Abstract
Mineralogical and chemical investigations were carried out on intra-craterial bedrocks (Lower Devonian sandstone) and regolithic residual soil deposits present around the Amguid structure, to discuss the hypothesis of its formation through a relatively recent (about 0.1 Ma) impact event. Observations with an optical microscope on intra-craterial rocks do not unequivocally confirm the presence of impact correlated microscopic planar deformation features (PDFs) in quartz crystals. Field observations, and optical and instrumental analysis (Raman spectroscopy) on rocks and soils (including different granulometric fractions) do not provide any incontrovertible pieces of evidence of high energy impact effects or products of impact (e.g., high pressure—temperature phases, partially or totally melted materials, etc.) either in target rocks or in soils. A series of selected main and trace elements (Al, Fe, Mg, Ni, Co and Cu) were analysed on rocks and soils to evaluate the presence in these materials of extraterrestrial sources. Comparative chemical data on rocks and soils suggest that these last are significantly enriched in Fe-poor Mg-rich materials, and in Co, Ni and Cu, in the order. A large number of EDAX-SEM analyses on separated soil magnetic particles indicate an abnormally high presence of Al-free Mg-rich sub-spherical or drop-like silicate particles, showing very similar bulk chemistries compatible with forsterite olivine. Some particles were found associated with a Ni-rich iron metal phase, and this association suggests a specific extraterrestrial origin for them. Electron microscope analysis made on a large number of soil magnetic particles indicates that 98% of them are terrestrial phases (almandine garnet, tourmaline and Fe-oxides, in abundance order), whereas, only a few grains are of questionable origin. One of the Mg-rich silicate particles was found to be a forsterite (Mg = 0.86) Mn-rich (MnO: 0.23%) Cr-free olivine, almost surely of extraterrestrial sources. Electron microprobe analysis of three soil particles allowed identification of uncommon Cr-rich (Cr2O3 about 8%) spinels, poorly compatible with an origin from terrestrial sources, and in particular from local source rocks. We propose a specific extraterrestrial origin for sub-spherical olivine particles characterised by quite similar magnesian character. Excluding any derivation of these particles from interplanetary dust, two other possible extraterrestrial sources should be considered for them, i.e., either normal micrometeorite fluxes or strongly un-equilibrated, or the Vigarano type Carbonaceous (CV) chondrite meteorite material. In this case, further studies will confirm an impact origin for Amguid, as such magnesian olivine components found in soils might represent the only remnants of a vaporised projectile of ordinary non-equilibrated meteoritic composition.
Highlights
Europeans discovered the circular structure known as the Amguid crater (26°05’17” N, 04°23’49”E, Figure 1) in 1948 and it was confirmed from an aircraft in 1954
Mineralogical investigation on selected particles was obtained at the University of Florence, Department of Earth Sciences, by micro-Raman spectroscopy using a confocal microscope equipped with a Horiba/Jobin–Yvon spectrometer coupled with an 1800 g/mm single holographic grating
The main aim of optical microscope observation was the search for evidence of microscopic planar deformation features in quartz crystals of target rocks, as a consequence of a possible high pressure-temperature impact metamorphism [9,10,11], and for evidence of impact products in soils from the Amguid area
Summary
Europeans discovered the circular structure known as the Amguid crater (26°05’17” N, 04°23’49”E, Figure 1) in 1948 and it was confirmed from an aircraft in 1954. Europeans discovered the circular structure known as the Amguid crater Jean-Philippe le Franc made the first first scientific description of it in 1969 [1]. One of the best‐preserved impact craters on Earth—the scientific description of it in 1969 [1]. One of the best-preserved impact craters on Earth—the Amguid. Amguid crater in Algeria—is one of the hardest to access crater in Algeria—is one of the hardest to access (https://www.wondermondo.com/amguid-crater/):
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