Ultra-depleted 2.05 Ga komatiites of Finnish Lapland: Products of grainy late accretion or core-mantle interaction?
Ultra-depleted 2.05 Ga komatiites of Finnish Lapland: Products of grainy late accretion or core-mantle interaction?
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芬兰拉普兰的超贫化 2.05 Ga 科马提岩：颗粒状晚期吸积或核幔相互作用的产物？
DOI：
10.1016/j.chemgeo.2020.119801
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发表时间：
2020
期刊：
影响因子：
3.9
通讯作者：
Walker, Richard J.
中科院分区：
文献类型：
--
作者：
Puchtel, Igor S.;Mundl-Petermeier, Andrea;Horan, Mary;Hanski, Eero J.;Blichert-Toft, Janne;Walker, Richard J.
本文报道了芬兰芬诺斯堪的纳维亚中部拉普兰绿岩带2.05 Ga Jeesiörova和Kevitsa科马提岩的铼-锇、铂单键锇、钐单键钕、Lusingle单键铪和铪单键钨同位素数据，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。这两个科马提岩在遗传学上密切相关，凯维察岩脉曾作为岩浆输送管道输送到Jeesiörova枕状和块状熔岩。据估计，父母科马提岩岩浆含有约25重量%的MgO，因此，来自地幔源至少热的一些晚太古代同行。一套Jeesiörova和Kevitsa的全岩科马提岩样品以及橄榄石和铬铁矿分离物定义了一个内部Resingle键Os等时线，年龄为2049 ± 13 Ma，初始γ 187 Os = −0.2 ± 0.2（2SE），表明地幔源中存在长期的热液作用Re/Os。相比之下，一组Jeesiörova铬铁矿分离物的Pt单键Os数据定义了平均初始μ 186 Os = +29 ± 2（2SE），表明地幔源中的超球粒陨石Pt/Os的长期历史。Jeesiörova-Kevitsa科马提岩系地幔源区的绝对HSE丰度估计为现今块状硅酸盐地球（BSE）的120 ± 5%。147 Sm单键143 Nd和176 Lusingle键176 Hf的等时线年龄和初始比值分别为2046 ± 22 Ma，ε 143 Nd = +3.7 ± 0.3，2072 ± 20 Ma，ε 176 Hf = +8.7 ± 0.4（2SE），表明Nd相对Sm、Hf相对Lu有长期亏损历史。测得的μ 182 W = +1.5 ± 3.3与现代地幔值没有区别。尽管强烈亏损的高度不相容的亲石微量元素，Th-Nb-La系统的科马提岩表明~1%的地壳污染的原始科马提岩岩浆，假设污染物是类似的成分计算Fennoscandian英云闪长岩平均（FTA）。这种程度的污染也会显着修改的Nd，Hf，和W同位素组成的原始科马提质岩浆，但不是Os同位素组成或HSE丰度。计算得到的原始科马提岩岩浆，经地壳混染校正后，其初始ε 143 Nd ~+4.9，ε 176 Hf ~+10.2，μ 182 W ~-10。我们的模拟表明，初始186，187 Os/188 Os同位素组成和超球粒陨石HSE丰度，加上预测的负μ 182 W，最好的解释是：（1）起源于地幔域，其特征是晚期增生的、分异的星子核金属过多，即，“粒状”晚期增生;（2）化学分馏的地球核金属加入科马提岩的地幔源区。Jeesiörova-Kevitsa科马提岩地幔源中这些特征的存在为地幔内化学多样性域的早期创建和长期生存提供了进一步的证据。
Rhenium-Os, Ptsingle bondOs, Smsingle bondNd, Lusingle bondHf, and Hfsingle bondW 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 Jeesiörova 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 Jeesiörova 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 Jeesiörova and Kevitsa whole-rock komatiite samples and olivine and chromite separates define an internal Resingle bondOs 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, Ptsingle bondOs data for a set of Jeesiörova 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 Jeesiörova-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.The147Smsingle bond143Nd and176Lusingle bond176Hf data yield isochron ages and initial ratios of, respectively, 2046 ± 22 Ma with ε143Nd = +3.7 ± 0.3, and 2072 ± 20 Ma with ε176Hf = +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 ε143Nd ~ +4.9, ε176Hf ~ +10.2, and μ182W ~ −10.Our modeling indicates that the initial186,187Os/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 Jeesiörova-Kevitsa komatiite mantle source provides further evidence for the early creation and long-term survival of chemically diverse domains within the mantle.
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