N-MORB和E-MORB数据挖掘——玄武岩判别图及洋中脊源区地幔性质的讨论

摘要

MORB分为N-MORB和E-MORB,二者的区分通常是以LREE亏损和富集为标志.玄武岩的基本理论主要是在MORB研究成果的基础上建立起来的.通常认为,N-MORB和OIB是两个独立的端元,E-MORB是N-MORB与OIB不同程度混合形成的.在20世纪70 ～ 80年代,玄武岩构造环境判别图的建立极大促进了玄武岩在地球动力学及大地构造背景研究的进展,其方法应用持续盛行了几十年.然而,国外学者现在已逐渐淡化玄武岩判别图解的应用,但在中国仍有方兴未艾之势.本文利用PetDB数据库5万多个MORB数据的投图表明,全体样品投图的效果显然比抽样和典型样品得出的结果更扎实可靠,许多原先被广泛应用的主量、微量元素判别图无论从实践上还是理论上均存在一些问题.本文的研究结果进一步表明,MORB样品既有LREE亏损的,又有富集的,但亏损和富集程度与早先定义的差异较大,揭示了洋脊下地幔不均一性要比早先认识的更为复杂,成分范围变化更大,仅少数是强烈亏损的,部分可能是相当富集,甚至可以与OIB源区相类比;玄武岩构造环境判别图需要重新审视,基于岩浆成因机理的多维度、高置信度的判别体系有待于建立.%MORB can be divided into N-MORB and E-MORB,N-MORB is LREE depleted and E-MORB is enriched.Studies on MORB have greatly boosted the fundamental theories of basalts.N-MORB and OIB are generally believed to be two separate end members,and E-MORB is mixed by N-MORB and OIB in varying degrees.The diagrams for discriminating tectonic settings of basahs were established during the 1970s and 1980s,which are tremendous progress for analyzing earth dynamics and tectonic background of basalts and has thrived for decades especially in China.We have compiled more than 50000 MORB data in PetDB database for tectonic setting discrimination diagrams.The plotting diagrams applying whole samples exhibit more accurate and reliable results than that of using representative samples.Therefore,previous discrimination diagrams of major and trace elements require reexamination.Our study indicates that MORB can be either LREE depleted or enriched,and the degree of depletion or enrichment is in large difference from previous definition,which reflect that the mantle beneath ocean ridge is more heterogeneous in composition than previous understanding.In general,our study provides new,different insights to basalts,and we propose multi-dimension,high-confidence level discrimination system to be established for more research on the origin of magmas.