¹Jiří Borovička,²Olga Popova,¹Pavel Spurný
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13259]
¹Astronomical Institute of the Czech Academy of Sciences, , CZ‐25165 Ondřejov, Czech Republic
²Institute for Dynamics of Geospheres, Russian Academy of Sciences, , 119334 Moscow, Russia
Published by arrangement with John Wiley & Sons

High entry speed (>25 km s⁻¹) and low density (<2500 kg m⁻³) are the two factors that lower the chance of a meteoroid to drop meteorites. The 26 g carbonaceous (CM2) meteorite Maribo recovered in Denmark in 2009 was delivered by a bright bolide observed by several instruments across northern and central Europe. By reanalyzing the available data, we confirmed the previously reported high entry speed of (28.3 ± 0.3) km s⁻¹ and trajectory with slope of 31° to the horizontal. In order to understand how such a fragile material survived, we applied three different models of meteoroid atmospheric fragmentation to the detailed bolide light curve obtained by radiometers located in Czech Republic. The Maribo meteoroid was found to be quite inhomogeneous with different parts fragmenting at different dynamic pressures. While 30–40% of the (2000 ± 1000) kg entry mass was destroyed already at 0.02 MPa, another 25–40%, according to different models, survived without fragmentation up to the relatively large dynamic pressures of 3–5 MPa. These pressures are only slightly lower than the measured tensile strengths of hydrated carbonaceous chondrite (CC) meteorites and are comparable with usual atmospheric fragmentation pressures of ordinary chondritic (OC) meteoroids. While internal cracks weaken OC meteoroids in comparison with meteorites, this effect seems to be absent in CC, enabling meteorite delivery even at high speeds, though in the form of only small fragments.

^1,2Emilie T. Dunham,^3,4,5J. Brian Balta,^1,2 Meenakshi Wadhwa,² Thomas G. Sharp,⁵ Harry Y. McSween Jr.
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13262]
¹Center for Meteorite Studies, Arizona State University, Tempe, Arizona, 8528 USA
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 8528 USA
³Department of Geology & Geophysics, Texas A&M University, College Station, Texas, 77843 USA
⁴ Department of Earth and Environmental Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania, 15260 USA
⁵Department of Earth and Planetary Sciences, Planetary Geosciences Institute, University of Tennessee Knoxville, Knoxville, Tennessee, 37996 USA
Published by arrangement with John Wiley & Sons

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine‐phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine‐phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low‐ and high‐Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low‐Ca pyroxene, which is one of the earliest formed REE‐bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last‐formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed‐system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.