INSIGHTS INTO IMPACT MELT CONDITIONS FROM DISSOCIATED ZIRCON IN MEDIEVAL TIN SLAG

Zircon in impact melt show a range of microstructures, including inherited grains with shock features [1] and evidence of dissociation [2]. Dissociation results from thermal decomposition [3] and/or chemical disequilibrium between zircon and melt, the former occurring experimentally at 1670 °C at 1 atm. To better understand dissociation, we describe partially dissociated zircon in glassy slag produced during tin smelting from a medieval archeological site in Devon, UK [4,5]. The slag grains were subject to high temperature at 1 atm during smelting, and have microstructures similar to zircon from impact melt. Electron backscatter diffraction mapping identified phases and orientation relationships of zircon and zirconia in a chip of glass from a smelter in Eggworthy [4,5]. Two types of zircon were documented: (1) Small (<20 µm), euhedral, neoblastic zircon with oscillatory zoning that appear to have crystallized in the melt, and (2) inherited grains with evidence of partial dissociation, including conspicuous coronas of zirconia surrounding zircon cores.

Dissociated zircon grains contain 5 to 20 µm-wide coronas of 1-5 µm-size grains of rounded and twinned baddeleyite surrounded by glass. Two types of cores were documented in zircon grains that partially dissociated. Most cores preserve inherited oscillatory zoning, with evidence of localized infiltration of melt along discrete fractures. In one dissociated zircon, primary igneous zoning in the core is absent, and high-strain domains are irregularly interspersed with newly formed low-strain domains that contain cryptic oscillatory zoning. Regions of the modified core recrystallized to a granular texture of interlocking neoblasts. Many features observed in the slag zircon population are observed in zircon from impact melt, such as coronas of twinned baddeleyite surrounding inherited cores [2]. As shown here, both dissociation and granular texture can occur when zircon is immersed in high temperature melt, and are not unique to shock deformation. We are now characterizing the glass to evaluate chemical disequilibrium.

 

Supported by NSF (EAR-1145118), NASA Astrobiology, a Curtin Research Fellowship, and SUNY Geneseo.

 

[1] Gibson et al. 1997 [2] Timms et al. 2016 in review [3] Kaiser et al. 2008 [4] Tylecote et al. 1989 [5] Farthing and Pivarunas 2015