geochemistry Flashcards
what does HFSE stand for?
why does it have this name?
high field strength elements
they get their names from the high charge and small ionic radius thus high field strength
are HFSE compatible or incompatible during mantle melting? why?
incompatible
because its hard for them to change with 2+ charged ions
what are the four main HFSE? which are moderaltely incompatible and which are highly incompatible?
Zr4+ (zirconium) - moderatley
Hf4+ (hafnium) - moderately
Nb5+ (niobium) - highly
Ta5+ (tantalum)- highly
by mass what are the ratios of mantle core and crust?
mantle ~2/3
core ~ 1/3
around what percentage are trace elements in common rocks?
trace elements <0.1%
what can and cant trace elements do?
trace elements can substiute for major elements in minerals but they are not themselves essential structral constituents
in terms of mass what does 1ppm and 1ppb equal?
1ppm = 1 microgram of element per gram of rock
1ppb = 1 nano gram of element per gram of rock
what are the major elements in silicate rocks?
O
Si
Al
Na
Mg
Ca
Fe
what are is the percentage range of minor elements?
present at concentration of 0.1% to 1%
what element can behave like a major and trace element?
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
what is meant by lithophile elements?
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).
what is meant by siderophile elements?
Siderophile elements
are metal-loving, as they partition into
metallic liquids. They are strongly enriched in the core but depleted in the silicate Earth.
what is meant by chalcophile elements?
Chalcophile elements
are sulfur-loving and they partition into
sulfide liquids.
what is meant by atmophile elements?
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.
what causes the goldschmidts classification?
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.
chalcophile does not automatically imply that an element is found in sulfide ore deposits. what does it mean then?
Many sulfide deposits are precipitated
from aqueous solutions but chalcophile means that an element
partitions into magmatic sulfide liquids/phases.
what does HSE stand for? how do they behave in the mantle?
HSE - highly siderophile elements
the HSE behave as chalcophile elements in the mantle
the partitioning of trace elements into minerals is primarily a function of…
the partitioning of trace elements into minerals is primarily a function of ionic charge and radius
The ions of one element can extensively replace those of
another element in ionic crystals (such as silicates) if…
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%.
how can ions whose charges differ by one substiute for each other?
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+.
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.
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.
Substitution may be limited, even when the size and charge
criteria are satisfied, when the competing ions have different…
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.
what is the camouflage principle?
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
what is meant by the capture principle?
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.
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
what is the formula for KD?
KD = Csolid/ Cliquid
where C is the concentration of a particular element
what are elements with KD that are:
>1
<1
<<1 (~<0.1)
>1 compatable
<1 incompatiable
<<1 (~<0.1) highly incompatible
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.
which three mineals dominate the earths upper mantle?
olivine
orthopyroxene
clinopyroxene
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
describe PGEs
platinum group elements (PGE)
a group of noble metals
partition strongly into mantle sulfides with KD values of >104
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.
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.
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.
describe REE
**rare earth elements (REE) **(or lanthanides)
The REE are incompatible elements in the mantle and they are dominantly trivalent (Ln3+).
what is another name for REE
lanthanides
what is the use of REE?
REE are important as trace element indicators of magmaic processes.
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
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)
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.
both U and Th partition into_______ they are _________ elements
both U and Th partition into silicates they are lithophile elements
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.
explain the decoupling of U from Th in the earths hydrosphere
The uranyl ion UO<span>2</span><span>2+</span> 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.
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+.
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).
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.
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.
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.
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.
what is batch melting involve?
Batch melting involves complete equilibration between the solid
phase (rock) and the melt (liquid).
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
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
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
what equation is used to calculate the trace element concentration of the residual rock that is in equilibrium with a partial batch/ equilibrium melt.
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.
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.
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.
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.
what is the equation for an aggregate liquid?
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
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)
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.
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?
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.
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
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.
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
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
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
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.
what is the use of CI chondrites?
generally used to define the average composition of the solar system.
describe the main consituents of the earth.
depleted in gaseous consituents - no H and He.
consist mainly of the elements O, Fe, Si, Mg
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
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.
what is the opposite of volatile?
refactory
describes at which temperatures elements condense
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
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.
what is meant by the BSE
bulk silicate earth (BSE)
BSE = mantle + continental crust + oceanic crust
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
where is nearly all of the earths iron?
in the inner and outer core.
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)
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
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
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.
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.
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
give an example of beta (negatron) decay
give an example of positron decay
give an example of alpha decay
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.
what is the line on this graph called and what can be worked out from it?
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.
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)
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.
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.
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
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
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.
after using the common Pb method how can we solve to find the parent daughter ratio of the system?
what has the concordia diagram been developed to take advantage of?
takes advatange of the high mu ratios found in zircon
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.
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.
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
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.
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.
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
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.
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).
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.
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
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.
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.
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.
what does this graph show?
liquid water is heavier so as more is condensed the vapour left behind becomes lighter
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.
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
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.
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
