Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (TRD). By comparison, olivine megacrysts from Udachnaya, sometimes called “megacrystalline peridotites”, often yield Archean TRD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine 1 cm) dunite, olivine megacrysts and low-orthopyroxene (11–21% opx) harzburgites equilibrated at 783–1154 °C and 3.9–6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917–0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg#WR; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37 Ga, same as eruption age of host kimberlite. Robust TRD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02–0.13). TRD ages for low-opx harzburgites (1.9–2.1 Ga; average 2.0 ± 0.1 Ga, 1 σ) are manifestly lower than for dunites and megacrysts (2.4–3.1 Ga); the latter define two subsets with average TRD of 2.6 ± 0.1 Ga and 3.0 ± 0.1 Ga, and TMA of 3.0 ± 0.2 Ga and 3.3 ± 0.1 Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6 Ga and 3.0 Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8–2.0, 2.4–2.8 and 3.0–3.4 Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ≥0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the ‘Archean’ (4–2.5 Ga) to modern tectonic regimes.

中文翻译：

---

克拉通岩石圈地幔的年龄和起源：太古代沙丘与西伯利亚克拉通 Udachnaya 金伯利岩的古元古代方辉石

克拉通岩石圈地幔据信形成于太古代，但来自西伯利亚克拉通中部 Udachnaya 的金伯利岩粗橄榄岩通常产生古元古代再耗竭 Os 同位素年龄 (TRD)。相比之下，来自 Udachnaya 的橄榄石巨晶，有时被称为“巨晶橄榄岩”，通常会产生太古代 TRD 年龄，但这些稀有材料的性质仍然是个谜。我们提供了来自 Udachnaya 的 24 种富含橄榄石的捕虏体的全岩 (WR) Re-Os 同位素和 PGE 分析，以及模态和岩相数据、WR 以及矿物主要和微量元素组成。样品的选择基于 (a) 手标本中的高橄榄石丰度和 (b) 足够的新鲜度和大小以产生具有代表性的 WR 粉末。它们包括中到粗粒（橄榄石 1 厘米）纯晶，橄榄石巨晶和低斜方辉石 (11–21% opx) 方辉石在 783–1154 °C 和 3.9–6.5 GPa 下平衡；Udachnaya 以前没有报道过粗沙丘；两个捕虏体含有钛铁矿。方辉石和纯辉岩具有相似的 WR 变化范围，Ca、Al、Fe、Cr 和 Mg#（0.917-0.934）是耐火克拉通橄榄岩的典型特征，但纯辉岩往往具有较高的 MgO、NiO 和 Mg/Si。矿物丰度以及Ca和Al的丰度与Mg#WR无关；它们不是由于熔化程度的差异，而是与交代作用有关。几个具有高 187Re/188Os 的样品与 187Os/188Os 呈正线性相关，表观年龄为 0.37 Ga，与宿主金伯利岩的喷发年龄相同。对 16 个具有低 187Re/188Os (0.02–0.13) 的捕虏体获得了稳健的 TRD 年龄。低 opx 菱镁矿的 TRD 年龄（1.9-2.1 Ga；平均 2.0 ± 0.1 Ga, 1 σ) 明显低于纯晶和巨晶 (2.4–3.1 Ga)；后者定义了两个子集，平均 TRD 为 2.6 ± 0.1 Ga 和 3.0 ± 0.1 Ga，TMA 分别为 3.0 ± 0.2 Ga 和 3.3 ± 0.1 Ga。橄榄石晶粒尺寸（粗晶与超晶）的差异与年龄无关。年龄关系表明，纯辉石和巨晶不能通过重新熔化方辉石（例如在弧形环境中）产生，也不能是方辉石中的熔体通道材料。相反，它们是太古宙中形成的岩石圈地幔的残余碎片（可能在 2.6 Ga 和 3.0 Ga 时或之后的两次事件中），在古元古代西伯利亚克拉通的最后组装期间并入克拉通岩石圈。西伯利亚岩石圈地幔的多阶段形成与来自 Udachnaya 地壳包体的锆石和来自北西伯利亚克拉通 (1.8-2.0, 2.4-2.8 和 3.0-3.4 Ga) 的碎屑锆石的 U-Pb 定年的地壳基底年龄一致. 来自西伯利亚和其他克拉通的新数据表明，强烈熔毁的克拉通岩石圈（如Mg#≥0.92）的形成并没有像人们普遍认为的那样在太古代-元古代边界停止，而是在古元古代继续形成。对于从“太古代”（4-2.5 Ga）到现代构造体制的过渡，这可能同样有效。来自西伯利亚和其他克拉通的新数据表明，强烈熔毁的克拉通岩石圈（如Mg#≥0.92）的形成并没有像人们普遍认为的那样在太古代-元古代边界停止，而是在古元古代继续形成。对于从“太古代”（4-2.5 Ga）到现代构造体制的过渡，这可能同样有效。来自西伯利亚和其他克拉通的新数据表明，强烈熔毁的克拉通岩石圈（如Mg#≥0.92）的形成并没有像人们普遍认为的那样在太古代-元古代边界停止，而是在古元古代继续形成。对于从“太古代”（4-2.5 Ga）到现代构造体制的过渡，这可能同样有效。