Oceanic and Continental Lithospheric Mantle in the 1.95-Ga Jormua Ophiolite Complex, Finland: Implications for Mantle and Crustal Evolution
Oceanic and Continental Lithospheric Mantle in the 1.95-Ga Jormua Ophiolite Complex, Finland: Implications for Mantle and Crustal Evolution
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芬兰 1.95 Ga Jormua 蛇绿岩杂岩中的海洋和大陆岩石圈地幔：对地幔和地壳演化的影响
DOI：
10.1093/petrology/egad080
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发表时间：
2023
影响因子：
3.9
通讯作者：
Walker, Richard J
中科院分区：
文献类型：
--
作者：
Finlayson, Valerie A;Haller, Mitchell;Day, James M;Ginley, Stephen;O'Driscoll, Brian;Kontinen, Asko;Hanski, Eero;Walker, Richard J
芬兰约1.95 ga Jormua蛇绿岩杂岩（JOC）是一种罕见的古元古代蛇绿岩，保存了多种上地幔物质和熔融过程的记录。四个JOC露头的米尺度网格采样，以及非网格采样，可以评估JOC内米到公里尺度的地幔非均质性。在四个网格之间以及在一些非网格样本中观察到显著的异质性。在不同的样品和地点之间，流体可移动元素的浓度变化特别大。蛇纹岩的新的全岩主要、亲石微量和高亲铁元素数据（HSE: Os、Ir、Ru、Pt、Pd、Re），包括187re - 187os同位素数据，表明JOC内存在两种不同的成分类型和可能的起源模式。这与先前的研究结果一致。类型1类似于现代难熔深渊型地幔。2型比1型难熔性更强，很可能是来自次大陆岩石圈地幔（SCLM）的样品。在米、公里尺度上，1型地幔的主微量元素和Os同位素组成均呈中等非均质性。相比之下，2型地幔在米尺度上比1型网格均匀得多，但在千米尺度上则更不均匀。1.95 Ga时，1型地幔初始γ - os值的中位数为~−2.0（其中γ - os为187os /188Os相对于特定时间的球粒体参考值的偏差%）。这一同位素组成与原始地幔相对于JOC形成之前的Re/Os的长期适度下降是一致的。该γ - Os值与现代深海地幔的γ - Os值以及若干显生宙蛇绿岩的初始值的相似性表明，上地幔在约2ga时达到了与球粒岩参考值相似的Re/Os比值，此后沿近平行轨迹向球粒岩参考值演化。要发生这种情况，至少从JOC形成以来，只有有限的Re可能从上地幔永久移除。约2 Ga的局部次生交代事件，加上JOC的估计逆冲年龄和随后的斯韦芬尼期造山运动，影响了一些1型样品的HSE系统。相比之下，2型岩石的晚太古宙o期trd模式年龄表明，深海地幔的长期Re枯竭叠加了一个枯竭事件。这一事件的确定时间不晚于~2.6 Ga，可能发生在~2.7 Ga时卡累利阿克拉通的重要地壳生产时期。
The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRDmodel ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.