Stable Chromium Isotope Fractionation During the Alteration of Abyssal Peridotite: Implications for Marine Chromium Isotope Mass Balance

The stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater-peridotite reaction on the global marine Cr isotope mass balance, we report high-precision Cr isotope data (delta 53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a delta 53Cr value of -0.08 +/- 0.06 parts per thousand (2SD, n = 4) for the oceanic mantle which is consistent with the established delta 53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive delta 53Cr values (up to 0.04 parts per thousand). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of similar to 3.5 x 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108-109 mol/yr. Furthermore, the MC results suggest that the peridotite-sourced net Cr flux has a negative delta 53Cr signature (-0.33 +/- 0.21 parts per thousand, 2SD). Thus, seawater-peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance. The stable chromium isotope system is an emerging paleoredox proxy for tracing Earth's atmospheric oxygenation over geological timescales. However, accurately inferring past Earth's atmospheric oxygenation levels using chromium isotopes requires a more precise understanding of the elemental and isotopic cycling of chromium, particularly within the oceans. By studying abyssal peridotites which are rocks found beneath the oceanic crust, we find that specific types of alterations in these rocks, particularly the process of serpentinization, have an impact on the chromium isotopes. These findings provide insights into the mechanisms involved in transporting chromium from the deep Earth mantle to the oceans. We discover that these altered rocks release a flux of isotopically light chromium into the ocean, which is comparable to the amount coming from rivers. Our study sheds new light on the complex interactions between seawater and abyssal peridotites, which potentially exert a profound impact on the global marine Cr isotope mass balance. The first data set of stable Cr isotopic compositions for abyssal peridotites Seafloor weathering has insignificant impact on the Cr isotopic compositions of abyssal peridotites Seafloor alteration exerts a key control on the Cr isotopic composition of seawater