Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly
Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly
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DOI：
10.1016/j.epsl.2009.06.013
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发表时间：
2009-08
影响因子：
5.3
通讯作者：
B. O’Driscoll;J. Day;J. Daly;R. J. Walker;W. McDonough
中科院分区：
文献类型：
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作者：
B. O’Driscoll;J. Day;J. Daly;R. J. Walker;W. McDonough
朗姆层状套是一个层状镁铁质-超镁铁质岩体，侵位于古近纪北大西洋边缘裂谷期。它是一个典型的开放系统岩浆房，由16个重复耦合的橄榄岩-橄榄岩单元构成，其中一些在其底部有横向延伸的约2 mm厚的铂族元素（PGE）富集（约2 µg g−1）的铬尖晶石层。为了研究铬尖晶石煤层岩石成因和富集的PGE，这些元素和Re-Os同位素的丰度已被确定在三个地层水平的朗姆酒层状套房，代表主要的岩浆补给事件。个别单位保留一系列的初始187 Os/188 Os的比例，证明了在补充岩浆的成分的异质性。铬尖晶石煤层和上覆硅酸盐的数据表明，形成铬尖晶石的过程也集中了铂族元素，岩浆补充。没有证据表明铬尖晶石中存在结构结合的PGE。相反，朗姆酒层状套件的铂族元素预算与贱金属硫化物，特别是镍黄铁矿，以及与铬尖晶石接缝内所含的铂族元素合金有关，但其作为单独的相存在于铬尖晶石晶界处。朗姆层状岩套中初始Os同位素组成的范围（γOs=3.4 ~ 36）可以通过5-8%的刘易斯片麻岩同化作用以及与硫化物去除有关的补充岩浆中PGE含量的变化来成功模拟。初始187 Os/188 Os比值的朗姆酒岩石范围从0.1305到0.1349，是非典型的对流上地幔，但最近爆发的苦橄岩和玄武岩从冰岛和古近纪苦橄岩和玄武岩巴芬岛，格陵兰岛和苏格兰的范围内。因此，锇同位素数据表明，北大西洋火成岩省岩浆集体产生的地幔源的组件，保持相对不变的锇同位素组成在过去的60马，并可能包含回收的岩石圈成分。
The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~2 mm-thick platinum-group element (PGE) enriched (~2 µg g−1) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial187Os/188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions (γOs=3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial187Os/188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.