ORIGIN OF PLANAR MICROSTRUCTURES IN DETRITAL MONAZITES FROM HEAVY MINERAL CONCENTRATES FROM THE NAMAKWA SANDS MINE, ATLANTIC COAST OF SOUTH AFRICA

Detrital monazite [(Ce,Th,La)PO₄] and zircon [ZrSiO₄] are highly resistant to erosion and can preserve microstructures formed during shock metamorphism from a meteorite impact after erosion and fluvial transportation [1]. Studies have revealed a high abundance of detrital shocked monazites in the Vaal River of South Africa that originated from the Vredefort Dome, the oldest (2.020 Ga) and largest (~90 km in diameter) impact structure on Earth [2]. To further explore sedimentary environments where shocked monazite might reside, detrital monazite grains from the Namakwa Sands mine, a Cretaceous heavy mineral deposit located on the Atlantic coast of South Africa ~1300 km from Vredefort Dome, were observed using scanning electron microscopy and optical microscopy (transmitted light and reflected light) to search for planar microstructures that may record shock deformation. A total of 64 monazites were observed in a polished thin section of heavy mineral concentrate from which four grains (6%) were found to have planar microstructures. An additional five grains (8%) contained zircon inclusions, a common feature of shocked monazites from Vredefort [1,2]. The ability to resolve planar microstructures, including possible deformation twins, in the Namakwa Sands monazites using a variety of imaging methods shows that these monazites contain similar features to shocked monazites from the Vaal River. However, planar microstructures, including deformation twins, have recently been documented in monazites from granulite-facies rocks in India that are unrelated to impact processes [3]. In either case, the planar microstructures in the Namakwa Sands detrital monazites preserve features that can be used to better understand regional deformation events that affected their host rocks.

Support was provided by NSF (EAR-1145118) and the NASA Astrobiology Program.

[1] Cavosie et al. 2010 GSA Bulletin. [2] Erickson et al. 2013 GCA. [3] Erickson et al. 2015 Geology.