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