Na/K diversity of primary basaltic magmas induced by the separation of slab-derived supercritical liquid: Implications from alkali basaltic lavas from Rishiri Volcano, southern Kuril Arc
Na/K diversity of primary basaltic magmas induced by the separation of slab-derived supercritical liquid: Implications from alkali basaltic lavas from Rishiri Volcano, southern Kuril Arc
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板状超临界液体分离引起的原生玄武岩浆的 Na/K 多样性：南千岛弧利尻火山碱性玄武岩熔岩的影响
DOI：
10.1093/petrology/egab099
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发表时间：
2021
影响因子：
3.9
通讯作者：
Nakagawa Mitsuhiro
中科院分区：
文献类型：
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作者：
Taniuchi Hajime;Kuritani Takeshi;Nakagawa Mitsuhiro
俯冲带的原生玄武岩岩浆即使在个别火山中也表现出化学多样性。本文通过对南千岛岛弧Rishiri火山高Na/K的Horyosawa熔岩和低Na/K的Nozuka熔岩的岩石学对比研究，阐明了决定原生玄武岩岩浆化学成分特别是Na/K的主导因素。两种熔岩流均为玄武岩，可用于岩浆生成条件的估计。高na /K的堀泽熔岩全岩sio2含量为51.9% ~ 53.4%，是本次研究的主要靶岩。岩浆的岩石学和地球化学特征表明，岩浆混合和地壳物质同化作用有限，低sio2岩浆主要来源于原生岩浆的橄榄石分馏作用。据估计，原始岩浆是在~1300℃、~ 2.4 GPa条件下，以~ 0.10 wt%的水对源幔进行~ 2.4%的部分熔融而产生的。通过对比高Na/K的堀泽熔岩与低Na/K玄武岩（野冢熔岩）的结果，我们发现：(1)它们的形成过程中所涉及的板源流体是由俯冲板块释放出来的超临界流体，具有相似的Na/K成分；(2)堀泽原生岩浆的生成位置比野冢原生岩浆更浅。从这些观测结果可以推断，堀泽熔岩的原始岩浆是由浅于临界点深度的超临界液体分离的含水流体流入熔融形成的，而野冢熔岩是由深于临界点深度的超临界液体熔融产生的。这些发现表明，在板状超临界流体分离成含水流体和含水熔体的过程中，元素分配可以引起单个火山体系原生玄武质岩浆的化学多样性。
Primary basaltic magmas in subduction zones exhibit chemical diversity even in individual volcanoes. In this study, we aim to elucidate the dominant factors that determine the chemical composition, especially the Na/K, of primary basaltic magmas by conducting a comparative petrological study of the high-Na/K Horyosawa lava and low-Na/K Nozuka lava from Rishiri Volcano in the southern Kuril Arc. Both lava flows are basaltic and are suitable for estimating the magma generation conditions. The whole-rock SiO2content of the high-Na/K Horyosawa lava, the main target of this study, ranges from 51·9 to 53·4 wt%. The petrological and geochemical features of this lava indicate that the effects of magma mixing and assimilation of crustal materials were limited, and the low-SiO2magmas were essentially derived via olivine fractionation from the primary magma. The primary magma is estimated to have been generated by ~2·4 % partial melting of the source mantle with ~0·10 wt% water at ~1300 °C and ~2·4 GPa. By comparing the results of the high-Na/K Horyosawa lavas with those published for the low-Na/K basalts (Nozuka lava), we find that (1) the slab-derived fluids involved in their formation were supercritical liquids with similar compositions, including Na/K, when released from the subducting slab, and (2) the Horyosawa primary magma was generated at a shallower level in the mantle than the Nozuka primary magma. From these observations, it is inferred that the primary magma of the Horyosawa lava was formed by the influx melting of aqueous fluid separated from the supercritical liquid at depths shallower than the critical point, whereas the Nozuka lava was generated by supercritical liquid-fluxed melting at depths deeper than the critical point. These findings show that elemental partitioning during the separation of slab-derived supercritical liquid into aqueous fluid and hydrous melt can induce chemical diversity within the primary basaltic magmas of a single volcanic system.