geochemistry Flashcards
1
Q
what does HFSE stand for?
why does it have this name?
A
high field strength elements
they get their names from the high charge and small ionic radius thus high field strength
2
Q
are HFSE compatible or incompatible during mantle melting? why?
A
incompatible
because its hard for them to change with 2+ charged ions
3
Q
what are the four main HFSE? which are moderaltely incompatible and which are highly incompatible?
A
Zr4+ (zirconium) - moderatley
Hf4+ (hafnium) - moderately
Nb5+ (niobium) - highly
Ta5+ (tantalum)- highly
4
Q
by mass what are the ratios of mantle core and crust?
A
mantle ~2/3
core ~ 1/3
5
Q
around what percentage are trace elements in common rocks?
A
trace elements <0.1%
6
Q
what can and cant trace elements do?
A
trace elements can substiute for major elements in minerals but they are not themselves essential structral constituents
7
Q
in terms of mass what does 1ppm and 1ppb equal?
A
1ppm = 1 microgram of element per gram of rock
1ppb = 1 nano gram of element per gram of rock
8
Q
what are the major elements in silicate rocks?
A
O
Si
Al
Na
Mg
Ca
Fe
9
Q
what are is the percentage range of minor elements?
A
present at concentration of 0.1% to 1%
10
Q
what element can behave like a major and trace element?
A
potassium does not form its own mineral in MORB or mantle thus K is a trace element in the ocean crust and mantle
K is a major element in granites and continental crust where it forms K feldspars
11
Q
what is meant by lithophile elements?
A
lithophile elements
are rock-loving, as they like to partition into
silicate phases. They are concentrated in the silicate portions of the
Earth (the crust & mantle).
12
Q
what is meant by siderophile elements?
A
Siderophile elements
are metal-loving, as they partition into
metallic liquids. They are strongly enriched in the core but depleted in the silicate Earth.
13
Q
what is meant by chalcophile elements?
A
Chalcophile elements
are sulfur-loving and they partition into
sulfide liquids.
14
Q
what is meant by atmophile elements?
A
Atmophile elements
are generally highly volatile, which means that
they like to form gases or liquids at the Earth’s surface. They are
concentrated in the atmosphere and the hydrosphere.
15
Q
what causes the goldschmidts classification?
A
differenes in chemical and physical properties of the elements.
elements with low electronegativity have a tendency to form ionic bonds with elements of high electronegativity. thus they form oxides and silicate minerals so are lithophile.
The siderophile and chalcophile elements have intermediate
electronegativities and they prefer covalent bonds (as those in
sulfides) and metallic bonding (which both feature sharing of
electrons) to ionic bonds.
The noble gases are very unreactive and they do not form bonds.
Thus concentrated in the atmosphere.
16
Q
chalcophile does not automatically imply that an element is found in sulfide ore deposits. what does it mean then?
A
Many sulfide deposits are precipitated
from aqueous solutions but chalcophile means that an element
partitions into magmatic sulfide liquids/phases.
17
Q
what does HSE stand for? how do they behave in the mantle?
A
HSE - highly siderophile elements
the HSE behave as chalcophile elements in the mantle
18
Q
the partitioning of trace elements into minerals is primarily a function of…
A
the partitioning of trace elements into minerals is primarily a function of ionic charge and radius
19
Q
The ions of one element can extensively replace those of
another element in ionic crystals (such as silicates) if…
A
The ions of one element can extensively replace those of
another element in ionic crystals (such as silicates) if their
radii differ by less than ~15%.
20
Q
how can ions whose charges differ by one substiute for each other?
A
Ions whose charges differ by one unit substitute readily for
one another, provided electrical neutrality can be maintained
in the crystal.
charge neutrality is maintained by the coupled
substitution of Al3+ for Si4+.
21
Q
When two different ions occupy a particular position in a
crystal lattice, the ion with … forms a stronger bond with the anions surrounding the site.
A
When two different ions occupy a particular position in a
crystal lattice, the ion with the higher ionic potential ( = higher
charge/radius ratio) forms a stronger bond with the anions
surrounding the site.
thus the ion with the higher ionic potential is preferred to be substiuted.
22
Q
Substitution may be limited, even when the size and charge
criteria are satisfied, when the competing ions have different…
A
Substitution may be limited, even when the size and charge
criteria are satisfied, when the competing ions have different
electronegativities and when they form bonds of different
ionic character.
23
Q
what is the camouflage principle?
A
The Camouflage Principle applies where two ions have virtually
identical charge, ionic radii, and electronegativity.
when a mineral accepts two minerals as readily as the other due to being so similar. for example zircon (ZrSiO4) accepts Zr4+ and Hf4+ as much as the other.
the Zr/Hf ratio is therefore similar for zircon crystals and the magmatic liquid from which the zircon crystallizes.
all rocks and mineral have nearly identical Zr/Hf ratios
24
Q
what is meant by the capture principle?
A
The Capture Principle applies when a trace element is
preferentially incorporated into a mineral relative to a major ion
because the trace element has a higher ionic potential.
25
what is meant by the admission principle?
The Admission Principle applies to the incorporation of ions
that have a lower ionic potential than the major elements.
The charges, ionic radii and electronegativities of K+ and Rb+
ions are similar.
thus when K-feldspar crystallizes from a magma, Rb is incorporated too.
the difference in ionic potential means that the larger
Rb+ ion is not incorporated in the same proportion as K+.
This means that the feldspar has a higher K/
Rb ratio than the magma
26
what is the formula for KD?
KD = Csolid/ Cliquid
where C is the concentration of a particular element
27
what are elements with KD that are:
\>1
\<1
\<\<1 (~\<0.1)
\>1 compatable
\<1 incompatiable
\<\<1 (~\<0.1) highly incompatible
28
what is the bulk partition coefficient?
it is the degree to which an element is distributed between the liquid and solid phases depending on the sum of the individual partition coefficients, weighed according to the relative proportions of the mineral phases present in the solid.
D = partition coefficients X mass fraction of a specific mineral.
29
which three mineals dominate the earths upper mantle?
olivine
orthopyroxene
clinopyroxene
30
why do we get low amount of Ni in mantle derived basalts that erupt on earth?
This is because Ni is a compatible element during partial melting of the mantle.
thus it will stay in the mantle rocks under ground
31
describe PGEs
platinum group elements (PGE)
a group of noble metals
partition strongly into mantle sulfides with KD values of \>104
32
describe HSE
highly siderophile elements (HSE)
is made up of PGE and Re
low concentrations in mantle and crust because they are concentrated in the earths core.
33
describe LILE
**large ion lithophile elements (LILE)**
Elements such as rubidium (Rb) and strontium (Sr) with low
charges and large ionic radii do not substitute readily into the
main silicate phases of the mantle.
They display incompatible behavior during mantle melting, and they are thus enriched in the melt phase.
34
what type of silicates is the only one that Sr is compatible?
what happens to the concentration of Sr and Rb when magma erupts?
feldspars
as Sr is compatible in feldspar when magma erupts it will be low in Sr and high in Rb as the Sr has been removed from the magma to make feldspars.
35
describe REE
**rare earth elements (REE) **(or lanthanides)
```
The REE are incompatible elements in the mantle and they are
dominantly trivalent (Ln3+).
```
36
what is another name for REE
lanthanides
37
what is the use of REE?
REE are important as trace element indicators of magmaic processes.
38
what does this graph show?
link this to compatibilty
the lowest atomic mass has the largest ionic radius
the larger light ions are less compatible than the smaller heavier ions
39
give an important mantle phase which shows the gradual change in compatibilty in REE
The most important mantle
phase for REE fractionation is
garnet – the heavy REE
(HREE) are significantly more
compatible in garnet (KD ~ 5)
than the light REE (LREE; KD
~0.02)
40
explain the zig zag shape graph and how we are able to make it into a straight line.
The zig-zag pattern is a consequence
of variations in nuclear stability during
the formation of the elements –
elements with even atomic number
have higher abundances throughout
the solar system.
the rock
concentrations are divided by the
concentrations in meteorites (CI
chondrites). This cancels out the
cosmochemical variations and
highlights the geological effects.
41
both U and Th partition into\_\_\_\_\_\_\_ they are _________ elements
both U and Th partition into **silicates** they are **lithophile** elements
42
lead partions into \_\_\_\_\_, \_\_\_\_\_\_\_, and _______ bearing compounds.
Pb thus displays \_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_\_, and _______ behavior
Lead (Pb) partitions into **silicates** but it also displays affinities for
**metal** and **sulfur-bearing** compounds.
Pb thus displays **lithophile,
chalcophile and siderophile** behavior.
43
explain the decoupling of U from Th in the earths hydrosphere
The uranyl ion UO22+ is highly soluble in water whereas U4+ and Th4+ are highly insoluble.
This leads to a pronounced decoupling of U from Th in the Earth’s hydrosphere.
44
what can substiute for K+ in feldspars?
what does it require for this to happen?
Pb is typically divalent in silicates. It can readily substitute for K+ in feldspars, just like Sr2+ and Ba2+
this requires that **charge neutrality** is maintained by coupled **exchange of Al3+ for Si4+.**
45
how do we get the concentrations of highly incompatible elements like Th, U, and Pb to increase?
The concentrations of incompatible elements
increase during **magma differentiation** (crystal fractionation).
46
why do mantle derived magmas tend to have high U/Pb ratios?
U is slightly more incompatible than Pb during mantle melting
thus mantle-derived magmas tend to have high U/Pb ratios.
47
why is Th4+ more incompatible than U4+
The ionic radius of Th4+ is slightly larger than that of U4+ and hence Th is slightly more incompatible.
48
is Th and U more or less compatible in the mantle than the REE?
Both Th and U are more incompatible in the mantle than the REE.
49
how are trace elemetns useful in the modeling of igneous processes?
because trace element fractionation is
governed by the mineral phases present, which in turn are
governed by p, T and chemical composition.
50
what is batch melting involve?
Batch melting involves complete equilibration between the solid
phase (rock) and the melt (liquid).
51
what is the equation for batch melting?
C0 = CLF + CS (1 - F)
C0 = original conc.
CL = concentration in liquid
CS = concentration of the solid
F = melt fraction = mass of melt/mass of system
52
give the equation that describes the enrichment or depletion of a trace element in a melt relative to the original state in the unmolten solid as a function of the degree of melting
53
what does this graph show?
where are the D value calculated from?
at the top is the incompatible elements and the bottom is the compatible
eventually they all converge to one as when completely melted it will have the same conc as the source rock.
D = 1/F
54
what equation is used to calculate the trace element concentration of the residual rock that is in equilibrium with a partial batch/ equilibrium melt.
55
what is meant by fractional melting?
If melting is very rapid and the liquids are sufficiently low in
viscosity, the melts can be quickly driven away from the source
rock by buoyancy forces. In this case, the solid will be unable to
equilibrate with the total amount of melt that is produced during the melting process.
Hence we have incomplete equilibration between
solid and melt.
56
what equation is used for fractional melting that describes the composition of a single melt increment that is formed at a particular value of F.
57
describe this graph on fractional melting
highly incompatible elements (D\<\<1) are depleted for the rock quickly .
they are so incompatible that when the elements form the first melt and this is driven away thus leaving none of the incompatible elements in the rock so the curve drops to zero.
58
what is meant by aggregate liquid?
what is it similar to?
**aggregate liquid**
produced by mixing the various melt increments formed in fractional melting over the melting interval F = 0 to F
aggregate fractional melts have similar compositons to batch melts for the same value of F.
thus it has a similar graph to batch/ equillibrium melting
real melting lies inbetween these two extremes.
59
what is the equation for an aggregate liquid?
60
what is equilibrium crystallization?
Equilibrium crystallization occurs when the total liquid and the total
solid remain in equilibrium throughout the crystallization
(differentiation) of a magma.
it uses the same equation as batch melting
61
when **cant** the model of equilibrium crystallization be used?
This model cant be used in the formation of solid
mineral crystals in a magma chamber because **it requires that
the crystal interiors are in continuous equilibrium with the
melt.**
This can only be achieved by solid-state diffusion which is
too slow over mineral-sized distances (mm to cm)
62
when is the equilibrium crystallization model useful?
it can be applied to the equilibration of two immiscible
melts, which can both rapidly equilibrate internally by diffusion:
e.g
Equilibrium between a silicate melt and a metallic melt –
relevant for core formation on Earth, where an Fe-rich melt
segregated from the molten silicate mantle.
Here we would use KD values for the system metallic meltsilicate
melt.
63
Fractional crystallization assumes that an instantaneous
equilibrium is achieved between the liquid and an (infinitesimally)
thin surface layer of a solid crystal. In this case, what is the relevant partitioning equation?
64
describe this fractional crystallization graph.
imagine where they converge is the start.
as it cools the fraction of melt decreases thus the concentration of the higly incompatible elements increases.
the compatible elements go into the solid crystal straight away and thus are depleted in the melt very quickly.
65
in terms of fractional crystallisation, what is the equation that shows the mean trace element concentration of a crystal that is formed by crystallization between F = 1 and F?
intergrates over all the layers formed giving the mean trace element concentration of a crystal
66
the solar system and our sun probably formed from a...
The Solar System and our Sun probably formed from a **dense
molecular cloud of gas and dust.**
The formation of the Sun and the Solar System began when a
fragment of a molecular cloud started to contract to form a
dense core, which evolved by further collapse into the protosun.
67
describe the process of the creation of the solar system
a) contraction of the molecular cloud
b) formation of protosun and a rotating disk which flattens out
c) condensation of gas to dust grains; dust aggregates into larger bodies
d) formation of sun complete; dissapation of nebula. frequent small collisions drive formation of planetesimals
e) planet formation through large and violent collisions between planetesimals
68
what is the compositon of the solar system lies in the sun?
thus what is the bulk solar system made up of?
sun has \>99% of the mass of the solar system
thus the bulk of the solar system consists of 99% of the elements hydrogen and helium
69
how do we determine the elemental composition of the sun?
can be determined by measuring the intensity of certain wavelengths of light that are emitted by the elements in the suns exterior layer
we can get additional info on the suns composition through meteorites
70
what is meant by carbonaceous chondrites?
they are a group of meteorites the have relative elemental abundances (normalized to Si - abundance of element/ abundance of Si) that are nearly identical to the suns photosphere.
71
what is the use of CI chondrites?
generally used to define the average composition of the solar system.
72
describe the main consituents of the earth.
depleted in gaseous consituents - no H and He.
consist mainly of the elements O, Fe, Si, Mg
73
the earth is made mainly of O, Fe, Si, and Mg.
where do we find these elements?
Fe is concentrated in a metallic core and usually found in the reduced form.
O,Si, and Mg form the silicate mantle
74
what, if any, differences are between the earths composition and CI chondrites.
many elements are strongly depleted in the bulk earth but are present in CI chondrites.
the highly depleted group of elements are the volatile elements.
75
what is the opposite of volatile?
refactory
describes at which temperatures elements condense
76
describe refactory elements.
high condensation temperatures \>1400K
these elements built the first solid stages of the solar system.
mostly metals and metals that form refactory oxides
77
explain why terrestrial planets if the inner solarsystem are all strongly depleted in volatile constituents?
this depletion reflects the conditions and processes of the solar nebula.
at an early stage it was too hot to fully condense the more volatile constituents.
the refactory elemtents that did condense went on to form the terrestrial planets but these planets were never able to acquire chondritic proportions of the volatile elements.
78
what is meant by the BSE
**bulk silicate earth (BSE)**
BSE = mantle + continental crust + oceanic crust
79
the composition of the BSE is essentially identical to the **primative mantle**, which represents...
the composition of the BSE is essentially identical to the **primative mantle**, which represents the primative state of the earths mantle prior to the formation of the continental crust
80
where is nearly all of the earths iron?
in the inner and outer core.
81
Based on studies of iron meteorites (most of which are probably
remnants of asteroid cores), the Earth’s core also contains large
amounts (~5%) of
Based on studies of iron meteorites (most of which are probably
remnants of asteroid cores), the Earth’s core also contains large
amounts (~5%) of **nickel (Ni)**
82
how do we know the earths core also contains other light elements?
From geophysics we know that the Earth’s core is not as
dense as pure Fe-Ni alloys. Such studies imply that the core also
contains ~10% of one or several currently unidentified light
elements.
likely to be O, Si, S, C
83
why does the compositon of the BSE differ from the solar system average, as defined by CI chondrites?
- volatile depletion in the solar nebula to produce a volatile depleted bulk earth composition.
- depletion of siderophile elements in the silicate earth by core formation
84
explain this graph in terms of the increase in refactory elements, the decrease in siderophile elements and decrease in volatile elements.
the graph shows the abundances of the elements in the primative mantle compared to CI chondrites.
increase in refactory because they are not affected by core formation and when other materials move to the core their concentration increases.
the decrease in siderophile elements is due to them being drawn into the core.
highly volatile due to not condense quick enough in early stages of formation of earth.
85
compare the elemental composition of the continental crust and the mantle.
The CC is low in Mg with a high Si/Mg ratio. In addition it has
relatively high contents of Ca, Al and the alkali elements Na and K.
The present (upper depleted) mantle of the Earth has a high Mg
content and thus low Si/Mg. The elements Ca, Al, Na, and K
are depleted in the mantle relative to the CC.
Ca, Al, Na, and K are thus important constituents of the CC but
only minor constituents of the mantle and the bulk Earth.
86
how is there a net transfer of Ca, Al, Na, and K form the interior of the earth to the surface?
The elements Ca, Al, Na, and K are enriched in basalts.
Basalts are produced by partial melting of the mantle
Ca, Al, Na, and K partition into mantle melts.
Basaltic magmas thus provide a net transfer of Ca, Al, Na, and K
from the interior of the Earth (mantle) to the surface.
87
not all mantle melt basalts that form the oceanic crust is subducted back into the mantle.
what are the major mechanisms of crustal growth?
In subduction zones, mantle-derived melts form island arc
volcanoes. Such island arcs are not subducted but they
eventually become parts of continents.
Some basalts are added directly to the continental crust from
the mantle.
88
what is the average continental crust composition similar to?
average continental crust has a composition akin to
**andesites.**
Andesites are rocks that are mainly found in island arc
settings and active continental margins above subduction zones.
89
describe the differences between the primative and depleted mantle.
why is there little difference between their composition?
The primitive mantle is the silicate portion of the Earth following
volatile depletion and core formation. This is essentially identical to
the BSE.
The depleted mantle is the residue that remains today following
the extraction of the continental crust from the mantle
their compositions are relatively similar because the continental resevoir is really small compared to the mantle. (~0.6% of the BSE) which is insufficient to generate strong depletions for the major mantle constituents
biggest differences is to do with incompatible elements.
90
how can we make a model of the mantle?
from **direct samples** of the upper mantle. such as abyssal peridotites and xenoliths.
**seismologists** study the velocities of seismic waves through the mantle which are effected by the mineral phases present
**experiments** show which phases are stable in the mantle
**cosmochemistry** - looking at meteorites
91
is the mantle layered or as a whole?
seismolgy has shown that there is no barrier at 660km but that the cold slabs can go all the way down to the core.
92
what are the uses of isotopes in earth sciences?
date objects
provide constraints in the composition of the earth
to study the geochemical evolution of the earth e.g. formation of the core
to investigate change in the earths past climate and mechanisms of climate change.
93
what is meant by **radiogenic isotope geochemistry?**
**radiogenic isotope geochemistry**
Studies of isotopic variations from the radioactive decay of
unstable nuclides.
e.g. 238U to 206Pb
94
what is meant by **stable isotope geochemistry?**
**stable isotope geochemistry**
Studies of isotopic variations, from physical, chemical or
biological processes, in a mass dependent manner.
Such changes are commonly called isotope fractionations.
Example: The evaporation of water enriches the light
molecule H2
16O in the vapor phase compared to the heavier
molecule H2
18O, which shows a complementary enrichment
in the liquid phase.
95
what does mass number =
mass number = protons + neutrons
96
define isotopes
same element but different N.
They have almost identical chemical properties but
different masses.
Example: 110Cd, 111Cd, 112Cd, 113Cd, 114Cd, 116Cd
97
define isobars
**isobars**
same mass number but different element thus different number of neutrons.
Example: 110Cd and 110Pd, 113Cd and 113In
98
what is the atomic mass unit?
what is it equal to?
Atomic mass unit: **amu**
1 amu is 1/12th of the mass of 12C
99
describe mass defect
the nuclide masses are less than the sum of proton, neutron, and electron masses
this **mass defect **is equivalent to the binding energy that holds the nucleus together.
E=MC2
a larger mass defect is equivalent to more binding energy thus more stable.
100
how do we calculate the atomic weight of an element with several isotopes?
it is the sum of the masses X abundance
101
what does this table show?
this is the chart of nuclides.
the chart provides information on all known stable isotpes, long lived radioactive nuclides and a large number of short and very short lived radioactive nuclides.
the shaded area shows stable atoms.
horizontal is trends of same element and its isotopes
diagonal downward shows isobars
this is only a small part of a much larger table
102
give an example of beta (negatron) decay
103
give an example of positron decay
104
what is meant by branched decay?
decay of a nuclide to two or more daughter atoms
e.g.
decay of 40K to both 40Ca (89%) and 40Ar(11%)
105
give an example of alpha decay
106
what is meant by spontaneous fission?
occurs only for the heaviest element e.g. 235U
it is the spontaneous breakup of a nuclide into two or more fairly heavy daughter nuclides
107
when using radioactivity equations why do we use ratios instead of absolute abundances?
using ratios is much more accurate and precise than absolute abundances/ concentrations.
the ratio is measured relative to a stable non radiogenic daughter isotope.
108
what does this graph show?
shows three minerals that crystallized from a melt.
after some time and decay the 87Sr has increased due to the decay of 87Rb to 87Sr.
the mineral with the highest amount of 87Rb then displays the most 87Sr.
109
on a graph what do the components of D = Do + N(elamdaT - 1) represent?
D = Do + N(elamdaT - 1)
y = mx + c
y = D
M = (e lamda T - 1)
x = N
c = Do
110
what is the line on this graph called and what can be worked out from it?
111
what conditions are necessary for successful dating with the isochron method?
isotopic equilibrium at t = 0
closed system from T= 0 to present
sufficient trace element fractionation between samples. as it reduces error bars
112
what is meant by an extinct decy system?
are based on short lived parent isotopes that were alive only during the early history of the solar system.
thus the radioactive parents are no longer present today.
their prescence can only be inferred from abundance variation in the daughter isotopes, which we can measure today.
113
what is the 87Rb --\> 87Sr system useful for?
it is particularly useful for dating felsic igneous and metamorphic rocks of the continental crust.
114
what problems can we have with the 87Rb-87Sr decay system?
Sr and particularly Rb are fluid mobile elements.
thus dating them can be a problem even when there is low temperature aqueous alteration of the samples
115
igneous differentiation is associated with increasing Rb/Sr ratios when ....
Igneous differentiation is associated with increasing Rb/Sr ratios
when **Sr is removed from the melt by the crystallization of
plagioclase** whilst Rb remains in the melt.
116
when dating igneous rocks with 87Rb/87Sr. is it better to use individual mineral isochrons or a whole rock isochron?
its better to use a whole rock isochron because it is not easily changed in a metamorphic event due to the two elements being fluid mobile.
this only works if the sample is large enough to act as a closed system.
117
how would you measure the ages from this graph?
Model ages are obtained by extending the isotope evolution line of a sample (mineral) back to its initial Sr isotope composition using the measured 87Rb/86Sr ratio.
Drawback: the initial Sr isotope composition must either be know or
assumed/estimated.
**take the gradient and apply M = (elamda t - 1)**
118
compare the BSE in terms of Rb and Sr to the CI chondrites.
the BSE is enriched in Sr and depleted in Rb. thus has a lower raton than the CI chondrites.
119
why is Rb depleted in the BSE?
it is thought due to Rb volatility and the BSE is depleted in volatiles
120
why is the BSE easier to make estimates on than the BE?
the bulk earth includes the core while the BSE doesnt.
as we are not fully sure on the composition on the core there is more error in the BE
121
why does core formation increase the amount of Rb and Sr in the BSE?
due to them being lithophiles they prefer to be in the silica based BSE
122
are Rb and Sr compatible or incompatible?
they have low D values thus are incompatible.
123
why does the continental crust have a large amount of the Rb and Sr?
why does it have a high Rb/Sr ratio?
they are incompatible.
Rocks that formed as small degree partial melts of the mantle thus
have particularly high Rb and Sr concentrations. Examples are Ocean
Island Basalts (OIBs) and the continental crust.
Rb is more incompatible than Sr thus has a high ratio.
124
what is meant by N MORBs?
why do they have a low Rb/Sr ratio?
N-MORBs (normal mid-ocean ridge basalts) are high-degree
partial melts of the depleted upper mantle (also called depleted
MORB mantle).
These liquids have Rb/Sr ratios lower than the
BSE, because the upper mantle was depleted (over time) in Rb
relative to Sr, by the extraction of the continental crust.
125
explain why the upper continental crust has a higher Rb/Sr ratio than the lower CC?
This is because the differentiation of the CC produced
• a lower crust that is dominantly mafic, rich in cumulates, and which
has suffered the extraction of upper crustal melts.
• an upper crust that is silicic and highly enriched in incompatible
elements.
126
why compared to CI chondrites is there a decrease in Rb and an increase in Sr
Rb is volatile,
Sr is refractory.
127
if Rb and Sr are lithophile elements they should be depleted in the core and enriched in the BSE.
why is it then that the ratios of Rb and Sr are the same for the BSE and the BE?
there is more in the the BSE than the BE but the ratio is the same.
128
over time what will happen to the amount of Rb and Sr in the crust compared to the mantle?
it will increase in the crust and decrease in the mantle.
129
why is the enrichment of Rb in the crust greater than Sr?
**Rb is highly incompatible,**
**Sr is moderately incompatible**
the continental crust is enriched in Sr and Rb relative to the depleted mantle.
The enrichment is stronger for Rb
130
describe the compatibility if both Sm and Nd.
why the difference?
Both Nd and Sm are moderately incompatible elements.
Nd is slightly more incompatible than Sm during mantle melting
because it has a **slightly larger ionic radius.**
131
the REE including Nd and Sm are relatively resistant toward ... thus are called...
The REE including Nd and Sm are generally considered to be relatively **resistant
toward mobilization by fluids**– they are**fluid-immobile elements**
132
why is the Sm - Nd ratio good for dating mafic and ultramafic rocks but not the CC in general (in particular siliceous rocks?
The **continental crust** in general and siliceous rocks in particular
**have low** and relatively uniform **Sm/Nd ratios** (= parent/daughter
ratio)
thus the system is no useful
**Mafic and ultramafic rocks have** variable and **high Sm/Nd ratios**
–› they are good candidates for Sm-Nd dating.
133
is Sm - Nd ages and initial Nd isotope composition sensitive or not to weathering ad metamorphism? why?
**Due to the fluid immobility of the REE**, Sm-Nd ages and initial Nd isotope compositions are not very sensitive to weathering and
metamorphism.
134
why does the BE and BSE have the same Sm/Nd ratio as the CI chondrites?
Sm, Nd, and the other REE are refractory and lithophile
elements
135
In absolute concentrations, the REE (Nd and Sm) are enriched in the BSE relative to chondrites (by about a factor of x 2.7).
why?
The higher concentrations reflect
(i) the loss volatile elements from the
material that built the Earth and
(ii) the loss of siderophile elements (particularly Fe) from the silicate Earth due to core formation.
136
Sm and Nd are __________ elements and they are therefore
depleted in the mantle and enriched in the continental crust (CC)
relative to the BSE.
Sm and Nd are **incompatible** elements and they are therefore
depleted in the mantle and enriched in the continental crust (CC)
relative to the BSE.
137
the LREE are more or less compatible than the HREE?
why?
thus the CC is enriched in _______ and thus the mantle is depleted in \_\_\_\_\_\_\_\_\_
the LREE are more incompatible than the HREE due to the LREE having larger atomic radii
thus the CC is enriched in LREE and thus the mantle is depleted in LREE
138
The Sm/Nd ratio of OIB’s and the continental crust are relatively
low because...
The Sm/Nd ratio of OIB’s and the continental crust are relatively
low because clinopyroxene and garnet are present in the magma
source. These minerals (particularly garnet!) preferentially retain
the HREE relative to the LREE.
139
N-MORB's typically have Sm/Nd ratios _______ than the BSE, because...
N-MORB's typically have Sm/Nd ratios higher than the BSE, because **Nd has become depleted** in the upper mantle relative to Sm over time.
140
The Sm-Nd isotope system can be used to study the temporal
evolution of terrestrial reservoirs, such as the crust and the
mantle.
The Sm-Nd system is far superior to the Rb-Sr isotope system
for such investigations because
• the composition of the bulk Earth (BE) and the bulk silicate
Earth (BSE) are much better constrained
• the Sm/Nd ratios and Nd isotope compositions of older rocks
have generally not been altered significantly by
metamorphism and weathering
141
why is the Nd isotope evolution of the earth the same for chondrites, bulk solar system adn BSE?
**Sm and Nd are refractory and lithophile elements**
the BE, the BSE and chondrites (which are representative of
the bulk solar system) have the same Sm/Nd ratio.
Hence, they also all display the same Nd isotope evolution over time.
142
define **CHUR**
**CHUR**
chondritic uniform reservoir
is defined by the average present day Sm/Nd ratio and Nd isotope composition of chondritic meteroistes
143
what do End values show us?
denote relative differences in Nd isotope compositon (relative to CHUR) in part per 10,000
allows us to consider past and present variations in Nd isotope compositions relative evolution of the BSE and CHUR.
144
what is meant by a depleted reservoir interms of LREE and Sm/Nd?
depleted reservoir (has high Sm/Nd and is depleted in LREE) has radiogenic 143Nd/144Nd and thus an ENd \>0
it is depleted due to the extraction of the CC
145
describe this graph.
MORB is at the depleted end.
this is because the MORB source is the residual of melt extraction processes which formed the continental crust.
near the BSE readings is due to deep mantle that has not been as effected by CC creation.
the OIB (ocean island basalts) like kerguelen are more enriched than the BSE. (BSE = 0)
this could be due to the recycling of continent - derived sediments into the deep mantle.
due to the different compositons we can say the mantle is not fully homogenized by convective stirring.
we can use the isotope compositions of OIB to investigate the origin of the inferred source hetrogeneities.
146
what is the problem with useing 176Lu?
there is an uncertainty on its half life.
about 4% error between results obtained from meteorites, terrestrial minerals, and direct counting experiments.
147
describe Lutetium (Lu).
Lutetium (Lu) is a **moderately incompatible heavy REE** that occurs in dispersed form in nature.
It forms **the smallest trivalent ions of the REE** and it is thus the **most compatible REE** during partial melting of the mantle.
148
what does Lu decay to?
hafnium (Hf)
149
what are the carrier phases of Lu and Hf?
Lu = garnet, zircon
Hf = zirconium (they are geochemical twins, as they have the same ionic charge (+4) and almost the sam ionic radius
150
what is the Lu-Hf system useful for?
is most useful for dating mafic rocks, which have relatively high Lu/Hf ratios.
in particular, to date eclogites,
The ages of the eclogites mark the closure of the Hf isotope system to diffusion during exhumation of the rocks (~100km?).
as it raises it cools thus closes the system.
151
compare Lu-Hf and Sm-Nd systems.
they are similar
* All four elements are not easily mobilized by meteoric or
metamorphic fluids.
• The radiogenic daughter elements Nd and Hf are more
incompatible than the radioactive parents Sm and Lu. Rocks
and reservoirs with high Sm/Nd ratios should thus also have
high Lu/Hf.
• only refractory and lithophile elements.
The distribution of Lu and Hf in the Earth is
broadly similar to the distribution of Sm and Nd.
thus there is a correlation between Nd and Hf isotope compositions.
152
what does this graph show?
why the correlation?
Explain why the EHf values are more variable (by about a factor of 2) than the ENd data.
shows the correlation between Hf and Nd
This correlation reflects that:
•! Both element pairs are
relatively immobile with
respect to fluids.
•! The radiogenic daughters
(Nd, Hf) are more
incompatible than the
radioactive parents (Sm,
Lu) during magmatic processes.
the EHf values are more variable due to
i) the shorter half life of 176Lu
ii) the larger magmatic fractionation of Lu-Hf ratios compared to Sm-Nd
153
the BE and BSE have:
* Lu/Hf ratios similar to chondritic meteorites
* elevated absolute abundances of Lu and Hf relative to chondrites
why is this?
As both Lu and Hf are refractory and lithophile elements (like Sm & Nd) the BE and the BSE have:
• Lu/Hf ratios similar to chondritic meteorites
• elevated absolute abundances of Lu and Hf relative to
chondrites (by about x 2.7 for the BSE).
154
both Lu and Hf display relatively high abundances in small degree mantle melts such as the continental crust (CC) and OIB’s.
thus they must be\_\_\_\_\_\_\_\_
both Lu and Hf display relatively high abundances in small degree mantle melts such as the continental crust (CC) and OIB’s.
thus they must be **incompatible**
155
OIB’s and the CC have relatively low Lu/Hf ratios because cpx and garnet are present in the magma source.
why the low ratio then?
Cpx and particularly
garnet (!) preferentially retain Lu (which is a HREE) relative to the
more incompatible element Hf
156
for calculating the Hf isotope evolution of the earth do we use CHUR or CHON?
we use CHON as in CI chonrites
157
compare the Hf isotope evolution of the earth to the Sm- Nd system
they are similar with the formation of an enriched CC with low Lu/Hf and unradiogenic 176Hf/177Hf.
formation of a complementary depleted mantle with high Lu/Hf and radiogenic 176Hf/177Hf.
thus there is still the anti-correlation between EHf and 87Sr/86Sr
thus must be a correlation between EHf and ENd
158
what makes Re/Os system different to others?
Most parent and daughter elements of long-lived radioactive decay systems are incompatible lithophile elements (Rb, Sr, Sm, Nd,...).
Re and Os are siderophile (Fe-loving) elements, such that they are concentrated in the Earth's core
159
what are the compatibilities of Re and Os?
Re is a mildly incompatible element (similar to the HREE,
D ~ 0.1), whereas Os is generally strongly compatible (D ~ 10)
As a result, partial melting can
generate very strong fractionations of the Re/Os ratio.
160
what is the Re-Os system useful for?
due to being different to other decay isotopes the system is useful when it is difficult to use other means.
e.g. dating iron meteorites (as they dont have lithophile trace elements in) precious metal ores.
particulary useful in high degree mantle melts as there are relatively high abundances of Re and Os
161
why are Re and Os enriched in the Be compared to the chondrites.?
because they are refactory. thus when the volatiles have exsolved they will have a higher concentration.
162
the BSE has low abundances of Re and Os. why?
because they are siderophiles they are concentrated in the earths core.
163
mantle melts typically have low Os contents and high Re/Os.. why?
Re is generally mildly incompatible (similar to the HREE, D ~ 0.1), whereas Os is generally strongly compatible
164
explain why even though Re and Os are siderophiles, they concentration in the CC is equal to the CI chondrites.
core formation (and metal silicate equilibration) was completed before the accretion of the earth was entierly finished.
the present budget of siderophile elements in the mantle was established by the late addition of chondritic material.(after the completion of the core)
165
describe the behaviour and valency of U, Th and Pb
U (+4) [(+6) under oxidising conditions]- lithophile
Th(+4) - lithophile
Pb (+6)- lithophile, chalcophile, siderophile
166
describe the compatibilty of U Th Pb
Th is slightly more incompatible than U - both more incompatible thean REE
Pb has the same incompatibiltiy as the LREE.
167
why is it hard to use the isochron method with Th, Pb and especially U?
what does the dating techniques take advantage of then?
even at low grade metamorphism and during weathering they are mobile e.g. the creation of UO22+ which is soluble.
this open system behaviour is enhanced by the radiation damage that occurs from alpha decay.
the fact that two U isotopes decay to two different Pb isotopes
168
what does 238U, 235U and 232Th decay to?
238U- 206Pb
235U- 207Pb
232Th - 208Pb
169
link this equation for the common Pb method to the gradient on a graph. and how the gradient of said grap can be used in terms of time.
the left hand side is equal to dy/dx on a graph of 207Pb/204Pb vs 206Pb/204Pb.
thus the gradient must be equal to the right hand side which is in terms of time.
170
after using the common Pb method how can we solve to find the parent daughter ratio of the system?
171
what properties of zircon make it useful for geochronlogy?
1. very hard - resistant to physical weathering
2. resistant to chemical weathering and metamorphism
3. it concentrates U and excludes Pb, so has a high 238U/204Pb ratios (mu values)
4. its common accessory phase in many crustal rocks
172
where in a zircon mineral is it most accurate?
it is most accurate in the centre due to less changes by outer environment.
173
what has the concordia diagram been developed to take advantage of?
takes advatange of the high mu ratios found in zircon
174
describe the concordia diagram and how it is created?
using the two equations shown below we get x and y coordinates by using time.
it shows how the two ratios are proportional to time.
207Pb\*/235U would have initially increased much more
rapidly than 206Pb\*/238U. This is because at 4 Ga there was still a lot of 235U around and this has a much shorter half-life than 238U. The initial evolution thus has a flat slope.
As time passed, radioactive decay slowly depleted most of the 235U. After ~2 Ga, we therefore see large changes in 206Pb\*/238U per unit of time, whereas 207Pb\*/235U does not change much any more.
175
what are the problems in using the concordia diagram?
for any zircon to plot on the concordia diagram it must be a completely closed system. this hardl ever happens and thus we usually get Pb loss.
176
what is the impact of Pb loss on the concordia diagram?
An important point is that 206Pb and 207Pb will be lost in exactly the same proportions at which they are present in the zircon, as these two isotopes are chemically essentially identical.
this means the ratio of 206Pb and 207Pb will remain constant.
177
once a discordia has been formed on a concordia diagram and we wait some time, what can we do?
what do the intercepts represent in terms of events and the amount of Pb loss
over time the concordia will look as shown.
the line intercepts at two point i) the zircon formation/ crystallization and ii) the metamophic event.
we can thus obtain the ages of both events with a single measurement.
at crystallization point there is 0% Pb loss
at metamorphic event there is 100% Pb loss
178
what technique is used in selecting and preparing zircon crystals for use in dating?
why is this process used?
select several different populations of zircons from a
larger pool, based on size, color, shapes, magnetic properties. zircons are treated by gentle abrasion to remove the
outer layers, becaus ethe outer layers are more likely to be changed while the inner are likely to have values close to the concordia.
zircons are then dissolved completely and analyzed.
179
what is meant by stable isotope geochemistry?
**stable isotope geochemistry **is concerned with isotopic variation that are created by physical- chemical rather than nuclear processes.
180
what is meant by stable isotope fractionations (or isotope fractionations)?
isotopes of the same element display small but significant differences in their behaviour during physical, chemical and biological processes, thus these processes produce small changes in the isotope compositions of the elements.
181
what is the main cause of differences in isotope fractionations?
variations are generally due to mass dependent stable isotope fractionation
this means the magnitude of the isotope effect depends on the mass difference of the isotopes involved.
182
what type of elements are useful for stable isotope geochemistry?
light elements
- low atomic mass
- the relative mass difference between the isotopes is large
- they form bonds with a high degree of covalent character
- they exist in more than one oxidation state, form a wide range of compounds
- they are abundant enough (\>0.1%) to facilitate the isotope ratios measured.
183
what is the difference between equilibrium and kinetic isotope fractionation processes?
**equillibrium**
arise from reversible isotope exchange reactions that run to completion, so that full equilibrium is achieved
**kinetic**
arise from irreversible, incomplete process
kinetic fractionations are typically larger and the reaction product is often enriched in light isotopes because these move or react faster than the heavy isotopes.
184
equilibrium isotope fractionations typically arise from quantum mechanical effects on the vibrations of molecules.
explain
an isotope exchange reaction will run to minimize the energy of a given system.
the vibrational energy of molecules and teh discrete vibrational energy levels anre thus the primary reason for the occurance of equilibrium isotope fractionation in many reactions
185
explain this diagram of energy levels due to quantum effects?
This plot shows the potential energy as a function of interatomic distance for H-H, HD, and D-D molecules.
Quantum theory tells us that the system can only assume certain discrete energy levels
The lowest energy level is denoted by the zero-point energy (ZPE) – this is the ground state (n=0) of the oscillator
Ground state relevant for most molecules at room
temperature.
The ZPEs of D-D, H-D, and HH are different.
The ZPE is lower for bonds involving heavier isotopes.
This means that bonds of heavier isotopes are stronger.
at equilibrium the heavy isotope will tend to occupy the site with the stronger bond.
186
what is the effect of temperature on equilibium isotope due to quantum effects.
```
At low (room) temperature most molecules are in the vibrational ground state (n=0). At higher temperatures, many molecules will be in exited states, characterized by higher quantum numbers n. At high n,
the differences between the energy levels of the different isotopes become smaller.
```
**equilibrium isotope effects are smaller at higher temperature.**
187
equlibrium isotope fractionation requires that a system actually attains __________ equilibrium
thus why do we use knietic isotope effects?
equlibrium isotope fractionation requires that a system actually attains thermodynamic equilibrium
systems rarely reach equilibrium due to the reaction being too slow or the product is removed.
kinetic isotope effects are associated with incomplete processes.
188
describe a mechanism of kinetic equilibrium due to kinetic energy
Ek = 0.5MV2
differnt isotopes must have the same kinetic energy thus a lighter element will have higher velocity and thus diffuse faster.
using the equation below we can calculate the kinetic isotope fractionation factor
189
when a dissociation reaction does not reach equilibrium, the lighter isotopes are typically enriched in the reaction products.
why?
it takes more energy to dissociate D2 than H2. (as shown on the energy level diagram) The dissociation reaction is also slower for
D2. Thus: when a dissociation reaction does not reach
equilibrium, the lighter isotopes are typically enriched in the
reaction products because the reactant molecules with lighter
isotopes react faster.
190
what are the main reasons for changes to diffusion in equillibrium?
faster diffusion of light isotopes
bonds of light isotopes react quicker.
191
the oxygen isotope composition of the mantle is best defined by data for fresh....
why are they used? and why must they be fresh?
the oxygen isotope composition of the mantle is best defined by data for **fresh peridotite xenoliths and mid ocean ridge basalts**
these samples are suitable because partial melting and melt extraction does not significantly alter O isotope compositions
they must be fresh because O isotope compositions are readily changed by weathering and reaction with sea water
192
what does this diagram show?
Olivines and clinopyroxenes from peridotite xenoliths show a narrow range of O isotope compositions
this indicates that the mantle has a well-defined and homogeneous O isotope composition of delta18O = +5.5%
The small systematic difference between coexisting olivine and
clinopyroxene is consistent with the expected fractionation at mantle temperatures (~1200-1300º C).
193
what does this diagram show?
explain the differences in the graphs.
MORB have a narrow range of O isotope compositions with a mean of delta18O = +5.7%. This confirms the delta18O mantle average derived from xenoliths and suggests that the mantle is fairly homogeneous and well-mixed with regard to its O
isotope composition.
The other basalts have a larger range of delta18O values.
For the continental and the subduction-related basalts this will be mainly due to the assimilation of crustal materials (which have a much wider range of 18O values) and/or contributions from the subducting slab.
For the OIB, this could reflect (minor) mantle
heterogeneities, or the assimilation of altered basaltic
ocean crust during magma ascent.
194
what does this diagram show?
explain the differences
This variability primarily reflects O isotope fractionations that occur through interaction with (meteoric) fluids. Large fractionations are observed at or near the surface of the Earth during weathering and in hydrothermal systems.
lower temps = larger changes thus lerger spread in low temp affected rocks where as mantle has smaller changes
195
with refrence to the diagram how can O isotopes be used as tracers in a similar manner to radiogenic isotopes?
Igneous rocks which have O isotope compositions that differ from the inferred primordial mantle value of delta18O = +5.5‰ must have either been
(i) directly affected by low-temperature processes or
(ii) contain a component that was once at or near the surface of the Earth.
196
what process is the principal cause of C isotope variations
in nature?
**Biological processes** are the principal cause of C isotope variations in nature. The most important process is photosynthesis.
The C isotope fractionation during photosynthesis is primarily of
**kinetic origin** (thus based on mass) and it occurs in several steps.
197
In marine plants, the photosynthetic fractionations are
superimposed on the equilibrium C isotope effects that occur during the ...
In marine plants, the photosynthetic fractionations are
superimposed on the equilibrium C isotope effects that occur during
**the dissolution, hydration and dissociation of CO2 in water.**
198
this graph shows the amount of dissolved inorganic carbons in the north atlantic. explain its shape.
Surface water is depleted in DIC due
to biological uptake.
Biological processes cease at deeper levels –› higher DIC contents.
Deep water is enriched in DIC: dead biological material rains down and is remineralized (dissolved) at depth.
The DIC of the surface water is enriched in the heavy 13C isotope (high "13C values).
This is due to the preferential uptake of 12C by the primary
producers (plankton) during photosynthesis.
Deep water has lower "13C values.
This is due to the remineralization of biological material (which is enriched in 12C and has low "13C) at depth.
199
other than carbon what else is fractionated by biological processes?
N, H, O, S are also fractionated by biological processes
200
what does this diagram show you?
This reflects differences in the
uptake mechanisms of the
elements and the biological
reactions by which the
elements are processed.
201
what is the effect of evaporation and condensation on stable isotope fractionation?
when evaporated the liquid left behind will be enriched in the heavier isotopes
202
what does this graph show?
liquid water is heavier so as more is condensed the vapour left behind becomes lighter
203
explain the correlation between 18O and D
Hydrogen isotopes are fractionated in the same manner as O isotopes in hydrological systems.
We therefore observe an
excellent correlation of delta D with delta18O for
precipitation from worldwide locations.
The fractionation is greater for H, because the relative mass
difference of the isotopes is larger.
204
what is the bases of the determination of palaeotemperatures of the oceans?
The determination of paleotemperatures for the oceans is based
on the observation that the O isotope composition of CaCO3
(calcite and aragonite) differs from that of water when the
compounds precipitate from water under equilibrium conditions.
based on the O isotope composition of calcite shells
that precipitated from this seawater.
205
what is the relationship between the oceans and ice caps in terms of delta 18O
glaciers store isotopically light water
(low delta18O values) and this enriches
the oceans in 18O.
thus an increase in the size of the caps would decrease the amount in the oceans
206
other than oxygen what else can we look for in ice cores to give palaeotemperatures?
describe the presicion of younger and older cores
we can look for CO2 as it has a strong correlation to temperature.
we can also look for isotopes of H.
young cores are more precise than old cores.
207
what is the reason for the periodicity of glacial and interglacial?
**milakovitch cycles**
eccentricity
obliquity
precession
note :variations in the orbital parameters do not alter the average annual insolation received by the Earth, but only its distribution in space and time.
e.g. so even with eccentricity you are further away at one part of the year and the other you are closer so over a year you will have the same average.
208
what is meant by insolation?
**insolation**
the intensity of incoming solar radiation.
209
what is the key factor that drives changes in climate, due to milankovitch forcing function is not very strong?
The key factor that is driving changes in climate appears to be the insolation (radiation) received during the summer months at high northern latitudes (~65º N).
Glaciers apparently develop in the northern hemisphere when the
summers are not warm enough to melt the snow of the previous winter.
In contrast, the southern hemisphere
is taken up mainly by oceans, which
cannot be covered by glaciers.
The Milankovich forcing function is not
very strong but **its effects are
amplified by feedback mechanisms.**
**e.g.** when glaciers build up they reflect more radiation so it gets even more cold.
210
Fractional crystallization is efficient at fractionating ... of melts, whilst ... are less affected.
Fractional crystallization is **efficient** at fractionating **compatible element** abundances (Ni) of melts, whilst **incompatibles (Sm, Nd) are less affected.**
211
what does a possitive End show you? (when using CHUR)
a positive End shows you that it came from a depleted source.
212
what are the major points of explanation of this graph?
Rb/Sr and Sm/Nd
to do with the formation of the CC.
Rb is more incompatible than Sr so CC has high Rb/Sr
Sm is more compatible than Nd thus CC has low Sm/Nd and develops adiogenic Nd isotope composition thus -ve ENd
the mantle and particularly the depleted MORB is the residual mantle after the extraction of CC. thus will have the opposite (low Rb/Sr, high Sm/Nd and develops radiogenic Nd isotope composition (high Nd)
the correlation decribes this systematic behaviour
