Serpentinization of olivine in troctolites and olivine gabbros from the Hess Deep Rift

To understand the similarity and diversity of serpentinization processes in different rock systems, gabbroic rocks recovered from IODP Site U1415 at the Hess Deep Rift were examined and compared with peridotites from the adjacent ODP Site 895. Textural observations, micro-Raman spectroscopic analyses and electron microprobe analyses indicated that most of the olivine-replacing serpentine in the gabbroic rocks lack the mixing with brucite, which is common in peridotites. At least three stages of serpentinization are observable in the gabbroic rocks; each generation is characterized by different submicroscopic mixtures or solid solutions of sheet silicates: 1) Mg-Fe^2 + lizardite + ferri-lizardite + chlorite, 2) Mg-Fe^2 + lizardite + ferri-lizardite, and 3) Mg-Fe^2 + lizardite + ferri-lizardite + saponite. The first and third generations of serpentine and mixed minerals are relatively Fe-rich, whereas the second generation is Fe-poor and associated with abundant magnetite and pyrrhotite. The major difference between the alteration of gabbroic and peridotitic systems is probably best explained by the iron content and modal abundance of primary olivine and by rock-dominated fluid compositions with a high silica activity due to the alteration of plagioclase in gabbroic rocks. The mineralogical variations between the reported three generations of mixed sheet silicates in gabbroic rocks can be ascribed to variations of silica and/or oxygen activities in the associated fluids under decreasing temperature conditions. The abrupt increase of magnetite crystallization during serpentinization in gabbroic rocks could be caused by oxidation at a relatively high SiO₂ activity without the olivine-serpentine-brucite buffering assemblage.