^1,2Aryavart ANAND,³Anuj Kumar SINGH,¹Klaus MEZGER,^3.4Jayanta Kumar PATI
Meteoritics & Planetary Science (in Press) Open Access Link to Article [doi: 10.1111/maps.139821Ó2023]
¹Institut für Geologie, Universität Bern, Bern, Switzerland
²Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
³Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Prayagraj, India
⁴National Centre of Experimental Mineralogy and Petrology, University of Allahabad, Prayagraj, India
Published by arrangement with John Wiley & Sons

The Dhala structure in north-central India is a confirmed complex impactstructure of Paleoproterozoic age. The presence of an extraterrestrial component inimpactites from the Dhala structure was recognized by geochemical analyses of highlysiderophile elements and Os isotopic compositions; however, the impactor type hasremained unidentified. This study uses Cr isotope systematics to identify the type ofprojectile involved in the formation of the Dhala structure. Unlike the composition ofsiderophile elements (e.g., Ni, Cr, Co, and platinum group elements) and their inter-elementratios that may get compromised due to the extreme energy generated during an impact, Crisotopes retain the distinct composition of the impactor. The distincte54Cr value of0.310.09 for a Dhala impact melt breccia sample (D6-57) indicates inheritance from animpactor originating within the non-carbonaceous reservoir, that is, the inner Solar System.Based on the Ni/Cr ratio, Os abundance, and Cr isotopic composition of the samples, theimpactor is constrained to be of ureilite type. Binary mixing calculations also indicatecontamination of the target rock by 0.1–0.3 wt% of material from a ureilite-like impactor.Together with the previously identified impactors that formed El’gygytgyn, Zhamanshin,and Lonar impact structures, the Cr isotopic compositions of the Dhala impactites arguefor a much more diverse source of the objects that collided with the Earth over itsgeological history than has been supposed previously.

¹Laura H. Lark,¹James W. Head,¹Christian Huber
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2023.118192]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
Copyright Elsevier

Low-reflectance material (LRM) on the surface of Mercury is thought to be darkened by 2-7 wt.% carbon, making Mercury’s surface the most carbon-rich among the terrestrial planets, but the origin of this carbon is debated. We observe exposures of LRM within large impact basins, which naturally sample Mercury’s outer layers, to produce the first observationally constrained estimate of the absolute quantity of carbon present in Mercury’s shallow interior. We observe LRM and other spectrally distinct material associated with craters within large basins and use scaling laws to relate these observations to the stratigraphy and composition of the subsurface. We find that many large basins across Mercury’s surface have thick layers of LRM in their subsurface. Based on inferences regarding the thickness of these layers, we estimate the absolute quantity of carbon present in Mercury’s crust and upper mantle to be at least
kg, which permits evaluation of hypotheses as to its origin. This quantity rules out the hypothesis that carbon near Mercury’s surface was delivered during late accretion of carbon-rich material, with implications for the delivery of carbon and volatiles to the terrestrial planets. It is also only marginally compatible with a magma ocean origin. Therefore, if Mercury’s core and mantle equilibrated with respect to carbon, we infer that Mercury was probably carbon-saturated early in its evolution and that carbon is an abundant light element in its core, with important implications for Mercury’s thermal and geological evolution.