Isotopic composition of Mg and Fe in garnet peridotites from the Kaapvaal and Siberian cratons
Isotopic composition of Mg and Fe in garnet peridotites from the Kaapvaal and Siberian cratons
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DOI：
10.1016/j.gca.2016.11.041
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发表时间：
2017-03
影响因子：
5
通讯作者：
Y. An;J. Huang;W. Griffin;Chuan-Zhou Liu;F. Huang
中科院分区：
文献类型：
--
作者：
Y. An;J. Huang;W. Griffin;Chuan-Zhou Liu;F. Huang
我们提出了镁和铁同位素数据的全岩和分离的矿物（橄榄石，单斜辉石，斜方辉石，石榴石，金云母）的石榴橄榄岩，平衡在深度为134-186公里的Kaapvaal和西伯利亚的火山。这两种岩石中石榴橄榄岩的δ 26 Mg和δ 56 Fe没有明显的差异。全岩δ 26 Mg变化范围为−0.243‰ ~ −0.204‰，平均值为−0.225 ± 0.037‰（2σ，n= 19）; δ 56 Fe变化范围为−0.038‰ ~ 0.060‰，平均值为−0.003 ± 0.068‰（2σ，n= 19）。这两个值与肥沃的上地幔没有区别，表明浅部和深部上地幔之间没有明显的Mg-Fe同位素差异。石榴橄榄岩的δ 26 Mg值与科马提岩和玄武岩的δ 26 Mg值相似，表明在不同程度的深地幔部分熔融和科马提岩形成过程中，Mg同位素分馏不明显，Mg和Fe同位素数据的精度（δ 26 Mg和δ 56 Fe，2σ为±0.05‰）使我们能够区分矿物间的同位素分馏。橄榄石在Mg和Fe同位素方面与opx处于平衡。石榴子石的δ 26 Mg和δ 56 Fe在共存的地幔矿物中最低，表明晶体结构对石榴子石的Mg-Fe同位素组成起主导控制作用。元素组成和矿物学表明，单斜辉石和石榴石是由后来的交代过程，因为他们没有在橄榄石或斜方辉石的化学平衡。这与斜方辉石/橄榄石和石榴子石/单斜辉石之间的Mg和Fe同位素不平衡是一致的。结合一个样品的δ 26 Mg稍重和δ 56 Fe轻得多的特征，石榴橄榄岩中的这些不平衡特征揭示了Kaapvaal克拉通中橄榄岩与熔融熔体反应期间Fe-Mg相互扩散引起的动力学同位素分馏。
We present Mg and Fe isotopic data for whole rocks and separated minerals (olivine, clinopyroxene, orthopyroxene, garnet, and phlogopite) of garnet peridotites that equilibrated at depths of 134–186 km beneath the Kaapvaal and Siberian cratons. There is no clear difference in δ26Mg and δ56Fe of garnet peridotites from these two cratons. δ26Mg of whole rocks varies from −0.243‰ to −0.204‰ with an average of −0.225 ± 0.037‰ (2σ,n= 19), and δ56Fe from −0.038‰ to 0.060‰ with an average of −0.003 ± 0.068‰ (2σ,n= 19). Both values are indistinguishable from the fertile upper mantle, indicating that there is no significant Mg-Fe isotopic difference between the shallow and deep upper mantle. The garnet peridotites from ancient cratons show δ26Mg similar to komatiites and basalts, further suggesting that there is no obvious Mg isotopic fractionation during different degrees of partial melting of deep mantle peridotites and komatiite formation.The precision of the Mg and Fe isotope data (⩽±0.05‰ for δ26Mg and δ56Fe, 2σ) allows us to distinguish inter-mineral isotopic fractionations. Olivines are in equilibrium with opx in terms of Mg and Fe isotopes. Garnets have the lowest δ26Mg and δ56Fe among the coexisting mantle minerals, suggesting the dominant control of crystal structure on the Mg-Fe isotopic compositions of garnets. Elemental compositions and mineralogy suggest that clinopyroxene and garnet were produced by later metasomatic processes as they are not in chemical equilibrium with olivine or orthopyroxene. This is consistent with the isotopic disequilibrium of Mg and Fe isotopes between orthopyroxene/olivine and garnet/clinopyroxene. Combined with one sample showing slightly heavy δ26Mg and much lighter δ56Fe, these disequilibrium features in the garnet peridotites reveal kinetic isotopic fractionation due to Fe-Mg inter-diffusion during reaction between peridotites and percolating melts in the Kaapvaal craton.