Download Geology 103

Geology 103
Composition and Stability of Carbonate Particles
Boggs,
5th
Lecture #18
ed., p 147-154
A) Intro:
- Carbonate rocks make up <10% of all sedimentary rocks
- Include limestones, dolostones
- Mineralogy: is relatively simple:
- Calcite, dolomite, some evaporates
- By comparison: clastic mineralogy is complex, many different mineral families
and rock types are involved
- Source of the particles (provenance):
- Source of carbonate grains is exclusively marine
- Biogenic vs. inorganic source is debated and not fully understood
- Transport:
- Is minimal in carbonates- they are too soft and ends up in powder form after
only a short distance
- My rule: you can generally look around and see the source (a few km distance at
most)
- Economic potential:
- Carbonates are major petroleum reservoirs
- Carbonates are common building stone, gravel
- Some ore deposits (skarn deposits)
- Depositional environments:
- Are all variations on open marine
B) Mineralogy/Chemical composition
- We will see: different carbonate minerals are common in different environments- pay
attention to the environments.
- Have class help: name some carbonate minerals:
See Table 8.1 from Boggs, older edition (not in our textbook)
1) Aragonite (CaCO3)
- The most common component in modern sediments
- Relatively rare in ancient sedimentary rocks
- Is metastable (alters easily to calcite)
- Is probably mostly of biogenic origin, although this is debated:
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- Ocean “whitings” may be inorganic aragonite precipitation
- Is orthorhombic (crystal system)
- Density is 2.94
- Has Calcium ions in octahedral coordination with oxygen ions, Carbon lies at
the center of equilateral triangles with oxygen's on the edges
2) Calcite (CaCO3) - a polymorph of aragonite
- Ask class: What is a polymorph?
- The most common component of ancient sedimentary rocks
- Is also common in modern sediments
- Belongs to the rhombohedral crystal system, crystals form rhombs (acute angles)
- Density = 2.71
- Is relatively stable from a crystal standpoint, but dissolves easily
- Biogenic vs. inorganic origin has probably varied through time
- Calcite can precipitate directly from saturated seawater, although the
relative importance of inorganic calcite and conditions that lead to
direct precipitation aren't well understood
See Figure 6.10 from Boggs, 5th edition, p. 150
- Geochemists have shown that high Mg concentrations in seawater may inhibit
calcite precipitation, lead to aragonite precipitation
- Production of Mg at mid-ocean spreading centers may add Mg to the ocean
during times of rapid oceanic plate production, and tip the balance toward
aragonite production
- Calcite precipitation may have been more common in the geologic past when the
Mg concentration of the world's oceans was lower
- Calcite has two forms:
High Mg calcite (>4% Mg)
Low Mg calcite (<4% Mg)
- High Mg vs. low Mg calcite can be distinguished in thin section with stains
- This is disordered: random substitution of Mg for Ca in the crystal lattice
3) Dolomite
- Crystal system is rhombohedral, forms pronounced rhombic crystals
- Mg substitutes for Ca in specific (regular) sites in the crystal lattice
- Is rare in modern environments
- Commonly forms as a replacement of aragonite or calcite in Recent or Ancient
sedimentary environments (we will spend more time on this later)
- Most dolomite is a result of Mg-rich fluid movement through sediment or
sedimentary rocks
- 2 time scales for replacement
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a) Replacement may be almost immediately after deposition:
- 1,000's of years time scale.
- This is called penecontemporaneous dolomite
b) Replacement after deep burial
- Millions of years time scale (requires burial and subsidence)
- This is called secondary dolomite
- There is still some debate over whether dolomite can precipitate directly from
seawater. Most geochemists would say that direct (inorganic)
precipitation of dolomite isn't likely or significant.
4) Others
- Siderite (FeCO3), ankerite Ca (Fe, Mg) (CO3)
(iron carbonates)
- And many more!
Note: most limestones are relatively pure (>95% calcite); this includes biogenic fossils.
See Figure 8.1 from Boggs, earlier edition (not in our book)
C) Role of organisms in Calcite precipitation
See Table 6.5 from Boggs, 5th edition, p 150
Several factors may mediate or enhance calcite production:
1) Direct extraction of Ca++ and CO3-2 ions from seawater by organisms
- Used to form shells
- Doesn't required saturated conditions
- Produces carbonate material in a wide range of sizes: from barrier reefs that
extend for 1,000's of km to silt and silt-sized particles (pteropod tests)
- Results in a constant production and rain of skeletal debris in most oceans
- Production is highest in shallow oceans (photic zone) and shelf environments
2) Photosynthesis
- Ocean plants (phytoplankton) remove dissolved CO2 from seawater, produce
carbohydrates (cellular material)
- ex: bacteria (not true plants), diatoms, coccoliths, dinoflagellates
- Causes CO2 concentration to vary in the surface of the ocean on a daily basis
(photosynthesis only occurs when the sun is shining)
- This affect the pH of water, too! (CO2 contributes to carbonic acid formation)
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3) Bacterial mediation
- A relatively new idea
- Bacteria may promote calcite precipitation, help algal mats calcify
4) Others:
- Decay of dead organisms:
- affects pH
- pH often decreases with organic decay
- Sulfur-reducing bacteria may have the opposite effect
- Generation of pellets
- Fecal pellets package carbonate
- May help with preservation: pellets sink faster than smaller particles
D) Factors that affect carbonate stability:
- Several physical and chemical factors affect the stability of carbonate in the ocean:
1) Saturation
- Surface waters and shallow shelf environments are often saturated with
Ca++ and CO3-2
- Deeper ocean waters (colder) are not saturated
2) Solubility
- Deeper, colder ocean water has more CO2, is slightly more acidic
- Carbonates tend to dissolve
- Calcite is also more soluble as temperature decreases!!
- Increased (hydrostatic) pressure in deep oceans also tends to promote
dissolution
The result: Calcite is more stable at the surface, tends to dissolve with depth
- Leads to the concept of the CCD (Calcite Compensation Depth)
- Also called the CCCD (calcium carbonate compensation depth)
- At ~ 4500 m calcite dissolves faster than it is added from above
- Average ocean depth ~ 3800 m
- The result:
- Calcite can accumulate on many parts of the (shallow) ocean
floor
- Calcite dissolves in the deep ocean
See Figure 2.13 from Oceanography textbook
- This process can be driven either way through a chain of reactions
- The net result: pH is relatively stable in the ocean
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- Reservoirs ("sinks") of CO2 on the atmospheric side and CaCO3 on the
sediment side keep the H+ ion concentration relatively constant
- Rates of reaction are the limiting factor:
- The dissociation of carbonic acid to bicarbonate is relatively fast and
"easy"
- The dissociation of bicarbonate to carbonate is slower, more difficult
from a kinetic standpoint
- Constant sources of raw reactants on both ends of the systems results in
buffering of pH in the world's oceans.
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