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kaberettSorry for dropping the ball on these - first-week-itis etc. As ever, if you have questions, I will do my best to answer them.
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") 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 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") 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