Diamond crystallization in the Fe-S-C system

Abstract

729 Sulfides are among of the most abundant inclu sions in natural diamonds and diamondiferous xeno liths of eclogite and peridotite [1–4]. They are found in microdiamonds from ultrahigh pressure metamor phic complexes as well [5]. The wide abundance of sulfides in diamonds has raised the issue on their participation in diamond gen esis. Some authors suggested that natural diamonds were formed from melts of sulfide composition in the Earth’s mantle [4, 6]. There is an assumption about the selective capture of the sulfide melt by growing diamond crystals as well [1]. Experimental studies of the sulfide melt as a prob able diamond forming medium have covered a wide compositional range of sulfur bearing systems from pure sulfur (the S–C system) [7] to the sulfide–C sys tems [8, 9]. It was established that graphite crystallized in sulfide melts at 5–6 GPa and 1200–1600°C, whereas synthesis and growth of diamond crystals required higher temperatures and pressures. On the other hand, pressures of 5–6 GPa and temperatures of ~900–1400°C are the geologically reasonable range of diamond formation [10]. However the pre eutectic area of the Fe–S system was not studied in experiments on diamond synthesis and growth. In our previous studies [11, 12], diamond crystals were obtained in the systems Fe–Co–S–C and Fe–Ni–S–C at 5.5 GPa and 1300°C in melts with S concentration of <14 wt %. In this paper we report the results of investigation of diamond growth in the Fe–S–C system with S concentrations in a melt of 5 wt %. Experiments were carried out on a multi anvil high pressure apparatus “split sphere” (BARS) in the Fe–S–C system at 5.5 ± 0.2 GPa and 1350 ± 25°C for 21–25 h. Pressure was controlled by the calibration curve plotted at room temperature against the phase transitions in PbSe and Bi. The temperature was esti mated by the calibration curve reflecting the depen dence of the current power at the heater on the behav ior of the PtRh 30/6 thermocouple. The construction of the apparatus, experimental techniques, and mea surement of P–T parameters are described in detail in [13].