Tenoumer impact crater, Mauritania: Impact melt genesis from a lithologically diverse target

Abstract

Abstract-Impact melt rocks from the 1.9 km diameter, simple bowl-shaped Tenoumer impact crater in Mauritania have been analyzed chemically and petrologically. They are heterogeneous and can be subdivided into three types based on melt matrix color, occurrence of lithic clast components, amount of vesiculation (melt degassing), different proportions of carbonate melt mingled into silicate melt, and bulk rock chemical composition. These heterogeneities have two main causes (1) due to the small size of the impact crater, there was probably no coherent melt pool where a homogeneous mixture of melts, derived from different target lithologies, could be created; and (2) melt rock heterogeneity occurring at the thin section scale is due to fast cooling during and after the dynamic ejection and emplacement process. The overall period of crystal growth from these diverse melts was extremely short, which provides a further indication that complete chemical equilibration of the phases could not be achieved in such short time. Melt mixing processes involved in the generation of impact melts are, thus, recorded in nonequilibrium growth features. Variable mixing processes between chemically different melt phases and the formation of hybrid melts can be observed even at millimeter scales. Due to extreme cooling rates, different mixing and mingling stages are preserved in the varied parageneses of matrix minerals and in the mineral chemistry of microlites. 40 Ar 39 Ar step-heating chronology on specimens from three melt rock samples yielded five concordant inverse isochron ages. The inverse isochron plots show that minute amounts of inherited 40 Ar* are present in the system. We calculated a weighted mean age of 1.57 AE 0.14 Ma for these new results. This preferred age represents a refinement from the previous range of 21 ka to 2.5 Ma ages based on K/Ar and fission track dating.

Silicate liquid immiscibility in impact melts

“…; Schultze et al. ). Specifically, it has been shown that they are the result of mixing of not less than six different target components in variable proportions, that is, 50% granitoids, 17–19% mica schist, 15% amphibolite, 10% limestone, and 6% ultrabasite (Pratesi et al.…”

“…The most recent age determination is that of Schultze et al. (), who obtained a weighted mean age of 1.57 ± 0.14 Ma by 40 Ar/ 39 Ar step‐heating analysis of three impact melt rock samples.…”

“…More recently, Schultze et al. () gave a short description of silicate emulsion textures (comprising Fe,Ca‐rich droplets disseminated in Si,K‐rich matrix) in the glassy mesostasis of the Tenoumer impact melt rocks. They attributed impact melt rock microtextures and compositions of constituent minerals (mainly hopper‐shaped olivine and acicular plagioclase and pyroxene in glassy matrix) to strong disequilibrium crystallization due to poor mixing and fast cooling of melts derived from different target lithologies.…”

“…For a brief sample description of Ten_7 and Ten_8, see Schultze et al. (); for a brief sample description of MSP_2314 and MSP_2315, see Pratesi et al. ().…”

“…), and Tenoumer (Schultze et al. ) and have recently been described from inclusions of silicate impact melt in the Netschaëvo IIE iron meteorite (Van Roosbroek et al. ).…”