抄録
Sapphirine is one of the major minerals ubiquitously found in ultrahigh-temperature (UHT) metamorphic rocks. Mineral assemblage of sapphirine + quartz was found in metamorphic rocks from Napier Complex, East Antarctica [1], and this paragenesis of the two minerals implies a possibility of extreme high-grade metamorphic conditions as follows: >1030°C at 9.5 kbar [2] or: >1050°C at 11 kbar [3]. Sapphirine is recognized as an important phase, which suggests UHT condition; however, nucleation and growths processes have not been well understood.
Our research group with Dr. T. Miyamoto and Prof. T. Kawasaki experimentally demonstrated sapphirine formation in the system of UHT granulite (sample no. TM981229-03E; [4]) from the Napier Complex. The granulite sample is almost H2O-free; and it contains mineral assemblages of "sapphirine corona including spinel grains" and "sapphirine + quartz". This experiment revealed that sapphirine formed at around 1100°C and 9 kbar under decreasing temperature condition through series of reactions: (1) spinel + melt = orthopyroxene + sapphirine; (2) spinel + melt = fluorphlogopite + sapphirine; and: (3) orthopyroxene + spinel + melt = sapphirine [5 and 6]. The molten phases showed granitic composition. The results mean that the sapphirine is a reaction product, which is crystallized under retrograde metamorphic condition. Moreover, it is also implied that molten phase, which is partially generated from UHT metamorphic rocks, is not completely segregated from its bulk; and the residual melt is essential for the sapphirine formation in the granulite system.
Grew et al. [7] pointed out that the distribution of trace elements in sapphirine could be used as a significant indicator for investigations of the partial melting. In order to understand that, we are conducting determination of minor elements of the sapphirine and other phase's in the granulite using an ICP-MS coupled with a laser ablation system at Tokyo Institute of Technology. Thus, it is expected that our analytical results should also give new information of that. In the presentation, we'll discuss genetic relationship between molten phase and sapphirine in the UHT granulite system, based on major and trace elements distributions in the sapphirine and other phases.
References: [1] Dallwitz, Nature, 219 (1968) 476-477; [2] Hensen and Green, Contrib. Mineral. Petrol., 38 (1973) 151-166; [3] Bertrand et al., Contrib. Mineral. Petrol., 108 (1991) 55-71; [4] Miyamoto et al., Polar Geosci., 17 (2004) 88-111; [5] Sato et al., J. Mineral. Petrol. Sci., 99 (2004) 191-201; [6] Sato et al., Abst. 24th Symp. Antarct. Geosci. NIPR (2004) pp. 87 (in Japanese); [7] Grew et al., Abst. 24th Symp. Antarct. Geosci. NIPR (2004) 27-28.
