[SMP44-P13] Grs50Prp50 garnet-bearing composite inclusion in Cr-rich pyrope from Garnet Ridge, the Colorado Plateau
キーワード：Grossular-pyrope garnet, Garnet Ridge, Kimberlite, Laser Raman spectroscopy, Colorado Plateau, Cr-rich pyrope
Garnet having near Grs50Prp50 composition is very rare in nature because of the large difference in ionic radii between Ca2+ and Mg2+. So far, only two occurrences have been reported from Garnet ridge, Arizona (Wang et al., 2000) and the Kokchetav UHP Massif, Kazakhstan (e.g., Ogasawara et al., 2000; Sobolev et al., 2001). At Garnet Ridge, Wang et al. (2000) described four grains of Grs50Prp50 garnet as a constituent of composite inclusions in pyrope-rich garnet in kimberlitic diatremes. In the Kokchetav UHP Massif, Grs50Prp50 garnet is a major constituent mineral of UHP dolomite marble, and contains abundant microdiamonds. Takebayashi et al. (2017) has stated that CaO8 and MgO8 clustering around a SiO4 tetrahedron stabilized ca. Grs50Prp50 compositions on the basis of the overlapping of R(SiO4)4- Raman bands corresponding to Grs (372 cm-1) and Prp (364 cm-1), and considered that two main factors controlled the formation of this strange garnet; (1) the bulk chemistry of the host rock (Ca:Mg = 1:1) and (2) UHP conditions.
Recently, we discovered one grain of Grs50Prp50 garnet from the Garnet Ridge; the garnet occurs as a constituent of composite inclusion in the host Cr-rich pyrope (Group A by Sakamaki et al., 2016), which is of garnet lherzolite origin. Cr-rich pyrope (Group A) is an original material for Cr-poor pyrope (Group B) during mantle metasomatism. The found composite inclusion, which shows spherical form measuring 150 mm across, consists of pargasite and dolomite with minor Cr-spinel, phlogopite and apatite. The other composite inclusions consist of pargasite, dolomite, Cr-spinel with minor apatite and magnesite. We conducted laser Raman spectrometry on this Grs50Prp50 garnet, and focused on the band attributed to R(SiO4)4- at 365 cm-1. The overlapping of R(SiO4)4- bands corresponding to Grs and Prp in a single Grs50Prp50 crystal was observed. Our results of Raman spectrometry were consistent with those of the Kokchetav Grs50Prp50 garnet by Takebayashi et al. (2017).
Almost all composite inclusions contain dolomite/magnesite and show rounded or spherical form. This suggests that these composite inclusions was trapped carbonate-silicate melt during the mantle metasomatism. The Grs50Prp50 garnet in the found composite inclusion was formed from such trapped melt which had the bulk chemistry, near Ca:Mg = 1:1, at very high pressure.
The Grs50Prp50 garnet described by Wang et al. (2000) could have formed by the same process from trapped carbonate-silicate melt, and the inclusion Grs50Prp50 garnet was not in equilibrium with the host pyrope-rich garnet. Their interpretation about the genesis of Grs50Prp50 garnet including very low formation temperature based on the coexistence with the host may be wrong.
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References
Ogasawara, Y., Ohta, M., Fukasawa, K., Katayama, I., Maruyama, S., 2000, Island Arc, 9, 400-416.
Ogasawara, Y., Sakamaki, K., Takebayashi, T., Suzuki, H., Saito, T., 2016, AGU Fall Meeting, SR33A-2673.
Sakamaki, K., Sato, Y., Ogasawara, Y., 2016, Progress in Earth and Planetary Science, 3, 1-17.
Sobolev, N.V., Schertl, H.-P., Burchard, M., Shatsky, V.S., 2001, Doklady Earth Science, 380, 791-794.
Takebayashi, T., Saito, T., Suzuki, H., Sakamaki, K., Ogasawara, Y., 2017. JpGU Meeting Abstract.
Wang, L., Essene, E.J., Zhang, Y., 2000, American Mineralogist, 85, 41-46.
Recently, we discovered one grain of Grs50Prp50 garnet from the Garnet Ridge; the garnet occurs as a constituent of composite inclusion in the host Cr-rich pyrope (Group A by Sakamaki et al., 2016), which is of garnet lherzolite origin. Cr-rich pyrope (Group A) is an original material for Cr-poor pyrope (Group B) during mantle metasomatism. The found composite inclusion, which shows spherical form measuring 150 mm across, consists of pargasite and dolomite with minor Cr-spinel, phlogopite and apatite. The other composite inclusions consist of pargasite, dolomite, Cr-spinel with minor apatite and magnesite. We conducted laser Raman spectrometry on this Grs50Prp50 garnet, and focused on the band attributed to R(SiO4)4- at 365 cm-1. The overlapping of R(SiO4)4- bands corresponding to Grs and Prp in a single Grs50Prp50 crystal was observed. Our results of Raman spectrometry were consistent with those of the Kokchetav Grs50Prp50 garnet by Takebayashi et al. (2017).
Almost all composite inclusions contain dolomite/magnesite and show rounded or spherical form. This suggests that these composite inclusions was trapped carbonate-silicate melt during the mantle metasomatism. The Grs50Prp50 garnet in the found composite inclusion was formed from such trapped melt which had the bulk chemistry, near Ca:Mg = 1:1, at very high pressure.
The Grs50Prp50 garnet described by Wang et al. (2000) could have formed by the same process from trapped carbonate-silicate melt, and the inclusion Grs50Prp50 garnet was not in equilibrium with the host pyrope-rich garnet. Their interpretation about the genesis of Grs50Prp50 garnet including very low formation temperature based on the coexistence with the host may be wrong.
.
References
Ogasawara, Y., Ohta, M., Fukasawa, K., Katayama, I., Maruyama, S., 2000, Island Arc, 9, 400-416.
Ogasawara, Y., Sakamaki, K., Takebayashi, T., Suzuki, H., Saito, T., 2016, AGU Fall Meeting, SR33A-2673.
Sakamaki, K., Sato, Y., Ogasawara, Y., 2016, Progress in Earth and Planetary Science, 3, 1-17.
Sobolev, N.V., Schertl, H.-P., Burchard, M., Shatsky, V.S., 2001, Doklady Earth Science, 380, 791-794.
Takebayashi, T., Saito, T., Suzuki, H., Sakamaki, K., Ogasawara, Y., 2017. JpGU Meeting Abstract.
Wang, L., Essene, E.J., Zhang, Y., 2000, American Mineralogist, 85, 41-46.