Wednesday, January 19, 2011

5ias deadblogging - day 1

This is a brief description of the session speakers at the 5th international Archean Symposium, held in Perth, Western Australia in September 2010. I attended the meeting as an exhibitor- but because it was a single session conference, there was not much point in sitting in an empty booth during the talks, so I managed to catch a number of them. The following are basically my notes as taken, with just enough cleanup to make them comprehensible to me.

If they are not comprehensible to y’all, then I’ll think about expanding them. But as y’all may have noticed, I’ve been flat chat with other things the past few months, so don’t hold your breaths for any blogging that requires extensive thought or writing.

5th International Archean Symposium speakers:

Alec Trendall, who graduated in 1949 and has been mapping around the world ever since. 1970 inaugral Perth international Archean symposium proposed in the late 60's as a backup plan when a carbonate platform conference proposal fell through.
Second meeting in 1980, once a decade ever since.

Review on geoconference websites

Advances from 1970 to 2001- geochron; Archean life.

No centre for archan studies, in Australia or anywhere else in the world.
No university has an Archean research centre, despite the innumerable climate research centres that spring up like mushrooms.

John Valley:
What can we agree on before 4 Ga?

Only rock record- detrital zircons
As old as ~4.4 Ga
What have we done to them?
O isotopes
Li isotopes
REE, Ti concentrations
Mineral inc.

Zircons come from chemically mature (95% SiO2+) conglomerates
Zircon + chromite
Chemically mature sediment with 5-10% hadean zircons

D18O in Hadean zircons 5-6 range “mantle” 6-7.5 surface processes, typical for archean. Some sort of surface processes (weathering). No very high s-type clay-rich oxygen isotopic signatures until Proterozoic.

Planetary differentiation
Moon formation essentially 182Hf dead (at least 62 Ma after CAI (4567 Ma).

Earth different to various chondrites, which differ from each other.

Estimate from 30-125 Ma after tZero.

142Nd early silicate mantle differentiation (30 Ma) smaller based on carbonaceous chondrite instead of ordinary chondrite

Confusingly early.

Not a clear talk, I need to go look all this stuff up. (speaker said it would be rushed and confusing- he’s right!)

Magma ocean- ample evidence on moon. Assumed for core formation early from 26Al.

Magma ocean assumed for Earth as is hard to explain how moon can have one without earth having one. A matter of convenience.

Speaker: Tarduno:

Magnetism in the archean
Metamorphism is an issue (lots of equations)
Single mineral analyses (god knows how they preserved orientation during rock disaggregation).
(They didn’t; looking for field intensity, not direction.)
Quartz good for this (micro magnetite inclusions)

(Tarduno et al. 2007)

Reheat in field to see what was removed. More crazy math; This is way outta my knowledge space.

When did solid inner core grow (3.5-1 Ga are estimates)

So mag field seems to be ~modern as old as 3.2 Ga.

Hypothesis that no mag field to explain atmospheric N isotopes (want solar wind stripping).

Theory that lunar magma ocean thermally inverted mantle- no core cooling- no geodynamo.

3.45 sediment- low temperature metamorphic field, high temperature random, consistent with expected for sediment.

Squid magnetometers, whatever they are.

Magnetopause standoff distance- solar wind vs. magnetic field.
Shedding angular momentum during solar spin-down (sun losing angular momentum).
More energetic solar wind- smaller standoff distance for same field strength.

Magnetic poles interact with atmosphere, water loss.

Atomic magnetometers for single zircon grains under development.

Vickie Bennett:

Age, volume, importance of early cont. crust

142Nd memory in archean rocks widespread.
Interpreted as continental growth.

176Hf vs 143Nd decorrelation in early archean- why?
Ask that question 10 years ago.
142Nd developed for terrestrial use since then.
Ppm level variations
2.3 ppm external reproducibility

Modern Earth 20ppm offset from chondrites

142Nd achean offsets inherited from hadean into archean- widespread.
176Hf- Hf from zircons, dated using shrimp

Contintntal crust is not the compliment for early depleted mantle- compliment is something where Lu is not compatible.

Early positive Hf data in Harrison 2005 not reproduced by anyone (including Harrison 2008).

Greenland rocks require early differentiation.

hadean crust
Same as Goldschmidt talk
Garnet cummingtoniteites (faux-amphibolites)
142Nd depletion in lower ‘tholiite’ more consistent with old age than inheritance.

Cooling of Earth
Emergent land
Continents weather, dump elements into ocean
All marine carbonates- 87Sr excess seawater evolution suggest 2.8 Ga start of long-lived seawater 87Sr excess.

Suggest Taylor & McClennan curve, assumes constant freeboard (continents emergent)
Suggest that continents are mostly submerged in archean.- pillow basalts ratio for continental basalt.

Hotter mantle-thinner lithosphere- weaker and can’t support topography.
Unaddressed points:
** Emerged land, or weathering flux? **

** continental profile governed by erosion? **

-depleted mantle reservoir
-magma oceans
-sub-continental lithospheric mantle

Diamond incl. with d33S- correlated to d14N depletion- is the best record for the archean atmosphere the deep diamond-bearing lithosphere?
Crazy stuff

Clark Friend:
Mantle slivers in Isua
Greenland mapping & age constraint review- we haven’t looked at actual rocks for a while now.
First geologic map since O’Neil.

Narryer monazite
Lots of metamorphic, nothing older than 3.6

Overview of Archean in Northern Territory
Implications for economic U mineralization
Summary of what NTGS was doing when I was at GA

S isotopes through time

Low early in earth history, larger later.

Sulphate concentration (1950’s) grown under different sulphate conc.
Larger fractionations at larger sulphate concs

Below 100uM low fractionation. (less than 5 permil)
Sediment-water systems the same as experiments
Ancient ocean- less than 1% of current marine sulphate

Predicted ratio based on extrapolation from 34.

D33S small for younger than 2.4
D33S -2 to +11 for archean rocks.

Actually discovered 20 year ago by a guy who couldn’t explain why it happened, and not followed up.

Mass-independent fractionation- gas phase reactions related to long lifetimes of excited states.

Sometimes also made by amino acid reactions.
Not necessarily relevant to Archean- very specific lab setup.

D33S in Archean rocks most likely atmospheric.

32S reaction wavelengths self-shielded, but not for lower concentration isotopes.

D33S fractionations large in archean, low afterwards- late archean- biggest effect, but also largest sample, esp. 2-8-2.4

Connection to atmosphere and oxygen.

Available UV depends on ozone, which depends on O2 concentration. Full screening at around 10^-2 PAL (present atmospheric levels).

With O around, S in atmosphere becomes H2SO4, dwarfed by hydrosphere sulphate.

Variations during the Archean

D33S 7 permil until ~3300, 1 permil 3300-2850, huge (13) until 2400.

Could be changes in :
–amount of S entering atmosphere
-methane and oxidation state of atmosphere
-organic haze
CO2-CH4 greenhouse with organohaze feedback
Haze blocks UV
More CH4 in mesoarchean due to methanogens. (iceage?)

Shielding by CO2
MIF photolosis vs non-MIF (oxidation and H2S)
Geologic preservation

S. Foley:
What do we know about the Archean atmosphere?
We know it was there.
Volcanic degassing of the mantle- ignoring the crust for this talk.

Greenhouse gasses needed for faint young sun problem.
Nitrogen in atmosphere.
Modern N is subducted
Goldblatt et al. put 1.4 PAN C. crust + lithosphere 1.25 PAN.
Archean could have 2x present N
Modern volcanics:
H2O 60%
CO2 10-40%

Archean crust- how much?
Composition of oceanic crust?
Oxidation state of the mantle
Many papers state unchanged.
Lots of plots presumably from volumetrically minor sources.
Hartzburgite reduced, arc oxidized- duh.
Doesn’t look at archean volumetrically significant magmas.
Very reduced magmas have methane, not CO2.
Ignores all transition metal valence arguments.
Not as strongly argued as some other talks


Sulfate in archean mantle
No oxidative weathering- volcanic degassing
Eaten by sulfate reducing critters
Stored in barite (which can exist at lower fO2 than other sulphates)
Source of sulfate?
Paleoarchena barites South Africa, India, Pilbara
Esp Barberton GB.
Small d34S
Small d33S
No mantle S
Older than 3.4- larger D33S
Metamorphic reset?
Temporal trend
D33S and D36S decrease together.
Barites different than pyrites.
Barite controlled by local process
D33S/D36S slope is -1; photolysis
Barite S from atmosphere- same ages different continents similar.
Atmosphere change at 3.4 GA
Dilution of old more fractionated with biogenic non-atmospheric S
What is geologic context of Barite?

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