Partial melting process in the upper mantle deduced from morphological and chemical variations of the chromian spinel two-pyroxene symplectite due to progressive partial melting of the Horoman peridotite, Hokkaido, Japan

Abstract

The symplectite is characteristic of the Horoman peridotite of Hidaka belt, Hokkaido, northern Japan. The symplectite is composed of vermicular chromian spinel, orthopyroxene and clinopyroxene, and is surrounded by a fine-grained aggregate, which is composed of coarser and equant grains of the same minerals as in the symplectite. Textural and chemical characteristics of the symplectite systematically change depending on the degree of refractoriness of the peridotite. With an increase of refractoriness of the peridotite, the symplectite minerals become coarser and more refractory. The Cr# (=Cr/ (Cr+Al) atomic ratio) of a discrete spinel is distinctly higher and increases more rapidly with an increase of the Fo content of olivine than the Cr#'s of spinels in the symplectite and fine-grained aggregate.
     The symplectite was formed by a breakdown of garnet+olivine, combined with partial melting and melt extraction. The differences of spinel composition depending both on the texture and on the degree of refractoriness of rock may indicate a process of inhomogeneous melting and melt extraction. The coarse-grained olivine-rich part of peridotites, which has simple grain boundaries and is relatively easy to be wetted by melt, was more effectively melted, while the symplectite and surrounding fine-grained aggregate, which have complicated grain boundaries and are enriched with pyroxenes, were less easily wetted by melt, and were less effectively melted. The relationship between the symplectite and the surrounding fine-grained aggregate may be inherited from the garnet-kelyphite rim relationship. Preservation of a delicate texture of the symplectite may mean that the heating duration was short on melting.