3
$\begingroup$

Is porphyritic texture always indicative of a 2 stage cooling process? Can't the phenocrysts and groundmass be formed at the same time depending upon the chemical constituents forming the rock? The groundmass being made up of constituents least in proportion and the phenocrysts being made up of the constituent in greatest proportion?

$\endgroup$
  • $\begingroup$ Crystal growth takes time so no I don't see how this would be possible. Crystal size is essentially a function of how long the crystal has been growing. $\endgroup$ – bon May 20 '16 at 11:25
4
$\begingroup$

Is porphyritic texture always indicative of a 2 stage cooling process?

Not necessarily. While it is a nice simplification for undergrad textbooks and it nicely explains phenocrysts in some simple basalts, there is much more to it.

Availability of chemical constituents

Can't the phenocrysts and groundmass be formed at the same time depending upon the chemical constituents forming the rock? The groundmass being made up of constituents least in proportion and the phenocrysts being made up of the constituent in greatest proportion?

In, principle, yes. Think of a granitic porphyry rock with huge feldspars and tiny zircon, apatite and oxides (magnetite, ilmenite, etc) in the groundmass. It could very well be that the feldspars are huge because the rock was rich in Na, Ca, K, Al and Si and it had little Zr, P, Ti and Fe3+, as you would expect from a granite. Even if all minerals were crystallising together, the minerals with the trace components will not grow any larger because there is simply not enough of the elements that make them.

Visual bias

Even if the rock looks porphyrytic, it may not be. Rocks with equally distributed grain size (also called seriate) may sometimes appear to be composed of phenocrysts and groundmass because this is what our brain sees. A seriate rock will have large and small crystals, and usually you will have lots of small crystals sitting just next to the big ones. Our brain tends to ignore the fact that the big ones actually come in all sizes, medium and larger.

Growth rate

It was mentioned in the comments that "Crystal size is essentially a function of how long the crystal has been growing". This is mostly true for a specific crystal, in a known cooling rate, in a defined chemical magma composition, when it perfectly obeys thermodynamic rules, etc. In real natural systems this is not always the case.

For crystals to grow, elements have to be removed from the silicate melt structure and attached to an existing crystal. What if there is no crystal yet? Usually, the formation of crystal nuclei requires a certain amount of undercooling of the silicate melt (up to a few tens of degrees, depending on conditions). Crystals can also nucleate on other minerals or vapour bubbles in the magma (just like bubbles in beer nucleate on the glass and not somewhere in the beer itself).

enter image description heresource: http://www.tulane.edu/~sanelson/eens212/textures_igneous_rocks.htm

However, temperature ideal for nucleation is not always ideal for crystal growth. So in this case, you will end up having lots of small crystals. This varies for different minerals, so one mineral may be in the "growth" field while another will be in the "nucleation" field. This will cause two different mineral sizes as the same time.

Dynamic magma chambers

Recent research shows that our view of a single magma chamber that crystallises slowly or erupts is basically wrong. There are abundant processes that mix and mingle magmas, inject magmas to other magmas, and this sort of stuff. Think of a shallow magma chamber under a volcano that has crystallising clinopyroxene and has some amount of volatiles (CO2, H2O) dissolved in it. This magma is sitting there minding its own business, when suddenly a fresh hot mafic magma is injected from below. Our pyroxene phenocryst-bearing magma is now heated, expands and tries to escape upwards. It decompresses while doing so, nucleating bubbles of gas. The magma is also very undercooled and undersaturated with respect to olivine, so this olivine now crystallises in abundant tiny crystals on the bubbles. In this case, the groundmass actually crystallises when the magma is heated, not when the magma is cooled. Note that this is only an example, not all rocks that look like this formed in the way I described, but it's a possibility.

This is also true for plutonic rocks where the mineral assemblage you see rarely corresponds to the exact composition of the magma from which it crystallised. Crystals can and do move around in magma chambers. This can bring together crystals of different sizes into one place.

Large crystals may not necessarily crystallise from magma

Phenocrysts may nucleate and grow rapidly in water bubbles that exist in the magma, during magmatic conditions (so I'm not talking about zeolites). While the minerals are those you would expect to see in the solid rock after magma solidified, it was favourable for those crystals to grow in the coexisting bubble. This is one explanation (out of many) for the formation of Rapakivi granites.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.