Abstract
The chemical compositions of the residues of the mantle melting that produces mid-ocean ridge basalt can be altered by fluid–rock interactions at spreading ridges and, possibly, during seawater penetration along bending-related faults in plates approaching trenches. This chemically modified rock, if subducted deeply and after long-term residence within the deep Earth, is a potential source of chemical heterogeneity in the mantle. Here, we demonstrate that peridotites from the Horoman massif preserve the chemical signatures of sub-seafloor hydrothermal (SSH) alteration at a mid-ocean ridge approximately one billion years ago. These rocks have evolved chemically subsequent to this SSH alteration; however, they retain the SSH-associated enrichments in fluid mobile elements and H2O despite their long-term residence within the mantle. Our results indicate that ancient SSH alteration resulting in the production of sulfide leads to Pb enrichment that could affect the present-day Pb isotopic evolution of the silicate earth. Evidence from the Horoman massif of the recycling of hydrous refractory domains into the mantle suggests that both the flux of H2O content into the mantle and the size of the mantle H2O reservoir are higher than have been estimated recently.
Highlights
IntroductionMid-ocean ridges (MORs) mark divergent plate margins where new oceanic lithosphere is created, with basalt (mid-ocean ridge basalt; MORB) and complementary peridotite residue (depleted MORB Mantle, DMM1–3) resulting from partial melting within the upper mantle
Mid-ocean ridges (MORs) mark divergent plate margins where new oceanic lithosphere is created, with basalt and complementary peridotite residue resulting from partial melting within the upper mantle
We investigated the concentrations of H, B, Cl, F, and S, isotopic compositions of H, Li, O, and Sr, and modal abundances of the major and trace minerals of massive peridotite (MSP) and thin-layer peridotite (TLP) samples, which were examined in previous studies[17,18] for their major and trace element compositions and Nd–Hf–Pb isotope systematics (Supplementary Tables S1, S2 and S3)
Summary
Mid-ocean ridges (MORs) mark divergent plate margins where new oceanic lithosphere is created, with basalt (mid-ocean ridge basalt; MORB) and complementary peridotite residue (depleted MORB Mantle, DMM1–3) resulting from partial melting within the upper mantle. The Horoman massif, exposed at the southwestern end of the Cenozoic Hidaka metamorphic belt in northern Japan (Supplementary Fig. S1), is a well-preserved Alpine-type peridotite massif containing fertile plagioclase-lherzolite and refractory spinel-lherzolite to highly depleted spinel-harzburgite with subordinate dunite, pyroxenite, and gabbro[12,13,14,15,16] In terms of their field occurrence, the peridotites of the massif are classified as massive peridotite (MSP) and thin-layer peridotite (TLP); the former occurring as massive rock with thickness >1 m and the latter interlayered with gabbro at millimeter to centimeter scales[17,18]. We investigated the concentrations of H, B, Cl, F, and S, isotopic compositions of H, Li, O, and Sr, and modal abundances of the major and trace minerals of MSP and TLP samples, which were examined in previous studies[17,18] for their major and trace element compositions and Nd–Hf–Pb isotope systematics (Supplementary Tables S1, S2 and S3)
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