PRESENCE OF THE RARE MINERAL CHROMFERIDE IN IMPACT MELT ROCKS FROM THE EL'GYGYTGYN IMPACT STRUCTURE (CHUKOTKA, RUSSIA)

The El’gygytgyn structure in Chukotka (Northeastern Arctic Russia), 18 km in diameter and 3.58 Myr old, is one of the best preserved impact structures on Earth. The impact origin of the El’gygytgyn structure was confirmed by Gurov and colleagues in the late 1970s through the discovery of shock metamorphosed rocks and impact melt rocks in the crater. The crater basin is occupied by the Lake El’gygytgyn Lake, which is surrounded by lacustrine terraces. Impactites are preserved within these lake terrace deposits. Various shock-metamorphosed rocks and glass bombs were (re)deposited as ejecta, while flows of impact melt on the inner walls of the crater might have been the source of these rocks in the terrace deposits. In 2009, El'gygytgyn was the target of a large international drilling project of the International Continental Scientific Drilling Program. Here we report on the discovery of the rare mineral chromferide in impact melt rocks from a lake terrace in the southern part of the El’gygytgyn crater. Chromferide occurs in micrometer-sized cooling-related contraction tracks and in gas vesicles within the glassy matrix of impact melt rocks. This location of chromferide indicates that it crystallized late, after solidification of impact melt and formation of contraction cracks during its cooling. The source of the Cr is not clear, but it is possible that it was derived from the impactor, and thus the presence of a rare Cr mineral is of interest. Early analyses of some siderophile elements (Cr, Ni, and Co) in impact melt rocks and volcanic target rocks at El’gygytgyn noted a significant enrichment of impact melt rocks in Cr and Ni, which was interpreted as indicating an impactor of possible ureilitic composition. Later analyses (partly based on the 2009 drilling project) indicated both ordinary chondritic as well as ureilitic signatures. This is the first report of the mineral chromferide from impact melt rocks.

Dedication: The first author would like to dedicate this presentation to his co-author and friend, who passed away on the date of submission of this abstract (July 19th, 2022) at the age of 88.