In order to distinguish the primary microstructures developed under mantle conditions from the secondary phenomena after xenolith entrainment in the host magma, this study intends to discuss the genesis of spongy, sie...In order to distinguish the primary microstructures developed under mantle conditions from the secondary phenomena after xenolith entrainment in the host magma, this study intends to discuss the genesis of spongy, sieve-textured, and reaction rims on mineral grains of mantle xenoliths in the Cenozoic basalts from the western North China craton. The spongy rims on primary clinopyroxene show neither obvious compositional zoning nor preferential development towards the host basalt and probably suggest an origin via partial melting within the lithospheric mantle or pressure release as the xenoliths were carried upwards. The sieve-textured rims on primary spinel show clear chemical zoning with increasing Cr# and decreasing AI towards the host basalt. They are interpreted as the result of partial melting due to heating of the host basaltic magma and decreasing pressure during ascent proc- ess. Post-entrainment reaction mainly generated secondary minerals at contacts between the host ba- saltic melt and xenoliths. The secondary clinopyroxene in reaction rims develops on primary clinopy- roxene and has higher Ti, Ca, and Fe contents and lower Mg# and Si contents than primary one, while the secondary spinel on primary Cr-AI spinel is titanomagnetite. The secondary olivine and clinopy- roxene in the reaction rims on primary orthopyroxene are enriched in Fe, Al, and Ti. The occurrence of reaction rims in mantle xenoliths reflects disequilibrium processes after xenolith entrainment in the basaltic melt. The spongy rims on primary clinopyroxene may not be related to the interaction with thehost basaltic melt, while the sieve-textured rims on primary spinel and reactions rims on primary clinopyroxene, spinel, and orthopyroxene may result from post-entrainment reaction between the host basaltic melt and xenolith minerals.展开更多
基金supported by GEMOC and the Department of Earth and Planetary Sciences,Macquarie University,Australia
文摘In order to distinguish the primary microstructures developed under mantle conditions from the secondary phenomena after xenolith entrainment in the host magma, this study intends to discuss the genesis of spongy, sieve-textured, and reaction rims on mineral grains of mantle xenoliths in the Cenozoic basalts from the western North China craton. The spongy rims on primary clinopyroxene show neither obvious compositional zoning nor preferential development towards the host basalt and probably suggest an origin via partial melting within the lithospheric mantle or pressure release as the xenoliths were carried upwards. The sieve-textured rims on primary spinel show clear chemical zoning with increasing Cr# and decreasing AI towards the host basalt. They are interpreted as the result of partial melting due to heating of the host basaltic magma and decreasing pressure during ascent proc- ess. Post-entrainment reaction mainly generated secondary minerals at contacts between the host ba- saltic melt and xenoliths. The secondary clinopyroxene in reaction rims develops on primary clinopy- roxene and has higher Ti, Ca, and Fe contents and lower Mg# and Si contents than primary one, while the secondary spinel on primary Cr-AI spinel is titanomagnetite. The secondary olivine and clinopy- roxene in the reaction rims on primary orthopyroxene are enriched in Fe, Al, and Ti. The occurrence of reaction rims in mantle xenoliths reflects disequilibrium processes after xenolith entrainment in the basaltic melt. The spongy rims on primary clinopyroxene may not be related to the interaction with thehost basaltic melt, while the sieve-textured rims on primary spinel and reactions rims on primary clinopyroxene, spinel, and orthopyroxene may result from post-entrainment reaction between the host basaltic melt and xenolith minerals.
