Laboratory Constraints on the Volume Increase of Serpentinization

Serpentinization of peridotite leads to a decrease in density of ~21%. However, whether the decrease in density is due to an increase in rock volume or export of mass has remained a matter of debate for more than a century. Because serpentinized peridotite is widespread at mid-ocean ridges, continental margins and subduction zones, the implications of volume changes and element mobility are of global geological importance. This study integrates results from time-series hydrothermal laboratory experiments and high-resolution 3-D x-ray micro-computed tomography (μ-CT) to provide quantitative constraints on volume changes and mass transfer during serpentinization. Cores of unaltered natural dunite and harzburgite were scanned using μ-CT before and after reacting them with artificial seawater at 300 °C and 35 MPa in gold capsules for 5, 10, and 18 months. Measured fluid compositions indicate Mg uptake, Ca loss, and conservation of Si and Fe. The volume changes, as calculated from 3-D displacements of inert internal markers, increased with systematically time, extent of reaction, and fluid composition. We propose that the volume increase involves a substantial increase in buoyancy force and water carrying capacity when compared with isovolumetric serpentinization, which has key implications for subduction zone processes. The volume increase led to fracturing, which created new fluid pathways, and allowed serpentinization to progress. Chemical exchange reactions between serpentinizing peridotite and reacting fluid can influence seawater composition, most notably Mg/Ca ratios. These findings have strong implications for the formation of serpentinites and their interactions with aqueous fluids in a wide range of major geodynamic settings on and possibly beyond Earth.