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Igneous & Metamorphic Petrology Spring 2013 > Test #2 > Flashcards

Test #2 Flashcards

1
Q

Phaneritic

A

Rock grains are large enough to be seen with the naked eye

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2
Q

Aphanitic

A

Individual crystals can’t be seen with the naked eye

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3
Q

Glassy

A

Molten liquid quenched so quickly that crystals do not have time to form

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4
Q

Porphyritic

A

Large phenocrysts are visible in the crystal matrix (either phaneritic or aphanitic)

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5
Q

Pegmatite

A

Very large minerals that grow quickly out of residual melt

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6
Q

Nucleation

A

Initial formation of crystal nuclei (small cluster of compatible ions)

Must reach critical size before further growth can take place

Requires supersaturation or undercooling

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7
Q

Diffusion

A

Movement of ions though magma to surface of growing crystal (heat moves away from surface of growing crystal)

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8
Q

Why are crystal nuclei unstable?

A

Many have very high surface area / volume ration.

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9
Q

What is undercooling?

A

Cooling of a melt below the theoretically predicted (“true”) crystallization temperature of a mineral

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10
Q

What degree of undercooling (low, moderate, high) generally occurs during slow cooling? Rapid cooling? Extremely rapid cooling? What textures result?

A

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.

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11
Q

What is the typical pattern of growth for chain silicates? Sheet silicates?

A

Chain silicates (such as pyroxenes) grow fastest along length of chains.

Sheet silicates (such as micas) grow along direction of silicate sheets.

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12
Q

How and why does the rate of growth differ on corners vs. edges vs. faces of growing crystals?

A

Corners > Edges > Sides

Volume of liquid available to growing crystal is greatest on corners, least on faces.

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13
Q

Poikilitic texture

A

Inclusions of one mineral within another

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14
Q

Ophitic texture

A

Special case of poikilitic texture, large pyroxene grain contains numerous plagioclase crystals

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15
Q

Subophitic

A

Plagioclase crystals are only partially enclosed in pyroxene.

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16
Q

What is compositional zoning and why does it occur?

A

Changes in mineral composition as crystal is growing

Common in solid solution minerals

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17
Q

Normal zoning

A

Occurs as predicted by phase diagram of solid solution mineral (changing composition with falling temperature)

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18
Q

Reverse zoning

A

Zoning that occurs opposite of what is predicted in phase diagram of solid solution minerals with falling temperature.

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19
Q

Oscillatory zoning

A

Alternating normal & reverse (most common in plagioclase)

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20
Q

Degree of Crystallization

A

Determined by rate of cooling

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21
Q

Holocrystalline

A

Entire rock composed of crystals

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22
Q

Hypocrystalline

A

Rock composed of both crystals and glass

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23
Q

Holohyaline

A

Rock is essentially all glass

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24
Q

Euhedral

A

Crystal is dominantly bounded by its crystal faces.

Rock is euhedral-granular or idiomorphic.

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25
Q

Subhedral

A

Crystal is partially bounded by crystal faces.

Rock is subhedral-granular or hypidiomorphic.

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26
Q

Anhedral

A

Crystal lacks any characteristic crystal faces.

Rock is anhedral-granular or allotriomorphic.

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27
Q

What determines grain shape?

A
  • 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.
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28
Q

Skeletal textures

A

Rapid growth, envelopes melt

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29
Q

Embayed texture

A

‘Corroded’ margins to phenocrysts infer that they were being resorbed by the magma and may imply addition of fresh, hotter magma.

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30
Q

Spherulitic texture

A

Spherulitic texture is the result of cooling and nucleation of material in a magma which has achieved supersaturation in the crystal component.

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31
Q

Tephra

A

Pyroclasts: ash, lapilli, blocks, bombs

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32
Q

What are glass shards and how do they form?

A

Glass shards form in air bubbles in pumice (interstitial liquid).

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33
Q

Welded texture

A

Welded textures occur when pyroclastic material is hot enough at the time of formation to weld together.

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34
Q

Eutaxitic texture

A

Layered, banded texture shown in welded tuff

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35
Q

Fiamme

A

Squashed fragment found in tuff

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36
Q

Graphic granite

A

Typically quartz intergrowths in microcline.

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37
Q

Myrmekite

A

“Wormy” intergrowth of quartz in plagioclase

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38
Q

Corona textures

A

Reation rims

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39
Q

Oxyhornblende

A

Grain of hornblende oxydizes, forming dark rim around the grain

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40
Q

Rapakivi

A

Typically plagioclase-mantled K-feldspar phenocrysts (very large)

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41
Q

What are the 8 major elements?

A

Si (+4)

Al (+3)

Mg (+2)

Fe (+2/+3)

Ca (+2)

K (+)

Na (+)

O (-2)

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42
Q

What are the 3 minor elements?

A

Ti (+3/+4)

P (-3)

Mn (+2)

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43
Q

Felsic

A

65-75% silica

Typically rich in Al, Na, K

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44
Q

Intermediate

A

52-65% silica

Na vs. Ca content

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45
Q

Mafic

A

45-52% silica

Rich in Mg, Ca, Fe

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46
Q

Ultramafic

A

<45% silica

Rich in Mg, Fe

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47
Q

What is the basic principle upon which spectroscopic methods of mineral anaylsis work?

A

The ability of atoms to either absorb or emit radiation with frequencies characteristic of the specific element.

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48
Q

Primary magma

A

The magma composition that is first melted

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49
Q

Evolved magma

A

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)

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50
Q

Parental magma

A

Least evolved magma found (generally lowest silica content)

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51
Q

Harker diagrams typically use a differentiation index of SiO2. What are some other usesful indices?

A

MgO (useful in basaltic rocks)

Mg-Fe rations (useful in basaltic rocks)

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52
Q

AFM Diagram

A

Used to furhter subdivide subalkaline magma series into tholeiitic or calc-alkaline series

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53
Q

Feldspar Ternary

A
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54
Q

Anorthite

A

CaAl2Si2O8

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55
Q

Albite

A

NaAlSi3O8

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56
Q

Orthoclase feldspar

A

KAlSi3O8

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57
Q

Plagioclase Feldspars

A

Anorthite

Bytownite

Labradorite

Andesine

Oligoclase

Albite

58
Q

Alkali Feldspars

A

Orthoclase

Sanidine

Mircrocline

Anorthoclase

59
Q

Forsterite

A

Olivine solid solution end member

Mg2SiO4

60
Q

Fayalite

A

Olivine solid solution end member

Fe2SiO4

61
Q

Which olivine compositions form at higher temperatures vs. lower temperatures?

A

Forsterite (higher T) –> Fayalite (lower T)

62
Q

What igneous rocks commonly contain olivine?

A

Typically found in mafic and ultramafic rocks

63
Q

Pyroxene Quadrilateral

A
64
Q

Augite

A

(Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al)2O6

65
Q

Clinopyroxenes

A

Dioside (Mg) –> Hedenbergite (Fe)

66
Q

Orthopyroxenes

A

Enstatite (Mg) –) Ferrosilite (Fe)

67
Q

Diopside

A

MgCaSi206

68
Q

Hedenbergite

A

FeCaSi2O6

69
Q

Enstatite

A

Mg2Si2O6

70
Q

Ferrosilite

A

Fe2Si2O6

71
Q

What igneous rocks commonly contain pyroxenes?

A

Intermediate to mafic igneous rocks.

72
Q

Lithophile

A

“stone loving”

elements that prefer silicate phases

73
Q

Chalcophile

A

“copper loving”

elements that prefer sulfide phases

74
Q

Siderophile

A

“iron loving”

elements that prefer metallic phases

75
Q

Goldschmidt’s Rule #1

A

2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overal proportions.

76
Q

Goldschmidt’s Rule #2

A

If 2 ions have a similar radius and the same valence:

the smaller ion is preferentially incorporated into the solid over the liquid

77
Q

Goldschmidt’s Rule #3

A

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

78
Q

Chemical Fractionation

A
  • uneven distribution of an ion between two competing phases
79
Q

Distribution or partition coefficient

A

KD = Xisolid/Ximelt

80
Q

What is meant by incompatible?

A

If KD < 1:

Element is incompatible with solid, so is concentrated in the melt.

81
Q

What is meant by compatible?

A

If KD > 1:

Element is compatible with the solid, so is concentrated in the solid.

82
Q

What are high field strength elements?

A
  • 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
83
Q

What are large ion lithphiles?

A
  • K, Rb, Cs, Ba, Pb2+, Sr, Eu2+
  • Tend to be mobile, especially if fluids are involved
84
Q

Bulk distribution coefficient

A

Di= ΣWADiA

WA=weight % of mineral A in the rock

DiA=partition coefficient of element i in mineral A

85
Q

What is the ‘europium anomaly’?

A

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+)

86
Q

Rare Earth Elements

A
  • 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)
87
Q

What mineral that may occur in mantle rocks at certain pressures really likes the heavy rare earths?

A

Garnet

88
Q

What is a spider diagram?

A

Samples are compared to average MORB (MORB=mid ocean ridge basalt–most abundant igneous rock at Earth’s surface)

89
Q

Ni as a trace element

A

Highly compatible

Concentrated in olivine

90
Q

Ba as a trace element

A

Incompatible element

Substitutes for K in K-feldspar, mica, or hornblende

91
Q

Rb as a trace element

A

Incompatible element

Substitutes for K in K-feldspar, micas, or hornblende (less readily in hornblende)

92
Q

Zr as a trace element

A

Very incompatible

May occasionally replace Ti in sphene or rutile

93
Q

REEs as trace elements

A

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

94
Q

What are the three main types of basalt?

A
  1. Tholeiitic–(most common) MORBs, primitive island arcs
  2. Alkaline–within-plate settings (hot spots)
  3. Calc-alkaline–mostly restricted to complex, evolved arc settings
95
Q

Characteristics of tholeiitic basalt

A

Groundmass: No olivine, OPX common, intersititial glass and/or quartz common

Phenocrysts: OPX reaction rims, early plagioclase

96
Q

Characteristics of alkaline basalt

A

Groundmass: Olivine common, no OPX, no quartz

Phenocrysts: Zoned olivine common, plagioclase less common

97
Q

What sources of information do we have on mantle composition?

A
  1. Ophiolites
  2. Dredge samples from oceanic fracture zones
  3. Nodules and mantle xenoliths in some basalts
  4. Xenoliths in kimberlite
98
Q

Xenolith

A

Large piece of rock embedded in a different, larger rock

99
Q

What is the maximum % of melting we think can occur under normal conditions?

A

20-25% (produces tholeiite, leaves dunite residuum)

If only 15-20% melts–produces tholeiite, leaves harzburgite

100
Q

What is lherzolite?

A

Fertile, unaltered mantle

101
Q

Aluminous phases in lherzolite at various depths

A

Plagioclase–<40-50km

Spinel–50-80km

Garnet–80-400km

Si–VI coordination, >400km

102
Q

Three ways the mantle could melt

A
  1. Temperature increase by radioactive decay–but not enough radioactive elements in mantle–occurs locally at hot spots
  2. 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
  3. Add volatiles–lowers melting point–fluids can be added at subduction zones
103
Q

What melting processes favor tholeiitic formation?

A
  • Shallow source of melting
  • Relatively high degree of melting (20-25%)
  • Presence of H2O-rich volatiles
  • Olivine fractionation during rise of melt
104
Q

What melting processes favor alkaline formation?

A
  • Deep source of melting
  • Low degree of melting
  • Presence of CO2-rich volatiles
  • Al-silicate fractionation during rise of melt
105
Q

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?

A

OIB: typical negative slope (enriched in incompatibles)

MORB: low positive slope, requires source already depleted in incompatibles by previous melting episode

106
Q

Fertile mantle source rocks

A

Nothing has been removed by prior melting episode

Lower mantle

107
Q

Enriched mantle source rocks

A

Mantle to which something has been added

108
Q

Depleted mantle source rocks

A

Residuum mantle after certain elements have been removed

Upper mantle

109
Q

Magmatic Differentiation

A
  1. Creates a compositional different in one or more phases
  2. Preserves chemical difference by segregating (or fractionating) chemically distinct portions
110
Q

What are common processes that accomplish magmatic differentiation?

A
  1. Crystal Fractionation
  2. Gravitational settling
111
Q

What is cumulate texture?

A

Mutually touching phenocrysts with interstitual crystallized residual melt

112
Q

Stoke’s Law

A

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

113
Q

How do pegmatites form?

A

Late-stage fractional crystallization

Late melt is enriched in incompatible, LIL, and non-lithophile elements, crystallize rapidly

114
Q

What factors control magma mixing?

A
  • Temperature
  • Composition
  • Viscosity
  • Volatile content
  • % of each parent magma
115
Q

What are some pieces of evidence that magma mixing occurs?

A

Cross-cutting dikes or layers

Changes in mineral composition (reverse zoning)

Layered mafic intrusions

Granitic plutons

116
Q

Assimilation

A

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

117
Q

Slow Spreading Ridges

A

< 4 cm/a

Indian Ocean

118
Q

Fast Spreading Ridges

A

>4 cm/a

East Pacific Rise

119
Q

Half Spreading Rate

A

Movement of one side of the ridge away from the other

120
Q

Differences in Morphology Between Fast Spreading & Slow Spreading Ridges

A

Slow Spreading: Older deposits carried away from axis, cut by normal faults

Fast spreading: Rapid, continuous volcanism

121
Q

Black smokers

A
  • Hydrothermal vents associated with spreading ridges
  • Circulating fluids ready ~350ºC
122
Q

White smokers

A

Hydrothermal vents, cooler than black smokers (~300ºC)

123
Q

Ophiolite Stratigraphy

A

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

124
Q

Harzburgite

A

Formed deep in the mantle; composed of olivine and OPX

125
Q

Typical Crystallization Sequence of MORB

A

olivine –> olivine + plagioclase –> olivine + plagioclase + CPX

126
Q

What trend do MORBs following on an AFM diagram?

A

Tholeiitic

127
Q

What evidence do we see that a typical MORB is not formed from a primary magma?

A

Typically MORB is formed from derivative magmas formed from fractional crystallization.

128
Q

What are N-MORBs and E-MORBs?

A

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)

129
Q

Petrogenesis of MORBs

A
  1. Separation of plates, upward motion of mantle
  2. Decompression–partial melting (adiabatic rise)
  3. Focused region of melting
  4. Lower enriched mantle resevoir may also be drawn upward (E-MORB)
130
Q

Island arc

A

Chain of volcanoes formed along tectonic boundary where oceanic crust subducts beneath more oceanic crust

131
Q

Continental arc

A

Chain of volcanoes formed along tectonic boundary where oceanic crust subducts beneath continental crust

132
Q

What is the most common type of volcano, and how does it erupt?

A

Stratovolcano (composite)

Explosive eruption, silicic composition

133
Q

Island Arc Thermal Structure

A
134
Q

What rock types are common in island arcs?

A

Basaltic andesite or andesite

135
Q

What trend on an AFM diagram is restricted to subduction zones?

A

Calc-alkaline

136
Q

What arcs could show a tholeiitic trend?

A

Low-K (low K2O content) such as Tonga-Kermadec

137
Q

What does a typical arc volcanic rock look like?

A

Phyric: >20% phenocrysts

Plagioclase phenocrysts ubiquitous!

Augite & olivine in mafic rocks

Magnetite common in most compositions

138
Q

Magnetite

A

Fe3O4

OR

FeO • Fe2O3 (varying oxidation states of Fe!)

139
Q

Ilmenite

A

FeTiO3

140
Q

Island Arc Petrogenesis

A
  1. Dehydration of altered oceanic crust (chlorite, phyllosilicates @ <50km, amphibole at ~100km(
  2. Dehydration provides components that enrich overlying mantle in particular trace elements
  3. Fluids rise into overlying mantle wedge
  4. Hydrated mantle dragged down to greater depths
  5. Differentiation in shallower magma chambers produces calc-alkaline trend
141
Q

In what ways do continental arcs differ from island arcs?

A
  1. Mantle-derived magmas must rise through thick layer of continental crust (SiO2-rich crust, incompatible element-enriched, crustal contamination common)
  2. Continental crust is low density (mafic magmas may be more dense–may “pond” at base or within crust–allows for extensive differentiation or assimilation)
  3. Continental crust has low melting point (may see considerable partial melting of crust)

Igneous & Metamorphic Petrology Spring 2013 (2 decks)

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