2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 117-2
Presentation Time: 9:00 AM-6:00 PM


TEGTMEIER, Eric L.1, BREARLEY, Adrian2, ELSTON, Wolfgang E.1, and SPILDE, Michael N.3, (1) Department of Earth and Planetary Sciences, University of New Mexico, Northrop Hall, MSC03 2040, 1 University of New Mexico, Albuquerque, NM 87131, teggy@unm.edu, (2) Earth and Planetary Sciences, University of New Mexico, MSC03 2040, Albuquerque, NM 87131, (3) Institute of Meteoritics, University of New Mexico, MSC03-2050, Albuquerque, NM 87131

Rhodes (1975) cited evidence that the ≤5 km siliceous Rooiberg Group of the ~2.6 Ga Bushveld Complex, widely accepted as an epicrustal volcanic succession, originated as an impact melt. This proposal was widely rejected because French (1990) found no evidence for shock. This study revisits the impact hypothesis with focus on Zr-bearing phases in the basal ~130 m of the Rooiberg Group (for setting, see Elston and Tegtmeier, this session). The rocks contain ~75 wt % SiO2 (in rhyolite range) and 200-300 ppm Zr (Schweitzer, unpubl. Pretoria PhD diss. 1998), but lack optically resolvable and extractable zircon. Characteristics which suggest quenching from abnormally high temperature include unusual quench textures and an absence of phenocrysts.

WDS Zr X-ray maps of samples from the basal Rooiberg, obtained by EM, located numerous <10 µm, unzoned, unshocked, and subhedral-anhedral zircon grains. Of 50 grains examined in ~2 cm2 of sample, 8 were irregularly shaped, 3-8 µm long, and composed of intergrown zircon and baddeleyite (ZrO2). A FEGSEM and FIB-TEM investigation of one of these composite grains has revealed several distinct subgrains of ZrO2 with the same crystallographic orientation, ± 2-3°. Dark field STEM imaging shows that the ZrO2 subgrains are irregularly-shaped, and are either fully or partially enclosed by metamict zircon, suggesting that zircon partially replaced the ZrO2. EM analyses found > 2,000 ppm U in zircon, accounting for its metamict state in a > 2 Ga rock (Ewing et al. 2000).

Zircon-ZrO2 associations have been cited as evidence for “impact induced high temperatures” (French and Koeberl, 2010). In all previous reports of this association in impactites, zircon is the primary phase, inherited from the target rock and dissociated to ZrO2 and SiO2 at ~1680°C. These preliminary results suggest that ZrO2 is the primary phase in basal Rooiberg. Zircon may have replaced ZrO2 by reaction, T ≤1680°C, or by later alteration. If confirmed, this would be the first example of primary ZrO2 crystallized from a SiO2-oversaturated melt at T >1680°C. All of our conclusions are preliminary. Ongoing work will examine additional grains, determine ZrO2 crystal structure, and test alternative interpretations. For example, there is a possibility that the ZrO2 was inherited, but, unlike zircon, ZrO2 is a rare accessory phase.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting


Session No. 117--Booth# 332
Impact Cratering on the Earth, Moon, and Planets: Remote, Field, and Lab Studies (Posters)
Minneapolis Convention Center: Hall C
9:00 AM-6:00 PM, Monday, 10 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 305

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