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kaberettThis is the talk in which the research I carried out in summer 2011 is referenced so incredibly obliquely that there are probably only about 5 people on the planet, myself included, who'd know I featured at all. :D
M ALBERTO SAAL -- HYDROGEN ISOTOPES IN LUNAR VOLCANIC GLASSES IMPLICATIONS OF THEIR TERRESTRIAL HERITAGE
Why is it important to determine the volatile contents of planets?
1. Provides information on the conditions of planet formation.
2. Affects dynamics of planetary interiors (modifying mantle, melts viscositiies, core segregation and longevity)
3. decreases the melting point of solids
4. lava reuption
5. ?
Why study the Moon?
- frozen record of first 500Ma of Solar System formation
- uses lunar volcanic glasses - fire fountaining, erupted REALLY fast, minimum contamination, so have some sense of how long exposed to solar rays (orange glasses == 30Ma, green glasses 300Ma)
- also they are the most primitive composition we get
In 2008 presented first report of lunar water in single lunar glass beads and concentration provfiles across beads of very-low-Ti glasses (yes thank you I recognise that diagram :-)
- volatile profiles indicated degassing processes... yep...
- degassing models with cooling rates etc until bead is quenched, giving an initial water concentration of ~750ppm, with minimum 260ppm at 95% confidence level
After 2008 concentrated on:
1. primitive volatile contents of lunar magmas
- use melt inclusions in olivine - H will diffuse but provides minimum amount
- find lunar volcanic glasses equivalent to those found in MORB...
- volatiles depleted by factor 5-6 compared to refractory elements
2. distribution of volatiles within the Moon - how representative are samples?
- new volatile analyses for more than 360 individual glasses
- two distinctive trends defined by the glasses and the melt inclusions for all lunar glasses that suffered degassing
-- represents initial concentrations esp for elements that did not degas much
Doesn't think we can go beyond 800ppm for water content of bulk Moon.
LMO evolution might provide starting point
- as crystallise LMO increase concentration of volatiles etc
Other evidence:
- McCubbin et al, GReenwood et al, Boyce et al estimated that 100s to 1000 of ppm water was present in the lunar basaltic melts from which the apatite crystallised. These results suggest that the volatile inventory for at least some lunar rocks is similar to comparable terrestrial rocks
- Hui et al detected up to 2.7ppm water in plag
Since 2011, focus on:
1. where the water came from
- D/H ratios as fingerprint for origin of lunar water (!!)
- Greenwood had already done that for apatite; found significant variation even in the same sample
- high delD indicate a cometary origin, from Oort cloud?
-- difficult to determine history of magmas previous to crystallisation of apatites - late in thingy. No correction applied for secondary processes.
- hence continuation with lunar glasses.
-- actually rather nicer! Negative correlation between delD and Ln H2O (ppm) - degassing control?
--- corrected for spallation based on exposure times
- equilibrium or Rayleigh fractionation cannot be applied to the lunar volcanic glasses - beads themselves have diffusion profile
-- so use diffusive degassing as our badger [observe the extent to which I am knackered by this point]
- but H speciation depends on amount of water as well as T and uh the other thing. What controls diffusion depends on H species concentrations. (MY WORK YESSSS)
- also used nitrogen isotopes of chondrites and coments -- Moon looks like chondrites and NOTHING LIKE comet.
- so source is carbonaceous chondrite or something very similar ("like Earth, for example")
- the Earth formed with water and the Giant Impact event did not evaporate all the H away
2. when the water came to the Moon
- the H was added ~50-100Ma after the formation of the Moon during the final stages of aggretion: "late veneer of carbonaceous chondrites" OR
-- PGE contents in the Moon suggest that E/M mass flux ratio of ~2700, which is considerably greater than the ratio of 30 estimated from the gravitational-attraction potential of the two bodies (Day et al 2007)
- brings up Jupiter's migration - the Grand Tack - Jupiter migrates inwards to 1.5AU, Saturn migrates inward, gets captured in 2:3 resonance w/ Jupiter, migration back out
- or how about a less processed lunar mantle reservoir? Ancient igneous intrusions and early expansion of the Moon revealed by GRAIL gravity gradiometry (Andrews-Hanna et al 2013, on which we will hopefully see at least a little bit tomorrow)
Conclusions:
1. all the evidence we have today about water in the Moon interior suggest an amount equivalent to that of the Earth's upper mantle ~100 ppm
2. lunar water as Earth's water originated from carbonaceous chondrites rather han comets. Simplest explanation is that water was in the Earth from its birth (consistent with the Grand Tack model) and it was not compltely lost during the giant impact event that formed the Moon
3. Our new observations imply that the Earth's water budget and isotopic composition did not change much since the formation of the Moon
-- wow, that's a BIG deal, huh - this is the case IF we believe the Earth is around 500ppm
Discussion:
- glasses volatile-rich in terms of other parameters. To what extent are moderately volatile elements depleted? Other volatile enrichments come in from the edges, i.e. infusing rather that diffusing out. Also he cares about Europium. Whatever you see as an enrichment of incompatible trace elements and volatiles, is mainly because when you melt the interior, you end up passing through the KREEP component & getting contamination. SPECULATIVE is that last sentence, says Alberto.
- where does it come from? Impact velocities of comets ~40km/second, so Earth's gravity not relevant, says questioner. However if the source is closer in and the impact velocity less, THEN Earth's gravity + focussing effect becomes important - so it matters where the Moon is at time of impact. Can use that information to work out where things come from? -- no, Alberto Saal thinks it's chondrites, NOT comets. (Except this is important if you get material from Earth...?)
- questioner says: no unifying theory associating source of volcanic glasses to mare basalts. So what do either of them say about the interior? Well, they're not from the same source nor related by a single process. So can we definitely use this to estimate bulk water content of Moon? Well, no, not necessarily.
- Alberto is p sure water arrived before or at time of giant impact, NOT with the late veneer
M ALBERTO SAAL -- HYDROGEN ISOTOPES IN LUNAR VOLCANIC GLASSES IMPLICATIONS OF THEIR TERRESTRIAL HERITAGE
Why is it important to determine the volatile contents of planets?
1. Provides information on the conditions of planet formation.
2. Affects dynamics of planetary interiors (modifying mantle, melts viscositiies, core segregation and longevity)
3. decreases the melting point of solids
4. lava reuption
5. ?
Why study the Moon?
- frozen record of first 500Ma of Solar System formation
- uses lunar volcanic glasses - fire fountaining, erupted REALLY fast, minimum contamination, so have some sense of how long exposed to solar rays (orange glasses == 30Ma, green glasses 300Ma)
- also they are the most primitive composition we get
In 2008 presented first report of lunar water in single lunar glass beads and concentration provfiles across beads of very-low-Ti glasses (yes thank you I recognise that diagram :-)
- volatile profiles indicated degassing processes... yep...
- degassing models with cooling rates etc until bead is quenched, giving an initial water concentration of ~750ppm, with minimum 260ppm at 95% confidence level
After 2008 concentrated on:
1. primitive volatile contents of lunar magmas
- use melt inclusions in olivine - H will diffuse but provides minimum amount
- find lunar volcanic glasses equivalent to those found in MORB...
- volatiles depleted by factor 5-6 compared to refractory elements
2. distribution of volatiles within the Moon - how representative are samples?
- new volatile analyses for more than 360 individual glasses
- two distinctive trends defined by the glasses and the melt inclusions for all lunar glasses that suffered degassing
-- represents initial concentrations esp for elements that did not degas much
Doesn't think we can go beyond 800ppm for water content of bulk Moon.
LMO evolution might provide starting point
- as crystallise LMO increase concentration of volatiles etc
Other evidence:
- McCubbin et al, GReenwood et al, Boyce et al estimated that 100s to 1000 of ppm water was present in the lunar basaltic melts from which the apatite crystallised. These results suggest that the volatile inventory for at least some lunar rocks is similar to comparable terrestrial rocks
- Hui et al detected up to 2.7ppm water in plag
Since 2011, focus on:
1. where the water came from
- D/H ratios as fingerprint for origin of lunar water (!!)
- Greenwood had already done that for apatite; found significant variation even in the same sample
- high delD indicate a cometary origin, from Oort cloud?
-- difficult to determine history of magmas previous to crystallisation of apatites - late in thingy. No correction applied for secondary processes.
- hence continuation with lunar glasses.
-- actually rather nicer! Negative correlation between delD and Ln H2O (ppm) - degassing control?
--- corrected for spallation based on exposure times
- equilibrium or Rayleigh fractionation cannot be applied to the lunar volcanic glasses - beads themselves have diffusion profile
-- so use diffusive degassing as our badger [observe the extent to which I am knackered by this point]
- but H speciation depends on amount of water as well as T and uh the other thing. What controls diffusion depends on H species concentrations. (MY WORK YESSSS)
- also used nitrogen isotopes of chondrites and coments -- Moon looks like chondrites and NOTHING LIKE comet.
- so source is carbonaceous chondrite or something very similar ("like Earth, for example")
- the Earth formed with water and the Giant Impact event did not evaporate all the H away
2. when the water came to the Moon
- the H was added ~50-100Ma after the formation of the Moon during the final stages of aggretion: "late veneer of carbonaceous chondrites" OR
-- PGE contents in the Moon suggest that E/M mass flux ratio of ~2700, which is considerably greater than the ratio of 30 estimated from the gravitational-attraction potential of the two bodies (Day et al 2007)
- brings up Jupiter's migration - the Grand Tack - Jupiter migrates inwards to 1.5AU, Saturn migrates inward, gets captured in 2:3 resonance w/ Jupiter, migration back out
- or how about a less processed lunar mantle reservoir? Ancient igneous intrusions and early expansion of the Moon revealed by GRAIL gravity gradiometry (Andrews-Hanna et al 2013, on which we will hopefully see at least a little bit tomorrow)
Conclusions:
1. all the evidence we have today about water in the Moon interior suggest an amount equivalent to that of the Earth's upper mantle ~100 ppm
2. lunar water as Earth's water originated from carbonaceous chondrites rather han comets. Simplest explanation is that water was in the Earth from its birth (consistent with the Grand Tack model) and it was not compltely lost during the giant impact event that formed the Moon
3. Our new observations imply that the Earth's water budget and isotopic composition did not change much since the formation of the Moon
-- wow, that's a BIG deal, huh - this is the case IF we believe the Earth is around 500ppm
Discussion:
- glasses volatile-rich in terms of other parameters. To what extent are moderately volatile elements depleted? Other volatile enrichments come in from the edges, i.e. infusing rather that diffusing out. Also he cares about Europium. Whatever you see as an enrichment of incompatible trace elements and volatiles, is mainly because when you melt the interior, you end up passing through the KREEP component & getting contamination. SPECULATIVE is that last sentence, says Alberto.
- where does it come from? Impact velocities of comets ~40km/second, so Earth's gravity not relevant, says questioner. However if the source is closer in and the impact velocity less, THEN Earth's gravity + focussing effect becomes important - so it matters where the Moon is at time of impact. Can use that information to work out where things come from? -- no, Alberto Saal thinks it's chondrites, NOT comets. (Except this is important if you get material from Earth...?)
- questioner says: no unifying theory associating source of volcanic glasses to mare basalts. So what do either of them say about the interior? Well, they're not from the same source nor related by a single process. So can we definitely use this to estimate bulk water content of Moon? Well, no, not necessarily.
- Alberto is p sure water arrived before or at time of giant impact, NOT with the late veneer