Thursday, August 11, 2016

Evidence of an Impact Produced Polymorph in NeoArchean Spherules

Shock-metamorphosed rutile grains containing the high-pressure polymorph TiO2-II in four Neoarchean spherule layers


Smith et al


At least 17 spherule layers are presently known within stratigraphic units deposited between ca. 3.47 and 2.49 Ga. The spherule layers contain varying amounts of formerly molten, millimeter-sized and smaller spherules. The aggregate thickness of spherules in these layers commonly ranges from ∼1 cm to as much as a few decimeters. Several lines of evidence support the interpretation that the spherule layers represent distal impact ejecta layers. Previously, only one shock-metamorphosed grain (quartz) had been documented from the spherule layers. Therefore, a key diagnostic criterion for the impact origin of these layers has remained elusive for 30 years. We report the discovery, using micro-Raman spectroscopy, of shock-induced TiO2-II, a high-pressure polymorph of TiO2, in 34 grains from four Neoarchean spherule layers deposited between ca. 2.65 and 2.54 Ga. As all the TiO2-II-bearing grains contain rutile, we interpret them as shock-metamorphosed rutile grains. Shock-metamorphosed rutile grains, which may be more abundant in the upper parts of three of the layers, provide unambiguous physical evidence to further support an impact origin for these four layers. Our results demonstrate that TiO2-II can survive for >2.5 b.y. in supracrustal successions that have undergone low-grade metamorphism. Because TiO2-II transforms to rutile at a temperature ≥440 °C, TiO2-II in impact ejecta layers is a potential geothermometer. To our knowledge, this is the first report of a shock-induced, high-pressure polymorph formed by an Archean impact.

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