Turner Simon, Tonarini Sonia, Bindeman Ilya, Leeman William P, Schaefer Bruce F
GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales 2109, Australia.
Nature. 2007 Jun 7;447(7145):702-5. doi: 10.1038/nature05898.
Evidence for the deep recycling of surficial materials through the Earth's mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent-daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that delta(18)O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in delta(11)B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth's surface. Moreover, delta(11)B is negatively correlated with (187)Os/(188)Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, (11)B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from approximately 3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands approximately 3 Gyr later.
长期以来，人们一直在寻找有关表层物质通过地球地幔进行深度再循环及其古老性的证据，以了解俯冲板块和地幔柱在地幔对流中的作用。由于母子体分馏的年龄和位置未知，这种再循环的放射性同位素证据仍然模棱两可。相反，虽然稳定同位素可以为低温分馏提供无可辩驳的证据，但它们在大多数未改变的大洋玄武岩中的范围有限，且任何变化的年龄都没有限制。在这里，我们表明亚速尔群岛玄武岩中的δ(18)O比率通常低于原始地幔中的比率。这与增加的Nb/B比率和较大范围的δ(11)B比率相结合，为在地球表面附近经历过分馏的物质的再循环提供了令人信服的证据。此外，δ(11)B与(187)Os/(188)Os比率呈负相关，该比率延伸至亚球粒陨石值，将高Nb/B、富含(11)B的端元的年龄限制为超过25亿年（Gyr）。我们推断该组分是来自俯冲大洋板块的熔体和流体亏损的岩石圈地幔，而亚速尔群岛的其他玄武岩含有约3 Gyr老的熔体富集玄武岩的贡献。我们得出结论，这两种组分很可能都来自一个太古宙大洋板块，该板块被俯冲，可以说是俯冲到深部地幔中，在那里它被储存起来，直到热浮力导致它在大约3 Gyr后在亚速尔群岛下方上升。
