Low-pressure melting relations of a basalt from ODP Hole 127-797C

One-atmosphere melting experiments, controlled to approximately the fayalite-magnetite-quartz oxygen buffer, performed on a basalt from Hole 797C crystallized olivine and plagioclase nearly simultaneously from about 1235°C and augite from about 1175°C. The liquid compositions indicate systematic trends of increasing FeO and TiO2 and decreasing Al2O3 with decreasing MgO. Experimental olivine compositions vary from Fo90 to Fo78, plagioclase from An79 to An67, and augite from En49 to En46. The KD value for the Fe2+ and Mg distribution between olivine and liquid is 0.31. The KD value for the distribution of Fetotal and Mg between augite and liquid averages 0.24. These KD values suggest experimental equilibrium. The KD values for Na and Ca distribution between plagioclase and liquid range between 0.55 and 0.99 and are dependent on crystallization temperature. Projected on pseudoternary basaltic phase diagrams, the liquid line of descent moves toward increasing quartz normative compositions, revealing a typical tholeiitic crystallization trend with marked Fe and Ti enrichments. Such enrichments are a reflection of the dominance of plagioclase in the crystallizing assemblage. The experimental results can explain the marked Fe- and Ti-enrichment trends observed for the sills of the lower part of Hole 797C, but have no direct bearing on the origin of the relatively evolved high-Al basalts of Hole 794C.

本数据集开展了常压熔融实验（One-atmosphere melting experiments），实验受控于近似铁橄榄石-磁铁矿-石英氧缓冲剂（fayalite-magnetite-quartz oxygen buffer）的条件，实验样品取自797C钻孔的玄武岩。该玄武岩中，橄榄石与斜长石约在1235℃时近乎同时结晶，普通辉石则约于1175℃结晶。熔体成分随MgO含量降低呈现出FeO与TiO₂含量升高、Al₂O₃含量降低的系统性变化趋势。实验得到的橄榄石成分范围为Fo₉₀至Fo₇₈，斜长石为An₇₉至An₆₇，普通辉石为En₄₉至En₄₆。橄榄石与熔体间Fe²+和Mg的分配系数（Distribution Coefficient, KD）为0.31；普通辉石与熔体间全铁（Fetotal）和Mg的分配系数平均值为0.24，上述结果证明实验达到了平衡状态。斜长石与熔体间Na和Ca的分配系数范围为0.55至0.99，且其数值随结晶温度变化而改变。将实验结果投影至伪三元玄武岩相图（pseudoternary basaltic phase diagrams）中可见，液相演化线（liquid line of descent）向标准石英组分升高的方向移动，呈现出典型的拉斑玄武岩结晶趋势，伴随显著的Fe、Ti富集现象。此类富集现象反映了结晶矿物集合体中斜长石占据主导地位。本实验结果可解释797C钻孔下部岩床所观测到的显著Fe、Ti富集趋势，但与794C钻孔中相对演化的高铝玄武岩成因无直接关联。