Temporal evolution of primordial tungsten-182 and 3He/4He signatures in the Iceland mantle plume
Temporal evolution of primordial tungsten-182 and 3He/4He signatures in the Iceland mantle plume
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DOI：
10.1016/j.chemgeo.2019.07.026
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发表时间：
2019-10-20
期刊：
影响因子：
3.9
通讯作者：
Halldorsson, S. A.
中科院分区：
文献类型：
--
作者：
Mundl-Petermeier, A.;Walker, R. J.;Halldorsson, S. A.
对短寿命放射性成因同位素系统和羽状岩稀有气体同位素组成的研究表明，地球现今地幔中存在原始域。在大火成岩省和洋岛玄武岩显生宙岩石中发现的钨-182异常和高He-3/He-4表明，现代地幔柱挖掘的早期（太阳系历史的前60 Ma）地幔域保存完好。事实证明，很难将原始域的证据与最近过程（如再循环）的地球化学证据联系起来。格陵兰-冰岛烟柱系统始于古新世北大西洋火成岩省的喷发，后来在中新世中期到冰岛的现代火山产物中表现出来。在此，我们报告了在类似于格陵兰-冰岛羽流60 Ma历史的不同时期喷发的玄武岩样品的Pb同位素组成、mu W-182（样品的W-182/W-184与实验室参考标准的百分之几偏差）和He-3/He-4，以及高亲铁元素浓度和Re-Os同位素系统。格陵兰岛古新世样品代表地幔柱的早期阶段，其特征是He-3/He-4在7 ~ 48 R/R- a（实测He-3/He-4与大气比值归一化）之间变化，平均μ W-182为-4.0 +/- 3.6 (2SD)，现代上地幔样值为0 +/- 4.5。根据其Pb同位素组成，冰岛玄武岩可分为两类。其中一组以中新世玄武岩为主，Pb-206/Pb-204值介于18.4 ~ 18.5之间，He-3/He-4值介于17.8 ~ 40.2 R/R- a之间，mu W-182值介于+1.7 ~ -9.1 +/- 4.5之间。另一类以更新世和全新世玄武岩为主，具有较高的Pb-206/Pb-204值，范围为18.7 ~ 19.2，He-3/He-4值范围为7.9 ~ 25.7 R/R- a， mu W-182值范围为-0.6 ~ -11.7 +/- 4.5。总的来说，研究的格陵兰-冰岛套件需要在至少三个以不同的铅-氦-钨同位素组成为特征的地幔源域之间进行混合，以解释这一范围的同位素数据。随着系统的演化，这些岩石的同位素数据的时间变化似乎跟踪了主要的羽流成分。其中一个域与周围的上大洋地幔难以区分，并在整个时间进程中贡献了大量物质。另外两个域极有可能是经过少量脱气的原始储层。考虑到某些岩石的负W-182值，其中一个域可能形成于太阳系历史的前60 Ma，并且是最年轻玄武岩的主要贡献者。格陵兰-冰岛柱状岩的同位素特征揭示了源组分比例的幕式变化。
Studies of short-lived radiogenic isotope systems and noble gas isotopic compositions of plume-derived rocks suggest the existence of primordial domains in Earth's present-day mantle. Tungsten-182 anomalies together with high He-3/He-4 in Phanerozoic rocks from large igneous provinces and ocean island basalts demonstrate the preservation of early-formed (within the first 60 Ma of solar system history) mantle domains tapped by modern mantle plumes. It has proven difficult to link the evidence for primordial domains with geochemical evidence for more recent processes, such as recycling. The Greenland-Iceland plume system, starting with eruptions of the Paleocene North Atlantic Igneous Province, is later manifested in the mid-Miocene to modern volcanic products of Iceland. Here, we report Pb isotopic compositions, mu W-182 (deviations in W-182/W-184 of a sample from a laboratory reference standard in parts per million), and He-3/He-4, as well as highly siderophile element concentrations and Re-Os isotopic systematics of basaltic samples erupted at different times during the similar to 60 Ma history of the Greenland-Iceland plume. Paleocene samples from Greenland, representing the early stage of the mantle plume, are characterized by variable He-3/He-4 ranging from 7 to 48 R/R-A (measured He-3/He-4 normalized to the atmospheric ratio) and an average mu W-182 of -4.0 +/- 3.6 (2SD), within modern upper mantle-like values of 0 +/- 4.5. The basalts from Iceland can be divided into two groups based on their Pb isotope compositions. One group, consisting mostly of Miocene basalts, is characterized by Pb-206/Pb-204 ranging from similar to 18.4 to 18.5, He-3/He-4 ranging from 17.8 to 40.2 R/R-A, and mu W-182 values ranging from +1.7 to -9.1 +/- 4.5. The other group, consisting mainly of Pleistocene and Holocene basalts, is characterized by higher Pb-206/Pb-204, ranging from similar to 18.7 to 19.2, He-3/He-4 ranging from 7.9 to 25.7 R/R-A, and mu W-182 values ranging from -0.6 to -11.7 +/- 4.5. Collectively, the Greenland-Iceland suite examined requires mixing between a minimum of three mantle source domains characterized by distinct Pb-He-W isotopic compositions, in order to account for this range of isotopic data. The temporal changes in the isotopic data for these rocks appear to track the dominant contributing plume components as the system evolved. One of the domains is indistinguishable from the ambient upper oceanic mantle and contributed substantial material throughout the time progression. The other two domains are most likely primordial reservoirs that underwent limited de-gassing. Given the negative mu W-182 values in some rocks, one of these domains likely formed within the first 60 Ma of solar system history and is a major contributor to the youngest basalts. The isotopic characteristics of Greenland-Iceland plume-derived rocks reveal episodic changes in the source component proportions.