Serpentinites, rocks made mostly of serpentine, are formed when water reacts with the earth’s mantle at moderate to low temperatures. Serpentinites are not rare, but they have a number of unusual geochemical features. These include:
- Low oxygen activity- nickel metal allows sometimes occur in serpentinites.
- Magnetic susceptibility- despite the low fO2, they can contain abundant magnetite.
- Low S fugacity- sulphides, when formed, are generally low S minerals.
- Association with other strange calcic rocks, such as rodingites.
- Whacky hydrothermal alteration.
Frost and Beard show that all of these unusual features are a result of the (very low) silica activity, which is controlled by the various olivine hydration reactions, specifically those between olivine, brucite, and serpentine. Some of it is qualitative, as the necessary thermodynamic data for some of the key reactions is not currently available, but the arguments are nevertheless quite robust.
If you like phase diagrams, this paper is a must read. But even if getting lost in a maze of stability fields isn’t your cup of tea, the paper is still quite readable in figure skimming mode. The thing I like about it is that it give a method of trying to predict the weirdness that serpentinization often produces.
And there is no doubt that serpentines are strange. In fact, they are often invoked as explanations of last resort for nay number of things, from methane on Mars to VMS base metal mineralization, to the origin of life (on Earth). By changing serpentinite geology from a black box of reduced alkali joy into a silica activity problem, the authors provide a powerful tool to constrain some of the more outlandish ways in which this process is invoked.
I do have one complaint, though. I despise scientific papers that have titles that begin with a preposition. "On" tells us nothing about the study.
Also, the paper is open access, so anyone can read it.
B. Ronald Frost and James S. Beard. 2007. On Silica Activity and Serpentinization Journal of Petrology 48(7):1351-1368.