Dear Raghav,

These cavities are most likely caused by the degassing of the basaltic magma, and now form irregularly shaped cavities. The rapidly cooled contact zone to the host rock shows no coarse pores. Towards the middle of the dyke pore volume increases significantly.

Best regards,

Guenter

Dear Guenter Sir

Any insights as to why such cavities could be found only at the center of the dyke? Such vesicles are uncommon in dykes that I have studied. Does it signify that I am looking at the shallowest part of the dyke (near the crust)  which is now exposed with less erosion?

Best

Dear Raghav

There are several feasible explanations to the examples you have shown. As you are aware, degassing in a magma can be triggered by pressure drop, reaching the saturation concentration for each volatile component at different pressures and that is the explanation you suggest (upper parts of the dykes, at lower pressure). Other possibility is volatile exolution as the magma solidifies; the concentration of volatiles in the melt increases as a consequence of volume reduction of the melt phase, reaching saturation and exolving. In confined dyke systems, this second option seems more probable and assuming a contact-inwards cooling, the vesicles are going to be concentrated in the center of the dyke, irrespective of the orientation of the dyke and even at higher pressures.  

Best regards 

Dear Raghav,

Both explanations above are correct.  If you have thin dikes you will most probably not have these vesicles as the dike material is cooled too rapidly.  Furthermore, the quantity of volatile material may not be the same in different dolerite dike systems.  So, the presence of vesicles from degassing will not be a good measure of pressure or depth during solidification of the dike.

Kind regards

Thank you for your explanations. I will redirect myself to reading relevant papers on this process.

Excellent answers, surely degassing will concentrate in the central parts of the dikes forming vesicles, instead in lava flows, degassing occurs only in the top or the flows, as it happens in the Deccan Trapps. An important "world class example" are the Triassic dolerite sills in New Jersey, those not only had produced large vesicles, due to their great thickness, but the cavities were hydrothermally filled with all sorts of secondary minerals, many of them quite famous and common in museum and private collections, such as prehnite, apophyllite, chabacite, heulandite, datolite, and many others.

Dear Raghav,

            Is the host rock of these dolerites, basalt?

                                    Regards.

Dear Jayabalan Sir

The host rock for these dolerites is an argillite-quartzitic rock.

Regards

Dear Sebastian Sir

Thanks for your answer. Any paper for those Triassic Dolerite Sills from New Jersey? I could not find any neither on ResearchGate nor Google Scholar. It would be great if you can provide one.

Regards

Dear Raghav:

There are endless references about the Triassic-Jurassic Central Atlantic Magmatic Province (CAMP), you can start looking in Wikipedia, and Google, but these are a few of them, in works done in the North American Atlantic Coast:

Wilson, M. (1997). "Thermal evolution of the Central Atlantic passive margins: Continental break-up above a Mesozoic super-plume". J. Geol. Soc. London 154 (3): 491–495. doi:10.1144/gsjgs.154.3.0491

McHone, J.G. (2003). "Volatile emissions of Central Atlantic Magmatic Province basalts: Mass assumptions and environmental consequences" (PDF). In Hames, W.E.; Mchone, J.G.; Renne, P.; et al. The Central Atlantic Magmatic Province: Insights from Fragments of Pangea. American Geophysical Union Monograph 136. pp. 241–254. doi:10.1029/136GM013. Retrieved August 2015.

Marzoli, A.; Renne, P.R.; Piccirillo, E.M.; Ernesto, M.; Bellieni, G.; De Min, A. (1999). "Extensive 200 million-year-old continental flood basalts of the central Atlantic magmatic province". Science 284 (5414): 616–618. Bibcode:1999Sci...284..616M. doi:10.1126/science.284.5414.616. PMID 10213679

With regards. Sebastián Grande

Thank you Sebastian Sir.

Yes Raghav, I agree with Enrique.

These features resemble classical gas pipes indicating the escape way of the vapour phase.

michele

Hi Raghav,

Many great answers have been already given regarding the presence of vesicles in dykes. I have similar vesiculation visible in Archean-aged mafic dykes in my thesis area.  If I could just elaborate on how the vesiculated dykes are distributed in my field area:   The vesicles are present in dykes of varying thicknesses, grain sizes and compositions but we do see a higher abundance of vesicles in dykes towards the stratigraphic top of one assemblage and therefore think it's more prevalent in dykes that were emplaced at shallow levels.   

Lyndsay

Hi Lyndsay

That answer was very informative. Actually when I saw the phenomenon in field, I had thought of its shallow intrusion exposure, but just wanted to confirm by asking here.

Thanks for your answer.

Raghav

Hello,

Just a small clarification please: Are these dykes emplaced in basalts of Deccan Traps?

If so, from your photographs, I suspect they are squeeze-up features commonly observed in pahoehoe flows; in which case the vesiculation would be a norm, rather than an exception. To that extent, then, your conclusion of 'shallow intrusion' also holds, since they form in the upper zone (often along tumuli) of inflated flows.

The first photograph (as also indicated by others) appears to be a pipe-cylinder. A typical vapour escape feature, again characteristic of inflated pahoehoe flows.

However, if the dykes are emplaced in other rocks, I would ascribe the vesiculation to a a 'passive' rather than 'force-full' emplacement of the doleritic lava/magma into openings of the country rock. Since the emplacement is lacking in any dynamic force, the chilling along the walls of the dyke (which will prohibit escape of vapour in the lava / magma) will leave behind a gas-enriched fluid in the core, and that can very easily explain the vesicles along the core of the dyke.

Hello Vivek Sir

These dykes have been intruded into metagreywackes and not Deccan Traps. Thank you for your two-possibilities explanation of the features.

I have a doubt. Why 'passive' intrusions would leave a vesiculated core? A dyke is a forceful injection into the rocks (So I don't really understand 'passive' types). Can you please mention some reference papers so that I could understand the process more clearly?

That would be of great help.

Regards

Raghav

Hi, very interesting answers. I learn a lot. An easy way (not necesarry correct) is to think that in a dyke intrusion, the magma is faster and hoter in the center than in the border, so the vesicles and crystals could be concentrate in the center. The upper and center photographs are a little rare for me because the dyke contact is not easy to see (cooling contact?)...so, they or it could be a different process (lithofase?), so from 10 000 kms is difficult to tell; the lower one is clearly a shallow dyke intrusion. 

A forceful injection would definitely have a different geometry.

Photo 3 for example indicates that the dolerite has simply come and 'sat' in the existing joints (apparently a pair of joint sets that are also evident in the country-rock in the photo). Does not appear to have any dynamic impact on the wall-rock, nor does it display a 'forceful injection geomtery'.... Not surprising, seen commonly in the 'Newer dolerites' found in the Dharwar craton and surrounding areas...

Yes - the emplacement is interpreted to occur at very shallow crustal levels....

These are passive intrusions....

Trust it clarifies....

Dear Vivek Sir and Alvarado Sir

Thanks for the enlightenment. More literature review would be my next task.

Regards