Major and trace element composition of clinopyroxenes from an olivine gabbro of ODP Hole 176-735B (Table 1)

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick, 2001, doi:10.1016/S0377-0273(01)00211-6) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.本研究采用离子探针（ion probe）与电子探针（electron microprobe）联用的原位（in situ）分析方法，针对取自大洋钻探计划（Ocean Drilling Program, ODP）735B钻孔的大洋辉长岩（oceanic gabbros），对其中单斜辉石（clinopyroxene）颗粒的主量与微量元素特征开展了系统研究。与均一的主量元素组成不同，微量元素（稀土元素（Rare Earth Elements, REE）、Y、Cr、Sr及Zr）呈现出从核部到边部的连续环带分布特征。单斜辉石中观测到的微量元素地球化学特征，无法通过熔体与辉长岩沿颗粒边界的简单扩散交换来解释。尽管瑞利分馏结晶（Rayleigh fractional crystallization）模拟可以复现该环带分布的整体形态，但要形成此类环带，需要熔体成分同时发生演化改造。在晶体堆积过程中，堆晶晶体（cumulate crystal）与孔隙熔体（porous melt）之间发生的同步交代作用，是解释该环带特征最合理的成因机制。晶粥（crystal mush）固结过程中发生的变形作用，可能会将富集不相容元素（incompatible element）的残余熔体（间隙液相（interstitial liquid））挤出。熔体沿颗粒边界的运移可将这些熔体带出体系之外。这一被称为同构造分异作用（synkinematic differentiation）或变形分异作用（differentiation by deformation）的过程（Natland与Dick，2001，doi:10.1016/S0377-0273(01)00211-6），可能是洋壳中重要的岩浆演化机制，至少在慢速扩张脊（slow-spreading ridges）环境中如此。