Hadal aragonite records venting of stagnant paleoseawater in the hydrated forearc mantle

Ryosuke Oyanagi,Atsushi Okamoto,Katsuyoshi Michibayashi,Yumiko Harigane,Madhusoodhan Satish-Kumar,Masayo Minami

doi:10.1038/s43247-021-00317-1

Abstract

The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. Circulation of seawater in the mantle rock induces carbonate precipitation within the deep-sea forearc mantle, but the timescale and rates of the circulation are unclear. Here we investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400 m water depth in the Izu–Ogasawara Trench. On the basis of microtextures, strontium–carbon–oxygen isotope geochemistry, and radiocarbon analysis, we found that the aragonite is sourced from seawater that accumulated for more than 42,000 years. Aragonite precipitation is triggered by episodic rupture events that expel the accumulated fluids at 10−2–10−1 m s−1 and which continue for a few decades at most. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior.

Highlights

The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion
The chemical environments for inorganic aragonite precipitation are limited by the aragonite saturation depth (ASD), which is defined by the thermodynamic equilibrium between aragonite and seawater
This aragonite is from the hadal zone (>6000 mbsl), which is far deeper than the ASD (1000 mbsl)[27], aragonite compensation depth (ACD; 500–1500 mbsl)[32], and carbonate compensation depth (CCD; 4500 mbsl;[33] Fig. 1c), suggesting that the aragonite was formed by fluid–rock reactions

Summary

Introduction

The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. We investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400 m water depth in the Izu–Ogasawara Trench. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior. Aragonite precipitation can occur at depths greater than the ASD23,24,26,30,31, where it is induced by the interaction between ultramafic rocks and circulating seawater[13,23]. The hadal aragonite potentially records fluid flow and chemical reactions during the circulation of seawater within the shallowest parts of the forearc mantle, and its investigation should provide insights into the transport of carbon from the surface to Earth’s interior. We used radiocarbon dating to estimate fluid residence time in the hydrothermal system[34,35]

Methods

Results

Conclusion