Test #2 Flashcards
Phaneritic
Rock grains are large enough to be seen with the naked eye
Aphanitic
Individual crystals can’t be seen with the naked eye
Glassy
Molten liquid quenched so quickly that crystals do not have time to form
Porphyritic
Large phenocrysts are visible in the crystal matrix (either phaneritic or aphanitic)
Pegmatite
Very large minerals that grow quickly out of residual melt
Nucleation
Initial formation of crystal nuclei (small cluster of compatible ions)
Must reach critical size before further growth can take place
Requires supersaturation or undercooling
Diffusion
Movement of ions though magma to surface of growing crystal (heat moves away from surface of growing crystal)
Why are crystal nuclei unstable?
Many have very high surface area / volume ration.
What is undercooling?
Cooling of a melt below the theoretically predicted (“true”) crystallization temperature of a mineral
What degree of undercooling (low, moderate, high) generally occurs during slow cooling? Rapid cooling? Extremely rapid cooling? What textures result?
Low degree of undercooling: moderate crystal growth, low nucleation, moderate diffusion. Produces a phaneritic texture. Slow cooling.
Moderate degree of undercooling: low crystal growth, high nucleation, low to moderate diffusion. Produces an aphanitic texture. Rapid cooling.
High degrees of undercooling: very low crystal growth, nucleation, and diffusion. Rock is quenced and very little, if any, crystals form. Produces a glassy rock, or holohyaline texture. Extremely rapid cooling.
What is the typical pattern of growth for chain silicates? Sheet silicates?
Chain silicates (such as pyroxenes) grow fastest along length of chains.
Sheet silicates (such as micas) grow along direction of silicate sheets.
How and why does the rate of growth differ on corners vs. edges vs. faces of growing crystals?
Corners > Edges > Sides
Volume of liquid available to growing crystal is greatest on corners, least on faces.
Poikilitic texture
Inclusions of one mineral within another
Ophitic texture
Special case of poikilitic texture, large pyroxene grain contains numerous plagioclase crystals
Subophitic
Plagioclase crystals are only partially enclosed in pyroxene.
What is compositional zoning and why does it occur?
Changes in mineral composition as crystal is growing
Common in solid solution minerals
Normal zoning
Occurs as predicted by phase diagram of solid solution mineral (changing composition with falling temperature)
Reverse zoning
Zoning that occurs opposite of what is predicted in phase diagram of solid solution minerals with falling temperature.
Oscillatory zoning
Alternating normal & reverse (most common in plagioclase)
Degree of Crystallization
Determined by rate of cooling
Holocrystalline
Entire rock composed of crystals
Hypocrystalline
Rock composed of both crystals and glass
Holohyaline
Rock is essentially all glass
Euhedral
Crystal is dominantly bounded by its crystal faces.
Rock is euhedral-granular or idiomorphic.
Subhedral
Crystal is partially bounded by crystal faces.
Rock is subhedral-granular or hypidiomorphic.
Anhedral
Crystal lacks any characteristic crystal faces.
Rock is anhedral-granular or allotriomorphic.
What determines grain shape?
- Some minerals have specific forms (plagioclase forms tabular or lath-like grains, quartz typically forms anhedral shapes)
- Order of crystallization: early formed minerals are more euhedral, late formed minerals are more anhedral (fill in between earlier formed minerals)
- Rate of cooling: slow cooling produces large euhedral crystals, rapid cooling may product skeletal, hollow, dendritic, or spherulitic grains.
Skeletal textures
Rapid growth, envelopes melt
Embayed texture
‘Corroded’ margins to phenocrysts infer that they were being resorbed by the magma and may imply addition of fresh, hotter magma.
Spherulitic texture
Spherulitic texture is the result of cooling and nucleation of material in a magma which has achieved supersaturation in the crystal component.
Tephra
Pyroclasts: ash, lapilli, blocks, bombs
What are glass shards and how do they form?
Glass shards form in air bubbles in pumice (interstitial liquid).
Welded texture
Welded textures occur when pyroclastic material is hot enough at the time of formation to weld together.
Eutaxitic texture
Layered, banded texture shown in welded tuff
Fiamme
Squashed fragment found in tuff
Graphic granite
Typically quartz intergrowths in microcline.
Myrmekite
“Wormy” intergrowth of quartz in plagioclase
Corona textures
Reation rims
Oxyhornblende
Grain of hornblende oxydizes, forming dark rim around the grain
Rapakivi
Typically plagioclase-mantled K-feldspar phenocrysts (very large)
What are the 8 major elements?
Si (+4)
Al (+3)
Mg (+2)
Fe (+2/+3)
Ca (+2)
K (+)
Na (+)
O (-2)
What are the 3 minor elements?
Ti (+3/+4)
P (-3)
Mn (+2)
Felsic
65-75% silica
Typically rich in Al, Na, K
Intermediate
52-65% silica
Na vs. Ca content
Mafic
45-52% silica
Rich in Mg, Ca, Fe
Ultramafic
<45% silica
Rich in Mg, Fe
What is the basic principle upon which spectroscopic methods of mineral anaylsis work?
The ability of atoms to either absorb or emit radiation with frequencies characteristic of the specific element.
Primary magma
The magma composition that is first melted
Evolved magma
Magma whose composition has changed from that of the primary due to a process such as fractional crystallization (evolves from lowest to highest silica content)
Parental magma
Least evolved magma found (generally lowest silica content)
Harker diagrams typically use a differentiation index of SiO2. What are some other usesful indices?
MgO (useful in basaltic rocks)
Mg-Fe rations (useful in basaltic rocks)
AFM Diagram
Used to furhter subdivide subalkaline magma series into tholeiitic or calc-alkaline series
Feldspar Ternary
Anorthite
CaAl2Si2O8
Albite
NaAlSi3O8
Orthoclase feldspar
KAlSi3O8
Plagioclase Feldspars
Anorthite
Bytownite
Labradorite
Andesine
Oligoclase
Albite
Alkali Feldspars
Orthoclase
Sanidine
Mircrocline
Anorthoclase
Forsterite
Olivine solid solution end member
Mg2SiO4
Fayalite
Olivine solid solution end member
Fe2SiO4
Which olivine compositions form at higher temperatures vs. lower temperatures?
Forsterite (higher T) –> Fayalite (lower T)
What igneous rocks commonly contain olivine?
Typically found in mafic and ultramafic rocks
Pyroxene Quadrilateral
Augite
(Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6
Clinopyroxenes
Dioside (Mg) –> Hedenbergite (Fe)
Orthopyroxenes
Enstatite (Mg) –) Ferrosilite (Fe)
Diopside
MgCaSi206
Hedenbergite
FeCaSi2O6
Enstatite
Mg2Si2O6
Ferrosilite
Fe2Si2O6
What igneous rocks commonly contain pyroxenes?
Intermediate to mafic igneous rocks.
Lithophile
“stone loving”
elements that prefer silicate phases
Chalcophile
“copper loving”
elements that prefer sulfide phases
Siderophile
“iron loving”
elements that prefer metallic phases
Goldschmidt’s Rule #1
2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overal proportions.
Goldschmidt’s Rule #2
If 2 ions have a similar radius and the same valence:
the smaller ion is preferentially incorporated into the solid over the liquid
Goldschmidt’s Rule #3
If 2 ions have a similar radius, but different valence:
the ion with the higher charge is preferentially incorporated into the solid over the liquid
Chemical Fractionation
- uneven distribution of an ion between two competing phases
Distribution or partition coefficient
KD = Xisolid/Ximelt
What is meant by incompatible?
If KD < 1:
Element is incompatible with solid, so is concentrated in the melt.
What is meant by compatible?
If KD > 1:
Element is compatible with the solid, so is concentrated in the solid.
What are high field strength elements?
- Small, highly charged cations
- Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta, and most rare earth elements
- Tend to remain immobile during most types of alteration
What are large ion lithphiles?
- K, Rb, Cs, Ba, Pb2+, Sr, Eu2+
- Tend to be mobile, especially if fluids are involved
Bulk distribution coefficient
Di= ΣWADiA
WA=weight % of mineral A in the rock
DiA=partition coefficient of element i in mineral A
What is the ‘europium anomaly’?
Negative Eu anomaly occurs in a rock where plagioclase is left behind during melting, or plagioclase phenocrysts were removed from magma as they formed.
Positive Eu anomaly occurs in a rock where plagioclase accumulated in the rock. Eu3+ –> Eu2+ (for Ca2+)
Rare Earth Elements
- Group IIIA
- 57-71
- All have 3+ oxidation states but Eu is easily reduced to Eu2+, Ce easily oxidized to Ce4+
- Heavy (HREE) are more compatible than light (LREE)
What mineral that may occur in mantle rocks at certain pressures really likes the heavy rare earths?
Garnet
What is a spider diagram?
Samples are compared to average MORB (MORB=mid ocean ridge basalt–most abundant igneous rock at Earth’s surface)
Ni as a trace element
Highly compatible
Concentrated in olivine
Ba as a trace element
Incompatible element
Substitutes for K in K-feldspar, mica, or hornblende
Rb as a trace element
Incompatible element
Substitutes for K in K-feldspar, micas, or hornblende (less readily in hornblende)
Zr as a trace element
Very incompatible
May occasionally replace Ti in sphene or rutile
REEs as trace elements
Garnet accomodates HREE more than LREE (OPX and hornblende do as well, to lesser degree)
Sphene and plagioclase accomodate more LREE
Eu2+ is strongly partitioned into plagioclase
What are the three main types of basalt?
- Tholeiitic–(most common) MORBs, primitive island arcs
- Alkaline–within-plate settings (hot spots)
- Calc-alkaline–mostly restricted to complex, evolved arc settings
Characteristics of tholeiitic basalt
Groundmass: No olivine, OPX common, intersititial glass and/or quartz common
Phenocrysts: OPX reaction rims, early plagioclase
Characteristics of alkaline basalt
Groundmass: Olivine common, no OPX, no quartz
Phenocrysts: Zoned olivine common, plagioclase less common
What sources of information do we have on mantle composition?
- Ophiolites
- Dredge samples from oceanic fracture zones
- Nodules and mantle xenoliths in some basalts
- Xenoliths in kimberlite
Xenolith
Large piece of rock embedded in a different, larger rock
What is the maximum % of melting we think can occur under normal conditions?
20-25% (produces tholeiite, leaves dunite residuum)
If only 15-20% melts–produces tholeiite, leaves harzburgite
What is lherzolite?
Fertile, unaltered mantle
Aluminous phases in lherzolite at various depths
Plagioclase–<40-50km
Spinel–50-80km
Garnet–80-400km
Si–VI coordination, >400km
Three ways the mantle could melt
- Temperature increase by radioactive decay–but not enough radioactive elements in mantle–occurs locally at hot spots
- Lower the pressure at constant T–adiabatic rise of mantle with no heat loss–requires rapid rise to prevent heat loss–decompressed mantle buoys up at rift zones, asthenosphere moves upward to fill gaps
- Add volatiles–lowers melting point–fluids can be added at subduction zones
What melting processes favor tholeiitic formation?
- Shallow source of melting
- Relatively high degree of melting (20-25%)
- Presence of H2O-rich volatiles
- Olivine fractionation during rise of melt
What melting processes favor alkaline formation?
- Deep source of melting
- Low degree of melting
- Presence of CO2-rich volatiles
- Al-silicate fractionation during rise of melt
In terms of trace element patterns, how do MORBs (tholeiitic) differ from OIBs (alkaline)? What does a positive slope on an REE or trace-element spider diagram indicate about the source rock?
OIB: typical negative slope (enriched in incompatibles)
MORB: low positive slope, requires source already depleted in incompatibles by previous melting episode
Fertile mantle source rocks
Nothing has been removed by prior melting episode
Lower mantle
Enriched mantle source rocks
Mantle to which something has been added
Depleted mantle source rocks
Residuum mantle after certain elements have been removed
Upper mantle
Magmatic Differentiation
- Creates a compositional different in one or more phases
- Preserves chemical difference by segregating (or fractionating) chemically distinct portions
What are common processes that accomplish magmatic differentiation?
- Crystal Fractionation
- Gravitational settling
What is cumulate texture?
Mutually touching phenocrysts with interstitual crystallized residual melt
Stoke’s Law
V= 2gr2 (ρs-ρl)
9η
V=settling velocity (cm/s)
g=acceleration due to gravity
r=radius of spherical**ASSUMED** particle
ρs=density of solid spherical particle
ρl=density of liquid
η=viscosity of the liquid
How do pegmatites form?
Late-stage fractional crystallization
Late melt is enriched in incompatible, LIL, and non-lithophile elements, crystallize rapidly
What factors control magma mixing?
- Temperature
- Composition
- Viscosity
- Volatile content
- % of each parent magma
What are some pieces of evidence that magma mixing occurs?
Cross-cutting dikes or layers
Changes in mineral composition (reverse zoning)
Layered mafic intrusions
Granitic plutons
Assimilation
Incorporation of chemical constituents from wall rocks by diffusion (chemical alteration) or xenoliths (physically breaking)
Controlled by: heat available in magma, composition and melting temperature of wall rock
Occurs where mantle-derived magmas pass through continental crust
Slow Spreading Ridges
< 4 cm/a
Indian Ocean
Fast Spreading Ridges
>4 cm/a
East Pacific Rise
Half Spreading Rate
Movement of one side of the ridge away from the other
Differences in Morphology Between Fast Spreading & Slow Spreading Ridges
Slow Spreading: Older deposits carried away from axis, cut by normal faults
Fast spreading: Rapid, continuous volcanism
Black smokers
- Hydrothermal vents associated with spreading ridges
- Circulating fluids ready ~350ºC
White smokers
Hydrothermal vents, cooler than black smokers (~300ºC)
Ophiolite Stratigraphy
Surface-.3km: deep sea sediments
.3-.7km–Basaltic pillow lava
1.0-1.5km–Sheeted dike complex
2-5km–layered gabbro, wehrlite
Up to 7 km: ultramafics
Harzburgite
Formed deep in the mantle; composed of olivine and OPX
Typical Crystallization Sequence of MORB
olivine –> olivine + plagioclase –> olivine + plagioclase + CPX
What trend do MORBs following on an AFM diagram?
Tholeiitic
What evidence do we see that a typical MORB is not formed from a primary magma?
Typically MORB is formed from derivative magmas formed from fractional crystallization.
What are N-MORBs and E-MORBs?
N-MORB (normal): depleted upper mantle source (K2O < 0.10, TiO2 < 1.0)
E-MORB (enriched): deeper (fertile) mantle source (K2O > 0.10, TiO2 > 1.0)
Petrogenesis of MORBs
- Separation of plates, upward motion of mantle
- Decompression–partial melting (adiabatic rise)
- Focused region of melting
- Lower enriched mantle resevoir may also be drawn upward (E-MORB)
Island arc
Chain of volcanoes formed along tectonic boundary where oceanic crust subducts beneath more oceanic crust
Continental arc
Chain of volcanoes formed along tectonic boundary where oceanic crust subducts beneath continental crust
What is the most common type of volcano, and how does it erupt?
Stratovolcano (composite)
Explosive eruption, silicic composition
Island Arc Thermal Structure
What rock types are common in island arcs?
Basaltic andesite or andesite
What trend on an AFM diagram is restricted to subduction zones?
Calc-alkaline
What arcs could show a tholeiitic trend?
Low-K (low K2O content) such as Tonga-Kermadec
What does a typical arc volcanic rock look like?
Phyric: >20% phenocrysts
Plagioclase phenocrysts ubiquitous!
Augite & olivine in mafic rocks
Magnetite common in most compositions
Magnetite
Fe3O4
OR
FeO • Fe2O3 (varying oxidation states of Fe!)
Ilmenite
FeTiO3
Island Arc Petrogenesis
- Dehydration of altered oceanic crust (chlorite, phyllosilicates @ <50km, amphibole at ~100km(
- Dehydration provides components that enrich overlying mantle in particular trace elements
- Fluids rise into overlying mantle wedge
- Hydrated mantle dragged down to greater depths
- Differentiation in shallower magma chambers produces calc-alkaline trend
In what ways do continental arcs differ from island arcs?
- Mantle-derived magmas must rise through thick layer of continental crust (SiO2-rich crust, incompatible element-enriched, crustal contamination common)
- Continental crust is low density (mafic magmas may be more dense–may “pond” at base or within crust–allows for extensive differentiation or assimilation)
- Continental crust has low melting point (may see considerable partial melting of crust)
