Kimberlites are deep and relatively rare mantle-derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle-derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U–Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70–2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb–Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology.

中文翻译：

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锆石异晶的脱硅边缘记录了金伯利岩就位的时间

金伯利岩是深部和相对稀有的地幔衍生火成岩，它为岩浆就位时大陆下方的岩石圈地幔成分（即，来自夹带的地幔物质）提供了重要的见解。此外，整个地球历史上金伯利岩的时间分布可能揭示全球地球动力学的长期变化。然而，不同年龄的蚀变和丰富的外来物质（即捕虏体和异种晶体）都是金伯利岩的典型特征，常常阻碍这些岩石的可靠测年。在这项研究中，我们研究了俄罗斯卡累利阿克拉通变质的 Kimozero 金伯利岩中的锆石异晶及其被脱硅反应边缘 (DSR) 取代的情况，该边缘主要由斜锰矿组成。先前已在许多金伯利岩中的地幔衍生锆石上发现了类似的 DSR，但这些边缘形成的确切时间和位置仍不清楚。Kimozero 金伯利岩中地幔锆石异晶的 U-Pb 同位素测年表明这些岩石位于约_ 1.97 Ga。在这些金伯利岩中的 1.97 Ga 地幔和 2.70-2.38 Ga 地壳锆石异晶上都出现了具有相似结构和斜锰矿化学性质的 DSR。DSR 的 Pb-Pb 同位素测年得出与地幔锆石相同的年龄（即约1.97 Ga）。数据将 DSR 的形成限制在地壳水平，更重要的是，表明这些边缘记录了金伯利岩就位的时间。因此，我们认为这种斜锰矿反应边缘是金伯利岩年代学的一种新的有前途的材料。