Chemistry of plagioclase of ODP Hole 106-648B

ODP Legs 106 and 109 drilled zero-age crust at Site 648 on the axis of the Mid-Atlantic Ridge and recovered sparsely to moderately phyric but chemically homogeneous basalt. Plagioclase phenocrysts in Site 648 basalts exhibit a variety of zoning patterns and compositions including: (1) tabular (An84) cores mantled by more sodic zones (An80 and An75), (2) tabular (An80) cores mantled by narrow (An84 and An75) zones, (3) skeletal cores (An80-78) enclosing sodic (An75) patches, and (4) rounded cores (An75) surrounded by skeletal (An80-78) mantles. Backscattered electron images have been used to define zone morphologies in 28 crystals; major element compositions of the zones have been determined by electron microprobe; and Mg, K, Ti, Fe, and Sr concentrations have been determined by ion microprobe. Equilibrium partitioning of minor and trace elements between crystals and liquids occurred during stable growth of tabular cores and inner rims. Disequilibrium partitioning occurred during rapid growth of crystal rims, resulting in enrichment of incompatible trace and minor elements, and during growth of calcic zones with skeletal and acicular morphology, resulting in enrichment of Fe and Sr and depletion of Mg, K, and Ti. Crystal/liquid partition coefficients estimated from the composition of inner rims and glass are DMg = 0.03, DK = 0.15, DTi = 0.028, DFe = 0.05, and DSr = 1.82. Using these partition coefficients, we have calculated the composition of primitive liquid in equilibrium with phenocryst cores. The estimated liquid composition is similar to primitive basalt compositions from the Kane Fracture Zone except for a much higher Ca/(Ca + Na) ratio which resembles values for MAR plume basalts. The diversity in plagioclase morphologies and compositions is explained by mixing of melts at different stages of evolution and stored in separated but interconnected reservoirs, as previously suggested by Kuo and Kirkpatrick (1982, doi:10.1007/BF00376957).

大洋钻探计划（Ocean Drilling Program, ODP）第106和109航次在大西洋中脊（Mid-Atlantic Ridge）轴部的648站位钻取了零年龄地壳（zero-age crust），并回收得到稀疏至中等斑晶发育、但化学组分均一的玄武岩（basalt）。648站位玄武岩中的斜长石斑晶（plagioclase phenocrysts）展现出多样的环带结构与组分特征，具体包括：(1) 板状核心（钙长石分子占比An84）被更富钠的环带（An80与An75）包裹；(2) 板状核心（An80）被窄幅环带（An84与An75）包裹；(3) 骸晶状核心（An80-78）内含富钠（An75）斑块；(4) 浑圆状核心（An75）被骸晶状（An80-78）边部包裹。研究人员借助背散射电子图像（Backscattered Electron Images, BSE）明确了28颗晶体的环带形貌；通过电子探针（electron microprobe）测定了各环带的主量元素组成；利用离子探针（ion microprobe）分析了Mg、K、Ti、Fe及Sr的元素浓度。在板状核心与内边缘的稳定生长过程中，晶体与熔体间发生了微量与痕量元素的平衡分配（equilibrium partitioning）；而在晶边快速生长阶段则出现非平衡分配（disequilibrium partitioning），导致不相容痕量与微量元素富集；在骸晶状、针状形貌的钙长石环带生长过程中，同样发生非平衡分配，致使Fe、Sr富集，Mg、K、Ti亏损。基于内边缘与玻璃质的组分估算得到的晶体-熔体分配系数（partition coefficients）分别为：D_Mg=0.03，D_K=0.15，D_Ti=0.028，D_Fe=0.05，D_Sr=1.82。利用上述分配系数，研究人员计算出与斑晶核心平衡的原始熔体（primitive liquid）组分；估算得到的熔体组分与凯恩断裂带（Kane Fracture Zone）的原始玄武岩组分近似，仅Ca/(Ca+Na)比值显著更高，这一特征与大西洋中脊柱玄武岩（Mid-Atlantic Ridge plume basalts）的数值特征一致。斜长石形貌与组分的多样性可通过不同演化阶段的熔体混合得到解释，且熔体存储于分隔但相互连通的岩浆储库（reservoirs）中，这与Kuo与Kirkpatrick（1982, doi:10.1007/BF00376957）此前提出的假说相符。