Isotope geochemistry Flashcards
Big Bang
14Ga
Produced protons, neutrons and electrons
Within minutes, 25% of Helium was created
Fusion
Primary source of energy in a star is the fusion of hydrogen into helium. EXOTHERMIC
Later in a star’s life cycle, it fuses heavier elements.
Temperature too high for neutral atoms so there’s a plasma.
Fusion becomes endothermic at Fe56.
Neutron capture
Process that creates elements that are heavier that Fe56.
Too many neutrons in the nucleus leads to beta decay. Neutron –> proton and electron
S-process: small flux of neutrons from other reactions. Neutron capture events are infrequent and atoms have time to undergo beta decay before receiving another neutron.
R-process: large flux of neutrons. Neutron capture events so frequent that atoms don’t have time to undergo beta decay before receiving more neutrons.
Some isotopes can only be made by the s-process as they are ‘shielded’ by stable isotopes.
Solar system
Formed from a nebula.
- Gravitational contraction of a rotating gas cloud leads to a dense central region and a more diffuse flattened region.
- Dust particles from the nebula form a disc
- Dust comes together to form planetisimals. Planetisimals collide: disintegrate, come together and alter orbits.
- Large bodies capture small ones.
- Uncondensed gas blown away by solar wind
How did Sun retain 98% of mass in solar system?
Planets have 98% of angular momentum
Inner planets formed from elements that condensed above 600K; outer planets in regions cold enough for ammonia, methane and water to condense.
Carbonaceous chondrites
Similar elemental abundances as the Sun
Contain chondrules: mineral inclusions
Basaltic Achondrites
Similar to terrestrial basalts.
Age close to carbonaceous chondrites.
Planetary differentiation and magmatic activity started very soon after formation of planetary bodies.
Al16 in Allende
Chondrules contain plagioclase with excess Mg26.
Al26 decayed to Mg26.
Half-life of Al26 is 0.74My and is formed when explosive He fusion ejects material from stars. These chondrules formed at the start of the solar system from material from nearby stars.
Rhenium-Osmium
Most abundant in iron meteorites.
Os187 is primarily made by the S-process. But, some is created by beta-decay of Re187.
Relative proportions of Os187 from the s-process and r-process can be calculated using using their capture cross-sections. The relative abundance of the excess Os187 in iron meteorites to the abundance of Re187 shows when the meteorites formed 4.55Ga, Re187 was already decaying.
MINIMUM AGE OF THE UNIVERSE is 8.65Ga.
Earth’s atmosphere
I129 has a half-life of 17Myr. Natural I129 is now extinct on Earth. Atmosphere formed by degassing Earth. Mantle has greater abundance of Xe129 that the atmosphere, so iodine stayed in the mantle. Atmosphere formed when I129 was still active.
Earth’s core
Hf182 has a half-life of 9Myr and is now extinct on Earth. Hf preferentially fits in silicate minerals, but its daughter isotope W182 prefers iron and nickel.
Earth’s mantle has a higher abundance of W182 than carbonaceous chondrites.
Iron meteorites have a smaller abundance of W182 than carbonaceous chondrites.
Cores of planetisimals and Earth formed when Hf182 was active. Cores of planetisimals estimated at within 3Myr of birth of solar system. Earth’s core within 30Myr of the birth of the solar system
Model age
Average age of the continents (2.8-3.0Ga)
Sm147 decays to Nd143
Continental crust separated from mantle by melting
Nd has a preference for liquid phase, Sm has preference for solid residue.
Measure relative abundance of Nd143 to Nd144
Measure ratio of Sm147 to Nd144 in carbonaceous chondrites to set initial value
Measure Nd143 : Nd144 and Sm147 : Nd144 currently, extrapolate back and estimate age when continental crust separated from the mantle.
Sediment subducted into the mantle
Rb87 decays to Sr87
Rb/Sr ratio higher in continental crust than in the mantle.
Sr87/Sr86 is c. 0.7028 in mantle and c. 0.7100 in continental crust
Sediment taken down into mantle is like a trace. Big Sr87/Sr86 anomaly in volcanic arc above.
Island arcs have similar Sr87/Sr86 ratios as the mantle, so very little sediment is subducted.
