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

Abstract

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.

Raman microspectroscopic mapping with multivariate curve resolution-alternating least squares (MCR-ALS) of the high-pressure, α-PbO2-structured polymorph of titanium dioxide, TiO2-II

“…Recently, shock-metamorphosed grains containing rutile and TiO 2 -II have been documented from four Neoarchean spherule layers-Carawine spherule layer (CSL), Jeerinah spherule layer (JSL), Monteville spherule layer (MSL), and Bee Gorge spherule layer (BGSL)-deposited between ~2.65 and ~2.54 billion years ago. [16] These grains, which provide unambiguous physical evidence to further support an impact origin for these layers [16], comprise the samples for this study.…”

Geochemistry of a confirmed Precambrian impact ejecta deposit: The Grænsesø spherule layer, South Greenland

“…All of these sedimentological features observed in the Graensesø spherule layer have been observed in debris-flow deposits (see Nardin et al [1979] and references therein). It is now clear that the Graensesø spherule layer has several sedimentological characteristics in common with several other Precambrian spherule layers (see, e.g., Simonson 1992;Simonson et al 1999;Hassler and Simonson 2001;Hassler et al 2005) that have a confirmed impact origin (Rasmussen and Koeberl 2004;Simonson et al 2009;Smith et al 2016). Rudnick and Gao (2003).…”

Extending the paleogeographic range and our understanding of the Neoarchean Monteville impact spherule layer (Transvaal Supergroup, South Africa)

“…In fact, only one quartz grain with planar deformation features has been reported from any APB or Early Archean spherule layer and it is from the Jeerinah layer in the Hamersley Basin (Rasmussen and Koeberl 2004). However, the heavy mineral fraction of the Monteville layer has yielded multiple grains of an a-PbO 2 -structured TiO 2 phase (TiO 2 II); this is a highpressure polymorph produced by shock metamorphism during large impacts (Smith et al 2016). Grains of this high-pressure TiO 2 polymorph also occur in three Hamersley spherule layers including the two correlated with the Monteville layer (Smith et al 2016).…”

“…However, the heavy mineral fraction of the Monteville layer has yielded multiple grains of an a-PbO 2 -structured TiO 2 phase (TiO 2 II); this is a highpressure polymorph produced by shock metamorphism during large impacts (Smith et al 2016). Grains of this high-pressure TiO 2 polymorph also occur in three Hamersley spherule layers including the two correlated with the Monteville layer (Smith et al 2016). attributed the formation of the Monteville layer to a succession of related events triggered by a large (roughly Chixculub-scale) marine impact.…”

“…Grains of this high‐pressure TiO 2 polymorph also occur in three Hamersley spherule layers including the two correlated with the Monteville layer (Smith et al. ).…”