Abstract Rhenium-Os, Pt Os, Sm Nd, Lu Hf, and Hf W isotope data, together with lithophile trace element and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances, are reported for 2.05 Ga Jeesiorova and Kevitsa komatiites from the Central Lapland Greenstone Belt, Fennoscandia, Finland. Both komatiites are closely genetically related, with the Kevitsa dikes having served as feeding magma conduits to the Jeesiorova pillowed and massive lavas. The parental komatiite magma is estimated to have contained ~25 wt% MgO and was, thus, derived from a mantle source at least as hot as those of some of its late Archean counterparts. A suite of Jeesiorova and Kevitsa whole-rock komatiite samples and olivine and chromite separates define an internal Re Os isochron with an age of 2049 ± 13 Ma and an initial γ187Os = −0.2 ± 0.2 (2SE), indicating long-term chondritic Re/Os in the mantle source. By contrast, Pt Os data for a set of Jeesiorova chromite separates define an average initial μ186Os = +29 ± 2 (2SE), indicating a long-term history of suprachondritic Pt/Os in the mantle source. The absolute HSE abundances in the mantle source of the Jeesiorova-Kevitsa komatiite system are estimated to have been 120 ± 5% of the present-day Bulk Silicate Earth (BSE). This is the first komatiite system for which excess HSE in the mantle source, relative to modern BSE, has been documented. The 147Sm 143Nd and 176Lu 176Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with e143Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with e176Hf = +8.7 ± 0.4 (2SE), indicating a long-term history of depletions of Nd relative to Sm, and Hf relative to Lu. The measured μ182W = +1.5 ± 3.3 is indistinguishable from the modern mantle value. Despite being strongly depleted in highly incompatible lithophile trace elements, the Th-Nb-La systematics of the komatiites indicate ~1% crustal contamination of the original komatiite magma, assuming the contaminant was similar in composition to the calculated Fennoscandian Tonalite Average (FTA). This level of contamination would have also significantly modified the Nd, Hf, and W isotope compositions of the original komatiitic magma, but not the Os isotope compositions or HSE abundances. The calculated original komatiite magma, corrected for the effects of crustal contamination, would have had initial e143Nd ~ +4.9, e176Hf ~ +10.2, and μ182W ~ −10. Our modeling indicates that the initial 186,187Os/188Os isotopic compositions and suprachondritic HSE abundances, coupled with the projected negative μ182W, are best explained by either (1) derivation from a mantle domain characterized by an excess of late accreted, differentiated planetesimal core metal, i.e., “grainy” late accretion, or (2) addition of chemically fractionated terrestrial core metal to the mantle source domain of the komatiites. The presence of these characteristics in the Jeesiorova-Kevitsa komatiite mantle source provides further evidence for the early creation and long-term survival of chemically diverse domains within the mantle.

中文翻译：

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芬兰拉普兰超耗竭的 2.05 Ga 科马提岩：颗粒状晚期增生或核-地幔相互作用的产物？

摘要 铼-Os、Pt Os、Sm Nd、Lu Hf 和 Hf W 同位素数据，以及亲石微量元素和高度亲铁元素（HSE：Re、Os、Ir、Ru、Pt 和 Pd）丰度报告为2.05 Ga Jeesiorova 和 Kevitsa komatiites 来自芬兰芬诺斯坎迪亚中央拉普兰绿岩带。两种科马提岩在遗传上密切相关，凯维察岩脉曾作为输送岩浆管道到 Jeesiorova 枕状和巨大熔岩。据估计，母体科马提岩浆含有约 25%（重量）的 MgO，因此，它来自地幔源，至少与其一些晚太古代对应物的地幔源一样热。一套 Jeesiorova 和 Kevitsa 全岩科马提岩样品以及橄榄石和铬铁矿分离物定义了内部 Re Os 等时线，年龄为 2049 ± 13 Ma，初始 γ187Os = -0.2 ± 0.2 (2SE)，表明地幔源中的长期球粒状 Re/Os。相比之下，一组 Jeesiorova 铬铁矿分离物的 Pt Os 数据定义了平均初始 μ186Os = +29 ± 2 (2SE)，表明地幔源中球粒上 Pt/Os 的长期历史。Jeesiorova-Kevitsa komatiite 系统地幔源中的绝对 HSE 丰度估计为当今大块硅酸盐地球 (BSE) 的 120 ± 5%。相对于现代 BSE，这是第一个在地幔源中存在过量 HSE 的科马提岩系统。147Sm 143Nd 和 176Lu 176Hf 数据产生的等时线年龄和初始比率分别为 2046 ± 22 Ma，e143Nd = +3.7 ± 0.3，以及 2072 ± 20 Ma，e176Hf = +8.7 ± 0.4，表明长期（2SE） Nd 相对于 Sm 的消耗历史，以及 Hf 相对于 Lu 的消耗历史。测得的 μ182W = +1.5 ± 3。3 与现代地幔值无法区分。尽管在高度不相容的亲石微量元素中强烈消耗，科马提岩的 Th-Nb-La 系统学表明原始科马提岩浆的地壳污染约为 1%，假设污染物的成分与计算的 Fennoscandian Tonalite 平均值 (FTA) 相似。这种污染水平也会显着改变原始科马提质岩浆的 Nd、Hf 和 W 同位素组成，但不会显着改变 Os 同位素组成或 HSE 丰度。计算出的原始科马提岩浆，校正了地壳污染的影响，初始 e143Nd ~ +4.9、e176Hf ~ +10.2 和 μ182W ~ -10。我们的模型表明，最初的 186,187Os/188Os 同位素组成和球粒上 HSE 丰度，加上预计的负 μ182W，最好的解释是 (1) 来自地幔域的推导，其特征是晚期吸积、分化的微行星核心金属过量，即“粒状”晚期吸积，或 (2) 添加化学分馏的地球科马提岩地幔源域的核心金属。Jeesiorova-Kevitsa 科马提岩地幔源中这些特征的存在为地幔内化学多样化区域的早期创造和长期存在提供了进一步的证据。