The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

The next fenitized gneiss example:

Fenitized gneiss. I’m not sure what feature was marked in blue pen- I didn’t observe anything unusual in that region. This sample is weak F1 and moderate F2, so alterations preferentially show effects of F2. This second metasomatic event is much less well defined (both in terms of originating fluid(s) and succession of mineral growth) as compared to F1. For example, whereas the source material for F1 is known (fluids and volatiles driven by the ijolite intrusion), the source material(s) driving F2 is unknown and may actually consist of successive pulses of fluids, each with differing metasomatic chemistries.

A later paper (https://www.cambridge.org/core/jour...implications/00AA1631F359FD10A73327CB7BEEB65F) subdivides the Fenitization-2 metasomatic event into discrete fenitization events. “Verschure and Maijer (1984) have distinguished between two metasomatic events in the fenite aureole, the first (here called A1) producing acmite or sodic amphibole, the second (A2) forming stilpnomelane at the expense of these minerals. A third, later metasomatic event of aureole extent (A3), was caused by interaction between rocks and groundwater-derived hydrothermal fluids infiltrating the eastern part of the complex, leading to oxidation of ferrocarbonatite to hematite carbonatite, locally known as 'Rødberg', i.e. 'red-rock’.”

Fenitization-2 (meaning both A2 and A3) is a hydrothermal process resulting in the replacement of Fenitization-1 minerals, primary gneiss minerals, and also minerals of the Fen Complex (Ijolites and carbonatites). Fenitization-2 produced low-temperature crystallization of hydrous minerals (Na-amphiboles, new greenish biotite, new chlorite, sericite and stilpnomelane), carbonates, quartz and opaque minerals. Fenitization-2 minerals are fine grained and seldom exceed 10% of the total rock volume.

These images show “Biotite replaced during Fenitization-2 by radiating needles of bluish Na-amphibole [A2], subsequently partially replaced by fine- grained magnetite and hematite. [A3] 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. “

Because fenitization is so variable, there's a lot of samples to work through. While the next several samples are examples of F2 (meaning both A2 and A3), there is another whole set of fenitization examples formed at the contact between intrusion and existing wall-rock (so-called "contact fenitization" and denoted C1 through C4), with the majority of contact fenitization occurring in carbonatites.

Lots to learn! #thinsectionthursday

 

[Andy Resnick]

Here's another example of a 'pulaskitic fenite', one that exemplifies the type:

Strongly Fenitized-2 Pulaskitic fenite sample from the Håtveittjørn Section, located 10m from contact between country rock and the Fen Complex. Classified as a strongly Fenitized-2 (low temperature hydration-carbonation, F2) fenite; acc Ap; F2 Op veins; 2% stilpnomelane in veins; 88% mesoperthite, 2% F2 Sphene, 2% sericite. Perthite unmixing texture in orthoclase grains with discrete albite rims (co-oriented albitization of K-feldspar). 2% F2 biotite, 2% F2 carbonate.

Below is F2 titanite (high relief, high backscatter), stilpnomelane (brown fibrous appearance) and feldspar:

Here is stilpnomelane, magnetite, and feldspar

The albitized K-feldspar is quite photogenic: this is within the region outlined in blue marker:

‘Pulaskitic’ means this rock is classified as a nepheline-bearing syenite composed of alkali feldspar (perthite) and nepheline (altered to sericite). Interestingly, unlike the previous examples of F1 gneisses shown before, in MA 68 the replacement of K-feldspar with albite is co-oriented rather than hetero-oriented. Hetero-oriented replacement occurs at grain boundaries between two minerals, while co-oriented replacement takes place inside the replaced mineral, a difference attributed to “intense sodium metasomatism”. F2 minerals include stilpnomelane, sericite, F2 carbonate and F2 titanite.

Perthite inclusions in sericite (altered nepheline?) do not show albitization, biotite inclusions altered to chlorite?

Veinlet of chloritoid (high relief, grey-green color, low birefringence)?

 

[Andy Resnick]

Here are 2 thin sections (presumably) from the same rock:

Fenitized Gneiss. Prof. Verschure classified this sample as weakly fenitized-1 (1% by volume) and moderately fenitized-2 (10% by volume). According to Andersen, this sample seems to exhibit A3 metasomatism rather than A2 metasomatism, evidenced both by the metasomatic replacements Albite + microcline + quartz + biotite + hornblende to Quartz + calcite + albite + chlorite + hematite and the lack of Na-amphibole.

Much of the primary gneiss remains intact- anhedral grains of quartz and plagioclase comprise about 50% of the rock volume, with primary perthite and additional 30%. What may have been vein-like aggregates of Na-pyroxene have been replaced by palimpsestic magnetite, calcite (possibly ferrocarbonatite), and quartz.

This sample presents one of the difficulties challenging easy understanding of the Fen complex geology: while veinlets of aegirine (Na-pyroxene) aggregates are characteristic of strong F1 metasomatism, there is significant primary gneiss present and almost a complete lack of K-feldspar albitization, a metasomatic alteration which is definitively characteristic of even weak F1 metasomatism.

Biotite has been completely replaced by chlorite and magnetite, and where primary biotite was in contact with quartz, an inner corona of (I think) diagenic quartz and outer halo of chlorite/opaques now exists.

Vershure reports that the biotite -> chlorite pseudomorph occurred pre-fenitization while the chlorite halos are F2 chlorite. That paper also claims that some opaques are associated with the primary gneiss but have been completely altered via F2 metasomatism, in addition to 3% post-F2 opaques. I am not sure if these claims are consistent with Andersen’s paper or not.

 

[Andy Resnick]

Another sample classified as fenitized gneiss:

Fenitized Gneiss. Prof. Verschure classified this sample as weakly fenitized-1 (1%) and moderately fenitized-2 (9%). However, according to Andersen, this sample seems to be A3 rather than A2, evidenced by the metasomatism Albite + microcline + quartz + biotite + hornblende to Quartz + calcite + albite + chlorite + hematite. Zircon is present as an accessory. Primary gneiss is nearly equal amounts of quartz, perthite, and plagioclase, and a prominent vein consisting of quartz, calcite, and bundles of needlelike opaques intrudes.

Biotite altered to chlorite with opaques and, where in contact with quartz, a halo of opaques and (possibly) hornblende. Also in the image: grain of zircon, K-feldspar with sericite inclusions.

Lastly, a grain of what was likely horneblende and a grain of either epidote or fenitized titanite (it's hard to tell the difference):

 

[Andy Resnick]

Yet more fenitized gneiss:

These gneisses are not the most photogenic of samples; I keep reminding myself that characterizing the samples is also an educational process... anyhow:

Fenitized Gneiss. Prof. Verschure classified this sample as weakly fenitized-1 (2%, primarily chessboard albite) and strongly fenitized-2 (11%).

However, according to Andersen, this sample seems to be A3 rather than A2, evidenced by the metasomatism Albite + microcline + quartz + biotite + hornblende to Quartz + calcite + albite + chlorite + hematite.

The presumption is that original biotite/hornblende altered to aegirine during the Fenitization-1 event, and during the fenitization-2 process, the aegirine altered into chlorite, carbonate minerals, and metal oxides (opaques).

 

[Andy Resnick]

Now the samples are more strongly fenitized and are also more photogenic:

Fenitized gneiss. This sample was been classified as having strong fenitization-1 and moderate fenitization-2. Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation. This sample’s fenitization-1 minerals (chessboard albite and aegirine, 20% and 10% of rock volume, respectively) comprise 30% of the total rock volume and occur as bands separating grains of the primary gneiss, sometimes referred to as a "mortar texture":

Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the (nearly) complete disappearance of primary plagioclase. Of the primary gneiss minerals, plagioclase alone shows a significant reduction in volume (10% final volume). Na-pyroxene now occurs entirely as irregular aggregates. Fenitization-2 minerals comprise 2% of the rock volume: opaques (1%) and arfvedsonite (1%). Titanate is present (1% volume), likely fenitzation-2 altered.

Here, a grain of feldspar shows Carlsbad twinning:

 

[Andy Resnick]

More fenitized gneiss samples:

Fenitized gneiss. This sample was been classified as having strong fenitization-1 and moderate fenitization-2. Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation. This sample’s fenitization-1 minerals (chessboard albite and aegirine, 30% and 5% of rock volume, respectively) comprise 35% of the total rock volume. In this sample, strong Fenitization–1 is characterized by a major reduction in the amount of primary quartz, primary microcline perthite and primary plagioclase (60% final volume).

In places, the K-feldspar is heavily sericitized. Na-pyroxene (aegirine) now occurs entirely as veinlets formed of irregular aggregates. Fenitization-2 minerals comprise 5% of the rock volume: opaques (2%) and stilpnomelane (3%).

Titanate and sericite are present as accessory minerals. The anhedral grains of titanite (presumably) show evidence of metasomatic alteration.

Lastly, an inclusion (unknown mineral) inducing stress birefringence in the host grain (K-feldspar):

 

[Andy Resnick]

Coming up on the end of Fenite samples, only a few more to go:

Fenitized gneiss. This sample was been classified as having strong fenitization-1 and moderate fenitization-2. Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation. Apparent breccia zones (most likely formed during metasomatic events), some containing stilpnomelane.

This sample’s fenitization-1 minerals are exclusively chessboard albite (co-oriented albitized K-feldpsar) comprising 53% of the total rock volume.

In this sample, strong Fenitization–1 is characterized by the complete replacement of primary quartz, accompanied by significant reductions in primary microcline perthite and primary plagioclase (40% final volume). Fenitization-2 minerals comprise 6% of the rock volume: opaques (3%), F2 chlorite (2%) and stilpnomelane (1%). Titanate and sericite are present as accessory minerals. The largely cryptocrystalline grains of titanite (presumably) shows evidence of metasomatic alteration.

The boundary between a breccia zone and country rock is quite sharp:

#thinsectionthursday

 

[Andy Resnick]

I think this will be the final example of Fenite:

Fenitized gneiss. This sample was been classified as having strong fenitization-1 (45% rock volume) and strong fenitization-2 (15% rock volume). Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation.

This sample’s fenitization-1 minerals are chessboard albite and aegirine, (40% and 5% of rock volume, respectively). Aegirine now occurs entirely as acicular aggregates, either fibrous or radiating. In addition, the quartz appears to have undergone 'dynamic recrystallization', as evidenced by the lobate grain boundaries:

Fenitization-2 minerals include opaques (2%), arfvedsonite (4%) and fenitized carbonate (9%). Here is an image showing partial transformation of Na-pyroxene (aegirine) to Na-amphibole (arfvedsonite):

The carbonate occurs as a vein, consisting of aggregated anhedral grains, possibly indicating a post-emplacement metasomatic event (in the paper, the carbonate is categorized as fenitization-2).

The carbonatite itself, as part of the Fen complex ijolite intrusion, providing the fluids driving the fenitization-1 event. So, I think the next few samples will be carbonatites. Stay tuned!

 

[Andy Resnick]

Moving on from Fenites, I have several samples of carbonatites- the source magma for fenitization. Carbonatites are still the subject of active research, and I confess I'm way out of my depth. But, the optical properties of carbonates make the samples fairly photogenic. Here's the first one:

Carbonatite. It is currently thought that this material is mantle-derived (possibly from the deep mantle) and is uncontaminated by country rock. Carbonatite magmas have physical properties very different from silica magmas- carbonatite magmas are highly conductive and have very low (by comparison) viscosities- honey is more viscous, actually. While this is an igneous rock and carbonate occupies at least 95% of the rock volume, the texture is reminiscent of augen gneiss- a metamorphic texture. Augen (from German word for "eyes") are lenticular mineral grains or mineral aggregates surrounded by a finer-grained matrix. Augen form by growth of a mantle on a porphyroblast with preferential growth occurring in low stress regions in the pressure shadow.

The optical properties of carbonates are fun to explore. One example is 'twinkling': in PP, the sample's relief changes under sample rotation:

The birefringence is so high that interference colors can appear even in PP images- if you look closely, you can find them in most of the images.

Of note, apatite grains in carbonatite are nearly always anhedral, often an elongated oval shape, and generally occur as aggregates. This is nothing like how apatite appears in other host rocks:

Unfortunately, optically there is no way to definitively determine if the carbonatite is calcio-, magnesio- or ferro-carbonatite. The preponderance of inclusions (‘dusty’ appearance) may be hematite and thus indicative of ferrocarbonatite.

It's unclear what these inclusions are:

 

[Andy Resnick]

A second example of a carbonatite:

Carbonatite. The literature on carbonatites is generally too advanced for me (for now), so I will simply quote some relevant sentences from a recent review (Annu. Rev. Earth Planet. Sci. 2022. 50:261–93) that may be open source:
“Carbonatites are igneous rocks formed in the crust by fractional crystallization of carbonate-rich parental melts that are mostly mantle derived. […]Their emplacement into the crust is usually accompanied by fenitization, alkali metasomatism of wallrock caused by fluids expelled from the crystallizing carbonatite. […], carbonatite melts can coexist immiscibly with carbonated silicate melts. […] Carbonatite melts continuously evolve by fractionating calcite, apatite, and dolomite, leading to concentration of other incompatible components such as alkalis, halides, and sulfate ± H2O.”

While this is an igneous rock and carbonate occupies at least 95% of the rock volume, the texture is again reminiscent of augen gneiss- a metamorphic texture. Augen (from German word for "eyes") are lenticular mineral grains or mineral aggregates surrounded by a finer-grained matrix.

Unfortunately, optically there is no way to definitively determine if the carbonatite is calcio-, magnesio- or ferro-carbonatite. Apatite present as loose aggregrates of anhedral grains (a peculiar crystal habit characteristic of carbonatites), and the presence of stilpnomelane may indicate a ferrocarbonatite.

The high birefringence and optical index of carbonates accentuates optical properties, for example the effect of condenser numerical aperture on the depth of field. First pair is using a low NA condenser (NA 0.3, IIRC) and the second a high-NA condenser (0.9, IIRC); both images using the same 16X 0.35 NA objective lens

Opaques are likely magnetite and pyrite.

 

[Andy Resnick]

Carbonatite, from the Fen complex. This sample has a significant amount of apatite, generally arranged in radiating aggregations of elongated anhedral grains. While most grains of apatite are characteristically anhedral, there are some large well-developed euhedral crystals.

Biotite has possibly been fenitized, a phenomenon (carbonatite-on-carbonatite crime, so to speak) I am starting to recognize in other carbonatite samples. Apparently, cabonatite magma is quite chemically active, with respect to silicate magmas.

In PP images, high-order interference colors may be generated by twin lamellae (2 closely spaced twin planes), some have constant color while others have a hue that varies (see below). While interference colors are usually associated with viewing through crossed polarizers, carbonates have such large optical constants that interference colors can appear even when using a single polarizer. Below, on the left is an image with only the analyzer inserted, while on the right is the view with only the (lower) polarizer inserted.

I don’t (yet) fully understand the mechanism that produces this optical effect because it is somewhat complex and based on optical anisotropy. In the left image, randomly polarized light (from the lightbulb) approximating plane waves first passes through birefringent carbonate (this sample may be magnesio-carbonate, based on the altered biotite) with the optical indicatrix in a given orientation with respect to the optical axis of the microscope. Then the light successively encounters two closely spaced planar interfaces (the twin lamellae), each interface characterized by discontinuous change to the orientation of the optical indicatrix, with the second interface returning the optical indicatrix to the original orientation. This (somehow) induces a phase difference between the two polarization states. The light then passes through a linear polarizer and interference colors are produced. On the right, linearly polarized light passes through the sample without encountering the analyzer.

While (I think) I understand how birefringence leads to interference colors between crossed polarizers, I’m not sure how interference colors can sometimes be visualized with a single polarizer. I may have to just calculate the Mueller matrix and obtain the resultant Stokes vector, something I am not in a hurry to do.

On a side note, the optical phenomenon of 'twinkling' behaves the same way: with no polarizers, there is no twinke. With just the analyzer, there is twinkle. With just the polarizer, there is twinkle.

In any case, Here's an image showing crystal defects surrounding a small inclusion (unidentified mineral):

 

[Andy Resnick]

Carbonatite, Fen Complex. This sample has a significant amount of apatite, generally arranged either in aggregations of elongated anhedral grains or larger isolated subhedral crystals.

Minor amount of altered phlogopite, which generally occurs as medium-to-large, isolated crystals:

Unclear what the ovoid (Poikilitic texture, but unclear either on the inclusions or surrounding melanocratic microgranular material.

I’ve mentioned that (for me) learning carbonatite petrology is hard because carbonatite geology is still an active area of research. For help, I recently turned to Notebook LM. Notebook LM, if you are unfamiliar, is an AI chatbot that lets me use peer-reviewed sources as training data, a constraint that (I hope) prevents/reduces AI hallucinations. I uploaded several primary sources, review articles, PhD dissertations, and textbooks about carbonatites and the Fen complex specifically, and posed questions that would typically result in links to peer-reviewed references, for example:

What process causes phlogopite to alter into a fine-grained aggregrate of carbonatite?

I can't make sense of the articles google sends me, but I can upload them into Notebook LM, which gave me this:

The alteration of phlogopite to an aggregate including calcite is described in the context of the C4 contact metasomatism in the Fen Complex:
• Process: The C4 process, associated with heterogeneous (pyroclastic) ferrocarbonatite (Group 4 carbonatite), induced pseudomorphic replacement of phlogopite. This process involves hydration and leaching of alkalis and CO2 from the rock.
• Starting Material: This alteration affects phlogopite phenocrysts (a mica closely related to biotite) set in a dolomite carbonatite matrix.
• Resulting Aggregate/Products: The phlogopite phenocrysts are transformed into an aggregate of chlorite, calcite, and quartz. The phlogopite crystals become disjoined along the basal cleavage, where lamellae of calcite have been formed.


This kind of output is enormously helpful (to me). There is still is the matter of fact-checking, which I confess I have not done. But, I present:

For my next trick, I hope to better understand this optical phenomenon Interference colors generated by twin lamellae (in a carbonate):

Right now, the best I can figure is that it's the result of [very high interference order] - [very high, but slightly different, interference order] = 1st or 2nd order interference colors (I don't trust my eyes on this, tho).