GEOS102 Final

Big bang

13.7 billion years ago

Earth dimensions

• Circumference ~40,000km

• Radius ~6350km

Earth density

• ~5400kg m^-3

Age of Earth

4.56Ga

Oldest minerals on Earth

Zircons West Aus ~4.4Ga

Earliest water on Earth

~4.4Ga

Earliest life on Earth

Stromatolites West Aus ~3.7Ga

Oldest rocks on Earth

~4.3Ga

Plate tectonics begin

~3Ga

Mineral definition

Naturally occurring inorganic substance with a definite chemical composition and a regular internal crystlline structure

Crystalline definition

Having an orderly internal structure made up of regularly repeating arrangement of atoms, ions or molecules in 3 dimensions

Rock definition

Naturally occurring multigranular aggregate of one or more minerals

Crystal structure is determined by . . .

Constituent atoms (chemistry) and how they are bonded together (structure)

O+Si Earth measurements

• 75% crustal composition by weight

• 90% of crustal composition by volume

Lustre

Way light interacts with the surface of a mineral

Hardness

Resistance to abrasion

Cleavage

Tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice

Crystal form/habit

Characteristic external shape of an individual crystal or groups of crystals

Crystallography definition

Experimental science of determining the arrangement of atoms in the crystalline solids

X Ray Diffraction

• Spacing of diffracted X-Rays function of atoms they impact

Polymineralic XRD

• Powders gained by crushing whole rocks

• type and abundance of minerals present

Single-Crystal XRD

• Shows patterns related to scattering of X-rays by atomic arrangement

Isometric (cubic)

• Axes a₁=a₂=a₃

• angles are all 90^o

• equant shape

Isometric (cubic) examples

• Pyrite

• galena

• gold

Tetragonal

• axes are a₁=a₂=/c

• Angles are all 90^o

• tall or short with square base

Tetragonal examples

• Zircon

• Rutile

• Idocrase

Hexagonal

• axes are a₁=a₂=a₃=/c

• 3 a axes in a plane at 120^o to eachother, c axis at 90^o to that plane

• tall or short with six sided or triangular cross sextion

Hexagonal subsystems

• Hex subsystem - 6 fold

• Trig subsystem - 3 fold

Hexagonal examples

• Graphite

• Calcite

• Dolomite

Orthorhomic

• axes are a₁=/b=/c

• Angles are all at 90^o

• Cereal box shape

Orthorhombic examples

• sulphur

• orthopyroxene

• barite

Monoclinic

• axes are a₁=/b=/c

• angles are one not at 90^o

• leaning ceral box shape

Monoclinic examples

• orthoclase

• gypsum

• hornblende

Triclinic

• axes are a₁=/b=/c

• Angles are none at 90^o

• Low symmetry form

Triclinic examples

• plagioclase

• turquoise

• kyanite

Silicate building block

SiO₄^-4 tetrahedron

Structural groups

• isolated

• single chain

• double chain

• silicate sheet

• framework silicate

Electromagnetic spectrum visible light

1-7700-3900 A, red-violet

Monochromatic light

Light of a single wavelength

White light

Mixture of all visible colours

Speed of light

• velocity and wavelength change in different media but frequency does not

Refractive Index

Speed of light in a medium

Higher RI

• N

• slower light in mineral

Lower RI

• n

• faster light in mineral

Refractive index equation

n=velocity in air/velocity in mineral

Refraction

Light refracts/changes direction when passing from one substance to another

Snells Law

n₁sin(0)=n₂sin(0₂)

Polarised light

Light is filtered/polarized so there is only one plane of vibration

Cross polarised light

Light is doubly filtered/polarized

Opaque minerals in optical mineralogy

• Black in XPL and PPL

Isotropic minerals in optical mineralogy

• affect light the same way in all directions

• Black in XPL

• Visible in PPL

Anistropic minerals in optical mineralogy

• affect light differently in different directions

• Visible in XPL and PPL

Igneous rock temperatures

Between ~1300 C - 700 C

Core proportion of Earth

~15%

Mantle proportion of Earth

~84%

Lithosphere proportion of Earth

~1%

Where is the highest thermal gradient?

Lithosphere

Oceanic crust

• Thin, young and hot

• Magma easily extrudes along MOR

Continental crust

• Old, thick, cold

• Difficult for magma to extrude

Magma definition

Molten rock (usually silicate) produced by partial melting in Earths interior

Igneous rock classification

• Chemistry

• Texture

Chemistry of igneous rocks is dependent on . . .

Magma generation and differentiation

Texture of igneous rocks is dependent on . . .

Cooling and emplacement environment

Percentage of magma from deep interior which erupts

~10-15%

Which type of magma is more likely to stall and form intrusions

High viscosity magma

Diversity in magma composition

• What is melting (source)

• Melting conditions

How does what is melting (source) impact magma composition

• Upper mantle is dominated by olivine & pyroxene = mafic magma

• Crust is dominated by plagioclase, quartz and alkali feldspar = felsic magma

How does melting conditions impact magma composition

• Felsic minerals melt at lower temperatures than mafic

• Low degree partial melts are more silicic than high degree partial melts

Flux melting

Water can drastically lower melting temperature

Diversity in Magma Differentiation

• Fractional crystallization

• Assimilation

• Magma mixing/mingling

Fractional crystallization

Fractional removal of early-formed crystals leaves residual melt more evolved

Assimilation

• Hot melt intruding into country rocks can break them off and then melt/assimilate

How is partial assimilation recognized

• xenoliths (foreign rock)

How is complete assimilation recognized

• Chemical signitature

Magma mixing/mingling

• Injection of fresh batch of magma into older batch of magma

Enclaves

Magmas mingle but do not mix due to large thermal and viscosity differences

Diversity in magma generation

• Magma composition

• Magma differentiation

Controlling variables for metamorphism

• Protolith

• Temperature

• Pressure

• Deformation

• Fluid

• Time

Metamorphic variable: protolith types

• Peletic

• Mafic

• Calcerous

• Quartzo-Feldspar

Metamorphic variable: temperature

• increases with depth along geothermal gradiant

• Increases in proximity to intrusions

Metamorphic variable: temperature upper limits

Melting >750C (dependent on compositon)

Metamorphic variable: temperature lower limits

“diagenesis” <200C

Lithostatic pressure

Total pressure exerted at a point in the crust due to the weight of the overlying material

Confining pressure

Assume pressure is uniform in all directions, and equal to lithostatic

Average pressure by depth

3 kilobars/10km

Deformation + Differential pressure

Pressure or stress is not uniform in all directions

• shear stress

• normal stress

Shear stress

Smearing of objects in direction of applied stress

Normal stress

Causes compression in direction of maximum differential stress and extension in direction of minimum

Fluid

React with rocks and cause element mobility, results in metasomatism and easiest way to achieve open system

Metamorphism variable: time

For equivalant T-P, coarser grained rocks indicate longer time and finer grained rocks indicate shorter time

Metamorphic grade

Overall degree to which a protolith has changed

Metamorphic P-T path

Order in which a protolith has changed

• prograde

• peak

• retrograde

Prograde metamorphism

Occurs during increase in T and P

Peak metamorphism

Highest temperature acheived

Retrograde metamorphism

Occurs during cooling and decompression

Regional metamoprhism

Metamorphism occurs over large areas subject to high confining and differential stress. Usually results from tectonic forces that produce compression.

Contact metamorphism

Magmatic intrusion subjects rocks to high temperature

• contact aureole around intrusion

Dynamic metamorphism

Occurs in narrower zone near faults due to mechanical shear deformation

Index minerals of pelites during regional prograde metamorphism from Low-High

• chlorite

• biotite

• garnet

• starulite

• kyamite

• silimanite