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speaking of silicon iron cook passivity

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today I'm going to show you some

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experiment results I'm silikal I still

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fractionation between simulated

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precambrian sea water and iron silica

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gel so we know the Precambrian was

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dramatic different from the model ocean

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in particular there was no silica

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secreting organism by the time so so the

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sim cellular concentration the

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Precambrian seawater was over an order

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of magnitude higher than the modern

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seawater so that actually leads to a

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silica flux from the seawater into

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segments that's why we see abounded

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unusually abounded deposition of silica

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rich rock student Cambrian like Church

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and bending our information so as we

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know the life actually started in this

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silicon rate environment so it's quite

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important to understand the spherical

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psycho during that time so silicon

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isotope is a potential proxy for the

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silica sonico so silicon has three

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stabilized health 28 29 and 30 a silicon

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star is expressed by the traditional

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doubt a notation in per mil so the high

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temperature process do not significantly

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fractionated silicon stealth as you can

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see is a point 5 ml changing the igneous

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rocks but the its variation can be

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pretty large at low temperature

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environment and over 7 premieres bread

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was reported for the Precambrian beefs

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and the church so the question is how to

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interpret this large spread in the Delta

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silica 30 venues so a previous study

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proposed the silicon still in the pre

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Cambridge church can reflect seawater

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temperature at the time based on the

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observation that silicon I still follow

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a similar train there's oxygen my cell

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if you still believe oxygen isotope came

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with flexi water after here in jax talk

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but later a latest study kind of Charlie

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this proposal was challenged by some

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later studies so this group of guys

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basically look at the silicon stove in

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the church against their chemical

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composition alumina oxide in this case

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and as you can see from this figure and

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those data set seems can be explained by

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three different end members so so the

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disco

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just proposed silicon stuff in the

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church can can can be used to trace

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celica from different sources but

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obviously there are other process may

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also affect silicon still preserved in

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this geological records for example this

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to a figure shows the completion of

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silicon is the convolution in the

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proterozoic bend your information church

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and the the black sail the open circle

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and the grip are indicated data from

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church and the black circle and the

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black bars indicate data from the

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depending on information so common

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observation is that the delta silicates

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30 venue in the independent our

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formation seems lower than the data

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recorded by the church even at the same

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time so it's still unclear why this is

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the case and maybe it's due to for a

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different fractionation factor

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associated with formation of these

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different type of rocks or due to

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different diagenesis so all this

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interpretation actually fundamentally

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depends on how well we know the

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fractionation in different eye on

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different systems so this study will

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just try to determine the fractionation

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factor between the acreage silica and

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our silica gel so the the method we use

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is called 3i stone method so basically

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this method involves ice tub exchange

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between two component one are in silica

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gel and one a quiz silica so for the

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folder are silica gel which has normal

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are silicon solution which falls on this

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mess dependent fractionation line so all

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natural sample should plot on this line

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but for another aqueous component we

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intentionally add some artificially

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enraged silica 29 spike so its

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completion just plot of this

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fractionation light up here so during

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the ice tub exchange the two component

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will gradually evolve together and

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finally reach the secondary

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fractionation line so a good cool thing

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about this technique it's one we sample

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at different time point we can actually

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extrapolate this partially exchange data

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point to a hundred percent exchange so

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this method actually guarantees do you

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get the the fractionation factor

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at a complete I silicone ice tub

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exchange between the two components so a

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little bit details about the experiment

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setup so I prepare the the silica 29

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spiked a quiz silica in a seawater like

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matrix which and try to mimic some some

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key feature of the ark in seawater

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including high silica concentration and

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low sulfate and no auction and the

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terminal species silica species in the

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aqueous as a monastery stochastic

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tetrahedra so silica concentration bow

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is about 1 million mo silica 29,000

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silica 29 because we spiked it with

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silica 29 and artificially in Rachel

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silica 29 so it has a venule close to

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100 per mil but Delta silica 30 was not

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affected by adding this 2029 slick spike

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so it has a kind of normal venues cloud

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20 per mil so I set up to comparative

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group of experiment and one group I just

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used the artificial seawater without any

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ferris oriental in the sea in the

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aqueous phase but for another group I

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just adding 1 million mole of a very

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fair of first iron in the in the aqueous

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phase in the aqueous phase because we

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know that our King seawater is kind of

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enraged in the in the in the fe 2 so

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each group of experiment contain

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actually contain to experiment so the

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solid phase is the amorphous Anna

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Ferrick silica gel as you can see from

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the figure here just a kind of amorphous

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cool stuff so the silicon atoms in this

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in this Joe is actually presenting

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different bounding environment silicon

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hydroxyl group silicon auction are

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inbound in silicon oxygen silicon not a

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network so the the are silica gel has

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normal siliconized to completion so both

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the Delta 29 silica powder silica 30 has

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0 per mil so the experiment was

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conducted at room temperature and pH new

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circum- mph and all the experiment was

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done in anaerobic chambers there is a

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free of oxygen which to prevent from

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prevent the oxidation

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fe to ya so here show some results so we

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can actually look at the the kinetics of

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the siliconix I study change between the

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two components by looking at Delta

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silica 29 so 442 group of extreme

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experiment you can see the the one color

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indicate data from the solid from the

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aqueous phase and the coat color

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indicator data from the from the solid

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phase so for both experiment you can see

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the the to compile data from two

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component just converge which indicate

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the ice tub exchange occurred between

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these two two components so so we can

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actually quantify sorry so we can

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actually quantify the extent of this I

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stop exchange of this I stock exchange

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relative to the equilibrium state so

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that's the earth manual as you can sure

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as you see here so we can see for the

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for the group without any first ironing

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in the art if you see water the the

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experiment reach about seventy percent I

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step exchange and after about 50 to 70

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days but for the for the group were

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using containing 1 million my affairs

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are in the artifice seawater in reach a

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higher percentage of I stock exchange

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reaching about eighty to ninety percent

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I stock exchange within the same amount

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of time so that suggests that the

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presence of fe to actually make the iso

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silicone ice tub exchange between two

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components faster so we can expand this

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by the fe 2 Fe 3 electron transfer Adam

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exchange interaction so imagine so

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imagine that's the our silica gel in the

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in the eggs to face and that stuff fe 2

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in the in the aqueous so when the f you

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to absorb on the solid so the electron

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from the one election from the f you too

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can actually moving into the solid and

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moving around so one fe 3 in the solid

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can eventually get this lecture and

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being reduced and

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then released back to the solution so

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this process actually leads to the

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breaking of silica are impounding so

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that's a potential way to accelerate the

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silica is the exchange between the

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aqueous and solid so we can also look at

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the natural fractionation between the

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solid and the aqueous by luke and outer

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silica 30 venues the same the warm color

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indicate data from the from the aqueous

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and the code color indicate data from

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the solid so we can from both experiment

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we can see the the heavy silicon stir

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preferentially goes to the a quest and

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the device silica preferentially goes to

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solid and we can see the two data point

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the two curve just diverge so that's

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indicated fractionation cup between the

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two component and we can extrapolate

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this partially exchange data to a

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hundred percent exchange so that will

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give us the fractionation factor so for

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the for the experiment without any in

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first iron in in the April's we've got a

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fractionation factors about a minus 2.3

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pml but for the experiment containing Fe

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2 in the APIs we've got slightly larger

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fractionation factor at minus 3.2 per

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mil so we can compare our results with

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some previous studies so this great

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that's just a fractionation factor

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determined and during the precipitation

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of pure silica silica gel from

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relatively pure oversaturated silica

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solution and as you can see our error

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fractionation factor is larger than

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those previously determined

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fractionation factors so we can also

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expand that by electron donation theory

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so some theoretical calculations

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indicate the fraction that the magnitude

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of silica isotope fractionation actually

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depends on the overall the average

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silicon auction bond strength and so the

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weights a weaker the silicon oxygen the

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average silicon oxygen bound the larger

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the fractionation factor will be so

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different atom can be characterized by

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these electronegativity values which is

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a measure for the tendency that

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Adam attract the electron towards itself

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so if there's a cat I'm presenting the

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the in the system if the qatar has an

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election activity value lower than the

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silica that means that there will share

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more election to this oxygen so that

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will shorten this silicon action bound

259
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by the same time that will lengthening

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the other three silica action bank so

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the overall the average silicon ocean

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bound energy will decrease so so so for

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our experiment we have learned here so

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so that's the that's the reason why the

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presence of aren't actually lower the

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average silicon oxygen bounding that

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leads to larger fractionation factors so

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the fe 2 has an activity

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electronegativity value even lower than

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the fe 3 so the effect of fe 2 in

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weakening this RN silica in the silica

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action bounding is the stronger than

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they feed three so that's why we see

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even larger fractionation in the fu two

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in fe 2 system so if you still remember

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those two figure just show you earlier

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so our experiment seems suggests that

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there are actually different

279
00:12:21,890 --> 00:12:18,080
fractionation factor associated with

280
00:12:24,200 --> 00:12:21,900
precipitation of pure silica as you can

281
00:12:26,300 --> 00:12:24,210
see from this church sample represented

282
00:12:29,120 --> 00:12:26,310
by this church samples and larger

283
00:12:31,490 --> 00:12:29,130
fractionation associated with this bend

284
00:12:34,160 --> 00:12:31,500
your information samples so maybe that's

285
00:12:37,640 --> 00:12:34,170
why we see lower venues in this in this

286
00:12:40,190 --> 00:12:37,650
spending our information so just to

287
00:12:42,830 --> 00:12:40,200
conclude so our experiments suggest the

288
00:12:45,080 --> 00:12:42,840
first provide first evidence for the fe

289
00:12:47,540 --> 00:12:45,090
to catalyze the siliconized of exchange

290
00:12:49,280 --> 00:12:47,550
and we determine the clipper and

291
00:12:52,070 --> 00:12:49,290
fractionation factor between the iron

292
00:12:55,220 --> 00:12:52,080
silica gel and the quests silica is

293
00:12:57,590 --> 00:12:55,230
about the at two or three per ml and the

294
00:12:59,720 --> 00:12:57,600
presence of fe to actually leads to two

295
00:13:01,970 --> 00:12:59,730
larger fractionation factor so our

296
00:13:04,010 --> 00:13:01,980
experiment kind of suggests the early

297
00:13:06,860 --> 00:13:04,020
diagenetic process and the formation of

298
00:13:10,520 --> 00:13:06,870
these different rock types actually play

299
00:13:13,070 --> 00:13:10,530
a major role in in in the delta celica

300
00:13:19,400 --> 00:13:13,080
30 venules actually proceed preserved in