Petrology of forearc peridotite from the Hahajima Seamount, the Izu-Bonin arc, with special reference to chemical characteristics of chromian spinel
Petrology of forearc peridotite from the Hahajima Seamount, the Izu-Bonin arc, with special reference to chemical characteristics of chromian spinel
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DOI：
10.1180/0026461067010310
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发表时间：
2006-02
影响因子：
2.7
通讯作者：
H. Okamura;S. Arai;Yung-Uk Kim
中科院分区：
文献类型：
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作者：
H. Okamura;S. Arai;Yung-Uk Kim
摘要：与普通的深海橄榄岩相比，在给定的铬尖晶石的Cr#（=Cr/[Cr + Al]原子比）下，弧前橄榄岩通常具有较低的Mg#（=Mg/[Mg + Fe²⁺]原子比）。这可能是由于（1）弧前橄榄岩中尖晶石的模态丰度较小和/或（2）平衡温度较低。弧前橄榄岩中尖晶石的含量与深海橄榄岩相同，排除了第一种可能性。伊豆 - 小笠原弧前的母岛海山的方辉橄榄岩和纯橄榄岩中的尖晶石具有较大的Cr#，>0.57，并且在给定的Cr#下，Mg#朝着较低的值稍有变化。尖晶石核的Mg#随着尺寸减小而大幅降低。这是由于冷却以及水化作用，导致母岛橄榄岩中钙角闪石的成分从浅闪石质角闪石（高温）变为透闪石（低温）发生变化。母岛橄榄岩的平均二辉石温度为921°C，低于深海橄榄岩的温度（1138°C），这与深海橄榄岩中尖晶石的尺寸相关的Mg#情况不符。弧前橄榄岩已被俯冲板块释放的H₂O有效地冷却，导致其尖晶石的Mg#较小。
Abstract Forearc peridotite is generally characterized by low Mg# (= Mg/[Mg + Fe2+] atomic ratio) at a given Cr# (= Cr/[Cr + Al] atomic ratio) of chromian spinel compared to common abyssal peridotite. This may be due to (1) smaller modal abundance of spinel and/or (2) lower equilibrium temperature, for the forearc peridotite. Forearc peridotite has the same amount of spinel as abyssal peridotite, eliminating the first possibility. Spinel in harzburgite and dunite from the Hahajima Seamount at the Izu-Bonin forearc, has a large Cr#, >0.57, and the Mg# is slightly variable towards low values at a given Cr#. The Mg# of spinel cores decreases strongly with a decrease in size. This is due to cooling along with hydration, which gave rise to a compositional variation of Ca-amphibole, from edenitic hornblende (high-temperature) to tremolite (low-temperature) in the Hahajima peridotite. The average two- pyroxene temperature of the Hahajima peridotite, 921°C, is less than that of the abyssal peridotite (1138°C), which is not consistent with a size-dependent Mg# of spinel for the latter. Forearc peridotite has been cooled effectively by H2O released from the subducted slab, causing a small Mg# of their spinels.