Crystal Growth & Volcanic Textures and Primary Magmas Francis 2013

1. Crystal Growth & Volcanic Texturesand Primary MagmasFrancis 2013

2. Volcanic A few coarse-grained phenocrysts in a fine to extremely fine-grained matrix Plutonic Coarse-grained equigranular rocks

3. olivine basalt Volcanic Rocks – relatively fine-grained but commonly phenocyrstic andesite Mafic Intermediate Felsic rhyolite

4. Little happens to a superheated melt that is cooled to its liquidus temperature because, although ΔGoxyl = 0, there is a positive free energy associated with maintaining an interface between melt and a newly formed crystal nucleii, and thus homogeneous nucleation does not occur. Crystallization begins when the degree of undercooling ΔT is such that the negative ΔGxyl offsets the positive Gsurface. For a spherical crystal: ΔGxyl× 4/3 πr3 / Vm>Gsurface× 4 π r2 r* = 3 × Vm × Gsurface / ΔGxyl Crystal nulceii with radii greater than r* will grow. As the degree of undercooling ΔT increases, the size distribution of spontaneously forming and dissolving nucleii shifts to larger r, while the magnitude of r* decreases. Thus as ΔT increases, the proportion of spontaneous nucleii with r > r* will increase. These will continue to grow into larger crystals rather than being resorbed.

5. Boundary Layer Effects A boundary layer develops at the interface of a growing crystal, across which both chemical constituents and heat must diffuse. Boundary layer stagnation may occur when the residual liquid produced by crystallization in the boundary layer becomes too evolved to crystallize at the ambient temperature. Crystallization ceases, but begins again once the relic boundary layer has been dissipated by convection in the body of the intrusion.

6. Chemical Boundary Layers

7. Diffusion versus Growth Rate • In most cases crystal nucleation and growth is delayed to some degree of undercooling ΔT. The greater the ΔT, however, the higher the crystal growth rate in comparison to the rate of diffusion of chemical constituents and heat in the melt. • The shape and size of crystals is controlled by the density of nucleii and the ratio of crystal growth rate to the diffusion rate of chemical constituents in the adjacent melt. • At low ratios of growth rate to diffusion rate, surface nucleation is the controlling factor, and crystals tend to grow layer by layer and have equant dimensions with well developed crystal faces. • High ratios of growth rate to diffusion rate leads to diffusion-controlled growth in which layer by layer growth breaks down and any protuberance on a crystal face will grow and be enhanced because it has access to higher nutrient concentrations and lower temperatures.

8. Cooling Rate Versus Grain Size and Shape ~ 1500 oC/hr Rapid cooling rates leads to large undercooling, high nucleation and growth rates and therefore diffusion controlled growth 0.5 oC/hr ~ 80 oC/hr olivine ~ 20 oC/hr

10. aa pahoehoe

12. glassy pillow margin palagonite

13. OLIVINE : #l: MgO(NM) + 1/2SiO2(NF) = MgSi0.5O2 #2: FeO(NM) + 1/2SiO2(NF) = FeSi0.5O2 NM: Network modifying cations NF: Network forming cations K1 = aFo/ (aMgOLiq × (aSiO2Liq)1/2) =XMgoliv / (XMg(NM))×(XSi(NF))1/2 ) K2 = aFa/ (aFeOLiq × (aSiO2Liq)1/2) =XFeoliv / (XFe(NM))×(XSi(NF))1/2) ln K = a + b a b T K1 6,700 ‑3.73 K2 6,874 ‑4.97 aFo = XMgoliv= Mg / (Mg + Fe)oliv aFa = XFeoliv = Fe / (Mg + Fe)oliv NF = Network Formers Σ Si + Na + K NM = Network Modifiers Σ Mg + Fe + Ni + Ca + Mn + Ti + Cr + Al - (Na + K) aMgOliq = Mg / ∑NM aSiO2liq = Si / ∑NF