Final exam Flashcards
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
40Ar/39Ar age equ.
t = 1/lam * ln ( 1 + j (40Ar*/39Ark))
J-Factor
Contains the abundances of K isotopes, decay constants, and neutron fluence, reaction across the section and duration of irradiation (aka irradiation parameter
Complications and limitations with chronometers
Incorporation of daughter (common lead)
Inheritance of daughter (radiogenic precursor)
Susceptible to alteration
Partial open-system behaviour
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
Goldschmidt classification of Re-Os
Both are siderophile (affinity for iron) and chalcophile (affinity for sulfur), Platinum group elements, and are considered highly siderophile
Mantle compatibility
Os is compatible with mantle (high ratios in peridotite). Re is incompatible with mantle (high in continental basalt)
3 major geochronological applications
- Re-Os dating of sulfides in ore deposits
- Isochron dating of sedimentation
- Model-age dating of mantle partial melting
Re isotopes
185, 187
Os Isotopes
184, 186, 187, 188, 189, 190, 192
Re-Os model age calculation
No initial Os (molybdenite):
187Os = 187 Re((e^lamt)-1)
ln(187Os/187R +1) /lam = t
δ13C trends
(+) excursions related to enhanced burial or organize matter
(-) excursions related to shut down of biologic pump (extinction
Biologic pump
Upper 1 km: CO2 increases and then plateaus below 1km, δ13C decreases and then plateaus
Isotopes: similarities and differences
Same chemical behaviour because same number of protons, different atomic mass because different number of neutrons hence different reaction rates
Relative abundance
Percentage of atoms with a specific atomic mass found in a naturally occurring sample of an elements
average atomic mass
a weighted average by multiplying relative abundances by atomic masses and then sum products
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
