Unlocking the Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

TL;DR Summary

petrology

(Edit: since the thread title was changed, this first sentence is too cryptic: the original title referred to a Tool song....)

Besides being a favorite song by a favorite band, the thread title is a straightforward play on words. This summer, as a present to myself for being promoted, I purchased a collection of thin sections that I believe comprise the research materials of Prof. Rob Verschure, who at the time was faculty in the Geological Institute in Amsterdam.

What changed this purchase from eccentric (although, at $2 per sample, also very affordable) to something more elevated is that Prof. Verschure published his findings on many of these samples, primarily in:

https://www.ngu.no/FileArchive/NGUPublikasjoner/NGUnr_380_Bulletin_70_Verschure_35_49.pdf

and

https://research.vu.nl/en/publicati...-parameter-for-metasomatism-and-its-applicati

i.e. I have a "Rosetta stone" for explosion breccias, carbonatites, and damtjernites. Many of the collected samples have been fully characterized, for example this thin section:

Sample Hor 1 has been classified as a calcite-bearing clinopyroxene-hornblende lamprophyre that has been dated to 313 Ma. This sample contains abundant augite and brown hornblende.

This sample (Fen 23) consists of zoned augite (I think...) and carbonates, dated to 594 Ma:

Many of the samples are carbonatites, but there are also many examples of schists and gneisses; here's a schist with pronounced folding:

I'm hoping to learn quite a bit as I get more familiar with the variety of samples...

 

[Andy Resnick]

I realized that while the microstructure of these rocks are interesting, the mesoscopic structure is often more photogenic- here are some samples photographed at 1:1 using a 55mm Micro Nikkor:

The other advantage is that I have the label in the image so I can more easily keep track of what I want to image at higher magnifications....

 

[Andy Resnick]

Here's my first 'official' presentation of one of the samples:

This is an example of a "Tveitan carbonatized damtjernite-like explosion breccia". It was obtained from UTM coordinates ⁵356-⁶⁵419, is 3.71% Potassium, 0.0888 ppm Wt of radiogenic ⁴⁰Ar (5% of which is 'atmospheric' ⁴⁰Ar) and was dated to 316 Ma. From the paper: "[The breccia consists of] a wide variety of xenoliths and xenocrysts in a very fine-grained groundmass consisting mainly of carbonate, green biotite, opaques, and apatite. [...] Many of the xenoliths and xenocrysts are strongly altered, but the abundant apatite phenocrysts and the cores of biotite phenocrysts and perthite xenocrysts do not show any alteration. Numerous veinlets of carbonate transect both the xenoliths and the groundmass"

The primary feature (I would call it a 'phenotype') I have come to associate with damtjernite are the rounded masses ("pelletal lapilli"). Here are a couple of higher-magnification views of this feature (crossed polars):

and, from a different sample:

The origin of these has (apparently) been a longstanding mystery- I found this paper claiming to 'understand' them as 'fluidised spray granulation':

https://www.nature.com/articles/ncomms1842

definitely not intro geology!

 

[Andy Resnick]

Next up, I was able to reproduce a few images appearing in:

Verschure, R. H. & Maijer, C. 2005: A new Rb-Sr isotopic parameter for metasomatism, ∆t, and its application in a study of pluri-fenitized gneisses around the Fen Ring Complex, South Norway. Norges geologiske undersøkelse Bulletin 445, 45–71.

Here's a piece of the published image (this is an open-source paper, btw.)

and mine (I have 3 of the 4 above samples):

Here's the relevant part of the figure caption:

"Fig. 10. Photomicrographs of fenitized gneisses. […].

(c) Ma 88: Biotite with reaction rim of massive Na-pyroxenes and alkali feldspar due to Fenitization-1. The rim was formed where biotite was in contact with quartz (clear) and not where biotite was in contact with feldspar (turbid). Other biotites within the same sample (not visible on this photomicrograph) appear stronger or even completely replaced. Location of the sample: Small quarry for road material near Steinsrud on road Holla-Steinsrud, Økonomisk Kart Foreløpig Utgave 1971 (BV 030-5-4) coordinates: 51502- 656890.1.

(d) Fen 125: Biotite replaced during Fenitization-2 by radiating needles of bluish Na-amphibole, subsequently partially replaced by fine- grained magnetite and hematite. The replacement occurred where biotite was in contact with quartz. Other biotites within the same sample appear unaffected or completely replaced. The sample was taken 1750 m south of the Fen Complex, far outside the zone of Fenitization–1. Location of the sample: HSP post M00059, Økonomisk Kart Foreløpig Utgave 1971 (BW 020-5-1) coordinates: 138965-51753. […]

(f ) Fen 33’’’: Biotite with a rim of a massive aggregate of blue Na-amphibole pseudomorphing Na-pyroxenes. Stilpnomelane (brown radiating flakes) occurs at the edge of the blue Na-amphibole aggregates. Location of the sample: Håtveittjørn Section 375 m from the contact with the Fen Complex. "

For me, this kind of information is really useful- not for the technical detail- I don't yet have enough knowledge to fully appreciate them- but rather (1) I learn how a geologist looks at samples, and (2) for the experience observing how color is used as a identification tool- I can't rely on color names to identify the mineral. For example, now when I read "bluish Na- amphibole", I understand what that color looks like to my color-deficient eyes. That holds especially for green/brown minerals.

Also, "stilpnomelane" :) .

 

[Andy Resnick]

This sample, "TVE 15", was obtained from UTM coordinates ⁵356-⁶⁵419 and is classified as a "sheared gneiss":

(All images are taken with crossed polarizers). This sample is one of a group of "crustal xenoliths from the carbonatized damtjernite-like explosion breccia near Tveitan, Bamble region". This particular sample is considered "country rock", rock that was already present during the explosion event and is dated to 1.4 Ga based on ⁸⁷Sr/⁸⁶Sr measurements.

It's clear there is something unusual about this sample- notice the elongated bright 'streaks' throughout the rock. These are sheared/strained quartz grains:

Note the presence of 'slip planes', which are visualized as undulose extinction. These generally lie parallel to the strain direction. The major components of this sample are quartz, K-feldspar (perthite) and plagioclase. Much of the plagioclase feldspar has undergone either sericitization or albitization. Minor amounts of biotite are also present (for example, there's a small biotite crystal located below the center of the second image).

I tried to calculate the pressure required to deform the quartz crystals, assuming the crystal started as a cuboid and was deformed via shear, but I got numbers that are way to large- about 10⁶atm. Clearly, slip planes reduce the yield stress and there are other considerations I am yet unaware of.

In any case, I started thinking about grain deformation/recrystallization dynamics and wondered about feldspar grains- quartz always has clearly demarcated grain boundaries, but feldspar does not seem to. For example, look at the (microcline) feldspar grains located just below the smaller deformed quartz crystal, left of center:

There are two bright-ish 'arcs' that separate the grains, and I have seen this particular pattern repeated in other samples. A closer look shows that the boundaries between feldspar crystallites have a much different appearance than quartz:

In contrast, neighboring quartz grains are (almost) always separated by a dark line:

The last image is of multiple microcline crystallites with a large feldspar grain that has undergone sericitic alteration, also showing symplectic alteration (symplectite):

Does anyone know of a good reference that discusses various grain deformation/recrystallization processes? The next sample I want to show has me thoroughly confused....

 

[Andy Resnick]

Today's sample is "Fen 59", mostly because I get to use a bunch of weird words....

This carbonatite sample was obtained from the Håtveittjørn section of the Fen complex, an inferred distance of -230m from contact with the country rocks... not sure how to interpret a negative distance. As I mentioned, this sample was a little confusing. At 1:1 zoom of the above image, it's clear that most of the material is a carbonate (for example, calcite and dolomite), but there are also what appear to be numerous subhedral and anhedral oikocrysts enclosing (most likely) anhedral carbonate chadacrysts:

What little I have read about pokilitic textures typically state that chadacrysts are euhedral, so one confusion that I still have is understanding how these formed. The other challenge was identifying the oikocryst mineral: neither quartz nor feldspar nor pyroxene nor garnet nor leucite. here are PPL (parallel polarizers) and XPL (perpendicular polarizers) views using a 4x objective:

In the PPL image, I decentered the illumination to improve the relief. The carbonate is grey and the oikocrysts are clear and in higher relief. It took some effort, but I finally identified the oikocrysts as apatite- the appearance of apatite in thin section for every kind of rock *other* than a carbonatite is "small crystals, hard to detect". Note also the carbonate vein and the small green/brown (remember- I don't do colors very well) crystals.

A closer look at some of those smaller crystals (surrounded by carbonate) in PPL and XPL is next. Make note of the birefringence of the crystals, the crystal on the left edge that is clear, and the corona of very small crystallites throughout:

So now we go to the relevant paper. The composition of this rock was reported as 60% carbonate, 15% biotite, 20% apatite (confirmation!), 1% quartz, 4% chlorite, and accessories of opaques, rutile, allanite, stilpnomelane, and dispersed carbonate. Since the crystals above are clearly not biotite, there's more to the story: "Fenitization"- an alkali metasomatic event.

In the "Remarks" data column, this rock is characterized as "M_F2carbonatite; Bt ⇒_F2Chl, _F2Qtz,_F2Carb rims", and now I can understand what I am looking at: a carbonatite, moderately Fenitized twice.

The original biotite, which appeared 583 Ma ago during the volcanic event, was subsequently altered first by a metasomatic Fenitization (F1) event that broke down the biotite, and then altered again (Fenitization-2), a late hydrothermal process replacing the F1 minerals, producing new fine-grained crystals of chlorite, quartz, and carbonates. And indeed, in the images it can be seen that the biotite crystals were replaced with chlorite (preserving the micaceous habit) and quartz, and also contain a reaction rim consisting of small carbonate crystallites.

Phew! That's a lot. Just getting warmed up, tho....

 

[Andy Resnick]

This, according to the paper, is a sample of 'melteigite', obtained from the Mjølteig A section of the Fen complex, -114 meters from country rock:

One thing I am learning is that the names of these 'exotic' rocks are often in flux- another name for melteigite is 'nephelinolite'. Melteigite comprises one end of the ijolite series, a type of plutonic rock consisting primarily of cliopyroxene and nepheline. Melteigite is at or near the melanocratic end, with the lowest fraction of nepheline while urtite, the most leucocratic, has the highest fraction of nepheline.

This sample is 'officially' described as a "strongly fenitized (F2) meltegeite, with coarse-grained zonal aergirine with titanaugite coronas". Without this, I would have absolutely no way to make sense of the images. For example, here's a pair (PP and XP) using a 4X objective:

I can maybe recognize a zoned-aergirine crystal in the center and (probably) a Ti-Aug crystal in the lower right. Other than that....???

Here's an image pair taken with a 16X objective from a different location:

I'm slightly more comfortable making mineral identifications here- for example, the central (zoned) aergirine crystal is partially surrounded by chlorite. I can maybe see a partial corona of crystallites, but I can't identify them. Perhaps surprisingly, there are virtually no opaques in this sample.

According to the paper, this sample has 63% aergirine, 3% each of biotite and melanite, 2% each of titanite, apatite, and carbonate. The F2 process created 9% each of chlorite and sericite, I'm guessing by altering the nepheline since I couldn't find any nepheline crystals in the sample. Carbonates make up the the remainder.

I can't find much information about ijolites (or melteigite or urtite). On one hand, I found "Ijolite is a rare rock type of considerable importance from a mineralogical and petrological standpoint.", but no references to back this statement up. By contrast, I also found "Ijolites are common alkaline rocks composed predominantly of nepheline (30–70 modal %) and clinopyroxene (~ 40 modal %), mainly diopside and aegirine-augite." So.... ?

In any case, it never fails that when I read the word 'foid', I giggle.

 

[WiredBuilder]

I'm getting the hang of identifying minerals, like those central aergirine crystals surrounded by chlorite. It's kinda cool that there aren't many opaques in this sample.

Now, about the composition, it's like 63% aergirine, with 3% each of biotite and melanite, and 2% each of titanite, apatite, and carbonate. That F2 process threw in 9% each of chlorite and sericite, probably by messing with the nepheline since I couldn't spot any nepheline crystals. The rest? Well, carbonates take up the slack.

I've been digging around for info on ijolites, melteigite, or urtite, but it's been a mixed bag. Some say ijolite's this rare deal with mineralogical clout, while others call it common alkaline rock mostly made up of nepheline and clinopyroxene.

 

[Andy Resnick]

Lately I've turned my attention to microcline, a form of feldspar, because I'm fascinated by the iconic 'tartan plaid' twinning pattern.

My impression is that the crystallization kinetics of feldspar is quite complex, resulting in a range of different crystal forms (monoclinic, triclinic) depending on temperature. I don't yet understand the details of albite/pericline twinning, but the twinning pattern seems (to me) a type of phase transition similar to spinoidal decomposition - a long-wavelength crystallization process (in contrast to the short-wavelength nucleation process).

One way microcline is altered (hydrothermally, for example) results in spontaneous unmixing of the two components (Na-feldspar and K-feldspar), forming two minerals called 'perthite' and 'antiperthite', depending on which component is the host mineral. Here's an example of perthite:

The thin lamellae are sodic feldspar residing within a larger K-feldspar crystal. The larger blobs are either (I think) orthoclase (center and bottom center) or sericite (right edge, bottom right), another type of feldspar alteration product.

It's possible that a third type of alteration results in "chessboard albite"- this is (I think) an example:

Maybe that would look better with crossed polars :)

Feldspare and quartz can also spontaneously unmix via spinoidal decomposition, forming myrmekite (a symplectite):

I'll end this post with a mystery:

Three microcline and a sericite-altered feldspar crystals surround a central crystal that may seem to be microcline, but notice the twinning does not follow the albite-pericline twinning laws. albite-pericline results in twinning patterns oriented at (nearly) 90 degrees, but the central one shows twinning at 120 degrees, so I don't (yet) know what this mineral is.

Suggestions are always welcome :)