Serpentinization parageneses controlled by lithological variations among olivine gabbro and peridotite from the Atlantis Massif, Mid-Atlantic Ridge

Serpentinization of olivine impacts the geochemical and ecological environment via molecular hydrogen production, promoted by iron oxidation to form magnetite and ferric iron in serpentine. Previous studies suggested that serpentinization proceeds with various reactions and the extent of iron oxidation is variable depending on the reaction pathways. To examine the effect of lithological variation on reactions at an incipient stage of serpentinization, we carried out microscopic observations, electron-probe analyses, and Raman spectroscopy of serpentine veins cutting olivine in gabbroic rocks and peridotites from the Atlantis Massif, Mid-Atlantic Ridge. The results indicate that the serpentine veins in the proximity of olivine dominantly comprise lizardite mixed with variable minor phases or components depending on igneous lithology: brucite in peridotites, troctolites, and primitive olivine gabbros; cronstedtite-bearing serpentine, commonly without brucite, in some troctolites and slightly evolved olivine gabbros; and neither brucite nor cronstedtite component in evolved olivine gabbros. This suggests that incipient-stage serpentinization paragenesis is dominantly controlled by silica activity reflecting lithological variation. In addition to magnetite, the formation of cronstedtite in gabbroic rocks has the potential to produce hydrogen. The presence or absence of brucite and cronstedtite could affect the timing and magnitude of iron oxidation and hydrogen production during serpentinization. Considering the predominance of gabbroic rocks in the lower oceanic crust, the variation in incipient-stage serpentinization paragenesis in gabbroic rocks may have an impact on the redox state and ecological environment around the (sub)seafloor.