Lecture 3: Rocks and Minerals References: Principles and applications of Geochemistry (Ch. 7 & 8) Igneous Petrogenesis (Ch. 2)

How are rocks formed: the rock cycle

Granite: intrusive igneous rock quartzhornblendefeldspar

What defines a mineral? Naturally Occurring Inorganic Solid Specific composition (e.g., Gold - Au, Salt - NaCl, quartz - SiO 2 ) Definite crystalline structure – atoms are arranged in a specific pattern

Mineral Identification Colour: caused by trace elements or impurities within a mineral Lustre: how a mineral surface reflects light Texture: how the mineral feels to the touch Streak: the colour of a mineral when it is scratched on a streak plate (i.e., colour when broken up) Hardness (Moh’s scale: 1-10 – diamond is 10, talc is 1) Cleavage: how a mineral breaks (typically along planes of weakness – related to bonding Fracture: splitting with no orientation Density Flame: colour under a flame Special properties like double refraction, radioactivity, taste, pleochroism, fluorescence

Mineral groups 1) Silicates (SiO 4 ) – make up 96% of minerals, e.g., olivine 2) Carbonates (CO 3 ): e.g, calcite CaCO 3 3) Oxides: metal and oxygen (e.g., hematite, magnetite) 4) Sulfides: element + S 2 (pyrite – FeS) 5) Sulfates: element + SO 4 (gypsum – CaSO 4 nH 2 O) 6) Halides: element + halide (salt - NaCl) 7) Native elements: e.g., Cu, Au, Ag pyrite gypsum

Crystal Habit appearance – shape and size of crystals Botryoidal: grape-like Bladed Dendritic: tree-like stibnite hematite

Crystal Form Any grouping of crystal faces or facets that are arranged in the same symmetry is referred to as a crystal's "form." There are approximately 48 unique crystal forms.

Atomic structure of crystals The relative size of ions determine how atoms pack and which ions can serve as substitutes.

Crystal structure: determined by radius size…

Silicates Silicate tetrahedron olivine, quartz Single chain structure pyroxene Double chain structure hornblende Sheet silicate structure micas Framework silicate structure Feldspars O 2- Si 4+ SiO 4 4- : although it is geometrically balanced, it is not charge balanced – needs ions or other tetrahedra to balance charge

How are minerals formed? 1)Solution: if a solution is supersaturated, minerals will precipitate 2)Magma: minerals form during cooling of a magma – the slower a magma cools, the larger the crystals Intrusive: cools slowly beneath Earth’s surface (e.g., basalt) Extrusive: cools rapidly at Earth’s surface (e.g., granite)Metamorphism 3)Metamorphism: transformation due to changes in pressure and temperature

Phase Diagrams A phase diagram is common way to represent the various phases of a substance and the conditions under which each phase exists. A phase diagram is a plot of pressure (P ) vs temperature (T). Lines on the diagram represent conditions (T,P) under which a phase change is at equilibrium. That is, at a point on a line, it is possible for two (or three) phases to coexist at equilibrium. In other regions of the plot, only one phase exists at equilibrium. Phase diagram for water Triple point: where 3 phases coexist

Binary phase diagram for a solid solution of Olivine Solidus: the temperature below which the substance is stable in the solid state Liquidus: the temperature above which the substance is stable in the liquid state Lever Rule: to determine quantitatively the relative composition of a mixture in a two-phase region in a phase diagram f liq f solid Fayallite (Fa) Forsterite (Fo)% Fo (Mg 2 SiO 4 )

Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting Mantle melts between ~ ºC due to: 1)Increase in temperature 2)Decrease in pressure 3)Addition of volatile phases Upwelling mantle plumes – hotspots Hawaii, Iceland geotherm solidus liquidus

Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting Mantle melts between ~ ºC due to: 1)Increase in temperature 2)Decrease in pressure 3)Addition of volatile phases Adiabatic rise of mantle material with no heat loss – decompression melting Mid-Ocean Ridges Partial melting

Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting Mantle melts between ~ ºC due to: 1)Increase in temperature 2)Decrease in pressure 3)Addition of volatile phases (e.g., water) Mantle solidus is depressed by addition of water Subduction zone settings Wet mantle plumes Depth (km)

Mantle melting: endmember models Batch melting: Melt remains in contact with residual crystals at all times, so the bulk composition remains constant Fractional melting: Melt leaves the system as soon as it is formed, so the bulk composition of the residual solid changes continuously. Incompatible elements: preferentially partition into the melt phase (D<1) Compatible elements: preferentially partition into the solid phase (D>1) Partition or distribution coefficient (D) = C solid /C liquid Spider diagram showing depleted MORB vs. enriched OIB sources Most incompatible Less incompatible Concentrations normalized to bulk earth, C1 chondrites, or primitive mantle

Relating trace element concentrations to melt fraction (F) Batch melting equation: C liq /C sol = 1/(F+D(1-F)) Fractional melting equation: C liq /C sol = (1/D)*(1-F) (1/D-1) E. Klein, “The Crust”, T.I.G series

Spider diagram of crust vs mantle Workman and Hart, 2005

Rare Earth Element diagrams REE are a group of 15 elements with atomic numbers ranging from 57 (La) to 71 (Lu) – LREE vs. HREE Although they are geochemically similar, they have different partition coefficients so are sensitive tracers of source enrichment, the degree of melting and/or fractional crystallization Shaw et al., 2009

Samples of the mantle 1) Ophiolites –Slabs of oceanic crust and upper mantle –Thrust at subduction zones onto edge of continent 2) Dredge samples from oceanic fracture zones 3) Nodules and xenoliths in basalts 4) Kimberlites –Diamond-bearing pipes blasted up from the mantle carrying xenoliths from depth Hacker Oman ophiolite

Mafic Rocks – Magnesium, Iron rich, usually dark coloured

Felsic or SiAlic Rocks – Silicon, Aluminum rich, usually light coloured