Week 11 diagenesis and metamorphism

What is protolith

Rock before metamorphism occured

Define metamorphism

changes in the mineralogy and texture occurring in the solid state (no melting)

→ addition of thermal + mechanical energy causing change in equilibrium state

What are the principal changes that occur in metamorphism

Metamorphism doesn’t change bulk comp of rock, is closed system

→ unless volatiles are lost and added

→ Metasomatism, addition or removal of elements through pumping pore waters

What are the two main processes that occur during metamorphism

• Recrystallisation- existing minerals adopt new textures

  → may have preferred orientation

• Neomineralisation- formation of new minerals at the expense of existing (no bulk comp change, only new formula)

  → involves chem reaction between mineral + fluid

  → (x+y)→(z+a)

2 reasons for importance of metamorphism

1. most abundant rock type

   → important economical mineral deposits

2. do not undergo retrograde metamorphism (revert back to OG state)

   → study interior earth processes

Explain the 2 ways metamorphism can occur with heat

1. original heat- geothermal gradient increases 25-30c per km

   → depends on tectonic setting

   → cold subduction (3-5c/km) vs mid ocean ridge (50c/km)

2. Local heat- igneous intrusions w intense local heat source

   → distributes geothermal gradient

   → very localised, distance away further= heated less

Explain the 2 ways metamorphism can occur with stress

1. Load pressure- result from weight of overlying rock

   → uniform in all directions (P=pgh)

   → negligible with tectonic setting compared to temp

2. Deviatoric stress- max/min principal stress (sigma)

   → changes texture e.g compression causes rock elongation

   → cleavage (minerals with preferred orientation)

what are mineral facies

mineral assemblages forming under distinct pressure + temp fields

→ found through T ( as P increases linearly w depth)

→ regional, contact and dynamic

Explain regional metamorphism

increase in both T and P

→ subduction or continent/continent ocean collisions

→ increasing grain size with T and P

→ deviatoric stress, so cleavage also increase with T and P

How can grade of metamorphism be determined through index materials

Rocks w index materials good (e.g mudstone + basalt)

→ e.g if staurolite and garnet then probably medium grade

Explain contact metamorphism

increase in high T but no P change

→ through contact w intrusion, e.g cont/ocean crust extensional setting, hot spot, where igneous rocks made

→ distinct mineral assemblage (comp) + grain size w T

→ no stress so no foliation (layers)

What are products of contact metamorphism called

Hornfels

→non foliated (layers) with ‘spots’ (porphyroblasts)

→ distinct increase in crystal growth size

→intruded igneous rocks nearby

Explain dynamic metamorphism

Increase in stress pressure w little T, deviatoric stress

1. At faults w intermediate stress focussed, grain size reduced

   → cataclastic rocks (brittle, crackling, no recrystallization)

   → mylonites (ductile, dynamic recrystallization)

2. Meteor impacts

Identifying shear from mylonites

mylonite orientated parallel to shear

→ ‘eye’ at center, rock at fault

→ so shear left above and shear right below

Give two reasons why minerals do not retrograde

1. Rocks lose energy as it cools

   → no activation energy

2. Prograde (OG) involves volatile release

   → volatiles no longer available after metamorphism

Define diagenesis

processes leading to lithification (loose sediments→ rock)

→ do not convert to new minerals like meta

→ burial of rocks can lead to change in bulk comp

→ sedimentary features still exist unlike meta

How does diagenesis occur

Mineral growth in porous space between grains, ‘cement’ grains together

→ spaces can be reservoirs for oil, gas, etc

Explain mechanical compaction

Overburden causes grain rotation to reduce volume

→ pore space reduced, rock volume same, expulsion of water

→ long axis perpendicular to compaction direction (shales)

Sandstone dykes

→ sand grains brought up w water, injected through overlying sediment

Explain compaction through pressure solution

Material at grain contacts perpendicular to compaction dissolve, becomes elongated

→ material re-precipitates as cement at grain contacts parallel to stress

→ less porous

Explain dissolution

Changes in T and P/pore water (water inside pore of rock) composition (porosity change)

→ dissolves minerals in solution (e.g co2 and carbonic acid)

Dissolution without re-precipitation

→ prolonged pumping of pore water w no dissolved mineral

→ stops water saturating and cement forming from precipitating

→ common in carbonates

Explain cementation

Chemical precipitates (new crystals) form in sediment pores and binds grains together

→ e.g quartz, clay, carbonates

→ from dissolution, reprecipitation and saturated waters

Explain what concretions are

Spherical mass of sediments more cemented/different mineral phase than surrounding

→ cement grows from nucleus then outwards

Explain recrystallization

crystal orientation of grain is changed, but mineral composition stays same

→ result of solution + reprecipitation of mineral phase present

→ no pore change

Explain mineral replacement

Newly formed mineral replaces pre existing one

→ same composition but polymorph (same chemical, diff internal structure), or entirely new mineral phase

→ change in volume can affect porosity

what diagenetic process increases porosity?

dissolution (remove minerals from rock from porous water)