[SCG54-P13] Major and trace element compositions of peridotites from the Maqsad diapir area: implication for the melting and melt-mantle reaction at mid-ocean ridge
キーワード：オフィオライト、マントル、ダイアピル、中央海嶺軸、部分溶融、マントルーメルト反応
We investigated the relationship between the mantle diapir structure and the spatial distribution of partial melting and the mantle-melt reaction using the harzburgites from the mantle section from the Maqsad area of Samail massif in the southern Oman ophiolite.
From the planar and linear structures of harzburgites with relation to the analysis of flow direction and shear sense the upward flow in the central part of the diapir and the horizontal flow in the peripheral part were reproduced almost as same as those reported by Jousselin et al. (1998). As a result, the center of mantle diapir locates the vicinity of Maqsad and Mahram and in the periphery a flow along the paleo ridge axis extending in the NW-SE direction and a horizontal flow normal to the paleo ridge axis.
On the central part of the mantle diapir and on the paleo ridge axis, the spinel Cr# is as high as 0.55-0.58, and the relatively depleted peridotites are distributed compared to the peripheral part. On the other hand, in the peripheral part the spinel Cr# is as low as 0.41-0.55 indicating the distribution of less-depleted peridotites. Also, the REE abundance of clinopyroxene in the central part of the diapir is low whereas high in the peripheral part. Since the degree of melting and temperature are higher in the central part of the diapir relative to the peripheral part, the amount of melt impregnation was presumed to be small, and as a result, the residual peridotites were depleted in highly incompatible elements. On the other hand, it is thought that the peripheral part consisted of residual peridotites which moves in the horizontal direction after passing through the corner flow and gradually decreases in temperature resulted in the impregnation of melt due to percolation along grain boundaries. It caused crystallization of a small amount of clinopyroxene and spinel in equilibrium with MORB melt along grain boundaries of harzburgites.
From the planar and linear structures of harzburgites with relation to the analysis of flow direction and shear sense the upward flow in the central part of the diapir and the horizontal flow in the peripheral part were reproduced almost as same as those reported by Jousselin et al. (1998). As a result, the center of mantle diapir locates the vicinity of Maqsad and Mahram and in the periphery a flow along the paleo ridge axis extending in the NW-SE direction and a horizontal flow normal to the paleo ridge axis.
On the central part of the mantle diapir and on the paleo ridge axis, the spinel Cr# is as high as 0.55-0.58, and the relatively depleted peridotites are distributed compared to the peripheral part. On the other hand, in the peripheral part the spinel Cr# is as low as 0.41-0.55 indicating the distribution of less-depleted peridotites. Also, the REE abundance of clinopyroxene in the central part of the diapir is low whereas high in the peripheral part. Since the degree of melting and temperature are higher in the central part of the diapir relative to the peripheral part, the amount of melt impregnation was presumed to be small, and as a result, the residual peridotites were depleted in highly incompatible elements. On the other hand, it is thought that the peripheral part consisted of residual peridotites which moves in the horizontal direction after passing through the corner flow and gradually decreases in temperature resulted in the impregnation of melt due to percolation along grain boundaries. It caused crystallization of a small amount of clinopyroxene and spinel in equilibrium with MORB melt along grain boundaries of harzburgites.