![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kaberettConference conference conference MUSHROOM MUSHROOM.
M NICOLAS DAUPHAS -- ARE THE EARTH AND MOON ISOTOPIC TWINS? (first brown speaker!)
(Dude I am sorry you are too French.)
(I am so entertained by the number of people who are using images I used in my SURF talk. His intro picture - Earth from the Moon - is one I used.)
Quantities of Earth contributing to Moon from various forms of collision:
F_earth < 30$ canonical
F_Earth < 60% head-on, icy, large core
Mass fractionation and isotopic anomalies:
Most natural processes can impart mass-dependent fractionation to isotopic ratios. Exceptions:
1. nuclear transmutations (radioactivity, cosmogenic effects)
2. nucleosynthetic anomalies
3. unusual chemistry (e.g. ozone, Hg)
--> both mass-dependent fractionation and isotopic anomalies can provide important clues on the formation of the Moon
- some space for Hf/W initial ratios to be different
Nucleosynthetic anomalies:
a. Mapping of ISM heterogeneity onto the solar disk
b. Vapourisation and gas/dust coupling
c. Grain-size sorting. Size sepration by aerodynamic drag can create bulk isotopic anomalies. Dauphas et al 2010
Earth, Moon and chondrites
- Earth and Moon map exactly on top of each other
- no chondrites match
Titanium isotope anomalies
- Moon doesn't QUITE match Earth but it's... fairly close. (Aubrits, Enstatite chondrites the only two that look anything like it - but they're not brilliant - but might explain lunar variations)
- cosmogenic effects on Ti isotopes: ??
- titanium isotope homogeneity - Zhang et al 2012
Plausibility of isotopic equilibration:
- what is the timescale for magma disc-vapour atmosphere isotope exchange? Common vapour atmosphere etween Moon, Earth.
-- Hertz-Knudsen equation - at equilibrium, the flux of atoms impinging the surface is given by the kinetic theory of gases and it must be equal to the flux of atoms leaving the surface --> exchange timescale? Variables are T and equilibration pressure... for Mg, Si, Cr ~a few weeks. For W ~-.2 years. Ti ~1 year (~1000 at 2000 K rather than 3000 K). Ca 25 year (250000 at 2000 K instead of 3000 K)
Evaporation coefficient for Ti?
What's next?
- equilibration timescale for Ca longer than for Ti
- newly-discovered planetary-scale 48Ca isotope anomalies in chondrites correlate with 50Ti
-> the ultimate test for Moon-Earth equilibration...
Reasons for having isotopic twins...
- nature of earth's accretion: nature of accreted material did not change at core formation (nor should it)
-- look at Ru - it's highly siderophile, so stuff at surface arrived as late veneer after core formation - vs Mo, which arose at main stage of accretion. Ratio... [Questions say that Mo *also* goes into the core.]
Stable isotopic fractionation:
- no measureably K isotopic fractionation
- measurable Zn isotopic fractionation
- "The average of 14 lunar basalts and highland plutonic rocks (...) is heavier by ~0.1ppm in del57Fe relative to Earth's mantle"
--> kinetic isotope fractionation during evaporation of metallic iron? Poitrasson et al
Silicate Earth:
- abyssal peridotites have d56Fe identical to chondrites
Moon:
- lunar mare basalts have variable d56Fe values most likely reflecting isotopic fractionation during lunar magma ocean differentiation or partial melting
--> no clear evidence for isotopic fractionation by volatilisation
- there can be a mineralogical control to iron isotopic fractionation during lunar magma ocean cyrstallisation (Craddock et al, 2010 & in prep)
Stable isotope fractionation: a complex problem to tackle
- isotopes can be mass-fractionated by many processes, making it difficult to use them as tracers of evaporation during the giant impact
-- nebular/disk processes, mantle-core partitioning, partial melting/fractional crystallisation, evaporation, LMO crystallisation, partial melting
Conclusions:
Mass-dependent fractionation and isotopic anomalies trace different aspects of the formation of the Moon
- refractory element Ti has the same isotopic composition in the Earth and Moon to within 1/150th of the total range in meteorites
- isotopic anomalies have been found for 48Ca, which will provide even more stringent constraints on Earth-Moon equilibration scenarios
- the Moon-forming impactor may have had Earth-like isotopic composition
- the Mg and Fe isotopic compositions of the Moon are still uncertain
- Mg and Fe isotopic variations among lunar rocks may reflect fractionation during LMO crystallisation
Discussion:
- is impactor having same composition as Earth a condition, and if so how did that happen? -- actually he claims to only be saying that material being accreted before & after giant impact was same. But uh. Uh?
- do we have high-precision Ti measurements for Mars? - there is some, it just isn't displayed here.
- this questions session is... actually kind of painful. There is a LOT that was in this that is being... challenged; I am not convinced it was well-received (... oh dear someone has just said "since this has become a bit of a feeding frenzy") -- is this due to the stable isotopes qua stable isotopes, or just the way it's being presented? I... think just the way it's being presented/conclusions drawn. But we are thank goodness probably to One Last Question. And I hope my career never involves being shredded at the RS...
======================================
M HUGH O'NEILL -- A COMPARISON OF THE CHEMICAL COMPOSITIONS OF THE BSE AND THE MOON [title not as booklet]
(speaker w/ disabilities! single stutter huge part of the way through the talk)
"Perhaps I'm one of the last surviving people who understands the petrological convolutions Ted went through to show..." (was a postdoc for Ted Ringwood at the time of Kona)
What are the similarities and differences between the chemical compositions of the BSE (Primitive Mantle) and the Moon?
What can these similarities and differences tell us about how the Moon formed?
1. How might the ompositions of the rockey planets relate to the solar composition?
2. How do we estimate the BSE composition?
3. What kinds of things should we look for in the Moon's composition for signatures of the Earth?
Chemical fractionations during accretion may b studied by examining as many types of solar-system objects as possible.
- achondrites harder, so start w/ the chondrites
- getting lots more types of chondrite! work started in Antarctica, but is moving into hot desert regions
- exampleof something with terrestrial oxygen, chromium - olivine w/ Mg#69.7 - but other aspects of chemistry VERY unEarthlike, v old
Chondrites all:
- derived from solar composition
- constant isotopic ratios of eheavy elements (up to a point)
- refractory lithophile elements occur in constant ratios relative to each other
- depletion in volatile elements (except CI chondrites)
- pattern of depletion of moderately volatile elements follows roughly the sequence of calculated condensation temperatures from the solar nebula
but there are differences in REEs if you look carefully, and some other places
Cosmochemical fractnation process from chondrites
1. RLEs not fractionated w/ respect to each other
2. Mg & Si NOT RLEs, Mg/Rle can be more or less than solar
3. volatiles???
4. ???
Chonrite assumption: "building blocks of the planets" -- actually, the rubble left over on the building site - assume composition of terrestrial planets matches chondrites is to ignore the last two stages of planet creation (olighargic growth, high-energy... stuff)
During growth from collisions: NON-CHONDRITIC FRACTIONATION PROCESSES
Mn/Na proportional to fourth root of fO2 - fO2 set in solar nebula by H-C-O - and ratio is the same in ALL chondrites
- REGARDLESS of how much they have lost volatile component
- ... achondrite parent bodies are EVERYWHERE ELSE on the plot. Lost Na relative to Mn. Moon is about as depleted in Mn as Earth, but has... lost Na.
- lose Na by volatilisation at higher fO2 after H2 of solar nebula has dispersed
BSE
-average composition of whole Earth minus core
- 68% of Earth
- sum of all "reservoirs" divided by relative mass - cc, atm, mantle - last makes up ~99.5% of mass!
How do we know the chemical composition of the mantle?
- cosmochemical constraints
- indirectly, through mantle melts & geophysics
- directly, through studies of mantle samples
BSE - cosmochemical constraints
To avoid circular arguments, minimise assumptions
1. composition of Earth broadly reflects that of the solar system, with a few obvious fractionations
2. RLEs have their solar (CI) ratios, as in ALL chondrites (but metachondritic fractionations such as collisional ones upset this)
Default chonritic model of BSE:
- more olivine, less px in Earth
- BSE has enrichment of FLEs
- depleted in SiO2
- BSE has ~15% of Earth's total Fe. Why particular oxygen? Why do we have a certain size of core and certain size of mantle?
- non-solar Fe/Mg ratio, so might core be ~10% too large for rest of Earth?
We have high-silicon steels made at atm p. So why do we assume ~5% Si in core?
S + Se half as much as HSE - which makes it much harder to argue for a volatile-enriched late veneer...
- but if you get rid of late veneer, no sulphur at ALL as we see...
- so... add it make in from the "Hadean matte" - last bit of sulfide liquid separating to the core
- less 16% of CI-like component - "oxidised impactor" - might explain why FeO so much higher on Moon...
- proto-Earth contains ~2wt% FeO in mantle - highly reducing - small amounts of least volatile "moderately volatile" elements plus something else...
Moderately volatile element depletion in Earth. "Peculiar pattern".
- even most depleted chondrite doesn't look anything like BSE!
- heavy halides etc v heavily depleted in BSE
- in other small telluric bodies we have non-chondritic Mn/Na ratios
-- don't have that in the Earth, but do have other badgers
How can we apply this methodolgy to the Moon?
1. we have no lunar mantle samples
2. even if we did, is the Moon's mantle mixed well enough to provide anything that is representative?
3. what proportion of the lunar mantle is complimenatry to the lunar crust?
4. do we include the lunar core in making comparisons with the BSE?
Moon is 5% crust, 95% mantle, 1% core vs 0.3%, 68%, badger for Earth
Geophysical estimates:
- Moon is 30%-oxidised but volatile-free impactor + 70% proto-Earth?
Fe in the Moon: higher iron content, possibly MUCH higher iron content.
- cannot simply make it out of Earth-like material
- have to add iron to it
- if adding oxidised iron to Earth, can we add Even More to make the Moon?
basalt-to-basalt type comparisons (cannot look at proper mantle samples)
- different petrological conditions on moon mean Cr and Va behave very differently on Moon vs Earth
- Ni is lower on the Moon. Small secondary metallic core?
- Co - works well
- W "everyone is interested in W" - one of the favourite elements of the 1970s
-- AMOUNT of W in Moon (compared with RLE) appears to be the same as in Earth's mantle
- Mo appears not to work at all (not sure how good data are) -- oxidation-state change?
- is there a depletion in Nb inherited from the proto-Earth? (Nb must be inherited from massive core-forming body, ditto Si)
-- can potentially get from basaltic rocks. Is Ta reduced as per in Earth? "Has been argued to be the case"
- Li depletion does not follow Na, K etc - about the same as in the Earth - so we assume the Moon should lose its volatile elements, but it hasn't lost Li and Mn, least volatile of the moderately-volatile elements
- lunar depletion in non-siderophile volatiles - but Mn and Li are much less depleted
Conclusions:
- volatile elements (Na, K but NOT Li, Mn) are lost during the later stages of planetary accretion
- taking this into account and postulating the presence of a small metallic core, there are many geochemical features of the Moon that bear an uncanny resemblance to those of the Earth's mantle
- similarities and differences can be accounted for if BOTH the BSE and the Moon are a mix of protoEarth and Impactor (BSE ~15% impactor, Moon ~30% impactor)
- the impactor ("Theia" [erm, might have this wrong, it might be Vesta]) was a highly oxidised body (Very high FeO); this implies that it was volatile-rich
-- from the outer part of the solar system?
Discussion:
- how while maintaining other isotope ratios do we get SO much more oxidised iron in the Moon, if we don't add an iron-rich impactor? (Says lecturer, in response to q about what one can say about impactor) CI carbonaceous chondrites - Orgueil - v sulphur-rich (smells like it), analyses going down over time
M NICOLAS DAUPHAS -- ARE THE EARTH AND MOON ISOTOPIC TWINS? (first brown speaker!)
(Dude I am sorry you are too French.)
(I am so entertained by the number of people who are using images I used in my SURF talk. His intro picture - Earth from the Moon - is one I used.)
Quantities of Earth contributing to Moon from various forms of collision:
F_earth < 30$ canonical
F_Earth < 60% head-on, icy, large core
Mass fractionation and isotopic anomalies:
Most natural processes can impart mass-dependent fractionation to isotopic ratios. Exceptions:
1. nuclear transmutations (radioactivity, cosmogenic effects)
2. nucleosynthetic anomalies
3. unusual chemistry (e.g. ozone, Hg)
--> both mass-dependent fractionation and isotopic anomalies can provide important clues on the formation of the Moon
- some space for Hf/W initial ratios to be different
Nucleosynthetic anomalies:
a. Mapping of ISM heterogeneity onto the solar disk
b. Vapourisation and gas/dust coupling
c. Grain-size sorting. Size sepration by aerodynamic drag can create bulk isotopic anomalies. Dauphas et al 2010
Earth, Moon and chondrites
- Earth and Moon map exactly on top of each other
- no chondrites match
Titanium isotope anomalies
- Moon doesn't QUITE match Earth but it's... fairly close. (Aubrits, Enstatite chondrites the only two that look anything like it - but they're not brilliant - but might explain lunar variations)
- cosmogenic effects on Ti isotopes: ??
- titanium isotope homogeneity - Zhang et al 2012
Plausibility of isotopic equilibration:
- what is the timescale for magma disc-vapour atmosphere isotope exchange? Common vapour atmosphere etween Moon, Earth.
-- Hertz-Knudsen equation - at equilibrium, the flux of atoms impinging the surface is given by the kinetic theory of gases and it must be equal to the flux of atoms leaving the surface --> exchange timescale? Variables are T and equilibration pressure... for Mg, Si, Cr ~a few weeks. For W ~-.2 years. Ti ~1 year (~1000 at 2000 K rather than 3000 K). Ca 25 year (250000 at 2000 K instead of 3000 K)
Evaporation coefficient for Ti?
What's next?
- equilibration timescale for Ca longer than for Ti
- newly-discovered planetary-scale 48Ca isotope anomalies in chondrites correlate with 50Ti
-> the ultimate test for Moon-Earth equilibration...
Reasons for having isotopic twins...
- nature of earth's accretion: nature of accreted material did not change at core formation (nor should it)
-- look at Ru - it's highly siderophile, so stuff at surface arrived as late veneer after core formation - vs Mo, which arose at main stage of accretion. Ratio... [Questions say that Mo *also* goes into the core.]
Stable isotopic fractionation:
- no measureably K isotopic fractionation
- measurable Zn isotopic fractionation
- "The average of 14 lunar basalts and highland plutonic rocks (...) is heavier by ~0.1ppm in del57Fe relative to Earth's mantle"
--> kinetic isotope fractionation during evaporation of metallic iron? Poitrasson et al
Silicate Earth:
- abyssal peridotites have d56Fe identical to chondrites
Moon:
- lunar mare basalts have variable d56Fe values most likely reflecting isotopic fractionation during lunar magma ocean differentiation or partial melting
--> no clear evidence for isotopic fractionation by volatilisation
- there can be a mineralogical control to iron isotopic fractionation during lunar magma ocean cyrstallisation (Craddock et al, 2010 & in prep)
Stable isotope fractionation: a complex problem to tackle
- isotopes can be mass-fractionated by many processes, making it difficult to use them as tracers of evaporation during the giant impact
-- nebular/disk processes, mantle-core partitioning, partial melting/fractional crystallisation, evaporation, LMO crystallisation, partial melting
Conclusions:
Mass-dependent fractionation and isotopic anomalies trace different aspects of the formation of the Moon
- refractory element Ti has the same isotopic composition in the Earth and Moon to within 1/150th of the total range in meteorites
- isotopic anomalies have been found for 48Ca, which will provide even more stringent constraints on Earth-Moon equilibration scenarios
- the Moon-forming impactor may have had Earth-like isotopic composition
- the Mg and Fe isotopic compositions of the Moon are still uncertain
- Mg and Fe isotopic variations among lunar rocks may reflect fractionation during LMO crystallisation
Discussion:
- is impactor having same composition as Earth a condition, and if so how did that happen? -- actually he claims to only be saying that material being accreted before & after giant impact was same. But uh. Uh?
- do we have high-precision Ti measurements for Mars? - there is some, it just isn't displayed here.
- this questions session is... actually kind of painful. There is a LOT that was in this that is being... challenged; I am not convinced it was well-received (... oh dear someone has just said "since this has become a bit of a feeding frenzy") -- is this due to the stable isotopes qua stable isotopes, or just the way it's being presented? I... think just the way it's being presented/conclusions drawn. But we are thank goodness probably to One Last Question. And I hope my career never involves being shredded at the RS...
======================================
M HUGH O'NEILL -- A COMPARISON OF THE CHEMICAL COMPOSITIONS OF THE BSE AND THE MOON [title not as booklet]
(speaker w/ disabilities! single stutter huge part of the way through the talk)
"Perhaps I'm one of the last surviving people who understands the petrological convolutions Ted went through to show..." (was a postdoc for Ted Ringwood at the time of Kona)
What are the similarities and differences between the chemical compositions of the BSE (Primitive Mantle) and the Moon?
What can these similarities and differences tell us about how the Moon formed?
1. How might the ompositions of the rockey planets relate to the solar composition?
2. How do we estimate the BSE composition?
3. What kinds of things should we look for in the Moon's composition for signatures of the Earth?
Chemical fractionations during accretion may b studied by examining as many types of solar-system objects as possible.
- achondrites harder, so start w/ the chondrites
- getting lots more types of chondrite! work started in Antarctica, but is moving into hot desert regions
- exampleof something with terrestrial oxygen, chromium - olivine w/ Mg#69.7 - but other aspects of chemistry VERY unEarthlike, v old
Chondrites all:
- derived from solar composition
- constant isotopic ratios of eheavy elements (up to a point)
- refractory lithophile elements occur in constant ratios relative to each other
- depletion in volatile elements (except CI chondrites)
- pattern of depletion of moderately volatile elements follows roughly the sequence of calculated condensation temperatures from the solar nebula
but there are differences in REEs if you look carefully, and some other places
Cosmochemical fractnation process from chondrites
1. RLEs not fractionated w/ respect to each other
2. Mg & Si NOT RLEs, Mg/Rle can be more or less than solar
3. volatiles???
4. ???
Chonrite assumption: "building blocks of the planets" -- actually, the rubble left over on the building site - assume composition of terrestrial planets matches chondrites is to ignore the last two stages of planet creation (olighargic growth, high-energy... stuff)
During growth from collisions: NON-CHONDRITIC FRACTIONATION PROCESSES
Mn/Na proportional to fourth root of fO2 - fO2 set in solar nebula by H-C-O - and ratio is the same in ALL chondrites
- REGARDLESS of how much they have lost volatile component
- ... achondrite parent bodies are EVERYWHERE ELSE on the plot. Lost Na relative to Mn. Moon is about as depleted in Mn as Earth, but has... lost Na.
- lose Na by volatilisation at higher fO2 after H2 of solar nebula has dispersed
BSE
-average composition of whole Earth minus core
- 68% of Earth
- sum of all "reservoirs" divided by relative mass - cc, atm, mantle - last makes up ~99.5% of mass!
How do we know the chemical composition of the mantle?
- cosmochemical constraints
- indirectly, through mantle melts & geophysics
- directly, through studies of mantle samples
BSE - cosmochemical constraints
To avoid circular arguments, minimise assumptions
1. composition of Earth broadly reflects that of the solar system, with a few obvious fractionations
2. RLEs have their solar (CI) ratios, as in ALL chondrites (but metachondritic fractionations such as collisional ones upset this)
Default chonritic model of BSE:
- more olivine, less px in Earth
- BSE has enrichment of FLEs
- depleted in SiO2
- BSE has ~15% of Earth's total Fe. Why particular oxygen? Why do we have a certain size of core and certain size of mantle?
- non-solar Fe/Mg ratio, so might core be ~10% too large for rest of Earth?
We have high-silicon steels made at atm p. So why do we assume ~5% Si in core?
S + Se half as much as HSE - which makes it much harder to argue for a volatile-enriched late veneer...
- but if you get rid of late veneer, no sulphur at ALL as we see...
- so... add it make in from the "Hadean matte" - last bit of sulfide liquid separating to the core
- less 16% of CI-like component - "oxidised impactor" - might explain why FeO so much higher on Moon...
- proto-Earth contains ~2wt% FeO in mantle - highly reducing - small amounts of least volatile "moderately volatile" elements plus something else...
Moderately volatile element depletion in Earth. "Peculiar pattern".
- even most depleted chondrite doesn't look anything like BSE!
- heavy halides etc v heavily depleted in BSE
- in other small telluric bodies we have non-chondritic Mn/Na ratios
-- don't have that in the Earth, but do have other badgers
How can we apply this methodolgy to the Moon?
1. we have no lunar mantle samples
2. even if we did, is the Moon's mantle mixed well enough to provide anything that is representative?
3. what proportion of the lunar mantle is complimenatry to the lunar crust?
4. do we include the lunar core in making comparisons with the BSE?
Moon is 5% crust, 95% mantle, 1% core vs 0.3%, 68%, badger for Earth
Geophysical estimates:
- Moon is 30%-oxidised but volatile-free impactor + 70% proto-Earth?
Fe in the Moon: higher iron content, possibly MUCH higher iron content.
- cannot simply make it out of Earth-like material
- have to add iron to it
- if adding oxidised iron to Earth, can we add Even More to make the Moon?
basalt-to-basalt type comparisons (cannot look at proper mantle samples)
- different petrological conditions on moon mean Cr and Va behave very differently on Moon vs Earth
- Ni is lower on the Moon. Small secondary metallic core?
- Co - works well
- W "everyone is interested in W" - one of the favourite elements of the 1970s
-- AMOUNT of W in Moon (compared with RLE) appears to be the same as in Earth's mantle
- Mo appears not to work at all (not sure how good data are) -- oxidation-state change?
- is there a depletion in Nb inherited from the proto-Earth? (Nb must be inherited from massive core-forming body, ditto Si)
-- can potentially get from basaltic rocks. Is Ta reduced as per in Earth? "Has been argued to be the case"
- Li depletion does not follow Na, K etc - about the same as in the Earth - so we assume the Moon should lose its volatile elements, but it hasn't lost Li and Mn, least volatile of the moderately-volatile elements
- lunar depletion in non-siderophile volatiles - but Mn and Li are much less depleted
Conclusions:
- volatile elements (Na, K but NOT Li, Mn) are lost during the later stages of planetary accretion
- taking this into account and postulating the presence of a small metallic core, there are many geochemical features of the Moon that bear an uncanny resemblance to those of the Earth's mantle
- similarities and differences can be accounted for if BOTH the BSE and the Moon are a mix of protoEarth and Impactor (BSE ~15% impactor, Moon ~30% impactor)
- the impactor ("Theia" [erm, might have this wrong, it might be Vesta]) was a highly oxidised body (Very high FeO); this implies that it was volatile-rich
-- from the outer part of the solar system?
Discussion:
- how while maintaining other isotope ratios do we get SO much more oxidised iron in the Moon, if we don't add an iron-rich impactor? (Says lecturer, in response to q about what one can say about impactor) CI carbonaceous chondrites - Orgueil - v sulphur-rich (smells like it), analyses going down over time
(no subject)
Date: 2013-10-02 11:41 am (UTC)Got any geochemistry-and-mercury 101 links to get me started?
(no subject)
Date: 2013-10-02 07:19 pm (UTC)RLEs
REEs [EDIT: Oh, are these Rare Earth Elements?]
FLEs
(no subject)
Date: 2013-10-02 10:28 pm (UTC)I suspect RLE is "refractory lithophile elements"; LMK if you want that unpacked more.
... I genuinely can't work out what FLEs is supposed to have been, though I suspect it is [something] Lithophile Elements...