.png){kind=icon}
.png){kind=icon}
Return
Magnetite production in mesh texture during serpentinization, a marker of H2 diffusion
.png){kind=icon}
PRE
AI
B
P
A
F
M
H
M
DOI：10.1093/petrology/egag021.png)
.png){kind=icon}
Original request for help.png){kind=icon}
Share.png){kind=icon}
Save
Abstract
En 中文
The detailed mineralogical study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies shows the formation of two alteration zones around olivine grains. At the olivine contact, a fine-grained mixture of serpentine, Fe-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.66 and 0.82) and awaruite occurs (Reaction Zone 1). X-ray mapping indicates limited mass transfer during Reaction Zone 1 formation, suggesting isochemical serpentinization except for the addition of water. The measured distribution of Fe and Mg between serpentine and brucite in Reaction Zone 1 is well reproduced with thermodynamic modelling incorporating the latest data for the Fe$\left ( OH ight )_{2}$ endmember. Thermodynamic modelling also reveals that, at low water to rock ratio, awaruite formation limits H$_{2}$ production to values more than one order of magnitude lower than previous estimates. The predicted H$_{2,aq}$ concentrations are comprised between 10$<^>{-3}$ and 10$<^>{-2}$ mol/kg, that is in the same range than the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At a water to rock ratio of 1, the updated thermodynamic model only predicts magnetite formation after olivine at temperatures above 300 $<^>{\circ }$C, that is above the temperature estimates for serpentinization based on published oxygen isotope data. Nevertheless, a second assemblage composed of Ni-bearing magnetite, serpentine and Mg-brucite ($\frac{Mg}{Mg + Fe}$ molar ratio comprised between 0.83 and 0.98) can be found at the mesh rim (Reaction Zone 2). Reaction Zone 2 can display a symplectite microtexture. Transmission electron microscopy reveals the presence of porosity and Fe-brucite relics in the symplectite, suggesting Reaction Zone 2 formation after Reaction Zone 1 by dissolution-precipitation. Significant mass transfer is observed in Reaction Zone 2 at the mesh rim, probably associated with the solid volume decrease of 7$\%$ during Reaction Zone 1 reaction to form Reaction Zone 2. The mineralogy and the composition of Reaction Zone 2 are reproduced with thermodynamic modelling of olivine serpentinization not by increasing the water to rock ratio but rather by removing H$_{2}$ from the system. This indicates that H$_{2}$ diffusion is the main driver for magnetite formation during serpentinization. The H$_{2,aq}$ concentrations at the equilibrium with Reaction Zone 2 fall in the 10$<^>{-7}$-10$<^>{-3}$ mol/kg range. The relative proportion of Reaction Zone 1 and Reaction Zone 2 in serpentinized peridotites has first-order impacts on H$_{2}$ production.
Keywords:
hydrogen
awaruite
RedOx
symplectite
thermodynamics
Journal
IF:
3.5
Papers:
2.8K
Citations:
1.7W
Organization
Cited Papers