Geochemistry
Dewashish Upadhyay
Origin of elements
Some factsHydrogen makes up about 73% of the mass of the visible universeHelium makes up about 25% of the massEverything else represents only 2%Abundance of heavy (A > 4) elements quite low
Important to remember that most matter on Earth is a part of this small portion of the matter of the universe
Nuclide chartAtomic number vs. Neutron number
Horizontally-isotopes
Vertically-isotonesValley of beta-stability-central dark regionRight of the valley-neutron rich nuclides-undergo - decayLeft of the valley-proton rich nuclides-undergo positron-decay or electron capture
Origin of elements
Three principal astrophysical settings for the synthesis of elements
Cosmological Big Bang
Stars
Supernovae
Big Bang TheoryThe Big Bang Theory is the cosmological model that best explains the origin of the universe
According to the standard theory, our universe came into existence 13.7 billion years ago as an infinitesimally small, infinitely hot, infinitely dense discrete point - a singularity
The pressure is thought to be so intense that finite matter is actually squished into infinite density
Where did it come from? We don't know. Why did it appear? We don't know.
Big Bang TheorySingularities defy our current understanding of physics. They are thought to exist at the core of "black holes (areas of intense gravitational pressure)
About 10-35 seconds after its initial appearance, the universe expanded exponentially (cosmic inflation)
After inflation stopped, the universe consisted of a quark-gluon plasma
It continued to expand and cool from very, very small and very, very hot, to its current size and temperature. It continues to expand and cool to this day
Big Bang Theory
There was no giant explosion. Rather there was and continues to be an expansion
Space didn't exist prior to Big Bang
Theory of Relativity says that time and space had a finite beginning that corresponded to the origin of matter and energy. The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy-nothing.
Where and in what did the singularity appear if not in space? We don't know. We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we
Evidence supporting Big Bang Theory
Cosmological red shift
Doppler effect: When an observer is moving relative to the source of waves, the wavelength of the wave changes, becoming longer if the source is moving away from the observer and vice-versa
The electromagnetic spectrum of stars is shifted to longer (redder) or shorter (bluer) wavelengths, which has been attributed to Doppler effect in light
Red shift- when star is moving away from EarthBlue shift- when star is moving towards Earth
Cosmological red shift
Hubble's Law: linear relationship between distance and red-shift
Galaxies are moving away from us at speeds proportional to their distance. This supports uniform expansion of the universe and implies that it was once compacted to a point- source
2. Cosmic microwave background radiation
If the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat
2. Cosmic microwave background
During the initial stages after the Big Bang, all particles and photons in the universe were in thermal equilibrium. Because electrons were unbound to nuclei, photons were continuously scattered by the electrons, making the early universe opaque to light
Temperature fell due to expansion, electrons and nuclei combined to form atoms, scattering of photons ceased and radiation became decoupled from matter, photons could travel unimpeded and the universe became transparent to light
2. Cosmic microwave background
Young universe filled with a uniform radiation (photon) from its plasma
As it expanded, both plasma and radiation grew cooler
When the universe cooled enough that stable atoms formed, these could no longer absorb the radiation
Photons existing at that time have been propagating ever since, growing fainter and less energetic as they fill a greater and greater universe. These photon form the Cosmic Microwave Background radiation (CMB)
In 1965, Radio astronomers Arno Penzias and Robert Wilson discovered a 2.73 K CMB which pervades the observable universe
3. The relative abundance of H, He and Li in the universe
Hydrogen makes up about 73% of the mass of the visible universe
Helium makes up about 25% of the mass
Abundance of the "light elements" H and He predicted during Big bang nucleosynthesis matches that of the observable universe
Big bang nucleosynthesisCalled primordial nucleosynthesisStarted 3 minutes after the beginning of space expansion and lasted for just 17 minutes- after 20 minutes temperature and density of the universe fell below that which is required for nuclear fusion
In the first few micro-seconds, because of the high energy density, universe existed as a quark-gluon plasma in which particle- antiparticle pairs of all kinds e.g., quark-anti quark, electron-positron continuously created & destroyed in collisions
Quark: elementary particle and fundamental constituent of matter (combine to form protons & neutrons)Gluons: elementary expressions of quark interaction, are messenger particle of the strong nuclear force, which binds quarksQuarks interact by emitting and absorbing gluons, just as electrically charged particles interact through the emission and absorption of photons
Big bang nucleosynthesis
By about 10-4 seconds, the universe had cooled for the primordial quark-gluon plasma to freeze out
An unknown process called baryogenesis produced excess of particles (MATTER) (quarks, leptons) over antiparticles (ANTI MATTER)
At 10-6 seconds quarks and gluons combined to form protons and neutrons (baryons)
Temperature no longer high enough to create new proton-antiproton pairs, mass annihilation(total destruction) occurred, leaving excess protons and neutrons and none of their antiparticles
Big bang nucleosynthesis
By approx. 3 minutes, further expansion and cooling allowed some neutrons and protons to fuse to D and He nuclei. Most protons remained uncombined as hydrogen nuclei
After about 379000 years, electrons and nuclei combined into atoms, mostly H and the universe became transparent to light
The 73% H and 25% He abundances that exists throughout the universe today come from that condensation period
Big bang nucleosynthesisFusion reactions during these initial stages formed elements up to LiNo nuclides heavier than Be, Li was formedNo neutral atoms existed at this timeHow did heavier elements form?
The evolution of the universeNeutral atoms formed in the time scale of 379,000 years once the universe was cool enough for free electrons to be captured by nuclei
Stellar nucleosynthesisUntil stars formed, there was nothing except H and He
StarsA star is a luminous ball of plasma held together by gravityStars shine due to energy radiated from thermonuclear fusion in their coreAlmost all elements heavier than H and He are created by fusion processes or neutron capture in stars and supernovaeThe evolution and eventual fate of a star is determined by its mass
Stellar evolution Formation of starStars form when a molecular cloud collapsesdue to gravitational instability or supernovashock wave
Molecular cloud is rotating cloud of:Molecular H2 gas, He, and molecules such as COAbout 1% by mass submicron-sized dust grainsAnother 1% gaseous molecules and atoms of elements heavier than HeT: 715 K and gas densities: 103105 mol/cm3Mass: a solar mass to thousands of solar massesRegions of active star formation are located within molecular clouds
Collapse of molecular cloudDense rotating cloud cores supported against their own self-gravity by a combination of turbulent motions, magnetic fields, thermal (gas) pressure, and centrifugal force
Conglomerations of dense dust and gas formed Bok globulesGlobules collapse, density increases, gravitational potential energy converted into thermal energy temperature rises
Collapse can be of two types:Self collapse - standard model of star formationMagnetic support gradually lost through ambipolar diffusion (neutral molecular gas slips past the small fraction of ionized gas and magnetic field lines) allowing bulk of cloud to contract gradually & eventually undergo a dynamic collapse phase
Collapse takes place on time scales of the order of 10 Myr
Presence of short-lived nuclides (e.g., 26Al, 60Fe) with half-lives much less than 10 Myr in early solar system indicates collapse was quicker
To explain the presence of these short-lived nuclides, a triggered collapse model was proposed
Shock triggered collapseShort-lived nuclei impose a time limit of at most about 1Myr between their nucleosynthesis and their incorporation in the solar nebula
Collapse of molecular cloud by a supernova shock wave travelling at velocities between 20-40 km/sec
Supernova triggers the collapse of molecular cloud and injects short-lived nuclides in to it on time scales of about 1 Myr
Most of the collapsing mass collects in the centre, forming a star, while the rest may flatten into a protoplanetary disc (nebula) out of which the planets, moons, asteroids, and other small bodies form
Protostar and protoplanetary disk
As material within the nebula collapses, atoms within it collide with increasing frequency converting kinetic/gravitational potential energy into heat
Most of the mass collects in the centre which becomes increasingly hotter than the surrounding disc
In about 100,000 years the interplay of gravity, gas pressure, magnetic fields, and rotation leads to the flattening of the nebula into a spinning protoplanetary disc
A hot, dense protostar (T-Tauri star) (has not begun fusing H) forms at centre (shines by radiating gravitational potential energy)
Stars with discs of pre-planetary matter with masses of 0.0010.1 solar masses are called T Tauri stars
These discs extend to several hundred AUthe Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter (200 AU for our solar system) in star-forming regions such as the Orion Nebula
Within 50 Myr, T & P at core of star become so great (106 K) that its H begins to fuse, creating an internal energy source which counters gravitational contraction until hydrostatic equilibrium is achieved - Main sequence star
Classification of stars: HerzsprungRussell diagram Main sequence90% of all stars (>0.7 Mo) at some stage in their life, they are in H burning stage H He
GiantsMid sized stars (0.7 to a few Mo) after main sequence
White dwarfsfinal evolutionary state of stars whose mass is not too high (< 0.7 Mo)
Supernovae- either massive stars or white dwarfs in a binary star system where one star gains mass from a companion starStar's absolute luminosity is plotted against its surface temperature (or spectral type)
Main sequence stage
After formation, star creates energy in its core through the nuclear fusion of H atoms into HeHydrostatic equilibrium outward radiation pressure from the hot core is balanced by the inward gravitational pressure
Position on main sequence depends on mass, chemical composition
Giant
Period of stellar evolution undertaken by all low to intermediate mass stars (0.6-10 solar masses) late in their life (after main sequence)- Red giantMain sequence star exhausts H by fusion in itscore- the core contracts, T increasesOuter layers of star expand and cool,Thermal instability causes convection whichbrings to the surface the product of Hburning- dredge upLuminosity increases greatly red giantOnce temperature in the core reaches 3x108 K, He fusion starts giving C, O
Asymptotic giant branch starWhen He is exhausted in the corethe C, Ocore again contracts
The envelope becomes convectiveagain- second dredge up
Core contraction continues with H burningin a shell with He in between the H shell andthe core
When T gets high enough, He ignites and burns in a runaway flash for some time (thermal pulse). This process is repeated intermittently
The star is called a thermally pulsing Asymptotic giant branch star
Gradually the star has inert core of C and O, a shell where He is undergoing fusion to form C (He burning), another shell where H is undergoing fusion forming He (H burning) and a very large envelope of material of composition similar to normal stars
Nuclear fusion in low mass starsHydrogen burning
Proton-proton (PP) chainAt T less than 5 million K (PPI)1p + 1p 2He2He 2D + + + neutrino2D + 1p 3He3He + 3He 4He + 21p
At T of 15 million K (PPII)3He + 4He 7Be7Be + - 7Li + neutrino7Li + 1p 8Be8Be 24He
At T above 25 million K (PPIII)7Be + 1p 8B + gamma ray8B 8Be + - + neutrino8Be 24He
Nuclear fusion in massive starsIn massive or higher generation stars, H converted to He by several reactions that use atoms of C, N, and O as catalystThe cycle results in the fusion of four hydrogen nuclei (1H, protons) into a single helium nucleus
Nuclear fusion in massive main sequence and AGB starsTwo important fusion reactions in AGB stars produce slow neutrons-nucleosynthesis of heavy elements 13C + 4He 16O + n22Ne + 4He 25Mg + nWhy does fusion stop with Fe-56 or Ni-56?
Supernovae
A supernova is an explosion of a star
Happens in massive stars or when a white dwarf gains mass from a companion in a binary star system
Intense amount of energy and radiation released
Star expels most of its mass at high velocity-generates shock wave into the interstellar medium
Supernovae
In later stages of a stars life, increasingly heavier elements undergo nuclear fusion
Binding energy of the nuclei increases, fusion produces progressively lower levels of energy
Once Ni-56 is produced, fusion becomes endothermic
Ni-56 decays to Fe-56Ni-Fe core builds up, fusion stops, and the outward thermal pressure cannot counterbalance gravitation
SupernovaeThe core collapses in on itself with velocities reaching 70,000 km/s sending out a shock wave
Fe breaks down to electrons, protons, neutrons. Because of the high P, electrons in core combine with protons forming neutrons and neutrinos
Intense gamma radiation & high energy neutrons produced
Nucleosynthesis ofneutron-rich and proton-richheavier nuclides take placeIron core (a) collapses (b). The inner part of core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red).The surrounding material is blasted away (f), leaving a neutron star
Nucleosynthesis of heavier elements
S-process (slow neutron capture) in AGB stars
13C + 4He 16O + n22Ne + 4He 25Mg + n
Fe-Ni seed nuclei undergo slow neutron capture accompanied by beta decayForms nuclides along the valley of beta stability
S-process pathwayTermination of s-process
R-process (rapid neutron capture)-core collapse supernovae
High neutron densities and fluences (1022 neutrons per cm per second)
Neutron captures occur much faster than beta minus decaysProduces neutron rich nuclides to the right of the valley of stability
Need for a P-processSome nuclides to the left of the valley of beta stability shielded from s- and r-process pathways
P-process needed to explain their formation
P-process
Photodisintegration of nucleus during interaction with gamma rays
Occurs in supernovae
Two types of photodisintegration:neutron-photodisintegration (, n)
Alpha photodisintegration (, )
During core-collapse supernova explosion, T reaches up to 2109 to 3109 Kelvin
Intense gamma radiation is produced that can disintegrate seed nuclei created by s-process & r-process
P-process operates for only a short time: p-nuclei less abundant