Disequilibrium growth of olivine in mafic magmas revealed by phosphorus zoning patterns of olivine from mafic-ultramafic intrusions
Disequilibrium growth of olivine in mafic magmas revealed by phosphorus zoning patterns of olivine from mafic-ultramafic intrusions
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镁铁质-超镁铁质侵入体橄榄石的磷分带模式揭示了镁铁质岩浆中橄榄石的不平衡生长
DOI：
10.1016/j.epsl.2017.09.005
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发表时间：
2017
影响因子：
5.3
通讯作者：
Tan Wei
中科院分区：
文献类型：
--
作者：
Xing Chang-Ming;Wang Christina Yan;Tan Wei
镁铁质-超镁铁质侵入体中的橄榄石很少出现主量元素和微量元素（如Fe、Mg和Ni）的生长环带，这是由于这些元素在高温下快速扩散造成的。因此，橄榄石中的这些元素在解释岩浆房过程，如岩浆对流，多次注入和混合方面没有用处。高分辨率X射线元素强度填图揭示了中国西南地区两个镁铁质-超镁铁质岩体中橄榄石的不同P分带模式。阿不浪当岩体中二辉橄榄岩和纯橄榄岩中的多面体橄榄石颗粒显示出类似于火山岩中所观察到的富磷枝晶。白马层状侵入体网状结构的铁钛氧化物矿石中的圆形橄榄石颗粒具有不规则的富磷斑块/条带，并被贫磷橄榄石域横切和互锁。富磷斑块/条带含有250至612 ppm P，远高于贫磷橄榄石域（123至230 ppm P）。在电子背散射衍射（EBSD）图中，单个橄榄石晶粒内的富P斑块/带具有相同的晶体学取向，表明它们是同一晶体的残余。因此，这两个P丰富的补丁/带和P-穷人橄榄石域在同一个晶粒显示不平衡的纹理，并清楚地记录两个阶段的增长。富磷补丁/带可能是残留的多面体橄榄石晶体，形成在第一阶段，而贫磷橄榄石域圆形富钛磁铁矿和富铁熔体包裹体可能已形成从富铁环境熔体在第二阶段。橄榄石复杂的P分带是由于早期多面体橄榄石的溶解和富铁环境熔体的再沉淀作用。早期的多面体橄榄石与周围的熔体处于化学不平衡状态，这可能是由于晶体糊状物中硅酸盐液体的不可溶解性而形成的。我们的研究表明，橄榄石晶体在火成岩堆积与平衡外观可能经历了缓慢冷却过程中的不平衡生长过程。因此，基于结构关系的镁铁质岩浆结晶序列应慎重对待。
Olivine from mafic–ultramafic intrusions rarely displays growth zoning in major and some minor elements, such as Fe, Mg and Ni, due to fast diffusion of these elements at high temperatures. These elements in olivine are thus not useful in deciphering magma chamber processes, such as magma convection, multiple injection and mixing. High-resolution X-ray elemental intensity mapping reveals distinct P zoning patterns of olivine from two mafic–ultramafic intrusions in SW China. Polyhedral olivine grains from lherzolite and dunite of the Abulangdang intrusion show P-rich dendrites similar to those observed in volcanic rocks. Rounded olivine grains from net-textured Fe–Ti oxide ores of the Baima layered intrusion have irregular P-rich patches/bands crosscut and interlocked by P-poor olivine domains. P-rich patches/bands contain 250 to 612 ppm P, much higher than P-poor olivine domains with 123 to 230 ppm P. In electron backscattered diffraction (EBSD) maps, P-rich patches/bands within a single olivine grain have the same crystallographic orientation, indicating that they were remnants of the same crystal. Thus, both P-rich patches/bands and P-poor olivine domains in the same grain show a disequilibrium texture and clearly record two-stage growth. The P-rich patches/bands are likely the remnants of a polyhedral olivine crystal that formed in the first stage, whereas the P-poor olivine domains containing rounded Ti-rich magnetite and Fe-rich melt inclusions may have formed from an Fe-rich ambient melt in the second stage. The complex P zoning of olivine can be attributed to the dissolution of early polyhedral olivine and re-precipitation from the Fe-rich ambient melt. The early polyhedral olivine was in chemical disequilibrium with the ambient melt that may have been developed by silicate liquid immiscibility in a crystal mush. Our study implies that olivine crystals in igneous cumulates with an equilibrium appearance may have experienced disequilibrium growth processes during slow cooling. Therefore, the crystallization sequence of mafic magmas based on textural relationships should be treated with caution.