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kaberettM NORM SLEEP -- TERRESTRIAL AFTERMATH OF MOON-FORMING IMPACT
Starting out with a joke about "calcite sequestration" & the Tea Party...
(Hah, heard people talking afterwards - I wasn't the only one to find it hard to follow)
Basic principles:
- atmosphere was opaque (i.e. had to convect): cooling was slow, ended with mostly solid mantle, transition likely to plates
- needed to subduct CO2 once crust present
- fast subduction of old plates works best ("looks like an oxymoron"): requires local episodes, life at ~3.9Ga is only evidence of Earth being habitable; subduction did the job
Atmosphere opaque:
- silicates are not highly volatile
- NaCl (KCl) and S species important
- atm opaque even ignoring clouds
- radiated at runaway greenhouse threshold
- atm was blanket except at start (i.e. immediately after impact): Earth radiated available heat production at very high surface temperatures, like star -- cooling protracted
Thermal history:
- initial heat and tides comparable (? tides cause things to get heaed again? evection?? drawing energy from system rotational energy???)
- this heat was gone in ~10Ma
- water & CO2 escaped to surface in final freezing
- geochem heterogeneities and solid crust were very early
Tidal dissipation:
- liquid magma was ineffective at dissipation
- cold rock was also ineffective
- needed mush: similar between high tides ~3 hours, viscosity is 10^13 Pa.s
Earth melting:
- pressure mattered
- Earth froze in lower mantle
- on adiabat: first freezing was at bottom of mantle
- may freeze from middle: consequently lower mantle isolated, tides heat first layer to freeze (Aubault et al, but several of yesterday's talks were... unconvinced)
Tidal dissipation, again:
- mush was buffer: excessive tidal heating remelts, kills dissipation; more mush forms with low dissipation, increasing dissipation
- depth range of dissipation zone increased to maintain heat flow as Moon moved away
- lunar orbit was controlled by climate (??!!!!) - this is shocking, but on the other hand weather has a strong control on plate tectonics, so...
- NB reckons whole mantle passed through boundary layer (multiple times) to cool -- chemical equilibrium w/ air
End of rapid convection:
- slurry with 10^9 Pa.s viscosity existed at surface
- dissipative mush at deeper upper mantle depth
- deep mantle mostly froze
- tendency for flows to erupt and lid to founer: overcooled mantle, Mantle heated up in Hadean and early Archaean (Herzberg et al), radioactivity
Tail of Moon-forming impact:
- continues with warm greenhouse; subduction of CO2
- how to get us to the point where the Earth is habitable? That's the central question of this talk, I _think_, how do we get the Earth livable by the time life shows up
Hadean and Archaean plates:
- thick oceanic crust hard to subduct
- once started, subducts fast
- slow overall rate, but rapid subduction and seafloor spreading when local activity occurred
- Hadean mantle heats up...
Slab temperature:
- CO2 stays in old, cold fast-downgoing slabs
- modern slabs; Hadean slabs were hotter - so could manage it with rapid plate tectonics all the time, otherwise can't subduct CO2
see Dasgupta 2013
Mush layer under crust:
- convenient way to start conventional subduction
- makes low-Ni granite (Archaean to Cainozoic, Jamaica)
Demise of warm greenhouse:
- maximum amount of carbonate in ocean crust
- still at 30 bars CO2, need to subduct ocean 4 times...
- argues for very early subduction
- thermophile environment at 25 bars CO2
- few bars CO2 means pH ~6 ocean, clement
Earth at 3.8 Ga:
- photosynthesis needed for black shale; land weathering & marine deposition
- banded iron formations
- mild greenhouse
- claims geochem evidence exists of especially deep mantle processes, but nobody has dupliated this work of his
Discussion:
- water-saturated mantle could give mush without needing plumes?
- "nice case to subduct carbon in the mantle - but what about degassing in a hot, young mantle? Arc volcanism?" - old, cold slabs, as above (but hey, noone much seems to want to ask questions on this, I'll forgive it)
- is... the argument that plate tectonics occurs because of our Moon? I think it might be? I found this quite hard to follow. Possibly because I am exhausted.
Starting out with a joke about "calcite sequestration" & the Tea Party...
(Hah, heard people talking afterwards - I wasn't the only one to find it hard to follow)
Basic principles:
- atmosphere was opaque (i.e. had to convect): cooling was slow, ended with mostly solid mantle, transition likely to plates
- needed to subduct CO2 once crust present
- fast subduction of old plates works best ("looks like an oxymoron"): requires local episodes, life at ~3.9Ga is only evidence of Earth being habitable; subduction did the job
Atmosphere opaque:
- silicates are not highly volatile
- NaCl (KCl) and S species important
- atm opaque even ignoring clouds
- radiated at runaway greenhouse threshold
- atm was blanket except at start (i.e. immediately after impact): Earth radiated available heat production at very high surface temperatures, like star -- cooling protracted
Thermal history:
- initial heat and tides comparable (? tides cause things to get heaed again? evection?? drawing energy from system rotational energy???)
- this heat was gone in ~10Ma
- water & CO2 escaped to surface in final freezing
- geochem heterogeneities and solid crust were very early
Tidal dissipation:
- liquid magma was ineffective at dissipation
- cold rock was also ineffective
- needed mush: similar between high tides ~3 hours, viscosity is 10^13 Pa.s
Earth melting:
- pressure mattered
- Earth froze in lower mantle
- on adiabat: first freezing was at bottom of mantle
- may freeze from middle: consequently lower mantle isolated, tides heat first layer to freeze (Aubault et al, but several of yesterday's talks were... unconvinced)
Tidal dissipation, again:
- mush was buffer: excessive tidal heating remelts, kills dissipation; more mush forms with low dissipation, increasing dissipation
- depth range of dissipation zone increased to maintain heat flow as Moon moved away
- lunar orbit was controlled by climate (??!!!!) - this is shocking, but on the other hand weather has a strong control on plate tectonics, so...
- NB reckons whole mantle passed through boundary layer (multiple times) to cool -- chemical equilibrium w/ air
End of rapid convection:
- slurry with 10^9 Pa.s viscosity existed at surface
- dissipative mush at deeper upper mantle depth
- deep mantle mostly froze
- tendency for flows to erupt and lid to founer: overcooled mantle, Mantle heated up in Hadean and early Archaean (Herzberg et al), radioactivity
Tail of Moon-forming impact:
- continues with warm greenhouse; subduction of CO2
- how to get us to the point where the Earth is habitable? That's the central question of this talk, I _think_, how do we get the Earth livable by the time life shows up
Hadean and Archaean plates:
- thick oceanic crust hard to subduct
- once started, subducts fast
- slow overall rate, but rapid subduction and seafloor spreading when local activity occurred
- Hadean mantle heats up...
Slab temperature:
- CO2 stays in old, cold fast-downgoing slabs
- modern slabs; Hadean slabs were hotter - so could manage it with rapid plate tectonics all the time, otherwise can't subduct CO2
see Dasgupta 2013
Mush layer under crust:
- convenient way to start conventional subduction
- makes low-Ni granite (Archaean to Cainozoic, Jamaica)
Demise of warm greenhouse:
- maximum amount of carbonate in ocean crust
- still at 30 bars CO2, need to subduct ocean 4 times...
- argues for very early subduction
- thermophile environment at 25 bars CO2
- few bars CO2 means pH ~6 ocean, clement
Earth at 3.8 Ga:
- photosynthesis needed for black shale; land weathering & marine deposition
- banded iron formations
- mild greenhouse
- claims geochem evidence exists of especially deep mantle processes, but nobody has dupliated this work of his
Discussion:
- water-saturated mantle could give mush without needing plumes?
- "nice case to subduct carbon in the mantle - but what about degassing in a hot, young mantle? Arc volcanism?" - old, cold slabs, as above (but hey, noone much seems to want to ask questions on this, I'll forgive it)
- is... the argument that plate tectonics occurs because of our Moon? I think it might be? I found this quite hard to follow. Possibly because I am exhausted.