不完美岩浆过程中的微量元素效应

TRACE ELEMENT GEOCHEMICAL EFFECTS OF IMPERFECT MAGMATIC PROCESSES

本文综述了国外近年来在不完美岩浆过程中微量元素效应研究方面的进展．在完美平衡过程（平衡部分熔融或者平衡部分结晶）中和在完美分离过程（分离熔融或者分离结晶）中，熔融或者结晶对于高度不相容元素和高度相容元素相对浓度变化的差异悬殊的影响效应是众所周知的．完美分离过程需要满足以下3个条件：（1）每一批次被分离的熔体或者晶体的量趋向于无穷小；（2）晶体与熔体的彻底分离；（3）体系相对于源区物质和残留物质是完全封闭的．本文探讨了偏离上述3个条件的后果．上述偏离将大大缩减完美分离过程所产生的微量元素浓度和完美平衡条件下微量元素浓度之间的差异．RXCT模型适用于分异良好、尚未受温染的熔岩；RXTC模型则适用于混杂岩浆喷发的情形，这种情形较RXCT模型所描述者更为常见．RXTC模型可以产生既高度富集不相容元素又同时兼具相容元素中等程度亏损的熔体．

Progresses in the research or trace element geochemical effocts in imperfect magmaticprocesses are reviewed and synthesized in this paper. The strongly contrasted effocts ofmelting or crystallization under perfect equilibrium conditions, under perfect fractionalpartial melting, and under perfcct fractional crystallization upon the relative concentra-hons of the highly incompatible and highly compatible elements are well known and aresurveyed at the start. For the perfect fractional processes to be operative three conditionsmust be met, inc1udirrgf (i) infinitesimal size of the batches of liquid or crystals removed,(ii) perfect separation of the crystal and liquid phases, and (iii) perfect closure of the sys-tem to fluxes of the source material and the residue. The consequences of departures fromthe above three conditions are explored. These departures can very substantially reduce thediffercnces between the trace element concentrations generated in the products and thoseexpected in a perfect equilibrium process. The RXCT model fits the situation where thevolcanic eruption taps the more differentiated magma before the mixing stage. An alterna-hve ideal model, i.e. the RXTC model, is applicable to the more common process of hy-brid magma eruption. In mid-ocean ridge basalts, the RXTC model may explain whylarge variations in incompatible elements coexist with moderate depletion of compatible el-ements.