The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

This sample is also a melteigite:

This sample consists of subhedral elongated grains of aegirine-augite, diopside, and apatite in a groundmass of (altered) nepheline, calcite, and titanite:

Nepheline altered to chlorite and sericite/muscovite (below, surrounded by apatite grains) so this sample is probably F2 fenitized to some degree.

Altered nepheline (center, below) contains small crystals of (possibly) epidote, if true then presence implies hydrothermal alteration.

A higher-magnification XP view of the crystalline inclusions- one in the upper left has the characteristic habit:

Some calcite in veins. Opaques: grains and a few “skeletal” arrangements of needlelike grains (ilmenite? magnetite? (60 degree symmetry? 90 degree symmetry? reflecting the primary mineral?) filled with pyroxene, calcite, and cryptocrystalline (probably)TiO2. These features are really striking and can best be seen with epi-darkfield imaging:

Biotite replaced with chlorite (anomalous blue birefringence), here with (F2?) titanite (very high relief) and pyroxene (high relief), a grain of carbonatite on the lower right:

Sample lacks melanite (see below).

From a relevant paper: Ijolites and melteigites (mela-ijolites)consist of euhedral prismatic crystals of clinopyroxene and apatite set in a matrix of nepheline and minor calcite. Sphene and strongly zoned (5-12% TiO2) melanite are common accessories. Pyroxenes are pleochroic in shades of light yellow-green to apple- green. Colorless to light brown cores of Al-diopside can be found, but the bulk of the pyroxenes are low Ti, low Al, Na-rich diopsides. The compositional zoning is one of Na and Fe enrichment along an aegirine-hedenbergite trend similar to that determined for the urtite pyroxenes, e.g.Di₇₀Hd₂₀Aeg₁₀ to Di₄₀Hd₄₀Aeg₂₀. Pyroxenes from ijolites which contain melanite are richer in Na and Fe on average than pyroxenes from rocks which lack melanite.

 

[Andy Resnick]

I originally thought this sample was also a melteigite:

because it superficially looks like the other samples. However, it's actually a pyroxene-hornblendite (or a hornblende-pyroxenite), an ultramafic rock lacking quartz and feldspar that consists of nearly equal amounts of hornblende and clinopyroxene (most likely a mix of diopside and augite):

The transparent minerals are apatite and carbonate, likely calcite. While this sample is about 5% apatite, the other Fen 12 sample I have is closer to 15% apatite. But that sample doesn't have photogenic crystals of hornblende:

Both hornblende and pyroxene grains are anhedral poikiolitic containing mostly apatite chadacrysts, but there are also pyroxene and biotite chadacrysts. Since everything is sub- or anhedral, I'm not sure if these are proper 'chadacrysts' or just inclusions. Hornblende is also a hydrothermal metamorphic product of pyroxene, so pyroxene inclusions could indicate incomplete alteration of the pyroxene. A couple of higher magnification views of the central inclusions above, showing apatite and clinopyroxene:

The opaques are interesting. They are a mix of red hematite (in veinlets), Illmenite (steel grey), magnetite (yellow/gold), and (I think) cryptocrystalline Titanium dioxide (white):

A super-duper magnification view of the lower right- these grains are tiny! Solidification must have happened extremely rapidly... but it's not a glass.

 

[Andy Resnick]

Instead of discussing another sample from the Fen complex, I want to present a sample from Crabtree Mine, in NC:

The nearby town of Spruce Pine has been in the news recently; Hurricane Helene dumped about 20 inches of rain into the area around Asheville which has frankly devastated the entire region. My family and I were vacationing there 14 months ago, and we took a side trip to the Crabtree emerald mine:
https://www.emeraldvillage.com/mines-activities/crabtree-emerald-mine/

I knew the area was full of interesting mineral deposits and opportunities for "rock hounding" (https://www.mindat.org/loc-26957.html), because the prior time I visited (also Crabtree) I was about 12 years old, pretty much when this picture was taken:

That's a load of emerald ore coming up from the mine for us 'civilians' to work through. All of those rocks you see in the image are pegmatite bearing large crystals of tourmaline, mica, cancrinite, beryl... and the occasional emerald . I have vivid memories of the place and was happy to re-visit the area. Even so, I was unaware that the region supplies most of the world's semiconductor industry with pure quartz (the Spruce Pine mine). I can't imagine what the area looks like now.... we joke about how flooding refreshed the mine dumps, but when I think about the people who live and work at Emerald village, or Little Switzerland, or the 1 1/2-lane roads that snake up and down the mountains... it's bad right now.

The region's geology mostly consists of a large intrusion of pegmatite, and the sample I have is primarily plagioclase containing abundant tourmaline (schorl). Last year I sent the sample off to Van Petro (https://www.vanpetro.com/) who prepared the thin section:

I would classify this particular rock as quartz-diorite, and the plagioclase often shows deformation twinning.

Confirmation of tourmaline- top row is PP, bottom is XP:

The top row shows pleochroism, the bottom shows extinction at 0 degrees- these properties are confirmatory for tourmaline.

One odd aspect about the plagioclase is that some of the grains are sort of blue-grey, while others are sort of yellow-brown. It's hard to show in photos, but hopefully you can see the faint colors, even in PP.

And finally, a grain of (what could be) ultrapure quartz:

Anyhow, I hope the residents are able to recover.

 

[Andy Resnick]

This is an example of Damtjernite from the type locality Damtjern:

Quoting from various papers:

“Damkjernites from the type locality at Damtjern are lamprophyric rocks containing phenocrysts of red-brown titanian phlogopite, yellow-brown titanian pargasite (amphibole), and clinopyroxene set in a groundmass of brown-green ferropargasite, pyroxene, green phlogopite, manganoan ilmenite, ulvospinel-magnetiite, and calcite."

Damtjernites are not found anywhere else in Norway, but they seem to be similar to Alnöites. Superficially they are similar to kimberlites, but there are significant differences in mineral content that argues against any simple relationship. In fact, the various damtjernite dikes surrounding the Fen complex all have very different appearances.

In the above view, a large pyroxene phenocryst is in the upper right corner, directly next to a grain of titanian phlogopite (pink-purple). Also visible are several zoned pyroxene phenocrysts (brown, mantled by yellow) and calcite. I'll zoom in on the bright purple phenocryst shortly.

Starting with the groundmass:

The opaques are Ilmenite, and it's (barely) possible to distinguish the amphibole and pyroxene by end facet angles- for example, in the center several of the grains show 60-degree angles (amphibole). The groundmass is also rich in carbonates. The grains in the upper left corner and lower right corner could be amphibole overgrown with pyroxene.

Zoned titanian phlogopite seems to be one diagnostic of Damtjernite. In PP, the grains are extremely pleochroic (clear - dark red/brown) and in XP, they are a bright turquoise:

In addition to titanian phlogopite, the complex pyroxene phenocrysts are another diagnostic.

"The pyroxene phenocrysts are composed of anhedral pale green cores mantled by anhedral to subhedral overgrowths of purple-brown pyroxenes. The pale green phenocryst cores are Al-Na diopsides which exhibit a wide range in Al content coupled with a low Ti content, i.e.the pyroxenes are rich in CaAl₂SiO₆ and poor in CaTiAl₂O₆. The Al-Na diopsides are complexly zoned with respect to Al₂O₃, which either increases or decreases from core to margin within individual crystals. The pyroxenes which mantle the Al-Na diopsides and which form the ground mass pyroxenes are Ti-Al salites (salite = Fe-bearing diopside), strongly zoned with increasingTi and Al from core to margin (Table l, anal. 6-9). This zoning reflects an increase in the CaAl₂SiO₆ component at the expense of the CaTiAl₂O₆. component.

Another example of complexly-zoned pyroxene phenocryst- I checked the extinction angle to verify this is pyroxene and not amphibole:

Another odd feature found in this sample (and a few other samples) are acicular masses of pyroxene surrounding a calcite phenocryst:

I like how the calcite looks like soap bubbles...

Finally, the opaques can also have a complex character. I believe this image shows ilmenite (the dark grey) lined with pyrite or magnetite (the bright yellow) and titanium dioxide:

I've identified 40 different examples of damtjernite in the collection, I hope to spend some time going through the different localities because the samples do show significant variations.

 

[Andy Resnick]

This next example of damtjernite comes from Brånan:

While there are some differences between Damtjern-damtjernite and Brånan-damtjernite, there are some similarities. For example, the presence of zoned titanian phlogopite:

and complexly-zoned pyroxenes (top of frame, titanian phlogopite on bottom):

From one of the papers: “The damtjernite dike near Brånan (Werenskiold 1910), 20 km NNW of the Fen complex, is also similar in appearance. The rock, which has been described by Brogger (1921) and Griffin & Taylor (1975), consists of phenocrysts of zoned augite, zoned biotite and aggregates of carbonate in a groundmass of pyroxene, biotite, opaques, melanite, sericitic pseudomorphs after nepheline, apatite and carbonate. Biotite produces a K-Ar age of 594 ± 20 Ma. “

I didn't find any melanite or sericritic pseudomorphs in this sample. In any case, the zoned pyroxenes are probably the most striking thing to image, the lower right image also has a crystal of phlogopite on the left side:

Another striking feature- I like how the artist made use of negative space within this calcite ocellus:

At higher magnification, we see the dark spots are not just opaques but something else (I don't know what...):

And finally, at super-duper magnification, I found what seems to be a metal oxide (likely ilmenite) partially covering the tip of a (likely) pyroxene crystallite- note how some of the dark spots are out of focus, so this really is a three-dimensional structure:

More to come...

 

[Andy Resnick]

Posting a day early this week... another damtjernite, this one from Steinsrud:

The appearance of this damtjernite differs somewhat from the other locales:

"A diatreme-facies damtjernite near Steinsrud, 1 km SW of the Fen complex, is likewise a less dark-coloured rock. Phenocrysts of clinopyroxene, large brown hornblende and biotite, along with phenocrysts or xenocrysts of feldspar (both sodic plagioclase and alkali feldspar) and aggregates of feldspar or feldspar-quartz are embedded in a groundmass of alkali feldspar, minor pyroxene, green amphibole, biotite, opaques, titanite and quartz. Biotite and hornblende give K-Ar ages of 523 ± 20 Ma and 597 ± 30 Ma, respectively.”

The following description is also appropriate- Sanna is a nearby locale with a damtjernitic plug that has an appearance similar to that at Steinsrud:

"The small plug at Sanna (Barth and Ramberg, 1966), 7km SSW of Fen, shows some similarities to the Fen damkjernites. Here, large titanian ferropargasite megacrysts are common and phenocrystal pyroxene cores are Na-salites. However, these cores are mantled by low-Na₂O salites with higher MgO contents than the cores. An outer thin discrete rim of apple-green acmitic pyroxene is commonly present on the phenocrysts, and a similar pyroxene together with green ferropargasite forms the bulk of the groundmass. Late-stage fluids have crystallized to alkali feldspars, zeolite (?altered nepheline), and calcite. At Sanna the majority of the phenocrysts are euhedral, country-rock xenoliths are uncommon, and spinel lherzolites appear to be absent, indicating that explosive activity was not so intense as at Fen and that the magma cooled relatively slowly. The Sanna "damkjernite" lacks the characteristic red titanian phlogopite and Ti-Al salites found at Fen."

The important point here is that all the damtjernites have been assigned the same age, suggesting they all formed during the same eruptive event. But then it's difficult to understand how the compositions could be so different. For example, here's a pair of images of the groundmass:

Along the top are three grains of sphene (very high relief), and the upper middle also shows a couple of hexagonal crystals of apatite. The bottom shows a crystal of pyroxene with the green acmitic rim. Unlike the previous examples, the groundmass here is primarily orthoclase rather than calcite.

An interesting object: randomly-oriented equigranular phlogopite and pyroxene arranged in a shell surrounding (I think) thin laths of feldspar:

Was this some sort of large amygdule? A closer PP view of the feldspar shows the relief in good detail:

Another interesting feature are (apparently) zoned opaques, here in reflected light:

The center is ilmenite (maybe chromite) surrounded by (most likely) magnetite with a thin rim of granular pyroxene.

Lastly, because this is not mentioned in either published report, I found what I think is stilpnomelane as an accessory in both veinlets and radiating needlelike crystals, here just some needles:

 

[Andy Resnick]

Another damtjernite, this one from the island of Presetoya:

"Very similar in mineral composition [to a dark damtjernite dike near Gulbrandstjern], but containing less altered, zoned biotite, is the damtjernite dike on the nearby island Presetoya in Hoseivatn (Klåy 1965), which probably forms the continuation of the Gulbrandstjern dike. Biotite gives a K-Ar age of 594 ± 20 Ma.

"The damtjernite dike near Gulbrandstjern (Klåy 1965), some 20 km SW of the Fen complex, consists of abundant phenocrysts of zoned pyroxene (Ti-augite, occasionally with aegirine-rich cores), biotite (strongly replaced by chlorite and apatite), opaques, zoned melanite and apatite in a groundmass of carbonate, chlorite, epidote, white mica, opaques, titanite, apatite and minor alkali feldspar. Biotite yields a K-Ar age of 601 ± 20 Ma."

There is comparatively less groundmass compared to other damtjernites. Another zoned pyroxene:

One of the pyroxene phenocrysts contains an amygdule:

It took a while for me to identify the andradite (var. melanite). I thought garnets were isotropic, but apparently andradite can show weak birefringence:

I think many of the melanite grains have sphene cores: melanite formula is Ca₃(Fe,Ti)₂(SiO₄)₃, sphene is CaTiSiO₅, so the chemical composition is similar, and it's possible to see a grain of something in the center of the garnet: