SIGNIFICANCE OF GRS50PRP50 GARNET IN PYROPE-RICH GARNET FROM THE GARNET RIDGE, COLORADO PLATEAU

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.