Six photomicrographs
Transformation of ultramafic mantle minerals (Ol – olivine; Opx – orthopyroxene; Spl – spinel; Chr – chromite; Cpx –clinopyroxene) to serpentine minerals (Srp) on the microscopic scale (the 500 μm bar corresponds to 1/2 millimeter). Fresh harzburgite – a mantle rock composed of olivine and orthopyroxene - begins to transform as serpentine minerals form along the olivine grain boundaries (b) which then gradually grow into a thickening mesh of serpentine-rich veins (c + d). Serpentinization of chromite and clinopyroxene-bearing mantle rocks are shown in (e) and (f). The olivine fluid inclusions, which contain the even more minuscule kuliginite crystals, measure only 1-30 micrometers in diameter and are invisible on this scale. Credit: Zhang et al. [2022], Figure 1
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

Serpentinization occurs when water infiltrates the ultramafic rocks of the Earth’s mantle and transforms the magnesium- and iron-rich mantle minerals olivine and orthopyroxene into a suite of secondary, hydrous Mg- and Fe- bearing minerals dominated by serpentine. The importance of serpentinization, which has been active on Earth for over 4 billion years, stems from its by-products hydrogen (H2) and methane (CH4). H2 is produced by water-driven oxidation of Fe2+ contained in the mantle minerals; the H2 then reacts with carbon dioxide (CO2) to CH4. The more Fe2+ is available, the more efficient the H2 production.

Zhang et al. [2022] report the first discovery of minuscule crystals of the Fe-rich hydroxychloride kuliginite [Fe3Mg(OH)6Cl2] in olivine-hosted secondary fluid inclusions of serpentinized mantle rocks that were exhumed in New Caledonia. Kulignite forms during serpentinization and binds a large amount of Fe2+, which stalls the H2 production. On the other hand, the later oxidative destruction of kuliginite can release the captured Fe2+ for a second stage generation of H2.

While kuliginite forms only in presence of moderately hot saline fluids, this condition is not extreme and environments conducive to its formation exist on Earth. Thus, if reliably identified at the sub-micron scale, kuliginite may emerge as a relatively common product of serpentinization in saline environments, which could profoundly change the understanding of the processes that support abiotic life and produce energy on Earth and on other water-rich rocky celestial bodies.

Citation: Zhang, L., Wang, Q., Mikhailenko, D. S., Ding, X., Li, W.-C., & Xian, H. (2022). Hydroxychloride-bearing fluid inclusions in ultramafic rocks from New Caledonia: Implications for serpentinization in saline environments on Earth and beyond. Journal of Geophysical Research: Solid Earth, 127, e2022JB024508.

—Susanne Straub, Associate Editor, Journal of Geophysical Research: Solid Earth

Text © 2022. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.