Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project

Abstract

Serpentinization plays a key role on the evolution of the physicochemical properties of the mantle lithosphere. The rate of serpentinization reactions idiscontinuities in the platelet separating ?1‐?m‐long segments of clinopyroxene. The presence of traces of magnetite in the discontinuities and their orientation in parallel to other magnetite grains suggest that the discontinuities were formerly filled with magnetite, which reacted during serpentinizations controlled by the transport of fluid, which itself depends on volume change during reaction. Element transfer can strongly modify the magnitude and sign of volume change. Here, we measure solid volume change and element transport perpendicular to a serpentine vein in a serpentinized dunite collected at depth during the Oman Drilling Project. The sample is extensively replaced (extent of reaction > 80 %) by a serpentine/brucite mixture parallel to a main serpentine vein network. The Mg content of serpentine and brucite indicates reaction with a small amount of fluid at temperatures below 100 °C. Concentrations of fluid‐mobile trace elements (Na, Ca, Sr, Rb, and Ba) decrease perpendicular to the main vein. Primary olivine contains parallel platelets of a clinopyroxene/magnetite symplectite. Tomography at the nanoscale reveals that these inclusions do not react during serpentinization but are cracked and displaced. We use these inert markers to measure a 59 % to 74 % positive volume change that is close to the 52 % expected for reaction in a closed system. Chemical data indicate no change in major element composition during reaction except for the addition of water. The initial olivine zoning in Al, Ti, V, Sc, and Cr is still preserved in serpentine and brucite. Serpentinization can thus be a local replacement process during which the solid volume homogeneously increases at the micrometer scale and the transport of aqueous species is limited.

Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration

“…The major element composition indicates that there has not been significant mass transfer during reaction, except for the addition of H2O. This is confirmed by profiles in trace elements, showing that the pre-serpentinization zoning in Al, Ti, V, Sc and Cr of olivine is preserved during the reaction (Malvoisin et al, 2020). The sample thus recorded isochemical serpentinisation with an associated solid volume increase corresponding to the value expected for reaction in a closed system.…”

“…At several micrometres to several tens of micrometres from the olivine surface, bright Fe-rich brucite appears to segregate in several micrometre wide zones with diffuse margins and in association with magnetite (Figures 1 A and C). Electron microprobe analyses always correspond to a mixture of serpentine and brucite, revealing that brucite segregation is never complete at the micrometre scale (Malvoisin et al, 2020). Near the olivine surface, the serpentine/brucite mixture is more homogeneous in colour suggesting a more intimate mixing.…”

“…The studied sample is dyke free. Malvoisin et al (2020) provide a full mineralogical and chemical description of the sample; a brief summary is given below.…”

“…This indicates that these inclusions have an orientation relationship with olivine acquired during a pre-serpentinization event, probably during symplectic exsolution in olivine at high temperature (Moseley, 1984). Malvoisin et al (2020) measured the deformation of these inclusions during serpentinisation to estimate a change in volume during reaction close to 50 %. The major element composition indicates that there has not been significant mass transfer during reaction, except for the addition of H2O.…”

“…Coleman and Keith, 1971) suggesting that either hydration occurred with very little export of dissolved components with different solubilities, or that there was large-scale, congruent dissolution of silicate minerals. Recent work on microstructures indicates that serpentinisation in experiments (Klein and Le Roux, 2020) and in natural rocks (Malvoisin et al, 2020) is accompanied by large increases in solid volume (30 to 110 percent). Because serpentinisation and carbonation rapidly add mass to altered peridotite as a result of limited solubility of Mg, Si, Fe and Ca in aqueous fluids, thermodynamic models indicate that hydration and carbonation produce increases of 40 to 60% in solid mass and volume (Malvoisin, 2015;Kelemen, 2018, 2020;Kelemen, 2021aKelemen, , 2021b.…”