State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China.
Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen, 518055, China.
Nat Commun. 2023 Mar 13;14(1):1382. doi: 10.1038/s41467-023-37072-0.
Olivine is the dominant phenocryst or xenocryst of Hawaiian tholeiitic basalts, and the general consensus is that lavas with MgO concentrations from 7.5 to about 15 weight percent were derived from their primary magmas, which contain ~18-20 weight percent MgO, by only olivine crystallization. However, the major element composition of estimated primary magmas through olivine crystallization correction is inconsistent with direct partial melting of either mantle peridotite or its hybrid with subducted oceanic crust. Our melting experiments on peridotite-derived melt composition show that this discrepancy can be resolved if the primary magmas experienced two other processes before abundant olivine fractionation. First, the primary magmas experienced crystallization of clinopyroxene and garnet in the chamber at the base of the lithosphere (approximately the depths of 90-100 km). Second, the evolved magmas re-equilibrated with harzburgite when passing through the lithospheric mantle (approximately the depths of 60-10 km). Different from the isotopic evidence, the major and rare earth element compositions of Hawaiian post-shield alkali basalts and shield tholeiites suggest that they form from the same source by assimilating different amounts of orthopyroxene.
橄榄石是夏威夷拉斑玄武岩的主要斑晶或异源晶，一般认为 MgO 浓度在 7.5 到约 15 重量%的熔岩是由原始岩浆形成的，原始岩浆中含有约 18-20 重量%的 MgO，仅通过橄榄石结晶。然而，通过橄榄石结晶校正估计的原始岩浆的主要元素组成与地幔橄榄岩的直接部分熔融或其与俯冲海洋地壳的混合不一致。我们对源自橄榄岩熔体组成的熔融实验表明，如果原始岩浆在大量橄榄石分馏之前经历了另外两个过程，这种差异可以得到解决。首先，原始岩浆在岩石圈底部的腔室中经历了辉石和石榴石的结晶（约 90-100km 的深度）。其次，演化后的熔体在穿过岩石圈地幔时与方辉橄榄岩重新平衡（约 60-10km 的深度）。与同位素证据不同，夏威夷后盾碱性玄武岩和盾状拉斑玄武岩的主量和稀土元素组成表明，它们由相同的源形成，通过同化不同数量的斜方辉石。
