Abstract During the past two decades significant progress has been made in understanding the origin and evolution of kimberlites, including relationships to other diamondiferous magma types such as lamproites and aillikites. However, the association of kimberlites and carbonatites on continental shields remains poorly understood, and two opposing ideas dominate the debate. While one school of thought argues that primary carbonatite melts transform into hybrid carbonated silicate magmas akin to kimberlites by assimilation of cratonic mantle material, others use geochemical evidence to show that carbonatite magmas can evolve from near-primary kimberlite melts within the cratonic lithosphere. The 1.15 Ga Premier kimberlite pipe on the Kaapvaal craton in South Africa hosts several kimberlite and carbonatite dykes. Reconstructions of magma compositions suggest that up to 20 wt.% CO2 was lost from near-primary kimberlite melts during ascent through the cratonic lithosphere, but the carbonatite dyke compositions cannot be linked to the kimberlite melts via differentiation. Geochemical evidence, including mantle-like δ13C compositions, suggests that the co-occurring kimberlite and carbonatite dykes represent two discrete CO2-rich magma batches derived from a mixed source in the convecting upper mantle. The carbonatites probed a slightly more depleted source component in terms of Sr-Nd-Hf isotopic compositions relative to the peridotitic matrix that was more effectively tapped by the kimberlites (87Sr/86Sri = 0.70257 to 0.70316 for carbonatites vs. 0.70285 to 0.70546 for kimberlites; eNdi = +3.0 to +3.9 vs. +2.2 to +2.8; eHfi = -2.2 to +0.7 vs. -5.1 to -1.9). Platinum-group element systematics suggest that assimilation of refractory lithospheric mantle material by the carbonatite melts was negligible ( Ɣ Osi = -12.7 to -4.5). The kimberlite and carbonatite dykes show similarly strong Nd-Hf isotope decoupling (ΔeHfi = -10.7 to -7.6 vs. -8.8 to -6.1), regardless of the variable lithospheric mantle imprints. This observation suggests a common sublithospheric origin of the negative ΔeHf signature, possibly linked to ancient recycled oceanic crust components in the convecting upper mantle to transition zone sources of CO2-rich magmatism. Mesoproterozoic kimberlite and carbonatite magmatism at Premier was coeval with subduction and collision events along the southern Kaapvaal craton margin during the 1,220 –1,090 Ma Namaqua-Natal orogeny associated with Rodinia supercontinent formation. Thermochronology suggests that the entire Kaapvaal craton was affected by this collisional tectonic event, and it appears that the changing lithospheric stress-field created pathways for deep-sourced kimberlite and carbonatite magmas to reach Earth’s surface. We find that collision-induced (e.g., Premier) and continental breakup-related (e.g., Kimberley) kimberlite magmas are compositionally indistinguishable, with the inference that plate tectonic processes aid solely in the creation of magma ascent pathways without a major influence on deep mantle melting beneath cratons. It follows that on-craton kimberlite magmatism in the hinterland of collision zones is not necessarily more likely to entrain large sublithospheric diamonds than kimberlite eruptions linked to continental breakup. This implies that Premier’s world-class endowment with ‘ultradeep’ Type-II diamonds is not causally related to its setting behind an active orogenic front.

中文翻译：

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大陆碰撞期间金伯利岩和碳酸盐岩的起源——超越解耦 Nd-Hf 同位素的见解

摘要 在过去的 20 年中，在理解金伯利岩的起源和演化方面取得了重大进展，包括与其他含钻石岩浆类型（如菱镁矿和铝镁石）的关系。然而，金伯利岩和碳酸盐岩在大陆地盾上的关联仍然知之甚少，两个对立的想法主导了辩论。虽然一种学派认为原生碳酸岩熔体通过吸收克拉通地幔物质转化为类似于金伯利岩的混合碳酸盐硅酸盐岩浆，但其他学派使用地球化学证据表明，碳酸岩岩浆可以从克拉通岩石圈内的近原生金伯利岩熔体演化而来。南非 Kaapvaal 克拉通上的 1.15 Ga Premier 金伯利岩管拥有多个金伯利岩和碳酸盐岩脉。岩浆成分的重建表明，在通过克拉通岩石圈上升过程中，近原生金伯利岩熔体损失了高达 20 wt.% 的 CO2，但碳酸岩脉成分不能通过分化与金伯利岩熔体联系起来。地球化学证据，包括类似地幔的 δ13C 成分，表明共生的金伯利岩和碳酸岩脉代表了来自对流上地幔中的混合源的两个离散的富含 CO2 的岩浆批次。相对于金伯利岩更有效开采的橄榄岩基质而言，碳酸盐岩探测到 Sr-Nd-Hf 同位素组成稍微更贫乏的源组分（87Sr/86Sri = 0.70257 至 0.70316 碳酸岩与 0.70285 至 6.70 陨石； eNdi = +3.0 至 +3.9 与 +2.2 至 +2.8；eHfi = -2.2 至 +0.7 与 -5.1 至 -1.9）。铂族元素系统学表明，碳酸岩熔体对难熔岩石圈地幔物质的同化作用可以忽略不计（Ɣ Osi = -12.7 至 -4.5）。无论岩石圈地幔印记如何变化，金伯利岩和碳酸岩脉都表现出类似的强烈 Nd-Hf 同位素解耦（ΔeHfi = -10.7 至 -7.6 与 -8.8 至 -6.1）。这一观察结果表明负 ΔeHf 特征的一个共同的亚岩石圈起源，可能与对流上地幔中古老的循环海洋地壳成分有关，这些成分与富含 CO2 的岩浆活动的过渡带来源有关。Premier 的中元古代金伯利岩和碳酸岩岩浆活动与沿 Kaapvaal 克拉通南部边缘的 1,220 – 1,090 Ma Namaqua-Natal 造山运动期间与 Rodinia 超大陆形成相关的俯冲和碰撞事件同时发生。热年代学表明，整个 Kaapvaal 克拉通都受到了这次碰撞构造事件的影响，而且岩石圈应力场的变化似乎为深源金伯利岩和碳酸岩岩浆到达地球表面创造了通道。我们发现碰撞诱发的（例如 Premier）和大陆分裂相关的（例如金伯利）金伯利岩浆在成分上无法区分，推断板块构造过程仅有助于岩浆上升通道的形成，而对深部地幔没有重大影响在克拉通下融化。因此，与大陆破裂相关的金伯利岩喷发相比，碰撞带腹地的克拉通上金伯利岩岩浆活动不一定更可能夹带大型亚岩石圈钻石。