Final exam Flashcards by Gryphen Goss

Types of Argon in a sample

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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40Ar/39Ar age equ.

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

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J-Factor

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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Complications and limitations with chronometers

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

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Closure temperature

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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Goldschmidt classification of Re-Os

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

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Mantle compatibility

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

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3 major geochronological applications

Re-Os dating of sulfides in ore deposits
Isochron dating of sedimentation
Model-age dating of mantle partial melting

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Re isotopes

185, 187

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Os Isotopes

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

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Re-Os model age calculation

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

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δ13C trends

(+) excursions related to enhanced burial or organize matter

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

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Biologic pump

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

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Isotopes: similarities and differences

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

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Relative abundance

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

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average atomic mass

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

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5 components of a mass spect

Introduction, ion source, filter, accelerator, collector

Beta factor

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

5 major controls for Fe isotope fractionation

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

Isotope fractionation

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

Isotope Effect + example

Fractionation of isotope molecules during a reaction or physical process.

ex. H2(18/16O) —-> CaC(18/16O3)

Three modes of molecule motion

Translational, rotational, and vibrational

Force eq (between the mass and the fixed point

F = -kx

K = force constant
x = displacement from equilibrium

Vibration frequency eq

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

k = force constant
mu = displacement from equilibrium

Isotopic substitution (heavier isotopes)

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

Isotope effects occur during what?

Evaporation Condensation Diffusion

Fractionation factor (alpha)

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)

Equilibrium isotope effects

relationship between equilirbrium constant and fractionation factor and free energy exchange Keq ~ 37a(clo-cl)

Equilibrium effects depend on what

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

Kinetic isotope effects

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

Barrier energy

Eb = Ediss - Ezp

Bonds with lighter isotopes

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

Residence time definintion and equation

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)

What controls the strength of the biological pump?

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

Carbon cycle: four reservoirs

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

Define diagenesis

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

Rayleigh fractionation

Combination of kinetic and equilibrium isotope effects. K- water molecules evaporate from the sea surface E- water molecules condense from vapor to liquid form

Enrichment process of 16/18O

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

Oxygen isotope as a thermometer

More 18O depleted (negative) oceans suggest more rainfall less ice (interglacial) less 18O depleted (positive) more ice, colder temps (glacier)

18O variatio is a function of what

Ice volume and ocean water temperature change