Importance of Silica Activity to the Serpentinization Processes: Insights From Microrodingites in IODP Hole U1309D.

Observation of small-scale alteration of olivine-rich troctolites from IODP hole U1309D has given us fresh insights into the processes of sea-floor serpentinization. Fresh troctolites are locally cut by mm-scale hydration veins. Where these veins cut olivine they contain serpentine, but where they cut plagioclase they contain prehnite +/- hydrogrossular. Chlorite always lies between the relict olivine and relict plagioclase. The alteration of plagioclase to prehnite + hydrogrossular is common in rodingites and is usually ascribed to Ca-metasomatism from associated serpentinite. We contend that the "micro-rodingites" in hole U1309D could not have formed due to Ca-metasomatism because the only calcic phase in the rock is plagioclase and in the micro-rodingites it has reacted to form phases that are even richer in CaO. Instead we postulate that they were caused by desilication of plagioclase in response to serpentinization of adjacent olivine. Possible net reactions are: (1) 4 anorthite + 5 olivine + 10 H2O = 2 prehnite + 2 chlorite + silica, (2) 5 serpentine + 9 prehnite = 6 grossular + 3 chlorite + 10 silica + 7 H2O Silica released by these reactions was used to produce serpentine by the reactions: (3) silica + 3 olivine + 4 H2O = 2 serpentine, (4) 2 silica + 3 brucite = serpentine + H2O Without a source for silica, olivine will hydrate to brucite + serpentine. The silica activity of this assemblage is extremely low (log a(SiO2) ~ -2.5 to -4, depending on temperature) and this has a major effect on the serpentinization process. Many of the distinctive features of serpentinites can be attributed to low silica activity. In addition to the formation of rodingites, this includes the extremely low oxygen fugacity of serpentinites and the associated formation magnetite and awaruite (FeNi3).