Circa 1 Ga sub-seafloor hydrothermal alteration imprinted on the Horoman peridotite massif

The residue of the mantle melting that produces mid-ocean ridge basalt is chemically modified by fluid–rock interaction at spreading ridge, and during seawater penetration along bending-related fault in plate approaching trench. If such a residual mantle material subducted deeply and resided for a long time within the deep Earth, it is a potential source of chemical heterogeneity in the mantle.

Modal abundances of the major and trace minerals, the concentrations of major and trace elements including H, B, Cl, F, and S, and isotopic compositions of H, Li, O, Sr, Nd, Hf, and Pb, of the Horoman peridotites suggest that the peridotites preserve the chemical signatures of sub-seafloor hydrothermal (SSH) alteration at a mid-ocean ridge approximately one billion years ago. These rocks have chemically evolved subsequent to this SSH alteration; however, they retain the SSH-associated enrichments in fluid mobile elements and water despite their long-term residence within the mantle. Evidence from the Horoman massif of the recycling of hydrous refractory domains into the mantle suggests that both the flux of water content into the mantle and the size of the mantle water reservoir are higher than have been estimated recently.

Figure 1: Trace element concentrations of the Horoman peridotites. Element concentrations are normalized to those of depleted MORB mantle, (DMM). Shaded fields show the ranges of element concentrations (±2σ deviations) of the DMM. (a) Massive peridotite. (b) Thin-layer peridotite.

Figure 2: Trace elements and Nd–Hf–Pb isotope compositions of the Horoman peridotites and global MORB. (a) Plot of (La/Sm)N vs 143Nd/144Nd. Element ratios are normalized to those of the depleted DMM. (b) Plot of 176Hf/177Hf vs 206Pb/204Pb. (c) Plot of 207Pb/204Pb vs 206Pb/204Pb.

Figure 3: Schematic cross-sections illustrating the evolution of the Horoman massif at different spatial scales of (a) upper mantle, (b) mid-ocean ridge, and (c) subduction zone. Symbols are the same as in Figure 2. The Horoman peridotites were formed as residues of partial melting of a MORB source at circa 1 Ga, and underwent the SSH alteration. The peridotites became part of lithospheric mantle, and recycled back into the mantle. With re-processing at different ridge systems, the peridotites interacted with partial melts of MORB mantle at circa 300 Ma. Since circa 150 Ma, the peridotites moved into the mantle wedge setting of the Hidaka subduction zone, and were metasomatized by a slab-derived fluid enriched in radiogenic Sr–Pb isotopes. Prior to unroofing at the surface, the Horoman massif was exhumed to crustal level, juxtaposed with the Hidaka metamorphic belt, and locally metasomatized by a slab-derived fluid at approximately 23 Ma.