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[SCG48-01] Mineral chemistry and origin of the olivine from the Ogi Picritic Dolerite Sill
Keywords:picrite, Cr-spinel, olivine, reverse zoning, magma mixing
Ogi Basalt occurs at the Ogi Peninsula, southwest Sado Island and is thought to be formed by subaqueous volcanism associated with Japan Sea opening (Fujibayashi and Sakai, 2003). Picritic dolerite sills (hereafter referred to as Ogi Picrites) are intruded into the tuffaceous mudstone of the Ogi basalt. Olivines are accumulated at the lowerpart of Ogi Picrite, its olivine mode is up to 55 vol.%, and it has komatiitic composition(whole-rock MgO content= 27 wt%).
Two hypotheses to explain the origin of reverse-zoned olivine crystals in Ogi Picrite have been proposed. Yokoyama et al. (1992) considered that in a magma chamber with a temperature gradient, Fe-rich olivines which crystallized at the roof of the relatively low-temperature magma chamber have accumulated at the bottom or center of the high-temperature magma chamber, then Mg-rich olivines have overgrown on the Fe-rich olivines. On the other hand, Fujibayashi et al. (2016) pointed out that the olivine compositional trends on the Fo-CaO(wt%) diagram of olivine cores (Fo84.5-90) roughly correspond to those of high-Ca boninites. They considered that this is due to the involvement of high-Ca boninitic magma (or its differentiated magma) and the Ogi Picrite had been formed by the mixing of an olivine basaltic magma and unsolidified olivine cumulates from a boninitic magma. Thus, it is difficult to say that a consensus has been reached on the origin of olivines from the Ogi Picrite.
Consequently, we attempted to re-examinate the origin of olivines from Ogi Picrite by observation and analysis focusing on the crystal size, shape, compositional zonation, and distribution of chromian spinel in olivine. As a result, olivines from Ogi Picrite can be classified into 3 types:
(i) Type-1 olivine (phenocryst): They are medium-grained (1-2 mm in major axis), subhedral crystal with a compositionally-homogeneous core (Fo89-90). They show normal zoning at the rim with sharply decreasing Fo content at the rim. CaO in olivine core is often high (> 0.25 wt%) and frequently includes Cr-spinel.
(ii) Type-2 olivine (megacryst): They are coarse-grained (more than 2 mm in major axis) and anhedral crystals. The Fo content gradually increases from core (Fo83.5-85) to mantle (Fo88-90) but sharply decreases at rim. CaO content in olivine core is low (0.11-0.20 wt%) and rarely includes Cr-spinel. On the contrary, the olivine mantle is rich in CaO (> 0.25 wt%) and frequently includes Cr-spinel.
(iii) Groundmass olivine: Their grain sizes are similar to groundmass plagioclase and clinopyroxene (less than 1 mm in major axis). They occur as subhedral to euhedral crystals and sometimes shows skeletal crystal shapes. They exhibit normal zoning, sharply decreasing from core (Fo85-87) to rim. The olivine cores rich in CaO (0.23-0.27 wt%) and frequently includes Cr-spinel.
The Mg-number of Cr-spinel within the core of the type-2 olivine is about 59, but that within the mantle is slightly higher (60-65). On the other hand, the Mg-number of Cr-spinel within the type-1 olivine and groundmass olivine is concentrated at 60-64, equivalent to that within the high-Ca mantle of the type-2 olivine.
Fresh olivine in chilled marginal basalt within a few meters of the wall rock is euhedral to subhedral, with a homogeneous composition of about Fo89-90, and commonly contains Cr-spinel. This feature is similar to that of Type-1 olivine. Using the olivine-melt Fe-Mg partition coefficient(Roeder and Emslie, 1970), the olivine composition in equilibrium with magma with the whole-rock composition of the chilled marginal basalt of the Ogi Picrite is Fo89-90.
From these results, we conclude the following about the origin of olivine. Type-1 olivine is a phenocryst crystallized from a Cr-rich magma with composition of chilled margin before the emplacement as a sill. The core of the Type-2 olivine megacryst, unlike the rim with comparable Fo contents, rarely contains Cr-spinel and differs in the amount of CaO. The features cannot be explained by the idea that the Type-2 cores have crystallized in low-temperature parts of the same magma reservoir as the mantle and rim. Therefore, we concluded that the low-CaO core of type-2 olivine is originated from another Cr-poor magma, and the type-1 olivine had overgrown on the core as high-Ca mantle. Groundmass olivine crystallized after the sill emplacement. The core of Type-2 olivine megacryst might have mixed with the magma that crystallized type-1 olivine at depth before the intrusion.
Two hypotheses to explain the origin of reverse-zoned olivine crystals in Ogi Picrite have been proposed. Yokoyama et al. (1992) considered that in a magma chamber with a temperature gradient, Fe-rich olivines which crystallized at the roof of the relatively low-temperature magma chamber have accumulated at the bottom or center of the high-temperature magma chamber, then Mg-rich olivines have overgrown on the Fe-rich olivines. On the other hand, Fujibayashi et al. (2016) pointed out that the olivine compositional trends on the Fo-CaO(wt%) diagram of olivine cores (Fo84.5-90) roughly correspond to those of high-Ca boninites. They considered that this is due to the involvement of high-Ca boninitic magma (or its differentiated magma) and the Ogi Picrite had been formed by the mixing of an olivine basaltic magma and unsolidified olivine cumulates from a boninitic magma. Thus, it is difficult to say that a consensus has been reached on the origin of olivines from the Ogi Picrite.
Consequently, we attempted to re-examinate the origin of olivines from Ogi Picrite by observation and analysis focusing on the crystal size, shape, compositional zonation, and distribution of chromian spinel in olivine. As a result, olivines from Ogi Picrite can be classified into 3 types:
(i) Type-1 olivine (phenocryst): They are medium-grained (1-2 mm in major axis), subhedral crystal with a compositionally-homogeneous core (Fo89-90). They show normal zoning at the rim with sharply decreasing Fo content at the rim. CaO in olivine core is often high (> 0.25 wt%) and frequently includes Cr-spinel.
(ii) Type-2 olivine (megacryst): They are coarse-grained (more than 2 mm in major axis) and anhedral crystals. The Fo content gradually increases from core (Fo83.5-85) to mantle (Fo88-90) but sharply decreases at rim. CaO content in olivine core is low (0.11-0.20 wt%) and rarely includes Cr-spinel. On the contrary, the olivine mantle is rich in CaO (> 0.25 wt%) and frequently includes Cr-spinel.
(iii) Groundmass olivine: Their grain sizes are similar to groundmass plagioclase and clinopyroxene (less than 1 mm in major axis). They occur as subhedral to euhedral crystals and sometimes shows skeletal crystal shapes. They exhibit normal zoning, sharply decreasing from core (Fo85-87) to rim. The olivine cores rich in CaO (0.23-0.27 wt%) and frequently includes Cr-spinel.
The Mg-number of Cr-spinel within the core of the type-2 olivine is about 59, but that within the mantle is slightly higher (60-65). On the other hand, the Mg-number of Cr-spinel within the type-1 olivine and groundmass olivine is concentrated at 60-64, equivalent to that within the high-Ca mantle of the type-2 olivine.
Fresh olivine in chilled marginal basalt within a few meters of the wall rock is euhedral to subhedral, with a homogeneous composition of about Fo89-90, and commonly contains Cr-spinel. This feature is similar to that of Type-1 olivine. Using the olivine-melt Fe-Mg partition coefficient(Roeder and Emslie, 1970), the olivine composition in equilibrium with magma with the whole-rock composition of the chilled marginal basalt of the Ogi Picrite is Fo89-90.
From these results, we conclude the following about the origin of olivine. Type-1 olivine is a phenocryst crystallized from a Cr-rich magma with composition of chilled margin before the emplacement as a sill. The core of the Type-2 olivine megacryst, unlike the rim with comparable Fo contents, rarely contains Cr-spinel and differs in the amount of CaO. The features cannot be explained by the idea that the Type-2 cores have crystallized in low-temperature parts of the same magma reservoir as the mantle and rim. Therefore, we concluded that the low-CaO core of type-2 olivine is originated from another Cr-poor magma, and the type-1 olivine had overgrown on the core as high-Ca mantle. Groundmass olivine crystallized after the sill emplacement. The core of Type-2 olivine megacryst might have mixed with the magma that crystallized type-1 olivine at depth before the intrusion.