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kaberettM RICK CARLSON -- AGE OF THE LUNAR CRUST: IMPLICATIONS FOR THE TIME OF MOON FORMATION
LMO = lunar magmatic ocean
Use the LMO to define the time of Morn formation
predicted LMO stratigraphy:
1 mafic cmulate sources of the mare basalts
2 formation of the ferroan anorthosite crust (talks about it being to do with anorthosite saturation???)
3 formation of KREEP
4 instrusion of Mg-suite magmas (orthositic), contaminated with KREEP, into the FAN crust
1. When did the Moon form? (FAN formation < 1 Ma?) - crystallisation occurs v quickly in magma ocean without crust/atmosphere lid - so this should give us the age of the Moon (ish)
2. when was LMO crystallisation complete? (KREEP ~150Ma?)
3. What led to renewed magmatism after LMO crystallisation was complete? (Overturn? Melting of primitive interior?)
4. What petrologic processes created the Moon (LMO necessary?)
Both FANs and Mg-Suite crustal rocks show a considerable range in ages
- prolonged crust formation or inaccurate ages? doesn't fit the idea that these were first-crystallised!
Lunar crustal mineralogy: not easy!
- FANs are nearly monomineralic plag (and separated EARLY, so very little incompatible inclusion)
- there is not much Rb or K on the volatile-depleted Moon, nor any other incompatible element in FANS
- most highlands rocks have experienced enough shock to disturb radiometric systems
- no zircons in FANS
- serious consequences of neutron irradiation for Sm-Nd and Lu-Hf systems
... shows us quite a beautiful diagram of a huge range of ages acquired... from JUST TWO ROCKS. but ~4.3 Ga, uncertainty 3--11Ma. (Axis the wrong way around TIME DOESN'T GO THAT WAY)
... alas it turns out that the lunar crust just... isn't very old. Rocks might have stayed deep in lunar crust i.e. remained "hot"? Yet Rb/Sm giving older age than the other one, but it's usually taken to have a lower closure temperature...
So initial Nd isotopic ratios: implications for source formation ages?
"The problem with having a single data point is you can't draw any useful lines through it"
Formation of FANS and KREEP must have been pretty much simultaneous -- according to Rb-Sr systematics
Other potentially relevant ages:
- lunar zicons - one at 4.2Ga (not at 4.56Ga...) (recent ones reset by impact)
- time of mare basalt source formation - Sm-Nd ratios. 4.323Ga...
When did the Moon form?
- 4.36Ga oldest concordant age for a FAN
- 4.32Ga peak in lunar zircon age distribution (4.417 oldest zircon)
- 4.38 - 4.48 for Zircon Hf model ages
- 4.323-4.417Ga for mare basalt source
- 4.42Ga Pb model age lunar highlands
4.36-4.4Ga KREEP Sm-Nd and Lu-Hf mnodel ages
- <4.5Ma - no evidence for live Hf
Possibly relevant ages from Earth:
- 4.35Ga peak in oldest Hadean zircons (W Aus)
- 4.450Ga U-Pb age of the Earth
- 4.47Ga I-Xe age of atmosphere
- 4.47Ga Sm-Nd model age for Isua
- 4.33704.406Ga Sm-Nd crystallisation age for Nuvvagittuq
... we're not seeing a cluster of 4.5Ga, we're looking at 4.4-4.3Ga
Giant impact model couples these two
argument for v late lunar formation
Discussion:
- Previous aussie bloke wants a story to reset ferroanorthosite dates (because KREEP doesn't form until the crust does because thermal diffusion) -- speaker doesn't agree. Top number (FAN) dubious. KREEP dates aren't dating when it crystallised, it's dating when it achieved its chemical signature (last 1% crystallisation)? oh goodness they are tau-squared kappa re the thickness of the crust they are YELLING THERMAL DIFFUSITIVTY AT EACH OTHER
- range in ages for the FAN -- but are they accurate, or just due to analytical difficulties?
- highly asymmetric/asynchronous crystallisation of the Moon -- Apollo samples all from the near side, far side might have cooled first??? - so we need go back to the Moon somewhere that ISN'T an impact crater and get more samples...
- would quite like a sample from somewhere that isn't the near side of the Moon, "but Venus is nice" ;) - but would also like an ocean-based geoneutrino detector...
LMO = lunar magmatic ocean
Use the LMO to define the time of Morn formation
predicted LMO stratigraphy:
1 mafic cmulate sources of the mare basalts
2 formation of the ferroan anorthosite crust (talks about it being to do with anorthosite saturation???)
3 formation of KREEP
4 instrusion of Mg-suite magmas (orthositic), contaminated with KREEP, into the FAN crust
1. When did the Moon form? (FAN formation < 1 Ma?) - crystallisation occurs v quickly in magma ocean without crust/atmosphere lid - so this should give us the age of the Moon (ish)
2. when was LMO crystallisation complete? (KREEP ~150Ma?)
3. What led to renewed magmatism after LMO crystallisation was complete? (Overturn? Melting of primitive interior?)
4. What petrologic processes created the Moon (LMO necessary?)
Both FANs and Mg-Suite crustal rocks show a considerable range in ages
- prolonged crust formation or inaccurate ages? doesn't fit the idea that these were first-crystallised!
Lunar crustal mineralogy: not easy!
- FANs are nearly monomineralic plag (and separated EARLY, so very little incompatible inclusion)
- there is not much Rb or K on the volatile-depleted Moon, nor any other incompatible element in FANS
- most highlands rocks have experienced enough shock to disturb radiometric systems
- no zircons in FANS
- serious consequences of neutron irradiation for Sm-Nd and Lu-Hf systems
... shows us quite a beautiful diagram of a huge range of ages acquired... from JUST TWO ROCKS. but ~4.3 Ga, uncertainty 3--11Ma. (Axis the wrong way around TIME DOESN'T GO THAT WAY)
... alas it turns out that the lunar crust just... isn't very old. Rocks might have stayed deep in lunar crust i.e. remained "hot"? Yet Rb/Sm giving older age than the other one, but it's usually taken to have a lower closure temperature...
So initial Nd isotopic ratios: implications for source formation ages?
"The problem with having a single data point is you can't draw any useful lines through it"
Formation of FANS and KREEP must have been pretty much simultaneous -- according to Rb-Sr systematics
Other potentially relevant ages:
- lunar zicons - one at 4.2Ga (not at 4.56Ga...) (recent ones reset by impact)
- time of mare basalt source formation - Sm-Nd ratios. 4.323Ga...
When did the Moon form?
- 4.36Ga oldest concordant age for a FAN
- 4.32Ga peak in lunar zircon age distribution (4.417 oldest zircon)
- 4.38 - 4.48 for Zircon Hf model ages
- 4.323-4.417Ga for mare basalt source
- 4.42Ga Pb model age lunar highlands
4.36-4.4Ga KREEP Sm-Nd and Lu-Hf mnodel ages
- <4.5Ma - no evidence for live Hf
Possibly relevant ages from Earth:
- 4.35Ga peak in oldest Hadean zircons (W Aus)
- 4.450Ga U-Pb age of the Earth
- 4.47Ga I-Xe age of atmosphere
- 4.47Ga Sm-Nd model age for Isua
- 4.33704.406Ga Sm-Nd crystallisation age for Nuvvagittuq
... we're not seeing a cluster of 4.5Ga, we're looking at 4.4-4.3Ga
Giant impact model couples these two
argument for v late lunar formation
Discussion:
- Previous aussie bloke wants a story to reset ferroanorthosite dates (because KREEP doesn't form until the crust does because thermal diffusion) -- speaker doesn't agree. Top number (FAN) dubious. KREEP dates aren't dating when it crystallised, it's dating when it achieved its chemical signature (last 1% crystallisation)? oh goodness they are tau-squared kappa re the thickness of the crust they are YELLING THERMAL DIFFUSITIVTY AT EACH OTHER
- range in ages for the FAN -- but are they accurate, or just due to analytical difficulties?
- highly asymmetric/asynchronous crystallisation of the Moon -- Apollo samples all from the near side, far side might have cooled first??? - so we need go back to the Moon somewhere that ISN'T an impact crater and get more samples...
- would quite like a sample from somewhere that isn't the near side of the Moon, "but Venus is nice" ;) - but would also like an ocean-based geoneutrino detector...