Planetary Materials

K-feldspar chemical formula

K Al Si_3 O_8

Albite chemical formula

Na Al Si_3 O_8 (plagioclase end member)

Anorthite chemical formula

Ca Al_2 Si_2 O_8

Plagioclase series

Continuous solid solution between anorthite and albite, subdivided arbitrarily based on Ca content. Replacement ions need to be coupled: Ca2+ -> Na+ & Al3+ -> Si4+

Why are there no intermediate minerals between K-feldspar and anorthite?

The miscibility gap is due to a difference in ionic radii and charges that would make intermediate compositions structurally unstable

Why are plagioclase feldspars more common on Earth than alkali feldspars?

Because calcium is more common in the crust than potassium (3.6 vs 2.8 %)

How does Raman spectroscopy work?

A sample is illuminated with a monochromatic laser. Frequency shift due to inelastic scattering is being measured: it corresponds to the energy of atomic bond vibration

Enstatite chemical formula

Mg_2 Si_2 O_6

Ferrosilite chemical formula

Fe_2 Si_2 O_6

Wollastonite chemical formula

Ca Si O_3

General pyroxene formula

X Y Z_2 O_6 where X€{Na+, Ca2+, Mn2+, Fe2+, Mg2+, Li+}, Y€{Mn2+, Fe2+, Fe3+, Mg2+, Al3+, Cr3+, Ti4+}, Z€{Si4+, Al3+}, X cation sites in general larger than Y

Pigeonite composition

5-20 % wollastonite, rest any ferrosilite-enstatite mix

Augite composition

20-40% wollastonite, rest any ferrosilite-enstatite mix

Diopside composition

Mg Ca Si_2 O_6, generally 40-50% wollastonite, rest with more enstatite than ferrosilite

Hedenbergite composition

Fe Ca Si_2 O_6, generally 40-50% wollastonite, rest with more ferrosilite than enstatite

Which pyroxenes have an orthorombic crystal structure?

Those with wollastonite content under 5%

Which pyroxenes have a monoclinic crystal structure?

Those with wollastonite content above 5%

Olivine chemical formula

(Mg, Fe) _2 Si O_4

Forsterite chemical formula

Mg_2 Si O_4

Fayalite chemical formula

Fe_2 Si O_4

What is the crystal structure of olivine?

Orthorombic. Iron and magnesium occupy edge sharing M1 and M2 octahedral sites

Spinel group general formula and crystal structure

A B_2 X_4. X anions (typically oxygen, sulfur) arranged in a cubic close-packed lattice, A and B cations occupying octahedral/tetrahedral sites in the lattice

Spinel chemical formula

Mg Al_2 O_4

Garnet general formula

X_3 Y_2 (SiO_4) _3. X€{Fe, Mg, Mn} & Y=Al || X=Ca & Y€{Cr, Al, Fe}

Three "pillars" of Planetary Materials study

Chemistry (elements), isotope chemistry, mineralogy

Properties of dark matter in Λ-CDM

Non-baryonic, cold (far slower than light at the radiation-matter equilibrium epoch), dissipationless (cannot cool by radiating photons), collisionless (only interacts by gravity and possibly weak forces)

What does Λ-CDM serve to explain?

Existence and structure of CMB, large scale distribution of galaxies, observed abundances of H, D, He & Li, accelerating expansion of the universe

Inflation phase of the Big Bang

First 10^(-37) s, universe volume increase ×10^20, temperature decreases from 10^27 K to 10^22 K

Post-inflation Big Bang phases

10^(-33) - 10^(-32) s: reheating, 10^(-6) s: forming of protons and neutrons from quarks and gluons, 10^(-2) s: nuclear fusion, light elements formed

Recombination of hydrogen

e- + p -> H +γ, recombination era occurred around the first 10^(-6) s after the Big Bang

Ground and excited states of the hydrogen atom

-13.6 eV (n=1), -3.4 eV (n=2), -1.51 eV (n=3)... (-13.6/(n^2))

Hydrogen state transition series

Kα,β... to ground state, Lα,β... to n=2, Mα,β... to n=3...

Dark Age in the beginning of the universe

First 0.4-1.5 Myr the particles were in a hot dense plasma of photons, leptons and quarks, which was opaque to electromagnetic radiation since the mean free path of electron was short due to Thompson scattering (same as inside the Sun)

Chemical element creation mechanisms

1. Big Bang nucleosynthesis (H, He, Li) 2. Spallation (Li, B, Be) 3. Nucleosynthesis (Z>3)

CNO cycle

C^12 + H^1 -> N^13 + γ, N^13 -> C^13 + e^+, C^13 + H^1 -> N^14 + γ, N^14 + H^1 -> O^15 + γ, O^15 -> N^15 + e^+, N^15 + H^1 -> C^12 + He^4 + γ

The alpha process

Adding an alpha particle and increasing mass number by 4: C^12 -> O^16 -> Ne^20 -> Mg^24 -> Si^28 -> S^32 -> Ar^36 -> Ca^40 -> Ti^44 -> Cr^48 -> Fe^52 -> Ni^56 -> Co^56 -> Fe^56 (last two by electron capture)

Chemical abundances in the Sun (based on photosphere emission spectra)

91% H, 8.9% He, 0.1% metals (among those 51.2% O, 24.5% C, 8.5% Ne, 6.4% N, 2.5% Mg, 2.4% Si, 2.2% Fe, 1.1% S, 1.1% rest)

Chemical abundances in the Sun compared to abundances in the solar system

They match for most elements. Exceptions: lithium, beryllium, aluminium, manganese - more stable away from the Sun.

Moseley's law

Proportionality between atomic number and √ν of an X-ray electronic transition

How does Auger effect work?

A vacancy on the innermost shell (corresponding to energy level E1) is filled by an electron from a higher shell (E2). The energy emitted is used to ionize the atom by ejecting an electron from the outermost shell (E3) with kinetic energy E1-E2-E3

Stokes shift in fluorescence

The emission spectrum is shifted towards longer wavelengths. This is due to energy loss in the time between absorption and emission (e. g. non-radiative decay to the lowest vibrational energy level of a given excited state)

Multi-element fluorescence

Elements in the sample are excited to emit fluorescence by γ rays emitted at 88keV & 22 keV by a radioactive cadmium 109. Each element can then emit a characteristic X-ray fluorescence photon

Spectrometer setup

Light source (e g tungsten-halogen lamp or laser) -> sample -> collimator (e g lens) -> monochromator (diffraction grating or prism, only a narrow wavelength range should be sent to the detector) -> detector (e g CCD camera) -> readout system

Characteristic spectrum

A sample is subjected to a voltage (50kV) which will cause some vacancies and transitions. X-ray peaks will correspond to Kα, Κβ etc. with intensity proportional to the probability of the transition. Continuum due to Bremsstrahlung - e kinetic energy loss

X-ray absorption spectroscopy concept

A sample is exposed to a beam of X-rays (emitted from electrons accelerated in a synchrotron). The amount of absorption is measured as a function of energy.

XAS spectra shape and information

XANES - X-ray Absorption Near Edge Structure - information about electronic structure of the absorbing atom. EXAFS - Extended X-ray Absorption Fine Structure - environment around the absorbing atom (cannot be obtained from diffraction except in crystals)

XANES subregions in X-Ray Absorption Spectroscopy

Pre-edge: information about the unoccupied states of the absorbing atom. Absorption edge: occupied states. Post-edge: excited states

Converting electron movement into radiation

1) Bending magnet: electron path bent, photon emitted 2) Wriggler - series of alternating magnets, proportionally enhanced emission spectrum 3) Undulator - magnetic fields of a wriggler tuned to cause interference, specified energies with N^2 intensities

Beer-Lambert Law

A = εcl, where A=log(I_0/I) is the absorbance of a solution, ε is the molar absorptivity (a.k.a. extinction coefficient), c is the concentration of the absorbing species in the solution and l is the path length of light through the solution

First 100 Myr of the Solar System

0-10^6 yr - stellar era, 10^6 -10^7 yr - disk era, 10^7-10^8 yr - telluric era

Typical conditions in a molecular cloud

10^2-10^3 ly diameter, 10-20 K temperature, 10^6-10^8 solar masses, turbulent dynamics: large scale gravitational forces counterbalanced by smaller scale (0.1pc) turbulent forces

Molecular cloud chemistry

Most common species are H, He and C bearing molecules. There are also dust grains with heavier elements (silicates, other hydrocarbons, ice)

What triggers star formation in a molecular cloud?

A local collapse can be triggered by: small density fluctuations due to turbulent forces, gravitational perturbation from passing bodies, shock wave from a nearby supernova

How is material structured in a molecular cloud and why?

Filaments along the magnetic field because about 1 in 10^7 particles in the cloud is electrically charged (electrons and ions), neutral ones are affected indirectly through collisions (ambipolar diffusion)

Dependence of molecular cloud collapse timescale on magnetic field

If weak (magnetic pressure < gas pressure), collapse is faster, star formed in ~10^4 yr. If strong,(magnetic pressure > gas pressure) star formed in ~10^5 yr or even longer

Molecular cloud collapse typical timeline

~10^4 yr - vast disk (10^3 - 10^4 AU), bipolar outflow, IR invisible, highly energetic radiation), ~10^5 yr - growing seed, more evident disk (IR visible), reduced to 500-10^3 AU, ~10^6 yr accretion-ejection, stellar wind inhibits further growth of a star

End of stellar era in the molecular cloud collapse

Close to the final mass of the star, confined to 0.1 AU by a magnetic cavity. Disk extends to ~50 AU in a cluster environment and ~10^2-10^3 AU in open/isolated environment. Still no planetesimals or planets

Constraint on gas giant formation

Mass loss during the disk era results in depletion of volatiles (by a factor of 10). Gas giants have to form before (the first 10 Myr of a planetary system)

Disk era characteristics

Mass loss from the disk, coagulation of dust into pebbles and their aggregation up to planetesimals

Mechanisms for planet formation

Core accretion (of dust by planetesimals), pebble accretion and coagulation (they slow down due to gas drag), disk instabilities (triggered by gas giant migration - Grand Tack)

What is a T-Tauri phase?

Another name for the disk era (10^6-10^7 yr of the star)

Why and how strongly is the pebble accretion mechanism more efficient beyond the snow line?

Ice acts as a sticky medium, making the process 10^3 times more efficient

Earth's atmosphere composition profile

Molecular oxygen, nitrogen and carbon dioxide up to ~10 km. Above a thin ozone layer, then atomic oxygen (up to ~1000 km), then atomic helium and then atomic hydrogen

Earth's atmosphere evolution history

5-4.5 Gyr CH_4 & H_2, plus some N_2, H_2O, H_2S, NH_3, Ar and a bit of He, 4.5-3 Gyr N_2, plus some SO_2, CO_2, H_2O, Ar and a bit of He, Ne, CH_2, NH_3, later similar but more O_2 and no SO_2 plus a bit of CH_4 and Kr

Formation modes of heavier elements in the universe

Dying low-mass stars, exploding massive stars, cosmic ray fission, merging neutron stars, exploding white dwarfs, human synthesis

What does "lithophilic" mean?

rock-loving (tending to stay in a mantle)

What does "siderophilic" mean?

iron-loving (tending to be sucked into the core of a planetary body)

What does "refractory" mean?

Non-volatile

What is the Late Veneer Model and what does it explain?

The Moon was formed from a part of Earth when the Earth was impacted by a Mars-sized object Theia. This impact (by assimilating Theia material into the Earth's mantle) can explain higher than expected abundances of siderophiles in the Earth's mantle

p-p fusion

H^1 + H^1 -> H^2 + e^+ + ν, H^2 + H^1 -> He^3 + γ, He^3 + He^3 -> He^4 + 2H^1

Dependence of element abundance in the universe on atomic number

H highest, He a bit less. Local minimum for Li-Be-B. Later from C on a more or less steady decreasing zig-zag: elements with odd atomic numbers are ~×10^2 less abundant than their even numbered neighbors, a local maximum for Fe & Ni (iron peak)

Oddo-Harkins rule and why it holds

Elements with odd atomic numbers are ~×10^2 less abundant than their even numbered neighbors. Reason: the α process, from carbon up, leads to a fusion of consecutive even-numbered elements. The odd-numbered can only be made from these by other processes

Why is there so little lithium in the universe?

It is not produced in the fusion process in the stars. It easily becomes involved in chemical reactions. In particular, it is consumed in a reaction with highly abundant hydrogen to form 2 highly stable helium atoms.

Apatite chemical formula

Ca_10(PO_4) _6(OH, F, Cl) _2

What is 1) an eutectic mixture and 2) eutectic point?

1) A homogenous mixture that has a melting point lower than that of the constituents. 2) The lowest temperature and mixing ratio for which a liquid phase of that mixture is stable at a given pressure.

What are isotones and isobars?

Isotones: nuclides with the same number of neutrons e. g. ^39_19 K, ^40_20 Ca. Isobars: nuclides with the same mass number e. g. ^40_16 S,^40_17 Cl,^40_18 Ar,^40_19 K,^40_20 Ca

What is the difference between "radioactive" and "radiogenic"?

Radioactive isotopes are those which undergo spontaneous radioactive decay. Radiogenic isotopes are products of radioactive decay, can be either radioactive or stable.

Mattauch isotope rule

For any pair of adjacent isobars only one of them can be stable, the other must be radioactive

Nuclear decay modes

α emission: ^4_2He, Z-=2, N-=2, β emission: β-: e- + ν_, Z+=1, N-=1, β+: e+ + ν, Z-=1, N+=1, ε decay == electron capture, Z-=1 N+=1, γ emission: from excited to lower state, photon emission, N, Z=const., p emission: Z-=1 N=const., n emission: N-=1 Z=const

How does electron capture work?

The nucleus of an ionized atom captures it's own K-shell (innermost shell) electron to form a neutron with one of it's protons. Released energy is split between a neutrino and gamma ray. Also known as K-capture or ε decay

What is spontaneous fission?

A highly energetic form of nuclear decay in which a heavy nucleus splits into two smaller nuclei

What is branched decay?

A radioactive nucleus can decay in multiple ways, each with a different probability (dependent on energy levels and spin states of the parent and daughter nuclei, e g a high spin-state nucleus may favor γ-decay) and half-life.

How was radioactivity discovered?

Radioactive substance in a lead block was emitting α,β,γ particles, the paths of which upon passing through a pair of electrically charged plates were bent towards -, +, and not bent, respectively (as confirmed by detections on a photographic plate)

Silicon isotopic abundances on Earth

^28 Si: 92.2%, ^29 Si: 4.7%, ^30 Si: 3.1%

How are the atomic masses calculated in the periodic table?

A weighted average of isotopic atomic masses, with abundances being the weights. Each isotopic mass should be reduced by the corresponding binding energy, otherwise (if we just added up protons and neutrons) the result would be overestimated.

How are stable isotopic compositions reported?

δ=(R_{sample}/R_{standard} -1)×1000 (permil) where R is the ratio of two isotopes of the same element which we want to compare. Analogously for smaller deviations ε would be for 1 per 10^4 and μ for 1 per million

What is used for hydrogen and oxygen as a standard reference when reporting δ?

SMOW - standard mean ocean water

How much micrometeorite material reaches Earth's surface on a daily basis?

~100 tons (mostly in the form of IPD - interplanetary dust particles)

What are and how far do Van Allen belts reach?

Van Allen radiation belts are zones of energetic charged particles, mostly coming from the solar wind, captured and held by Earth's magnetosphere. The inner belt reached ~5000 km and the outer one ~15000 km

Where do we get ^14 C from?

Continuously generated in the atmosphere due to cosmic radiation: ^14 N + n -> ^14 C +p, where n is a secondary thermal neutron created in the atmosphere due to cosmic radiation

What happens to ^14 C?

It decays back to ^14 N (β- decay) with a half-life of 5730 years. Human civilization time scale phenomena can be dated with ^14 C (e. g. an Egyptian mummy)

CO_2 isotopes in the Earth's atmosphere

^14 CO_2 vs ^12 CO_2 ratio is around 1 to 10^12. The former is created as follows: ^14 C +.5 O_2 -> ^14 CO, ^14 CO +.5 O_2 -> ^14 CO_2

Radioactive isotopes good for dating over large timescales

^235 U - half life of 700 Myr, ^238 U - half life of 4.5 Gyr

Water isotopic fractionation

H_2 ^16 O evaporates easier than H_2 ^18 O or D_2 ^16 O, leading to enrichment of liquid water in heavier isotopes. In ice, on the other hand, protium is preferred in the crystal structure as opposed to deuterium.

Which isotopes are preferred by plants in photosynthesis?

lighter ones (^12 C O_2 as opposed to ^13 C O_2, H_2 ^16 O as opposed to H_2 ^18 O)

What is the difference between equilibrium and kinetic isotope exchange?

The latter does not reach equilibrium (due to constraints such as diffusion rate, temperature, pressure) and is unidirectional (irreversible)

What is an equilibrium constant K for an isotopic exchange?

A ratio of δ parameters of two phases, each raised to the power of it's stoichiometric coefficient (number in the reaction equation in front of the compound). K^(1/n), where n - number of exchange sites, gives a fractionation factor of element swapped

Examples of kinetic isotope fractionation

Photosynthesis (more ^13 CO_2 in air than in living organisms), evaporation-precipitation (more H_2 ^18 O in liquid water than in vapor)

How does a mass spectrometer work?

A sample is vaporized and ionized, then the ions are accelerated by an electric field and subsequently their trajectories are bent by a magnet (Lorentz force). The bend radius will depend directly on mass to charge ratio (most ions have a charge of 1)

What mineral is commonly used to date old rocks and why?

Zircon (ZrSiO_4), because it easily traps radioactive isotopes of uranium (which substitute zirconium easily thanks to it's big radius) that decays to lead.

What detectors are used in mass spectrometers?

Photographic plates electron multipliers, Faraday cup (a metal cup designed to catch charged particles, which can then be measured by discharge in a measurable small current)

How does secondary ion mass spectrometry work?

Similar to standard mass spectrometry, but the ions are created by bombarding a sample with a primary ion beam (hence they are secondary)

What would be the point in keeping the ion beam at the same spot in SIMS?

To find out about the dependence of isotopic contents on depth

Quartz > albite > K-feldspar > ...>anorthite>diopside > wollastonite > zircon=garnet> olivine What is this series?

A sequence of decreasing ^18 O enrichment which is correlated to decreasing degree of polymerization and increasing melting point

What is δ^18 O in the Earth's atmosphere?

+24

Oxygen isotope ratios and TFL

If we plot δ ^17 O with respect to δ ^18 O for terrestrial rocks, we get the Terrestrial Fractionation Line which has a slope of 1/2=(17-16)/(18-16). This slope is indicative of equilibrium fractionation, all equilibrated bodies should land parallel

C chondrites on the oxygen isotope plot

Equilibrium fractionation never occurred, ^16 O dominates whereas ^17 O and ^18 O formed with equal likelihood => slope 1 rather than 0.5

Uranium decay series

A. k. a. the 4n+2 chain. Starts from ^238 U, which α decays to ^234 Th with a half life of 4.5 Gyr. Other processes have much shorter life times: 2 β- decays bring ^234 Th to ^234 U, then a series of α decays down to ^218 Po, then branched until ^206 Pb

Actinium series

A. k. a. the 4n+3 chain. Starts from ^235 U, which α decays to ^231 Th with a half life of 704 Myr. After much shorter life times: β- decay bring ^231 Th to ^231 Pa (protactinium), then α decay to ^227 Ac (actinium), then branched until ^207 Pb

Thorium decay series

A. k. a. the 4n chain. Starts from ^232 Th, which α decays to ^228 Ra with a half life of 14.1 Gyr. After much shorter life times: 2 β- decays bring ^228 Ra to ^228 Th, then a series of α decays down to ^212 Pb, then β- to 212^Bi and branched to ^208 Pb

Less commonly used in dating radioactive isotopes with long half-lives

^184 Os (osmium, 1.12×10^13 yr), ^87 Rb (rubidium, 49.2 Gyr)

What is the relationship between a daughter and parent radioactive/genic isotope abundances?

daughter_present = daughter_initial + parent_present(e^(λt)-1) where λ=1/(half life × log2)

Concordia and discordia

Concordia is a curve on ^206 Pb / ^238 U vs. ^207 Pb / ^235 U which portrays the dependence of these ratios on time (sample age). A discordia is a straight line under this curve, which intercepts it at T0 - crystallization age and T1 - metamorphism age

What is the difference between Patterson and Holmes-Houtermans model for lead isotopic dating?

The former uses initial lead isotopic ratios, the latter primordial ones, that is corresponding to the age of the solar nebula. The latter is more modern and precise

Mg/Si ratios in planetary bodies of the Solar System

Decreasing with heliocentric distance: 1.25 on Earth, 1.04 on Mars, 1 on Vesta

What are short lived radioisotopes, how do we know about them and what is their significance?

SLRs are radioactive isotopes with short half-lives which are extinct in the universe but we know they were in the past because we observe anomalously high abundances of daughter isotopes. Decay could have driven core differentiation in planets/asteroids

Short Lived Radioisotopes examples

^26 Al decaying to ^26 Mg with a half life of 7.17×10^5 yr; ^60 Fe decaying to ^60 Co with a half life of 2.6 Myr, which then decays to ^60 Ni in ~5yr (^60 Fe was found in fossilized bacteria suggesting a supernova near the Solar System)

How does a chromatograph work?

A sample is carried over a stationary phase (fixed solid or liquid material) by a mobile phase (liquid or gas). The interactions between the flowing sample and the stationary phase will be varied and a time shift will help differentiate between them

Describe organic compounds in meteorites

There are around 14000 different known molecular species, with mass to charge ratios over 400, with abundant C, H, N, O and also S and Mg

Radiogenic lead concentration in different materials

Stony meteorites > Terrestrial sediments > Iron meteorites

What is the difference between meteoroid, meteor and meteorite?

Meteoroid: small rock orbiting the Sun; meteor: entered Earth's atmosphere: meteorite: extraterrestrial rock found on Earth's surface

What is the difference between a meteorite fall and find?

Find: found on the surface but uncorrelated with a fall (not observed, arrival time unknown). Fall: an observed meteor correlated with a find of at least one fragment.

Meteorite fall statistics

chondrites: 85%, achondrites: 10%, iron: 4%, stony iron < 1%

What are micrometeorites similar to?

Most of them are similar to carbonaceous chondrites, although they are likely to have comet origin. Less than 1% resembles achondrites

Are micrometeorites common on other planetary bodies?

Yes, 40% of martian regolith is composed of micrometeorites

Micrometeorite general characteristic

Size: 50μm - 2 mm. Entry velocity: ~10 km/s. About 2700 tons fall to Earth each year

Example of a big meteorite fall

Sikhote-Alin, fell in Siberia on 12.02.1947. It's an iron meteorite type 2AB weighing 23 tons. Composed primarily of nickel alloys.

Largest meteoritic find ever

Hoba meteorite: iron meteorite, discovered in 1920 in Nigeria, weighing 60 tons, believed to have fallen over 80000 years ago and to have originated from the asteroid belt

What are typical depth/diameter ratios for impact craters and what do they tell us?

~0.2 for simple and ~0.1 for complex craters. The ratio increases with impactor velocity

Meteorite impact evidence at the site

Impact breccias, shocked quartz (shocked lamellae a. k. a planar deformations), impact melt (glass droplets in breccias), ejecta blanket, crater walls and rims, impact-related minerals (coesite, stishovite - both SiO_2 but formed at high pressures)

Quartz chemical formula

SiO_2

Largest impact crater on Earth

Vredefort crater in South Africa, 300 km diameter, formed by an impact of a ~10-15 km sized asteroid/comet over 2 billion years ago

Characteristics of the Allende meteorite

This >2 ton (in several pieces) carbonaceous chondrite (CV3) fell in Mexico on 08.02.1969 It is heavily studied due to it's array of refractory elements, including rare Earth elements, and organic compounds

What is the oldest known material on Earth?

The Murchison meteorite - an iron & water rich CM meteorite that fell in Australia on the 28.09.1969 and is dated to be 7 Gyr old

Heliocentric distance distribution of chondrites (believed origins)

Enstatite chondrites - closest, ordinary chondrites - slightly further, Rumuruti - even further, carbonaceous chondrites - way out

What chemical markers increase with heliocentric distance?

Iridium/Rhodium ratio and ε^100Ru

Chemical characteristics of chondrites

Silicate minerals (olivine, pyroxene, feldspar), Fe-Ni alloys, sulfides, volatiles, organics including amino acids

What are the chondrules?

Millimeter-sized spherules which must have formed before the meteorite itself and then accreted within the matrix. They have diverse textures and compositions

What's in the CAIs and how were they formed?

They are composed of refractory minerals, such as hibonite (Ca Al_12 O_19) and melilite (Ca_2 Al_2 Si O_7). They must have formed in the hot, inner regions of the solar nebula, where temperatures were high enough to melt these minerals

What is the overall chemical composition of chondrites like?

Similar to the solar photosphere

Example of presolar content in chondrites

Si C - diamond, formed by other stars

Prior's rules and the reasoning behind

The overall metal content in chondrites decreases proportionally with increasing Ni/Fe (overall) and Fe/Mg in the silicate phase. The former related to higher density of Fe wrt.Ni. The latter with Fe tending to oxidize if metal content is high

What was the interpretation of the linearity in the Prior's rules? What was the counterargument?

It was supposed to represent a common origin of chondrites by oxidization of a magma ocean or reduction of highly oxidized primordial material. Urey&Craig showed Fe/Si partitioning varied (0.6-0.8) which would not be consistent with a common magma source

Urey & Craig diagram: interpretation and peculiarities

Plotting (Fe in metallic or silicate phase)/Si vs. (Fe in oxides)/Si. Diagonal lines represent fixed Fe/Si ratios. Most chondrite groups form clusters around these lines (0.6 or 0.8) => different origins. Enstatite chondrites have metallic Fe only

How to differentiate between early and late core formation?

Based on short lived radioisotopes. Example: lithophytic ^182 Hf decays to siderophilic ^182 W with a half-life of 9 Myr. If W is found in the silicate (mantle), it means differentiation occurred after >9Myr, if in metal (core), then earlier

What is the interpretation of the chondrite lithophylie element abundances plot?

EH meteorites have more highly volatile elements (Cr, Mn, Na, K) which implies low temperature alteration. The ones which have more of less volatile and fewer highly volatile elements, had higher temperature metamorphism

Metamorphism among petrrologic index grades

Grade 3 is the most pristine. 3 -> 1: aqueous low temperature alteration. 3 -> 6: increasing thermal metamorphism

Which petrologic grades do carbonaceous chondrites have and why?

Between 1 and 3 - presence of carbon is due to interaction with water

How does matrix appearance depend on petrologic grade?

1 - fine, opaque. 2 - mostly fine, opaque. 3 - clastic, minor opaque. 4 - 6 transparent, recrystallized, increasing coarseness

How do chondrules depend on petrologic grade?

They are most sharply defined at grade 3 and become less defined with increasing metamorphism. In grade 1 no chondrules. In thermally metamorphized cases metallic materials become incorporated into chondrules, building metallic shells

Water and carbon content in chondrites, dependence on petrologic grade

Only in grades 4 and below. At grade 1 up to 22 wt% of water and up to 3 wt% of carbon

How does nickel content vary with petrologic grades in chondrites?

At lower grades Ni in sulfides. At higher grades more metallic Ni. For grades 4-6 an exsolution of alloys taenite and kamacite

What is the crystal structure of taenite?

Face centered cubic

What is the crystal structure of kamacite?

Body centered cubic

What does the petrologic grade tell about Kakangari chondrites?

They all have grade 3, which suggests a small parent body (no metamorphism)

How much vol% of chondrules/matrix/CAI in CI meteorites?

>99% matrix, <1% rest

What compounds and minerals are found in CI meteorites?

complex organic compounds, diamond, graphite, carbonates; 80% phyllosilicates - serpentine and saponite, triolite, pyrrholite, olivines and pyroxenes: no metallic iron

Serpentine chemical formula and how is it formed

(Fe, Mg, Mn, Al, Ni, Zn) _2-3 (Al, Si, Fe) _2 O_5 (OH)_4. Formed through hydrothermal metamorphism of olivine and pyroxene (on Earth near convergent plate boundaries between oceanic crust and the upper mantle

Saponite chemical formula and properties

Ca_0.25 (Mg, Fe) _3 ((Si, Al) _4 O_10)(OH)_2×n(H_2O). It belongs to the smectite group, it is soluble in sulphuric acid (H_2 SO_4). It is soft and plastic. It exists in veins and cavities of serpentine and basalt.

What characterizes smectites?

They have a mixture of phyllosilicates forming a 3 layer structure 2:1 TOT (tetrahedra-octahedra-tetrahedra), with the main cations in interlayers being Na+ & Ca2+

What are phyllosilicates?

Also known as sheet silicates, these minerals form from sheets of silicate tetrahedra in a Si_2 O_5 configuration, hydrated with H_2 O or OH (hydroxyl) and involving various cations

What are troilite and pyrrhotite?

Troilite: FeS, with Fe^2+ kations and a hexagonal structure. End member of pyrrhotites. Pyrrhotite: Fe_(1-x)S where x goes up to 0.125, can have Fe^3+

Are CI meteorites commonly found? Where?

They are rare because it is hard for them to survive the entry and impact without being altered. They are most commonly found at Antarctica

Si O_2 contents in the Earth's crust

Around 60-65 wt% on average, higher in granites, lower in basalts

What does chondrule rim thickness depend on?

H_2 O content, temperature and time - in a diffusion controlled growth it's square (width of a rim which grows both outwards and inwards) is proportional to time and to exp(-H/kT) where H is the activation entalpy

What is the consensus about CI chondrites?

They are a product of low T (~150°C) alteration of precursor anhydrous silicates, likely to have occurred on the parent body in the first 15 Myr of the Solar System

How to measure self-diffusion?

Example: CO_2. On one side of the chamber we place regular CO_2 (with ^12 C), on the other CO_2 with some ^13 C. We open the divider and allow diffusion, then after some time we close it and measure ^13 CO_2 based on IR absorption and Beer-Lambert Law

How to measure diffusion in solids?

Quench an experiment after a time of interest by turning off the furnace - at low temperature diffusion is not so effective

How does diffusion change with increasing pressures?

In solids the pressure constrains the space, leading to slower diffusion. In liquids it may be faster as the coordination number of ions may change introducing new diffusion mechanisms)

What is a coordination number?

The number of atoms/molecules/ions to which a central atom is bonded

Describe the Tagish Lake meteorite

It was a fall in Canada on 18.01.2000. The meteorites are classified among carbonaceous but not assigned to specific group. Contains pristine (4.5 Gyr) organic matter, maybe most unaltered meteorite ever recovered. Similar high ^13 C content as comet 67P

Pros and cons of optical microscopy

Pros: non-destructive imaging, real-time observation, little/no sample preparation, low cost (use of visible light). Cons: upper magnification limit ~1000× related to a minimum λ=400nm, proportional to achievable resolution

What should be a particle size in a ground sample investigated with optical microscopy and why?

~300nm, which corresponds to highest scattering efficiency for visible light. If a mineral were crushed to lower grain sizes, it would lose it's color and appear white

What is pleochroism?

A phenomenon in which a substance has different colors when observed at different angles with polarized light, due to anisotropy in crystal structure

Compare scanning and transmission electron microscopy

SEM uses an electron beam to scan the surface of a sample. TEM looks at electrons transmitted through the sample. TEM has higher resolution (atomic scale) but requires elaborate sample preparation: thinning to widths ~nm mechanically and with an ion gun.

SEM setup

Electron gun -> accelerating anode -> focusing magnet -> scanning magnet -> sample -> detectors (secondary electrons, backscattered electrons, X-rays)

What observable products does the electron beam generate in scanning electron microscopy and which information do they provide?

Secondary electrons (topography of the sample), backscattered electrons (atomic number), characteristic X-rays (elements in the sample), Auger electrons (some elements), cathodoluminiscence (photons trigger electrons), continuous X-rays (Bremsstrahlung)

Compare energy dispersive X-ray spectroscopy and wavelength dispersion spectroscopy

Both bombard the sample with an electron beam and measure emitted characteristic X-rays. EDX - the spectrum is acquired synchronously from γ energies converted to voltage. WDS - the wavelengths are measured directly (1 at a time) using crystal diffraction

How to analyze a SEM backscattered electron image?

The pixel value is going to be an indicator of the average electron density. High density metals will be dark. Low density phases (e. g. olivine) will be light. Glass phases will be darker and opaque.

How were chondrules formed?

An aggregate dustball in the molecular cloud heats up, melts and cools down in a span of under one hour (timescale indeed from sizes and phases found). Rapid cooling leads to the glassy appearance. Various theories for trigger mechanism

Which chondrule forming triggers have been proposed? Which are least likely?

Lightning, spontaneous combustion, nuclear shockwaves, magnetic current sheets, gamma ray bursts. Other: planetesimal collisions or radiative heating from their molten surfaces. But chondrules are older than planetesimal age and have no REE fractionation

What is the main counterargument for the chondrule formation due to lightning theory?

It is hard to control the cooling process, which would lead to amorphous spherules. It could work for obsidian (achieved in lab), but chondrule composition is different. So seemingly the process cannot work, at least on Earth.

What is the size limit on chondrules and why?

0.05 - 1 mm. Inward heat diffusion is proportional quadratically, and heat loss due to black body radiation is proportional linearly to dustball size. If the radius is big, heat diffusion is slower and the dustball cools down. If too small, it overheats

What is the formula for inward heat diffusion timescale?

a^2/(4α), where a is the radius and α is thermal diffusivity which depends on the number density of impurities and on elastic properties such as phonon sound velocity

Which elements are most abundant in chondrules?

O, Mg, Si, Fe

How does chondrule chemistry compare to CI meteorite matrices?

Similar for refractory and moderately volatile elements. Depleted in volatile and siderophile elements

Which is the crystallization sequence of mafic minerals with decreasing temperature and why?

First olivine, then orthopyroxene, then clinopyroxene. This is related to greedy use of Mg and Fe and gradually decreasing (Mg+Fe)/Si ratio

What is a phenocryst?

A large crystal

What is typical chondrule mineralogy?

Abundant olivine and pyroxene phenocryst. Glassy matrix frequently with microcrystalline clinopyroxene quench crystals.

Which types of chondrules are there?

Type I: FeO poor - Mg rich olivine and pyroxene, metallic iron. Type II: oxidized - no metallic Fe, lower Mg numbers in ferromagnesian silicates

Which setting corresponds to high chondrule number density?

High volume fraction, small chondrule radius

Which textures are found within chondrules?

Porphyritic (phenocrysts in a fine-grained matrix), granular (smaller crystals), radial (elongated crystals from center outward), barred (elongated or elliptical), banded (alternating layers), cryptocrystalline (fine grained, almost amorphous), porous

How was the porphyritic structure in chondrules formed?

There were few sites of nucleation. First, phenocrysts grow, likely due to rapid cooling. The matrix formed later. Temperature must have been close to the melting point

How was the radial structure in chondrules formed?

Rapid heating and cooling, possibly due to a brief, intense heating event

How was the barred/banded structure in chondrules formed?

The texture suggests varying cooling rates or changes in chemical composition during chondrule formation

How was the cryptocrystalline structure in chondrules formed?

Slow cooling and recrystallization of the chondrule. Presence of a different Si O_2 on the sides of chondrules suggests presence of at least two distinct liquids during formation

How was the porous structure in chondrules formed?

Escaping gas during chondrule formation

How was the granular structure in chondrules formed?

Relatively high number of nucleation sites, moving around as the crystals grow

What dictates the pyroxene to olivine ratio in a chondrule?

The (Mg + oxidized Fe)/Si ratio. If high, then more pyroxene, if low, more olivine

What dictates nucleation for a forming chondrule and why?

Peak temperature - affect the ability of molten iron to coalesce

Explain the glass transition

As we cool down a liquid, at the melting temperature it should reduce it's volume to turn to a solid crystal. If the cooling rate is too fast, it will become a metastable supercooled liquid. At a lower "glass transition temperature" it can become glass

How do CM and CR meteorites present themselves on the oxygen isotope diagram?

Intermediate slopes (~0.7), related to parent body water and alteration trying to bring the line closer to TFL but not succeeding all the way because it is low temperature alteration

Which meteorites have a high ^15 N content?

CR

CH meteorite characteristics

High metal fraction (20-40%), little oxidized iron, low CAI fraction (but well preserved), small (0.02 mm) but frequently-occurring cryptocrystalline chondrules. Petrologic grade 3

How to observe cryptocrystalline chondrules?

They are invisible under an optical microscope. They can be analysed with diffraction

CO meteorite characteristics

Highest CAI fraction (13%). Fe rich olivine (up to 60% fayalite). Low metal content (1-5%). Petrologic grade 3.

How are pyroxenes and olivines in CO chondrules like?

Pyroxenes are clustered mostly around diopside and enstatite, some also around Fe_(50-60). Olivines continuously between fayalite and forsterite

What drives iron content in olivines in chondrules?

Oxidation state

Which mineral is more iron-rich in chondrules and why?

Olivine (higher fayalite fractions in olivine compared to ferrosilite fractions in pyroxene). Olivine has the advantage because it crystallizes first.

Taenite chemical formula

Fe Ni

What decides between epitaxial and dendritic growth of crystals in chondrules?

For the epitaxial growth, metal seeds (droplets) are needed within the liquid phase, which in turn requires low oxygen fugacity

How does the melting point of olivine depend on iron content?

Decreasing. Forsterite has a melting point around 1900 °C, whereas fayalite around 1200 °C

What is a wetting angle and how it relates to crystal development in chondrules?

An angle between a tangent to the melt and the interface. Low θ means high melt phase radius, low surface to volume ratio and higher heterogenous nucleation chance

How does the free energy change depend on melt radius in crystal growth in chondrules?

ΔG has a local maximum, then drops to zero at r_0 and later below. Same behavior for homogenous and heterogenous nucleation, but below r_0 the homogenous has higher ΔG (less likely) and above lower.

CM meteorite characteristics

Low metal content (0.1%), oxidized, high matrix fraction (~70%) with presolar materials (e g diamond), strong variation of alteration

How are metals in CR chondrites?

High content (5-8%), reduced, Mg rich mafic minerals

Water dissociation mechanisms

H_2 O + Si O_2 = 2 Si O H; H_2 O = H_2 + 1/2 O_2

Which carbonaceous chondrites have lowest oxygen fugacity?

CR

What are indicators of water alteration in carbonaceous chondrites?

Phyllosilicates and PCP (poorly characterized phases) clumps

How were aminoacids formed in meteorites?

Equillibration of a mixture of simpler organic compounds, at low temperature (~60°C is enough) for some time (in experiments 6 days)

How do Mg/Si ratios in Renazzo meteorite look like?

Higher in chondrules (higher temperature at formation), lower in the matrix. The average comparable to CI meteorites.

What's the difference between CR and CM oxygen isotope plot?

Although both have a similar slope ~0.7, the pivot point for CR is higher. The lines are parallel, the difference due to varied isotopic signatures of secondary water source

What sets wollastonite apart from enstatite and ferrosilite?

The tetrahydra chain is distorted, which makes it a "pyroxinoid", not a true pyroxene

CV meteorite characteristics

Largest chondrules (1 mm), high CAI fraction (~10%)

CK meteorite characteristics

Temperature metamorphism (petrographic grades 3-6), high matrix content (like CM), no glass, varied oxidation/reduction states

How polymerized is plagioclase?

Completely - each oxygen is bridging between Si/Al tetrahedra

What are the diagnostics of increasing thermal metamorphism in CK chodrites?

Coarsening of plagioclace grains within chondrules and all mineral grains within the matrix

What would be a good method to measure the diffusion profile along a chondrule rim?

EDX

How do alluminium, chromium, titanium and iron diffusion profiles along chodrule rims compare to each other?

The higher ionic charge (4 for Ti, 3 for Al & Cr, 2 for Mg & Fe) the less efficient the diffusion, because the charge balance needs to be preserved, so letting in one alluminium and titanium cation would require 1.5 and 2 Mg vacancies, respectively

Dolomite chemical formula

Ca Mg (CO_3) _2

What is an equilibrium constant K for a chemical exchange?

Similar as isotopic, but instead of δ we have activities (a=γX, where X - mole fraction, proxy, γ - activity coefficient, may deviate from 1)

What is fugacity?

Activity of a gaseous phase - activity coefficient times partial pressure of that gas

What happens with iron depending on oxygen fugacity in controlled experiments?

Low fugacity - metalic Fe. Medium fugacity - higher Fe content in olivine. High fugacity - Low Fe in olivine, instead magnetite or even hematite formed.

What are magnetite and hematite?

Highly oxidized iron compounds, which form a high f_(O_2) solid state buffer: 2 Fe_3 O_4 (magnetite) + 1/2 O_2 = 3 Fe_2 O_3 (hematite)

Wustite chemical formula

Fe O

Solid state buffers from high to low oxygen fugacity

MH (2 magnetite + 1/2 O_2 -> 3 hematite), NNO (Ni + 1/2 O_2 -> Ni O), FMQ (2 magnetite + quartz + O_2 -> 3 fayalite), WM (wustite + 1/2 O_2 -> magnetite), IW (Fe + 1/2 O_2 - > FeO), QIF (quartz + Fe + O_2 -> fayalite)

In the pyrrhotite formula Fe_(1-x)S, why does x only go up to 0.2?

x=0.2 corresponds to 20% vacancies. In the Fe^2+->Fe^3+ exchange, each vacancy corresponds to two Fe^3+ kations, so x=0.2 corresponds to an equal number (40%) of Fe^2+ and Fe^3+ kations.

How does oxygen fugacity in solid state buffers depend on temperature?

Increasing; over a temperature range from 600°C to 1200°C it may increase by over 15 orders of magnitude

How to impose specific oxygen fugacity on an experiment?

Surround the sample with an appropriate solid state buffer

How to distinguish between Fe^2+ and Fe^3+ in Mössbauer spectroscopy?

Fe^2+ has higher quadrupolar splitting

What happens to quadrupolar splitting in Mössbauer spectra of minerals with symmetry distortions?

In octahedral coordination of Fe^2+ it decreases (going away from central symmetry), in tetrahedral of Fe^3+ it increases (distortion goes towards central symmetry)

Which minerals would be good water sources on Mars?

Jarosite (K Fe_3 (SO_4) _2 (OH)_6), Alunogen (Al_2 (SO_4) _3·17 H_2 O), Goethite (Fe O (OH)), Brucite (Mg(OH)_2)

How does Mössbauer spectrum change with increasing coordination number?

Increasing isomeric shift

How does Mössbauer spectroscopy work?

We have a ^57 Co source which decays with a half-life of 271 days into excited (5/2 spin) ^57 Fe which in turn goes down to ground state (1/2): 15% directly but 85% through intermediate 3/2 state leading to γ ray emission of 14.4 keV

Which measurable effects shape the Mössbauer spectra?

Isomeric shift (related to electron density and oxidation state), quadrupolar splitting (electric field gradient splitting 3/2 into 2 substates), magnetic spliting (3/2 into 4 and 1/2 into 2 substates, 6 different transitions allowed)

Explain the role of recoil elimination in Mössbauer spectroscopy

The emitted and absorbed spectra would be shifted to lower and upper energy, respectively, by E_R=E_γ^2/(2Mc^2). We need large mass to make that shift negligible and obtain resonance - work with solids, where M is the mass of the whole metal, not 1 atom.

How to study the structure of amorphous materials?

UV absorption due to d-orbital electron level splitiing (on the order of 2-3 eV) described by the crystal field theory

What dictates the anomalies in Europium concentration in CAIs? Why only for Europium?

Oxygen fugacity. Europium can be in a 2+ state, most RREs are in a 3+ state.

How were the oxygen fugacities when CAIs were formed?

Extremely low (~10^(-20) at 1200°C)

How to identify very low oxygen fugacities?

Presence of metallic iron can only give an upper bound. To go lower, we need to look at the presence of sulphides or e g Ti^3+/Ti^4+ ratio

How to control extremely low oxygen fugacities in experiments?

Gas mixing (e.g. specific proportions of CO and CO_2)

How does ionic radius change among the lanthenide contraction?

Decreases from left to right (from lighter to heavier elements)

Why are Rare Earth Elements mostly found in the mantle?

They are lithophylic and have large ionic radii, as a result they have trouble finding mineral hosts

How do Rare Earth Element concentrations vary in clinopyroxenes and garnets?

They decrease with ionic radii in clinopyroxenes. Lighter are more abundant in clinopyroxene, heavier in garnet. In garnets, there's slightly more in the core than in the rim, which likely experiences removal by mixing with later crystallizing pyroxene.

What drives high heavy RRE concentrations in ultrarefractory CAIs?

Temperature

Which are primary components of CAIs?

Melilite ((Ca, Na) _2(Mg, Fe, Al, Si) _3 O_7), Fassaite (K_5 Ca_8 (Si_6O_15) _2(Si_2O_7) Si_4 O_9 (OH)*_3(H_2 O)), rest (hibonite, spinel, perovskite, anorthite)

Which types of CAIs are there?

Type A: primarily melilite, occurs in CV, CO, CR, CM. Type B: mix of melilite, fassite and some "rest", CV meteorites only. Type C: much more "rest", i e oxides and anorthite.

Which short lived radioisotopes are encountered in CAIs?

^10 Be (linked to protoSun) and ^26 Al (late injection)

What does AOA stand for and what do they share with CAIs?

Amoeboid Olivine Aggregates, they have the same ^26 Al / ^26 Mg isochron

What are the main differences between ordinary and carbonaceous chondrites?

OCs: above TFL (high ^17 O/^18 O ratio), common thermal metamorphism, depletion of refractory litophile elements (Ca, Al, Ti) to 85% of CI abundances, few CAIs and AOAs

What happens to iron depending on OC subgroup?

As we move from H through L to LL there is less metallic Fe but more Fe in olivines and pyroxenes.

How are matrix and chondrule volume fractions in ordinary chondrites?

10-15% matrix, 60-80% chondrules, in all subgroups

How are Widmanstätten patterns formed?

When a mixture of Fe & Ni with ~10-40% Ni content is cooling between ~900-600°C, a mixture of low Ni kamacite and high Ni taenite precipitates

What happens to an Fe & Ni alloy with with ~10-40% Ni content if it cools down below ~600°C?

The kamacite/taenite ratio will no longer change, but the teanite will change it's structure (e. g. to tetrahedral)

What does CCAM stand for?

Carbonaceous Chondrite Anhydrous Mineral line (the slope 1 line of CCs in the oxygen isotope plot)

How do ordinary chondrites present themselves on the 3-isotope oxygen plot?

Slightly above TFL, parallel, little variation in ^18 O (anhydrous parent body)

RREs in ordinary chondrites

Depletion of Europium in mafic minerals, instead more europium in plagioclase

What happens to calcium and iron in ordinary chondrites as the petrographic grade increases?

More FeO and Fe in olivine, less CaO

How did we learn that some meteorites are older than the oldest chondrules?​ What is a theory behind it?

Xenon dating. Within the first 1.5 Myr or so, there was enough energy in the ^26 Al decaying in the protoplanetary disk to melt any chondrules that had formed.

What are the shock classes?

From 1, which corresponds to pressures under 5 GPa, through 6, which corresponds to pressures over 45 GPa

What pressure does schock class 3 correspond to?

~15-20 GPa, at which olivine can undergo transformations, it corresponds to 70-80 km depth within Earth's mantle

Assuming similar impactor and target size, at what impactor approach velocities do shock effects become present?

Around 10 m/s. If the impactor has a diameter ~100 km also lower

What are typical shock classes among ordinary chondrites?

For H and LL subgroups most finds in S2-S3, whereas for L in S3-S4

If both shock and thermal metamorphism evidence is present within a meteorite, which one was younger?

Shock. Heat metamorphism would have erased it's evidence otherwise.

What noble gas abundances in meteorites are like?

Option 1: solar profile - similar abundances to the Sun due to solar wind corrected by atmosphere, typical of regolith. Option 2: planetary profile - differentiation favoring heavier and depleting lighter, volatile ones.

Where does ^22Ne come from?

^22 Na decay

What is the formula for diffusion coefficient? What does it depend on?

log D = log(D_0) - E_A / kT. The activation energy E_A depends on the fluid (viscosity, structure)

How is noble gas diffusion in liquid minerals measured?

A ceramic tube with the mineral is put into the furnace, with gas being inserted above. Then quenching to glass and measurement of the vertical concentration profile

How does diffusion compare between gases, liquids and solids?

O_2 in air: 10^-1 cm^2/s, He in silicate liquid: 10^-6 cm^2/s, H_2 O in silicate mineral: 10^-10 cm^2/s

How to find cooling rates of H chondrites?

Measure the nickel diffusion profile, different concentrations correspond to different temperatures (taenite-kamacite miscibility), correlate with diffusion coefficient

What is the interpretation of different petrologic grades in H chondrites?

Different levels of thermal metamorphism corresponding to different depths in a parent body. The outermost (H4) would also have fastest cooling rates

What is the correlation between cooling rates, petrologic grades and body sizes?

The petrologic grade cannot be distinguished from the cooling rate alone. The cooling rate is more sensitive to the body size, and only secondarily do both correlate to petrologic grade

What's Δ ^17 O of ordinary chondrites?

For the H group ~0.7, for L ~1.1, for LL ~1.3

How are olivine/pyroxene contents and compositions among ordinary chondrites?

Going from H through L to LL, we have increasing ol/(ol+px) ratios (from 50 to 80) and Fe/(Fe+Mg) ratios in both (from 15 to 30)

Enstatite chondrites characteristics

Very reducing conditions, Fe & Si in the metallic phase => low f O_2. Little H_2 O (0.01%). Rare CAIs. EH - 30% wt Fe, EL - 25%

Do enstatite chodrites have forsterite? Why?

Generally no (only in low petrologic grades). Low f O_2 torether with high Si: O ratio leads to Si being incorporated in pyroxenes, and remainder in metallic phase.

Which sulphur bearing minerals can be found in enstatite chondrites and why?

Oldhamite (CaS) and niningerite ((Mg, Fe, Mn)S). Normally these atoms would bind with Si, but after the solidification of enstatite they cannot find it and bond with sulphur.

Which nitrogen bearing minerals can be found in enstatite chondrites and why?

Osbornite (TiN) and sinoite (Si_2 N_2 O) - nitrogen reduced but not volatalized

What causes the luster in EH meteorites?

Metals and sulphides

Chondrule sizes and volume fractions in enstatite chondrites

EH: 0.2 mm. EL: 0.6 mm. Both 60-80 vol% (similar as OCs)

What are similarities between the Earth and enstatite chondrites? What sets them apart?

Reduced iron, Δ ^17 O (enstatite chondrites on TFL), ε ^54 Cr, ε ^50 Ti. The mismatching bit is in Si contents, there would have to be 28% in Earth's core

Element abundance similarities and differences between enstatite and other chondrites

Similar Al/Ti (refractory litophylic elements), lower Sm/Nd and Ti/Sc

Protoplanetary disk temperature profile and related crystallization

Flatter until Mars, then steeper until ~20AU. Silicates until Mars, later Fe alloys, sulphides, hydrated silicates, ices, NH_3. At ~10AU crystallized methane (30K)

Which meteorites are associated with Mercury formation

Either enstatite chondrites (similar low f O_2, but not recreated in experiments - clinopyroxenes formed) or metal rich chondrites - Bencubbin find from 1930 with 60-80% vol metal in spherules

What are signature characteristics of Mercury?

Large metallic core, low FeO (2 wt%, Earth has 8 and Mars 18), Na & K in the atmosphere

What non-meteoritic models for Mercury are there?

Metal enrichment (aerodynamic fractionation or silicate removal by impact); evaporation (50% volatalized), refractory condensation or mixture with volatiles

What is the effect of cosmic ray exposure on meteorites?

Spoliation (cosmic ray spallation) creates new isotopes at fixed rates. Some of those are stable, many radioactive.

How do stable and radioactive cosmogenic istope abundances change with time in meteorites?

In space: linear for stable, proportional to (1-e^(-λt))/λ for radioactive. After the fall: constant for stable, exponential decrease for radioactive

How to find terrestial ages of meteorites (date an unobserved fall)?

Based on fossil beds they were found in

How is galactic cosmic radiation different than solar cosmic radiation?

It has a lower flux of higher energy particles (able to penetrate 1m deep, solar only centimeters). The flux is constant in the Solar System

What can be said about neon in meteorites?

Up to a "shielding depth" we have a contribution of cosmogenic ^21 Ne, below only trapped ^22 Ne. Cosmogenic neon production rates depend more on composition and less on depth

How do Rumuruti chondrites present themselves on the oxygen 3-isotope plot?

Δ ^17 O = 2.7 (very high)

How do Rumuruti chondrites present themselves on the Urey-Craig diagram?

Quite oxidized, comparable to CV

Rumuruti chondrite chondrule size and vol%

0.4 mm; 35-50%

How do pyroxenes and olivines differ between equillibrated and unequillibrated Rumuruti chondrites?

Pyroxenes are Ca rich in equillibrated and poor in unequillibrated. Olivines are Fe rich, especially for unequillibrated (Fa_(45-60))

What other parameter is indicative of mean chondrule sizes in enstatite and ordinary chondrites?

Vol% metal, varies inversely with mean chondrule size

What is the problem with the metal enrichment through late stage accretion model of Mercury?

It fails to account for low FeO

Why are cosmic ray exposure ages lower than true ages of meteorite materials?

They do not include the time on parent body

What measurement is needed to start CRE dating?

Mass spectrometry

Where does natural remnant magnetization come from in meteorites?

Either early solar magnetic field in the nebula, or magnetic field of a differentiated planetesimal

What happens to magnetic moments within a body with increasing temperature?

They become disordered. For ferromagnetic materials (e. g. Fe) it happens at Curie temperature, for antiferromagnetic (e. g. hematite) at Neel temperature

What is magnetic susceptibility and what values does it take?

It is a ratio of magnetization M of a material under a magnetic field H, to the intensity of that field (M=χH). It is usually a small number, can be negative (we then call a material diamagnetic)

What is an anisotropy degree of a magnetic meteorite?

Ratio of maximum and minimum magnetic susceptibility measured along different directions

Kakangari chondrite characteristics

Only 1 fall and 3 finds, very reduced (only enstatite chondrites more reduced), pyroxene rich, 70 vol% matrix, oxygen isotopes different in matrix (CR like) and chondrules (OC H like)

What does Zn-Al-Mn ratio plot tell us about meteorites?

Zn is chalcophile, Al lithophile, Mn siderophile. OCs: low Zn/Mn & Al/Mn. CCs both high, except CI where Zn/Mn is low. EH same as CI, EL low Al/Mn medium Zn/Mn. R and K medium Al/Mn, but Zn/Mn inconsistent among K (low for R)

Which meteorites came from Vesta? Which are more common?

Howardites, Eucrites, Diogenites. Most are eucrites (62%), more diogenites than howardites

Vesta characteristics

Basaltic, volatile poor surface, 550 km diameter, 220 km metallic core diameter, 2 very large craters Rheasilvia & Veneneia​. Rheasilvia is larger, younger (~1 Ga), formed V-type asteroids

Eucrite subgroups

Non-cumulate (most) - quickly cooled, extrusive basalt, breccia. Cumulate (rarer) - intrusive gabbro, slowly cooled

How do different Vesta meteorites correlate with depth in Vesta and how does it affect their mineralogy?

Eucrites were close to the surface, diogenites deeper. Diogenites have more olivine and more Mg in pyroxene. Howardites are mixtures of both (metamorphized breccia of their fragments)

Why do diogenites have more olivine than eucrites but fewer than terrestial deep mantle rocks?

Higher olivine content corresponds to higher crystallization temperature. Earth is bigger and has a deeper magma ocean

How is the mafic mineral composition of diogenites?

Orthopyroxenes (Mg rich) dominate, some meteorites have larger olivine fractions, but all of them have very little clinopyroxene

What are polymict eucrites?

Meteorites with >9:1 eucrite to diogenite content (diogenite material uplifted in small amounts to shallower depths)

How do Al & Ca ratios to magnesium vary between HED meteorites?

Highest for cumulate eucrites. Slightly lower for basaltic eucrites, lowest for diogenites, howardites lie on a perfect straight line between these two groups

What was inferred about history of Vesta from argon isotope dating of eucrites?

Cooling of the magma ocean completed 4.44 Gyr ago (unbrecciated eucrite ages), then three major volcanic events around 4, 3.8, 3.5 Gyr

What models for Vesta magma ocean are there?

Simple: crystallizing solids were sinking to the core/mantle boundary, until all became solid. Complex: only sinking to some point, deeper the liquid was more dense due to pressure. Maybe several solid layers sandwiched in liquid. Fe-rich layers deep down

Why does core radius have large impact on cooling of the magma ocean for small bodies?

The (surface area)/(magma ocean volume) has a stronger dependence on core size for small bodies

Which types of elastic behavior are there?

Elastic - instantaneous reversible volume change under applied force. Inelastic - reversible but not instantaneous. Plastic - irreversible

What is stress and strain?

Stress - applied force. Strain - resulting deformation

Hooke's law simple and tensor version

ΔP=-KΔV/V. K- stiffness. β=1/K - softness. Tensor version σ=cε, where stiffness tensor c has 81 elements

What is the stress tensor for hydrostatic pressure?

Pδ_(ij) (pressure times identity matrix)

How to reduce the stiffness tensor to 21 constants?

In static equillibrium the stress tensor, as well as the stiffness tensor itself, should be symmetric

Which crystal symmetries have non-diagonal stiffness tensor elements corresponding to non-diagonal stress?

Triclinic - all 21 constants independent, monoclinic - 13 constants with one non-diagonal direction affected by the diagonal ones and the other two non-diagonal affecting each other

Which crystal symmetries do not have non-diagonal stiffness tensor elements corresponding to non-diagonal stress?

Orthorombic (9 constants, with only diagonal terms for non-diagonal stress), trigonal/tetragonal (6-7 constants), hexagonal (5 constants - matrix like in orthorombic but relations between non-zero components), cubic (3 constants)

What are the 3 constants defining the stiffness tensor in cubic symmetry?

Modulus for axial compression (c_11=c_22=c_33), shear modulus (c44=c_55=c_66), modulus for dilation (c_12=c_13=c_23)

How does olivine react to hydrostatic pressure?

Since it has orthorombic symmetry and c_11, c_22, c_33 are pairwise distinct, it would compress by a different factor along each axis

What happens to crystals under stress at very high pressures?

The linearity of Hook's law is no longer applicable, materials become stiffer, sophisticated equations of state needed to correlate volume with pressure

How to modify Hook's law to get an equation of state that's applicable at somewhat larger pressures?

Make the stiffness depend linearily on pressure (K=K_0+K'P). Integrating such differential equation leads to ρ=ρ_0(1+PK'/K_0)^(1/K')

Why is stiffness relevant to magma ocean cooling?

Dependence of density on pressure will dictate whether the liquid-solid density crossovers would occur within the ocean (such as Vesta complex model)

How does the composition of urelites look like?

Coarse colorful olivine and pyroxene (miscibility gap between pigeonites and augites) grains, forming 120° junctions indicative of slow cooling, interstitial material in between (graphite, diamond, sulfides, fine grained silicates)

What are possible sources of diamond in urelite interstitial material?

Either presolar (nebular) origin, or formed from graphite through shock metamorphism

How do urelites present themselves on the 3-oxygen isotope diagram? What is the interpretation?

They are parallel to CCAM, not to TFL. It may be due to post differentiation changes such as a late impactor bringing volatiles (this also explains the look of grain boundaries

What subgroups of urelites are there?

Monomict - coarse grained, low Ca composition, no plagioclase and in 80% cases all pyroxene is pigeonite. Polymict (rare) - more mature, clasts with contrasting chemical composition

How does pressure smelting in a magma ocean work?

Arrival of carbon in molten silicate material (slag), pulls out iron from the metal into the silicate phase. Carbon becomes oxidized (CO)

Aubrite characteristics

Similar to enstatite chondrites, but without chondrules: no iron in pyroxenes, Si in the metallic phase, possible oldhamite and heidite (Fe Ti_2 S_4) all indicative of very low f O_2. Igneous, brecciated structure implies temperatures over 1500°

How do aubrites present themselves on the 3-oxygen isotope diagram? What is the interpretation?

Right on TFL. This suggests a theory that Theia might have been the aurbite parent body, leading to incorporation of similar materials into Earth (could also explain low Fe abundance on Earth)

What is viscosity and how does it depend on temperature?

Ratio between shear stress and shear rate. It decreases with increasing temperature

What is a dilatant fluid?

A non-newtonian luid which becomes more viscuous with increasing shear stress (e g corn starch solution)

What is a pseudoplastic fluid?

A non-newtonian luid which becomes less viscuous with increasing shear stress (e. g. ketchup)

What is an Arrhenean material?

One where the viscosity dependence on temperature follows the Arrhenius equation η=η_0e^(E_A/kT)

How does Earth's mantle viscosity compare to water, ketchup and peanut butter?

23, 19 and 18 orders of magnitude bigger, respectively (log η = 20)

How to measure viscosity of a hot material?

Rotate a spindle in the liquid. The higher viscosity, the greater torque at the spindle for a given rotational velocity

How to measure viscosity of a cool material?

Press a rod with a spherical tip, applying a specific force. The lower viscosity the faster will be the descent of the rod. High viscosity needed for this, hence low T only

How to measure viscosity at intermediate values?

Difficult, but can be done with a falling sphere. Once it reaches terminal velocity, viscosity can be inferred from that velocity. Sometimes a bigger g is needed, for this we can use a centrifuge.

What does ferric and ferrous iron mean, respectively?

Fe^3+ and Fe^2+

How does viscosity depend on pressure in silicate liquids?

It increases with increasing pressure for depolymerized, but decreases for highly polymerized because Si-O bonds can break and Si coordination number change. For intermediate polymerization it has a local maximum (complex behavior)

Why does molar volume increase with polymerization?

Because of the rigid structure, in depolymerized liquids tetrahedra can fit smaller spaces between each other

How does rhyolite compare to basalt?

More polymerized, more Si O_2 (70 vs 50%), less of other oxides, particularly much less hematite (3 vs 15%)

How does glass transition depend on temperature and sampling frequency?

With higher sampling frequency everything will appear as glass. The colder the temperature, the lower the threshold timescale (frequency)

Angrite characteristics

Unbrecciated, unschocked igneous balasts. Phenocrysts. Ca-rich pyroxenes - land on a diopside-hedenbergite pseudobinary, Fe favored over Mg

What meteorites other than aubrites have been proposed to be from Theia and why?

Angrites, because they have low (0.0087) Mn/Fe ratio, so the lunar ratio of 0.014 would fall between angrites and Earth (0.025)

Branchite characteristics

(30-35% Fe) Olivine rich, no plagioclase, oxidized Fe & Ni metal, parallel and slightly below TFL

What are the arguments for and against branchite formation by igneous cummulates?

High Ca content in olivine supports cooling from high temperature. But there is a preferred orientation in the olivine lattice, which contradicts cummulate origin

What is the main theory for branchite formation?

Partial melting and recrystallization of CI material. Supported by oxidized metals, high olivine content, and depletion of incompatible elements such as titanium

What are primitive achondrites and what is their metal content?

There are three families, with broad but increasing metal content ranges: lodranites (0-25), acapulcoites, winonites (>5 - >30). The first two might have had the same parent body

Lodranite characteristics

Coarse-grained low Ca-pyroxene and olivine. Plagioclase and sulphides depleted with respect​ to chondritic mineralogy. Metamorphic temperature over FeNi-FeS eutectic​

Acapulcoite characteristics

Fine-grained olivine, orthopyroxene, minor plagioclase, FeNi, troilite​ Partial melt textures but little or no loss of plagioclase and sulphides

Winonite characteristics

Bulk chemistry between H and E chondrites. Very similar chemistry and isotope signatures as silicate fraction in IAB iron meteorites

How to measure metamorphic temperatures in the 900°-1100° range

Pyroxene partictioning between low and high Ca pyroxene

How to measure metamorphic temperatures in the 600°-700° range

Euillibrium between fayalite with 3 Mg bearing spinels (Mg Cr_2 O_4, Mg Al_2 O_4, Mg Fe_2 O_4) and forsterite with these spinels with Mg swapped for Fe (the last one becomes magnetite)

How do primitive chondrites present themselves on the 3-oxygen isotope diagram?

Parallel and below TFL. Lodranites and acapulcoites are superimposed, winonites have higher δ^18 O

Where and when were lunar meteorites found?

Some found before Apollo disproven when the samples came. After Apollo many found in Antarctica. First non-Antarctic found in Australia in 1990

Lunar meteorite subgroups

LUN A - anorthosites, LUN B - mare basalts (younger), LUN G - gabbro basalts, somewhat similar to cumulate eucrites, rare

Moon history timeline

Formation 4.6 Gyr ago. Magma ocean solidified in 200 Myr, leaving an anorthosite crust & thick mantle. Still hot, melt phases percolate through mantle => outpouring of younger volcanic rock on the surface. Start of volcanic activity 4.2, end 2.5 Gyr ago

What is KREEP, where is it found on the Moon and why?

Potassium, rare Earth elements and phosphorus. Due to gravitational interaction with Earth during the cooling of Magma ocean, the dense KREEP accumulated on the near side of the Moon.

How much of the Moon's surface is mare basalts?

17% (1% volume of the crust)

What types of basalts are there on the Moon?

Mare: Very low (<1.5%), low (1.5-6%) and high (>6%) titanium. Old (pre-mare) basalts with KREEP

What's the difference between lunar and terrestial anorthosites?

Higher Ca content

How much of the impactor is in the Moon assuming the impactor was an aubrite parent body?

25-28%

Martian meteorite subgroups

Shergottites (most common, pyroxene but no olivine, young: 200-500 Myr), nakhlites (augite rich, 1.2-1.4 Gyr), chassignites (rare, similar age to nakhlites, anomalous noble gas abundances compared to martian atmosphere)

What can be said about shergottite origin?

Unqestionably Martian because noble gas concemtrations match martian atmosphere (also N_2 and CO_2 almost on the line). Likely from Mojave crater forming event based on CRE age (2-3 Myr)

What are shergottite pyroxenes like?

Miscibility between pigeonites and augites. Both found both in rims and cores, but cores more Mg and rims more Fe rich

Where do martian meteorites land on the 3-oxygen isotope plot?

Above TFL, parallel

What were the arguments for potential biomarkers in the Allan Hills meteorite?

Some grains of this martian meteorite had organic C & N compounds

How do crystals and glasses look in Raman spectroscopy?

Glasses have smoother spectra, crystals have sharper peaks.

What do different parts of the silicate glass Raman spectra correspond to?

Smaller shift <=> bigger polymerization

Why are Martian igneous materials relatively young?

Because as a larger body Mars had a bigger magma ocean volume / surface area ratio, so volcanic activity lasted longer than on e. g. Vesta

Stony Iron meteorite definition and main categories

Roughly equal proportions of silicates and FeNi. Pallasites (129) and mesosiderites (261)

How do pallasites look like?

Olivine gems within a metallic matrix

How do mesosiderites look like?

Metallic blobs in a fine grained igneous breccia matrix

What minerals are found within pallasites besides olivine?

schreibersite (Fe Ni) _3 P, chromite (Fe Cr_2 O_4, type of spinel), troilite (FeS) pyroxenes, phosphates​

Proposed pallasite origin and reasoning behind

An impact of olivine-rich body sampling the core-mantle boundary. Arguments: big olivine grains mixed with metal despite density discrepancy of a 2.4 factor, remnant megnetism which could be preserved if the parent body was ~200 km, not much bigger

What are pallasite cooling rates?

Inconsistent. Metallographic cooling rates on the order of a few °/Myr, based on olivine it would be few °/yr

Pallasite subgroups

Main group, Eagle Station group, pyroxene grouplet (1-5% pyroxene content)

How do pallasites present themselves on the 3-oxygen isotope plot?

Main group close to TFL. Pyroxene grouplet nearby but not all. Eagle Station much below TFL and to the left (low δ ^18 O), similar to CV & CO, also different texture

What are CRE ages of pallasites and what do they compare to?

50-200 Myr, coincident with IIIAB meteorite CRE ages

Mesosiderite subgroups

Class A: basaltic, abundant plagioclase, clinopyroxene, 50% orthopyroxene. Class B: abundant orthopyroxene (70-80%), but also some plagioclase and clinopyroxene. Class C: only orthopyroxene

What is a proposed scenario for mesosiderite formation?

Impact on Vesta during crust formation (match with eucrite ages)

Based on which element abundances are iron meteorites classified?

Nickel with respect to galium and germanium (volatile siderophile) and with respect to iridium (refractory siderophile)

What is the interpretation of the vertical and horizontal scale of Ni/Ga abundance classification of iron meteorites?

Vertical: galium is volatile, so it's ppm content should vary inversely with temperature. Horizontal: from Prior's rules, the more nickel, the less overall metal content. The ones far to the right (IAB) are going to be non-magmatic

What is the link between winonites and iron meteorites?

IAB meteorites have non-equillibrium plagioclase inclusions. Winonites have plagioclase and metal composition similar to IAB

Iron meteorites structural groups

Hexahedrites - kamacite only (Ni under 6%), octahedrites - Widmanstatten pattern, different lamallae widths depend on cooling rate, fastest for plessic (SEM scale), ataxides - taenite only (Ni over 18%)

Which iron meteorite groups have most representatives?

IAB (impact related) and IIIAB (magmatism related)

What is the rationale behind the iron meteorite subgroups?

The Roman numeral corresponds, roughly, to volatile siderophile content - higher number means lower content. The higher arabic number means higher nickel content

How do angles between crystals affect segregation and core formation?

From percolation theory, angle between crystals should be under 60° for melt to be able to move along the grains. Depends also on temperature, pressure, viscosity, chemistry. The angle is decreasing with oxygen content in a Fe, O, S liquid mixture

What is the problem with a model of Earth formation by accretion and what is a proposed alternative?

Accretion and differentiation would take too long. Alternative model is formation from collisions of bigger, already differentiated bodies

What parameter is correlated with FeO wt% in the mantle of a planetary body? Which bodies do not follow that trend and why?

Core fraction varies inversely (non-linearly). The Earth has disproportionately large and the Moon low core fraction, due to the Moon formation event (Theia impact).

How does oxygen fugacity vary among solar system bodies?

Inversely with heliocentric distance