Metasomatic formation of kosmochlor-bearing diopside in peridotite xenoliths from North Island, New Zealand
Metasomatic formation of kosmochlor-bearing diopside in peridotite xenoliths from North Island, New Zealand
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DOI：
10.2138/am-2004-1006
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发表时间：
2004-10
影响因子：
3.1
通讯作者：
K. Ikehata;S. Arai
中科院分区：
文献类型：
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作者：
K. Ikehata;S. Arai
摘要新西兰北岛Ngatutura火山岩区的上新世-第四纪夏威夷岩中发现了一种绿色Cr、Na的浅绿色透辉石（含钾氯透辉石）。透辉石的特征在于高Cr2 O3（高达5.87重量%）和Na 2 O（高达3.31重量%）和低Al 2 O3（1.09重量%，平均），并包含最高的kosmochlor组分（高达18摩尔%）已知的尖晶石橄榄岩捕虏体中的透辉石。稀土元素含量高，轻稀土相对于重稀土稍富集。含钾镁矾的透辉石通常在铬尖晶石周围形成边缘，并以橄榄石为代价取代从板状衍生流体/熔体中结晶的次生贫Al（<0.57 wt% Al 2 O3）和富Si（~58.1 wt% SiO2）斜方辉石。含钾氯透辉石可能是由含钠碳酸盐岩熔体通过交代作用形成的，该熔体选择性地与斜方辉石反应，并能携带大量的稀土元素。弧环境下形成的难熔橄榄岩中富铬尖晶石是碳酸盐岩交代作用下含钾氯透辉石成因的先决条件。碳酸盐岩熔体从更深的部分穿透地幔楔，这一直是在一个俯冲带设置，直到15 Ma，由于板盖层的情况下，在板内设置（~10 Ma）在北岛。
Abstract Pale green Cr- and Na-rich diopside (kosmochlor-bearing diopside) was observed in anhydrous Group I mantle xenoliths hosted by Pliocene-Quaternary hawaiite from the Ngatutura volcanic field of the North Island, New Zealand. The diopside is characterized by high Cr2O3 (up to 5.87 wt%) and Na2O (up to 3.31 wt%) and low Al2O3 (1.09 wt%, on average), and contains the highest kosmochlor component (up to 18 mol%) known for diopsides in spinel peridotite xenoliths. It also contains high REE concentrations and is slightly enriched in LREE relative to HREE. The kosmochlor-bearing diopside commonly forms rims around chromian spinel and replaces secondary Al-poor (<0.57 wt% Al2O3) and Si-rich (~58.1 wt% SiO2) orthopyroxene crystallized from slab-derived fluid/melt at the expense of olivine. The kosmochlor-bearing diopside may have been formed through metasomatism by an Na-bearing carbonatite melt, which selectively reacted with orthopyroxene and can carry a large amount of rare-earth elements. Cr-rich spinel in refractory peridotites that had been formed in an arc setting is a prerequisite for genesis of this kind of kosmochlor-bearing diopside upon carbonatite metasomatism. The carbonatite melt from the deeper part penetrated the mantle wedge, which had been at a supra-subduction zone setting until 15 Ma, due to the absence of slab cover at an intraplate setting (~10 Ma) in the North Island.