Search for shock-metamorphosed grains in Precambrian spherule layers
Smith, Frank C.
University of Delaware
There is minimal physical evidence in only a few of the ∼17 Precambrian spherule layers to support an impact origin. A search was done for shock-metamorphosed grains in the following spherule layers: Carawine, Jeerinah, and Bee Gorge (formerly Wittenoom) in Western Australia, Monteville in South Africa, and Grænsesø in South-West Greenland. Samples went through acid digestion, and the residues were wet sieved. The 63-125 μm (± 125-250 μm) size fractions went through heavy liquid separation. For most samples, the heavy mineral assemblages consist predominantly of anatase, rutile, tourmaline, and zircon (± chrome spinel) grains. Using micro-Raman spectroscopy, the high-pressure, α-PbO2 -structured polymorph of TiO2 (TiO2 II) was identified in 27 buff rutile grains from the Carawine, Jeerinah, Bee Gorge, and Monteville spherule layers. For three of the layers, rutile + TiO2 II grains were found only in their upper parts. For a sample or stratigraphic subdivision within a sample, rutile + TiO2 II grains comprise ∼1-5% of the rutile population. The TiO2 II polymorph is interpreted as a shock-induced phase that is syngenetic with respect to its host spherule layer. The rutile + TiO2 II grains provide physical evidence to support an impact origin for these four spherule layers. Using a universal stage microscope, measurements of the crystallographic orientations of planar microstructures in three quartz grains from the Carawine spherule layer support the interpretation that the microstructures are shock-induced planar deformation features. No unequivocal evidence of shock metamorphism was found in the white opaque zircon grains; instead, these grains appear to have varying degrees of metamictization. The physical properties of the chrome spinel, rutile, and zircon grains support the previously proposed hypothesis that the Carawine, Jeerinah, and Monteville spherule layers are parts of a single spherule layer that is older than the Bee Gorge spherule layer. This study has documented, apparently for the first time, a shock-induced, high-pressure polymorph (TiO 2 II) from Archean terrestrial rocks. The Grænsesø spherule layer (∼145 cm thick) is a dolomixtite that predominantly contains spherules and mm-to-dm-sized chert and carbonate intraclasts in a dolomitic matrix, and it records a singular debris flow within a low-energy marine environment.