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首页> 外文期刊>Contributions to Mineralogy and Petrology >A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt
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A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt

机译：来自日本西南部高岛市的一种新型的钙霞石异种石：演化出的碱性玄武岩富集地幔中的二氧化硅

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We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO2 activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al2O3 contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.
机译：我们从日本西南部高岛市发现了细粒丰富的富铁，富含邻苯二茂的异岩（主要是邻辉石），其中含有部分消化的I类榴du石碎片。钙钛矿脉石（其中一些中心含有斜长石）也代替了I组的榴辉岩和辉绿岩异岩中的橄榄石。这显示出对参与钙钛矿形成的熔体的橄榄石具有高反应性，表明其具有较高的SiO2 活性。这种类型的次生邻二甲苯的特点是Mg＃[= Mg /（Mg +总Fe）原子比]低（低至0.73）和Al2 O3 含量高（5–6 wt％） 。它在化学上不同于衍生自地幔楔的橄榄岩异岩中的其他次生邻苯二甲酚。富铁正火辉石中的亚斜辉石显示出向上凸起的REE模式，且波峰围绕Sm。这种模式与II型辉石岩异石质的斜辉石以及表晶和异晶斜石辉石的辉石非常相似，表明存在类似的碱性玄武岩熔体。来自高岛的富铁正辉石异岩，是由演化的碱性玄武岩熔体与地幔橄榄石反应形成的；最初在二氧化硅中略微不饱和的碱性玄武岩可能已通过在高压条件下分步结晶而演变为二氧化硅过饱和的成分。富含铁的正辉石以堤坝的形式出现在最上层的地幔中，该地幔覆盖在II组辉石岩的囊袋中，由辉光岩和白云母组成。这种类型的富含邻苯二茂基的辉石岩可能普遍存在于大陆裂谷带下方的最上地幔中，在该裂谷中碱性玄武岩岩浆已经普遍存在。

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