Module 6

Question 1

Describe how oceanic crust has differentiated from the mantle and where this occurs.

Answer 1

Primary magma (melted from mantle):

1. mid-ocean ridges
   -ultramafic mantle forms mafic magma
   -forms mafic oceanic crust
   - partial melting (decompression)
2. At mantle plume hot spots
   -ultramafic mantle forms mafic magma

Question 2

Describe how continental crust has differentiated from the mantle and where this occurs.

Answer 2

Secondary magma (melted from a crustal rock):

1. subduction zones
   -mafic oceanic crust forms
   intermediate and felsic magma

-wet oceanic plates subducts under
continental plate
-partial melting (flux)
-produce mafic magma

• fractional crystallization and heat from mafic magma produces SiO2-rich magma & rocks at base of continental crust (think compatible/incompatible)

Question 3

Where are felsic and mafic rocks found in the Earth’s crust?
Why are the rock minerals different in terms of composition?

Answer 3

Felsic rocks are found in continental crust

Mafic in oceanic crust

Mafic also in continental crust due to upwelling (rock getting thrown up)

Magmas formed from partial melting of different rock types have different
compositions

Question 4

Partial Melting of Rock and fractional crystallization

Answer 4

localized partial melting of rock

Decompression melting
-Reduction in pressure

Flux melting
-Addition of volatiles, e.g., water

Decompression more common than flux

 incompatible elements go into the melt
 Solid phase remains ultramafic/mafic with the compatible elements

Magma becomes increasingly felsic (enriched in Al, Na, K, Si) as crystallization continues

 Compatible elements are removed into earliest-formed minerals
 Incompatible elements are left behind in the liquid and solidify last

Question 5

How do you determine if an element will be higher in the crust or mantle (and what data you need to determine this)

Answer 5

Compatible elements will be higher in the mantle and incompatible elements will be higher in the crust

Distribution constant (KD) of an
element
KD= X(solid)/X(liquid)
X = mol fraction of element X
KD > 1 = compatible
KD < 1 = incompatible

Question 6

 Explain the general trend in oxygen-sharing, Fe and Mg concentration, and incompatible element abundance along Bowen’s reaction series and Goldich’s weathering series

Answer 6

Silicates with more shared oxygens tend to crystallize out at cooler temperatures
More shared oxygens -> lower on BRS, more stable at the surface, incompatible

Compatible to incompatible from high T to low T

Igneous minerals that formed at
higher magmatic temperatures are the
least stable at Earth’s surface
->Unstable (prone to weathering) to stable from hight T to low T

Weathering of SiO2
 One of most stable silicates at Earth surface p and t
 Dissolves in water to form silicic acid:
 Almost always under-saturated in natural waters – very slow reaction!

Question 7

 Describe the four most common types of chemical weathering

Answer 7

1. Dissolution:
   -dissolve mineral into its ions with water
2. Acid dissolution:
   -carbonate system
3. Redox:
   -electron transfer
4. Hydrolysis:
   -“water splitting”,
   -silicate minerals react with water and acid
   -new mineral forms
   -irreversible
   -carbonate system

Question 8

 Explain why differential weathering occurs in silicate rocks

Answer 8

Felsic silicates weather slower

Intermediate silicates weather faster because igneous minerals that formed at higher magmatic temperatures are the least stable at Earth’s surface

Question 9

 Briefly describe how hydrocarbons are formed and which chemical properties infer good
hydrocarbon quality

Answer 9

Petroleum:
hydrocarbon-rich fluid derived from kerogen

Crude oil: liquid petroleum
Natural Gas: gaseous petroleum, mostly methane (CH4)

Formation conditions
1. abundant organic carbon
2. adequate depth (high T)
3. high porosity & permeability
4. Structural trap to seal petroleum in reservoir

Main stages involved:
1. Diagenesis (< 60ºC)
-organic matter to kerogen
-increasing t and p
-least formed here

2. Catagenesis (60-225ºC)
   -most petroleum formed here
   -Thermal “cracking” of kerogen
   -produce crude oil and natural gas
3. Metamorphism (>225ºC)
   -breakdown of crude oil to methane and graphite
   -high t and p

Question 10

Ore Deposit geochemistry

Answer 10

 Magmatic
 Hydrothermal
 Laterites and Bauxites
 Placers

Question 11

Describe the 3 magmatic ore deposits and which elements in each

Answer 11

Chromitite
-fractional crystallization of ultramafic or mafic magma
-compatible
-Removal and concentration of
early-formed crystals of dense
chromite

 Pegmatites
-late-stage cooling of a felsic magma rich in H2O
-rich in incompatible elements, B, Be, Li,
REE, gemstones

 Sulfides
-immiscible (don’t mix) and
separate by density
-dense, crystallize and sink to bottom of magma chamber
-source of Cu, Ni, and platinum-group elements

Question 12

describe hydrothermal Ore Deposits and which elements in each

Answer 12

 Hot, salty brines leach sulfide & oxide
minerals & native metals from rocks
 Brines transport the metals
 Lower T & P, or change redox
conditions/pH of brine causes precipitation of minerals

Often quartz! Can be native metals and
sulfides