Abstract
Highly depleted peridotites from the Yap Trench in the western Pacific Ocean have been studied for Re-Os elements and Re-Os isotopes. These peridotites have a low Re-Os content and variable 187Os/188Os ratios (0.12043–0.14867). The highest 187Os/188Os ratio is far higher than that of the primitive upper mantle and the lowest 187Os/188Os ratio is comparable to the most unradiogenic 187Os/188Os ratio (0.11933) discovered in subduction zone peridotites. The suprachondritic 187Os/188Os ratios of the Yap Trench peridotites results from modification of the mantle wedge by slab-derived fluid and melt. This is consistent with the observation that high 187Os/188Os ratios generally occur in oceanic peridotites with low Os content (<2 ppb) since Os may be reduced during late processes such as fluid alteration and melt refertilization. The sub-chondritic 187Os/188Os ratios of the Yap Trench peridotites correspond to a Re depletion age of 0.24–1.16 billion years, which means that these peridotites represent old mantle residue of ancient melting events. This ancient melting, combined with probable back-arc melting and forearc melting during subduction initiation, indicates that the Yap Trench mantle has a complex evolutionary history. The amount of old mantle residue in the oceanic asthenosphere was underestimated because the 187Os/188Os ratio in mantle peridotites is elevated during late processes. Therefore, old depleted mantle fragments may contribute substantially to the chemical heterogeneity of the oceanic mantle.
Highlights
As 187 Re decays to 187 Os with a half-life of about 42 billion years (Ga) [1], the 187 Os/188 Os ratios of basaltic rock and mantle residue diverges
The highly depleted Yap Trench peridotites are rich in light rare earth element (LREE) and highly incompatible elements, implying that they were affected by late processes such as fluid alteration and melt refertilization
As the 187 Os/188 Os ratio will be increased during late processes, the lowest 187 Os/188 Os ratio in the Yap
Summary
As 187 Re decays to 187 Os with a half-life of about 42 billion years (Ga) [1], the 187 Os/188 Os ratios of basaltic rock and mantle residue diverges. The 187 Re–187 Os system in mantle-derived rock is widely used for detecting mantle melting and post-melting processes, which are essential for constraining the geochemical evolution of the oceanic mantle. Os behaves as a compatible element, whereas Re is a moderately incompatible element with a bulk partition coefficient similar to that of Al [2,3]. The residual peridotites will be depleted in Re and evolve to sub-chondritic 187 Os/188 Os. In contrast, Re-enriched basaltic rock will develop highly suprachondritic. Mid-ocean ridge basalts generally have higher 87 Os/188 Os ratios (0.125–0.23) relative to abyssal peridotites (0.12–0.13) [6].
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