GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 42-7
Presentation Time: 9:00 AM-5:30 PM


SUZUKI, Hiroshi and OGASAWARA, Yoshihide, Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo, 169-8050, Japan

Garnets having near Grs50Prp50 compositions are 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 the Garnet Ridge, Arizona (Wang et al. 2000: Am. Min.) and the Kokchetav Massif, Kazakhstan (Ogasawara et al. 2000; Sobolev et al. 2001). At the Garnet Ridge, Wang et al. (2000) described four grains of near Grs50Prp50 garnet as a constituent of composite inclusions in pyrope-rich garnet in a kimberlitic diatreme. In the Kokchetav, Grs50Prp50 garnet occurs in UHP dolomite marble and contains abundant microdiamonds. In Raman spectroscopy, Ogasawara et al. (2016: AGU 2016F) found the overlapping of two kinds of R(SiO4)4- bands corresponding to Grs (372 cm-1) and Prp (364 cm-1), and concluded that CaO8 and MgO8 clustering around a SiO4 tetrahedron stabilized near Grs50Prp50 compositions. They considered that two main factors controlled the formation of such strange compositions; (1) bulk chemistry of the host rock (Ca:Mg = 1:1) and (2) UHP conditions. Recently, we discovered one grain of Grs50Prp50 garnet in Prp-rich garnet, “garnet inclusion in garnet”, from the Garnet Ridge; the garnet is a constituent of composite inclusions in Cr-bearing Prp-rich garnet (Group A by Sakamaki et al. 2016: PEPS), which is of garnet lherzolite origin. The inclusion garnet is Prp48Grs38Alm14 and the host is Prp71Grs13Alm16. We conducted laser Raman spectrometry on this inclusion garnet and focused on the band attributed to R(SiO4)4-. Our results were consistent with those of the Kokchetav Grs50Prp50 garnet by Ogasawara et al. (2016). Moreover, the found composite inclusion, with rounded or spherical form, consists of pargasite and dolomite with minor Cr-spinel, phlogopite and apatite. These suggests that such composite inclusions were trapped as a carbonate-silicate melt with a relic of near Grs50Prp50 garnet during the mantle metasomatism. Although Wang et al. (2000) understood that the inclusion garnet was in equilibrium with the host pyrope-rich garnet, we concluded that Grs50Prp50 garnet was not in equilibrium with the host and is a relic of (1) the host rock having bulk chemistry, Ca:Mg = 1:1, formed at (2) very high pressures. This discovery is significant evidence for the chemical heterogeneity of the mantle underneath the Colorado Plateau.