Melting of the Fe [sbnd]FeO and the Fe [sbnd]FeS systems at high pressure: Constraints on core temperatures

Abstract

Melting temperatures of FeO, FeS and FeS 2 have been accurately determined in a hydrostatic, inert pressure environment up to about 0.5 Mbar using a new yttrium-lithium-fluoride (YLF) heating laser. The melting curve for FeO is in perfect agreement with data obtained from multi-anvil experiments at 160 kbar [1], but is in stark disagreement with previous laser heating experiments [2]. Preliminary measurements show strong melting depression on mixtures of Fe and FeS, whereas mixtures of Fe and FeO were observed to melt close to the melting curve of iron. The Kraut-Kennedy melting relationship [3], in which the melting temperature is a linear function of volume, is successfully tested in this study for Li, Na, K, Fe, FeS, FeS 2 and FeO, to compressions up to > 30%. At 1.36 Mbar (core-mantle boundary, CMB) FeO, Fe, and FeS are estimated to melt at 3670, 3260 and 3060 (± 100) K respectively. Assuming outer core compositions with about 60% Fe and about 40% FeO or FeS, and a solid solution system, the melting temperature at the CMB, on the core side, would be 3300 K (± 200 K), compared to a temperature at the bottom of the mantle of 2650 ± 100 K. If these systems exhibit eutectic behaviour, the melting gradient through the outer core would have to be substantially higher than the adiabatic gradient in order to maintain a thermal boundary at the CMB. The present melting data, and experimental constraints on the adiabatic gradient in the outer core suggest a temperature at the inner core-outer core boundary of nearly 4200 K.