13:45 〜 15:15
[MIS13-P07] Apatite geochemistry: a case study from Miocene granitoids in SW Japan
★Invited Papers
キーワード：燐灰石、中新世花崗岩、微量元素
Apatite occurs widely as an accessory mineral in various igneous rocks. The structure of apatite allows for numerous chemical substitutions, including metal cations and anions. As such, apatite can record the geochemical compositions of the melt at the time of its crystallization. Apatite is an early crystalizing and long-lasting phase during the evolution of a range of silicate melts. In addition, because apatite has relatively high resistance against alteration processes, it can serve as an indicator widely occurring in the range of host igneous rocks. In order to track whole-rock composition and evolution in granitic rocks, some previous studies investigated apatite compositions (e.g., Sha and Chappell, 1999; Chu et al., 2009; Miles et al., 2014). However, little studies have focused on M-type granite that is formed from the most oxidized magma.
Miocene granitoids are widely distributed throughout southwestern Japan. We collected samples from M-type Tanzawa tonalitic plutons, M-/I-types Kofu granitic complexes, I-/S-types Ohmine granitoids, and granitoids exposed in Kagoshima prefecture . We separated apatite from the specimens, and chemical compositions (F, Cl, Mn, Fe) of the grains were determined using a Jeol JSM-6060LV scanning electron microscope (SEM) with an Oxford Instruments INCA X-act energy-dispersive spectrometer (EDS).
Most hydroxyapatites in M-type granite are characterized by high Cl (> 1 wt%) and low F (<0.18 wt%) abundances. These Cl contents are clearly higher than those of I-type granites, reported in Sha and Chappell (1999). Thus, Cl content might become an indicator of the apatite in M-type granite. Mn content in apatite is often used as an oxybarometer in granitic rocks (Sha and Chappell, 1999; Miles et al., 2014). The higher Mn and Fe abundances in apatites from S-type granites result from the lower oxygen fugacity and higher aluminosity of the S-type magmas, in which abundant divalent Mn and Fe cations can substitute for Ca in apatite more easily. In our data-set, apatites from most S-type granitoids have higher Mn (>0.2 wt%) than those from I-type granitoids. On the other hand, apatites from M-type granitoids have high Mn (>0.2 wt%), whereas apatites from some S-type granitoids are characterized by low Mn (<0.04%). Because whole-rock Mn contents are not so different, any parameter other than oxygen fugacity also dominates Mn content in apatite.
Miocene granitoids are widely distributed throughout southwestern Japan. We collected samples from M-type Tanzawa tonalitic plutons, M-/I-types Kofu granitic complexes, I-/S-types Ohmine granitoids, and granitoids exposed in Kagoshima prefecture . We separated apatite from the specimens, and chemical compositions (F, Cl, Mn, Fe) of the grains were determined using a Jeol JSM-6060LV scanning electron microscope (SEM) with an Oxford Instruments INCA X-act energy-dispersive spectrometer (EDS).
Most hydroxyapatites in M-type granite are characterized by high Cl (> 1 wt%) and low F (<0.18 wt%) abundances. These Cl contents are clearly higher than those of I-type granites, reported in Sha and Chappell (1999). Thus, Cl content might become an indicator of the apatite in M-type granite. Mn content in apatite is often used as an oxybarometer in granitic rocks (Sha and Chappell, 1999; Miles et al., 2014). The higher Mn and Fe abundances in apatites from S-type granites result from the lower oxygen fugacity and higher aluminosity of the S-type magmas, in which abundant divalent Mn and Fe cations can substitute for Ca in apatite more easily. In our data-set, apatites from most S-type granitoids have higher Mn (>0.2 wt%) than those from I-type granitoids. On the other hand, apatites from M-type granitoids have high Mn (>0.2 wt%), whereas apatites from some S-type granitoids are characterized by low Mn (<0.04%). Because whole-rock Mn contents are not so different, any parameter other than oxygen fugacity also dominates Mn content in apatite.