Kimberlitic olivines derived from the Cr-poor and Cr-rich megacryst suites
Kimberlitic olivines derived from the Cr-poor and Cr-rich megacryst suites
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DOI：
10.1016/j.lithos.2016.04.022
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发表时间：
2016-08
期刊：
影响因子：
3.5
通讯作者：
A. Moore;G. Costin
中科院分区：
文献类型：
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作者：
A. Moore;G. Costin
逆带橄榄石（岩心比岩缘富铁）在金伯利岩中普遍存在，分布广泛。这些橄榄石通常包括相对于占优势的正常分带橄榄石的从属种群。而在Kaapvaal和津巴布韦克拉通的中白垩世修道院和早元古代巨像金伯利岩中，它们的含量更为丰富。这两个地方的逆带橄榄石确定的成分场与修道院金伯利岩中记录的贫铬组合的两个橄榄石巨晶群的成分场非常相似。这指出了巨晶岩和反转带橄榄石之间的遗传联系。富铁橄榄石在金伯利岩中普遍存在，且地理分布广泛，这反过来又证明了巨晶岩与寄主金伯利岩之间存在着密切的联系。一些大型橄榄石含有圆形富铬斜辉石、石榴石和/或尖晶石包裹体，其内部成分分带尺度精细，不稳定。具有这种化学非均质富cr包裹体的橄榄石并非来自分解的地幔橄榄岩，而是与富cr巨晶套件有关。因此，它们不能作为大多数金伯利岩橄榄石的岩心来自分解的地幔橄榄岩的证据。
Reversed-zoned olivines (Fe-richer cores compared to rims), appear to be ubiquitous in kimberlites with a wide distribution. These olivines generally comprise a subordinate population relative to the dominant normally zoned olivines. However, they are notably more abundant in the megacryst-rich mid-Cretaceous Monastery and early Proterozoic Colossus kimberlites, located on the Kaapvaal and Zimbabwe cratons, respectively. The reverse-zoned olivines at these two localities define compositional fields that are closely similar to those for two olivine megacryst populations of the Cr-poor association which have been documented in the Monastery kimberlite. This points to a genetic link between megacrysts and the reversed zoned olivines. The ubiquitous, occurrence of the Fe-rich (relative to the field for rims) olivines in kimberlites with a wide geographic distribution in turn argues for an intimate link between megacrysts and the host kimberlite. Some large olivines have inclusions of rounded Cr-rich clinopyroxenes, garnets and/or spinel, characterized by fine-scale, erratic internal compositional zoning. Olivines with such chemically heterogeneous Cr-rich inclusions are not derived from disaggregated mantle peridotites, but are rather linked to the Cr-rich megacryst suite. Consequently, they cannot be used as evidence that cores of a majority of kimberlitic olivines are derived from disaggregated mantle peridotites.