Bonjour,

pourrait-il s'agir d'un élément organique qui aurait disparu, mais dans les cavités duquel un minéral diagénétique aurait pu se former ? Cela me fait penser à une coupe dans des faisceaux conducteurs de sève (trachéides) et la matière organique aurait été reminéralisée. Toutefois, la MO continentale est en général résistante, pourquoi aurait-elle ici disparu ?

A voir !

Cordialement,

NT

Salut Nicolas

MErci pour ta reponse!  et je dois dire qu'elle est assez compatible avec le contexte. En realite, ces roches sont du Toarcien inferieur, donc dans l évennement anoxique... Il y a peu de matiere organique dans le sediments (TOC assez faible il me semble) mais par contre nous avons de nombreux vestiges de vegetaux venu du continent...et les sigantures isotopique du carbon tres tres negative... donc ca colle assez bien avec tes "faisceaux conducteurs de seve" et MO remineralisée.

le mineral qui brille en haut a gauche est en fait un oxide de fer riche en Ti, V, et Ba.

JE vais chercher des cliches de "tracheides" pour voir a quoi ca ressemble.

Merci pour le tuyau!!

amicalement

eric

Hello Eric

To me it appears to be trachied of plant may be burnt.

Regards

Bandana

HI Bandana

nice to hear from you!

thank you!! It seems that you and Nicholas are pointing to the same conclusion: trachied... and it make sense since these sediments contain high amount of continental plants

thank you! I hope you are doing well

cheers

eric

Hi Eric, it  can be allophane, typical in  volcanic soils (short-range order mineral); generally protected by increased organic matter. What is the size (diameter)? It should be about 4nm and Al/Si ratio may vary between 1.6-2.6 ....Good luck

Hana Gison

Coal fly ash has such spherical particles which are formed during the burning of coal at high temperature from the mineral residue of the coal. These spheres are rich in Si and Al. Can there be a connection? was your sample exposed to high temperature?

I have also seen something like this in carbonates which were hydrocarbon reservoirs. The spheres I have seen have metals (oxides or sulfides) inside them. The spheres in your image also look like they may have other minerals inside them. Bitumen or organics which have been preserved in fractures may host fe and sulfur reducing bacteria and these may also represent one of those feeding on the organics of the sediments. If these are in volcanic derived soils could also be a clay mineral coating or replacing the  outer sphere surface. 

Hello everybody , and thank you for your numerous suggestions.. I will try to respond step by step:

@ Kenneth: It is very difficult to calculate the ratio of Si and Al under the SEM and using EDS, because samples are rock fragments (not polished section) and has not been coated by carbon. This means that the EDS compositional analyses may be not precise enought. and unfortunately, at the time I looked at this, I did not acquired EDS spectra...What I did is a mapping (I can send the photo later) and it is quite difficult to see the exact composition of the sphere)

and yes pyrite is an iron sulphur. I have a lot of them, mostly in the form of framboids. And this is quite normal since the age of the rock is Lower Toarcian, i.e. in the OAE (oceanic anoxic event). D13C are very negative. TOC is low but we have a lot of plants preserved.

@Fialova: I would have been very happy to link this features to allophane, but unfortunately the rock are marine sediment (proximal platform) with turbidites... so there are no soils here... Most intriguig is the cavity ("tunel") where these spheres are located... this is why I think that the idea of nicholas and bandana, which is that these are trachied, sounds more compatible with the sedimentary environment

@ Mingelgrin: idem as Fialova, no volcanic ash in the sediments nor in the area. No exposure to high temperature as well.

@Angelo: I ve already heard about these "coccus", but I don´t remember them as being so perfectly organised and distributed within such "elongated" cavity (tunel). Are these coccus can be remineralised as Si and Al? because the coccus I've heard about was composed mostly by iron oxide..

@ Anita: interesting...and yes we can observe some smaller minerals within the sphere... but I can tell you that it is not iron oxide or iron sulphur, otherwise they should be as bright as the Ti-bearing iron oxide located just at the top of the sphere. And  also, no volcanic soils in these marine sediments nor in the area.

.

@ Kenneth: independently, allophane is not compatible with the sedimentary context: no volcanic ash or soils, marine sediments in proximal platform...

Thank you very much to all of you for these interesting suggestions and constructive discussion

eric

PS: I attached the compositional mapping of the photograph

No comment

Cher Eric ... toujours aussi curieux, bravo. Pour la nature de ces oeufs, je donne ma mangue au chat, à moins que ce soit ceux du poissson d'Avril ... que tu as malicieusement placés là pour que les copains réagissent. Amicalement Jean-Luc

Maybe KEnneth ;).. actually, these "eggs" come out of the blue, I was not specially looking at them... but at iron oxide and sulfur morphology (not specially composition). But under SEM, there is always something unexpected that comes to the observator eyes ;).

 et non JEan-Luc, c est pas un poisson d'avril, mais ca aurait pu hehe amities!

I agree with Kenneth, try to polish the section, coat with carbon and your EDS data will be better.

Hi Eric,

Once upon a time I has been working with so called peloids, including siliceous peloids in sulphide deposits. They (paloids) are spherical forms with size from 80 - 150um, so generally larger as the spheres you'v found. However, peloids have complex inner composition, which can be revealed after etching, you can see many smaller spheres with um sub-um size. The photo you attached shows spheres about 10um in diameter,  but the spheres of yours are not homogeneous and show smaller composite details. I would suggest you prepare normal polished section from your sample, but please polish it without water as lubricant (use diamond spray to allow water soluble um-minerals to survive). Once you have your section polished, look at it, find spheres and go to etching. Use HF fumes as etching agent to reveal sub-um details. If fumes are not strong enough you would have to apply a droplet of HF for 0.5 to 3 minutes or more. Good luck. Harry Kucha.  If you need more discussion I am willing to help further on.

Are they truly spheres? They look like they might be filled channels (cylindrical) in either organic material, or left as surrounding calcite/aragonite was dissolved away.  The numerous rhombs in the image suggest dolomitization, which often leaves molds of fossil fragments.  At 20kV the imaging depth for BSE can make organic material look somewhat transparent, though it looks more to me like an etched fossil fragment.

Well, there are many possibilities to interpret is. More scanning research is needed. Cheers, Harry

I also noticed the rhombs and thought the same. The compositional mapping (2nd image) shows Ba concentrated in the filled space above the spheres with the Ti, Fe and Si. Barite is often associated with low temperature hydrothermal dolomitization. 

Being a marine geologist (not being a biologist) my first impression is that the spheres in the tube are typical eggs of some small marine organism. It is very common to see in sediment samples long beads of eggs in a transparent tube. Also the actual nucleus inside looks like an embryo.

The surface of your sample seems to be unconsolidated. Has the surface been prepared in any way or was it scanned without sputtering (normally used when preparing samples for SEM?