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Isotope Geochemistry > Final exam > Flashcards

Final exam Flashcards

1
Q

Types of Argon in a sample

A

Atmospheric 40Ar/39Ar = 298.6
Radiogenic 40Ar* from 40K decay ec (HL 11.93Ga)
Trapped/inherited atmospheric/excess mixture
Cosmogenic Ar produced from cosmic ray bombardment of Ca Ti and Fe
Irradiation induced Ar from irradiated K Ca Cl

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2
Q

40Ar/39Ar age equ.

A

t = 1/lam * ln ( 1 + j (40Ar*/39Ark))

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3
Q

J-Factor

A

Contains the abundances of K isotopes, decay constants, and neutron fluence, reaction across the section and duration of irradiation (aka irradiation parameter

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4
Q

Complications and limitations with chronometers

A

Incorporation of daughter (common lead)
Inheritance of daughter (radiogenic precursor)
Susceptible to alteration
Partial open-system behaviour

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5
Q

Closure temperature

A

The temperature at which a given parent-daughter decay chain is set, because no diffusion of the parent or daughter isotopes takes place at lower temps

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6
Q

Goldschmidt classification of Re-Os

A

Both are siderophile (affinity for iron) and chalcophile (affinity for sulfur), Platinum group elements, and are considered highly siderophile

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7
Q

Mantle compatibility

A

Os is compatible with mantle (high ratios in peridotite). Re is incompatible with mantle (high in continental basalt)

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8
Q

3 major geochronological applications

A
  1. Re-Os dating of sulfides in ore deposits
  2. Isochron dating of sedimentation
  3. Model-age dating of mantle partial melting
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9
Q

Re isotopes

A

185, 187

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10
Q

Os Isotopes

A

184, 186, 187, 188, 189, 190, 192

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11
Q

Re-Os model age calculation

A

No initial Os (molybdenite):
187Os = 187 Re((e^lamt)-1)
ln(187Os/187R +1) /lam = t

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12
Q

δ13C trends

A

(+) excursions related to enhanced burial or organize matter

(-) excursions related to shut down of biologic pump (extinction

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13
Q

Biologic pump

A

Upper 1 km: CO2 increases and then plateaus below 1km, δ13C decreases and then plateaus

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14
Q

Isotopes: similarities and differences

A

Same chemical behaviour because same number of protons, different atomic mass because different number of neutrons hence different reaction rates

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15
Q

Relative abundance

A

Percentage of atoms with a specific atomic mass found in a naturally occurring sample of an elements

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16
Q

average atomic mass

A

a weighted average by multiplying relative abundances by atomic masses and then sum products

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17
Q

5 components of a mass spect

A

Introduction, ion source, filter, accelerator, collector

18
Q

Beta factor

A

Beta is the isotopic affinity for a compound to incorporate heavy isotopes. Can be compared to another compound to predict isotope fractionation.

19
Q

5 major controls for Fe isotope fractionation

A

1) Differences in redox chemistry between min-min, min-fluid, and fluid-fluid species
2) Covalent bonds preferentially incorporate the heavier isotope of Fe
3) Coordination state of Fe in the aqueous complex or in the mineral structure
4) Bond partner strength
5) Temperature

20
Q

Isotope fractionation

A

Offset in the isotopic composition of two substances as a result of physical or chemical processes. Differences in mass lead to partial separation

21
Q

Isotope Effect + example

A

Fractionation of isotope molecules during a reaction or physical process.

ex. H2(18/16O) —-> CaC(18/16O3)

22
Q

Three modes of molecule motion

A

Translational, rotational, and vibrational

23
Q

Force eq (between the mass and the fixed point

A

F = -kx

K = force constant
x = displacement from equilibrium
24
Q

Vibration frequency eq

A

f = (1/2pi) sqrt(k/mu)

k = force constant
mu = displacement from equilibrium
25
Q

Isotopic substitution (heavier isotopes)

A

Heavier isotopes form lower-energy bonds that do not vibrate as violently, thus form stronger bonds

26
Q

Isotope effects occur during what?

A

Evaporation
Condensation
Diffusion

27
Q

Fractionation factor (alpha)

A

The ratio of the number of atoms of two isotopes within a given molecule or chemical compound divided by the corresponding ratio in another compound

ex: 13a(co2Hco3) = (13Rco2)/(13Rhco)

28
Q

Equilibrium isotope effects

A

relationship between equilirbrium constant and fractionation factor and free energy exchange

Keq ~ 37a(clo-cl)

29
Q

Equilibrium effects depend on what

A

temperature, decreases in magnitude as temperature increases, scaling 1/T^2

30
Q

Kinetic isotope effects

A

Accompany dynamic, processes that are fast, imcomplete, or unidirectional

31
Q

Barrier energy

A

Eb = Ediss - Ezp

32
Q

Bonds with lighter isotopes

A

have lower barrier energies (e.g. breaking bonds with lighter isotope requires less energy

33
Q

Residence time definintion and equation

A

Average time that a substance remains disolved in seawater

Rt (years) = Total amount in seawater(kg)/input rate (kg/year)

Input rate is average concentration in rivers (kg/km3 x river discharge (km3/yr)

34
Q

What controls the strength of the biological pump?

A

Oceanic nutrient inventory, remineralization length scale of organic carbon, ocean circulation

35
Q

Carbon cycle: four reservoirs

A

Atmosphere, continental (biosphere, freshwaters, and soil), ocean, and geosphere

36
Q

Define diagenesis

A

Chemical changes that occur following sediment deposition, pore fluids, mineral recrystallization, meteoric alteration, deep burial

37
Q

Rayleigh fractionation

A

Combination of kinetic and equilibrium isotope effects.

K- water molecules evaporate from the sea surface
E- water molecules condense from vapor to liquid form

38
Q

Enrichment process of 16/18O

A

Light 16O evaporates first, heavier 18O condenses out first

39
Q

Oxygen isotope as a thermometer

A

More 18O depleted (negative) oceans suggest more rainfall less ice (interglacial)

less 18O depleted (positive) more ice, colder temps (glacier)

40
Q

18O variatio is a function of what

A

Ice volume and ocean water temperature change

Isotope Geochemistry (2 decks)

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