Hydration processes in the arc mantle; petrology of the Megata peridotite xenoliths, the Northeast Japan arc
Hydration processes in the arc mantle; petrology of the Megata peridotite xenoliths, the Northeast Japan arc
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弧地幔中的水化过程；
DOI：
10.2465/ganko.87.305
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发表时间：
1992
期刊：
影响因子：
--
通讯作者：
Yasuhiro Saeki
中科院分区：
文献类型：
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作者：
N. Abe;S. Arai;Yasuhiro Saeki
产自 Megata 火山 Ichinomegata 火山口的橄榄岩捕虏体具有广泛分布的铬尖晶石 Cr#（Cr/(Cr+Al) 原子比），从 0.07 到 0.53。它们经常含有次生含水矿物、细闪石和金云母，取代或脉化原生矿物。斜方辉石的量与单斜辉石的量呈负相关。金云母仅出现在富含磷灰石的岩石中。石蜡石的K/(K+Na)比值随着岩石水化程度的增加而增加。磷灰石和金云母的 TiO2 含量较低 (<1.6 wt. %)。原生矿物已根据所添加的含水矿物的量进行了化学改性；橄榄石和辉石富含 Fe（低至 Fo83），铬尖晶石富含 Al 和 Fe3+。含水矿物的交代形成至少伴随着 Na、Ti 和 K 的添加。交代作用是由含水弧岩浆渗透引起的，这些岩浆以含金云母的角闪岩细脉的形式固结到最初无水的橄榄岩中。随后释放的液体扩散到周围环境中，产生含水矿物。 Sannomegata 陨石坑的橄榄岩捕虏体的铬尖晶石 Cr # 分布非常窄，从 0.10 到 0.25；即，一切都是二辉橄榄石。它们通常含有辉石-尖晶石辛辉石或其粗化的等价物，带有或不带有残骸斜长石。它们几乎不含含水矿物，并且它们的主要矿物没有经过交代改性。 Ichinomegata 和 Sannomegata 橄榄岩之间的岩石学对比表明，日本东北弧下方的上地幔在无水原生矿物学和水化程度方面都存在很强的异质性。
Peridotite xenoliths from the Ichinomegata crater, the Megata volcano, have a wide spread of Cr# (Cr/(Cr+Al) atomic ratio) of chromial spinet, from 0.07 to 0.53. They frequently have secondary hydrous minerals, pargasite and phlogopite, replacing or veining primary minerals. The amount of pargasite is negatively correlated with the amount of clinopyroxene. Phlogopite occurs only in pargasite-rich rocks. The K/(K+Na) ratio of pargasite increases with an increase of degree of hydration of the rocks. Pargasite and phlogopite are depleted in TiO2 (<1.6 wt. %). Primary minerals have been chemically modified depending on the amounts of the hydrous minerals added ; olivine and pyroxenes have been enriched with Fe (down to Fo83), and chromian spinel, with Al and Fe3+. Metasomatic formation of the hydrous minerals was accompanied with addition of, at least, Na, Ti and K. The metasomatism was caused by infiltration of hydrous arc magmas, which consolidated as phlogopite-bearing hornblendite veinlets, into initially anhydrous peridotites. The fluids subsequently released were diffused into the surroundings to produce hydrous minerals. Peridotite xenoliths from the Sannomegata crater have a very narrow spread of Cr # of chromian spinel, from 0.10 to 0.25; i.e., all is lherzolitic. They usually have pyroxene-spinel symplectite or its coarsened equivalent, with or without relic plagioclase. They are almost free from hydrous minerals and their primary minerals have been devoid of the metasomatic modification. The petrographical contrast between the Ichinomegata and the Sannomegata peridotites suggests a strong heterogeneity, in terms both of the anhydrous primary mineralogy and of the degree of hydration, for the upper mantle beneath the Northeast Japan arc.