SILICON ISOTOPE RECORD OF EARLY SILICIFIED PALEOARCHEAN (~3.3 GA) KOMATIITES FROM THE ONVERWACHT GROUP, BARBERTON GREENSTONE BELT, SOUTH AFRICA

Mafic and ultramafic volcanic rocks interbedded with thin (dominantly cherty) sedimentary units form an important component of the upper part of the Onverwacht Group, Barberton Greenstone Belt, South Africa. These sequences may be extensively altered or silicified. Previous studies indicated that silicification resulted from initial extrusive activity or from syn-formational low-temperature fluid interaction. Below the contact with the overlying Fig Tree Group, silicified komatiites have present compositions that range from ~80 to >99 wt % SiO₂. Despite pervasive replacement by quartz, delicate spinifex textures are well preserved. Here we report O and Si isotope compositions from quartz crystals that have replaced original phases within a single well-defined spinifex bladed-crystal morphology. The original spinifex crystal was ~4 mm long by 1 mm wide. The exterior margin is now fringed by a 200 µm thickness of multiple, µm-scale, thin, bladed quartz crystals. Three separate quartz "bridges" cut across the mid line of the spinifex crystal. All are composed of micro-quartz, two finer-grained bands flank a central medium-grained one. This results in the formation of two triangular-shaped compartments bordered by the bridges and outer fringe and that are filled with increasingly larger quartz crystals. Data from multiple transects across the spinifex "ghost" crystal were collected utilizing the high spatial resolution of the Cameca 1280 SIMS at the Swedish Museum of Natural History. Regardless of position or crystal size, oxygen isotope values show a narrow range δ¹⁸O_VSMOW = 12.5 ± 0.2 to 14.7 ± 0.2 ‰), which corresponds to calculated temperatures of 131-191 °C. In contrast, silicon isotope values span a relatively larger range (δ³⁰Si = -6.5 ± 0.2 to 0.9 ± 0.2 ‰), with values systematically linked to spatial position. Based on experimental data, silicon isotope heterogeneity can be linked to varying precipitation rate changes, while oxygen isotope values reflect either mixed hydrothermal-seawater fluid temperatures acquired during formation, or diagenetic temperatures reset during burial. Decoupled O and Si isotope values in these silicified komatiites associated with well-preserved chert questions their use as a paired paleo-thermometer.