橄榄石微量元素:分析、地球化学行为与地球科学应用

Trace elements in olivine:Analysis,geochemical behavior,and case studies

橄榄石化学成分记录了幔源岩浆性质、结晶演化、喷发速率等重要信息。近十年来,橄榄石微量元素分析技术飞速进步,积累了丰富的数据。本文系统综述了电子探针和激光剥蚀电感耦合等离子体质谱法在橄榄石微量元素分析领域的最新研究进展,总结了橄榄石中微量元素的分配系数与扩散系数等地球化学行为,分析了全球不同岩石单元的橄榄石微量元素特征,重点介绍了岩浆铜镍矿床中橄榄石微量元素的研究进展。实验岩石学表明,控制元素在橄榄石中分配系数的主要因素有温度、压力、岩浆成分等,且不同微量元素行为的主控因素各不相同。在确定的温压条件下,橄榄石中多数元素的配分可由晶格应变模型预测,而Ni、Co和轻稀土元素主要受晶体场效应和结晶动力学控制。橄榄石中元素的扩散速率由快到慢依次为:H和Li(快)→Fe-Mg-Ni(中等)→Ca(较慢)→P-O-Si(慢)。对比全球不同岩石单元中橄榄石微量元素得出:(1)火山岩系列的科马提岩-苦橄岩-玄武岩中橄榄石的Fo值和Ni、Cr、Al、Mn等元素呈演化趋势,但在二元相关性图中不同岩石类型间存在较大重叠区域,机器学习等多维分析手段可大大提升分类准确率;(2)地幔橄榄岩与侵入岩系列(地壳堆晶岩)中的橄榄石微量元素特征相近,难以使用单元素指标区分,例如常用的Ca亏损;(3)碳酸岩中橄榄石的Ni、Mn含量有别于其他岩石,可能与其来源和元素在橄榄石-碳酸岩熔体中特殊的分配行为有关;(4)石榴子石地幔橄榄岩与尖晶石地幔橄榄岩的橄榄石微量元素存在一定差异,橄榄石V/Sc比值和Fe-Mg-Mn-Ca-Ni含量是判别地幔橄榄岩类型和来源深度的重要变量。岩浆铜镍矿床橄榄石微量元素的研究进展有:(1)堆晶-后堆晶阶段再平衡作用驱使Ca-Cr-Al元素向其他矿物迁移,使橄榄石普遍亏损上述元素;(2)橄榄石Ni-Co变化可记录其与硫化物的元素交换反应,Ni-Co正相关可用于指示岩体深边部的找矿潜力;(3)与硫化物共生橄榄石的Ni富集(>5000×10^(-6))或亏损(<500×10^(-6))可能与较高或较低氧逸度有关。未来橄榄石重点研究与突破方向有:堆晶-后堆晶阶段元素行为、多元素-多矿物扩散联合限定岩浆时空演化、机器学习判别橄榄石成因类型与镁铁-超镁铁岩体含矿性、挥发份与新兴非传统金属稳定同位素等。

The chemical composition of olivine records valuable information on mantle-derived magma properties,crystallization evolution,and eruption rates.Over the past decade,there have been significant advancements in trace element analysis techniques for olivine,resulting in a wealth of data.This paper systematically reviews the latest research progress in the field of trace element analysis of olivine using electron probe and laser ablation inductively coupled plasma mass spectrometry methods,summarizes the geochemical behaviors of trace elements in olivine,such as the distribution coefficient and diffusion coefficient,analyzes the trace element characteristics of olivine in different rock units around the world,and highlights the research progress of trace elements in olivine from magmatic Ni-Cu deposits.Experimental petrology indicates that the main factors controlling element distribution coefficients in olivine include temperature,pressure,and magma composition,with different trace elements being influenced by different primary factors.The partitioning of most elements in olivine can be predicted by the lattice strain model,whereas Ni,Co,and light rare earth elements are primarily controlled by crystal field effects and crystallization kinetics.The diffusion rates of elements in olivine from the fastest to the slowest are:H and Li(fast)→Fe-Mg-Ni(medium)→Ca(slower)→P-O-Si(very slow).A comparison of trace elements in olivine from different rock units around the world reveals:(1)In volcanic series such as komatiite-picrite-basalt,the Fo value and Ni,Cr,Al,Mn,and other elements of olivine show evolutionary trends,but there is a large overlap between the rock types in binary correlation diagrams and multidimensional analysis methods such as machine learning can greatly improve the classification accuracy;(2)The trace element characteristics of olivine in mantle peridotites and intrusive rocks(crustal cumulates)are similar,making it difficult to distinguish them using single-element indicators like the commonly used Ca depletion;(3)The Ni and Mn contents of olivine in carbonatites differs from other rocks,which may be related to their source and the unique partitioning behavior of elements between olivine and carbonatite melts;(4)There are certain differences in the trace elements of olivine between garnet mantle peridotite and spinel mantle peridotite.The olivine V/Sc ratio and Fe-Mg-Mn-Ca-Ni content are important variables for distinguishing the type and source depth of mantle peridotite.Research progress on trace elements in olivine from magmatic Ni-Cu deposits includes:(1)The re-equilibrium effect during cumulus and post-cumulus stages drives the Ca-Cr-Al elements to migrate to other minerals,resulting in olivine depletion of these elements;(2)Variations in Ni and Co in olivine record the Fe-Ni-Co exchange reactions with sulfides,where a positive correlation between Ni and Co can indicate the presence of sulfides at deeper locations of the intrusion;(3)Ni enrichment(>5000×10^(-6))or depletion(<500×10^(-6))in olivine coexisting with sulfides may be related to higher or lower oxygen fugacity.Finally,the paper outlines future research directions and breakthroughs in the field of trace elements in olivine:element behavior during cumulus and post-cumulus stages,multi-element and multi-mineral diffusion to constrain the spatiotemporal evolution of magma,machine learning to discriminate olivine genesis types and mineralization potential of mafic-ultramafic complexes,volatiles and untraditional metal isotopes in olivine.