Accomplished research
My Master's project supervised by late Prof. L. Perchuck was dedicated to the origin and history of the Limpopo Granulite Belt (South Africa) and associated metasomatic processes. The study of the Paleoarchean Sand River gneisses from the Central Zone of the Limpopo Complex revealed fluid-induced dehydration of the gneiss at the local scale (Rajesh et al., 2013, JP). I have reproduced these orthopyroxene-forming reactions experimentally by subjecting unaltered Sand River gneiss samples to high-pressure high-temperature conditions under the alkali fluids of various compositions (Safonov et al., 2012, GF).
My Doctoral project supervised by Prof. U. Klötzli was also related to the metamorphism and deformation in the crust but focused on the aspect of crystal-plastic deformation and recrystallization of minerals in high-temperature shear zones. The final goal of my project was to develop a tool for isotopic dating of high-temperature deformation events in the deep crust. Such dating can be potentially achieved by using plastically-deformed and recrystallized accessory minerals found in high-temperature shear zones. During my PhD studies, I have done comprehensive work toward understanding the crystal-plastic deformation processes in zircon, and their effects on trace elements and isotopic systems in this mineral (e.g., Kovaleva et al., 2014, SE; Kovaleva et al., 2016, Lithos; Kovaleva et al., 2017, ChemGeo; Kovaleva and Klötzli, 2017, AmMin).
Another part of this project was a study of seismically-deformed zircon. I have found that zircon grains, adjacent to pseudotachylite (i.e., friction melt) veins, develop planar microstructures, such as planar fractures (PFs) and planar deformation bands (PDBs) (Kovaleva et al., 2015, AmMin). Later on, these samples were analyzed with transmission electron microscopy (TEM). We have discovered Pb oxide nanospheres in detrital cores of these zircon grains and showed that Pb nanospheres are a product of mineral-fluid interaction and annealing of the zircon metamict lattice (Kusiak et al., 2019, GCA).
During my Postdoctoral appointment, I have gained interest in shock metamorphic processes and impactites from the Vredefort impact structure in South Africa. I studied two major melt rock types from the Vredefort impact structure, namely, Vredefort granophyre (impact melt) and pseudotachylite (in situ melt) (e.g., Kovaleva et al., 2018, SAJG; Kovaleva et al., 2019, JP). My investigations are a useful addition to our knowledge about the formation and development of large impact craters.
Further, I discovered granular neoblastic zircon grains in impact melt rock (Kovaleva et al., 2019, Geology). After conducting a nano-scale study with transmission electron microscopy, I demonstrated that the granular textures result from the complete decomposition and melting of zircon under shock and subsequent crystallization from a liquid. The presence of cubic zirconia preserved in perfectly round nano-sized inclusions demonstrates that the shock-related temperatures possibly exceeded 2700 °C (Kovaleva et al., 2021, EPSL).
My ongoing research can be divided into two related topics:
(1) Shock deformation of accessory minerals, with an emphasis on geothermobarometry of the impact process. My current main research interest is in shock deformation microstructures in accessory minerals, such as zircon, monazite, apatite, titanite, rutile, and xenotime. I collaborate with Prof. Hugues Leroux, Université De Lille, France, on investigating experimentally-shocked zircon with transmission electron microscopy (TEM). To a greater extent, I collaborate with Dr. Richard Wirth, Helmholtz Centre Potsdam, Germany, on the TEM investigations of the accessory minerals deformed under extreme conditions in nature (e.g., Kusiak et al., 2022 CMP; Kovaleva et al., 2023 AmMin). I plan to further study shocked zircon and other accessory minerals (monazite, apatite, titanite, rutile) from various impactites. Combined EBSD and TEM investigations help to reconstruct pressure-temperature conditions of the impact event, and shock pressure localization phenomenon, and to better understand the large-scale impact processes on Earth, Moon, and Mars, as well as asteroids and other planetary bodies. A large dataset of shocked accessory minerals is currently being compiled in the frames of my Alexander von Humboldt project. I have gathered a wide collection of samples from various terrestrial impact structures such as Araguainha (Brazil), Kara (Russia), Rochechouart (France), Ries (Germany), Tswaing (South Africa), Chicxulub (Gulf of Mexico). All samples but the ones from Chicxulub I collected myself during expeditions and field trips over several field seasons of 2019-2022. The results are being in preparation for publication, such as papers with working titles: “Multiple P-T-t paths revealed by granular zircon open new horizons in impact research,” by Kovaleva and co-authors; “Phase distribution maps of Zircon and Reidite in shocked zircon with unsurpassed spatial resolution using EELS spectra” by Roddatis and co-authors; and "Shock experiment on zircon (ZrSiO4) at 20, 40, and 60 GPa revised: deformation microstructures, amorphization and reidite formation" by Zamyatin and co-authors.
We have just finished analyzing the deformed and recrystallized zircon from a natural fulgurite (the product of a lightning strike). The results are exceptionally interesting and show that the degree of zircon decomposition depends on the location of the glass tube created by lightning. We have not found any evidence of elevated pressure, but indicators of a very high temperature of the fulgurite formation. A manuscript titled "Zircon dissociation, melting, and new crystallization under extreme conditions of lightning strike" by Kovaleva and co-authors is being in preparation for submission.
I plan to further study shocked zircon and other accessory minerals (zircon, monazite, apatite, titanite, rutile) from various impactites. Combined EBSD and TEM investigations help to reconstruct pressure-temperature conditions of the impact event, and shock pressure localization phenomenon, and to better understand the large-scale impact processes on Earth, Moon, and Mars, as well as asteroids and other planetary bodies.
(2) Since 2016, I have ongoing studies of the impact-generated melts and other impactites related to the Vredefort impact structure. I served as a coordinator of the GRAVITAS (Geological Research and Analysis of Vredefort Impact with Timely Anthropological Studies) research, which is a multidisciplinary collaboration project. GRAVITAS aimed to develop scientific research, education, and public communication of planetary science from South Africa. The primary scientific goal of GRAVITAS was to better understand the impact melt dykes of the Vredefort Impact Structure in South Africa, by employing geophysical, geochemical, remote sensing, petrological, geochemical, and (micro)structural methods, as well as anthropological methods. Three manuscripts with me as a co-author are being currently finalized for submission: “Cooling history of a superheated impact melt dike based on fracture and clast evidence from Lesutoskraal Granophyre Dike, Vredefort Dome, South Africa” by Clark and co-authors, and “Paragenesis of the Vredefort Granophyre” by Huber and co-authors. All these studies provide details on the development of a large impact crater through time and the interaction of different components within the crater.
Moreover, a manuscript by Huber and co-authors titled “Can Archean Impact Structures be Discovered? A Case Study from Earth's Largest, Most Deeply Eroded Impact Structure” has been recently published in JGR-Planets in collaboration with the leader of IODP-ICDP expedition 364 Sean Gulick and other colleagues. It describes the porosity and permeability of the basement rocks that were collected along a profile of the Vredefort central uplift. The results are compared with predicted values and with those measured for the Chicxulub crater.
Finally, a supposed proximal ejecta deposit generated by the Vredefort impact event was recently discovered by my colleagues and myself in the Northern Cape, South Africa, and the results are being in preparation for submission. The suspected ejecta deposit is located at the basal portion of the Gamagara Formation, namely the Doornfontein Member, and preserves breccias that are age-constrained coincident with the Vredefort impact structure. We have collected and petrographically and geochemically analyzed samples from numerous field sites and drill cores above the Pre-Gamagara Unconformity. The studied samples are polymict breccias, overlain by the bedded accretionary lapilli, which are, in turn, covered by a paleosol layer. Previous models of the formation of the Doornfontein Member do not adequately explain the observations made in this study, and therefore, we reinterpret it to represent the Vredefort proximal impact ejecta. In that case, is proven, that the Doornfontein breccia represents a time-stratigraphic horizon that was deposited at the precise time of the Vredefort event and can be used in constraining the age of iron ore mineralization. This study is of particular importance because it discovers the oldest proximal ejecta known on Earth, which is linked to an impact crater, and represents one of the richest terrestrial iron ore deposits. A manuscript titled “Discovery of proximal ejecta from the 2.02 Ga Vredefort impact event” is in preparation for submission to Earth and Planetary Science Letters.