High-precision Pb and Hf isotope and highly siderophile element abundance systematics of high-MgO Icelandic lavas
High-precision Pb and Hf isotope and highly siderophile element abundance systematics of high-MgO Icelandic lavas
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DOI：
10.1016/j.chemgeo.2021.120436
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发表时间：
2021-11
期刊：
影响因子：
3.9
通讯作者：
R. Nicklas;A. Brandon;T. Waight;I. Puchtel;J. Day
中科院分区：
文献类型：
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作者：
R. Nicklas;A. Brandon;T. Waight;I. Puchtel;J. Day
洋岛玄武岩（OIB） 87sr /86Sr、143Nd/144Nd、176Hf/177Hf、186187os /188Os、206207pb /204Pb组成差异显著，表明其成因有多种地幔源。为了进一步限制OIB中不同长寿命同位素系统之间的关系，新的双峰Pb和Hf同位素数据，以及同位素稀释高亲铁元素（HSE: Os, Ir, Ru, Pt， Pd和Re）丰度，报告了19种冰岛硬橄岩和玄武岩，其中3he /4He，87Sr/86Sr，143Nd/144Nd和186,187os /188Os数据之前已报道（Brandon et al., 2007.GCA）。样品组的铪- nd - he同位素系统表明存在低3he /4He的古难熔源，而He-Pb同位素相关性表明冰岛羽流中存在从脱气较少的地幔源添加的高3he /4He的再循环海洋地壳成分。在北部和西部火山区取样了高度枯竭的低3he /4He特征，可能是由于这些地区的部分熔融程度较高，使得这种难熔端元的取样成为可能。187os /188Os比值与Pb、Hf和Nd同位素比值相关，表明贫源和富源之间存在二元混合，没有硫化物交代导致的解耦的证据。187os /188Os-206,207,208Pb/204Pb数据的扩散可以用1-11%的年轻再循环海洋地壳添加到与大西洋MORB源不同的枯竭地幔源来解释。北部和西部火山岩中的HSE丰度与其他MgO含量相似的OIB中的HSE丰度重叠，呈现出富集更不相容的Re、Pd和Pt而不是相容的Ru、Ir和Os的分选模式。这些熔岩的母体熔体HSE丰度与其他OIB的计算结果也有重叠。东部火山带的熔岩相对于其他冰岛熔岩来说，铂和钯的含量是高度枯竭的。冰岛熔岩中Pt和Pd丰度与同位素参数（如206pb /204Pb）相关，表明在东部火山岩带采样的同位素富集端元在HSE中被耗尽，与基性相一致。岩浆中MgO的丰度也与同位素参数大致相关，支持贫端为超基性，富端为基性的特征。新的和先前发表的双尖峰Pb同位素数据显示，在火山Öræfajökull附近主要存在二元混合和附加成分。冰岛207pb /204Pb和208pb /204Pb数据分别位于大部分中大西洋脊玄武岩数据的下方和上方，表明羽流中贫端和富端都比周围地幔具有更低的U衰竭年龄和更高的Th/U。这与冰岛地幔柱内富集端元和亏缺端分别为<2 Ga再循环洋壳和岩石圈地幔相一致。
Ocean island basalts (OIB) exhibit significant variations in their87Sr/86Sr,143Nd/144Nd,176Hf/177Hf,186,187Os/188Os, and206,207,208Pb/204Pb compositions that indicate contributions from multiple mantle sources. To further constrain the relationships between diverse long-lived isotopic systems in OIB, new double-spike Pb and Hf isotope data, as well as isotope dilution highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances are reported for nineteen Icelandic picrites and basalts for which3He/4He,87Sr/86Sr,143Nd/144Nd, and186,187Os/188Os data have been reported previously (Brandon et al., 2007.GCA). Hafnium-Nd-He isotope systematics for the sample set reveal the presence of an ancient refractory source with low-3He/4He, while He-Pb isotope correlations indicate that recycled oceanic crustal components with high-3He/4He added from a less degassed mantle source are present in the Iceland plume. The highly depleted, low-3He/4He signature is sampled in the Northern and Western Volcanic Zones, likely due to higher degrees of partial melting at those localities enabling sampling of this refractory endmember. The187Os/188Os ratios show correlations with Pb, Hf, and Nd isotope ratios indicating binary mixing between depleted and enriched sources, with no evidence for decoupling due to sulfide metasomatism. The spread of187Os/188Os-206,207,208Pb/204Pb data can be explained by addition of 1–11% young recycled oceanic crust to a depleted mantle source distinct from the source of Atlantic MORB. Abundances of the HSE in Northern and Western Volcanic Zone lavas overlap with those from other OIB with similar MgO contents, showing fractionated patterns that are enriched in the more incompatible Re, Pd, and Pt over the compatible Ru, Ir, and Os. Calculated parental melt HSE abundances of those lavas also overlap with those of other OIB. Eastern Volcanic Zone lavas are highly depleted in Pt and Pd relative to other Icelandic lavas. Abundances of Pt and Pd correlate with isotopic parameters, such as206Pb/204Pb, in Icelandic lavas, indicating that the isotopically enriched endmember sampled in the Eastern Volcanic Zone is depleted in the HSE, consistent with it being mafic in nature. The abundances of MgO in the lavas also roughly correlate with isotopic parameters, supporting the identity of the depleted endmember as ultramafic and the enriched endmember as mafic. New and previously published double-spike Pb isotope data show chiefly binary mixing with an additional component present in the vicinity of the volcano Öræfajökull. Icelandic207Pb/204Pb and208Pb/204Pb data plot below and above the majority of Mid-Atlantic Ridge basalt data, respectively, indicating that both depleted and enriched end-members in the plume have a lower U depletion age and a higher Th/U than ambient mantle. This is consistent with the enriched and depleted end-members within the Iceland plume being <2 Ga recycled oceanic crust and lithospheric mantle, respectively.