Abstract
Of the isotopically distinctive mantle domains, the so-called HIMU (“high-μ”; μ = 238U/204Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206Pb/204Pb (reaching 56) but unchanged 208Pb/204Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208Pb/204Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum.
Highlights
The inventory of U dissolved in seawater is orders of magnitude higher than that of Th and Pb (e.g., Chen et al, 1986)
The resulting Pb-isotope evolution over 1.9 Ga shows that a mantle reservoir metasomatised by altered oceanic crust (AOC)-derived supercritical slab fluids beyond subarc depth can yield a composition similar to the most radiogenic HIMU ocean island basalt (OIB) for both 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb (Fig. 3a,b)
AOC formed over 2.5 and 1.8 Ga age interval could have variable initial 207Pb/204Pb, which might be reflected in HIMU
Summary
The inventory of U dissolved in seawater is orders of magnitude higher than that of Th and Pb (e.g., Chen et al, 1986) After their emplacement along mid-ocean ridges, oceanic basalts and mantle rocks become enriched in U over Th and Pb, owing to interaction with seawater (e.g., Niu, 2004; Staudigel et al, 1995). Ocean floor serpentinites are arguably the best recorders of seawater-derived fluid alteration of igneous oceanic lithosphere This is because previous melt extraction leaves these rocks (originally representing depleted MORB source mantle; DMM) severely depleted in all incompatible elements, including U, Th, and Pb. Any later incompatible element addition from seawater during serpentinisation is readily quantified (Kodolányi et al, 2012; Niu, 2004; Paulick et al, 2006)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
