moon, we may conclude that Venus and Earth accreted from a common pool. But differences in �18O and Δ17O can be attributed to escape rather than to genetics. If the differences are large enough, they can be used to constrain when escape took place and the extent of volatile exchange with the lithosphere. Neon and argon systematics are most consistent with minimal escape, especially if an Ar-rich source, possibly derived from comets, is added. However, we also find a novel class of solutions in which Ne and Ar of Venus, Earth, and Mars are evolved from a common source material subject to different vigors of hydrodynamic escape, least extreme for Earth and most extreme for Mars. These alternative models require that Venus was always rather dry (<10% of an Earth ocean) and its water lost very early (before <100 Myrs). The two styles of escape – minimal or extreme – should be readily distinguished by an unambiguous measurement of the Ar/Kr ratio. Finally, we find that predicted D/H enrichments are of order 100 for almost all model parameters. This result, a direct consequence of diffusion-limited escape of H and D, provides support for the overall scenario.

^1,2Elizaveta Kovaleva, ³Hassan Helmy, ^4,5Said Belkacim,²Anja Schreiber, ²Franziska D.H. Wilke,²Richard Wirth
American Mineralogist 108, 1906-1923 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P1906.pdf]
¹Department of Earth Sciences, University of the Western Cape, Robert Sobukwe Road, 7535 Bellville, South Africa
²Helmholtz Centre Potsdam—GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany
³Department of Geology, Minia University, 61519-Minia, Egypt
⁴LAGAGE Laboratory, Department of Geology, Faculty of Sciences, Ibn Zohr University, P.O. Box 28/S, 80 000, Agadir, Morocco
⁵Research Institute on Mines and Environment (RIME), Université du Québec en Abitibi-Témiscamingue, 445 Boul. Université, Rouyn-Noranda, Québec J9X 5E4, Canada
Copyright: The Mineralogical Society of America

The origin of Libyan Desert Glass (LDG) found in the western parts of Egypt close to the Libyan
border is debated in planetary science. Two major theories of its formation are currently competing:
(1) melting by airburst and (2) formation by impact-related melting. While mineralogical and textural
evidence for a high-temperature event responsible for the LDG formation is abundant and convincing, minerals and textures indicating high shock pressure have been scarce. This paper provides a
nanostructural study of the LDG, showing new evidence of its high-pressure and high-temperature
origin. We mainly focused on the investigation of Zr-bearing and phosphate aggregates enclosed within
LDG. Micro- and nanostructural evidence obtained with transmission electron microscopy (TEM) are
spherical inclusions of cubic, tetragonal, and orthorhombic (Pnma or OII) zirconia after zircon, which
indicate high-pressure, high-temperature decomposition of zircon and possibly, melting of ZrO2. Inclusions of amorphous silica and amorphous Al-phosphate with berlinite composition (AlPO4) within
mosaic whitlockite and monazite aggregates point at decomposition and melting of phosphates, which
formed an emulsion with SiO2 melt. The estimated temperature of the LDG melts was above 2750 °C,
approaching the point of SiO2 boiling. The variety of textures with different degrees of quenching immediately next to each other suggests an extreme thermal gradient that existed in LDG through radiation
cooling. Additionally, the presence of quenched orthorhombic OII ZrO2 provides direct evidence of
high-pressure (>13.5 GPa) conditions, confirming theory 2, the hypervelocity impact origin of the LDG.