14:15 〜 14:30
[SVC30-03] Inverse crystallization order of phenocrysts in high-Mg basalt from Ryozen Formation, Fukushima prefecture
キーワード：結晶化順序、玄武岩マグマ、霊山
Since the proposal of Bowen’s reaction theory (Bowen, 1928), chemical and mineralogical diversities of basaltic rocks have been attributed to crystallization differentiation. In primitive basalt magmas, olivine is assumed to be the initial crystallization phase followed by crystallization of pyroxene and plagioclase. The orders of crystallization are often determined with microscopic observations for natural basalt samples (e.g., Aoki, 1989). Recently, an inverse crystallization order (pyroxenes - olivine - plagioclase) from basaltic magma was identified by Rafren et al. (2020) from Manguao basalt, Palawan Island, Philippines. The inverse crystallization order was accounted for by decompression crystallization during magma ascent. They explained the crystallization process based on mineral chemistry, mineral textures, and MELTS simulation (Gualda and Ghiorso, 2014). Decompression crystallization models have been applied for some basaltic volcanoes (Kuritani, 1999; Ohba et al., 2009), but is still not a widely accepted model. Ohba et al. (2009) pointed out that chemical variation trends from Hakkoda are not consistent with the fractionation of phenocryst minerals, and explained that cryptic crystallization at MOHO resulted in the chemical trends whereas phenocrysts were formed in the shallow crust during ascent.
The Miocene Ryozen Formation, Fukushima, northern Honshu Japan, is a basaltic volcanic suite composed of high-Mg basalt. Shuto et al. (1985) identified four types of phenocryst mineral assemblages. The least differentiated basalt is one of the basaltic samples containing olivine phenocrysts solely and is one of the highest in MgO content of the samples. However, a pyroxene-bearing rock has a similar while rock chemistry. When whole rock compositions of Ryozen Formation are plotted on variation diagrams, pyroxene-free rocks show a wide MgO range, and pyroxene-bearing rocks overlaps the entire trend of pyroxene-free rocks. An evolved mineral assemblage in the gauge of Bowen’s reaction theory (olivine + clinopyroxene + orthopyroxene + plagioclase) has a similar chemical composition to the rocks with the primitive mineral assemblage (olivine alone). This fact indicates that the variation of observed phenocryst assemblage is not necessarily relevant to the chemical differentiation.
We re-examined whole rock chemistry and mineral chemistry of some basaltic rocks from Ryozen. The analyzed basaltic rock samples contain olivine and clinopyroxene as phenocrysts. Plagioclase is contained in a sample. In these samples, clinopyroxene phenocryst cores have higher Mg# than those of olivine. At the basaltic temperature of 1200 C which is consistent with the plagioclase-melt geothermo- hygrometer for the plagioclase-bearing sample, Mg-Fe partitioning between olivine and clinopyroxene is close to 1. The difference of Mg# between olivine and clinopyroxene cores indicates earlier crystallization of clinopyroxene than olivine. The observation is consistent with MELTS modeling of isothermal decompression, resulting in the early crystallization of clinopyroxene and later crystallization of olivine. The model calculation also resulted in early crystallization of Low-Ca pyroxene (pigeonite or orthopyroxene) which is contained in some other samples but not in the examined samples. Low-Ca pyroxene may be replaced by olivine through the peritectic reaction during the decompression.
The Miocene Ryozen Formation, Fukushima, northern Honshu Japan, is a basaltic volcanic suite composed of high-Mg basalt. Shuto et al. (1985) identified four types of phenocryst mineral assemblages. The least differentiated basalt is one of the basaltic samples containing olivine phenocrysts solely and is one of the highest in MgO content of the samples. However, a pyroxene-bearing rock has a similar while rock chemistry. When whole rock compositions of Ryozen Formation are plotted on variation diagrams, pyroxene-free rocks show a wide MgO range, and pyroxene-bearing rocks overlaps the entire trend of pyroxene-free rocks. An evolved mineral assemblage in the gauge of Bowen’s reaction theory (olivine + clinopyroxene + orthopyroxene + plagioclase) has a similar chemical composition to the rocks with the primitive mineral assemblage (olivine alone). This fact indicates that the variation of observed phenocryst assemblage is not necessarily relevant to the chemical differentiation.
We re-examined whole rock chemistry and mineral chemistry of some basaltic rocks from Ryozen. The analyzed basaltic rock samples contain olivine and clinopyroxene as phenocrysts. Plagioclase is contained in a sample. In these samples, clinopyroxene phenocryst cores have higher Mg# than those of olivine. At the basaltic temperature of 1200 C which is consistent with the plagioclase-melt geothermo- hygrometer for the plagioclase-bearing sample, Mg-Fe partitioning between olivine and clinopyroxene is close to 1. The difference of Mg# between olivine and clinopyroxene cores indicates earlier crystallization of clinopyroxene than olivine. The observation is consistent with MELTS modeling of isothermal decompression, resulting in the early crystallization of clinopyroxene and later crystallization of olivine. The model calculation also resulted in early crystallization of Low-Ca pyroxene (pigeonite or orthopyroxene) which is contained in some other samples but not in the examined samples. Low-Ca pyroxene may be replaced by olivine through the peritectic reaction during the decompression.