T117 Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies (Posters)

Monday, 10 October 2011
9:00 AM - 6:00 PM
Hall C

SEARCH FOR EXTRATERRESTRIAL SIGNATURES OF A POSSIBLE IMPACT EVENT IN THE DIAMICTITE/CAP CARBONATE TRANSITIONS AFTER SNOWBALL EARTH GLACIATIONS (STURTIAN, MARINOAN), OTAVI GROUP, NW-NAMIBIA

Paper number: T117-13
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The idea of this topic is based on earlier results of our group, who in 2004 found an iridium (Ir) anomaly within drill-core-samples from the diamictite/cap carbonate transition in Zambia, which was explained as accumulation of cosmic dust on an ice sheet with low sedimentation rates. The so-called Snowball Earth hypothesis states that the Sturtian (710 Ma) and Marinoan glaciations (635 Ma) were of global extent and may have lasted for several million years. Our samples were collected from conspicuous transition layers on top of the glaciogenic Chuos (Sturtian) and Ghaub (Marinoan) formations of the Neoproterozoic Otavi Group in NW-Namibia.

The mineralogical composition of the samples was studied using the petrographic microscope, X-ray powder diffraction, cathodoluminescence microscopy, and micro-Raman spectrometry. Instrumental neutron activation analyses were utilized for the geochemical study.

Normal impact ejecta are well-known to contain marker minerals, such as Cr- and Ni- spinels, as well as microtektites and a variety of spherules. In our samples, however, only minerals characteristic for the environment of the local carbonate platform, such as quartz, detrital dolomite, mica, clay minerals, and several minerals of a green-schist facies tectono-metamorphic overprint during break-up of Rodinia, e.g., pyrite and chlorite, were found until now. According to the cathodoluminescence studies, all quartz is mostly of diagenetic origin (no CL color) and occurs as fracture-filling mineral with several generations of carbonate-cement (micrite and block cement). In terms of geochemistry, the Fe/Ni, Ni/Cr, Co/Cr ratios are all near or below normal crustal ratios and the concentration of Ir is under the detection limit in bulk samples. However, element concentrations of the bulk samples are highly diluted with carbonate, study of the residue after carbonate dissolving procedure might provide better results.

Thus, at this time we have not been able to confirm the presence of any definitive impact markers in our samples. Before abandoning the impact hypothesis, additional analyses are planned to look for impact evidence in the Snowball Earth cap carbonates. These include the search for geochemical anomalies in terms of trace element measurements of the separated silt fraction, and isotope analyses.