SEARCHING FOR EVIDENCE OF SHOCK METAMORPHISM IN A 3.2 GA SPHERULE BED: PRELIMINARY RESULTS

S4 is a 3.2 Ga chert spherule bed located in the Barberton Greenstone Belt, South Africa. Iridium concentrations and meteoritical Cr isotopic ratios suggest S4 to be part of a large meteorite impact ejecta deposit. This research focuses on the search for high-pressure polymorphs, high-temperature decomposition phases, and planar deformation features (PDFs) in a 282-g sample of S4.

The sample was cut into three separate sublayers. The lower sublayer consists of three spherule layers, containing ~30-40 % spherules (0.2-1.2 mm in size), interlayered between very-fine grained rock and has an irregular contact with the middle sublayer. The middle sublayer consists of angular to subangular, silt to fine sand-sized grains, in a dark gray, silicified matrix, no spherules are present and it displays no obvious grading or bedding. The top sublayer is medium to dark gray, very fine-grained, strongly silicified, has no visible spherules and appears identical to the very-fine grained rock in the lower sublayer.

The sublayers were crushed below 125 µm and sieved into <63 µm and 63-125 µm fractions. The 63-125 µm fractions went through heavy liquid (ρ >2.96 g/cm³) separation to recover heavy minerals. Single-grain X-ray diffraction (XRD) and micro-Raman spectroscopy were done on 23 and 269 grains, respectively. Grain mounts of the 63-125 µm light fractions (ρ <2.96 g/cm³) will be searched for PDFs in quartz grains. Some <63 µm fractions will be searched for shock-produced diamonds.

The most abundant heavy minerals vary within the sublayers and they are probably of secondary origin: the lower sublayer has rutile, pyrite and hematite both separate and intergrown with quartz; the middle sublayer has chlorite and pyrite; the top sublayer has chlorite. Zircons are <1 % of total heavy minerals and 96 % of them have been found in the middle sublayer. Some zircons in the middle and top sublayers show xenotime outgrowths. Four zircons from the lower and middle sublayers display X-ray asterism in their Debye-Scherrer XRD patterns, possibly indicating distorted lattices via shock waves. Trace heavy minerals (<<1 %) include anatase, anglesite, apatite, barite, diopside, fluorapatite, graphite, hydroromarchite, jarosite, magnesiochromite and tourmaline. No PDFs or high-pressure phases have been observed thus far.