Igneous Rocks Flashcards
(210 cards)
1
Q
what colour are primary igneous rocks?
A
dark/black
ie gabbro
2
Q
what is a primary igneous rock?
A
basalt that has been erupted from a volcano
3
Q
how can we classify rocks?
A
whole rock chemical composition
mineralogical composition of rock
modal composition of rock
4
Q
what do we call a rock of pure olivine?
A
dunite
5
Q
what minerals do you expect within a basalt?
A
plagioclase
clinopyroxene
mica
feldspar
olivine
6
Q
what is removed from olivine when crystalising?
A
Mg
Fe
7
Q
what is removed from pyroxene when crystalising?
A
Mg
Fe
Ca
8
Q
what is removed from plagioclase when crystalising?
A
Ca
Na
Al
9
Q
what is removed from amphibole and micas when crystalising?
A
K
Al
10
Q
What percentage of olivine is SiO2?
A
40%
11
Q
what happens when there is more silica content than cations?
A
doesnt have anything to react with therefore reacts with itself SiO2 to form quartz
12
Q
what are the sources of igneous rocks?
A
crust: igneous, sediment, metamorphic
mantle: fertile or depleted
erupts at surface
13
Q
what end member of plagioclase has the highest melting point?
A
Anorthite (instead of albite)
Calcium rich
stronger bonds need more energy to break
14
Q
what does 5GPa equate to?
A
when diamond forms
150km depth
15
Q
why do we refer to mantle as fertile?
A
contains minerals that can melt
ie has pyroxenes –> basalt
16
Q
why do we refer to mantle as depleted?
A
contains only olivine, needs 1800 degrees C to melt therefore typically doesnt
17
Q
what happens everytime you melt the mantle?
A
purify it ie makes it more olivine in composition
spew out what earth doesnt what ie Al,
18
Q
what do we call slow and fast growth?
A
slow growth = monocrystalline (large crystals)
fast growth = polycrystalline
19
Q
what is quenched texture?
A
needle like shapes formed by fast cooling
crystals floating in a sea of liquid(now solid) rock
typical of basalt
20
Q
what is the evolution of basalt?
A
basalt -> andesite -> rhyolite
21
Q
at what speeds do basalts and rhyolites flow?
A
basalts very slow - iceland
rhyolites very fast get out - mt pinnatubo
22
Q
What is the BADR Series?
A
Basalt-Andesite-Dacite-Rhyolite
23
Q
what produces BADR series?
A
evolution of magmas
mixing of magma
partial melting
24
Q
what does basalt form from?
A
the melting of periodite
25
what does a linear array of data points imply?
rocks are inter-related, forming an igneous rock series
26
what kind of igneous rock series do we see in trinidade?
nephelinite and phonolite
27
what kind of igneous rock series do we see in tenerife?
basanite, tephrite, phonolite
28
where do we see a rock series of four rock types?
st helena
iceland
29
how does mixing create hybrid magmas?
magma A + magma B = magma C (hybrid)
dioriric magma intrudes into chamber, viscosity differences cause mingling
30
what causes mingling?
viscosity differences
31
how does mingling lead to mixing?
ionic inter-diffusion between melts (making a homogeneous melt)
32
what do we call the hybrid magmas that form within a chamber?
pillows
ie basaltic andesite pillows (incoming preexisting pillows)
33
what causes convection in a magma chamber?
fountaining of incoming magma - heat difference causes convection (stirring)
34
what causes a volcanic eruption?
new magma injection into chamber causes instability = seismicity = eruption
35
what makes a magma/rock felsic?
Si rich
36
what makes a magma/rock mafic?
Mg and Fe rich
Olivine, pyroxene
37
how do magmas of different composition equilibrate?
elements diffuse across a gradient as a function of temperature
38
what does a diffusion profile tell us?
how long elements were in contact before they erupted
39
what is a nucleation point?
where an mineral grows rapidly from a seed
40
how does fractional crystallisation work?
removal of material from Magma A --> Magma B --> removal of material from Magma B --> Magma C
41
How is the composition of magma changed in fractional crystallisation
crystals form from magma cooling and settle to floor of chamber
crystals from early cooling accumulate
42
what crystallises first out of a melt?
olivine (1200°C)
pyroxene & plagioclase (1100°C)
plagioclase (1000°C)
quartz
mafic first
43
what removes crystals from a melt?
crystal settling
eruption
44
why is there no quartz in basaltic rocks?
lots of cations for SiO4 to bond with
quartz only forms when there are no cations and bonds with itself
45
what does a typical histogram of lavas by fractional crystalisation from basic parent magma look like?
basic intermediate acid
high to very low
46
what does a histogram of lavas from a mid ocean ridge look like?
massive basalt
vanishingly small amount of andesite and dacite. no rhyolite
consistent with bowen prediction
46
what processes result in uneven/untypical histogram shape?
more plumbing systems
hydrated basalt
partial melting of subducting basaltic oceanic crust = andesite
47
why are mid ocean ridges purely basaltic?
there is no system provided to allow basalt to evolve
48
what produces rhyolite?
quartz rich melt
49
what is hydrothermal alteration?
the chemical replacement of original minerals in a rock with new secondary minerals where a hydrothermal fluid delivers the chemical reactants and removes the aqueous reaction products
50
give examples of secondary phases from hydrothermal alteration?
carbonates
hydroxides
oxides
zeolites
smectites
silicates
51
is a diamond stable at earth surface?
metastable - needs a lot of energy to return to graphite
52
by what degree does plagioclase alter hydrothermally in a granitic and gabbro rock?
plagioclase alters a lot in granites as is it the least stable phase
minimal alteration in gabbros as it is the most stable phase
52
what is the process of serpentization?
hydrothermal water enters a mineral and flows through fractures in olivine, altering the minerals
53
what is the equation for serpentinization?
olivine + water = serpentine + opaque
53
what are potential opaques produced by serpentinization?
brucite (Mg rich)
magmatite (Fe rich)
hematite (Fe rich)
54
how does serpentinization appear?
mesh texture
54
what is iddingsite alteration?
hydrothermal alteration of olvines and plagioclases with iron rich or clay fluids, resulting in a red rim around the mineral
55
what is sericitization?
feldspars being hydrothermally altered to sericite, in concentric shapes
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what is sericite?
fine grained white phyllosilicate with an unknown composition
typically micas. appears dirty in ppl
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where do we find hydrothermally altered rocks?
subduction zones
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what causes a phreatic eruption?
large quantities of water within the magma chamber
water is the most abundant gas released from volcanoes
58
what is a geyser?
sub aerial black smoker(high thermal energy and chemicals)
immense weathering beneath
59
what what point does water become a supercritical fluid?
374°C
218 bars
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what is the benefit of supercritical fluids?
can transport mass quickly and far
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what do we know about the behaviour of elemental groups?
large ion lithophiles dissolve easily in water at low temperature
high field strength and rare earth elements do not dissolve at low temperature
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what elements are the most compatible in mafic minerals?
Ca
Mg
Fe2+
60
why are certain elements not compatible with mafic minerals?
too big (Large ion lithophiles ie K)
too charged (high field strength ie U)
61
what are the classification of visible alteration?
F type (fresh) - black, all primary minerals
L type (low temp alteration) - dark grey to grey, reddish mm-cm halos
H type (High temp alteration) - dark green to light green = presence of chlorite
61
what is loss on ignition?
crude proxy used to access the degree of alteration.
powered and weighed -> melted and weighed -> mass difference expressed as % mass difference = abundance of volatiles (secondary minerals)
62
how do you identify a granite from a granodiorite?
count the abundance of quartz, plagioclase and K feldspar in a thin section that is representational and plot on triange
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what is meant by silisic?
high quartz content
63
on what graph do gabbros plot?
A (kfeld)-P(plag)-F(foid)
no quartz
63
what are the stages of crystalisation of plagioclase?
1500-1100 degrees c
big crystals = anorthite
small crystals = albite (Ca used up by anorthite)
63
how does equilibrium crystalisation differ from fractional crystallisation?
equil = homogeneous, same composition as the inital melt
fract = zonation , calcic cores, sodic rims . disequilibrium is unstable
64
what is the order of density of the crust?
basalt 3.08
granite 2.71
peridotite 3.34
eclogite 3.54
64
can granite be subducted?
no
65
what are the tools of petrogenesis?
mineralogy/petrography
major oxides
trace elements
isotopes
65
what are the assumptions for granitic melts?
composition of a rock reflects the composition of the magma from which it crystalised
composition of the magma provides information on source, contaminations, fractional crystallisation.
65
what differentiates a granite from an alkali granite?
granite: 20-42% qtz, 10-65% plag, 90-35% K feld
alkali granite: 20-42% qtz, <10% plag, >90% k feld
65
what are the four granite types?
Sedimentary
Anorogenic
Mantle
Igneous
65
what separates granites and rhyolites if they have the same composition?
rhyolites crystallise on the surface, granites crystallise underground with temperature and pressure
65
if quartz is present is the rock felsic or mafic?
felsic
65
where is there a lot of granite production?
subduction zones ie lesser antiles
65
what major oxides are found in granites?
SiO2
Na2O, K2O
CaO
Fe2O3, MgO, MnO, FeO
Al2O3
TiO2
P2O5
H20
66
what are the chemical classifications of granite?
Peraluminous Al2O3>CaO+Na2O+K2O
Metaluminous Na2O+K2O
66
What are M type granites?
ocean island arcs + mid ocean ridges + ocean islands
66
what are I type granites?
Metaluminous
Ocean island arc +continental arc + continental collision +post orogenic uplift/collapse
66
what are S type granites?
Perluminous
continental arc + continental collison + post orogenic uplift/collapse
66
what are A type granites?
peralkaline
post orogenic uplight/collapse + continental rifting, hot spot
66
what is hot stokes diapir model?
ascent Velocity = 2Gravity x Radius of model sphere(difference in density) / 9x viscosity
66
what are mechanisms of granite magma ascent?
10^5 - 10^9 slow =segregation by gravity driven compaction faults, major continental shear zones, diapirism
10^-1 - 10^2 fast = 1-50m wide dykes, partial melting by dehydration, segregation by deformation-enhanced flow, enplacement along structural traps
66
what increases viscosity?
decreased temperature
increased %crystals
increased SiO2
decreased H2O
66
What increases density?
increased pressure
decreased temperature
reduced water
composition
66
what is the coordination number?
number of oxygens bonded to a cation
66
what are the characteristics of a interlocking structure?
sticky, prevents diffusion, explosive (big ash clouds)
rhyolite
66
what are the characteristics of a open structure?
runny, allows diffusion.
basalt
66
what is the basic rule of equilibrium?
any two things in contact will equilibriate
66
what does a glassy matrix suggest about the history?
quenched from an eruption
66
what does partial melting lead to?
trace element partitioning
66
why does OIB have a great abundance of incompatible trace elements?
comes from deeper and more enriched mantle
66
what happens to the mantle and crust during melting?
depletion of the mantle
enrichment of the crust
66
where on bowens reaction series does potassium become compatible?
incompatible above biotite
compatible below amphibole
66
when does simple substitution occur?
when cations are of the same size and charge
66
what is omission substitution?
charge balance maintained by leaving a site vacant
66
what are Goldschmidts rules?
1. ions of one element can replace those of another in a crystal if their radii differ by less than 15%
2. ions whose charges differ by one unit substitute readily for one another provided electrical neutrality of the crystal is maintained. if the charges differ by more than one unit, substitution is generally slight
3. when two different ions can occupy a particular position in a crystal lattice, the ion with the higher ionic potential forms a stronger bond with the anions surrounding the site
66
Where do we find Nickel deposits?
in laterite deposits
contained in peridiorite which weathers very quickly and easilt
66
why does compatibility change?
changes at different stages of igneous evolution cooling
66
is mars from Mg or Fe rich?
Fe -
66
where does melt cooless?
in corners - moves along grain boundaries
66
what is a D value?
how much a mineral prefers a melt to a solid
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