Thursday, May 5, 2011

Water and mineral complexity, and a South African xenolith

These past few days I've been playing around with THERIAK DOMINO, a program for thermodynamic calculations in metamorphic petrology. My goal was to get a general idea of the P-T region in which both garnet and plagioclase are stable for these Sierran metaquartzite xenoliths. First, I generate a few pseudosections in DOMINO for a given bulk composition in a simplified components (i.e., CMAS - Ca-Mg-Al-Si), then I incrementally add another component (i.e., CFMAS - with Fe, to name one of many possibilities), until I get to the real system (which is something like CFMKNASH, or NCKFMASH as they say in the literature - I guess NCK F MASH is easier on the tongue!). Then to refine these general plots, I pick a few reactions to investigate, or a specific PT point, or an isothermal or isobaric line - which can be done in THERIAK. Anyway, the results have been pretty cool, but one thing that struck me as really interesting was the addition of water to an extremely quartz-rich, almost pure sandstone composition (NOT your typical metapelite). Adding water greatly increases the complexity of the diagram, as the phase rule F = C-P+2 would dictate (increase the # of components in the system, and the variance or degrees of freedom increases). However, as the # of phases increases in the system, the variance of the system must also decrease (Goldschmidt's Rule), and we can never have more phases than components. For example, at the triple point for aluminosilicate, 3 phases (Kyanite, Andalusite, and Sillimanite) exist in equilibrium, so according to the phase rule for this 1-component (Al2SiO5) system F = 1-3+2 = 0; meaning if we change P or T we will no longer have 3 phases. So, when water is added to CKFMAS (CKFMASH), we have one additional component (H2O), and we can stabilize many new hydrous minerals. Does this imply that the stability field of a hydrous mineral coexisting with an anhydrous mineral is going to be restricted in size? If we have many more phases stable because of the addition of H2O, the phase rule forces us to minimize the variance of the system at all times - so this means that the "size" of a stability field of hydrous mineral + anhydrous mineral in PT space must be small. (This makes intuitive sense to me since hydrous minerals normally can't exist for large expanses of PT space because they will just break down). This seems like a simple question, but I am struggling with it! Below, the first figure is for the anhydrous system, and the second figure for the water-saturated equivalent. The numbers just refer to mineral reactions which are not shown on the diagram.

But anyway, after seeing the remarkable contrasts between these two plots, the ONLY difference being water, that got me thinking to a Loony Noon discussion way back when, concerning "life" and how to define it. Somehow, that got us on the idea that minerals, and the complexity of minerals on Earth, draws some interesting parallels to how life has evolved. For instance, in our ideas of how to define life, we agreed on such criteria as 1) life competes for what it needs to keep going (minerals also compete for cations they need for their structures. 2) life moves (minerals also move... on an atomic scale - but then so does everything, so maybe this is a vague definition). 3) life replicates itself but also mutates (minerals also replicate... given enough substrate, you could grow a crystal meters long. Sometiems minerals mutate too - meaning there are imperfections/defects in crystals). Anyway, the list can go on. But one thing that life needs (or life as WE know it) is water. Complex, eukaryotic life would not be possible with water as a solvent and facilitator for metabolism. Does having mineral complexity also require water? Once we get water involved, a single chain pyroxene can become a double-chain amphibole because now there is space for OH- groups to fit into the lattice. Making more complex chains - you get sheets and phyllosilicates, which all have water. Minerals that have water or OH- in their structures tend to be structurally complex. Did the availability of water on Earth's surface initiate the explosion of new minerals like phyllosilicates? I'm probably definitely not the first to think of this, but it's sort of a neat idea, isn't it? Plus... the more water you have around, the easier it is to dissolve cations, and the more possibilities you have to make new minerals.

Anyway, aside from forays into the world of metamorphic petrology, I come back again to my true love in petrology - the ultramafics. Cin-Ty had some xenoliths from South Africa, which I'm going to analyze for a small side project. These xenoliths are beautiful! Huge garnet megacrysts, huge phlogopites, huge diopside crystals... huge everything. Here's an example of one from the Venetian mine: